Question about time in QM vs. special relativity?

  • #1
This question is about both quantum mechanics and special relativity, so I wasn't sure which forum to post it in. If mods think it's in the wrong forum then please move it as you see fit!

My understanding is that in special relativity, we live in a four-dimensional "block universe" of space-time. Since simultaneity doesn't exist in SR, two events that I consider to both be happening in the present may occur in the present and the future (or the present and the past) for you. Therefore, since there's no privileged frame of reference, there is no absolute present for everyone, and the past, present and future all have equal reality, depending on who's observing - all of time and space exists at once in a four-dimensional block universe. In the same way as we think of distant galaxies far away in space as as "real" and "out there" as local places on Earth, we should also think of events in the distant past and future as "real" and "out there" too - just as real as the present. This is contrary to our intuitive "tensed" perception of time where the past has already happened and is no longer "real" and the future does not yet exist, but there's no other way to describe it in special relativity. As Einstein said, the distinction between past, present and future in SR is little more than "a stubbornly persistent illusion." So I'd say this kind of universe is inherently deterministic, since the future already exists, and we're simply riding the illusory "flow of time" created by the way our conscious brains interpret the world into that pre-existing future. The future is "closed" rather than "open" - a done deal. What will happen will happen. For some other observer, maybe it already has.

However, in quantum mechanics, there does seem to be a real difference between the present and the future. The future path of an electron or photon is unpredictable, and we can't say where it's going - indeed, in some sense, the Universe itself doesn't "know" where it's going. Nature is random and probabilistic, rather than deterministic. So in quantum mechanics, it seems to me that the future is not deterministic, cannot be predicted exactly, and therefore seems "open"? It seems to me that quantum uncertainty depends on a tensed theory of time, one where there really is a "present moment" distinct from a future that is uncertain and cannot be predicted until it happens. (You also couldn't "predict" the past too, since even if you knew the exact position of every particle in the Universe at the present moment you couldn't know their momenta and couldn't extrapolate back to find out where they were a second ago as you could in classical physics or special relativity. So it seems like there really is something special about the present.)

So how do we reconcile these two views? Is there a way to interpret special relativity's 4D space-time, time dilation and loss of simultaneity without having to live in a deterministic block universe where the past, present and future all exist on an equal footing and the flow of time from one "present moment" to the next is an illusion? Or is there a way to interpret quantum mechanics' uncertainty principle, the probabilistic nature of reality and inherent unpredictability of the future that's consistent with a block universe and doesn't rely on a tensed theory of time?

Answers and Replies

  • #2
I've dealt with this question too, and it has bugged me for awhile. I don't know that I've ever come to a completely satisfactory answer, but perhaps this will help.

Quantum mechanics doesn't necessarily do away with determinism, and the block-universe.

There are popular interpretations of the formulas which do suggest no determinism, but there are others which do not. As one particular example, Everett's relative state (or many-worlds) interpretation has the wavefunction evolve only deterministically according to the Schrodinger equation with no collapse upon measurement. The reason that we see random measurement outcomes is that we as physical beings would have to include ourselves, the measurement interaction between device and system (and everything else) to get the complete picture. I'm not entirely certain everything's been ironed out yet, but it's a promising alternative viewpoint.

Another more pessimistic, but respectably conservative viewpoint is that quantum mechanics serves only to describe measurement probabilities and has nothing to say on its own about an objective external reality. This interpretation (known largely as quantum-bayesianism, or qubism) has the the combined benefits of not being wrong, and assuming the least, but still isn't quite satisfying imho.

Though these theories (QM and SR) are rather accurate descriptions of what we see and measure, the map is not the same as the territory (as they say), and figuring out the nature of reality from experimental results is an ongoing challenge. These issues have yet to be settled.

Hope this helps:)

  • #3
A search for QBism will be more fruitful.
  • #4
we have an established policy about block universe policies: [Broken]

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