There is an often-repeated statement that different quantum mechanical theories are equivalent. This is an over-simplification. Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled?
Some smart people have tried to make theories where wave function collapse is a real thing. These new experiments are working to rule out those ideas.
My feeling is that Everett's "many worlds" interpretation is the clear favorite: there is no wave function collapse.
That's right: the "collapse" interpretation is an approximation.
You can derive if from the MWI. We already know that the "worlds" separate and almost completely cease interacting. The interactions decrease to the order of "two to the power of Avogadro's number" almost immediately. The collapse interpretation rounds that to zero (at some arbitrary point).
To make that more concrete you have to add a term to the equations, making it theoretically testable, but it's remarkable that anybody could manage to test it in practice. It's clever to use a neutrino detector -- something designed to measure ludicrously small amounts of interactions.
I don't think anybody is surprised by the results, just that they could get any. The un-physical arbitrariness of the cutoff caused problems from the get-go.
I was going to say that it doesn't make MWI any more palatable, though I suppose it does. They've detected the other branches, albeit in a very indirect way. They're real. But they're just as inaccessible as they ever were.
You still need the concept of decoherence to make it work. The unstable equilibrium can't hold for large objects. That's both mathematically and physically sound.
Not as an actual physical process, no. "Collapse" in the MWI just means that, once the "worlds" have decohered, within each "world" the wave function can be collapsed to the one that describes the measurement result observed in that "world", for purposes of predicting future results. But in the MWI this is just a mathematical convenience and doesn't correspond to anything physically happening.
In the theories being tested by the experiments described in this article, though, collapse is an actual physical process: there is only one "world". This contradicts the MWI.
> They've detected the other branches, albeit in a very indirect way.
Oh I misread your comment - I thought you were referring to decoherence as the mathematical convenience. Now that I see that it was collapse, definitely agree.
If other 'branches' are still detectable, then they're probably detectable by more than one means; say, gravitationally. Could this be an explanation of dark matter? That is, dark matter is the sum of tiny gravitational interactions between many mostly-split branches of the wave function.
“What actually happens in an experiment with electrons is kind of interesting. Since photons are massless they are easy to excite if they have long wavelength and hence low energy. Whenever an electron gets accelerated many "soft" (i.e., long wavelength) photons get emitted. But if the acceleration is weak, the photons have such long wavelength that they provide little information concerning which path, and interference is possible.
It is the same with gravitons. Except the probability of emitting a "hard" graviton (with short enough wavelength to distinguish the paths) is far, far smaller than for photons, and therefore gravitational decoherence is extremely weak.”
If quantum computers would work, it would be very difficult to explain "where" the computation happens without having some sort of MWI into picture. But if, somehow, quantum computers keep their noise levels up without producing any interesting acceleration I'll expect that a theory would emerge at some point explaining that. And it may disambiguate which interpretation is the one.
Quantum computers are easy to explain in CI as well: they simply exploit certain numerical properties of complex-valued probabilities (unlike the real-valued probabilities of classical probabilistic Turing machines).
No. They don't behave at all like dark matter. Among other things, there's no reason for them to be distributed in a halo, nor for some galaxies to lack them.
There's no reason for any dark matter to be distributed in a halo. Maybe it makes some math work better for the observations but nobody offers a dynamical explanation of how it would come to be a halo.
> There's no reason for any dark matter to be distributed in a halo.
This was one of the more eye-catching statements in this thread. To be blunt, you're wrong. A straightforward and almost inevitable dynamical explanation has been available (and explored in ever greater detail) since at least the mid-1990s, driven in part by the Saskatoon experiment and subsequently supported by BOOMERanG's observation of the first acoustic peak, and many others lines of evidence since then (see [1] for a less-heavy explanation than what follows).
There are of course other already-completed textbooks. For example, one can find an excellent pedagogic (if advanced) treatment of primordial Gaussian Random Fields[2] as applied to halo formation in chapter 7 of Mo, van den Bosch & White (2010) <https://www.cambridge.org/core/books/abs/galaxy-formation-an...> (you can find the chapter on scy hob via its DOI if you don't have institutional or otherwise affordable access).
[2] If you're more visually inclined and can't find the illustrated version of Mo et al., see the diagrams on p.3-5 of this slide deck on GRFs <https://www.astro.rug.nl/~weygaert/tim1publication/lss2016/l...> from Rien van de Weijgaert of the Kapteyn Institute at the University of Groningen.
I thought that the main mechanism that produces the 2D disk of matter in galaxies was gravity, and that dark matter is equally affected by gravity as normal matter? What is the prevailing theory as to why dark matter accretion around galaxies is different from normal matter?
(edit: to answer my own question, the rabbit hole starts here: https://physics.stackexchange.com/questions/485214/ -- basically, accretion happens because light matter can radiate and shed its (kinetic) energy that way. Dark matter does not radiate so it remains in a higher-energy state)
Dark matter affects some galaxies, but the rotation of others is completely explained by the observable matter. Why would one galaxy be influenced by its many worlds twins, but not another?
Well because that galaxy in that position might be a low probability event - not many universes have that galaxy at that position. So therefore that galaxy appears to have less dark matter.
Check out "superdeterminism," first proposed by Bell (of Bell's inequality fame) himself and currently being vigorously pursued by Sabina Hossenfelder. There is no "collapse" per se according to this model and everything is really classical with hidden variables, and the entanglement comes because everything shared the same origin (the big bang). A very fascinating and in my opinion the best explanation, though it's extremely controversial and taboo because of its implications on free will (e.g., I see no mention of it at all in this article).
I have never fully understood that idea, even after reading Hossenfelder's posts about it. The idea that there's a "block universe" and everything is predetermined is comprehensible, sure; but what's the exact mechanism by which Bell's inequality is violated in experiments? Doesn't that require the universe to have been carefully set up to look as if it's acting in a weird non-local way, on an experiment-by-experiment basis? I assume I am just misunderstanding the idea, though. Any suggested reading material much appreciated!
The idea is that the natural laws are fractal, which when mapped out trace many state space configurations. But with a fractal there are always gaps in state space that remain no matter how much you zoom in or out; fractals preserve certain structural invariants at all scales.
So the idea is that counterfactual reasoning sometimes fails in a fractal universe, because statistical independence no longer works as a general rule (your counterfactual could lie in one of those gaps). He first published this idea in 2009 under the name "invariant set postulate":
> Doesn't that require the universe to have been carefully set up to look as if it's acting in a weird non-local way, on an experiment-by-experiment basis?
Yes. That is a big reason why most physicists do not favor superdeterminism.
I am not so sure. In my understanding of the block universe, causality doesn't really exist, and the universe is simply a consistent solution with boundary conditions both at the beginning and at the end of time (as well as in space).
The game of life is perhaps a nice analogy here. On the 2d grid of the game of life, a glider seems moving in a certain direction when time moves forward. But in the 3d grid, where time is the third dimension, a glider is just a static volume of space.
The reason we experience time is that the evolution of the block universe does depend on the dimension you look at (e.g entropy generally increases in the positive time dimension), and because our consciousness is an emergent property of a self-sustaining structure within the block universe, it's plausible at least that we perceive movement in time different from movement in space. I realize this is very sloppy wording but I'm having trouble finding better words to describe my intuition here.
> In my understanding of the block universe, causality doesn't really exist
That's one way of interpreting it, but superdeterminism does not entail or require this. Superdeterminism can be formulated just fine for a universe that evolves in time from some initial state according to causal processes.
Agreed — superdeterminism is not so much a theory as a theory-for-a-theory at this stage (a vague guess at the form such a theory might take).
It is a sign of the much greater maturity of the physical collapse models that their parameters can be constrained by experiment. The ability to be constrained is a good sign, not a bad one.
I love John Preskill’s standard nod to superdeterminism in his writings: without mentioning it by name he says “I leave it up to the reader to decide how seriously to take this possibility” (get it?)
I think the right way to look at it is that superdeterminism is an umbrella term for a class of theories that have a certain property. Like some interpretations of quantum mechanics are "psi-ontic", meaning the wave function is considered physically real (like many worlds), vs. "psi-epistemic" where the wave function is considered a reflection of our knowledge of reality (Copenhagen).
> what's the exact mechanism by which Bell's inequality is violated in experiments?
It’s not violated - it simply doesn’t apply because it’s predetermined that the measurements that are done work well together. The experimenters follow the script.
That a serious scientist can believe this leads me to consider the economic benefits of believing such nonsense, rather than prompting me to examine it further. Just total rubbish
Free will is also incoherent in any religion or philosophy presupposing a bi-omni deity (which itself is an incoherent notion).
The point, that many ignore, however is that, free will does not matter. It is irrelevant. Regardless of whether one has free will or not, one still has to make choices and support the consequences of said choices. Lack of free will was never the get out of jail free card some fear it to be.
If you bring in to the discussion sociology and psychology, free will makes even less sense.
I get the impression that physicists who advocate “superdeterminism” are ignorant of the philosophical literature on “determinism” - because from a philosophical viewpoint, there is nothing “super-“ about their position, it is just plain old “determinism”.
Advocates of superdeterminism aren't always in favour of the term. Its most notable advocate, Hossenfelder, disapproves of the term, believing it to be misleading. She suggests the term 'implicate order' (crediting Bohm) be used instead.
> it's extremely controversial and taboo because of its implications on free will (e.g., I see no mention of it at all in this article).
"True" free will is ruled out by all existing physical theories, so that's not why it's controversial. The bigger problem with superdeterminism is that it wants to explain physics without assuming statistical independence, even for far away events.
Even without superdeterminism free will is a concept that only makes any sense in specific contexts. To say otherwise is to introduce God of the Gaps style woo.
The notion that a probabilistic world enables free will in a way that a deterministic one doesn't still confuses me. A coin I flip isn't exercising free will.
Could free will be explained as super-physical phenomena that "haunts" the physical world, w/ choice manifest as which of the branching many worlds' paths consciousness "chooses" to experience?
E.g., I am in some universe taking every possible action right now, but I'm only experiencing the actions which I "chose" to take.
What is the point of such a theory besides trying to relieve us from the uncomfortable idea of many worlds? It is no better at explaining our perceived world than the simpler theory that consciousness exists across all these states and we aren't following some particular path through them. All states of conscious existence are being perceived and the perception of it being a coherent path through time is just an illusion created by memory and the similarity of adjacent states.
Furthermore it still removes any actual willed choice of path within the context of our existence. I am not some meta-entity seeing all possible paths and choosing one for some unknown reason. That would be an existential alien "haunting" me and not "free will".
I am trying the reconcile the feeling of free-will we experience with its evident absence, per the theory.
(Of course, you could also just say the feeling is an illusion.)
I don't know if you can ever answer it one way or the other, because while consciousness certainly exists (again a fact I believe just because I feel it), it can't in any way influence the universe, at least per current theories of physics.
On the other hand, if you're right that many-worlds is real and our consciousness persists in all of them, then we will, individually, come to realize that is the case, as find ourselves still alive, forever, ever more improbably (since, I think, the odds of dying would never be 1, so you'll always be alive in some lucky world).
Consciousness does influence the universe by being part of it. That's why we have the illusion of choice within the context of each moment rather than some meta-alien's perspective. But no it can't defeat determinism if that's what you meant.
I believe someone called that idea "quantum immortality" and I find it conceptually interesting if not somewhat terrifying. However it does seem like the inevitable conclusion of these ideas.
I'm not sure I'm following the argument. Let me phrase it this way: can you devise an experiment which will determine if a given entity is conscious?
> I believe someone called that idea "quantum immortality"
(Thanks for the term; I'd heared the idea expressed before, but not that phrase.)
Skimming the wikipedia page, I didn't find any of the counter-arguments to be particularly compelling.
So, while I'm not sure how deeply I believe it (simply because it feels very unintuitive -- and I'm definitely not about to try to kill myself to test it!†), I agree with you: it really does seem like the inevitable conclusion of these ideas; I also agree it's somewhat terrifying!
Especially when you consider that as time goes on, probably in the majority of the universes where you are alive, you are just barely alive, in great pain. The only consolation is that, perhaps, you'll still have enough awareness to think to yourself, "incredible that I haven't died yet -- I guess many-worlds really is true!"
(† Even if you are knock-down 100% convinced in quantum immortality, you probably still don't want to engage in wildly risky behavior or try to kill yourself -- because you will be killing yourself in a large number of universes, and causing a lot of pain to your loved ones in those universes. A similar argument could be applied to engaging in any sneaky behavior which would cause significant pain if discovered.)
> can you devise an experiment which will determine if a given entity is conscious?
No, without being able to define what consciousness is we can't make an experiment to test for it. The simplest explanation in my view is that qualia are a base "element" of the universe and information systems and our consciousness is a loosely drawn subset of qualia associated with our brain/body's information system. To be clear this is not dualism, the qualia are not some magical thing riding on top of physics and cannot change the path of particles from their physically determined routes.
Regarding quantum immortality I don't know if it necessarily demands a universe in which some form of every person lives forever by some unexpected chance of quantum improbability. At least in terms of a continuity of normal life it may be that the probability of a person living longer than a human body physically can is zero. However some have argued there is a non-zero chance that random quantum fluctuations could lead to brief instances of a person arising from "nothingness" anywhere and anytime in the universe, and some of those persons could by pure luck happen to match the mind of a person elsewhere. However this does start getting into the realm of silliness and quantum randomness and infinite universes allowing for literally every possible combination of particles.
Well, I'm not sure how you square that definition(ish) of consciousness w/ the claim that consciousness can influence the universe. Because, how?
Yeah, intuitively somehow it seems absurd to think that "everything possible is happening", somewhere, in some dimension. It almost feels like a modern version of Zenos paradox. Like, in the limit, there are no universes in which you're alive...
't Hooft's version of superdeterminism, according to his book, can be falsified by demonstrating a large scale quantum computer. I don't know if Hossenfelder's version is something different or how she would account for this.
David Deutsch seems to think that a general intelligence running on a quantum computer ought to be able to observe itself existing in parallel worlds if many-worlds is true.
> observe itself existing in parallel worlds if many-worlds is true
If many-worlds is true, we are already observing ourselves in each of those universes in which we can observe, we just don't communicate with our replicas. And if the AGI can communicate with its other instances, the exponential replicas will quickly overload their channel.
I think the point of the claim is that a general intelligence running on a quantum computer would make different observations if many worlds is true versus if many worlds is false.
Which sadly means nothing, so many details being hidden behind the words "general", "intelligence", "running", "quantum computer", "ought", "oberve" and "existing".
If you could refine what these words all mean, I guess we could understand it as something more than "Deutsch seems to think that yes", which can replace your sentence entirely, "yes" describing it all as precisely.
I am myself, in some ways, a general intelligence running on a quantum computer but I dont feel like I can observe myself existing in a novel special way. Let alone being able to then express it for you in a way that is novel as well.
Most of those words don’t cause definitional problems here any more than they do for all other scientific tests which involve general intelligences (humans) making observations about things that exist. “Quantum computer” is a unique term that doesn’t show up in every description of a scientific experiment, but as far as I know it doesn’t have a particularly ambiguous definition.
And while you are indeed a general intelligence, I don’t think you’re running on a quantum computer.
He wrote a pretty well known article about it that as far as I can tell wasn’t written off-hand and hasn’t been retracted or disavowed. I’m pretty confident he meant it.
There has to be some misinterpretation going on here (or David Deutsch just hasn't thought this through). You can have a quantum computer in a superposition of states, but that isn't many worlds; to have many worlds, the computer has to interact with the outside world and split the outside world into a corresponding superposition of states. Such interaction with the outside world prevents a quantum computer from functioning - and it's precisely such information leakage to the outside world that is a major difficulty when trying to scale up quantum computers.
Yes, because it falsifies every interpretation of quantum mechanics not just MWI itself. I think in context here, "is superdeterminism falsifiable" is as silly a question as "is logic falsifiable" or "is causality falsifiable". Specific mmathematical, causal or superdeterministic models are always falsifiable, but the idea that superdeterminism as a whole is not falsifiable should be no more surprising or interesting than the fact that logic is not falsifiable.
If superdeterminism allows specific models. The idea is that the experimenter's behavior is fine tuned so he can't discover superdeterminism, which means falsification would be a paradox by definition.
Given we can enumerate all possible models because we can enumerate all possible Turing machines, we clearly have enough degrees of "freedom" in this universe that that isn't an issue.
There is non-linearity in the evolution operator though - it's called gravity. It's why MWI in its trivial form is falsified - though with a linear theory of everything (potentially string theory) - it might be possible.
Yes. You could, for example, allow a measured particle to interact with an isolated measuring apparatus of varying size - from a few atoms to a macroscopic object - and see how the size affects the system - so you get evidence of decoherence?
Also, many worlds interpretation as it currently exists is already falsified by its incompatibility with gravitational nonlinearity.
> Check out "superdeterminism," first proposed by Bell (of Bell's inequality fame) himself
“Proposed” - but as something that avoids the issue but it’s not worthy of much consideration.
- I was going to ask whether it is still possible to maintain, in the light of experimental experience, the idea of a deterministic universe?
You know, one of the ways of understanding this business is to say that the world is super-deterministic. That not only is inanimate nature deterministic, but we, the experimenters who imagine we can choose to do one experiment rather than another, are also determined. If so, the difficulty which this experimental result creates disappears.
- Free will is an illusion - that gets us out of the crisis, does it?
That's correct. In the analysis it is assumed that free will is genuine, and as a result of that one finds that the intervention of the experimenter at one point has to have consequences at a remote point, in a way that influences restricted by the finite velocity of light would not permit. If the experimenter is not free to make this intervention, if that also is determined in advance, the difficulty disappears.
I'm a fan of hers but I think many of her criticisms of string theory, multiverse stuff, etc, apply equally well to superdeterminisim.
That being said, in her latest book she has a pretty strong argument against free will that does not require superdeterminisim so if that sounds interesting check it out.
I don't see how relabelling "collapse" to "separation" and adding more or less infinitely proliferating universes solves the problem.
MWI also contradicts itself. Supposedly the universes are independent, but if their influence doesn't define the wave equation they do nothing to help explain it.
It's a very strong and exceptional claim with absolutely no evidence to support it. "Feeling" isn't enough.
> I don't see how relabelling "collapse" to "separation" and adding more or less infinitely proliferating universes solves the problem.
MWI isn't just renaming the collapse. Copenhagen is fundamentally different in that exactly one outcome is somehow chosen / selected and all others cease to exist. In other words Copenhagen has to add / invent some information: Which world to pick and when to do so. MWI simply avoids having to pick by continuing every branch simultaneously and recursively.
Meaning that MWI is actually a simpler theory and shows that the selection is unnecessary and all problems that come with it can be avoided. In that sense, the burden of proof lies with Copenhagen IMO and it just gives handwavy answers that the selection process somehow involves a conscious observer, whatever that is ...
> MWI also contradicts itself. Supposedly the universes are independent, but if their influence doesn't define the wave equation they do nothing to help explain it.
In MWI the wave function IS the integral of all possible outcomes / worlds / branches. In that sense they don't just influence it, they define it. Not sure how that is contradictory.
Btw, the same goes for Copenhagen in the state of super position as well. So, they are identical up to the point where Copenhagen selects one (collapses) and MWI simply carries on.
It follows by contradiction of the opposite statement: Lets say that all "you"s across all branches perceive all the other branches as well. That means that they do influence each other. In other words they are not separated and never where different outcomes to begin with.
But, all the interpretations of quantum mechanics start with the axiomatic assumption that the universe is modal and that there are different possibilities / outcomes. They only differ in if they chose to turn hypothetical outcomes into real outcomes or simply let everything be equally real from the get-go.
And yes, even superdeterminism has to deal with / model hypothetical outcomes. It just says that some of them cancel out each other early on as they would lead to inconsistencies in the future otherwise.
I don't believe "feeling" has anything to do with the MWI interpretation. At least I've never heard it described that way.
Given the equations that describe quantum mechanics (ie Schrodinger equation) MWI is essentially the "null hypothesis". No equations that describe collapse have ever passed the rigor of experiment and all collapse theories require modifications to the mathematics of QM. The burden of proof here is on theorists that support collapse theories not proponents of MWI.
Null hypothesis in the sense of not producing any predicition? /joking
I agree with the grand-parent that substituting a "collapse" that we cannot understand with a "separation" that we cannot understand doesn't seem a big step forward.
MWI doesn't posit that the universes are separated. Different classical states of a single quantum computer are technically in different "universes", but are still interacting with each other.
Separation that you are talking about boils down to different parts of wave function being uncorrelated with each other. If this is the case, then events in one "universe" can't have causal connection with another because that would imply correlation.
It's not just a relabelling. In collapse theories, the wavefunction stops obeying the Schrodinger equation for a moment and discontinuously jumps to a new state. The times when it performs a discontinuous jump are called "measurements", though this doesn't necessarily mean there's a scientist sitting there with a ruler, it just means that the system has interacted with its environment sufficiently. In the many worlds theory on the other hand, the wavefunction continues to obey the Schrodinger equation for all time, and the natural result of this is that the wavefunction becomes very complicated and entangled, so that the motion of atoms here on Earth is very entangled with photons heading away from us at the speed of light out into deep space, along with pretty much everything else. But there's no mention of "separation" or "worlds" in the basic description of the theory; the one sentence description of many worlds is "the wavefunction obeys the Schrodinger equation all the time with no exceptions".
Where the worlds come in is that it's impossible to do calculations on the wavefunction of the entire universe, so we need to come up with a way of dividing it up into manageable pieces. Not because the theory requires that it be divided into pieces, but because otherwise we couldn't handle the math. The worlds are one of the ways we do that: We break the wavefunction of the universe into approximately perpendicular components that don't interfere with each other very much and don't have too much entanglement making them hard to understand, and we call those worlds. We can further simplify things by just looking at a subsystem of the universe rather than the entire universe, which involves taking a partial trace (this tends to introduce randomness). As time goes on and entropy increases, the entanglement and complexity even within a "world" will continue to increase and at a certain point we may notice, "hey, this component is really complicated now, and it can itself be divided into subcomponents that are approximately orthogonal and don't really interact with each other much, I can simplify my calculations by treating those as separate worlds now". This is what we mean when we say that worlds tend to split apart, but since the worlds are only approximately orthogonal and independent, when you define splitting is really a matter of how much error you're willing to allow in your calculations. (Also, the process of splitting is driven by increasing entropy, so when (if?) the universe reaches a point of total heat death and entropy stops increasing, this will also imply that the worlds have stopped splitting.)
So I'm not sure what you mean by "strong and exceptional". It's just math, and can be compared with experiment just like any other piece of physics. Take the experiments done in the original article. If any kind of collapse had been observed, then that would have straight-up falsified the many worlds theory. Many worlds says that physical systems can become entangled with their environments, but their wavefunctions can never just collapse, and these two cases are distinguishable in a careful experiment. Since collapse wasn't observed when these tests were done, that provides a little bit of evidence in favour of many worlds.
Falsifiability is a little more complicated for collapse theories. People don't agree on the exact definition of a "measurement", and what level of interaction with the environment is required to trigger a collapse, but in order to have a falsifiable theory, it's important that we have a precise, mathematical definition of when a collapse should happen (this definition does not have to be deterministic, it could just give us a probability distribution). So various people have put forward different definitions, and it sounds like these experiments have ruled out a bunch of them, but obviously they haven't ruled out every collapse theory put forwards by every physicist ever. It's a bit like when the LCH failed to find any supersymmetry particles, and some physicists were like, "okay, but in my version of supersymmetry, the particles are heavier than the energies reachable by the LHC so of course we wouldn't expect to have seen them".
Well, the thing is that the MWI is actually simpler than all the other interpretations, because it simply removes the idea of wave function collapse entirely. Any theory involving wave function collapse is adding something extra to quantum physics that can't be demonstrated experimentally.
Because we live in the macro world, it feels like a single unitary reality is simpler, but actually, the MWI makes fewer assumptions. So I would turn that around and put the burden on the other theories to show that collapse is real, the default position should be MWI.
It is not simpler, since MWI still needs the Born postulate to actually predict the results of quantum experiments. It replaces this idea of the wave function collapsing with a redefinition of what a measurement apparatus / observer is (in MWI, a measurement apparatus exists in a single "branch" of the wave function).
This is especially problematic because QM doesn't predict any particular decomposition of a quantum state into particular classical states. That is, the Schrodinger equation doesn't actually predict that a particle has some amplitude here and some other amplitude there, as is often presented; instead, it predicts that it is described by some vector which can be decomposed in many different ways. You can say it has some amplitude X here and some amplitude Y there, but with any 2 points in space-time (adjusting the amplitudes). Or, it can have some combination of position and spin with amplitude X, and some other combination of position and spin with amplitude Y. You can choose any basis you like for the measurement, and you will get the corresponding answer.
But, for MWI to actually predict experimental results (and the classical world we live in) you not only have to choose to look at a single element of that basis, but you also have to believe that the classical basis is somehow preferred.
>This is especially problematic because QM doesn't predict any particular decomposition of a quantum state into particular classical states.
It does predict it, this decomposition is called decoherence, where a classical state splits into a superposition of several different classical states.
> this decomposition is called decoherence, where a classical state splits into a superposition of several different classical states.
No, that's not what decoherence is. Decoherence is the spread of entanglement over a very large number of untrackable degrees of freedom (usually referred to as the "environment"). This makes interference effects unobservable. But decoherence itself does not involve any "splitting"; the entanglement that decoherence spreads over a very large number of untrackable degrees of freedom has to already be there before decoherence can act on it.
AFAIK the largest molecule to show quantum behavior in experiments is somewhere at 100 atoms. I suppose that's all "environment" you need for a large number of degrees of freedom.
And yes, spread of entanglement does mean splitting. Entangled state is not factorizable, because all states in it are split. This splitting is the result of splitting of the initial state.
> I suppose that's all "environment" you need for a large number of degrees of freedom.
No. The 100 atoms is the largest molecule that we can do experiments on without having decoherence happen and ruining our attempts to observe interference.
The number of degrees of freedom in the environment is many orders of magnitude larger, something like 10^30 or more for a typical experiment.
Only to the extent that MWI is actually a position. My understanding of MWI is: shit happens, and there's no need to understand why specific shit happens because all shit "happens" (by a definition of "happens" that carries basically no meaning) and you should be happy just to prove that the shit that happens is among the full set of shit that could happen. It's the "don't worry your pretty little head over it" theory of reality.
Here's the best poem ever written, or that will ever be written:
Roses are red
Violets are blue
MWI stinks
And so do you.
Don't think it's that great? Well, that's because there exist other versions of me that wrote different words up there, and you just happened to read a version that wasn't that great.
I hereby accept my position as God Poet of the Universe.
Shit happens because that's how the state of matter changes over time, the wave function is the description of the state and the Schrodinger equation is the description of the state's change. That's as complete and happy understanding as it goes.
..And failing to find any evidence of collapse. Hence "can't be demonstrated". I didn't mean "can't be looked for" I meant that attempts to find it have not been successful.
You don't think postulating an unknown number of extra universes with a very poorly defined relationship with each other qualifies as an extra assumption?
The MWI does not postulate extra universes, although that pop science description is unfortunately common even though it is wrong. The MWI just says that the one universe that exists is actually very, very different from what we perceive. (To be clear, I'm not saying this correct description makes the MWI any more palatable. I am just clarifying exactly what kind of unpalatableness the MWI requires.)
One should not be trying to learn physics from Wikipedia, particularly not for something as complicated as QM, even more particularly not for an aspect of QM as contentious as the MWI.
The MWI says that there is one single universal wave function, which evolves in time by unitary evolution forever, and that that universal wave function is all that exists. That is one universe, not an infinity of them. It is just one universe that is nothing like what we think we perceive.
Not the parent, but I 'd say this amounts to how you define a "universe". There is a single wavefunction which says that one "copy" of you observes result A and another "copy" observes result B. Using the phrase "many universes" implies that these two scenarios are considered as "different universes". I personally do not like this phrase and find it misleading.
You are all the "copies", and, under certain conditions, can experience all their observations. Think of yourself more as an aspect of a multifaceted being experiencing the universe through you.
Practice lucid dreaming, so you can maintain a semi-awake meditative state. Practice remembering your dreams -some of them are memories from other aspects like you, others are your subconscious communicating through symbols and situations. Practice asking questions, dreaming, and remembering. Practice reaching the semi-awake meditative state from the awakened end of your day. Realize, what you think of as your body, is the 4D sensory organ of your hyperdimensional self, maintain meditative state, and open your other eyes. The effect is a lot like the collage view of regular stereoscopic vision, except out of the immediate overton-window of aspects of yourself similar enough to you.
Getting to that point can take several years. A quicker option seem to be drugs, based on conversations I had with friends, but I never wanted to artificially taint my perception, so I did it the hard way. My findings are that while there is no fate, there exist enough aspects in universes similar enough to yours, but perhaps chronically shifted, that you can learn from their mistakes. This is known as instinct, "guardian angels", premonition, deja-vue, etc.
Good luck.
If you can receive information from alternative universes, experiments can be devised to prove this, and would indeed answer a great many questions in quantum physics. Physicists and others interested in the field would be extremely grateful. As would cogitative scientists and statisticians, who would need to adjust their experimentally demonstrated explanations for instinct, "guardian angels", premonitions, deja-vue etc.
Those worlds are states in superposition, the term "world" is an allegory to help people understand that those states don't interact due to linearity of the Schrodinger equation, because not all properties of linearity are obvious.
Let's say we generally subscribed to the flat earth theory. But that causes many problems with our actual measurements. So someone proposes the "many earths" theory, where there are many flat earths connected at different angles.
If you could travel between these many earths, you could even end up in one - let's call this mythical land "Australia" - where South was up!
Sounds a bit crazy but some people believe it could really exist.
We perceived flat earth and geocentrism, so not the first time. I can recommend quantum electrodynamics to people who are too attached to corpuscular paradigm.
It is the other way around. The "universes" are there when you solve the equations, and you can either let them be or keep only one by invoking the collapse.
The collapse amounts to just setting some terms to zero.
The MWI is a simpler theory than other interpretations because it does not require "collapse" to be a physical process. The "realness" of the other worlds is untestable, but also not part of the theory.
"unreal" MWI should be the null hypothesis given what we know to be true about quantum mechanics. If we find evidence that collapse is a physical process then we can reject it, but otherwise it makes the fewest leaps. At the moment the Copenhagen intepretation is taught instead, which is a problem. To quote wikipedia:
> There is no uniquely definitive statement of the Copenhagen interpretation
And
> the device used to observe a system must be described in classical language, while the system under observation is treated in quantum terms
In other words, it does allow you to predict the result of an experiment, but we know it can't be right because it's impossible to formalize without building this "classical observer" into the model. Attempting to build on this inevitably leads to metaphysical nonsense about consciousness.
In "unreal" MWI there is a universal wave function, and something is "real" to us if it is entangled with us. The "many worlds" terminology makes it easier to visualize but doesn't mean those other worlds are real because whether something is real is subjective in this interpretation. Subjective reality may be a problem for some people, but if you look at the evolution of physical theories, we consistently find that everything is more subjective than we thought it was (see also: relativity).
Believers in Everett may not end up being right. But believers in Copenhagen aren't more right than believers in Everett. In fact, they "aren't even wrong" either, because they refuse to even engage with the question. So I think it's like the situation with democracy. You have to go with Everett until you find something better, providing you want to be scientific at all.
Wait, I'm pretty sure that if these measurements had shown a collapse, that would have to be taken as disproving many worlds. You can't very well have many worlds when your wavefunction is physically collapsing all the time before those worlds can diverge. So it is actually kind of testable, since a test was just performed that could have ruled it out. Maybe you're saying that it's untestable relative to unmeasurable collapses of the wavefunction. But if those collapses are contrived to be unmeasurable, doesn't that make them kind of, well, pointless?
Many worlds is just some peoples favorite because they get too weirded out but indeterminism. They prefer a billiard ball universe and so invented the hairbrained many worlds theory.
Or- and hear me out with this one- they prefer an explanation which follows naturally from existing experiments and doesn't require a hacked together non-linear irreversible operation which occurs only under bizarre conditions exactly when needed to patch over experimental results. No one in the whole world cares if the universe is deterministic or not, but collapse is embedded in an entirely deterministic system. MWI may not be right, but collapse is wrong.
It’s quite nice to have something irreversible though. It gives you time. Also nobody really thinks QM is the end of it, assuming semi classical physics under the hood is just odd to me. There is something below QM that we don’t understand yet (AdS/CFT looks like a good start to me) and personally I think the whole interpretation of QM debate will look stupid in retrospect. Yeah collapse is odd, but it just shows us this isn’t it. Reality is much weirder than we thought and giving up on realism is just the beginning.
You get time from all sorts of technically reversible things though! Even in a totally classical universe entropy gives us an arrow of time. Under MWI decoherence is reversible, but is functionally irreversible in the same way entropy is.
You're right though, I very much doubt QFT as it stands is the bottom. However, that doesn't mean the current debate is stupid. Whatever underlies QM, you'd still expect the measurement effect to also be an emergent property. The debate about whether atoms existed is still meaningful even though we now know that "atom" isn't a perfectly natural category. Indeed there are protons and such underling the physics, but the protons do pretend to be atoms much of the time, and thus pretend to do all the things we use atoms to predict.
Similarly, MWI and collapse (as well as, if less so, weirder theories) can be good explanations as to why a quantum phenomenon occurs even if there's also a reason they happen.
OK other than the several holy wars and church schisms and most of the physicists of the early 1900s including Einstein. And quantum-woo obsessed people on the internet. The point is that all the quantum stuff still adds up to mostly deterministic classical physics. Maybe only quantum physicists get to have free will, so long as they make decisions based on the exact results of their experiments?
Actually, if you eliminate collapse entirely as a non-linear operation, then you have a new huge problem for QM: there is plenty of non-linearity in the universe (e.g. double pendulum experiments, not to mention GR), and QM predicts that there shouldn't be any non-linearity at all, if you eliminate wave function collapse/the Born postulate.
Ahh, linear in the differential equations sense, not the affine function sense. Trigonometric functions are quite linear- they decompose into a basis! That's why, for instance, Fourier transforms are useful for more than signal processing.
Eh, these are models, not reality. The Many-Worlds interpretation makes the fewest assumptions, so it's probably the least prone to over-fitting.
Until we find philosophically or experimentally distinguished ways to pare down its ontological cost, working with most general theory is just keeping an open mind.
> Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled?
I've listened to enough Carroll to convince myself that I think MWI makes sense, but what I never got is that it doesn't seem to offer much in terms of explanation of how the entangled state ends up being what it is: We see a particular result of an experiment with many possible a priori outcomes, but is there a mechanism that determines which particular entangled state we end up in? Something like the phases of the wave functions on individual Hilbert space sectors conspiring to produce a certain outcome, so that an omnipotent observer with full access to both parts would be able to tell what happens a priori, no dice being rolled? Or is that just hidden variable theory?
> it doesn't seem to offer much in terms of explanation of how the entangled state ends up being what it is:
According to MWI, all possible results of an experiment are manifest and real. You're asking "which particular entangled state we end up in?". You end up in all of them. For a 2-state superposition system, there are 2 versions of "you" that exist after the experiment, both of which are equally real. Subsequent measurements of the quantum system will appear to be "collapsed" for both versions of you but each will see different and opposing values.
Except that I've only ever experienced ending up in one of them, so exactly how did the "I" that is typing this right now wind up here, and not in some other branch?
There's an unexplained bifurcation of consciousness implied by MWI which it cannot explain (although I think it suggests the appeal of MWI since it gives enough wiggle room that people think that their free will could control which universe they wind up in, which appeals to everyone's inner Malcolm Gladwell).
I’m currently reading Anathem so this is an interesting topic.
For simplicity, let’s say that all quantum interactions have two possible outcomes, 0 or 1.
The first point is that we can only talk about what interactions appear to have done in the past in our worldline/universe, which is randomly distribute themselves between 0 and 1. If we happened to be on a worldline where all interactions had always resulted in 0 for the whole history of the universe then that universe almost certainly wouldn’t support complex life. So perhaps there’s an observership bias here: if quantum interactions hadn’t decently distributed themselves between 0 and 1 then we wouldn’t be asking this question (just like the values of all the other fundamental constants).
As far as I can tell there’s nothing in current theories which would rule out all future interactions happening to result in a 0. Let’s hope our consciousness don’t end up on that worldline (unless you believe in quantum invulnerability).
But how does bifurcation of consciousness happen and how does it diagonalize the state so that we never see a cat in a complex linear superposition of |alive> and |dead> at the same time? 1/sqrt(2) |alive> + 1/sqrt(2) |dead> should be an equally valid outcome of the experiment via purely unitary evolution.
Consciousness is not a thing that gets bifurcated. It is just a word for an information state that is aware. If you create two information states, you have two awareness states. That is not bifurcation any more than copying a movie and editing one pixel on one frame (or as a better analogy having an effectively infinite number of frames with every possible pixel combination some of which describe aware states).
I don't see how that would allow or require seeing a cat in a complex linear superposition. You just have one set of frames which describe a live cat and another set of frames that describe a dead cat.
I feel like this is a pretty reasonable question being met with non answers. And it's a shame bc I think MWI is the frontrunner as of 2022, but this is a pretty significant question with Many Worlds I had not considered before.
Well I stole it from Roger Penrose (same Penrose as in the title article) which I think he presented in either The Emperor's New Mind or Shadows of the Mind. I think its the latter book which has more concrete arguments like this diagonalization argument. Also, the idea that collapse of the wave function could be mediated by virtual gravitons or something like that once the superposition state got sufficiently "massive" (which is the genesis of the ideas in the title article here).
Both are worthwhile if you like thinking about these kinds of things.
I've generally found nearly zero people who have read those books and of them most people poo-poo it all, and don't really engage with some of the issues presented in them.
He may be wrong. Einstein was also wrong about the EPR paper. But it seems like he's trying to engage with the problems and do science rather than just provide philosophical justification.
The state and result of observation depends on the method of interaction, i.e. hamiltonian. In principle it's possible to observe the cat in a mixed state, we just don't use interaction that gives that result.
> There is an often-repeated statement that different quantum mechanical theories are equivalent.
Worse than that, I'd say that "physical collapse" is usually presented to the general public as established scientific fact, even by prominent scientists that know better.
That oft repeated statement is really a lie except inasmuch as much as all interpretations / theories are intended to be compatible with reality as we observe it (and thus yield equivalent predictions to classical quantum mechanics for everyday quantum experiments).
Circular. You have to already accept many worlds to take this as evidence of worlds. You can argue for Many-Worlds on grounds of parsimony and clarity, but that's it.
No I don't think so. Many worlds is basically the hypothesis that we can put arbitrarily large systems in superposition. Like schroedingers cat that is both dead and alive, but instead it could be the whole lab, the whole planet, the whole galaxy that is simultaneously in two different states.
We could test this by trying to actually do it, by putting really large and complex systems in superposition. If they both evolve as we'd predict and cause the interference patterns we predict, then we must conclude that these "other-labs", "other-planets", "other-galaxies", "other-worlds" evolve just like ours, and have a causal effect on ours.
Saying that sure, they evolve like ours, and sure they affect ours, but even so they are not real - that is I suppose one stance, but then you are getting pretty close to solipsism.
I thought there's a quasi-classical approximation of quantum gravity, it doesn't work only for event horizon due to infinities there, but should work fine for diffuse matter, like how most calculations for electron orbitals use classical electromagnetic field.
> Saying that sure, they evolve like ours, and sure they effect ours, but even so they are not real - that is I suppose one stance, but then you are getting pretty close to solipsism.
No, you'd get something like Bohmian mechanics, or Rovelli's relational interpretation. Which is my point: you can only take this as evidence of other worlds if you basically smuggle those assumptions in.
Just go full Idealism. All things that can be - just are. A block universe, of all possible things and all possible interrelation of things. A giant timeless crystal of qualia. It's pretty obvious to me now.
One location in the latent space of a latent diffusion model is a view into a universe that is as real as ours. That half cat, half dog monstrosity you accidentally created. It exists as a drawing, or as a physical mass of flesh, with every possible back story as to how it got there. And also every possible future.
And what are the electrons? AFAIK, they are excitations within a quantum field. And what's a quantum field? Each layer down seems to get closer and closer to pure mathematics.
Me too but... It would still be some form of mathematics as soon as you tried to write it down in a neat, precise way for others to understand it. Maybe exotic math, but still math :p
Quantum computing is a means to avoid checking wrong solutions en route to solving certain types of problems. Seems very unlikely for this kind of decision to ever depend on a quantum computer.
Note that this was not the "mainstream" explanation. It was not even the most popular explanation.
Everyone agree about the math, but there are a few interpretations of quantum mechanics. All are weird and equivalente, so it's not possible to make an experiment to decide which one is the correct one.
There are a few attempt like this to extend QM and get a less weird theory, but it looks like this failed, at least with the more simple model for the extension. Anyway, most people just use "Shut up and calculate".
Yep: the article seems to say "there is no collapse" (in the title) but then it turns out to say "collapse as explained by these models may not be". Two very different things. I was caught as you by the title.
No, the mistake is in the natural interpretation of the title. The title seems to imply "collapse is not a thing", whereas the article is about "these explanations of collapse do not hold".
Don't tell anyone, but in the back of my mind I wonder if what we call quantum mechanics has something to do with the rendering engine in this simulation we find ourselves in.
Is quantum collapse just us "catching" the engine deciding what to render when we observe something? :-)
I have very little knowledge of quantum physics, but in my mind there is some similarity between quantum phenomena and backward raytracing in computer graphics.
In forward raytracing, images are rendered by "shooting" photons from all lightsources and seeing which ones end up hitting the player's camera. This is similar to what we think happens in nature, but the calculation is extremely inefficient, because the vast majority of photons never hit the player's viewpoint.
In backward raytracing, we only shoot "photons" backwards from the player's camera, and see which rays end up hitting a light source. This computation is much more efficient.
It makes me wonder if our "simulation" uses a similar shortcut
That's my interpretation as well, except I'd go a step further and say it's time-independent based on what we've seen of quantum entanglement. There's no reason to resolve a particle's state if it's not interacting with anything, but once you do you can safely define the entangled pair's state retroactively because it hasn't interacted with anything either.
So what we end up seeing is that after you measure one particle, the other behaves as if it always had the complementary value. It's not faster-than-light spooky action at a distance, it's more akin to reality "resolving" to a consistent time-independent state.
Maybe, but it would be a highly non-classical computer. Probably it could cheaply render the most physically plausible paths to specified outcomes, and quickly factor large primes.
Nb. quantum also highlights assumptions made by our brain's rendering engine.
The tricky part with using quantum physics as evidence for a simulation is that it's weird, but it's only a specific amount of weird. A simulated universe could easily be weirder; why isn't P=NP obviously true?
A lot of people who work with computers get this idea. :) But it seems rather unlikely, as quantum-mechanical models are actually more expensive to compute. Quantization is not at all similar to pixelization or rounding effect, it's fundamentally different. Quantum computers being more powerful than classic computers hints that it's not a simplification, on contrary.
Maybe. Considering that the simulation is taking place on a nearly infinite large white sand dessert with a turtle meticulously moving rocks according to a simple set of rules, there are bound to be hiccups in the physics engine part.
Falsification is hard work, and rarely reaches a tidy end point. Even now, according to Curceanu, Roger Penrose — who was awarded the 2020 Nobel Prize in Physics for his work on general relativity — is working on a version of the Diósi-Penrose model in which there’s no spontaneous radiation at all.
It's incredible how influential Penrose has been for such a long time, and continues to be. As far as I can tell (I'm interested but far from an expert) very few theorists are coming up with genuinely testable ideas in this area.
Most scientists either blindly accept collapse as a mechanism and then happily use the highly accurate math and don't care it isn't testable, or if they think about it at all they adopt something untestable like MWI and then happily use the highly accurate math and don't care it isn't testable. Thinking hard about testable theories in this area is incredibly slow progress and doesn't pay a lot of bills (https://www.theguardian.com/science/2013/dec/06/peter-higgs-...)
While reading about the various QM experiments I've always wondered why people use the word "measurement" for what looks more like to me as "particle collision with large enough macroscopic effect to be seen by human".
ugh the actual news event being covered in this article is a 2022 result[1] confirming a 2020 result[2] both of which 'set a lower bound' on this theory by penrose. 'set a lower bound' means they didn't find it.
this article is credulous -- yes, this may exist and semi-serious people are looking for it, but pls don't confuse 'we haven't found it yet' with 'current science suggests this is very small'.
how small?! at least admit that your theory doesn't predict an energy level. 'Current science suggests that bigfoot dwells in the places we have not yet looked'. Also put the newest paper in the first paragraph, don't make me dig through recirc links, ugh.
From a layman’s point of view, it seems like we grasping at straws when it comes to these thorny quantum questions. Is fair to say, for example, that we are about as clueless as our ancestors were with the bubonic plague?
To a non-expert it can be difficult to separate which theories lay on solid ground and which theories are highly speculative.
QM is hard to visualise, but we have extremely sophisticated equations and principles for determining how quantum systems will evolve, and can engineer complex functioning systems using that knowledge. For example transistors only work because we understand QM well enough to precisely engineer the energy level state behaviour of electrons in semiconductors.
It would be like accidentally discovering antibiotics during the plague. You might not know how it works or why, but you know what it does and it absolutely gets the job done.
That applies to a lot of medicine right now - we don't know how Tylenol or antidepressants work, and Semmelweiss's invention of handwashing was rejected because he couldn't explain why it was working.
That would not be fair, though it's understandable why a layman might feel that way. The fact is, most physicists don't particularly feel the need to have an explanation for that kind of thing. We have the math, and most people agree on how to use it to make very accurate predictions. Collapse was always a little silly, but there are other possibilities as to why you'd get that kind of effect just from wavefunctions.
"It's only a model" Patsy says, but then they cut to a whole big song and dance routine, so it must be a pretty good model. Or course it is unsurprising that Monty Python's Holy Grail would provide deep physics insights, they were a pretty clever bunch.
So QM itself is on very, very solid ground. You're using it now on your computer.
The interpretations of QM and the attempts to reconcile the exceptionally well tested mathematics of QM and the reality that we experience which is not-QM at all are all philosophical with zero evidence. Everyone just tries to make compelling arguments based on things like Occam's razor about why their horse is the best one in the race without actually knowing anything at all.
We have place a few bounds around things like Bell's inequality so we know that local hidden variable theories are ruled out, but that is about it.
The title article is very interesting because its one of the first few actual tests to probe if there really is a transition between QM reality and classical reality. Regardless of who actually wins the horse-race the important thing here is that there's slow progress being made on trying to experimentally test theories. This is why I've always liked the Penrose models of collapse better than the MWI models since the former have some chance of being actually testable, while with MWI you just blindly decide it is true or not and then you argue a bunch about philosophy and never do any experiments, which isn't science. Penrose models of collapse might be wrong but at least they're in principle testable, which is incredibly exciting about this article.
Many worlds is absolutely testable, since if we observe collapse in even a single one of these experiments then that completely falsifies many worlds. If one of these experiments discussed in the article had actually observed a collapse, then I have no doubt we'd be seeing headlines like "many worlds theory disproven", and Nobel prizes for the physicists involved. It would be the biggest discovery in physics for decades.
We "observe" collapse all the time, and can calculate the probabilities of the different possible collapsed states of a not-yet-observed superposed or entangled state using the Born rule. What we don't know, and can't tell, is whether we have seen an "objective collapse" (we live in just one universe, that undergoes a discontinuous change at the time of measurement) or in something like MWI. That is question for philosophers afaik. The experiment in the title falsifies certain models of objective collapse, but others are harder to falsify.
I agree that's it's possible to create objective collapse theories that are arbitrarily difficult to falsify, but the difference between objective collapse and regular old decoherence due to interaction with the environment has experimentally measurable implications. In particular, if you expect a certain probability of objective collapse in a certain period of time, then do an experiment with a coherence time longer than that, while keeping your system carefully isolated from interaction with the environment. Then if the wavefunction collapses anyway, that would prove objective collapse and disprove many worlds.
I thought an objective collapse theory was one where the observation (whatever that is) causes the collapse. An observation is necessarily an interaction with the system, where the observer is part of the environment. So if the system is that isolated from the environment, the collapse or lack of it can't be observed. I didn't think it meant the collapse happens after some amount of time like radioactive decay, even without an observation (interaction). Maybe I'm wrong.
> Is fair to say, for example, that we are about as clueless as our ancestors were with the bubonic plague?
“We” in the collective sense are not clueless about quantum mechanics at all. We have an extremely exact model, and pretty much every attempt
At proving that it’s insufficient fails.
The only “problem” with quantum mechanics is the same as the “problem” we had in Newtonian mechanics when we found that a tennis ball and a bowling ball falling from head height hit the ground at the same time. This was a problem because intuition would have the heavier object fall faster than the lighter. It wasn’t an actual problem with Newtonian mechanics though mind you. Our intuition was counter to reality, the problem was the intuition not the model of reality.
Naturally there were other actual problems with Newtonian mechanics, but none of that had to do with making it more intuitive. And the same is true of quantum mechanics. It doesn’t sound intuitive to most people, but that’s not problem, it just means your intuition about how systems should behave at these scales is wrong.
> From a layman’s point of view, it seems like we grasping at straws when it comes to these thorny quantum questions. Is fair to say, for example, that we are about as clueless as our ancestors were with the bubonic plague?
> To a non-expert it can be difficult to separate which theories lay on solid ground and which theories are highly speculative.
Sure, but isn't that the point of doing these experiments?
I'm all for trying to understand these phenomena and running these experiments. Just trying to get a sense for how much of a grasp we have on these phenomena.
Based on the other responses, it seems like we can mathematically model these phenomena very well and make very good predictions. However, when it comes to explaining why these phenomena exist in the first place, we are like a medieval doctor trying to explain why antibiotics work.
This tidbit about a prodigious undergraduate struck me. He double majored in math and physics and was about to start his PhD at Harvard. And in a sudden freak accident he was gone from existence. The universe will always remain unfathomable in some ways.
> In 1996, Qijia Fu of Hamilton College in New York — then just an undergraduate — proposed using germanium-based neutrino experiments to detect a CSL signature of X-ray emission. (Weeks after he submitted his paper, he was struck by lightning on a hiking trip in Utah and killed.)
Reminds me of a golden age (?) science fiction short story -- no idea the author or name -- about a spate of suicides and mysterious deaths among physicists who got too close to "the truth," externally caused by an alien force trying to keep humans in their petri dish.
"If there is indeed a background perturbation that provokes quantum collapse — whether it comes from gravitational effects or something else — then all particles will be continuously interacting with this perturbation, whether they are in a superposition or not."
This makes eminent sense to me.
I have a notion that I'll try to present in its simplest form which is that a quantum system exists in background 'sea' or 'substrate' akin to random noise whose maximum fluctuations are of themselves insufficient to disturb or perturb said quantum system.
Without this noise an 'event' such as a particle, photon etc. would perturb the quantum system in a predictable way thus it would 'collapse' into a predictable outcome (according to understood physics).
As the noisy substrate permeates and superimposes on everything it randomizes the 'collapse'. Of course, we'd have to invoke notions such as virtual particles zero point energy, Casimir-like effects—even then, saying that is a gross oversimplification without much additional amplification. Perhaps it's best just to say that essentially a noisy fluctuating background environment would perturb and randomize an otherwise predictable quantum system.
Again, like many of these theories, I've little to support my idea except intuition—and with quantum mechanics relying on it is a very dangerous thing to do.
The results of these experiments indirectly tell us something about our chances of ever meeting aliens:
* No physical collapse => Everett interpretation AKA "many worlds"
* Many worlds gives us the Anthropic Principle for free
* The Anthropic Principle explains the origin of the first living organism, or, to put it another way, the observed existence of the origin of our first living ancestor does _not_ set any lower bound on the probability of the first living organism developing from non-living molecules.
* A very low probability of the origin of life implies the non-existence of any other life in the observable universe.
* Therefore, no aliens.
(The aliens do of course exist in other parts of the total Universal Wave Function, but we never get to meet them.)
It should be noted that the Everett many worlds model is impossible due to the nonlinearity of gravity - different worlds cannot exist independently of each other without interacting gravitationally - you’d get the kind of collapse that Penrose’s theory predicted. Given these experiments, there isn’t any obvious theory that squares the kind of wavefunction collapse we observe with the nonlinearity of gravity.
> If there is indeed a background perturbation that provokes quantum collapse [...] then all particles will be continuously interacting with this perturbation, whether they are in a superposition or not.
This reminds me of how our own heart's peacemaker works.
In true collapse-of-the-gaps style, what if collapse happens when an entangled system becomes so large that the edges of it are outside of each others event horizons due to cosmic inflation?
If you have only one particle of the entangled pair, you can't do any experiment to check if they pair is still entangled or someone else has done something to the other particle and broke the entanglement. Someone else is not necessarily a person. May be a device or just a brick.
In a common case, when you measure your particle you get 50% "yes" and 50% "no". So if you have a stream of particles that are one half of an entangled pair, you just get a random sequence.
It doesn't matter if the other half is still flying happily in vacuum, got into a detector like the one you have, or it just hit a wall. You just get a random sequence. Otherwise, it could be used to build a FTL "walkie talkie".
If you later can talk with someone in the other side that measured the other half of the pairs, then both of you can compare notes and notice that the two random sequences are equal or oposite or something in between according to which experiment each of you have done.
Dang I low key have a problem here because I already mentioned it was a joke and gave the same explanation, but because this is the top reply and the comments both show n-hours ago it looks like I walked back my statement in response, and I think it's fair to say that this is why the original comment got a down vote and ended up at the bottom.
After reading your comment again, now I'm wondering how does the variation of QM discussed in the article handle the special cases you mentioned.
In the usual QM the explanation is "easy" because the collapse is magical.
But if they want to eliminate the magic and propose a underlying "physical" process for the collapse then they have nasty problems and FTL transmission.
I think you mean expansion rather than inflation (inflation is something that happened very briefly at the beginning of the universe). In models with collapse, the collapse happens instantaneously, no matter what
As with any unsettled questions there are pros & cons to the various takes on quantum reality. Part of the appeal of MWI (intrication of observer into observed system as part of the measurement process) to me was always that the instantaneousness you mention here is a very "ordinary" kind of "fastness by synchronization". It's basically just a kind of causal wavefront hitting.
There are plenty of "infinitely fast" synchronization effects like this..some perhaps less obvious than others, but many pretty pedestrian and accessible to lay people. Neither the junction of a scissors or the "spot light" on clouds need be constrained by light speed, for example. People routinely abuse "nothing" (or maybe "go") in the " 'nothing' can 'go' faster than light" saying. :-)
Yeah. Perhaps most charmingly treated here: https://arxiv.org/abs/2011.12671 { EDIT: though surely many other places! There are also YouTube videos of this one, though, and Sidney Coleman was really a Feynman-class edutainer, perhaps as under-recognized as von Neumann is relative to say Everett on this topic :-) }.
I should perhaps have used the word "entanglement" rather than "intrication". Oops.
Also of interest might be the treatment by Lev Landau in his Course on Theoretical Physics Volume 3: Quantum Mechanics - non-relativistic theory. Deriving the Born "postulate" is in Section7: "The wave function and measurements" in the most available 2nd edition (but it is admittedly a rather abbreviated derivation). I am not sure if that was in the original 1958 edition.
Quantum systems measuring other quantum systems is part & parcel of quantum computation. So that can lead to more clear thinking on (some of) these matters.
Why would that force collapse? I would almost expect that the lack of communication over those distances forces _more_ uncertainty (i.e. a wider probability distribution) rather than less uncertainty.
It's kind of a joke, which is what I meant by -of-the-gaps. If two particles are outside of each others event horizons, they can't communicate so you can't tell whether they are entangled (AA or BB) or whether the system has collapsed into say AA.
I like PWT because other theories don’t rule out FTL propagation of things, either, and PWT just assumes it. It is far less weird than for example the many worlds explanation.
No, none of the other explanations predicts anything like this. The theories that fell here predicted physical effects that went beyond standard quantum mechanics. The others are playing it safe and staying unfalsifiable.
Some superdeterministic theories are testable. Some of them aren't, or say things like experiments aren't capable of proving anything in the first place because the universe doesn't obey rules.
This article is a perfect example how science is getting led into dark areas by people who didn't learn quantum mechanics right or people who pretend to understand it, but can only blindly follow the formalism without much understanding of what they actually do. Every such article continuous to mysticize the whole subject by quoting famous scientists who were either puzzled by it at the time or scientists like John von Neumann who clearly gave a dumbed down view of the now called collapse (perhaps on request to skip the math).
I really appreciate this forum - as it is one of the last places that I know of where one can have a civil discussion - and therefore I will take the effort to show that pure quantum mechanics - with no additions - essentially explains the process of measurement which is not at all sudden as the name "collapse" would suggest. The reasoning comes from von Neumann himself, but now sometimes it's attributed also to Wojciech Żurek.
TLDR of below: All processes in nature, including the measuring process are unitary, the "collapse" is just an artifact of our ignorance about the exact state of the measuring aparatus.
Here it goes:
For simplicity, let's assume that psi describing our particle is a superposition of two eigenstates:
|psi> = c1 |1> + c2 |2>, i.e., |c1|^2 + |c2|^2 = 1.
Without loss of generality we can pick:
c1 = x and c2 = sqrt(1-x^2) exp(i phi), where x is a real number smaller than 1.
The density matrix of this pure state can then be written as rho = |psi><psi| and
one by writing the explicit form of this density matrix one can see that the diagonal terms are:
x^2 and 1-x^2, while the non-diagonal terms are:
xsqrt(1-x^2) exp(i phi) and xsqrt(1-x^2) exp(-i phi).
In the most complete scenario of a measurement, the density matrix of the system can change change in many ways including the diagonal terms of the density matrix. However in this simplistic example, a measurement will by necessity, bring only the non-diagonal terms to zero (I hope most of the interested readers will have enough background to understand why).
Now, the measuring device, as a macroscopic object, will have the number of degrees of freedom far greater than the simple particle which's state we're about to measure. This number will be the order of the Avogadro number (~ 10^23) - even the smallest human visible indicator will be this big. The measurement, by necessity, includes an interaction of our small system with the enormous measuring device.
Before the interaction the whole system (the particle and the measuring device) can be written as a tensor product of the two wavefunctions:
where |Xsi> represents the wavefunction of the measuring device and everything it interacts with before the measurement. When the interaction occurs the state of our measuring device changes unitarily (as everything in nature) according to the full Hamiltonian of the system, and with some regrouping of the terms, we can write the state after the interaction as:
This is the true state of the system as performed by nature.
The individual subsystems are no longer in pure states, but the whole system |Omega_after> (if we're able to completely describe it) - is.
Now, comes the final part, which some call the "collapse", but in reality it is just "an average" over all possible states of the bigger (measurement) system *which we declared apriori to not be the system of interest and states of which not able to follow because we measure with it*:
Tr_{over the degrees of freedom of Xsi} |Omega_after><Omega_after|
In result we obtain a matrix after the measurement which is just formed with the diagonal elements
x^2 and 1-x^2, i.e., the probabilities of the two measurement results and non-diagonal terms being equal to zero.
Why are they zero?
Let's inspect one of the non-diagonal elements over which the above trace is taken:
x*sqrt(1-x^2)exp(i phi) <Xsi_1|Xsi_2>
It is effectively zero, because the trace over the degrees of freedom of Xsi is a mutliple integral, again of the multiplicity of order of Avogardo number and a similar number functions which change in various ways. It is enough that only a fraction of such integrals will have a value lesser than 1 to guarantee that the product will be equal to zero.
And this is all. Any attempt to change this fact would need to reject quantum mechanics completely, because probability calculus is at the heart of it.
This is just _one_ interpretation of wave function collapse and the only thing it has going for it is that the dimensionality in which collapse happens can always require another particle, which adds another complex degree of freedom, and always remains out of the realm of what we can compute.
Two particle interactions show nothing like wave function collapse, neither to three of four. Until you say a reasonable number of particles that make up the measuring apparatus where we should see _something_ weird starting to happen theoretically you're not even wrong.
Nothing "weird" starts happening. Unitarity evolution is never broken, there are just rules in quantum mechanics that could perhaps be grouped under supplementary framework related to how we, macroscopic entities, extract information from it.
>Until you say a reasonable number of particles that make up the measuring apparatus where we should see _something_ weird starting to happen theoretically you're not even wrong.
Just saying something twice doesn't make it true. The "weird" thing you were perhaps referring too starts at very beginning of quantum mechanics framework. The Born rule is just a "conversion" of predictions of quantum framework to our classical language. The only option you have is to reject quantum mechanics as a whole and not try to patch it - because this clearly will not work.
The weird thing would be discontinuities showing up in wave function evolution without putting them there with the potential energy functions. In the few cases where we can get analytical solutions there are no discontinuities that look anything like wave function collapse.
You're whole thesis rests on the fact that this should fall out when we put 6e23 particles together for reasons.
So far we've not managed to simulate 1,000 quantum particles because the curse of dimensionallity means we run out of computers on earth rather quickly. Which makes anything you're saying pointless since we can't ever check it, even if we turned the whole observable universe into a computer.
I hope that you read this with a scientific attitude, i.e., critical, but open to the fact that not only your position is wrong, but also the unproductive enterprise of solving "the measurement problem" is wrong.
What I'm trying to explain to you is that:
1) The wavefunction is only the DESCRIPTION of the underlying phenomena.
2) Within this description everything, and I mean ::everything::, evolves unitarily. No exceptions ever.
3) Whenever you decide to measure, i.e., probe the microscopic system with an object that is not within your quantum description, that is you know that it's huge, but have no details about all the phase/amplitude information you're destined to average/trace over the unknown states. This can be done symbolically (as in my first post here) and shown to always give probabilities in the reduced density matrix. That's always what we're left with in case of a large system outside of our description interacting with a small system within our description.
On the other hand if you put a small quantum system, with another small quantum system (say two particles), there's no need to trace/average/apply the born rule immediately because your description can be complete both in principle and in practice. You can just unitarily evolve the system for as long as you wish/can compute for. However sooner or later you'll want to measure, because ultimately that's what physics is all about - verifying your predictions with experiment - and you go back again to small vs big, because that's the only way we, humans, can perceive this microscopic reality - through probing. The result will be completely analogous to the one before, the only change being that you'll now be able to predict probabilities of a two-particle system.
If you're familiar with electrodynamics it's quite similar there, but here it's brought to another level with the probabilistic interpretation. What are the similarities?
You can have your complete description with the four-potential, about which you know, from the Aharonov-Bohm effect, carries more information than electric/magnetic fields alone although we only measure fields not the electric/magnetic potentials.
The potentials were the side product of the formalism that turned out to have real consequences. Similarly as we learned the importance of the wavefunction/phases in the description, even though we only measure probabilities.
About the curse of dimensionality, the only thing I have to add is, that's true. We have a precise way to describe what is going down there but it's insanely expensive to simulate in all detail. That's still a lot to be happy with in my opinion.
Also, if you feel uneasy with wavefunctions which have the status of descriptions of reality,
go and study classical field theory in which the fields are to be thought of as real physical entities,
go step deeper and you're in quantum field theory in which you deal with descriptions again.
Would a theory in which we deal with "real physical entities" be better than that of "descriptions"?
I'd say, the hell with it. Go with whatever works best, not whatever fits your preconceived notions of reality.
Tiktok has been quietly amassing a huge library of educational content, and I’ve been diligently sorting those into collections. I’ll post several more related to physics.
I often post on HN that TikTok offers much more to the world than videos of girls dancing and silly pranks. It gives people what they want (for better or worse) and adapts very very quickly to tastes.
Once it learns you're a science nerd it fills your feed with some high quality educational content like this. The chemistry stuff they have is also A+.
Looking out for quantum consciousness in articles about quantum physics would be a good drinking game, though here it is handled with a proper amount of scepticism.
The pressure is enormous for writers to include mumbo-jumbo, because people click on these articles expecting to find out that it's possible to alter the Universe with psyhic abilities, which is how quantum physics is represented in social media, especially TikTok.
Having been passionate about this for awhile, I have a layman’s surface overview of the state of physics, or I like to believe so.
It amused me to no end when my elderly aunt came to ask me for a tl;dr; on Quantum Physics. She stopped me shortly asking how she can control the universe with her mind.
I tried to explain a bit about the famous interpretation (Copenhagen, not Von Neumann–Wigner) but she would have none of it because the business course she just attended had a segment on how to control the universe and proceeded to disregard and mock me.
To be fair, the Copenhagen interpretation doesn't really fit today's world view. We have largely moved away from viewing humans or "consciousness" as something special, so the notion that an observer collapses the wave function just seems weird now.
"Many worlds" or "the wave function never collapses, you are in a superposition" both make much more sense with how we currently view the world
The Copenhagen interpretation never defined an observer that way. It's a non-interpretation basically, just shut up and calculate, which is still the most widely accepted approach among scientists, since there are no other falsifiable interpretations.
I'm reminded of learning about the grandfather paradox as a kid. I recall hearing shit about scientists saying if you were to travel back in time, you would be compelled by a universal force to do things that don't alter the future and create a paradox. Even as a kid, that sounded so idiotic. In retrospect it was almost certainly the reporting that was wrong.
Is that really so weird? That's a very common version of time travel in SF, perhaps the most common. You can travel back in time, and you think you're changing history, but through a series of unlikely coincidences you end up being the person who created the history you were trying to change.
Come to think of it, maybe that's more common in short stories, where the "gotcha!" format works. It's probably harder to spin that unchangeable-history gimmick out to novel length, whereas "you can change history and it has endless weird side effects" can work well in long form. I can't think off-hand of any time travel novels with unchangeable history.
But it's stupid as a serious time travel proposal. Things like "travel in back in time 5 seconds unless you see your future self is already there" create a hard paradox that this logic cannot solve. Time loops are easily broken by anyone interested in testing them.
I found that link via a blog post that excoriates it for its supposed logical fallacies: https://loopingworld.com/2019/07/13/debunking-ted-chiang-rec.... I don't necessarily agree or disagree with that, but I do disagree that it's "obviously" logically wrong or inconsistent.
It's really just on them for trying to alter the future in large ways before conducting small experiments to understand the mechanics of the universe imo.
Time loops are only a problem if you insist on being able to simulate reality by computing the next time step from previous ones, without discarding any. Or if you for some reason insist on total free will.
> Or if you for some reason insist on total free will.
Like, the free will to do something other than what you saw your future self do? Yeah, you do have that. You have that ability even as a completely deterministic brain.
Heck you could send a machine to go back in time 5 seconds that displays a number 1 greater than the number it just saw its future self display from the previous travel and your time loop has infinitely increasing state with no human in the process. Or the mundane, send a bomb back in time to destroy the time machine and your past self. It's very easy to think of examples.
> Time loops are only a problem if you insist on being able to simulate reality by computing the next time step from previous ones, without discarding any
If you frame the system as "the universe conspires to create a series of coincidences such that no paradox exists", then it makes perfect sense that nobody chooses to run that experiment, because the simplest coincidence that prevents the paradox is for the characters in question to simply not think to run that particular experiment. This raises the obvious question of how the time machine got invented in the first place, which seems like a great story.
I also prefer the Many Worlds interpretation and I do think that it is gaining popularity in relation to Copenhagen, but I'm not sure what you mean by "today's world view". And who is this "we" that you write about?
I prefer Many Worlds because it is a simpler explanation, so I follow Occam's razor. That's it.
Consciousness and the existence of qualia remains a fundamental mystery. There is something definitely special about it, in the sense that it currently does not fit any scientific model. It is also a deep philosophical problem that started being considered millennia ago and dates at least all the way back to Plato vs Aristotle.
In my experience, people who think that consciousness is somehow a settled matter simply haven't thought about it enough and are perhaps a bit naive on the many ramifications of the issue that have been explored do far.
Exactly. A related deep millennia old observation is that no one has actually seen the real world. Everything we’ve ever experienced has been filtered through our minds. Every experiment, every measuring device, every meticulously crafted model of reality… it is all inescapably limited by what we are able to experience.
One could and many have argued that the reality we’ve been observing and operating in is consciousness. The “real” world could be entirely different and largely inaccessible.
These kinds of thought exercises don’t have much practical utility, apart from one very important feature; they humble you. At the end of the day we don’t fundamentally know anything, and should always recognize that at its most basic root level, everything we do is an educated guess. A fundamentally skeptical and curious outlook that acknowledges our perceptual limitations is how we got all of the sophisticated models of very difficult to observe phenomenon in the first place. If we want to continue to get the best understanding of whatever it is we’re experiencing, I think it’s very important to stay humble and ensure our knowledge is treated as a hard earned set of well reasoned guesses rather than unquestionable dogma. 99.9% of objections to well established ideas and models might be a waste of time entertaining, but you never know what might turn out to be the seeds of a whole new universe of understanding that invalidates huge swaths of our existing corpus of knowledge.
Most of what I’m saying here is probably obvious to a lot of readers, and I don’t think anyone in this thread is being particularly arrogant or dogmatic, but I think it’s worth reiterating. If people who understand the limits of knowledge aren’t constantly emphasizing the fact that we don’t know what we don’t know, that creates fertile ground for both dogmatic assertions and unreasonable skepticism, and I think a huge amount of dysfunction in the culture at large is explained by insufficient well calibrated humility amongst otherwise very intelligent people who set an example for others.
MWI is exactly as simple mathematically as CI, so not sure what you mean by "simpler".
MWI still postulates the equivalent of wave function collapse, but instead of it happening only for the quantum system being measured, it is happening in the mind of the observer, as each "version" of that mind gets entangled with a single "version" of the outcome.
Even if you were to accept that this process is more natural (so not an "assumption") than wave-function collapse in principle, that simplicity completely falls apart when you then need to recover the relationship between the probability of observing a certain outcome and the amplitude of that outcome in the wave-function.
CI just says "when a quantum system described by a wave-function interacts with a measurement apparatus that measures in a certain basis, the wave-function gets updated to one of the values of its decomposition in that basis, with a probability equal to the modulus of the square root of its amplitude in that basis". Of course "measurement apparatus prepared in a certain basis" does a lot of work here, as we don't know how to define this in terms of a quantum system.
To make a similar quantitative prediction, MWI needs to define something like "the number of worlds", so that it can then say something like "when a quantum system interacts with a measurement apparatus prepared in a certain basis, the measurement apparatus becomes entangled with the quantum system such that for each value of that basis state there is a number of worlds proportional to the square root of the amplitude of each value of the decomposition in which the apparatus sees that particular value; if we were to compute the probability that we happen to live in a world where the apparatus is showing the value X, that probability would naturally be higher the more worlds there are where it shows this value X". So, the MWI has to actually introduce extra elements (the worlds and their number, and the observer wanting to compute a probability) to explain the actual measured results of quantum experiments.
>MWI still postulates the equivalent of wave function collapse
It's not postulated, but deduced from the Schrodinger equation. MWI is simpler in a sense that it has one fewer axiom. But Occam's razor isn't really applicable here, because it selects from otherwise equal theories, which CI isn't. There are more important criteria to use before Occam's razor.
You can't deduce the Born rule from the Schrodinger equation. MWI can say "look at every basis separately", but you still have to postulate that the probability of finding yourself asking that basis vector (seeing that measurement outcome) is proportional to the square root of the amplitude.
> you then need to recover the relationship between the probability of observing a certain outcome and the amplitude of that outcome in the wave-function.
I have never understood how that is a strong objection. We've experimentally determined that the state you are more likely to find yourself in is based on the squared amplitude. How is this different from CI but with probability of observing given state - which was also determined empirically?
> Of course "measurement apparatus prepared in a certain basis" does a lot of work here, as we don't know how to define this in terms of a quantum system.
Yes, this is where the Occam's razor bit comes in.
> So, the MWI has to actually introduce extra elements (the worlds and their number, and the observer wanting to compute a probability) to explain the actual measured results of quantum experiments.
The worlds and their number are equivalent to the states & probability of CI without having to introduce the "measurement apparatus" that is distinct from the quantum system.
> How is this different from CI but with probability of observing given state - which was also determined empirically?
It's not different, that's the point. For both interpretations, the relationship between the probability of observing a certain outcome and the amplitude of that outcome in the wave function are an extra postulate, the exact same extra postulate in fact, the Born rule.
> The worlds and their number are equivalent to the states & probability of CI without having to introduce the "measurement apparatus" that is distinct from the quantum system.
The measurement apparatus as a separate thing from the quantum system was actually partially explained by decoherence (ironically discovered by MWI proponents), which needs this separation for the same reasons as CI: we need some way to explain why quantum phenomena don't happen at our scale. Now, MWI makes this concept relative to an observer, whereas in CI it is often assumed to be absolute.
Basically, we can deduce from the Schrodinger equation alone that after the interaction of a quantum system with the environment, coherence is lost, and the different "states" of the wavefunction can no longer interfere with each other.
CI postulates that, as a result of this interaction, only one of the states will remain, with the Born rule probability. This is the most direct way of interpreting our experimental results.
MWI says that nothing changes after this interaction. It postulates though that, if we were to ask how likely we are to be in the same "state" as a particular result, we should expect that to be the Born rule probability. This explanation takes the theoretical description of the wave function to be more real in some sense than the actual observations we make.
> The measurement apparatus as a separate thing from the quantum system was actually partially explained by decoherence
I disagree that the decoherence at all partially explains a different measurement apparatus separate from the system. After observation, we still have the the system is in a superposition of multiple states. How this system "collapses" to one state in the measurement apparatus is unexplained.
Decoherence explains why each state of the observer can't tell that the other states also exist.
> This is the most direct way of interpreting our experimental results.
Sure, except we have to invent an entirely new non-unitary transformation of "collapse" despite all observations and predictions of quantum mechanics showing that unitary evolution of the wavefunction continues even for large macro-scale objects.
> Decoherence explains why each state of the observer can't tell that the other states also exist.
Sure, but, again, decoherence doesn't explain (1) why the wave function is split in the classical states (position, momentum etc.) and not other states (linear combinations of position and momentum and spin etc.); and (2) doesn't explain why different states have the precise probabilities of being observed that they do. You still need to postulate these features.
> Sure, except we have to invent an entirely new non-unitary transformation of "collapse" despite all observations and predictions of quantum mechanics showing that unitary evolution of the wavefunction continues even for large macro-scale objects.
You're confusing the map for the territory. What we can see, plain as day, is that unitary evolution of the wavefunction does not continue for macro-scale objects. MWI explains this through the framework of the universal wavefunction and its mutually un-interacting "branches", CI explains it through wavefunction collapse. No one has ever successfully put a macroscopic object in superposition, so the idea that the universe itself could be in superposition remains highly theoretical at best.
Note also that movement in the classical world (and in GR) is often highly non-linear/non-unitary (such as the movement of a free pendulum, or many kinds of orbits). While far from perfect for this, the Born rule at least allows us to "sneak in" non-unitary evolution through this collapse; but, in the MWI, we predict that the movement of a free pendulum should in fact be non-chaotic. This is rarely discussed, but is an interesting observation that could be experimentally tested as we get better at creating larger and larger systems that remain coherent for longer and longer periods of time.
> Sure, but, again, decoherence doesn't explain (1) why the wave function is split in the classical states (position, momentum etc.) and not other states (linear combinations of position and momentum and spin etc.); and (2) doesn't explain why different states have the precise probabilities of being observed that they do. You still need to postulate these features.
I agree that (1) is a strong objection that I do not have an easy solution to. (2) I think is addressable in the same way that it is in CI - our theory has to explain our observed experience and the born rule (whether CI-flavored or MWI-flavored) is how we do so.
> No one has ever successfully put a macroscopic object in superposition
> (2) I think is addressable in the same way that it is in CI - our theory has to explain our observed experience and the born rule (whether CI-flavored or MWI-flavored) is how we do so.
Absolutely, but I would still say that it means we are adding an extra postulate that can't be derived directly from the Schrodinger equation even in MWI. I'm not trying to claim that the MWI is wrong: just that it is not simpler than the CI (nor it is more complex though - I'm arguing it has the same number of postulates).
> But getting larger and larger.
Sure, and I'm really hoping we'll advance closer to macroscopic objects in my lifetime, so maybe some of these questions may be elucidated.
What surprises me is not that many people think consciousness is a settled matter; but rather, people who, when faced with the claim that "consciousness" isn't settled, are so often tempted to assert that it is that they rapidly provide another example of how it isn't.
> We have largely moved away from viewing humans or “consciousness” as something special.
Who is “we” in this comment? Christianity and Islam emphasize consciousness and the uniqueness of human experience, and their adherents still account more than 50% of the population.
As much popularity as the many worlds theory is gaining, the generic “we” almost certainly still doesn’t believe it, and many (most?) probably don’t even know about it.
The CI has absolutely nothing whatsoever to do with consciousness. The CI view is simply "when a quantum system interacts with a measurement device, the wave function of the quantum system changes to one of some subset of real values, with a probability given by the modulus of the amplitude of that value in the wave function". Exactly what constitutes a measurement apparatus is undefined, but it certainly doesn't involve a human.
In fact, it is precisely the MWI that requires human observers for its explanations, at least to some extent, as it makes the entire notion of a classical world a fiction that only exists inside your own head because you are the one that's getting entangled with a quantum system. Any probability you compute in MWI is relative to you personally, since in MWI any possible event happens with probability 1 when checking at the universe level.
I’d say a good heuristic would be to include all mammals in any theory about consciousness, wave function collapse, quantum consciousness, etc. and then see if the theory still holds up. If it does, you are probably on to something.
I think scientifically we will look back on “humans are uniquely conscious” as a categorical differentiation instead of a gradient with other mammals to be as absurd as believing the earth is the center of the universe. “Unique consciousness” is a quasi-religious mechanism we use at a societal level to not run around all the time terrified of death.
No interpretation of quantum physics proposed by scientists ever required a conscious observer, that's more of a misinterpretation of what an observer is. It's an interaction, not a person or animal.
As for the theory of consciousness, of course we are not special, it's information-processing, and it looks like thermodynamics is responsible for the emergence of information-processing structures.
A mental trick that helped me understand the concept of “an observer” in particle physics is to imagine it like playing billiards in a pitch black room.
In normal human-scale billiards, there are immense numbers of photons flying around bouncing off everything. The photon interactions are far too small to affect the path of a moving billiard ball, but we can detect them easily with our eyes. So we can use photons to passively observe the balls rolling around.
But when you’re trying to observe a photon itself… there are no tiny photon-equivalents flying around. It’s like playing billiards in pitch black: the only way to know which direction a ball is rolling is to touch it. And you can’t touch a rolling billiard ball without changing its path somehow. Likewise, you can’t “observe” a single photon without interacting with it in some way.
This is a great analogy, but also in experiments the presence or absence of the measuring device like a beamsplitter determines the outcome. The measuring device, the which-way detector is the observer, and it can be regarded as a quantum mechanical system. It's correlations with the rest of the system causes the particle behavior.
> No interpretation of quantum physics proposed by scientists ever required a conscious observer, that's more of a misinterpretation of what an observer is. It's an interaction, not a person or animal.
Are you saying that the Von Neumann–Wigner interpretation[1] does not explicitly postulate consciousness to be necessary for the completion of the process of quantum measurement?
Those guys are excused, they were early on the floor, had no idea what's going on. They grew up believing that nature is deterministic, but quantum physics complicated that picture a great deal.
I should've said no interpretation in the 21st century.
> As for the theory of consciousness, of course we are not special, it's information-processing, and it looks like thermodynamics is responsible for the emergence of information-processing structures.
That's a really interesting paper, thanks for that. It's also a bit depressing to consider that there's a good chance that everything we think that makes us special is really just an emergent property of a thermodynamics memory and prediction system.
What I don't understand is why we call it "observation" when it should really be "interaction". The quantum weirdness resulting from the collapse of probabilities has nothing to do with a conscious observer, just whether and at what point the phenomena in question interact with something else in an observable way.
> What I don't understand is why we call it "observation" when it should really be "interaction" […] at what point the phenomena in question interact with something else in an observable way.
That’s why. If it’s observable it could be observed by an observer in an observation.
Because when an object "interacts" with another object, it goes into a quantum superposition with that object. It is only when we observe that we don't see the superposition.
The obvious implication that people don't like to talk about is that there is nothing special about observation, it is just that our own body goes into superposition and we only subjectively experience one of the quantum states.
> there is nothing special about observation [...] we only subjectively experience one of the quantum states
That subjective experience seem something special!
There is nothing special about quantum superpositions - they are pure quantum states like any other. They are superpositions when we consider them in a particular basis. How does the subjective experience project your body - and the rest of the universe - onto one element of the right basis?
I don't really understand much of quantum physics - I'd say as much as someone with a passing interest in chemistry would need to (and that's pretty focused on what groups of electrons get up to), but every once in awhile I'll read about the actual problems and experiments that the old Nobel laureates got up to and all of a sudden something will fall into place. A big key was reading about the ultraviolet catastrophe and how Max Planck basically just played around with equations and sort of hit on quantizing energy levels and almost accidentally invented quantum mechanics. If my education had just been a history of all the discoveries in physics from the end 19th to mid 20th centuries (and to be fair to my education that was mixed in a bit), I think I would have been served a lot better, but I also acknowledge that could be down to my individual way of learning.
Except that in this context it isn't mumbo-jumbo at all. It's a falsifiable theory proposed by one of the most distinguished mathematical physicists in the world, in the process of being falsified. Perfectly respectable, though highly speculative, science.
The article didn't include it for lulz and clicks. It's directly relevant.
No one apart from Penrose took it seriously. You might as well say that Russell's Teapot is falsifiable because you only need to look through a finite amount of space to prove it's not there.
If you read the article there is still plenty of room for those effects to hide in, just like there was before these experiments were done. No one has changed their minds because of this.
You mean the bit where he's working on a version of Orch OR that doesn't cause bremsstrahlung? That's coming up with a new theory. You can always go away and change the theory. This isn't one of those overparameterised theory generators that can trivially explain every possible observation. He will have to materially change it to accommodate this.
And why shouldn't he explore what changes would be needed to accommodate the new evidence? That's not the behaviour of a crank convinced his theory must be right regardless of any evidence. It's just a natural thing to think about when a theory is falsified. He's never described Orch OR as anything more than speculative.
And frankly, what else would you expect him to do? He's 91. It's astonishing that he's still doing this sort of work at all. He's hardly going to start from scratch with a completely fresh theory at this stage, although if anybody could it would be Penrose.
It's not a joke. Social media is heavily curated for a specific agenda/angle (at best) and generally full of dis/misinformation. Why on earth would anybody consider what they see on social media to be representative of reality?
Perhaps the collapse happens when all the quantum fields are activated at a particular point in their coordinate system.
I.e. if we have 3 fields, with respective coordinates [0, 1, 2], [1, 2, 3] and [2, 3, 4], a particle emerges only when the 3rd point of field A, the 2nd point of field B and the 1st point of field C, all with value = 2, are activated.
If the quantum field activations happen periodically, but their periods do not match, or the fields have different granularity when they are activated, then we may get a wave-like outcome for particles, since the interactions of all the fields happen only when the fields are synchronized at specific points in their history.
EDIT:
Writing the above made me realize that the underlying fabrique of the universe may work like a neural net, where each possible point in the universe is actually a node in a neural net, and each node has multiple fields connected to each node, and values flowing into the fields excite the nodes based on a function, for example accumulation. When a limit is exceeded in a node, a particle is created, or a particle disappears (for black holes).
A neural net of such proportions could be called ...God (yeah, I said it, sorry...I don't believe in a God but the parallelism is interesting, at least to me, from a philosophical perspective).
Some smart people have tried to make theories where wave function collapse is a real thing. These new experiments are working to rule out those ideas.
My feeling is that Everett's "many worlds" interpretation is the clear favorite: there is no wave function collapse.