- Classical mechanics: Basically views time as another spacial axis. In classical mechanics bodies can be viewed as being a 4D extrusion of their 3D shape along a 1D time path. High precision simulation of classical mechanics does not model time as steps / frames in a movie, but as a continuous 4D geometry.
- Relativity: Builds on classical mechanics idea that time is a geometric axis and using a 4D coordinate system. What is further added is the possible distortion of that 4D spacetime.
- Quantum mechanics: Strictly speaking quantum mechanics does not model time at all. It is more like algebra. It only describes equivalences / transformations which can occur. When we reformulate some algebraic expressions we don't think of the steps we take as time steps. It wouldn't work anyway because there are many possible paths not one. It is not a 1D sequence but a graph, which can even contain cycles.
- Thermodynamics: None of the other areas of physics make time "pass", because to them time is symmetric and past and future might as well be swapped. Only thermodynamics introduces an asymmetry by coupling it to information theory: On the macroscopic average, entropy can not decrease, only increase. Thus, we get a global gradient form low to high entropy, which is what makes time irreversible on a large scale.
Note that thermodynamics may be kind of a "meta" or "supra" theory: other areas of physics don't have a concept of entropy or direction of time because it is an emergent property of a great number of particles following more "basic" rules. The same way the game of go doesn't have a concept of "immortal shapes", those are just a consequence of its rules.
Congratulations, you’ve just explained relativity, QM and thermo more clearly and concisely than years and years of other courses and literature. Thank you.
What do you think about the notion that time as a dimension _competes_ with the space dimensions? So time is not the same thing as space (duh), so no generalized "spacetime" exist. If spacetime is like a wavefront, reconstructing its space dimensions as it goes, we can never get out of it, unless we subside into the underlying (timemore vs timeless) sea. A wavefront has rotation built in, but the wavefront itself could also be rotating, around a center, so you have your entangled oscillator(s) right there.. </agistimulus>
>High precision simulation of classical mechanics does not model time as steps / frames in a movie, but as a continuous 4D geometry.
Strong disagree on that part. High precision numerical analysis is always done with discrete time steps. It has been this way since the Finite Element method was invented. Anything non-linear (eg. collisions, large displacements, non-linear material behavior, non-linear boundary conditions, etc.) requires discrete steps to be calculated accurately.
The CCD research you linked below has very good uses cases (eg. robot control systems) but predicting the collision time and location of two simple solid shapes in a linear domain is absolutely not a "High precision simulation". There could be ways to use it to make a complex simulation more efficient (eg. dynamically adjust the steps to make them shorter right before a collision, densifying the mesh where you predict a collision, etc.) but at the end of the day your simulation is solved using discrete time steps.
And it's not like nonrelativistic quantum mechanics and Newtonian mechanics have that different a conception of time. Both describe the evolution of a system using differential equations in time, both treat the world as having a single shared time (like "frames in a movie"), etc. I think the main differences are what evolves in time, and also quantum mechanics possibly needing different rules for what happens to the system when a "measurement" happens.
Also, the idea that "none of the areas of physics make time pass [except thermodynamics], because to them time is symmetric", isn't true, because e.g. the standard model doesn't have time symmetry.
Also also, I don't think it's a helpful simplification to say that relativity simply allows for "distortion of spacetime". SR says that the symmetries we expect of spacetime are wrong, and that a very different group of symmetries applies.
> Both describe the evolution of a system using differential equations in time
All areas of physics use differential equations, and so does relativity which you are contrasting these two against next.
> both treat the world as having a single shared time (like "frames in a movie"),
I think you misunderstand what the "frames in a movie" meant. It was about the distinction of time being modeled as discrete or continuous.
What you are talking about ("single shared time") is indeed a shared property of all other areas, which are not relativity, by the virtue of not being relativity (which introduces the distortions that make a single shared time impossible). But that is a tautology.
> I think the main differences are what evolves in time
All four areas are somehow describing the evolution of matter and energy. Again, not a difference.
> and also quantum mechanics possibly needing different rules for what happens to the system when a "measurement" happens.
Measurement, collapse and conscious observers are unnecessary additions which are not needed for a complete description of quantum mechanics. I would instead argue that the main difference to classical mechanics and relativity is the superposition principle (which has nothing to do with measurement): The idea that the properties of things are not in one state at a time but in a combination of states simultaneously. And that in turn means you can not model time as a simple 1D axis anymore.
> because e.g. the standard model doesn't have time symmetry.
My point was about flipping the direction of the flow or the role of past and future. The standard model has time reversal symmetry if you also flip matter and anti-matter, which for the sake of the argument makes no difference because you can't tell which one you are made of. From the POV of anti-matter people we are the ones made from anti-matter or matter which moves backward in time. Which strengthens the point that only thermodynamics assigns a clear direction to the flow of time.
> SR says that the symmetries we expect of spacetime are wrong, and that a very different group of symmetries applies.
Can you elaborate on that? Are you referring to the free fall equivalence principle?
> All areas of physics use differential equations, and so does relativity which you are contrasting these two against next.
Yes, but the way in which Newtonian mechanics and quantum mechanics use DEs is similar was the point, and the way GR uses them is very different.
> Measurement, collapse and conscious observers are unnecessary additions
Maybe, I don't know. I meant to say "possibly", not "probably", sorry.
> All four areas are somehow describing the evolution of matter and energy... I would instead argue that the main difference to classical mechanics and relativity is the superposition principle
I think we're sort of in agreement? Quantum mechanics describes how superpositions evolve over time. The "what", for example, might go from particles to wavefunctions.
> My point was about flipping the direction of the flow or the role of past and future.
Quantum mechanics describe the evolution of the state of system given the initial conditions and the hamiltonian describing it. That's exactly what the Schrodinger equation says and why it has a partial derivate of time. Usually in introductory treatments only the "time independent Schrödinger equation" is addressed, but that's a sub product on a common technique used to solve some family of differential equations.
In classical mechanics you can interpret time as another coordinate, but with so many privileges that it's a little far reached call it in the same way as space dimensions. Without the metric introduced in special relativity there is no way to justify put them in the same footing. For example, in classical mechanics you cannot rotate in a way that makes the time coordinate fall in a space coordinate, while you can do that in relativity, and it's called a boost.
Well, the Schrödinger equation only describes the evolution of the wavefunction over time (which is indeed a similarity with classical mechanics). But how the wavefunction then further relates to the actual "state of system" is up to the interpretations of quantum mechanics. And there in lies my point: That because of the superposition the way time works in quantum mechanics (properties are simultaneously in a combination of states) is very different from classical mechanics (properties are in exactly one state at a time), even if both have a "t" in their equations.
> For example, in classical mechanics you cannot rotate in a way that makes the time coordinate fall in a space coordinate, while you can do that in relativity
That is a good point. Maybe it is fair to say that in classical mechanics we start out with an additional separate geometric dimension (which only translation / shifting is allowed in). And in special relativity it gets promoted to a full spacial dimension, allowing all sorts of transformations.
Because there's no way to convert energy from heat to mechanical, electrical, or chemical but they inevitably lose some of themselves to heat energy (through resistance, friction, and exothermic reactions).
Note that you can use a difference in temperatures to generate mechanical/electrical energy, but that's also an increase in entropy as the nicely sorted temperatures get muddled and weakened, and there's no way to undo that either (at least not without suffering a loss of efficiency).
Also note that you can always "cheat" by expanding your reference frame to include objects and energy you don't care about. That's why the first law of thermodynamics states that you can't get energy for free, but we can, for all intents and purposes, get "unlimited" "free" energy from the sun, and that a solar-powered machine could have what is effectively perpetual motion from our perspective. I don't know about you, but I'm not going to worry about the machine failing in five billion years when the sun dies.
In QM information can be created (by measurements), but not destroyed* (no-deletion/no-cloning theorem). To "store" that information you need to have enough possible physical quantum states (the kind bounded in Bekenstein bound) which is thermodynamic entropy.
[*] what made blackhole information paradox controversial
I think it originates from the observation that all machines "leak" power in the process of performing work. Vibrations, thermal conduction, infrared radiation, etc all tend to decrease efficiency and hence increase entropy (in the form of heat loss, mostly). Entropy can of course be lowered by bringing a measure of order to a given "isolated" system. At much larger scales however the vast number of possible microstates dominate, and thus the "inevitable march toward disorder". Also bear in mind that "entropy" itself is a fairly general concept which can be applied to just about any field. Information, for example, can expressed in terms of entropy. (Which obviously has nothing to do with the "thermal entropy" of physics.)
It is like asking "why can't things go faster than the speed of light", this is an observation of all known processes, not an absolute law. It is possible there are things we haven't observed that will change this.
Entropy can decrease, it's just statistically unlikely. It's a "law" due to the law of large numbers, not because the transitions are forbidden.
Another way to phrase this is that there's no law of nature the says your pool balls can't end up neatly racked after the break, but you'd still never expect to see that happen.
Within her discussion is the unscientific "humans have no free will" statement. Free will is experienced as a part of the consciousness, which we cannot point to, physiologically. (For those who point to the head region as "containing consciousness", this is also unscientific.) The most annoying part of the video is that her statement comes off as this being simple observable fact that "everybody knows" and is a good example of why articles on topics are best when kept on topic.
My own observation is that time is better described as human narrative description of events in the observable universe.
I have not read the paper referenced in the video and I have to really spend time on these things to recall the concepts such papers are typically breezing through, but when I have attempted in previous years to understand how scientists explain time, they seem to usually have little agreement and lots of guesses. I have not found scientists claiming they have discovered fundamental units of time. I have not found scientists claiming they can sample time. I have not found scientists claiming they can isolate time. I have not found scientists claiming to have derived time as some sort of force in the universe. I am not clear on how something that has no fundamental units, cannot be sampled, cannot be isolated, and is not a force can also be something in the physically existent universe. Yet, scientists seem to place time in their explanations and equations as something that de facto exists.
I think your observation can be generalised - all of physics is human narrative description of events in the universe, coupled with some principles for choosing one narrative over another.
I think when a scientist says "free will does not exist", they mean that in a very particular sense - a theory of physics (or narrative) that includes free will has as much predictive power as one that doesn't. So from a scientific point of view it's parsimonious to exclude it.
As you rightly point out this has nothing to do with the subjective experience of free will, which is an aspect of consciousness that I expect most people are familiar with (scientists or no).
By the way, this is not directly related, but who are these scientists you speak of who claim to be unable to measure time? Time, as it is used in theories like classical mechanics or relativity, has been measurable with clocks for millenia. The measurements fulfil the role of time in those theories; the predictions they make have been tested to an extreme degree. This is the strongest evidence for time as an ontological primitive that science can produce!
(which is not inconsistent with the fact that it may behave differently outside those regimes)
> I think your observation can be generalised - all of physics is human narrative description of events in the universe, coupled with some principles for choosing one narrative over another.
At least in regards to scientific study, this is not correct. The purpose of science is to discover the properties of the physically existent universe. The properties, themselves, cannot be "narrative". For example, we would not describe magnetic force as "narrative". We may use "words" to communicate what a magnetic for is, but this does not mean the magnetic force, itself, is narrative, and the magnetic force is what science is discovering.
> I think when a scientist says "free will does not exist", they mean that in a very particular sense - a theory of physics (or narrative) that includes free will has as much predictive power as one that doesn't. So from a scientific point of view it's parsimonious to exclude it.
This is an opinion on what this individual is claiming. However, I can also have an opinion on what this person is claiming, and based on my personal experience with both the body language and tone of the statement, my opinion is she is likely a Humanist or heavily influenced by Humanists (even without realizing this), as a critical part of Humanism is that free will does not exist.
> By the way, this is not directly related, but who are these scientists you speak of who claim to be unable to measure time?
It is easier for me to find them than to post a comment on HN. I am confident you experience will be similar.
> The purpose of science is to discover the properties of the physically existent universe. The properties, themselves, cannot be "narrative".
The purpose of science, as I understand it, is to discover predictive models of the extant universe. I don't think "magnetic force" is a property of the universe any more than "good" or "evil" is. It's a model that gives us good predictive power in certain regimes. It is completely inadequate in others. These models are typically operationalized using formal languages (such as mathematics, or programming languages), so I think calling them "narrative" is justified.
Science also typically proceeds in narrative fashion via papers, journals, conferences and review processes. Your comment strikes me as a map/territory confusion.
> It is easier for me to find them than to post a comment on HN. I am confident you experience will be similar.
This is nice rhetoric, but I know a lot of physicists and none of them claim that they cannot measure time. We literally do it all the time when we set alarms, run stopwatches, etc. It's only mysterious when you get to regimes that physics has yet to capture adequately (microuniverse or extremely high energy microuniverse).
I believe that human brains harness quantum uncertainty and chaos theory to combine stateful information (such as preference for chocolate icecream) and recursive processes to make 'free' decisions which aren't random and fundamentally unknowable to outsiders without destroying the human (similar to a quantum system which can't be measured in more than one dimension without destroying it).
At what point did the brain transition from not "harnessing quantum uncertainty and chaos theory to combine stateful information and recursive processes" to doing so? Do fish brains? What about the jellyfish nervous system?
My problem with these types of explanations is that they are so egotistical. We probably all agree that a rock doesn't have free will. Is there no quantum uncertainty and chaos theory happening inside a rock? What about in a solar system?
A rock certainly isn't going to surprise us in any time scale relevant for us. The processes shaping it will be more from the outside than inside it's boundaries. Certainly chaos theory and quantum uncertainty are at play, but the stone is rather just like a single pendulum which swings rather very predictably and humans seem to be more like dual pendulums which are so chaotic that they escape simulation beyond minutes.
We have come far to scan the brain for the manifestation of conscious decision making, but nobody serious is claiming that we can predict human choices from say 20 minutes before a decision is made. In the same sense that we can predict the weather now 7-10 days but would never think it viable to go for 100 day weather forecasts.
This phenomenon is certainly on a sliding scale and we might simulate rather well jellyfish, fly, and increasingly bigger brains, but the fundamental uncertainty just means we won't be able to get the outcome correct enough the longer we look into the future.
Our inability to predict something on an arbitrary timescale isn't the threshold for free will, right? Otherwise, we'd say tornadoes and hurricanes exhibit free will.
I believe that with perfect information, humans are predictable, but that doesn't make the will any less free.
Just because someone could predict that you would make a decision doesn't mean you didn't make the decision, it just means you're internally consistent.
We know that the universe doesn't permit perfect information. You have to pick if you want to know the speed of a particle or its location once you approach the tiniest scales (and various other trade-offs based on the fundamental uncertainty). Thus you can't predict perfectly how a system will behave.
Neuronal processes in the brain seem to be build in a way to amplify such tiniest scale difference to generate divergent outcomes from very similar sensory and memory inputs.
But you still get to make them yourself, too. If free will is synonymous only with unpredictability, than you'd necessarily have to make some choices that you don't want to in order to demonstrate free will, and that doesn't really make sense to me.
I figure you have the choice. You could choose to do otherwise, but you don't. If someone offers to give me a million dollars absolutely free, or to not do so, just because I could be predicted to take the million dollars doesn't mean I don't have the choice to reject it. I'm not being coerced to take it. It's just a very predictable decision.
I think you’re presupposing the existence of free will. Here’s a question for you: what explanatory or predictive value does the concept of free will have?
The goal of the concept of free will is to explain that there is a possibility of an event to be 'caused' by a human (or another life form sufficiently able).
Without the concept of free will you would have to assume that the cause for the event is originating from something else (a calculation of sorts based on previous states of the universe is a commonly stated alternative).
Free will walks the line of being a physics-based process which is neither to easily deterministic such as stone being heated in the sun or too random such as a radioactive element emitting a radioactive decay at a random/statistical time but still localizes the cause for an event in some assemblage of atoms such as a living being.
Exactly, this is my thinking as well; I've mentioned the recursive concept in another comment before.
And I suspect the question of free will is not resolvable until the hard problem of consciousness is solved. The cells in our bodies may not have free will, but we might. The issue involves characterizing what exactly does or does not have free will, which is bound up with the question of consciousness.
I like to assume both sides of the free will argument will be wrong in the future. The future answers will be wrong as well. This is consistent with history.
These clocks fundamentally measure how long certain processes at the quantum level take. We can measure time now so exactely that on the scale of the age of the universe we are less than 1/10th of a second inaccurate. Or as another example clocks are now so accurate that we can measure the time dilation by raising a clock in the gravitational field of the earth by millimeters.
How does this compare to confirming the existence of other things demonstrated to exist? For example, do we confirm the existence of a proton as some percent of the universe of protons? The same for gravity? Magnetism? Why is it that with time, we are always using some kind of tolerance statement based on some understanding of the entire existence of the universe (which I am unclear on how to test such theories), meaning that we still are not coming to a single statement "time is x" while also assuming time to exist (I haven't seen things published such as "we are less than 1/10th of a nuclear force inaccurate" because this is not how the nuclear force is demonstrated to exist)? To me, this suggests there may be certain broadly predictable properties of the universe such that time seems to explain some aspect what we observe, but it is merely concurrent with the mechanics of the universe and useful for human understanding, but not something that exists (note that this is my interpretation, not a scientific proposition).
(Conceptually similar to how the Ptolemaic model of the universe can predict the cycles of the stars, but does not provide a description of how the universe works, but I'm not trying to change the topic, just to stick strictly here to demonstrate things can be useful and scientifically not true.)
every single measurement is subject to measurement uncertainty. the mass of an electron will be quoted with some errorbar.
we had a quote in the lab at uni: errorbars or it didn't happen. errorbars aren't a weakness, on the contrary, they indicate how confident we are of some measurement.
What I am doing here, really, is demonstrating that time does not seem to be a particle, which is usually made up of some fundamental unit (molecules are made from atoms, atoms are made from nucleons, those are made from quarks, etc) or is a fundamental unit.
Something I've heard physicists say but doesn't make sense to me is, "gravity is a just a distortion in space-time, it's not like the other forces." Can someone with more knowledge than me explain this? As far as I can tell:
1) General relativity is just a model.
2) The force that was observed as gravity can be modeled as a a distortion in space-time.
3) Thus (circular logic incoming), gravity is not actually a force, it's just a distortion in space-time, and that why it looks like a force.
This is just begging the question. I could say the exact same thing about any of the other forces between particles, just instead of using the permanent[0], I'd use the determinant[1] for fermions or the immanant[2] for anyons.
I remember in my electrodynamics class applying relativity to Coulomb's law, and seeing the magnetic force just pop out in the Taylor expansion (and I just learned the Feynman lectures does this too: II_13-6[3]). So what do physicists mean when they say gravity is special, or general relativity doesn't play well with quantum dynamics?
The best understanding of matter is in the form of quantum field theory (QFT). QFT is always formulated with some background metric (geometry) of spacetime as an input.
One idea is that the metric (geometry, gravity) could be a field just as matter is a field and people tried to apply the standard rules of (perturbative) QFT to gravity but failed. This is because the theory of gravity is unrenormalizable [0]. An interesting avenue in saving this line of thought is asymptotic safety where the idea is that gravity coupled to the standard model could actually be renormalizable in a certain sense [1].
In any case general relativity and quantum theory have so far been irreconcilable and there is now consensus on how to bridge the gap between those two theories. It is exactly because of this that most physicists will think of gravity and the other forces to be of a different nature.
When people say that gravity is not a force they mean that there is no known particle which acts as the intermediary of said force. For all the other forces we have a theory that explains the exertion of force via a particle.
Your first point states that "general relativity is just a model". Many physicists believe that it is more than a model but a true description of what the world really is like. I understand your urge to label theories as models, but ultimately the question is whether or not there is some level of ground truth that can be accessed in the form of mathematical theories.
First of all, in some sense it is not that interesting of a statement. Whether gravity is a force or not, or whether light is a particle or a wave -- all that depends on definitions of the word force, particle, wave, etc. And nowadays that is the realm of philosophy instead of physics.
Ultimately physicists care not about the words but rather about the equations. Only equations allow them to predict the outcome of experiments (at least probabilistically). A physicists' ultimate dream is to find a single set of equations that predicts every experiment. Words are just used to describe what the equations do, to colleagues and laypeople alike.
So yes, our best model to describe gravitational phenomena is general relativity which describes these phenomena in terms of the bending of spacetime.
If you want to go back to a "force" description of general relativity you can do so as it's a free world. The description of all the other forces starts with a fixed background spacetime, so to do the same with gravity you'll have to contort the elegant covariant equations of general relativity into an ugly mess by expanding around such a background. And then your resulting "theory" will predict exactly the same experimental outcomes as general relativity. (Around flat space it is not so difficult, it produces Newton's law of gravity to the first approximation.)
All that just does not seem worthwhile. More importantly, the discovery of general relativity (and of the importance of general covariance) led to a paradigm shift in physics: all reasonable physicists now believe that any fundamental theory will not ultimately be a set of equations on a fixed background spacetime but rather a set of equations from which spacetime itself should somehow emerge. Very different, therefore, from quantum field theory which needs a fixed background and describes all the other forces in the standard model.
This was my attempt at addressing the "not like the other forces". For the quantum dynamics see the sibling comment https://news.ycombinator.com/item?id=41179419 . And thank you for teaching me about the "permanent" and "immanant" of a matrix, but I still have no idea what that comment is supposed to mean - sorry.
I'm out of my depth here, but I'm imagining something like a path integral[0] between n particles. For example, with two fermions, A and B, you'd get a fraction f of A moving to B's location, and vice versa, giving:
d[AB] = f * ([B] - [A])
The fraction is real as f^2 is the action A -> B -> A which shouldn't get a phase or A would self-destruct. If you had a bunch of anyons circling around in a magnetic field[1], then f could be a root of unity or something more complicated. You can generalize the boundary operator
d[123...n] = Σ(-1)^k * [123...k-1,k+1...n]
to be
d[123...n] = Σ χ(k) * [123...k-1,k+1...n]
where χ(k) is the character of your group. For example, if you have n particles interacting in a circle 1 -> 2 -> 3 -> ... -> n -> 1, then χ(k) = e^2πik/n. This is where the immanant comes from.
Whereas, for the other 3 fundamental interactions, the currently accepted model is in the form of a quantum field theory. For these 3 interactions, the models suggest force carrier particles, and these particles have been observed (electromagnetism: photon, weak interaction: bosons, strong interaction: gluons).
People have tried to construct a quantum field theory -like model for gravity, which would include gravitons as the force carrier particles. But all these attempts have failed to produce a consistent theory, and also nobody has ever measured or observed gravitons, so this is all hypothetical.
But if we get philosophical about this, gravity is not like other forces because the currently accepted model for gravity is not like the models for the other forces. Gravity is not a field with force carrier particles because we have not been successful at modelling it as such.
I’m not a physicist, but I think the comparison is made between gravity and e.g. strong nuclear force. In the latter, the “force” is modelled by interactions between hadrons, with virtual particles (distortions in various fields) mediating the interaction. With gravity, there are no particles (though graviton is a hypothesis).
One can say that space time is also “field”, and mass creates distortions, so gravity is just like other forces interacting via field distortions, but iiuc, unlike nuclear forces, gravity does not have a quantized mathematical model.
Everything in physics a model. Quantum mechanics, electromagnetism, Newton's laws... Arguably everything in science is a model. But at some point, a model does so well at explaining observations that we take it to be a Truth. At one point, genes were 'just a model/theory,' but after the discovery of DNA, we took them to be real and physical. Seen in this way, there's nothing circular about either case.
It's uncontroversial to say that gravity is special because it is non-renormalizable. This means that one can't write down a quantum theory of gravity the same way that we write a quantum theory of electromagnetism, because it predicts infinite quantities that can't be removed from the theory. However string theory does unify quantum mechanics and gravity, and gravity is only "sort of special" in string theory (it does look in some ways like the other forces; the force carrier is the graviton). But exactly how spacetime emerges from string theory is still not fully understood.
Because all masses accelerate at equal rates in a gravitational field either:
1) Inertial mass and gravitational mass are exactly equal without an explanation why
or
2) The acceleration is just an effect of a curvature of space.
Imagine two bodies thrown exactly Northward on a sphere. Although their paths are parallel they would approach each other, as if there was an acceleration. This acceleration would be the same whatever their mass is.
>Imagine two bodies thrown exactly Northward on a sphere. Although their paths are parallel they would approach each other, as if there was an acceleration. This acceleration would be the same whatever their mass is.
The thing that always confuses me is how the distortion explains their behavior when stationary.
Sure, sure, "stationary" doesn't exist and everything is a matter of reference frames etc etc etc, but still, if you and I are both standing still on a sphere, there's nothing drawing us towards each other. Why are we drawn together from rest?
I am just trying to understand this and not make any claim, so bear with me.
Let's say we have a cylinder with a hemispherical top instead of a sphere.
Say the two objects were thrown directly from the base of the cylinder towards what would be the equivalent of north on the hemisphere. Relative to each other they would be moving perfectly parallel and the distance between them would not be changing.
Once they reach the hemispherical section they would still be moving parallel to one another at the same speed but the distance between them would start to shrink, wouldn't this be the equivalent of acceleration due to gravity? Movement towards each other started at 0 and increased, right?
IIRC, Einstein actually spent quite a bit of effort trying to reformulate electromagnetism as a geometric curvature just like gravity, but eventually published a paper admitting failure. I recently saw some physicist talking about revisiting that with some new tricks because maybe it wasn't such a bad idea after all.
Whatever solution is simplest, it's probably better. Every rule has an associated cost and, if the universe itself can be seen as a state transformation engine going over a matrix of states, the fewer the rules, the more likely it is.
And yes, this is bordering some form of faith - that the universe principles should be as efficient and simple as they can possibly be.
That doesn't make sense to me because electromagnetism is both an attractive and repulsive force (can pull objects together or push them apart). Gravity is purely an attractive force which makes sense when explained as warped spacetime. But I'm also not a physicist...
Time is fun to think about. It seems that the smallest unit of time measures the smallest unit of detectable change in a phenomenon. If awareness is needed for time that hints to me that awareness and time are the same thing, dependently arisen like two sides of a coin. Space is also needed for time as a phenomenon exists in some form of space for it to be observed to change, so it seem that space = time = awareness.
I think you won't gain anything from conflating "detectable change" and "awareness". We kind of already had this discussion in Quantum mechanics with "observations" and the "consciousness" of an observer.
Nowadays the information theoretic point of view is prevalent among physicists and an observation is often seen as just a correlation between the observer system and the measured quantum system. No consciousness needed.
Nevertheless I agree, that it is interesting to think about time as an series of changes or events instead of just as a dimension of 4D spacetime. I didn't watch the video yet (I prefer text), though, so I refrain from bothering you with may opinion on this topic.
You would need memory as well. Without memory you wouldn't be able to detect change over time, and hence not the existence of time. And without that you can't have awareness (as I understand the word).
The past light cone of an object constrains what past events can affect an object regardless of whether the object has awareness of not. So you don’t need memory to experience time, even if you need memory to understand it.
Measurement, awareness, and memory, are not required for time to exist. These are all things that make observation possible, but time could still exist even if the universe were just a fog of loose atoms bouncing around with no self awareness.
If you're on a space ship travelling at 0.86c, your Lorentz factor is 2, and so time is moving half as fast for you. Yet if you measure the speed of light it will be the same. Then if you look out the window you will see the universe is 1/2 the size it used to be in the direction you are travelling.
I don’t see why space should be required for a change to occur. Now, particle spin may be associated with a physical direction, but I don’t see why it needs to be. Why not say “we have a unit vector in C^n evolving unitarily” (Where C^n is n-tuples of complex number) ?
In my philosophy, times would merely be various signal clocks. Time in this sense, then, will be the instantenous unit that relates one's environment-embedded consciousness' clockrate to the clockrate of the clock. Consequently, one can have at every moment multiple, changing times. One builds foundations of embedded agency partly on this, signal theoretic, interpretation—without confusing it with the physics notation which has uses elsewhere.
If you take the quantum field of view.. its the start of interaction between two particle-permutations forcing a breakdown into a observable and measurable interaction?
Time is the expansion of the involved while simultaneously collapsing complexity permanently.
If the universe has only a single particle, is time even possible?
I think a universe with one particle is the same as one with none. As soon as you have two particles they can move toward each other, and spin can be observed.
I think that's gonna depend heavily on your definition of particle.
It seems most likely that our current universe is a singularity, a single point that contains the entirety of space and time, such as they are. Viewed through a certain lens this could be considered a particle, but I'm not sure that it checks all the boxes for properties you associate with one.
If a universe only contains a single particle, then there can be no "position" within the universe as how could you possibly measure it? Similarly, how would you be able to determine if it was static or moving without comparing it to something else?
Yes, but that is irrelevant. Change happens, even if you can't recognize it. Recognizing change is a shortterm-problem. But longterm it can be relevant, if the particle some day will break down, disappear, burn out, or whatever happens to them.
> If the universe has only a single particle, is time even possible?
space is constantly expanding. particles have energy. energy decays. eventually into photos. colors shift red due to increasing wavelength and decreasing energy.
We don't know if space is expanding. The models and observations appear to support expansion.
This was just a thought experiment with a theoretical single partical in a universe of unknown size to demonstrate that the universe changes in non-obvious ways as you add energy/matter.
I wonder when Adams came up with the idea for the Agrajag character and storyline when he had written that line in book 1. Or did he decide to make Agrajag the bowl of petunias whilst writing book 3.
A simple middle school formula really spells it out clearly.
If you’ve no distance or velocity, you don’t have time.
If we pretend that velocity is heat (electron excitation, though technically a cloud), you can see why we can never get to absolute zero without discontinuity (black hole), so time increases or the universe gets larger. There is a reason being cold, slow, and dense are linked.
You can keep increasing the heat to “infinity”, making time slower and slower relatively.
I had a similar line of thinking. Just because you can measure something doesn't mean it fundamentally exists. We can measure things like pressure and temperature, but that doesn't mean they are fundamental in any way.
You shouldn't take 'Illusion' too strongly here. Ms Hossenfelder is saying that conceptually time can't really be separated from 'change' so the only way to measure time is to have some constantly changing but steady ticker like a pendulum. This has always been a problem in quantum mechanics because you can't study the a system independently of its clock, you have to add the clock to the system. The video is discussing a paper where they use entanglement and a tiny ticker to get around this problem. Arxiv: https://arxiv.org/abs/2310.13386
No need to invoke metaphysics here. Whether time is an illusion or not doesn't depend on its "existence," though I'm not sure what you mean by "exists." I'm not convinced anyone does.
I agree with your overall point though. All we really need to say, when we say something is an "illusion," is that the phenomenon in question is not as it intuitively appears. Our expectations were one thing, but because of x, y, and z, this can't be the case. We think of it one way, but it's actually another way.
It's tempting to draw this distinction upon metaphysical lines, e.g., we thought it was one way, but here's how it Really (TM) is. Again, this is totally unnecessary and adds nothing to our scientific understanding.
Everything which occurs involves change. Change implies time and motion and exchange only happen over time. Without time there is no big bang, no "work", no decay, no expansion, no energy propagation, no chemical reactions, no state changes... probably no resistance.
How can "the speed of light is the speed limit of the universe" and "time is an illusion" coexist?
This article is one of my favorites on Wikipedia. The last bits are maybe the best for this question.
Somewhere in the very far future, time will lose all meaning. In that, the universe will be so big and so empty that if you ran time backwards it would not look meaningfully different than it running forwards. All matter that is left will have devolved to the basic particles and the universe will be in the maximally entropic state. So much so that the page correctly stops labeling time in any unit. It's not that if you could pop into this future and then look at your watch, that it would not tick. All laws of physics still apply, 'time' is passing still. It's that for the rest of the universe, 'time' has become something different. I'm not in the field anymore, but this maximal entropy and time's use are obviously linked.
A nanosecond or a gigayear are both essentially noise in the time spans that these events occur in. Time, very truly, is meaningless to our future dead universe.
This sounds very similar to how Henri Bergson describes time in Time and Free Will.
Particularly chapter 2 the Multiplicity of Psychic States. And as he says the fault is making extensity out of duration and therefore producing nothing more than the ghost of space. I can't possibly do it justice here, highly recommended read for the curious of thought.
Time doesn't exist. This is a human concept to measure mutations. Brain preserves previous states - it is a feature of a brain. It doesn't require time - it just preserves previous states like a history pattern. What exist is constant change - here and now. There's no time.
What do you mean time is irreversible? Your movement in it is irreversible, just like your movement on a one-way road is irreversible in space. Time itself does not possess the property of irreversibility, at least in modern physics. All the equations describing natural phenomena can work both ways, forwards and backwards in time.
Maybe this analogy will be helpful for you. We have CPU with mutable RAM. You can create immutable language on top and start arguing that you can go both ways in mutations back and forth in RAM. But this is the feature of the abstraction you applied not the core feature of "reality".
This is what differs model from reality. You're talking about abstract concept which helps you to measure mutations but as I tried to explain above - is not a real thing. It's just working and useful idea.
I have a relative who holds the view that time is illusory, especially when hen is under the effect of some toxin.
I ask hen: when did you figure this out?
The answer inevitably starts with a story that happened many years ago and I point out that if time were illusory then there is no appropriate way to answer the question.
Because hen is drunk or stoned, I get to do this multiple times. It's not the time is illusory, it's that hen doesn't remember.
- Relativity: Builds on classical mechanics idea that time is a geometric axis and using a 4D coordinate system. What is further added is the possible distortion of that 4D spacetime.
- Quantum mechanics: Strictly speaking quantum mechanics does not model time at all. It is more like algebra. It only describes equivalences / transformations which can occur. When we reformulate some algebraic expressions we don't think of the steps we take as time steps. It wouldn't work anyway because there are many possible paths not one. It is not a 1D sequence but a graph, which can even contain cycles.
- Thermodynamics: None of the other areas of physics make time "pass", because to them time is symmetric and past and future might as well be swapped. Only thermodynamics introduces an asymmetry by coupling it to information theory: On the macroscopic average, entropy can not decrease, only increase. Thus, we get a global gradient form low to high entropy, which is what makes time irreversible on a large scale.