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I Back to Muller's 'Now and the Flow of Time'

  1. Jan 13, 2019 #1
    Folks, I'm back to reading Muller's paper (https://arxiv.org/pdf/1606.07975.pdf) about the flow of time. He postulates that time is expanding similar the way space is expanding (ok... so if you can swallow that). He asks: "why are the new nows created at the end of time, rather than uniformly throughout time, in the same way that new space is uniformly created throughout the universe?"

    One answer is he gives is "that a physics principle of causality accounts for the apparent asymmetry in the creation of new space and new time. In this view, we postulate that new time can be created only at the end of previous time, since its creation earlier would disrupt the causal connection of past events."

    The above is fine from a relativistic perspective, but I'm wondering if it also extends to a quantum mechanical perspective? That is, can someone give an example of an inconsistency in that would result in QM (QFT) through the creation of new time throughout the past? (e.g., what would change - measurements, observers, etc.?)

    I'm less familiar with causality in a quantum setting...
    Last edited: Jan 13, 2019
  2. jcsd
  3. Jan 13, 2019 #2
    Ok, how about this - can two quantum systems in different states ever produce identical measurement results?
    Last edited: Jan 13, 2019
  4. Jan 13, 2019 #3

    Vanadium 50

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    Of course. If I have a hydrogen atom in the 1S state and one in the 2S state, I can in both cases find an electron at a given r.
  5. Jan 13, 2019 #4
    Ok, going back to the original question, is there any reason why 'expanding time' would necessarily lead to inconsistencies with a quantum framework? Does the same problem Muller mentions with the "causal connection of past events" still occurs in QFT, where there is some probability of any measurement result?

    That's is, does the causal nature of relativity carry over to QFT?

    Or could someone point me to a place to look? (odd question I know - but I haven't seen it discussed anywhere)
    Last edited: Jan 13, 2019
  6. Jan 14, 2019 #5
    You don't need quantum mechanics to see a problem with this. "Changing the past" always contains seeds of paradox. If this "new time" actually has physical consequences, e.g. by changing how long it took light to travel from A to B, then the retrospective insertion of new moments of time is going to change what happened in the past.
  7. Jan 14, 2019 #6
    Thanks @mitchell porter - here's one QM-specific question:

    Take the light ray example - where the time required for a photon to travel from A to B has increased slightly. Is it possible that an observer at B would (could) measure the photon to arrive at the original time? That is, due to uncertainty in the position of the photon, is there not some probability that you would obtain the same measurement, even though time has expanded?
  8. Jan 15, 2019 #7
    If the "expansion of time" in the past has any physical meaning, it has to change something. For example, if there is a probability distribution of possible durations of some event, then the distribution might shift to favor longer durations, even if the range of possibilities itself has not changed... The moment that your theory of reality allows the past to change, it faces the risk of self-contradiction through paradox and will require special features to avoid that.
  9. Jan 15, 2019 #8
    Is there a simple toy QM example that would illustrate such a paradox?
  10. Jan 16, 2019 #9
    I suppose it's not a paradox - like traveling back in time and killing yourself before you make the time machine - because there's nothing in Muller's writing about controlling the "expansion of the past". But even having the past change spontaneously is akin to a contradiction, because what happens in the past has causal consequences for the present.

    Suppose that extra proper time makes it take longer for something to happen. At the quantum level, it means that de Broglie waves or wavefunctions would experience a greater amount of phase rotation. At a more macroscopic level, if the change is big enough, it could mean that whether two objects collide is different. Either way, there are causal consequences downstream. If the past is different, then something about the present must be different, indeed history must always have been different, starting from the moment that changed.

    The idea that the past can change, in any sense, is the real core of the problem. It's simply impossible unless you have some peculiar concept of time, like branching universes, or you suppose that for some reason the consequences of the change die out completely before the present. Since Muller seems to be pushing a concept of time like the "evolving block universe", where space-time is growing a new layer all the time, and the current top layer is "now", you might think that the past could change because you'd just be changing something deep inside the block, leaving "now" unaffected. But this is a peculiar concept of time, because time has actually been doubled - there's historical time, frozen in the block, and then there's evolving time, how the block changes.

    In the context of general relativity, and really any gravity theory based on Einsteinian space-time, the concept of expansion of the past is incoherent, because it implies this doubling of time. So Muller is right to want to make "expansion of time" a phenomenon of the present. But there's actually no need to have "expansion of time" at all, as John Rennie explains here. General relativity makes sense without it. It's really up to Muller and Maguire to come up with a new theory of gravity in which their desired effect occurs.
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