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I have never understood why physicists get so hung up on the arrow of time and entropy. If you have a casual system, then statistically, things will tend to be

I have never understood why physicists get so hung up on the arrow of time and entropy. If you have a casual system, then statistically, things will tend to become more disordered over time, because there are just more ways to be disordered than ordered. Lots of clever people seem to agonise about this, but I don't see any problem. What am I missing?

DiogenesKynikos 2y ago
That the laws of physics are symmetric under time reversal[0], so any statement you make about how things should evolve "over time" also holds if you go backwards in time. The apparent arrow of time is commonly thought to have to do with initial conditions. Entropy was low right after the Big Bang, creating an asymmetry between the two time directions. 0. Technically, the laws of physics are symmetric under stimultaneous inversion of time, flipping of all charges, and a mirror reflection of space.
JohnKemeny 2y ago
What is time?
- GenerocUsername 2y ago
  Our brains operate on the principle entropy, therefore we experience time as the macro evolution of the universe along the dimension of increasing entropy. Time itself does not exist, we merely experience it because it is how our consciousness experiences reality
  - geysersam 2y ago
    Do you mean to say that our experience of time depends on what we see happening around us? If I place you in a room with nothing but a video tape playing backwards, would you experience time going backwards? That doesn't sound plausible.
    - garciasn 2y ago
      On a universal scale, time is a distinctly human construct and it clearly warps based on location, distance from Earth, and is heavily influenced by our biology and location in the solar system (days, years, etc). It just doesn’t matter on a universal level.
      - nntwozz 2y ago
        This is not consensus. Kurzgesagt – In a Nutshell has a very good video on it: Did The Future Already Happen? - The Paradox of Time https://youtu.be/wwSzpaTHyS8 https://youtu.be/wwSzpaTHyS8 "To be honest, no one knows. What we’ve learned are two possibilities to describe time, but they're not the only ones. Some scientists think that the idea of “now” only makes sense near you, but not in the universe as a whole. Others think that time itself doesn’t even exist – that the whole concept is an illusion of our human mind. And others think that time does exist, but that it's not a fundamental feature of the universe – rather, time may be something that emerges from a deeper level of reality, just like heat emerges from the motion of individual molecules or life emerges from the interactions of lifeless proteins." They touch on block universe: https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time) https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time... …and growing block universe: https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time) https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time... "The growing block universe, or the growing block view, is a theory of time arguing that the past and present both exist, and the future as yet does not. The present is an objective property, to be compared with a moving spotlight." Excellent video worth your time.
    - GenerocUsername 2y ago
      OP here, and no, that's not what I mean. What I mean is that since our brains store memories and generally process information utilizing entropic systems, we perceive time as always moving forward. There is no fundamental physical direction to time... But since our experience slides along an axis of increasing entropy, we perceive the whole universe sliding on that axis as well.
  - MrMcCall 2y ago
    > Time itself does not exist, we merely experience it because it is how our consciousness experiences reality That's backwards: Time exists to allow our consciousness to experience change over time. This is because time exists to allow change over time to the physical universe. That said, there is only the ever-present Now in a constantly evolving universe.
    - GenerocUsername 2y ago
      Doubtful, but also perhaps a matter of perspective. Time does not exist so we experience it, our experience of existence is a function of our brains integrating and recalling state via entropy. Therefore, our moment of consciousness is sliding along in the direction of entropy, and our entanglements with the universe around us are likewise evolving with entropy
Trasmatta 2y ago
Some of what you perceive as "getting hung up" may must be physicists trying to explain the oft misunderstood concept of entropy to people who have misconceptions about it. That being said, there are still plenty of unanswered questions around time in physics. Even just open questions about block vs growing block universe are unanswered, and tied into the ultimate nature of time. Also, we know that entropy was much lower in the past, but we don't know WHY.
mitthrowaway2 2y ago
Liouville's theorem implies that for a causal system, the volume of phase space is constant; therefore the total configuration space of the system does not grow over time. However it can become harder and harder to precisely describe.
Nevermark 2y ago
> Lots of clever people seem to agonise about this, but I don't see any problem. What am I missing? I feel the same way. The past is just the direction where things were more ordered (to be glib about it.) Even though the large scale implications of the statistics are profound, star fusion, rivers, etc. the “cause” is statistics. The one place (that I am aware of) where the statistics -> time effect still needs explicit work is how the timal fabric dimension operates differently (but very similarly) to the 3 spacial fabric dimensions of space-time in the very large scale. Often represented by a negative (for time) and three positive (for space) signs in equations. But I assume when we understand how space in the small works, the topology will still have a statistical explanation. — A very close topic, and the same forehead slap feeling for me, is all the contortions many physicists go through to avoid the many worlds quantum interpretation. Which is the parsimonious interpretation. (Should more accurately be called the “quantum field web” interpretation, according to … me.) Again statistics: we just think there is a “collapse” in possibilities when super-positioned information becomes to mixed up for us to remix. The alternative: a mysterious collapse with yet unexplained cause, with deep problems regarding how this introduces another unique “time” effect, of nearly infinite new information constantly being injected into every interaction everywhere and everywhen in the universe! And this information is supposed to be causeless or self-caused in some mystical (but don’t ever say “mystical”), way. And don’t point out the mockery this makes of “conservation of information” if 99.99…99% of information now wasn’t there just one quantum event ago, for all quantum events. Again, statistics completely explains why us biggies experience fields making choices, based on the field equation defining field connection (we experience as event) point distributions, and … statistics. Yet every day physicists who can’t admit to themselves that we are ourselves constantly expanding webs of superpositions, take for granted that anything they can scale up in the lab will have superpositions, subject to experimental practicalities. Because anything else would violate the field equations.
- Trasmatta 2y ago
  > I feel the same way. The past is just the direction where things were more ordered (to be glib about it.) The only problem is that this ignores the fact that we still don't know WHY entropy was lower in the past. It's an open question in physics.
Etheryte 2y ago
If that's true, then where did all the order that is there come from? Lots of open questions in that regard, plenty to agonize over.
Buttons840 2y ago
Is entropy real, or is it a purely intellectual tool of thought?
- MrMcCall 2y ago
  With the CMBR being so low-temp (4K IIRC), combined with the vast distances of empty space (I know it's not really empty, but practically so), the tendency over time is that temperature will settle down to lower and lower energy states, as energy radiates outward. (Because no energy is being added to the system.) Order requires energy -- i.e. our highly thermodynamic body processes -- so as the temp lowers, disorder will tend to decrease. Maybe a physicist can correct me, but that seems like a simple yet accurate system-wide summation. Of course, local areas (such as Earth) can retain enough heat for a span of time to maintain complexly organized systems, but in the universe's time scale, it's going to keep winding down towards that CMBR, where ever little complexity will be possible.
  - zmgsabst 2y ago
    But steam has higher entropy than water has higher entropy than ice. I’m not sure I follow your description: as the universe empties, aren’t we at some point left with less and less happening per cubic meter?
    - MrMcCall 2y ago
      You're right. I inverted entropy's direction as temps decrease, forgetting that its positively correlated. Thanks for correcting me.
lisper 2y ago
> What am I missing? Your theory: > If you have a casual system, then statistically, things will tend to become more disordered over time, because there are just more ways to be disordered than ordered. contains a tacit assumption that states are chosen at random. But assuming that is begging the question. Why are states chosen at random? What is the source of the randomness? Newtonian mechanics doesn't have any obvious source of randomness, and it's an open question whether quantum randomness is "really random". Bohmian mechanics is completely deterministic, and so is (obviously) superdeterminism. It's true that "there are more ways to be disordered than ordered", but in any time-reversible dynamic there are exactly as many states where entropy decreases as there are states where entropy increases because for any entropy-increasing state, the time-reversed state has decreasing entropy.
- MattPalmer1086 2y ago
  Even a completely deterministic causal system will tend towards greater disorder (if it wasn't already maximally disordered I guess). There is no need for randomness, just statistics. > but in any time-reversible dynamic Now that is making an assumption that the dynamics of an entire system could be thrown into reverse. Everything flipped to its opposite, and then proceeding onwards causally from that point. It also presumes that the system started in a state of order, got more disordered, and then you reversed it and then order "magically" appears. Your eggs unscramble themselves. If you merely take a disordered system and throw it into reverse, you will still just see a disordered system becoming more disordered, because there was never any surprising ordered state built in to be uncovered later.
  - simiones 2y ago
    > If you merely take a disordered system and throw it into reverse, you will still just see a disordered system becoming more disordered, because there was never any surprising ordered state built in to be uncovered later. This is circular. If the system is becoming more disordered, it follows that playing it in reverse will make it less disordered. Sure, if the initial state wasn't very ordered to begin with, this won't look very different, but that's entirely irrelevant: as long as we accept that "disorder" is a measurable objective property of a system, then there is a quantifiable difference between moving forward and backward in time. And this doesn't match either a deterministic time-reversal symmetric theory like classical mechanics, nor a deterministic CPT-reversal symmetric theory like QFT.
cma 2y ago
ergodicity and Boltzmann brain parodoxes
layer8 2y ago
The question is why things become more disordered forward in time but not backwards in time, given that the physical laws have time-reversal symmetry. In other words, why does the macroscopic world seem to behave differently forward in time than backwards in time (e.g. biological life, or the evolution of stars), while the fundamental equations of physics don’t.
- MrMcCall 2y ago
  Well, we are a part of a physical system whose laws correspond to mathematical formulas, but aren't formulas, they're existing physical, energetic relationships maintained magically in realtime. And, yeah, it's magic, this universe we live in, my friend. So, while the math is reversable, time is a one-way street by virtue of the construction of the physical universe, which is not reversable. And it's that way because that's how this magnificent universe was instantiated. It may sound trite, but sometimes Occam is bang-on. As well, there are endless unknowns, but there are also unknowables. All matter is >99% empty, not counting field interactions. I don't know how much of it can be understood, but some of it is likely to be unknowable.
- s_m_t 2y ago
  Let's do a thought experiment. Floating in an otherwise completely empty region of space is a bomb. It explodes. For quite some time after the explosion entropy will be decreasing in that region. I really don't see the mystery here.
  - simiones 2y ago
    The point is this: say we see a video of an otherwise completely empty region of space, with two masses orbiting each other for a while, that then fly off in different directions. We reverse the video and see two masses coming towards each other until they get into an orbit. Can we tell which of the two videos was the original and which was the reversed one? The answer is that we can't. However, say we receive a video of a billion billion such masses all starting in a single point, staying more or less still for a few seconds, and then moving out at high speed away from each other. It is obvious that this video is almost certainly playing forward in time, since the reverse, a billion billion balls all coming together to form a single object, is very very unlikely.
tasty_freeze 2y ago
I have a different take on this, only half joking. What if time is a free variable - it can move forward and backwards freely, perhaps like a drunkards walk? To a conscious being, any memory of having been at time t will be erased when t decreases. When t increases again, but you'll have no memory by which to detect the revisitation. I know that I just postulated that there is a time sequence by which the motion of time can be described. But that is only to make the idea graspable to our ordinary sense of time. One interpretation of what I described is the slab model of the universe where time is just another axis and the evolution along that line is fixed. Another interpretation would be that as time moves forward and backwards the state of the universe is free to change. In either case, our perception of our personal timelines would always appear to be unidirectional.
- ClumsyPilot 2y ago
  > Another interpretation would be that as time moves forward and backwards the state of the universe is free to change What would be the measurable effect of this - would casualty actually be broken? Would we get Dejavu?
- n2e 2y ago
  Anecdote, but this is how my observer experiences time.
  - anodyne_one 2y ago
    What do you mean by this?
- UltraSane 2y ago
  The slab model of the universe is not compatible with quantum randomness.
  - mitthrowaway2 2y ago
    Isn't it?
  - 2y ago
rtpg 2y ago
> things will tend to become more disordered over time, because there are just more ways to be disordered than ordered. Isn't this a bit underspecified? Like a marble in a bowl has plenty of positions it can be in, but it tends to end up in the center due to the whole physical system doing its thing to roll a marble to the center. Or is there some deeper argument here?
- Crye 2y ago
  You just added gravity, which is another form of entropy.
  - deleted 2y ago
    [deleted]
jethkl 2y ago
Sharing a simple thought experiment that was shared with me years ago that explains (to me at least) why this is an interesting question. Imagine a billiard ball with nonzero velocity bouncing around an enclosed box. When the ball encounters a side of the box, it bounces off elastically. A replay of this ball's path over time is equally plausible if the replay were run forward or reversed. The preceding is also true if one imagines 2 or 3 balls, with the only difference being that the balls may also bounce off each other elastically. Even in this scenario, reversibility of playback holds no matter the configuration of the balls: they could all start clustered or be scattered and the replay would be plausible when played in either time direction. But this is no longer true when the box (now much larger) contains millions of billiard balls. If the balls start clustered together, they will scatter over time about the box and the replay of their paths has only one plausible time direction. This is because it is extremely unlikely that all the billiards will, simply by chance at some point in the future, collect together so they are contained within a very small volume. To summarize, in the "few scenario" we can plausibly reverse time but in the "many scenario", we cannot. The only difference between scenarios is the number of balls in the box, which suggests that time is an emergent property. layer8's answer elsewhere in this thread says the same, but more succinctly.
- aurareturn 2y ago
  But this is no longer true when the box (now much larger) contains millions of billiard balls. If the balls start clustered together, they will scatter over time about the box and the replay of their paths has only one plausible time direction. I don't understand how increasing the number of balls means you can't reverse the playback.
  - jethkl 2y ago
    you can reverse the playback, all the physics of billiards bouncing around works equally well in either time direction. > If the balls start clustered together, they will scatter over time about the box and the replay of their paths has only one plausible time direction. It is extremely unlikely that all the billiards will, simply by chance at some point in the future, collect together so they are contained within a very small volume. this shows that there is asymmetry in which direction time flows.
cjfd 2y ago
Seems accurate enough.
reedf1 2y ago
The reason that physicists get excited about entropy is that it is an extremely useful tool. "things will tend to become more disordered over time, because there are just more ways to be disordered than ordered." You can derive all of classical thermodynamics from this one observation. At a high level, physics is just symmetries and laws, and efforts to poke holes in them. We are "hung up" because they form the base of the core lines of inquiry in the field, not because we don't understand them.
GuB-42 2y ago
> things will tend to become more disordered over time Only if they weren't disordered to begin with. If you start with well stirred coffee and milk and stir it again, it won't be more "disordered" than what you started with. The problem is that our universe started as "ordered" for reasons we don't understand. We call it the big bang, and that's the origin of the arrow of time. No big bang means maximum entropy, no arrow of time, and no conscious observer. And if there is, that would be a Boltzmann brain scenario, and no one wants a Boltzmann brain scenario. The question of the arrow of time is literally the last question of life, the universe, and everything.

DiogenesKynikos 2y ago

That the laws of physics are symmetric under time reversal[0], so any statement you make about how things should evolve "over time" also holds if you go backwards in time. The apparent arrow of time is commonly thought to have to do with initial conditions. Entropy was low right after the Big Bang, creating an asymmetry between the two time directions. 0. Technically, the laws of physics are symmetric under stimultaneous inversion of time, flipping of all charges, and a mirror reflection of space.

JohnKemeny 2y ago

What is time?

GenerocUsername 2y ago
Our brains operate on the principle entropy, therefore we experience time as the macro evolution of the universe along the dimension of increasing entropy. Time itself does not exist, we merely experience it because it is how our consciousness experiences reality
- geysersam 2y ago
  Do you mean to say that our experience of time depends on what we see happening around us? If I place you in a room with nothing but a video tape playing backwards, would you experience time going backwards? That doesn't sound plausible.
  - garciasn 2y ago
    On a universal scale, time is a distinctly human construct and it clearly warps based on location, distance from Earth, and is heavily influenced by our biology and location in the solar system (days, years, etc). It just doesn’t matter on a universal level.
    - nntwozz 2y ago
      This is not consensus. Kurzgesagt – In a Nutshell has a very good video on it: Did The Future Already Happen? - The Paradox of Time https://youtu.be/wwSzpaTHyS8 https://youtu.be/wwSzpaTHyS8 "To be honest, no one knows. What we’ve learned are two possibilities to describe time, but they're not the only ones. Some scientists think that the idea of “now” only makes sense near you, but not in the universe as a whole. Others think that time itself doesn’t even exist – that the whole concept is an illusion of our human mind. And others think that time does exist, but that it's not a fundamental feature of the universe – rather, time may be something that emerges from a deeper level of reality, just like heat emerges from the motion of individual molecules or life emerges from the interactions of lifeless proteins." They touch on block universe: https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time) https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time... …and growing block universe: https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time) https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time... "The growing block universe, or the growing block view, is a theory of time arguing that the past and present both exist, and the future as yet does not. The present is an objective property, to be compared with a moving spotlight." Excellent video worth your time.
  - GenerocUsername 2y ago
    OP here, and no, that's not what I mean. What I mean is that since our brains store memories and generally process information utilizing entropic systems, we perceive time as always moving forward. There is no fundamental physical direction to time... But since our experience slides along an axis of increasing entropy, we perceive the whole universe sliding on that axis as well.
- MrMcCall 2y ago
  > Time itself does not exist, we merely experience it because it is how our consciousness experiences reality That's backwards: Time exists to allow our consciousness to experience change over time. This is because time exists to allow change over time to the physical universe. That said, there is only the ever-present Now in a constantly evolving universe.
  - GenerocUsername 2y ago
    Doubtful, but also perhaps a matter of perspective. Time does not exist so we experience it, our experience of existence is a function of our brains integrating and recalling state via entropy. Therefore, our moment of consciousness is sliding along in the direction of entropy, and our entanglements with the universe around us are likewise evolving with entropy

Trasmatta 2y ago

Some of what you perceive as "getting hung up" may must be physicists trying to explain the oft misunderstood concept of entropy to people who have misconceptions about it. That being said, there are still plenty of unanswered questions around time in physics. Even just open questions about block vs growing block universe are unanswered, and tied into the ultimate nature of time. Also, we know that entropy was much lower in the past, but we don't know WHY.

mitthrowaway2 2y ago

Liouville's theorem implies that for a causal system, the volume of phase space is constant; therefore the total configuration space of the system does not grow over time. However it can become harder and harder to precisely describe.

Nevermark 2y ago

> Lots of clever people seem to agonise about this, but I don't see any problem. What am I missing? I feel the same way. The past is just the direction where things were more ordered (to be glib about it.) Even though the large scale implications of the statistics are profound, star fusion, rivers, etc. the “cause” is statistics. The one place (that I am aware of) where the statistics -> time effect still needs explicit work is how the timal fabric dimension operates differently (but very similarly) to the 3 spacial fabric dimensions of space-time in the very large scale. Often represented by a negative (for time) and three positive (for space) signs in equations. But I assume when we understand how space in the small works, the topology will still have a statistical explanation. — A very close topic, and the same forehead slap feeling for me, is all the contortions many physicists go through to avoid the many worlds quantum interpretation. Which is the parsimonious interpretation. (Should more accurately be called the “quantum field web” interpretation, according to … me.) Again statistics: we just think there is a “collapse” in possibilities when super-positioned information becomes to mixed up for us to remix. The alternative: a mysterious collapse with yet unexplained cause, with deep problems regarding how this introduces another unique “time” effect, of nearly infinite new information constantly being injected into every interaction everywhere and everywhen in the universe! And this information is supposed to be causeless or self-caused in some mystical (but don’t ever say “mystical”), way. And don’t point out the mockery this makes of “conservation of information” if 99.99…99% of information now wasn’t there just one quantum event ago, for all quantum events. Again, statistics completely explains why us biggies experience fields making choices, based on the field equation defining field connection (we experience as event) point distributions, and … statistics. Yet every day physicists who can’t admit to themselves that we are ourselves constantly expanding webs of superpositions, take for granted that anything they can scale up in the lab will have superpositions, subject to experimental practicalities. Because anything else would violate the field equations.

Trasmatta 2y ago
> I feel the same way. The past is just the direction where things were more ordered (to be glib about it.) The only problem is that this ignores the fact that we still don't know WHY entropy was lower in the past. It's an open question in physics.

Etheryte 2y ago

If that's true, then where did all the order that is there come from? Lots of open questions in that regard, plenty to agonize over.

Buttons840 2y ago

Is entropy real, or is it a purely intellectual tool of thought?

MrMcCall 2y ago
With the CMBR being so low-temp (4K IIRC), combined with the vast distances of empty space (I know it's not really empty, but practically so), the tendency over time is that temperature will settle down to lower and lower energy states, as energy radiates outward. (Because no energy is being added to the system.) Order requires energy -- i.e. our highly thermodynamic body processes -- so as the temp lowers, disorder will tend to decrease. Maybe a physicist can correct me, but that seems like a simple yet accurate system-wide summation. Of course, local areas (such as Earth) can retain enough heat for a span of time to maintain complexly organized systems, but in the universe's time scale, it's going to keep winding down towards that CMBR, where ever little complexity will be possible.
- zmgsabst 2y ago
  But steam has higher entropy than water has higher entropy than ice. I’m not sure I follow your description: as the universe empties, aren’t we at some point left with less and less happening per cubic meter?
  - MrMcCall 2y ago
    You're right. I inverted entropy's direction as temps decrease, forgetting that its positively correlated. Thanks for correcting me.

lisper 2y ago

> What am I missing? Your theory: > If you have a casual system, then statistically, things will tend to become more disordered over time, because there are just more ways to be disordered than ordered. contains a tacit assumption that states are chosen at random. But assuming that is begging the question. Why are states chosen at random? What is the source of the randomness? Newtonian mechanics doesn't have any obvious source of randomness, and it's an open question whether quantum randomness is "really random". Bohmian mechanics is completely deterministic, and so is (obviously) superdeterminism. It's true that "there are more ways to be disordered than ordered", but in any time-reversible dynamic there are exactly as many states where entropy decreases as there are states where entropy increases because for any entropy-increasing state, the time-reversed state has decreasing entropy.

MattPalmer1086 2y ago
Even a completely deterministic causal system will tend towards greater disorder (if it wasn't already maximally disordered I guess). There is no need for randomness, just statistics. > but in any time-reversible dynamic Now that is making an assumption that the dynamics of an entire system could be thrown into reverse. Everything flipped to its opposite, and then proceeding onwards causally from that point. It also presumes that the system started in a state of order, got more disordered, and then you reversed it and then order "magically" appears. Your eggs unscramble themselves. If you merely take a disordered system and throw it into reverse, you will still just see a disordered system becoming more disordered, because there was never any surprising ordered state built in to be uncovered later.
- simiones 2y ago
  > If you merely take a disordered system and throw it into reverse, you will still just see a disordered system becoming more disordered, because there was never any surprising ordered state built in to be uncovered later. This is circular. If the system is becoming more disordered, it follows that playing it in reverse will make it less disordered. Sure, if the initial state wasn't very ordered to begin with, this won't look very different, but that's entirely irrelevant: as long as we accept that "disorder" is a measurable objective property of a system, then there is a quantifiable difference between moving forward and backward in time. And this doesn't match either a deterministic time-reversal symmetric theory like classical mechanics, nor a deterministic CPT-reversal symmetric theory like QFT.

cma 2y ago

ergodicity and Boltzmann brain parodoxes

layer8 2y ago

The question is why things become more disordered forward in time but not backwards in time, given that the physical laws have time-reversal symmetry. In other words, why does the macroscopic world seem to behave differently forward in time than backwards in time (e.g. biological life, or the evolution of stars), while the fundamental equations of physics don’t.

MrMcCall 2y ago
Well, we are a part of a physical system whose laws correspond to mathematical formulas, but aren't formulas, they're existing physical, energetic relationships maintained magically in realtime. And, yeah, it's magic, this universe we live in, my friend. So, while the math is reversable, time is a one-way street by virtue of the construction of the physical universe, which is not reversable. And it's that way because that's how this magnificent universe was instantiated. It may sound trite, but sometimes Occam is bang-on. As well, there are endless unknowns, but there are also unknowables. All matter is >99% empty, not counting field interactions. I don't know how much of it can be understood, but some of it is likely to be unknowable.
s_m_t 2y ago
Let's do a thought experiment. Floating in an otherwise completely empty region of space is a bomb. It explodes. For quite some time after the explosion entropy will be decreasing in that region. I really don't see the mystery here.
- simiones 2y ago
  The point is this: say we see a video of an otherwise completely empty region of space, with two masses orbiting each other for a while, that then fly off in different directions. We reverse the video and see two masses coming towards each other until they get into an orbit. Can we tell which of the two videos was the original and which was the reversed one? The answer is that we can't. However, say we receive a video of a billion billion such masses all starting in a single point, staying more or less still for a few seconds, and then moving out at high speed away from each other. It is obvious that this video is almost certainly playing forward in time, since the reverse, a billion billion balls all coming together to form a single object, is very very unlikely.

tasty_freeze 2y ago

I have a different take on this, only half joking. What if time is a free variable - it can move forward and backwards freely, perhaps like a drunkards walk? To a conscious being, any memory of having been at time t will be erased when t decreases. When t increases again, but you'll have no memory by which to detect the revisitation. I know that I just postulated that there is a time sequence by which the motion of time can be described. But that is only to make the idea graspable to our ordinary sense of time. One interpretation of what I described is the slab model of the universe where time is just another axis and the evolution along that line is fixed. Another interpretation would be that as time moves forward and backwards the state of the universe is free to change. In either case, our perception of our personal timelines would always appear to be unidirectional.

ClumsyPilot 2y ago
> Another interpretation would be that as time moves forward and backwards the state of the universe is free to change What would be the measurable effect of this - would casualty actually be broken? Would we get Dejavu?
n2e 2y ago
Anecdote, but this is how my observer experiences time.
- anodyne_one 2y ago
  What do you mean by this?
UltraSane 2y ago
The slab model of the universe is not compatible with quantum randomness.
- mitthrowaway2 2y ago
  Isn't it?
- 2y ago

rtpg 2y ago

> things will tend to become more disordered over time, because there are just more ways to be disordered than ordered. Isn't this a bit underspecified? Like a marble in a bowl has plenty of positions it can be in, but it tends to end up in the center due to the whole physical system doing its thing to roll a marble to the center. Or is there some deeper argument here?

Crye 2y ago
You just added gravity, which is another form of entropy.
- deleted 2y ago
  [deleted]

jethkl 2y ago

Sharing a simple thought experiment that was shared with me years ago that explains (to me at least) why this is an interesting question. Imagine a billiard ball with nonzero velocity bouncing around an enclosed box. When the ball encounters a side of the box, it bounces off elastically. A replay of this ball's path over time is equally plausible if the replay were run forward or reversed. The preceding is also true if one imagines 2 or 3 balls, with the only difference being that the balls may also bounce off each other elastically. Even in this scenario, reversibility of playback holds no matter the configuration of the balls: they could all start clustered or be scattered and the replay would be plausible when played in either time direction. But this is no longer true when the box (now much larger) contains millions of billiard balls. If the balls start clustered together, they will scatter over time about the box and the replay of their paths has only one plausible time direction. This is because it is extremely unlikely that all the billiards will, simply by chance at some point in the future, collect together so they are contained within a very small volume. To summarize, in the "few scenario" we can plausibly reverse time but in the "many scenario", we cannot. The only difference between scenarios is the number of balls in the box, which suggests that time is an emergent property. layer8's answer elsewhere in this thread says the same, but more succinctly.

aurareturn 2y ago
But this is no longer true when the box (now much larger) contains millions of billiard balls. If the balls start clustered together, they will scatter over time about the box and the replay of their paths has only one plausible time direction. I don't understand how increasing the number of balls means you can't reverse the playback.
- jethkl 2y ago
  you can reverse the playback, all the physics of billiards bouncing around works equally well in either time direction. > If the balls start clustered together, they will scatter over time about the box and the replay of their paths has only one plausible time direction. It is extremely unlikely that all the billiards will, simply by chance at some point in the future, collect together so they are contained within a very small volume. this shows that there is asymmetry in which direction time flows.

cjfd 2y ago

Seems accurate enough.

reedf1 2y ago

The reason that physicists get excited about entropy is that it is an extremely useful tool. "things will tend to become more disordered over time, because there are just more ways to be disordered than ordered." You can derive all of classical thermodynamics from this one observation. At a high level, physics is just symmetries and laws, and efforts to poke holes in them. We are "hung up" because they form the base of the core lines of inquiry in the field, not because we don't understand them.

GuB-42 2y ago

> things will tend to become more disordered over time Only if they weren't disordered to begin with. If you start with well stirred coffee and milk and stir it again, it won't be more "disordered" than what you started with. The problem is that our universe started as "ordered" for reasons we don't understand. We call it the big bang, and that's the origin of the arrow of time. No big bang means maximum entropy, no arrow of time, and no conscious observer. And if there is, that would be a Boltzmann brain scenario, and no one wants a Boltzmann brain scenario. The question of the arrow of time is literally the last question of life, the universe, and everything.