Afaik, if we introduce a special frame of reference (e.g. the cosmic microwave background frame) and this special reference frame is the only frame in which FTL is permitted, FTL does not violate causality
Put another way, travelling FTL in and of itself does not mean you're travelling backwards in time. But the geometry of the universe provides a barrier between STL and FTL, yet no barrier between forwards and backwards in time except inasmuch as you need to first rotate from STL to FTL before you can rotate from forwards to backwards-
So if you invent such a barrier, then sure, FTL is fine. It's just that there's geometrically nowhere to attach that barrier, except if you make use of the CMB.
Causality is not robust, and you can prove it to yourself with three sheets of light polarizing gratings. When we figure out how to put probability hooks on things other than photons, it’s going to get rowdy.
This is a nitpick. The warp drive concept doesn't allow for FTL travel, but it could allow for you to get from point a to point b faster than light could.
The idea behind a warp drive is that it utilises expansion of space, which is not subject to speed of light restrictions. The universe has already expanded faster than light without breaking causality.
The causality violation examples I've seen in relativity all involved going back to where you started. No return trips, which includes the cosmic horizon, no causality violation.
One question I've got though: does relativity (1) remove the need for universal frame of reference, or (2) preclude the possibility of a universal frame of reference?
Because if it's the former, then (I think) FTL doesn't need to also be a time machine?
> No return trips, which includes the cosmic horizon, no causality violation.
The drawback is that if a warp drive going from A to B leaves expanded space in its wake, a return trip will take longer than light would take to make a round trip. At the same time, an observer on B looking in the direction of the ship will see a shift to blue until the ship arrives, and, then, would see a marked shift to red when observing A.
For one thing it can create paradoxes, like your future you deciding they didn’t like where they ended up and stopping yourself from setting out on a journey in the first place.
This is true if and only if we live in a universe with a time continuum that is not branching (i.e. a line). Instead, if we live in a time continuum that can branch (which will result in a directed acyclic graph) the act of traveling faster than light simply creates a new branch in which you go to the past and in that branch your past self never traveled.
This relates to the different interpretation of quantum mechanics. The one that has paradoxes is the Copenhagen interpretation and the one with the branching is the many-worlds interpretation.
There is also the pilot wave interpretation, but I am unsure how to map the example you proposed to that one.
I thought branches happened when a wavefunction 'collapsed' -- i.e. for every possible value of the wavefunction, you get a branch with a unique answer for 'value of the wavefunction'?
The wavefunction decoheres; it doesn't collapse. Collapse is a function of collapse theories like Copenhagen, but has never been observed in reality.
That said, decoherence doesn't branch time in the sort of absolute manner you might be thinking of. "How many timelines exist" is a question similar to "how long is a coastline"; it depends on how closely you're looking. At the smallest scales, 'timelines' interfere and mingle in a way that distinct universes really shouldn't. It's only once decoherence has gone well out of control that they can be said to be truly separate, and even then the interference never goes to zero, only approximately zero.
If we exist in a block universe, then all time-travel effects are "already" baked in. Just because something's path through the block isn't always "forward" doesn't mean it can change its path, or anything else's.
In a growing block universe, I think time travel would be impossible, but I also think that the growing block universe theory is cope.
Not really at the quantum level, at least not in the form that is typically understood as causality. There are all kinds of ways that you can interfere with probability and when you do, the “no taksie-backsies” rule goes out the window. You can prove this to yourself with a flashlight and 3 diffraction gratings.
That’s where probability manipulation really gets interesting.
My preferred conjecture is that all things exist, at all times, in all places, but only a fraction of all of those infinite-dimensional probability vectors will produce the act of your observation, so those are all of the vector-spaces that you can detect. We can’t observe, for example, a probability vector-space where water boils at 15c/1bar, or where e=3 because we would never have existed to observe within that environment. Perhaps other things exist and observe those places, but not carbon based biological life as we know it.
I think that the Casimir effect and especially Casimir pressure in constrained geometries, Zero point energy, the uncertainty principle and other observable phenomena point towards the possibility of a continuum of multiple overlapping observable probability fields, but obviously that could be just a convenient hand wavy explanation.
In the bandwidth of overlapping probability fields flavour of MWI there is no branching or creation of new realities, merely extant vector spaces drifting in and out of observability based on probabilistic functions. Vector spaces within observability tend to be similar, and the overall aggregate sum is what we perceive as a unified observation.
As we make changes that manipulate this probability space, new probabilities dominate our perception. The apple was very unlikely to be seen on the table until we moved it there, making it then extremely probable to be observed in that position.
There are some very interesting ramifications that appear to be observable phenomena if this line of thought is drawn out to its extrema. It’s one of my favourite possible models.
Perhaps one day I or someone else will figure out how to experimentally test the idea, but so far it seems to be unfalsifiable (which obviously doesn’t mean that it is more correct, just that it’s hard to test, like the simulation conjecture and many others)
I should have also made it clear that the probability vector-spaces that become unobservable include those where you got hit by a bus, and all of the coherent/adjacent spaces to that event. So the bandwidth of perceivable “universes” keeps shrinking to lesser infinities as you go through life.
In addition, the perceivable vector spaces where you can talk to your friend also includes only those where she also got missed by the bus, and to a lesser extent all those who are connected and perceivable through communications… the act of communication being a constraint to the infinities of perceivable vectors where the communication would be possible, making us much more constrained as a society than we would have been when distant people had little or no contact.
There are a lot of interesting implications of this conjecture that may go some distance in explaining subjective human experience.
Um, sorry, how does that experiment prove anything quantum? It's explained entirely by Malus' law of I = I0 * cos^2(alpha) which has an entirely classical, simple wave-theoretical explanation (even if Malus himself was an emissionist and derived his law by introducing transversal asymmetry in the light corpuscles).
Malus’s law does indeed describe the amplitudes of the resulting light, but it does not say “why”, only how. In the same way that Newtonian physics assumes gravity but fails to explain its existence.
For more in depth look at this there is a 1995 paper by K Wódkiewicz contrasting the corpuscular formula with the quantum explanation IIRC.
Obviously, despite its explanatory power, we know that QM is incomplete so YMMV, but I’ll take QM over light corpuscles, until something better comes along.
Having light corpuscles that have poles and can experience fits of easy transmission/reflection is not much mathematically different from having photons with spins and wave phases, to be fair.
And Malus' original justification of his law straightforwardly carries over to the photons.
