10 ms·

Time’s Almost Reversible Arrow

118 points by alberto_ol 11y ago

mrfusion 11y ago
Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean time can't run in reverse. Also information might be lost in black holes. Wouldn't that also prohibit it reversing time?
- oneJob 11y ago
  Yeah. Totally what I was thinking. Other than that though, solid theory.
- deleted 11y ago
  [deleted]
- koverstreet 11y ago
  No. Consider what it would look like if you ran the solar system in reverse - all the planets and moons would go backwards in their orbits, but gravity still works exactly the same. The equations of mechanics don't care about time. And yes, information being lost in black holes would be an issue, if true - that's why that problem has been studied so much recently.
  - mrfusion 11y ago
    But if gravity isn't reversed how would planet formation be reversed? What would allow the matter in a planet to fly apart?
    - maho 11y ago
      Interstellar gas/dust forming a planet increases its temperature, and it radiates thermal photons out into space. For the planets to fly apart, you would have reverse the photons as well, and aim them super precisely, and define the initial state of the planet super precisely, and... It's essentially a problem of entropy. All weirdness of macroscopic time-reversal can be reduced to the weirdly low entropy of the big bang.
    - empiricus 11y ago
      Lets consider only the moment before the collision of the small planet pieces: these are all accelerating to the center of mass. If you revert time, you still have a lot of pieces, with high speed, but these are going away from the center (and getting slowed by the gravity). After collision, it gets more tricky, because the kinetic energy of a body will be lost (mostly transformed in thermal energy) and it will be less intuitive to imagine how to revert that.
    - danielsju6 11y ago
      You'd see radiation hitting the planet warming the mantel, complex chemicals breaking down & releasing heat, all warming the planet significantly to the point where it was a molten ball, and finally lots of instability where large explosions were throwing pieces of it into space. Basically take the movie and play it backwards.
      - bronson 11y ago
        I find it very hard to picture a molten ball throwing a giant chunk of cold comet deep into space.
        tbabb 11y ago
        That is what is so unsettling about T-symmetry. What you describe is merely improbable, not impossible. With appropriate starting conditions, it is certain. Basically we'd require all the particles in the comet to bounce into each other in such a way that their heat motion cancels and organizes into momentum in one direction, away from the planet. This is physically permitted in either time-direction, but we only see it in one.
        Sharlin 11y ago
        Yes, but that has nothing to do with gravity and everything to do with thermodynamics.
        ajross 11y ago
        Yes, but the reason you do is that it's merely (and extremely) unlikely, not impossible. The third law of thermodynamics has its own arrow of time (the one in which entropy increases), but the basic interactions of the universe don't need to change to make that happen. They work symmetrically (mostly, with some tiny exceptions as detailed in the linked article).
    - Strilanc 11y ago
      Gravity gets double-negated when you reverse time. Its units are over seconds squared instead of just seconds. A simple way to see this is to imagine taking a movie of a heavy back arcing upward through the air then back down. What do you see when you run the movie backwards? A ball arcing upward them downward. The acceleration still points in the same direction: down.
  - empiricus 11y ago
    So if I understand correctly, time reversal means that all the object speeds get inverted, but the fields remain the same. If you change the sign of the field (gravity becomes repulsive) then things will explode very quickly :)
    - pdpi 11y ago
      Nothing gets reversed at the equation level. It's just that every equation with a dt in it gives you reversed results with a negative dt.
- keeperofdakeys 11y ago
  https://en.wikipedia.org/wiki/Arrow_of_time https://en.wikipedia.org/wiki/Arrow_of_time If you go to the 'overview' section, there is a nice snippet explaining all this. It points out that gravity works fine in a universe that is running backwards, however entropy is decreasing instead of increasing.
rubidium 11y ago
Great read on some of the history and current efforts in particle physics by a leader of the field (Frank Wilczek). I was surprised to read such a well written article on quanta magazine (I've often been disappointed there) but it made sense when I saw who the author was. It read more like what might be published in a general audience Nature or Science article.
maho 11y ago
This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the result unmix itself again, even though the interactions between the individual molecules are perfectly reversible. In fact, you can build a toy-universe in your head, where all interactions are time-reversible, and you would still observe that the chaotic-ness increases with time, unless you start with a completely random initial state (in which the chaoticness stays roughly constant). This increase in chaoticness is comletely unsurprising: It's a simple consequence of the fact that there are much, much more "states" in which a system looks chaotic, than states in which it looks ordered. Therefore, a system transitioning from state to state is much more likely to be in one that looks chaotic, and very, very unlikely to ever go back to a state that is non-chaotic. The real mystery, for me, is: Why was the initial state of our universe so completely non-chaotic? Why did the universe not start out in a state that is essentially equal to the heat-death [1]? The microscopic processes that are not time-reversible might have something to do with this, but it's not clear at all how they can solve this mystery. [1] https://en.wikipedia.org/wiki/Heat_death_of_the_universe https://en.wikipedia.org/wiki/Heat_death_of_the_universe
- blaze33 11y ago
  As I understand it, the universe actually started in a heat-death state (very small temperature differences, homogeneous and isotropic), it's just that it is a very hot heat death state, not the usual cold one where everything freezes. Then it is the universe expansion that made the entropy per volume decrease, actually allowing complex systems to evolve. The expansion still has to occur faster than the entropy increase rate but I read somewhere that the universe total entropy did not increase that much since the Big Bang.
  - charlieflowers 11y ago
    Or whole universe was in a hot dense state. Then, nearly 14 million years ago, expansion started. Wait ... The earth began to cool.
mrfusion 11y ago
What s the current thinking on being able to run Schrödinger's cat backwards? How can time be reversed on something in an indeterminate state?
- jsprogrammer 11y ago
  The indeterminacy may not be real. It's just that you don't know the state. I don't believe there is a rule that says you get to know everything.
  - tbabb 11y ago
    Are you proposing a "hidden state" which is unobservable? Such a formulation is not consistent with quantum mechanics, as proven by Bell's theorem, unless you permit instantaneous influence from arbitrarily distant parts of the universe.
    - jarin 11y ago
      Put another way, aside from the possible "instantaneous influence" exception above, there are no "hidden variables" or mechanisms under the surface that we just haven't figured out yet. At that level it is truly probabilistic.
    - jsprogrammer 11y ago
      I'm saying that the construction "indeterminate" is only a statement about the knowledge of the person making the statement. Are you proposing that the universe doesn't act unless you are considering it?
      - tbabb 11y ago
        https://en.wikipedia.org/wiki/Bell's_theorem https://en.wikipedia.org/wiki/Bell's_theorem Certain observations are probabilistic, and the correlations in them predicted by quantum mechanics cannot be reproduced by ascribing them to the traits of particles. It means that at the lowest level, the universe is (barring faster-than-light communication, generally regarded to be false) "truly random". (No concept of an "observer" needs to be devised in order to arrive at this conclusion).
brudgers 11y ago
The modern history of T invariance begins in 1956. Or 1781, when Critique of Pure Reason was published. Kant proposed that time was an empirical fact based on experiencing events in time being a precondition of all human experience. The tradeoff for a well reasoned non-skeptical position was that we give up claims to "really really" know how things really are independent of human experience...i.e we give up claims to potential omniscience. That we normally take "knowledge" to mean human knowledge, doesn't change that this is what we mean. Or to put it another way, what would time be for a computer with knowledge or as the article mentions for creatures that experience time backward?
ilostmykeys 11y ago
With React/Redux you can reverse time! :D
mrfusion 11y ago
Wouldn't the expending of space prevent particles from being able to run backwards in time? Unless you also ran the expansion backwards?
djfm 11y ago
I'm very surprised no one posted this yet: http://xkcd.com/1621/ http://xkcd.com/1621/
Houshalter 11y ago
There is a hypothesis that universes with reversible time can't exist. Or time travel of any kind. Such universes, the hypothesis goes, can't have any meaningful cause and effect, like we know it. Causality is an extremely important property of our universe. And it's really weird that our universe has it, if the hypothesis isn't true. The space of possible universes without causality is much larger than restricted subset that obeys that rule. This is difficult to explain, so see here: http://lesswrong.com/lw/fok/causal_universes/ http://lesswrong.com/lw/fok/causal_universes/
nickpeterson 11y ago
This is an honest question, how are we so certain time exists? Does anyone have a few quick references to books or articles that would explain it to the layman? I always found the concept of time completely artificial, created by observation of cyclical systems.
- jberryman 11y ago
  "created by observation of cyclical systems". Can you elaborate on that? I'm not sure what you mean, but it sounds interesting. It seems to me we believe time "exists" because of causality; things that happen at "this moment" affect things that happen "in the future" but not things that happen "in the past". We've also predicted and measured certain weird things about time, such as how it can go faster or slower.
  - nickpeterson 11y ago
    So, you could condense my knowledge of physics to an index card, but here we go...time seems like it's always based on the measurement of a regular thing. Say the oscillation of a quartz crystal. So we have no proof there of time, just that certain phenomena can be very regular in intervals/cycles. I've heard things like, "if you go the speed of light, or at least really fast, time will move differently." But that seems like it would be a side effect of that local system moving at a rate that interferes with its physical interaction relative to what we consider normal. There are probably tons of holes in that. But my point is that we tend to think of time almost as an invisible "thing", and I really have trouble wrapping my brain around that. It just feels like things happen and we call it time, but really its ordered intervals of effects.

118 points by alberto_ol 11y ago

Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean time can't run in reverse. Also information might be lost in black holes. Wouldn't that also prohibit it reversing time?

oneJob 11y ago
Yeah. Totally what I was thinking. Other than that though, solid theory.
deleted 11y ago
[deleted]
koverstreet 11y ago
No. Consider what it would look like if you ran the solar system in reverse - all the planets and moons would go backwards in their orbits, but gravity still works exactly the same. The equations of mechanics don't care about time. And yes, information being lost in black holes would be an issue, if true - that's why that problem has been studied so much recently.
- mrfusion 11y ago
  But if gravity isn't reversed how would planet formation be reversed? What would allow the matter in a planet to fly apart?
  - maho 11y ago
    Interstellar gas/dust forming a planet increases its temperature, and it radiates thermal photons out into space. For the planets to fly apart, you would have reverse the photons as well, and aim them super precisely, and define the initial state of the planet super precisely, and... It's essentially a problem of entropy. All weirdness of macroscopic time-reversal can be reduced to the weirdly low entropy of the big bang.
  - empiricus 11y ago
    Lets consider only the moment before the collision of the small planet pieces: these are all accelerating to the center of mass. If you revert time, you still have a lot of pieces, with high speed, but these are going away from the center (and getting slowed by the gravity). After collision, it gets more tricky, because the kinetic energy of a body will be lost (mostly transformed in thermal energy) and it will be less intuitive to imagine how to revert that.
  - danielsju6 11y ago
    You'd see radiation hitting the planet warming the mantel, complex chemicals breaking down & releasing heat, all warming the planet significantly to the point where it was a molten ball, and finally lots of instability where large explosions were throwing pieces of it into space. Basically take the movie and play it backwards.
    - bronson 11y ago
      I find it very hard to picture a molten ball throwing a giant chunk of cold comet deep into space.
      - tbabb 11y ago
        That is what is so unsettling about T-symmetry. What you describe is merely improbable, not impossible. With appropriate starting conditions, it is certain. Basically we'd require all the particles in the comet to bounce into each other in such a way that their heat motion cancels and organizes into momentum in one direction, away from the planet. This is physically permitted in either time-direction, but we only see it in one.
      - Sharlin 11y ago
        Yes, but that has nothing to do with gravity and everything to do with thermodynamics.
      - ajross 11y ago
        Yes, but the reason you do is that it's merely (and extremely) unlikely, not impossible. The third law of thermodynamics has its own arrow of time (the one in which entropy increases), but the basic interactions of the universe don't need to change to make that happen. They work symmetrically (mostly, with some tiny exceptions as detailed in the linked article).
  - Strilanc 11y ago
    Gravity gets double-negated when you reverse time. Its units are over seconds squared instead of just seconds. A simple way to see this is to imagine taking a movie of a heavy back arcing upward through the air then back down. What do you see when you run the movie backwards? A ball arcing upward them downward. The acceleration still points in the same direction: down.
- empiricus 11y ago
  So if I understand correctly, time reversal means that all the object speeds get inverted, but the fields remain the same. If you change the sign of the field (gravity becomes repulsive) then things will explode very quickly :)
  - pdpi 11y ago
    Nothing gets reversed at the equation level. It's just that every equation with a dt in it gives you reversed results with a negative dt.
keeperofdakeys 11y ago
https://en.wikipedia.org/wiki/Arrow_of_time https://en.wikipedia.org/wiki/Arrow_of_time If you go to the 'overview' section, there is a nice snippet explaining all this. It points out that gravity works fine in a universe that is running backwards, however entropy is decreasing instead of increasing.

rubidium 11y ago

Great read on some of the history and current efforts in particle physics by a leader of the field (Frank Wilczek). I was surprised to read such a well written article on quanta magazine (I've often been disappointed there) but it made sense when I saw who the author was. It read more like what might be published in a general audience Nature or Science article.

maho 11y ago

This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the result unmix itself again, even though the interactions between the individual molecules are perfectly reversible. In fact, you can build a toy-universe in your head, where all interactions are time-reversible, and you would still observe that the chaotic-ness increases with time, unless you start with a completely random initial state (in which the chaoticness stays roughly constant). This increase in chaoticness is comletely unsurprising: It's a simple consequence of the fact that there are much, much more "states" in which a system looks chaotic, than states in which it looks ordered. Therefore, a system transitioning from state to state is much more likely to be in one that looks chaotic, and very, very unlikely to ever go back to a state that is non-chaotic. The real mystery, for me, is: Why was the initial state of our universe so completely non-chaotic? Why did the universe not start out in a state that is essentially equal to the heat-death [1]? The microscopic processes that are not time-reversible might have something to do with this, but it's not clear at all how they can solve this mystery. [1] https://en.wikipedia.org/wiki/Heat_death_of_the_universe https://en.wikipedia.org/wiki/Heat_death_of_the_universe

blaze33 11y ago
As I understand it, the universe actually started in a heat-death state (very small temperature differences, homogeneous and isotropic), it's just that it is a very hot heat death state, not the usual cold one where everything freezes. Then it is the universe expansion that made the entropy per volume decrease, actually allowing complex systems to evolve. The expansion still has to occur faster than the entropy increase rate but I read somewhere that the universe total entropy did not increase that much since the Big Bang.
- charlieflowers 11y ago
  Or whole universe was in a hot dense state. Then, nearly 14 million years ago, expansion started. Wait ... The earth began to cool.

mrfusion 11y ago

What s the current thinking on being able to run Schrödinger's cat backwards? How can time be reversed on something in an indeterminate state?

jsprogrammer 11y ago
The indeterminacy may not be real. It's just that you don't know the state. I don't believe there is a rule that says you get to know everything.
- tbabb 11y ago
  Are you proposing a "hidden state" which is unobservable? Such a formulation is not consistent with quantum mechanics, as proven by Bell's theorem, unless you permit instantaneous influence from arbitrarily distant parts of the universe.
  - jarin 11y ago
    Put another way, aside from the possible "instantaneous influence" exception above, there are no "hidden variables" or mechanisms under the surface that we just haven't figured out yet. At that level it is truly probabilistic.
  - jsprogrammer 11y ago
    I'm saying that the construction "indeterminate" is only a statement about the knowledge of the person making the statement. Are you proposing that the universe doesn't act unless you are considering it?
    - tbabb 11y ago
      https://en.wikipedia.org/wiki/Bell's_theorem https://en.wikipedia.org/wiki/Bell's_theorem Certain observations are probabilistic, and the correlations in them predicted by quantum mechanics cannot be reproduced by ascribing them to the traits of particles. It means that at the lowest level, the universe is (barring faster-than-light communication, generally regarded to be false) "truly random". (No concept of an "observer" needs to be devised in order to arrive at this conclusion).

brudgers 11y ago

The modern history of T invariance begins in 1956. Or 1781, when Critique of Pure Reason was published. Kant proposed that time was an empirical fact based on experiencing events in time being a precondition of all human experience. The tradeoff for a well reasoned non-skeptical position was that we give up claims to "really really" know how things really are independent of human experience...i.e we give up claims to potential omniscience. That we normally take "knowledge" to mean human knowledge, doesn't change that this is what we mean. Or to put it another way, what would time be for a computer with knowledge or as the article mentions for creatures that experience time backward?

ilostmykeys 11y ago

With React/Redux you can reverse time! :D

mrfusion 11y ago

Wouldn't the expending of space prevent particles from being able to run backwards in time? Unless you also ran the expansion backwards?

djfm 11y ago

I'm very surprised no one posted this yet: http://xkcd.com/1621/ http://xkcd.com/1621/

Houshalter 11y ago

There is a hypothesis that universes with reversible time can't exist. Or time travel of any kind. Such universes, the hypothesis goes, can't have any meaningful cause and effect, like we know it. Causality is an extremely important property of our universe. And it's really weird that our universe has it, if the hypothesis isn't true. The space of possible universes without causality is much larger than restricted subset that obeys that rule. This is difficult to explain, so see here: http://lesswrong.com/lw/fok/causal_universes/ http://lesswrong.com/lw/fok/causal_universes/

nickpeterson 11y ago

This is an honest question, how are we so certain time exists? Does anyone have a few quick references to books or articles that would explain it to the layman? I always found the concept of time completely artificial, created by observation of cyclical systems.

jberryman 11y ago
"created by observation of cyclical systems". Can you elaborate on that? I'm not sure what you mean, but it sounds interesting. It seems to me we believe time "exists" because of causality; things that happen at "this moment" affect things that happen "in the future" but not things that happen "in the past". We've also predicted and measured certain weird things about time, such as how it can go faster or slower.
- nickpeterson 11y ago
  So, you could condense my knowledge of physics to an index card, but here we go...time seems like it's always based on the measurement of a regular thing. Say the oscillation of a quartz crystal. So we have no proof there of time, just that certain phenomena can be very regular in intervals/cycles. I've heard things like, "if you go the speed of light, or at least really fast, time will move differently." But that seems like it would be a side effect of that local system moving at a rate that interferes with its physical interaction relative to what we consider normal. There are probably tons of holes in that. But my point is that we tend to think of time almost as an invisible "thing", and I really have trouble wrapping my brain around that. It just feels like things happen and we call it time, but really its ordered intervals of effects.