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Comments - How Fundamental is the Arrow of Time?

  1. Jun 15, 2015 #1


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  3. Jun 15, 2015 #2


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    I haven't seen an analysis of the experiment mentioned in the paper, but it looks like a variation of the good old delayed choice experiment. Before going any further I think a careful analysis of that experiment needs to be part of the debate:

    Like a lot of things in QM where people talk about sometimes its a particle and sometimes its a wave, the reality when you look at it carefully, is its got nothing to do with that outmoded idea. Its never really a paticle or a wave - its always just quantum stuff. Whats quantum stuff:

    QM IMHO, as I have posted many times, is simply the most reasonable probability model that allows continuous transformations between pure states. The real interpretive issue is not the formalism - we understand it and why its like that pretty well - it's a model of observations - observations are the primitive of the theory. This is what causes all the angst and worry. Everyday experience is the world is more than just observation. You see it all the time in discussions about QM - the deep need we seem to have that there is something behind those observations. I believe if you simply accept this one fact much fog and angst about it disappears.

    What's going on with these delayed choice type experiments - IMHO its in simple cases decoherence can be unscrambled, as has been discussed a number of times here on physics forums.

    Last edited: Jun 15, 2015
  4. Jun 16, 2015 #3

    jim hardy

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    No way am i smart enough.

    It is however reminiscent of Isaac Asimov'e papers on Thiotimoline
    http://danm.ucsc.edu/~phoenix/danm203/thiotimoline.pdf [Broken]

    What about Heisenberg's Indeterminacy principle ? Are their measurements of when and where this photon is within that uncertainty ?
    I have no doubt it's someplace at every instant, just we cant measure where..
    Last edited by a moderator: May 7, 2017
  5. Jun 16, 2015 #4


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    As I said, that's the problem that's particularly hard to shake.

    Once you do, progress is swift in understanding QM.

  6. Jun 16, 2015 #5
    I'm with Jim, except for the "not smart enough" part. Everyone is smart enough by their own means and ends.
    QM just exposes a fraction of existence which lies between the layers of the onion. You can slice through the layers but not between them.
  7. Jun 16, 2015 #6
    I'm new on this site and will have to look at the article later. But I have often wondered if the problem is not with the arrow of time but with the dilation of time. We model it that a photon of light leaves a source and travels thru space and arrives at a target. And to us, that seems reasonable. But from the point of view of the photon there is another story. No time elapses at the speed of light. The photon has no time, and consequently no spatial transition. The source and target experience a resonance transfer of energy, across no distance. Somehow we have to resolve the problem of why the source and target have no time or distance separation in one sense, and a different separation in our perspective. And obviously, we keep measuring a velocity for the photons thru our space, and finding a result ...

    Perhaps we need a model with higher spatial dimensions that do not have a time axis. So while we talk of the sequential order of events: photon emission, first slit, second slit (or not), and target (with location detected by us) ... the photon merely is a representation of energy transfer across a higher dimension connection, that does not have that same time sequence.

    That is all very half-baked, and I apologize. I spend a little time now and then thinking about the EPR position that there is an underlying reality that is incompletely described in quantum theories. I tend to agree with the side that says the theories are incomplete, and that we just have yet to elucidate a more complete model, that will then leave out the possibility that the photon is making mid-flight decisions on what it is.

    I am first time reader on this site and will be looking for any threads on EPR and Bell's Inequality ... any pointers on good threads to read for insights? If there is an etiquette issue on this site for commenting at the half-baked level prior to looking at the source material ... apologies in advance. I'll pick up on the rules later also.
  8. Jun 16, 2015 #7


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    This is again the "is matter a particle or a wave" argument (there is, at one point, "particle or atom", but I presume that is a typo). I think that the very concepts of "particle" and "wave" are "macro" concepts and do not apply to the micro world of quantum processes.
  9. Jun 16, 2015 #8


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    That's a common misconception.

    SR is the theory about transformations between inertial frames. No inertial frame can travel at the speed of light so the theory says nothing about time at the speed of light.

    Even plugging c into the equations you end up with the dreaded 1/0 which is undefined.

    Last edited: Jun 19, 2015
  10. Jun 17, 2015 #9
    Dear Josh Meyer
    I am not a physicist, so you can consider my views as the views of a common man about time (I beg kind permission from the authorities to participate).
    1. The experiment is almost like the Buckyball or Feyman's experiment, or the delayed choice experiment. The results of these experiments have been nicely interpreted by Stephen Hawking in his 'the grand design' in 'alternative histories' chapter.
    2. "Water does not spontaneously jump back into a glass, buildings do not materialize from rubbles"
    I think these things have nothing to do with the arrow of time. They occur simply due to 'things happen the way they can or the way they are allowed to happen by the laws of thermodynamics. To demonstrate time's arrow, people have been deliberately choosing the irreversible events. But there are reversible events as well- ice melts into water and water can freeze back into ice at suitable temperatures. Does it show reversible time's arrows?
    3. "It is also required by the second law of thermodynamics whereby perpetual increase of entropy in closed system requires a single direction of time"
    So, if we consider entire universe as a system, where a subsystem, like the life itself, grows by decreasing its entropy, though at the cost of increasing the entropy of the whole system, then should we think that this particular subsystem is going against the time's arrow (though the whole system is going towards time's arrow). This is an unacceptable picture.
    4." Without an absolute arrow of time, we end up with a universe where time travel is possible"
    In my view, every moment of the present universe is created with the rubbles of the past and every moment of future will be created with rubbles of present, so we can travel neither to past- it has been consumed to create the present, nor to future- it is yet to be created with rubbles of present. Thus time travel is simply a redundant idea.
    5. Minkowski and Einstein have at least partly solved the problem of time. They defined it as a dimension- the fourth dimension of space (though I personally believe that this definition too needs a little change in its language). I think a dimension has no independent existence. It is simply a property of the object (here space). The height of a boy (his first dimension) has no existence independent of the boy. The height of the boy does not grow. It is the boy who grows and adds to his height. Similarly, it is the universe and its components that evolve and add to their fourth dimension- time.
    Thus, the flow of time and time's arrow are, I think, redundant ideas.
    If my views seem to be non-scientific, I apologise.
    Thanks and Regards
  11. Jun 17, 2015 #10


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    A number of posts in this thread have been removed, as they were driving the discussion off-topic.

    I'd like to remind the participants about the Physics Forums rules about personal theories. They are not allowed unless supported by peer-reviewed publications or other scientifically accepted sources - and the books that pump out misleading half-truths like "time slows down as the speed of light is approached" aren't that).
    Last edited: Jun 17, 2015
  12. Jun 18, 2015 #11
    Time DOES slow down as the speed of light is approached.

    Where is there any evidence of the opposite?
  13. Jun 18, 2015 #12


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    @votingmachine : Thanks for reading, and welcome to PF! Your interpretation of the time dilation equations of Special Relativity is, as bhobba pointed out, a common misconception. See http://www.quora.com/If-a-photon-travels-at-the-speed-of-light-and-time-dilation-at-the-speed-of-light-is-infinite-does-this-mean-that-from-the-photons-perspective-it-takes-no-time-at-all-to-cross-the-entire-universe [Broken] for a good explanation.

    @dayalanand roy Thanks for reading, Dayalanand. Let me respond to your questions in order:

    1. This experiment is indeed another type of delayed choice experiment. You can read more about it here: https://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment

    2. I chose irreversible examples to imply that entropy is always increasing (the second law of thermodynamics). Entropy is one of the only quantities that requires a specific direction for time. Reversible thermodynamic processes are not particularly interesting to this end since they are performed at equilibrium and keep entropy constant. In fact, it is this conservation of entropy that allows them to be time-reversible! If the entropy had increased, the process would be irreversible, hence the arrow of time. Irreversible processes are primarily what occurs in nature.

    3. For a full explanation of why entropy implies an arrow of time, read this: https://en.wikipedia.org/wiki/Entropy_(arrow_of_time)

    4. On this point we can agree somewhat. I don't think time travel is in the cards, either.

    5. Spacetime is the fusion of three dimensions of space and one dimension of time into a single continuum. I don't think calling it the "fourth dimension of space" is technically correct. While spacetime was a major leap forward in our understanding of time, note that this experiment was dealing with quantum mechanics instead of relativity, and the two theories are famously incompatible. Our explanations of relativistic time do not account for many of the weird things in QM.
    When you get right down to it, I think you're correct that dimensions do not "exist" in a physical sense. They are descriptions we have applied to nature for the purposes of prediction, and they do a great job at that.
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  14. Jun 18, 2015 #13


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    Thanks for pointing out the error, I fixed it.
  15. Jun 18, 2015 #14


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    Its misleading. Proper time, for example, remains the same. Suppose you have two inertial frames moving close to the speed of light relative to each other. Each has a clock placed at the origin. Each will read the same time on their clocks, but see the other clock as running slower. Which is correct? Which runs slower?

    What's really going on is its like rotating a rod to fit through a door. The rod length remains the same - but its relation to other coordinate systems is different. In fact formally length contraction and time 'slowing down' can be viewed as hyperbolic rotation.

    But really this issue should be discussed on the relativity sub-forum - not here.

  16. Jun 18, 2015 #15


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    Does this experiment violate any Bell inequality?

    I can't find the article, but it seems there is only one particle involved, so it seems unlikely that a Bell inequality is violated. If no Bell inequality is violated, then we are not forced to abandon classical relativistic causality, so it seems that causality should still be intact.
    Last edited: Jun 18, 2015
  17. Jun 19, 2015 #16
    Highly grateful having such an illuminating reply.
    Being a non-physicist, it is hard for me to conceptualize about the said QM experiment, but I have tried many a times. After reading the explanations given by Stephen Hawking in his 'the Grand Design', I tried to understand it in following way- Suppose we fire a bullet from a gun. While leaving the gun, the bullet will affect the gun ( as a jerk in the opposite direction)- a causality, but after leaving it, we may not think that it will again affect the gun (no retrocausality). But let us suppose that the bullet is tied with an almost invisible long thread the other end of which is tied with the gun, then some time after leaving the gun, when the bullet travels the length of the thread, the bullet will again affect the gun that may appear to us as a retrocausality.
    I know that this situation does not simulate with the double slit experiment, but I have just tried to transform the bullet (a particle like thing) -by tying it with a thread- into a wave like thing and see the effect. The explanations of the results of double slit experiment also depend somewhat on the wave- particle duality. Apologies in advance, if my way of thinking is too wild.
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  18. Jun 22, 2015 #17
    Dear colleagues
    One more comment. It has been said that entropy is the process that requires 'arrow of time'. However, this requirement leads to many unanswerable qestions about time as it gives a sense of 'time as an entity' and then leads some people to say that time does not exist.
    In my view, it is more sensible to say that it is the entropy and the second law of thermodynamics and the irreversible processes that give us the feeling of the so called 'time's arrow'.
    Thanks and regards
  19. Jun 25, 2015 #18
    I just want to respond to say how much I love this kind of writing. The weirdness physicists encounter is a huge part of why I love this forum, even though I'm not a physicist. Its just amazing to read about.

    My impression (as a computer guy) is that the universe is explicable because the universe is in a sense, compressible. For instance, you could accumulate gigabytes of data on the locations on the path of a planet, or you could generate that same data from the comparatively small amount of math which predicts the path of that planet. In fact, because its compressible, if a physicist has enough information about a planet at one moment of time, she can expand the whole path, past and future, out of it. In this sense it collapses the path down from a set of previous possibilities. The reason this is possible is that the system is describable using less information that the universe actually manifests. Its compressible, thus is explicable. If it were not compressible, no point in the future or past would be predictable from any other point, just as an incompressible string is completely random.

    I haven't read the experiments linked to, but I've read about retro-causality in delayed choice quantum erasure, where the choice a photon makes down the road dictates whether the interference pattern will be observed earlier. You can choose to view that in terms of the choice retro-causing the pattern, or the pattern predetermining the choice, or you can see in terms of compressibility: The which-path information for the whole system, spread out over space and time, is described by only 1 bit, the seemingly spread out nature of it in space and time is the illusion, just as the planet is described by more simple equations. The actual reality is the underlying compressed form.

    Anyway, that's my two cents on making any sense of it. Thanks again for sharing!
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