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Why can we not see the future?

  1. Sep 19, 2006 #1
    Why cannot we see the future? You see, all the laws of relativity, quantum mechanics and the other physics interaction laws are time-reversible

    Then why cannot we see the future as well? I mean, when I look at the sky, I'm supposed to see ALL the stars that have zero-space-time-interval with me, not just those who have zero-space-time-interval AND of past.
     
  2. jcsd
  3. Sep 19, 2006 #2
    not that this has much to do with ur question but if the multiple universe theory isnt true I think quantum mechanics isnt reversible.
     
  4. Sep 19, 2006 #3
    o yea, and the answer is that light takes time to reach us. There is no way to see something before it happens.
     
  5. Sep 19, 2006 #4
    Actually we do not see the stars themselves at all. We only see the photons that traveled from the stars to our eyes.
    And from our frame of reference it takes time for the photon to reach us since their speed is finite.
     
    Last edited: Sep 19, 2006
  6. Sep 25, 2006 #5
    It remains a valid physics question (that may eventually be answered through GR/cosmology), why do we remember the past and not the future?
     
  7. Sep 25, 2006 #6

    DaveC426913

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    The short answer is that, while most of the laws of physics are time-reversible, entropy is not; it only travels from past to future. As do our experiences.
     
  8. Sep 25, 2006 #7
    An argument is as follows:

    The universe was apparently in a state of low entropy to start with. And since entropy can only increase in all thermal processes, time only flows in one direction.

    If the universe was not in such a low-entropy state, who knows, perhaps things would have been different.
     
  9. Sep 25, 2006 #8

    Astronuc

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    Because it hasn't happened yet. Because time is one way. Of course one can contrive a mathematical model that will permit anything, but then one might not be able to physically realize what one's model indicates.

    People should be careful not to confuse models with reality.
     
  10. Sep 25, 2006 #9
    The odd thing (from a fluctuation theorem standpoint) is not that we observe our solar system to have had such improbably low-entropy at one time (since our existance seems to demand roughly that much), but that at the same time conditions were just as locally-improbable across our entire galaxy and every other galaxy we observe beyond..
     
  11. Sep 25, 2006 #10
    Einstein famously made quite the business of that: QM sprang from "confusing" Planck's model with reality, and GR from refusing to distinguish between real gravity and a model of gravity (acceleration).
     
  12. Sep 25, 2006 #11

    JesseM

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    That's not really a physically meaningful answer. If the universe had a future low-entropy boundary condition and no past one, then we would remember the future according to the current understanding of physics, although presumably we would think of it as the past.

    Think of it this way--suppose we had a giant supercomputer that could simulate a collection of particles as large as those that make up the solar system, for billions of years, according to the fundamental laws of physics. And suppose we could do multiple multibillionyear simulations with different starting conditions, and we simply chose the initial conditions completely randomly. This would mean that on the vast majority of runs, the initial conditions would be at or very close to maximum entropy, and would stay at or close to maximum entropy throughout the entire run, so that there'd be no overall entropic arrow of time. With a mind-bogglingly vast number of runs, though, there'd occasionally be runs which happened to start out at very low entropy, similar to that of the nebula that our solar system formed out of, which could then form a simulated star and simulated solar system, perhaps one where sim-life would arise and evolve intelligence. There'd also occasionally be runs which started out at high entropy, but with just the right combination of positions and momenta so that entropy would continually decrease, such that the end of the simulation would be in a very low-entropy state similar to that of the nebula the solar system formed from. Both types of runs would be vanishingly rare--maybe 1 in a googolplex, who knows--but if the fundamental laws were time-reversible, they would have to be equally rare, the second type would be no rarer than the first. And the second type would really be a time-reversed version of the first type in every respect, including the fact that any intelligent life in the simulations with low-entropy final conditions would remember the future and not the past, in spite of the fact that these events "haven't happened yet" from our outside perspective watching the simulation.
     
    Last edited: Sep 25, 2006
  13. Sep 27, 2006 #12
    We don't remember the past. We remember events that happend to us, and their relationship to one another in relation to our existance. You can't look into the future (just liike you can't look into the past) because time is only a concept created by us to help us explain those relationships.
     
  14. Sep 27, 2006 #13

    JesseM

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    Do you disagree with my conclusions about the results of the thought-experiment where you simulate a solar-system sized collection of simulated particles with randomly chosen initial conditions? Specifically, do you disagree that runs where the system started at low entropy and went to high entropy, with any intelligent life that evolved during the run remembering the past and not the future, would be no more common than the reverse, runs which start at high entropy and go to low entropy with life in that run remembering the future and not the past? This is a necessary consequence of time-reversible laws, since there must be a one-to-one relationship between paths in the phase space that match the first description and paths that match the second, so that each type occupies an equal volume in phase space, and choosing initial conditions randomly corresponds to picking a random starting point in the phase space.

    It is also the position of all the physicists I have read addressing the "arrow of time" issue that all arrows of time, including the fact that we remember the past but not the future, are consequences of the low entropy of the universe around the time of the big bang. For example, on pp. 145-145 of A Brief History of Time, Stephen Hawking writes:
    For a good general discussion of the arrow of time issue, I also recommend Huw Price's book Time's Arrow and Archimedes' Point.
     
  15. Sep 27, 2006 #14
    I think I agree with jeryst. Jesse, all of the language you use contains preconceptions which colour your thoughts. Take a look at The direction of time in which a computer remembers the past is the same as that in which disorder increases. I think there is no time. And there is no past. What you really have is things moving, and being recorded. There isn't necessarily any disorder. There are no arrows. There is no future. Even now is a figment of our imagination. All there is is space. And things in it. Moving. Changing. We see this, we recall our records, and we invent time and talk about time travel. But it isn't really out there.
     
  16. Sep 27, 2006 #15
    According to current physics, time does exist (just as three spatial dimensions exist) and the "past" is correlated with the "future" (each is correlated with the present, and in a completely symmetric manner).

    Now, if you accept that, why do you say we remember events that "happened to us" but not events that "will happen to us"?

    JesseM: I borrowed that book after Huw Price gave a talk here on different interpretations of QM, it was especially interesting/curious after just having read Paul Davies' Physics of Time Asymmetry..
     
    Last edited: Sep 27, 2006
  17. Sep 27, 2006 #16

    JesseM

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    This seems like philosophical talk rather than anything with actual physical consequences. When physicists talk about time all they mean is that you can use clocks to assign time-coordinates to different events, when they talk about "arrows of time" all they mean is that for certain processes event X will consistently be assigned a later time-coordinate than event Y and not vice-versa, and when they talk about disorder all they mean is that there are well-defined rules for assigning a value of entropy to any given system. Unless you disagree that these procedures work fine in an operational sense, I can't see what the relevance of debating whether time or arrows of time or disorder "really exist", or what you think this philosophical talk is even supposed to mean.
     
    Last edited: Sep 27, 2006
  18. Sep 28, 2006 #17
    Jesse:

    I didn't intend to bring philosophy into it, but instead focus on the physics of what's actually there. I do agree with procedures working fine in an operational sense, and I wasn't challenging entropy.

    How can I clarify? Please try your paragraph above with different instead of later time coordinates. Or compare the arrow of time with the arrow of space.
     
    Last edited: Sep 28, 2006
  19. Sep 28, 2006 #18
    The key word here is "current". Just because theories are current, doesn't mean they are correct. They are current because it's the best we have been able to come up with, that the majority agrees with. Every time we send up a new satellite, or launch a new probe, we are "surprised" to find that what we currently believe may not be, or is not, correct. For all we know, the Theory of Relativity may be just as inaccurate as any of the thousands of other "current" views that man has held to be true.

    I don't accept that. Any time you accept something, you close your mind to what may be the truth. Our brain records events as they occur, as a survival mechanism, otherwise we would keep putting our hand in the pretty fire. We can't remember what will happen to us, because we have not experienced the events for our brain to record them. Imagine what it would be like, if time does exist according to "current physics". At the moment of your birth, you would know everything about every event that will take place in your life, and there would be nothing you could do to change it (How depressing). You would know everything that you will ever learn during your life (Now there's a paradox). You would know every mistake that you will ever make, and what the consequences will be, but you will make them anyway. The universe would be unchangeable. Obviously, that's not the way things are. There is no set future, which means that there is no past or present, which means that time doesn't exist, except as a concept.
     
  20. Sep 28, 2006 #19

    DaveC426913

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    (It smacks of condescension to imply that all of us do not already know this. It really does go without saying.)

    But it does not change anything. A theory being "our current understanding" does not reduce it to some sort of pseudo-theory.
     
  21. Sep 28, 2006 #20

    JesseM

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    That's nonsense, the idea that the future is set does not in any way imply that your physical brain would be capable of remembering future events in advance. We can certainly run computer simulations where the laws governing the simulated worlds are known in complete form, and they are both deterministic and time-reversible, but this doesn't mean we can look at any records that form within the simulation and find out about both past and future events within it! Any records in such a simulation will only record events in the direction of lower entropy, for reasons outlined by Hawking in the quote I posted earlier.
     
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