What causes the arrow of time ?

[lalbatros]

Sherlock,

I still try but cannot follow you:

The point is that phase space and Hilbert space representations aren't physical explanations for what we observe. They're just methods of accounting for the quantitative results of experiments.

What is then your criteria for a physical explanation or theory? And according to your criteria, can you give me an example of a physical theory? Would you reject classical mechanics as a physical theory?

In addition, I draw your attention to the fact that 'particles-in-a-box' arguments are precisely not pure algebraic accounting methods, but instead they are purely physical explanations. They precisely lack the quantitative aspect, and this is why many feel there is a vacuum in physics there to be filled with new develoments.

A better starting point for a physical explanation of the arrow of time would be to assume that what is never observed ...

Precisely, this would be a non-physical assumption. I am convinced that experiments on medium-scale systems (say 10 atoms in a box ) would reveal that the second law of thermodynamics may fail sporadically. It may be an experiment quite difficult to built, but you can very easily test this on a computer. Note that the classical theory of fluctuations is precisely dealing with such tiny deviations in large systems.

Finally, I do not exclude additional sources of irreversibility, like the expansion of the universe. But it is difficult to believe that this would play any role when I pour milk in my cofee.

 

[member 11137]

This will be once more the intervention of a "dreamer", only for the purpose to get some more scientific precisions... Time is going a little bit like the water of the river, from the top to the bottom; or a little bit like an elastic that would have been elongate: it must come back to the initial state. The chance to see a drop going in the opposite direction exists but is tiny. Could we see any analogy of this type in the irreversibility of the evolution of time? (I am here certainly rediscovering an illustration of the energy-time HUP and of the entropy). I would like to push the idea further. And what would have been hapen if the geometry of the universe would have been curved at the origin? With other words, cann't we consider that our universe is coming back to a more comfortable and economic geometric state? Or other original idea in the same direction: if as QFT pretends, there are permanent spontaneus fluctuations of the fields; are not these fluctuations in some way the "motor" of the time, due to the fact that each fluctuation must be followed by a relaxation? Sorry if this is a little bit confuse and thank you for giving a little bit order in my thoughts.

 

[Sherlock]

What is then your criteria for a physical explanation or theory? And according to your criteria, can you give me an example of a physical theory? Would you reject classical mechanics as a physical theory?

Ok, poor choice of words on my part. Of course qm and cm are physical theories, more or less.

What would be an approach to explaining the arrow of time that more closely approximates the way the universe actually works? Well, who knows -- but since dynamical, causal theories (where possible) are preferred over probabilistic ones, then I would start with the fundamental physical process and work from there. Of course there is some disagreement wrt what the fundamental physical process is. In my view, the fundamental physical process is the isotropic expansion of the universe.

Micro-processes aren't fundamental. They don't determine larger scale evolutions, but are, rather, carried along with and evolve according to the more fundamental, larger scale motions. Thus, the microstates of earlier universal configurations can't be duplicated or revisited as long as the universe continues to expand. The universe-scale expansion energy doesn't come from inside the boundaries of the universe. The expansion is the fundamental physical fact (the cause of the expansion is an unanswerable question), and the total kinetic energy was imparted before anything else inside the universe could happen. Wrt gravitation, the universe is not fundamentally driven by gravitation. Gravitational behavior emerged after the expansion began.

The conceptual, fundamental 'big picture' is of interacting waves in a hierarchy of media with the fundamental medium being a continuous medium with no particulate structure -- ie., the fundamental medium is undetectable, but it's a necessary metaphysical foundation for understanding the interconnectedness of all phenomena wrt the fundamental motion of the universe as a whole.

Instead of starting with very tiny things and giving them the combinatorial properties they need to account for experimental results, one might start at the most encompassing scale of motion (from which nothing is isolated) and work, via decreasingly encompassing and increasingly interactionally complicated scales, toward atoms and sub-atomic structures which can be effectively isolated from each other (eg., the behavior of atoms in Zurich right now is not affecting the behavior of the atoms in my computer in NYC right now).

Ok, I have very little idea what I'm talking about, but you get the general drift. Is it a silly way to approach things, or what?
Maybe you could say what I'm trying to say (if I've communicated anything of interest) in a better way. This is not a new idea. It comes from my more or less vague recollections of stuff I've read by a few physicists.

The ergodic hypothesis wrt the phase space of microstates might entail correct predictions in laboratory settings, but it's not a model of the way things actually work in nature , imho.

In addition, I draw your attention to the fact that 'particles-in-a-box' arguments are precisely not pure algebraic accounting methods, but instead they are purely physical explanations. They precisely lack the quantitative aspect, and this is why many feel there is a vacuum in physics there to be filled with new develoments.

Maybe I'm not thinking about the 'particles-in-a-box' argument correctly -- but as I understand it, say you have a few billion atoms of some gas in an enclosure inside another enclosure. Then you open smaller enclosure to let the gas escape and it expands to fill the larger enclosure, and you leave the opening to the smaller enclosure open. From what I understand, the ergodic hypothesis, via the phase space model says that it is possible for the gas to revisit the state in which it was entirely confined in the smaller enclosure -- and anything that can possibly happen, will, eventually, happen. Now, even though the model entails that the probability of this happening is so small that it will never be observed (by us anyway), the fact that it is a 'possibility' makes the model unacceptable to me. In the approach that I currently like, we will never see this happen because in an expanding universe it's dynamically impossible.

I am convinced that experiments on medium-scale systems (say 10 atoms in a box ) would reveal that the second law of thermodynamics may fail sporadically.
It may be an experiment quite difficult to built, but you can very easily test this on a computer. Note that the classical theory of fluctuations is precisely dealing with such tiny deviations in large systems.

If it's easily testable on a computer, then has it been done?

Anyway, if you build a computer simulation based on a theory that says a violation of the second law ot thermodynamics is possible but it will take (whatever time the theory says it will take to cycle through the possible microstates) some amount of 'time' only artificially accessible to a super computer, then 'observing' that to happen via the simulation would not necessarily be a confirmation of the physical truth of the model.

Finally, I do not exclude additional sources of irreversibility, like the expansion of the universe. But it is difficult to believe that this would play any role when I pour milk in my coffee.

In my view (which changes a lot) the expansion of the universe isn't an "additional" source of irreversibility. It's *the* source. There can be only one fundamental source. The expansion is the mother of all motion. Because of the expansion, all 'instantaneous' universal configurations that we observe are unique. Configuration number 10^50 is more like configuration number 10^50 - 1 than it is like configuration number 10^12. Configuration number 10^50 + 1 will be very much like configuration 10^50. And all configurations that have already happened can never happen again in an expanding universe.

Ok, it's difficult to connect the universal expansion to the milk in the coffee. Explaining gravitational behavior in terms of expanding waves and wave interactions will be difficult also. Nevertheless, what's wrong with taking the universal expansion as the starting point in attempting to *understand* the arrow of time and natural irreversibility of processes?

This post might well be moved to the la la land category, but I couldn't resist airing my really vague thoughts on this since I regard the discovery of the universal expansion as the single most important physical discovery of all time.

 

[member 11137]

Ok, poor choice of words on my part. Of course qm and cm are physical theories, more or less.
What would be an approach to explaining the arrow of time that more closely approximates the way the universe actually works? Well, who knows -- but since dynamical, causal theories (where possible) are preferred over probabilistic ones, then I would start with the fundamental physical process and work from there. Of course there is some disagreement wrt what the fundamental physical process is. In my view, the fundamental physical process is the isotropic expansion of the universe.
Micro-processes aren't fundamental. They don't determine larger scale evolutions, but are, rather, carried along with and evolve according to the more fundamental, larger scale motions. Thus, the microstates of earlier universal configurations can't be duplicated or revisited as long as the universe continues to expand. The universe-scale expansion energy doesn't come from inside the boundaries of the universe. The expansion is the fundamental physical fact (the cause of the expansion is an unanswerable question).

Don't you think that a set of expanding points (say millions of millions of millions of ...) could not give the same result than a global expansion? How do you explain galaxies present in the early history of our universe?

 

[Sherlock]

Don't you think that a set of expanding points (say millions of millions of millions of ...) could not give the same result than a global expansion? How do you explain galaxies present in the early history of our universe?

I have no idea. I just don't like the probabilistic explanation as a general explanation for the arrow of time and irreversibility. But we're probably stuck with doing it that way, so I'll just have to learn to love it.
There's a paper by Laughlin (iirc) about theories of everything wherein he talks about the limitations (and likely incorrectness?) of the reductionist approach. There are, apparently, organizing principles on many scales which aren't effectively captured by fundamental descriptions of nature which proceed from the sub-micro to the micro to the meso to the macroscopic.

Maybe nature actually works the other way around. Maybe the larger scales are the more fundamental, and the smaller scale stuff is the byproduct.

And, now I pledge to stop speculating about this because it's out of place in the quantum physics forum. Or is it? I don't know. I'm somewhat amazed that the thread hasn't been moved already. I guess it's that some of the proposed alternative explanations in the poll involve qm.

 

[member 11137]

I have no idea. .

Do you know what? I have also no explanation!

I just don't like the probabilistic explanation as a general explanation for the arrow of time and irreversibility. But we're probably stuck with doing it that way, so I'll just have to learn to love it.

I suspect that a wall results in the addition of stones, not conversely ... and exactly so the nation results from an addition of people living together (or at least trying to do it ...); when they don't, you get the intervention of a biggest order (the state) but I am not sure that such political, philosophical comparison can be extend to the quantum physics, you are right!

 

[RonLevy]

Arrow of Time poll-entropy aims the bow

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

 

[KKHausman]

Arrow of Time

One solution might be that the "arrow of time" results from a temporal form of momentum. If the singularity-emergence postulated by a Big Bang type event expanded not only in 3 dimensions, but also along the temporal event line, then half of the initial energy would have had a momentum forward and half back. This would allow a neat explanation as to the dominance of normal matter and the relative scarcity of antimatter, and also support the theories of early inflationary expansion - the relative temporal proximity would have allowed energy and matter to interact more than now. This would also account for the seeming "slowing down" of the expansion.

Just a few thoughts.

K Hausman

 

[Apollothe]

entropy
if there is no other proper explanation

 

[sameandnot]

time is the mind. there is no time without mind. do not just jump to defensives... consider how the idea that there is change occurring depends on the minds perception that there was a past, in time, and therefore a continuity of change, extending into the future. where is the past and where is the future? as soon as there is a thought about what is percieved, that thought, which is the expression of mind on sense awareness, becomes a part of the minds' past. what is, is never touched by the mind. can anyone see? truth is, and the mind is confined to what was and it can imagine a future. this past and future are imaginary and utterly incomplete. the present cannot even be touched by thought. all ideas of time are thoughts and therefore do not touch the truth of It. know that the mind is time.

 

[sweetser]

Hello:

Here is my "other" answer.

The "arrow of time" is good English, math nonsense. The member of the Lorentz group that flips the sign of time also keeps the three directions of space in place. The "arrow of spacetime" is good English, and good math. It is easy to imagine why spacetime has a handedness, the space part of spacetime is like that. Local transformations trump global ones.

doug
TheStandUpPhysicist.com

ps. I just filmed an episode on this topic (+ spin) this morning. Should be up on the site in January since post-production takes at least a month.

 

[Hermite]

i haven't takent the time to read all the posts above but couldn't one possible answer come from statistical physics: the claussius inequality.

 

[Amp1]

I haven't read all the replies. I voted some 'other' por exemplo, space is still expanding so is it reasonable to say the expansion of space causes the arrow, or is it posssible that some event of the distant future exerts an influence backwards through time drawing reality foward towards it. (Big Crunch, Big Rip, what ever). I am a spiritual fellow so I won't state what comes to mind as obvious just that hint.

Edit- I just noticed Lalbotros mention expansion. I'd like to add that Pelestration commented that his theory has something to say about the arrow of time.

 

[DaveC426913]

That still doesn't answer my question.

If time flies like an arrow, why do fruit flies like a banana?

 

[jfxjmn]

humanity

I think that the "arrow of time" can be explained by the necessity of freedom of choice in human interaction with the universe. It may be a result of intelligent design, but wherein the human mind IS the creator. The humans react to a past and make choices effecting the future. Although it appears that organization naturally deteriorates, humans (even our evolution) tend to be some driving force towards another state of organization. Once we have "re-ordered" the cosmos, then it is essentially "unified", whereas the relative state is similar to the "initial condition" of the universe... just an illusion, and again necessitating human (or Mind) involvement to perpetuate reality.

 

[gptejms]

I've logged in after a long time and read some of the exchanges between Juan R. & vanesch.I think,as vanesch has also said in one of his posts,that even if we were really going backwards in time(with the big bang having occurred sometime in future),we would 'experience' moving forward in time and find entropy to increase(i.e. the 2nd law of thermodynamics to hold).

Regarding irreversibility,Poincare recurrence times are indeed quite large for such a large system as our universe.And even if things start to reverse after a few billion years(which is unlikely) the 'experience' of time would still be that of moving forward in time(though it's anybody's guess!).

 

[exponent137]

Moving back in time

I am interested what happens when I move back in time. I think that this is not only movie, which turns back. Because divergent light becomes convergent, light is coming from eyes..

What do you think that happens when gas, which moves in opposite direction of time contact with gas, which moves in ordinary direction of time. How two persons in different directions of time or two gases comunicate?

Regards

 

[vanesch]

That still doesn't answer my question.
If time flies like an arrow, why do fruit flies like a banana?

This is probably the best statement in the whole discussion

 

[atulcontrarian]

We take second law of thermodynamics as granted due to some obvious reasons. Direction of time doesn't make any sense to me. The question is nothing but why does things happen in one direction. Time is not as such a physical quantity. The concept of time is itself derived from the consequences of The law of Thermodynamics II.

 

[lalbatros]

To my opinion, the arrow of time is understood by physics since a very long time. Simple considerations like particles-in-a-box are quite clear and theoretical developments can only bring some 'operational' power to the simple ideas.

But where the arrow of time still remains hard to assimilate is with respect to the human being. We are prisonners of time. We can not freely travel in time, unlike space. This is the result of humans being part of the physical world instead of outside observers. Our lives give us the illusion of free will and we have difficulties to accept the restriction 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.