What causes the arrow of time ?

[Sherlock]

Could it be increasing of Entropy that points the arrow of time? We don't see time running backward with all of the smoke heat, light, and ash coming back together to produce a fresh log in the fireplace, or chicks going back into eggs that are taken back into a hen's oviduct. All of these "natural processes" never seem to run backwards.
Wouldn't an ocean flow uphill through rivers to enter springs and go back into the ground, while raindrops emerge from the soil and go back into the sky to make clouds?
Ron

That the entropy associated with a closed system doesn't decrease, but will increase or remain constant, is a statement of what the arrow of time is. The standard, probabilistic model (via volumes in phase space) of this isn't an explanation or an identification of the fundamental physical cause of the arrow of time. It's just a way of mathematically describing, acausally, an apparently general feature of the observed evolutions of natural processes that we call the arrow of time.
How to generally define this (in terms of volumes in phase space, micro-configurations, macro-configurations, number of quantum states, etc.) is difficult enough -- but methods have been devised to deal with it which are satisfactory for many calculational problems.
However, none of these methods address the really interesting question, which, imo, is what is the fundamental physical cause of this feature of reality that we call the arrow of time.

In order to answer this question, some speculation regarding the fundamental physical
process(es), or fact(s), or force(s), or however it might be termed, is required. Is it gravity? Is it isotropic expansion? Whatever it is, it is, apparently, affecting everything in the universe, from the smallest to the largest length and energy scales.

 

[Sherlock]

To my opinion, the arrow of time is understood by physics since a very long time.

It's only understood that there is an arrow of time, and physics has developed some general mathematical descriptions of what the arrow of time is. But the descriptions are acausal or noncausal. They're probabilistic, and hence do not address the question posed in this thread. What causes of the arrow of time?
I don't think we can say that the arrow of time is understood -- unless we're satisfied with saying that, well, that's just the way things are.

 

[Sherlock]

Hello:
It is easy to imagine why spacetime has a handedness, the space part of spacetime is like that. Local transformations trump global ones.

I don't understand how this explains it, causally. In what sense do local transformations trump global ones? I've been thinking about it the other way around.

By the way, I visited your web page. It looks very interesting, and I hope to find time to read all of it.

 

[lalbatros]

It's only understood that there is an arrow of time, and physics has developed some general mathematical descriptions of what the arrow of time is. But the descriptions are acausal or noncausal. They're probabilistic, and hence do not address the question posed in this thread. What causes of the arrow of time?
I don't think we can say that the arrow of time is understood -- unless we're satisfied with saying that, well, that's just the way things are.

Sherlock, it is nice to revive this forum on this century-old topic!
But I don't see your point.
Why should the probalistic explanation be a poor explanation when it is really the explanation?
Why are you unsatisfied with the Poincaré-recurrence-time point of view?

I have little morivation to program a particle-in-a-box simulation during Xmas time. Low motivation because the result is well know and nearly trivial: with a moderate number of particles and generic initial conditions there is no hope to see a significant deviation from the second law. I could only enjoy some stats about these deviations and their statistical distribution (exp(S)). In addition, real physics gives the same insight without the programming effort: for example thermal noise is a common deviation from the second law with well-known statistics and irronically reversible.

But I agree with you that, indeed, things are the way they are.
Indeed, the world around us involves huge amounts of particles, and indeed this suffices to explain the second law.

Nevertheless, I am not necessarily satisfied by an answer to the question of this forum. Because it does not give us tools to understand thermodynamics in a new way without postulates (and trying without would make things much more complicated but unchanged in the end). Also because the systems studied in thermodynamics are much more complicated than a few thermodynamic potentials and, finally, thermodynamics tells us so little about time evolution.

 

[Sherlock]

Sherlock, it is nice to revive this forum on this century-old topic!
But I don't see your point.
Why should the probalistic explanation be a poor explanation when it is really the explanation?

Probabilistic models aren't explanations. An answer to the question posed in the title of this thread would begin with postulating some fundamental physical law that is not now a part of standard physics. I would begin with the isotropic expansion of the universe and work from there. A century ago the, apparent, fact of the expansion wasn't known.

Why are you unsatisfied with the Poincaré-recurrence-time point of view?

I'm not satisfied with the Poincare-recurrence-time point of view because I think it's wrong. It's wrong because any model that has phase space volumes recurring fails to take into account the fundamental physical fact of our universe -- it's expanding.
Any micro-configuration of an expanding universe is unique. As long as the universe continues to expand, then universal micro-configurations cannot repeat. There are good reasons to believe that the evolution of the universe is dominated by the energy of the expansion. There are, afaik, no good reasons to believe that the universe will ever reverse its expansionary trend and begin contracting. In any case, in an expanding universe, smaller phase space volumes will never be revisited.
We see similar kinematic patterns wrt all scales of physical phenomena. What is generally true wrt the largest scales should also be generally true for the smallest scales, because nothing in the universe is isolated from the energy of the expansion (indeed, everything is driven by it), and nothing is isolated from the general expansionary trend of the universe.

I have little motivation to program a particle-in-a-box simulation during Xmas time. Low motivation because the result is well known and nearly trivial: with a moderate number of particles and generic initial conditions there is no hope to see a significant deviation from the second law. I could only enjoy some stats about these deviations and their statistical distribution (exp(S)). In addition, real physics gives the same insight without the programming effort: for example thermal noise is a common deviation from the second law with well-known statistics and irronically reversible.

You don't actually have to program the simulation to have a good idea of what it will produce. If you program it so that configurations can recur, then they will. If you program it so that configurations can't recur, then they won't.

Indeed, the world around us involves huge amounts of particles, and indeed this suffices to explain the second law.

I don't think the number of particles has anything to do with it. Micro-processes should be as irreversible as macro-processes. The fundamental equations of motion are reversible in the sense that they can be applied to any direction in space. The isotropic expansion involves every direction in space. The evolution of wavefronts is always retarded, never advanced, on any scale. Anyway, the particle model will not suffice to actually understand the way things work. The fact that it is what is used to do calculations is more a statement of the limits of our capabilities rather than a statement of what the universe (and anything in it) actually is and the way it is actually evolving.

Nevertheless, I am not necessarily satisfied by an answer to the question of this forum. Because it does not give us tools to understand thermodynamics in a new way without postulates (and trying without would make things much more complicated but unchanged in the end). Also because the systems studied in thermodynamics are much more complicated than a few thermodynamic potentials and, finally, thermodynamics tells us so little about time evolution.

I think I agree with this, so let's have some new postulates. (But not until we're well done with the holidays. )

 

[Buckeye]

Time is a human concept. From the universe's point of view, there is no time, no arrow, no universal entropy. The so-called Big Bang, which, from the human perspective, appears to be the start of the arrow, is a counter balanced oscillating phenomenon which from our earthly point within all relativity (universe) appears to be, based on our local point in the universe, an expansion, a progression of time. Entropy and its' opposite (anti-entropy) exist side by side within the whole of the universe. Our section of the chaotic universe, which is currently undergoing Entropy, perceives time as an increase in the distance between particles.

If you want to think of the Big Bang as a "potential" start of time point, we need to step back and look at the universe from its' point of view, ie from a larger perspective. In other words, the Big Bang, which "occurred" in our neck of the universe, was/is simply a minor eddy in the balanced chaos of the universe, which from our human point of view is currently being thought of as a set of multi-verses, multi-times or multiple m-branes, instead of the sum total, ie the universe.

The question that displaces the question of the arrow of time is: How did the universe come to be? If you dare to ask the question within a religious context, then: Did one God or a group of Gods create the universe? If so, then who/what created that God or those Gods? I vote for "The No-Beginning No-End Theory" wherein the universe is ageless and with no beginning and no end. It simply exists.

 

[lalbatros]

Sherlock,

I don't understand why the expansion of the universe would be necessary to explain the irrevesible diffusion of milk in my cup of cofee.

I agree with you that we have no other choice than to live in the universe as it is, maybe expanding, and that therefore we might be experiencing one -and only one- of the two possible time evolutions of the universe. (But note that the expansion hypothesis is in big trouble today)

To be clear, we should agree on which "arrow of time" we are trying to understand. The most common meaning is regarding everyday irreversibility, like milk in the cofee or heat flowing to the cold side. Our lives too follow the same time direction as the warm to cold heat diffusion.

There is sure still more to discover in physics than what is understood. But there is little doubt that the "arrow of time" in its usual meaning is just as simple as a particles-in-a-box argument. This is the basics of statistical physics and this lead to many verifications including the fluctuations theory.

The fluctuation theory is remarquable in this context because it precisely deals with deviations from the second law: a small entropy decrease is not impossible, but its probability decreases fast enough so that macroscopic entropy decreases are practically impossible. Still, the microscopic level is easily observed sometimes: like in the thermal noise. The experimental results on fluctuations (around equilibrium) confirmed the theory.

Finally I cannot agree with your remark:

Probabilistic models aren't explanations

First you should give me the reasons and the criteria to discard probabilities as meaningless. But more important, it happens that the behaviour of large ensembles of particles are best described by probabilistic methods. These methods are our best tools to catch what is important for the understanding and discard what is not.

I thing that before any other facts of physics, maybe the most important one is that our world is made of an extraodinary large number of smaller parts with only limited coherence of behaviour (fundamental laws of physics don't display any 'collective' behaviour that could not be understood by the 'limited' interactions, but entanglement in QM is a questionmark here). The result of that is the second law of physics. Statistics has been devised to handle such situations where the number dictates the law and the details are only second. This is why it is the best tool to explain the second law.

 

[Sherlock]

Sherlock,
I don't understand why the expansion of the universe would be necessary to explain the irrevesible diffusion of milk in my cup of cofee.

Well, there would be at least a few steps from positing the isotropic expansion of the universe as the fundamental physical law of motion to explaining the irreversible diffusion of milk in a cup of coffee. If I understand the statistical approach correctly, then the diffusion isn't, strictly speaking, irreversible. It's just irreversible FAPP (for all practical purposes). That is, if the cup were around long enough, then it would happen that the milk and the coffee would eventually revisit a state where they were unmixed. But, I don't think that such unmixing is possible. I don't think that nature works that way. And, since we have to start somewhere, then why not start at the beginning and posit a new first law of motion. Something like, the propagation of any disturbance in any medium follows the direction of the expansion (which is any and every direction). Ok, that doesn't quite cut it. I don't have any good idea of how to reformulate the first law of motion, but I have no doubt that one is needed. Maybe you can brainstorm it and come up with something.

I agree with you that we have no other choice than to live in the universe as it is, maybe expanding, and that therefore we might be experiencing one -and only one- of the two possible time evolutions of the universe. (But note that the expansion hypothesis is in big trouble today)

I don't think the expansion hypothesis is in any trouble. Where did you read that?

To be clear, we should agree on which "arrow of time" we are trying to understand. The most common meaning is regarding everyday irreversibility, like milk in the cofee or heat flowing to the cold side. Our lives too follow the same time direction as the warm to cold heat diffusion.

If you have a container with two compartments, and you fill one compartment with 20 degree water and the other compartment with 80 degree water, and the container is in a room that is 70 degrees, then you open a door in the partition of the container to allow the different temperature waters to mix, then the 80 degree side will decrease in temperature and the 20 degree side will increase in temperature until, eventually, the temperature of both sides is the same as the 70 degree temperature of the room.

If you drop a very small pebble into a very large, smooth pool of water, then the resulting disturbance propagates omnidirectionally until eventually dissipating and the pool of water is once again smooth.

These are both examples of expansion to equilibrium. Of course the world at large is incomprehensibly more complicated than that, but isotropic expansion to equilibrium is the basis of all motion. If I was going to model the universe as a whole, then I would begin by representing its boundary as an expanding wave front.

There is sure still more to discover in physics than what is understood. But there is little doubt that the "arrow of time" in its usual meaning is just as simple as a particles-in-a-box argument. This is the basics of statistical physics and this lead to many verifications including the fluctuations theory.
The fluctuation theory is remarquable in this context because it precisely deals with deviations from the second law: a small entropy decrease is not impossible, but its probability decreases fast enough so that macroscopic entropy decreases are practically impossible. Still, the microscopic level is easily observed sometimes: like in the thermal noise. The experimental results on fluctuations (around equilibrium) confirmed the theory.

I suppose that I would define the arrow of time as expansion to equilibrium. The only state that can ever recur is the equilibrium state. The arrow of time and irreversibility is most clearly depicted as a single spherical wave front, in some more or less homogenous and isotropic medium, propagating away from its source and eventually dissipating.

Finally I cannot agree with your remark: "probabilistic models aren't explanations".
First you should give me the reasons and the criteria to discard probabilities as meaningless.

They're not totally meaningless. After all, they do help us determine the rates at which certain phenomena will occur -- and, in the absence of direct qualitative apprehension of underlying causes, it's really the only unambiguous way to talk about things. But in certain areas I have the idea that physics can do something a bit better than a strictly probabilistic accounting -- and the arrow of time and irreversibility is one of those areas.

But more important, it happens that the behaviour of large ensembles of particles are best described by probabilistic methods. These methods are our best tools to catch what is important for the understanding and discard what is not.

I can't argue with this.

I think that before any other facts of physics, maybe the most important one is that our world is made of an extraodinary large number of smaller parts with only limited coherence of behaviour (fundamental laws of physics don't display any 'collective' behaviour that could not be understood by the 'limited' interactions, ...

I see it just the other way around. The universe wasn't built from a bunch of small things interacting. It began as one humungous disturbance, and all the smaller stuff is a byproduct of that. There are organizing principles on every scale that seem scale-specific, but there are also similar phenomenological patterns that appear on every scale. Maybe "fundamental laws of physics don't display any 'collective' behavior that could not be understood by 'limited' interactions", but I think that nature does.
The reductionist approach has had its day. And I think we will see a somewhat different trend in physics in the future.

... but entanglement in QM is a questionmark here).

Everything in the universe is entangled wrt the general motion of the universe, which is away from (in every direction) the originating disturbance (which we can call the Big Bang).

The result of that is the second law of physics. Statistics has been devised to handle such situations where the number dictates the law and the details are only second. This is why it is the best tool to explain the second law.

But, dare I say it again , statistical models aren't explanations. We use them instead of explanations when explanations are either impossible or prohibitively difficult.

 

[lalbatros]

Sherlock,

I agree with you that the common origin of all small parts of the universe make little doubts. It is highly probable that this has an influence on the global evolution: the milk in my coffee indeed takes part to the motion of our galaxy. But it is also clear that the diffusion of milk in my cofee is the result of the 'brownian' motion of milk fat molecules.

I would take you starting point to say instead that since the common entangled origin, all parts of the universe have developed greater and greater autonomy and entanglement has decayed due to its fragility. Maybe gravitation is the sole remains from the common birth. I would be more motivated to consider this possibility: could gravitation be a remain of the initial quantum correlations?

Regarding the "recurrence time" point of view: why do you postulate it is wrong? You know that for 1 mole of molecules in a box the recurrence time exceeds by far the age of the universe. How are you going to imagine any experiment to prove your point? In addition, I stress again that fluctuations theory and its experimental verifications do support the recurrence-time point of view. Indeed, on smaller scales the entropy can decrease for short periods of time. This occurs because smaller system do not perfectly follow the second law of thermodynamics. The second law applies only in the statistical limit: for very large numbers of particles. When this condition is not satisfied, the second law can be violated. For small fluctuations around equilibrium it is the case and it is possible again to study the statistical distribution of these fluctuations.

Note also that in the limit of a two-particles system the second law does not apply at all. So, the "precision" of the second law decreases when the size of the system decreases. Any alternative theory would have to explain that too!

Michel

 

[Sherlock]

Sherlock,
I agree with you that the common origin of all small parts of the universe make little doubts. It is highly probable that this has an influence on the global evolution: the milk in my coffee indeed takes part to the motion of our galaxy. But it is also clear that the diffusion of milk in my cofee is the result of the 'brownian' motion of milk fat molecules.
I would take you starting point to say instead that since the common entangled origin, all parts of the universe have developed greater and greater autonomy and entanglement has decayed due to its fragility. Maybe gravitation is the sole remains from the common birth. I would be more motivated to consider this possibility: could gravitation be a remain of the initial quantum correlations?
Regarding the "recurrence time" point of view: why do you postulate it is wrong? You know that for 1 mole of molecules in a box the recurrence time exceeds by far the age of the universe. How are you going to imagine any experiment to prove your point? In addition, I stress again that fluctuations theory and its experimental verifications do support the recurrence-time point of view. Indeed, on smaller scales the entropy can decrease for short periods of time. This occurs because smaller system do not perfectly follow the second law of thermodynamics. The second law applies only in the statistical limit: for very large numbers of particles. When this condition is not satisfied, the second law can be violated. For small fluctuations around equilibrium it is the case and it is possible again to study the statistical distribution of these fluctuations.
Note also that in the limit of a two-particles system the second law does not apply at all. So, the "precision" of the second law decreases when the size of the system decreases. Any alternative theory would have to explain that too!
Michel

Michel, you've asked some questions that I don't have any ready answers for, and made some interesting points. Since I have only the basics of this stuff and no in-depth knowledge, then I don't want to nitpick about what you've written. There's much to study about and lots of time to do it, so maybe I'll return to this topic in the future. For now, I thank you for responding to my speculations.

 

[Juan R.]

Hi,

Next i detail some apparent misconceptions on the arrow of time:

1) The projection postulate / Born rule: Waves collapse only into one direction is not the basis for the arrow of time. In fact there is not dynamical consistent description of the projection inside standard QM, which is based in unitary dynamics (Schrödinger dynamics). In fact, the projection postulate arises from non-unitary dynamics. There are very-well detailed models on quantum lorentz gas and Friedrichs models published on relevant literature. The limits of aplicability of Schrödinger dynamics are also well-detailed.

2) Imperfect entanglement: The conservation laws are not exactly 100% is not the basis for the arrow of time. Any explanation of arrow of time strictly conserves probability and energy. However, when one passes to an approximate description (as very well-known valid in the limit of very big markovian reservoir) from the full conservative dynamics one obtains both apparent violation of conservation of probability and energy. From this apparent violation and via very well-known theorems follows Caratheodory statement, the theory of adiabatic transformations and other interesting stuff: Gamow generalized vectors, Weisskopf/Wigner decay theory, and others. today there is a debate on why E.C.G. Sudarshan received not the Nobel Prize. Well, Sudarshan generalized IRREVERSIBLE quantum mechanics describing Kaon systems follows from nonunitary dynamics in the limit nonunitarity is epsilon when epsilon -> 0 (but is NOT zero as in Abel kernel for the Zubarev equation). Note: Sudarshan was a coworker of my colleague Prigogine who last years was working in Prigogine theory of instable elementary particles.

3) Spontaneous symmetry breaking: We might see Universes where time goes the other way is not the basis for the arrow of time. In fact, universe evolution is clearly nonunitary and we can choose either an initial simmetric or non-simmetric vacuum. If one choose already a non-simmetric vacuum then breaking is unnecesary. If one chooses an initial simmetric vacuum (Brushel School option) then it has been proven on literature that spontaneus symmetry broken at big bang does that our universe follows one of semigroups (the so called Lambda_{+}) generated in the phase transition ('Big Bang') from vacuum. The possibility of other universes with the other semigroup is possible but i) there is not possibility for combined evolution and time asymmetry is always fundamental in the universe ii) each universe follow a different semigroup of the spectral decomposition in the RHS, but the evolution in each semigroup (+) or (-) is always irreversible, simply there is two different arrows. All of this is published in literature.

4) The time dimension itself is simply one way: The future does not yet exist. This precisely is incompatible with deterministic dynamics where future is already defined. Consistent models for time evolution follow only from non-unitary theory. Material is available on literature, including serious criticism to the time dimension concept, for example via Stueckelberg/Horwitz 5D theory.

5) I cannot offer detailed comments on the last option of the poll since is not defined.I have add some comments and references to recent work on non-unitary dynamics on why irreversible physics is superior to reversible one, the appearence of new properties on biological systems and the true nature of time which needs of a basic understanding of hiperreal numbers or similar (see xml page cited in Woit blog for some aditional data and references) in Peter Woit blog about Princeton http://www.math.columbia.edu/~woit/wordpress/?p=309#comment-6492". More data was available but erased by Woit due to the off-topic character of Andy query.I read and re-read again comment https://www.physicsforums.com/showpost.php?p=816792&postcount=25" and after of recent troubles in this thread i obtained my own conclusions.

I abandon this forum.

Marcus, CarlB, and others interesting guys, thanks by your valious comments!