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What *precisely* makes cause and effect asymmetric?

  1. Sep 24, 2015 #1
    My question is slightly philosophical, so be warned.

    If we could only experience the world through a recording played backward, what laws would we form?

    I am asking, because the strangest thing occured to me: at least some of these laws would be exactly like ours.

    Say, you have two coins. They travel through void. They hit. Bounce off. Now they travel in different directions. Now imagine you record the whole thing happening and you play it backwards. The funny thing is, your recording still makes sense. As much sense, as the 'proper' recording. Energy preserved. Cause and effect just "traded places". Cause and effect (= time?) is "symmetric".

    Earth revolves around Sun. We play it backwards.
    Still makes as much sense.

    But, obviously, not all causes and effects can be reversed like this. Let's say an asteroid hits the Moon and it explodes. Little pieces fly in all directions and come to rest at the surface of the Moon. If we reverse this, we get small pieces detaching themselves, for no apparent reason, from the surface; jumping together; forming the asteroid and then flying away into the space. It's hard to imagine what set of alternate-physics laws would it take to do that. Gravity working in opposite direction, that's for one (or maybe the void working as the source of gravity-like force). I am not sure if it could be explained by any laws... but maybe?

    What of entropy? In our world, it increases with time. In alternate reverse-time world, the laws would have to state "entropy decreases with time". Ironically, it appears just as logical as our "increases with time".

    After all, physics is generally like mathematics. It has equations... Symmetric, reversible. 2+2 is 4 and therefore 4 is 2+2. E=mc2 and mc2=E. And yet in real life, in physics, when we get to cause and effect, this symmetry gets broken.

    Does it?

    If we tried to form laws of physics for a "recording-played-backward world", where exactly would they lose symmetry with our laws? Or wouldn't they?
  2. jcsd
  3. Sep 25, 2015 #2
    Some interactions have time reversal symmetry, some do not.

    In other cases, it's not just adding a component like entropy increasing that breaks time reversal symmetry.

    A cold virus can cause certain symptoms, but certain symptoms can never cause a cold virus.

    Science often only answers what rather than why. It is enough that we can enumerate interactions that do and do not have time reversal symmetry.
  4. Sep 25, 2015 #3


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    The underlying laws of physics are time-symmetric, but these time-symmetric laws still give us two ways of recognizing when we're looking at a recording played backwards.

    The first is so easy and natural that we do it without even noticing: We know more about the physical system than just what's in the film we're watching, and we use that information to decide whether we're watching the real thing or a time-reversal. For example, a video of an object falling through an opened trapdoor under the influence of the attractive force of gravity could also be interpreted as an object being forced upwards by some repulsive force and being captured by the closing trapdoor - but if we assume that the video was filmed on the earth's surface where gravity is attractive we will reject that interpretation.

    The second is entropy, a concept that is much more precise than the mushy "increasing disorder" that you'll find in pop-sci writing. Google will find you the real definition, but here's an example that will show you how it works. Suppose I have a box with fifty coins neatly laid out on the bottom. We film me shaking the box so that coins flip back and forth between heads and tails more or less randomly. If we focus on the behavior of just one coin, it is completely time-symmetric - heads flipping to tails can be played backwards as tails flipping to heads and they're both equally sensible. However, if we're looking at all fifty coins, and the film shows them starting in in some random-looking configuration and ending up all heads... There is only one chance in ##2^{50}## that the film is being played in the forward direction; it's much more likely it's a film of fifty heads-up coins being shaken into a random pattern and played backwards. Thus, a very strong sense of the one-way direction of time (we can shake the box to destroy the all-coins-up pattern, but not to restore it) emerges from the completely time-reversible behavior of the individual coins.
  5. Sep 25, 2015 #4


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    The classic example: Imagine a cup of hot water and an ice cube dropped into it. The result: a cup of tepid water. Could you imagine a time reversal - a cup of tepid water suddenly getting hotter while an ice cube is created in the water?
  6. Sep 25, 2015 #5


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    Actually, I think the classic example is just a water glass shattering.
  7. Sep 25, 2015 #6


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    Possibly. I just quote from an example involving "Maxwell's demon".
  8. Sep 26, 2015 #7
    You guys made a good point, I can see that entropy is not time-symmetric. And I don't want to dismiss it, but... Well, entropy is more like an effect, an observed result, not a cause of why do things happen. So it seems to me. And if it is not just an effect -- is it a reason for all asymmetry?

    Is there any explanation that ties the causes of entropy to the fundamental forces? Some good article maybe?
  9. Sep 28, 2015 #8
    This example is obviously correct but it is far too complicated to be useful in any way, just like all examples involving living creatures.

    The simplest example of time-asymmetry and the simplest example of entropy I could think of is this:


    Two adjacent balls, simultaneously hitting a third ball. They bounce off and get separated. The same event played backwards just doesn't happen. There is something magic about the geometry in this example. You can exchange balls for atoms or elements or forces. It doesn't matter because the geometry is the same. State A changes into state B and it cannot be reversed simply by using the-same-but-opposite forces. I would love to understand if mathematics can explain this lack of symmetry. It is really like 2+2 is 4 but not the other way around.
  10. Sep 28, 2015 #9


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    Why not?
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