# Talk:Arrow of time

WikiProject Physics (Rated Start-class, Mid-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Start  This article has been rated as Start-Class on the project's quality scale.
Mid  This article has been rated as Mid-importance on the project's importance scale.
WikiProject Time (Rated Start-class, High-importance)
This article is within the scope of WikiProject Time, a collaborative effort to improve the coverage of Time on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Start  This article has been rated as Start-Class on the project's quality scale.
High  This article has been rated as High-importance on the project's importance scale.

How come, if Lochschmidt's paradox is resolved by the Fluctuation theorem, the problem of the arrow of time is not? i.e. how do these two actually differ?

Does inflation explain why entropy is not maximized in our universe?

No.
Roadrunner 06:15, 31 May 2004 (UTC)

Most possible worlds have higher entropy. Although each decoherence event is reversible, most possible events increase entropy. Isn't that the basis of the seond law? Fairandbalanced 04:16, 10 Oct 2003 (UTC)

There are websites that indicate that entropy and disorder are the same (ie. as described on this wiki page); and there are websites that indicate that they are not the same; and there are websites that say disorder in the entropy sense is not the same as disorder in the widely used sense. Please clarify.

Talking about entropy as if it were disorder is a rough way of understanding the concept, just like you can get a rough understanding of temperature by thinking of it as "hotness". However, it's just a rough analogy that gives you an intuitive feel for the concept, and as with all analogies, you get into trouble if you take it too literally. Essentially both temperature and entropy have very precise mathematical and physical definitions. You can get some intuition for which the definitions are, but that intuition can lead you astray.
I can easily think of things that are intuitively "hotter" but have less or the same temperature (hot iron at 350 F versus dry air at 350 F). Similiarly, if you give me a few hours I can probably come up with things that seem less disorganized but have higher entropy.

I support your main point but not your example -- linking temperature and "hotness" is misleading. Temperature is akin to the amount of motion in a confined space i.e. add energy, get more particles moving faster = higher temperature. That we perceive that as "hotness" on our skin or so forth is secondary, which is why you were able to find examples of equivalent temperature with different perceived "hottness" levels -- we assume. For entropy, think complexity vs. simplicity. It takes energy to build up ordered or complex forms and maintain them -- on the macro level -- thus, over time's forward arrow, you get less complexity... whether the heat death of the universe, the destruction of complex atoms into simpler forms, or eventually just the end of matter itself. Chesspride 66.19.84.2 (talk) 07:33, 28 February 2015 (UTC)

Roadrunner 06:15, 31 May 2004 (UTC)

Would a manifold explain — Preceding unsigned comment added by 184.17.111.128 (talk) 01:09, 19 September 2013 (UTC)

## Is gravity reversible ?

Sorry if the question is silly, but while it is true that I never saw a broken plate come together spontaneously, I have also never seen a stone going up spontaneously. Does it mean that the law of gravity is also irreversible, just as the law of entropy ? Can we use the fall of objects to infer the arrow of time ? Or is there something more to it ? Would it be useful to clarify this in the article ? Pcarbonn 11:25, 20 May 2004 (UTC)

Yes, it should be reversible (symmetrical). Feynman used the example of a planet orbiting a star, in an ellipse. If you reverse the motion of the planet, giving it the same speed in the opposite direction then it still gives the same ellipse, it just goes the other way. In the case of your stone, the reason the stone doesn't go up is that it has lost all its energy to the ground when it landed – it's unlikely that the ground would impart the energy back into the stone afterwards. If, however, the stone was falling with a particular speed at a particular height, then you might reverse its velocity and (all things being equal) it would move away from the Earth. This is just my bad explanation – a better one is required. Try a Google search for gravity+symmetry.  – Lee J Haywood 19:52, 20 May 2004 (UTC)

Thanks ! I think I've got it. The fall, i.e. the conversion of potential energy to kinetic energy, is indeed reversible. What is not reversible is the stop of the fall, i.e. the conversion of mechanic energy to heat. This is what the second law of thermodynamics says, and that's why I have never seen a stone go up...

Gravity is itself symmetrical, but the analogy with orbiting of the planets is flawed, in that the planets are slowing down due to friction (and possibly due to gravity aswell) Bobby1011 23:05, 7 February 2006 (UTC)

I guess I still don't get it. Gravity is always attractive in the forward direction of time, and always repulsive is the reverse direction. I guess electric charge can be either attractive or repulsive in both directions, but what about the other forces? Why isn't the irreversibility of attractive force as important to the arrow of time as entropy? --Jackdavinci 18:23, 5 August 2006 (UTC)

Gravity is not repulsive in the reverse direction. It is still attractive. The phenomenon of objects "falling" up is the result of vibrations in the ground below the object manifesting out of heat and giving the object an upward impulse. It is the reverse of what happens when the object hits the ground sending a compression wave into it which is soon lost to heat. Vyroglyph 20:11, 28 August 2006 (UTC)

## Remove this

I think this is irrelevant

Cosmologists have found that the Universe had a very smooth and ordered state in the distant past, with small fluctuations. Following a period of rapid inflation, the denser regions collapsed to form galaxies, stars, planets and life. As time marches on the Universe is cooling and its energy becoming more evenly distributed. In this sense there is a well-defined thermodynamic arrow of time.

Roadrunner 05:53, 31 May 2004 (UTC)

(I rewrote this after it had been removed. The original text quoted above was, in fact, wrong). — Lee J Haywood 19:39, 28 Sep 2004 (UTC)

hey — Preceding unsigned comment added by 124.83.11.178 (talk) 02:30, 14 July 2015 (UTC)

## This too

It has been argued that the arrow of time as we perceive it – giving a distinct past and future – results from the influence of the second law of thermodynamics on the evolution of the brain. To remember something, our memory goes from a disordered state to a more ordered one, or from one ordered state to another. To ensure that the new state is the correct one, energy must be used to perform the work and this increases the disorder in the rest of the Universe. There is always a greater increase in disorder than the amount of order gained in our memory, thus the arrow of time in which we remember things is in the same direction as that in which the disorder of the Universe increases.

This looks like something that the editor read somewhere, but didnt quite understand, and somehow avoided actually describing the basis for this analogy between the workings of the brain and the workings of universe being somehow related in physics. In describing a theory (perhaps valid, but why unattributed?) as to how our perception of time may be related to time via the analogy of work, the meaning of "work" here is far too general. In the end the idea seems to rests upon a notion that the expense of brain energy is somehow equivalent or related to an "expense" of temporal inertia, which in and of itself doesnt seem to hold water, regardless of the problems with the second part. -SV 23:06, 27 Sep 2004 (UTC)

Most of the article as it stands (following the introduction) is my attempt to summarise my understanding of the publications and books that I have read that cover the arrow of time. Previously there was no attempt to explain why there is an arrow of time. However, I am not a physicist and you're right that there should be some references to back up the assertions. The text concerning the brain is indicated as being a hypothesis and anyone is free to replace it with something better. I'm sorry if my contribution isn't good enough, but I'd hope that there were some useful attempt to actually explain the arrow of time here, rather than just having examples of it. — Lee J Haywood 19:39, 28 Sep 2004 (UTC)
This is very much in line with what I read recently in "Time's Arrow and Archimedes' Point : New Directions for the Physics of Time" but could probably use some clarification and a citation. I think that since it begins with "It has been argued..." that it's clear that it represents one theory, though perhaps not a mainstream one. Churchofmau 13:11, 27 December 2005 (UTC)
Just a layman's idea here... Time itself is the propagation of events and states into the future (which we measure by watching for predictably repeating cues, like clocks, daylight, the seasons, and so on). Our thoughts can confuse aspects of what the future is with the past as we learn to predict the outcomes of our actions; we can even imagine some parts of our future easier than remembering parts of our past. The arrow of time is any physical property that is different going foward in time than backward (breaking, burning, decomposing, exploding) which aids in reminding us that the two are quite different. So, while our brain function is limited by many things which are arrows of time (most importantly the foward-only flow of information), the term seems to apply mostly as a reminder to physicists that past and future aren't always interchangable. -- tussock 17 Jan 2005
Well ,yea ,it's called the second law of thermodynamics. I believe it is stated here.--Procrastinating@talk2me 00:46, 18 January 2006 (UTC)

## (Special - ) Relativistic Arrow of time

We (any inertial reference frame) always have non-zero complex speed vector in Minkowski space with constant norm c (speed of light in vacuum). The unit vector of that vector is our (that inertial reference frame's) Arrow of time.

In other words, even if we don't notice that while apparently at rest, we are always moving along our world lines toward the future with maximal (and minimal..., in fact only possible one) speed c.

I am aware that this, of course, is a circular definition, but it perhaps sheds some more light (or casts some more shadow, whatever) on the notion. As much as there is no absolute time, there also isn't neither any absolute Arrow of time nor absolute rest.

Are you referring to a known source of reference, or just giving us your own ideas? — Lee J Haywood 19:35, 17 August 2005 (UTC)
It is true that every particle is moving along its worldline (which follows a geodesy in general relativity) in one direction. The question dealt with in this article is what distinguishes the "forward" direction from the "backward" direction. Dan Gluck 19:22, 24 May 2007 (UTC)

## Irreversibility

"It is not impossible for the molecules in the large container to separate out, just so very unlikely as to never actually happen – even if given the lifetime of the Universe to do so. The liquids start out in a highly-ordered state and their entropy, or their disorder, increases with time."

If it's not impossible, it's possible. This section is self-contradictory, pick one side or the other.

It is both possible and impossible, but there's no contradiction. It is possible that they would separate out if given an infinite amount of time. But since the Universe is generally considered to have a finite lifetime, it is impossible that such a large collection of molecules would actually manage it during the lifetime of the actual Universe. The first is a hypothetical situation, the second is a real one. Note that the article never actually states that it is impossible, however. — Lee J Haywood 08:59, 3 September 2005 (UTC)
Oh I see - I assumed an infinite amount of time. Thanks for the clarification.
Interestingly, since it is possible, if a closed system were given an infinite amount of time, it would be certain to happen, since over an infinite amount of time the system would evolve in every possible way. But that is only because an infinite number of evolutions will eventually lead to every possible solution, even those of near infinite improbability and practical impossibility. Churchofmau 12:57, 27 December 2005 (UTC)
It is not entirely true that given infinite time all states will occur. It is entirely possible to have a mixed state of molecules which never seperate out - their combined state traces a periodic loop in the Phase space. Also if they are not in an infinite loop, certain states are still going to be unattainable in absolute. However, in that case we can be sure that given enough time we can go arbitrarily close to any state we imagine, and so the particles will seperate out. --Hirak 99 17:22, 27 December 2005 (UTC)
That's definitly true in a purely clasical system. In the above example of molecules seperating, the motions of those molecules might result in a closed loop that eventually results in an exact repetition of the positions and vectors of the starting state and thus a repitition of the loop. However the probabilistic (aka "random") nature of quantum effects -- for example the uncertain timing of proton evaporation -- would, given enough time, perturb the system out of it's closed loop. But then again, the proton evaporation (or what have you) may or may not be reversible in a closed loop of it's own. With our present understanding of quantum mechanics it seems the timing of the evaporation of the proton (again, just as an example of a probabilistic proccess) would be unlikely to reverse and recur at the same point in each iteration of the classical loop.
It seems that some quantum mechanical proccesses are either -- for lack of a better word-- "random", or that there are "hidden variables" that govern the proccesses in a more classical manner. If they are "random" then an infinitely recuring closed loop would be impossible, if there are "hidden variables" then it is quite probable that all such systems (purely hypothetical though they may be) would end up repeating themselves again. Either way we will certainly not live, nor the universe likely last, long enough to carry out our little hypothetical experiment. Churchofmau 08:18, 24 January 2006 (UTC)

## biological systems

Where does the excessive energy of biological evolution comes from ? doesn't the oredered state of things now ,as opposed to less orederly in the past contradicts the second law of thermodynamics? where does the system draw it's "order" from ? what is it that becomes less ordered as biological system evolve? Diza 09:26, 8 December 2005 (UTC)

This is mentioned briefly by the thermodynamics article, though without explanation, and is missing entirely from the laws of thermodynamics article. Life doesn't have any "excessive" energy, it is simply very good at hoarding energy available extracted from its environment. In reality life performs work, and that work increases the total rate at which energy is used – so entropy still increases when the surroundings are taken into account.   — Lee J Haywood 21:33, 8 December 2005 (UTC)

Actually biological system can introduce even more disorder. E.g. cow by eating grass destroys more order (enclosed in the grass) then creates new order (in a form of own tissues and giving birth to new cows). It is not true of the grass so. If there no grass whole solar energy will be absorbed by the soil and dissipated into the heat. The grass uses part of it build itself and create the order.

Diza 18:25, 16 December 2005 (UTC) Narutally all the excessive energy for the functions of life comes from the sun's rays,This cpudl be the answer. So all of the order that has been built in the whole history the biological evolution could be explained the the sum of all dicipated heat from the planet ? this still bugs me. becasue there are other plantes. maybe one could say the biological system CONCENTRATE their order in one location ,so that they have a higher order density.

Biological systems are like machines. They create order in one place but use work for this, meaning that they create more disorder in another place. Flow from order to disorder happens, of course, even without the biological systems, for example a black soil turns sunlight to disordered energy (heat) more rapidly than grass. Dan Gluck 19:17, 24 May 2007 (UTC)

## Removing a little, spaming a lot...

I am removing the following statement

" Like gravity most physical laws can be described by relatively simple mathematical equations (momentum can be described as F=ma for example), and most mathematical operations are reversible (if you multiply a value by a number you can take the result and divide it by the same number to obtain the original value). So it is no suprise that most physical processes are also reversible and that most laws are symmetrical. The fact that there are exceptions the symmetry is one of the greater mysteries that science is currently exploring. "

as it seems to be a mostly meaningless, somewhat misleading collection of words (I started out by correcting misspeled word "mathematical", but then realized that the whole thing is beyond repair). Indeed: "most physical laws can be described by relatively simple mathematical equations" - is this even a statement? Simple relative to what? Is ${\displaystyle F=ma}$ an example of simple? How about equations of motion in Lagrange form, are they simple? Is Hamiltonian approach simple? Are forces, or fields simple, are classical dynamical systems simple? The adjective most, this indeterminate quantifier, that we'll soon see again and again, effectively nullifies the argument. For, you see, until 1956 or so, not most, but all known laws of physics appeared to be T-invariant. The violations are pretty damn rare. Much more so, than "relatively complicated" physical laws...

Next, saying "momentum can be described as F=ma" is an awkward way to express an idea at best, for momentum (as well as coordinates, for example) is indeed contained in the equation, but indirectly, and what the equation is normally describing is, depending on what's given, is either force or acceleration. Of course.

Then there is a claim that most (again!) mathematical operations are reversible. Now what does this mean? Most by what measure? Finally, an extraordinary assertion is made that since "most" functions are bijections (right?), "it is no suprise that most (Ah!) physical processes are also reversible". Now, the claim that it is no surprise indicates, to me, an unsubstantiated POV, at best, a completely misleading analogy at worst.

Moreover, it's pure idealism. Fact of the matter is, that the laws of classical mechanics (and also Electromagentism, Special and General Relativity, and Quantum Electrodynamics) -- in any form, simple or not, for form is irrelevant here, we are talking substance -- appear, as a result of an observation, a posteriori, to possess certain symmetry, which is expressed mathematically not by reference to arbitrary invertible transformation, but very precisely: as a symmetry of all of the laws above w.r.t time reversal. This invariance, and again, a posteriori, appears not to be a universal law of nature, which is most spectacularly seen in CP-violation (that, due to CPT invariance, implies T-violation). It's fine to apply mathematics to look for an underlying reason for T-symmetry being so pervasive, being violated so ever slightly and (as far as I know) only in the weak interactions. But "reversibility" of "most mathematical operations" has nothing to do with the issue. It's fruitless pseudomathematical mysticism (formally, a POV). A claim that the time symmetry of (most of) the physical laws could be known a priori is simply factually false. Isn't it?

But, if you disagree, by all means, just put the piece back to the article. I just wanted to explain that I have no intention to vandalize it. To the contrary, I think it has already suffered enough...Feodor 22:38, 24 January 2006 (UTC)

== Removing Unclear Paragraph ==Consequently unless a native Chinese person can support the contention that these meanings somehow cause the Chinese to view time differently I think they are just artifacts of the differing usage of words like 'behind', 'after' 'front' in different languages. —Preceding unsigned comment added by 114.77.67.48 (talk) 09:05, 6 August 2009 (UTC) I'm removing this paragraph (emphasis added)

The problem with the thermodynamic argument is that the second law is based off of all the other laws of physics, which are fully time symetric, meaning that the second law, if totally applied to a system should predict that entropy should increase in the future and in the past.

I think that there may be an interesting point in there, but it's not presented clearly. Anyone know how to clean it up? 20:09, 24 March 2006 (UTC)

I've come across some entries for a few books which may be applicable to this subject, but as I haven't seen the books I can't say for definite. They are:

• Davies, Paul, "The Physics of Time Asymmetry", ISBN 0520028252
• Blum, Harold, "Time's Arrow and Evolution", ISBN 0691023549 (I get the feeling that this is a biology book, but I'm not sure...)
• Coveney, Peter; Highfield, Roger, "The Arrow of Time: A Voyage Through Science to Solve Time's Greatest Mystery", ISBN 0449907236
• Schulman, Lawrence, "Time's Arrows and Quantum Measurement", ISBN 0521567750

Mike Peel 19:05, 13 April 2006 (UTC)

Davies' books are too Religious-biased; Blum's book is good (from what I have read so far); I don't know about the other two.--Sadi Carnot 04:31, 25 July 2006 (UTC)

## recent addition to psychological arrow

The past is memory, the future is anticipation, now alone is real Saint Augustine.

Awareness is a present moment function which compares what seems to be experienced externally to what is being experienced internally. The difference attributed to an experience identified as memory is called the past. The psychological future, like a projection in a mirror, makes what is behind such as expeirences, anticipations, desires, dreams, hopes, and goals, seem ahead of the observer. The ability compare novel experienced events and imagined events to countable repeating events and chart them is called time. The consistant increase in memorable novel experiences creates the psychological illusion of an arrow of time since no current experience seems totally indentical, to any prior one and no retracing or reversing of even a single moment of the universe in total is ever noticed. Since the universe seems to be expanding it seems impossible for any current experience to be anything like a prior one since all prior experiences have an apparently smaller universe in total. Jiohdi 15:27, 3 September 2006 (UTC)

Yes, er, thanks for that. I removed it because it reads like the confused musings of someone who doesn't really know what they're talking about. I see you've reinstated it. The matter of human perception of the arrow of time is very interesting indeed, but this addition contains no reference to published sources. I'd like to hear what psychologists and philosophers of consciousness have to say about this, not what you think. Doesn't anyone with a brain keep an eye on Wikipedia's articles? Doesn't anyone here want to uphold scholarly standards? Reverting.... 86.136.7.160 12:07, 5 September 2006 (UTC)
so let me get this straight, it does not matter if an idea had merit or not in your mind, you are ignoring it because you need some re-assurance that someone with a degree concurs with it? if it is poorly written, then re-write it, but dont simply ignore a valid observation because you dont have a brain of your own and need someone elses to think for you. and btw, what was there was bland and without citation from anyone nor referance to anyone so until you treat it all fairly keep your hands off.Jiohdi 21:44, 22 September 2006 (UTC)

## Teleological arrow of time

Though the main article focuses on scientific aspects of the arrow of time, it seems odd that there is not even a passing reference to any teleological aspect. For theists this must surely be an important philosophical/theological consideration. Moreover, even science itself used to be known as natural philosophy. Many other articles in Wikipedia treat difficult theological and philosophical ideas with a fair degree of competence. For example, the articles on predestination and determinism – which, by the way, are not the same thing – so I would suggest that there is a gap in this page that needs filling. For the time being, I am content merely to draw this to your attention. DFH 16:27, 23 September 2006 (UTC)

This article is about a scientific concept. Time#Religion states "the Judaeo-Christian concept, based on the Bible, is that time is linear, beginning with the act of creation by God. The general Christian view is that time will end with the end of the world.". gmipTalk 14:16, 1 November 2011 (UTC)

## Missing references

Missing references is probably the major weakness of the whole article. I have made a start by citing a reference news article for my recent addition to the section on the psychological/perceptual arrow of time. I hope this will prompt other users to do likewise. DFH 19:55, 26 September 2006 (UTC)

## Planetary motion reversible ??

It is said in the article that such motions are reversible. There are 2 or more problems here. One is that chaos may be present, so that over a sufficiently long time, the orbits will have a divergence from each other that is so sensitive to initial conditions it is impossible to predict which set it the right one. (see Lyapunov Exponent). Another is that there are many small bodies present, from asteroids down to meteorites and even dust, that make it impossible to work backwards, and even make forward predictions diverge initially by meters a year, and eventually in a Lyapunov manner (exponentiallyI. See Standish E. M., and Fienga A.: 2002, Accuracy limit of modern ephemerides imposed by uncertainties in asteroid masses. Astron. and Astrophys, 384, 322 Carrionluggage 04:29, 29 September 2006 (UTC)

I don't see what your problem is. The direction of time makes no difference – the chaos is present in either case. And even though we might not be able to predict the outcome of a chaotic system with any accuracy, that doesn't mean that the motion isn't reversible – only that we cannot model it. The article only says that a movie of planetary motions looks realistic in both directions, which would be true no matter how long the movie ran for.   — Lee J Haywood 09:43, 29 September 2006 (UTC)

## Chinese perception?

I found the article referenced for the Andean tribe seeing time "backwards" quite interesting, but its only mention of the Chinese perspective is that they specifically are NOT an exception to the usual rule. I think we DEFINATELY need a citation for the Chinese claim, for as the above article itself mentions, words can be very tricky and misleading when trying to pin down what people really think from them, including a few odd examples of english seeming backwards in much the same way as these Chinese examples do... Elgaroo (talk) 16:32, 5 August 2008 (UTC)

I agree. The Chinese words may not be sufficient to attribute different cultural ideas of time. For example:

The Chinese word for 'day after tomorrow' 后天 hou tian does literally mean 'behind day' but it could be interpreted to mean that the 'day after tomorrow' is chronologically behind today just as the number 4 is behind the number 3 on a number line.

The Chinese word for 'day before yesterday' 前天 qian tian does literally mean 'front day' but it could be interpreted to mean that the 'day before yesterday' is chronologically in front of today just as the number 3 is in front of the number 4 on a number line.

Consequently unless a native Chinese person can support the contention that these meanings somehow cause the Chinese to view time differently I think they are just artifacts of the differing usage of words like 'behind', 'after' 'front' in different languages. —Preceding unsigned comment added by 114.77.67.48 (talk) 09:05, 6 August 2009 (UTC)

Isn't the difference just due to perceiving "in front" as being stuff you can see (i.e. what's already happened) while "behind" is stuff that you cannot? I doubt the Chinese have any fundamentally different view of the direction of time. I'm very tempted to remove that section Woscafrench (talk) 00:11, 25 October 2010 (UTC)

## Unreferenced Section

I add a unreferenced verification template stamp to the section about the radiative arrow of time since it seems to me to be very vague about what experiments it refering too in the sentance "This arrow has been reversed in carefully worked experiments".

I felt like one of the experiment is was implying was time-reversal of electrodynamic wave involving nonlinear optics (aka what wikipedia calls "optical phase conjugation"). Now would be a good time to read on optical phase conjugation at http://en.wikipedia.org/wiki/Nonlinear_optics#Optical_phase_conjugation . Especially note that 100% reflective mirror have never been made, so this mean that the phase conjugate beam is a fraction of the source beam. Hence, there is a energy loss during optical phase conjugation, which I believe but am not 100% sure can be interpreted as an increase in entropy. Thus I did not want to suggest optical phase conjugation since it may not violate the second law of thermodynamics.

However, if not optical phase conjugation then what experiment does the article mean? —Preceding unsigned comment added by 68.214.39.193 (talk) 20:24, 8 May 2009 (UTC)

## Memory and Entropy

It is now written in the article:

The psychological arrow of time is thought to be reducible to the thermodynamic arrow: it has deep connections with Maxwell's demon and the physics of information; In fact, it is easy to understand its link to the Second Law of Thermodynamics if we view memory as correlation between brain cells (or computer bits) and the outer world. Since the Second Law of Thermodynamics is equivalent to the growth with time of such correlations, then it states that memory will be created as we move towards the future (rather than towards the past).

This seems to me total nonsense, in more ways than one. First, there is a powerful tradition in the literature which says that memory erasure equals entropy increase. (Landauer and Bennett come to mind, see the final chapter of my http://philsci-archive.pitt.edu/archive/00002201/ ) This is the exact opposite of what the current article says, since it equates memory creation with entropy increase. This is strange. (It is also proof of the rampant confusions surrounding the notion of entropy!)

Second, the notions of "memory" and "correlation" used here must be made much more specific before the claim can even be understood. In a deterministic Hamiltonian world, any time slice of the Universe implies its state at all other times; no correlations can form which do not already exist at the outset. If an indeterministic (but still unitary!) quantum mechanics somehow changes this, tell us how. As it stands, the claim that "the Second Law of Thermodynamics is equivalent to the growth with time of such correlations" seems to me at least unclear and probably false.

Third, even if the physics and the information theory is sorted out, it is still not clear that the psychological arrow is linked to the thermodynamic arrow. We don't form memories of the future when we temporarily lower the entropy of our brain (for instance, by sticking our head in a fridge). If there is a link, it needs to be argued for in much more detail.

Fourth, the article claims that the psychological arrow "is thought to be reducible" to the thermodynamic arrow. Thought by whom? This is again too vague. Victor Gijsbers (talk) 16:51, 18 June 2009 (UTC)

I've traced this section to this edit Revision as of 19:52, 27 August 2005 (Add several other arrows). And I think you are right, this section is vague and simply incorrect in some places (i.e. equating memory creation with total entropy increase is incorrect). --Dc987 (talk) 00:06, 10 November 2009 (UTC)

## Unreferenced section - The quantum arrow of time

This section have been in the article at least since 2006, but it contains no reference to published sources or citations. --Dc987 (talk) 06:02, 5 November 2009 (UTC)

I've traced Arrow of time#The quantum arrow of time section to this edit Revision as of 19:52, 27 August 2005 (Add several other arrows) by JLM (talk). It sounds reasonable, but contains no references to published sources or citations. I've tried googling for references, but without much success. Anybody can reference this material?--Dc987 (talk) 00:12, 10 November 2009 (UTC)
Regarding time-irreversibility in QM, a source is Ch. 6, "Time Asymmetry in Quantum Mechanics" in _The physics of time asymmetry_ by P.C.W. Davies. JLM (talk) 20:33, 30 May 2010 (UTC)
Anybody can check this material? Especially the statement: "While at the microscopic level, collapse seems to show no favor to increasing or decreasing entropy, some believe there is a bias which shows up on macroscopic scales as the thermodynamic arrow." - in fact that's about "fine grained" vs "coarse grained" entropy, right? --Dc987 (talk) 09:10, 1 June 2010 (UTC)

────────────────────────────────────────────────────────────────────────────────────────────────────{{cite book|author=P. C. W. Davies|title=The physics of time asymmetry|url=http://books.google.com/books?id=9saLoa7by8oC&pg=PA154|accessdate=1 June 2010|year=1974|publisher=University of California Press|isbn=9780520028258|pages=154–184|chapter=6 Time Asymmetry in Quantum Mechanics}}

"There is a close analogy between the destruction of microreversibiliy in the classical system by coupling to the outside world stochastically through the walls of the container (see section 3.5), and the same destruction in a quantum system due to external coupling with the 'measureing apparatus'." p.168

"...the quantum system under observation is capable of changing in two distinct ways: (1) reversibly during the normal evolution of the system as described by the Schrödinger equation, (2) irreversibly during the measurement process, which cannot be described by the Schrödinger equation." p.170

"[Thermodynamic metastability] is an indispensable prerequisite for amplification of the quantum signal which triggers the [macroscopic measurement] apparatus. As a result of this amplification, the apparatus proceeds fairly rapidly to a stable equilibrium condition, which is correlated with the initial state of [the microscopic observable], and is macroscopically distinct from its neighbour states which are correlated with different states of [the microscopic observable]. Such amplification obviously involves a coarse-grained entropy increase (though not fine-grained). Thus, although the nature of quantum measurement irreversibility depends on coherence times of macroscopic systems, the origin of the irreversibility lies squarely in the domain of thermodynamics (as in the Everett interpretation), which in turn is related (as described in chapters 3 and 4) to the formation of branch systems, and ultimately to the origin of the universe, and the nature of gravity." p.174

Would any of these work?—Machine Elf 1735 (talk) 12:54, 1 June 2010 (UTC)

Well... JLM had rephrased the section, it's much better now. But AFAIK it is still one of the unsolved problems. Claims by that 1974 book:

"although the nature of quantum measurement irreversibility depends on coherence times of macroscopic systems, the origin of the irreversibility lies squarely in the domain of thermodynamics (as in the Everett interpretation), which in turn is related (as described in chapters 3 and 4) to the formation of branch systems, and ultimately to the origin of the universe, and the nature of gravity."

... well... that feels like speculation, rather than hard science. True, very likable speculation, but... --Dc987 (talk) 08:11, 3 June 2010 (UTC)
Not that speculative, AFAIK. The origin of time irreversibility lies in the initial conditions, and surely they must be generated by gravity and the origin of the universe.i.e. quantum gravity/ string theory or some TOE?
A good source for some of this would be Wheeler and Zurek's "Quantum Theory and Measurement" anthology, which has a selection of articles on irreversibilty. --Michael C. Price talk 09:06, 3 June 2010 (UTC)
@Dc987, I may have misunderstood—I thought the reference just needed verification. It's better to reference specific pages so I meant to ask if those page numbers would "work" (for the reference, if not the physics). Maybe I didn't pick the best available in the chapter preview... but quite a few citations seem to be available and none at all have been added to the section?—Machine Elf 1735 (talk) 22:00, 4 June 2010 (UTC)
I don't have any problems with the current version being unreferenced (after JLM had rephrased it). Mainstream... --Dc987 (talk) 05:29, 17 June 2010 (UTC)

## The radiative arrow of time

"This arrow has been reversed in carefully worked experiments which have created convergent waves": I recall reading something about this years ago, probably in Scientific American or Physics News Update. IIRC, these waves were emitted from a source and reconverged onto it. Paradoctor (talk) 02:53, 28 January 2010 (UTC)

## Flow of time

The article states that it often appears that there is "an obvious direction (or flow) of time." The words "(or flow)" should be removed. The arrow of time refers to an asymmetry of the universe with respect to time, and perhaps to some other flows with respect to time, but not to any flow of time itself.

Flow in relation to what?

Psychologically, and experientially, there is a present moment, and this moment changes, but this is not an objective phenomenon. Objectively, a moment in time is considered past, present, or future only in relation to other moments, and there is nothing dynamic about those relationships. Thus no flow. Certainly our experience of a dynamic universe is often modeled as though our awareness moves through time, but this is an apparent flow of awareness through time, not a flow of time itself. Poetically, time is the stream bed through which the stream of our awareness flows.

This movement of awareness (i.e. the seemingly dynamic character of our experience) cannot be a function of objective facts about the universe except as a kind of illusion because the objective facts about all particular points in space-time are fixed, indeed the objective facts are what define space-time. Lest I be misunderstood, I do not hold that our experience is an illusion, only that if it isn't an illusion then no objective description of reality can be complete.

Even if we take it that our awareness only apparently moves forward in time, in the direction of the arrow, then the flow of time with respect to our awareness would be in the opposite direction—not with the arrow of time but against it. So the reference to the flow of time in the article is less than helpful.

It is like thinking of the arrows on a one-way street as being the direction that the street moves. Mazzula (talk) 15:48, 15 March 2011 (UTC)

## Citation Needed: Why must the universe grow by definition?

Regarding the statement: :If this arrow of time is related to the other arrows of time, then the future is by definition the direction towards which the universe becomes bigger. Thus, the universe expands - rather than shrinks - by definition. on the section on Cosmological Arrow of Time.

I think the term "related to" is pretty vague and I cannot see how the relationship would require the universe to be expanding (positive scale factor)?

Indeed, the fact that negative energy has not been observed is suggests that the universe seems to be populated entirely with objects that travel in one direction in time, regardless of the reversibility of their interactions. That is in my opinion a more fundamental property.

--Jallberg (talk) 17:30, 16 July 2011 (UTC)

## Causal Effects

"For example, the causes of the resultant pattern of cup fragments and water spill are easily attributable in terms of the loss of manual grip, gravity, trajectory of the cup and contents, irregularities in its structure, angle of its impact on the floor, etc. However, applying the same event in reverse, it is difficult to explain how the various pieces of the cup come to possess exactly the nature and number of a cup before assembling, how they could assemble (as neither floors nor hands can create china cups unaided), why they should assemble precisely into the shape of a cup and fly up into the human hand (as immobile floors cannot throw and, without contact, the human hand lacks the capacity to move objects unaided) and why the water should position itself entirely within the cup."

This uncited statement completely and utterly threw me off. What I find confusing here, is that the example reverses time in a single case of the cause and effect, without reversing the entirety of cause and effect. I've always considered reversing cause and effect to be quite simple - in a "backwards" perception of time, cause follows effect, leading to a decrease in entropy. Floors don't throw cups, but time-reversed gravity pushes things up. So instead of cause = hand opening | effect = falling cup, you have rising cup "causing" the hand to open. Therefore the property that causes scattered pieces to form a cup are the very same forces that hold the cup together as a solid until the moment it hits the ground. The causal relationship already 'cannot be perceived' forwards, and it makes no more or less theoretical sense backwards. For an example, centrifugal force does not become centripetal force purely because you looked at in the order of cause following effect. gmipTalk 13:49, 1 November 2011 (UTC)

## Citation to Randy Wayne paper

I've tentatively removed the citation BinaryPhoton added to what appears to be his own paper, which was recently published in what is (as far as I can tell) a very low-impact journal. (I'm in no position to judge the scientific merits of the paper itself, though I can say that the style of it doesn't resemble any peer-reviewed journal article I've ever read.) The sentence to which this editor has added the citation long predates his paper; surely if the claim needs a citation at all then it had better be to the paper in which it was first advanced, or to a secondary source which summarizes the past research. —Psychonaut (talk) 12:22, 27 June 2012 (UTC)

I am unfortunately familiar with this person's work. Plainly WP:FRINGE and not notable here. a13ean (talk) 22:34, 27 June 2012 (UTC)

## Computational Arrow of Time?

Glaringly missing from this page. Some calculations are not reversible. Suppose that a computer accepts two inputs and adds them together. The result is 42. The inputs are erased from memory. There is no way to tell whether the numbers were 21 and 21 or any other particular combination. The past state of the machine cannot be computed from the present one.

It may be in fact this arrow to which the others are hinged.

Even in a totally deterministic universe, there can be a forward time arrow.

Suppose that a cup falls on the floor and breaks in a deterministic universe, resulting in a spill, fragments and various artifacts of the dissipation of energy such as vibrations and heat.

That result is like the number 42 in the earlier example. More than one pattern of fall of more than one cup could produce exactly the same results. If time is reversed, there is no way to get to the past state because, for instance, if any future state depends on addition calculations over the previous state, we run into the problem that sums cannot be decomposed into the original terms. The information is simply not there. The broken cup pieces can be reassembled into more than one cup because of ambiguities. As soon as we find two cup fragments which have an identical shape, but are distinguishable (by some surface imperfection, marking or embedded inpurity), it means that we do not know which of two possible cups were broken. Perhaps the place where those pieces landed could tell us, but that itself could be subject to ambiguities.

The basic idea is that the universe is based on processes and processes irretrievably replace previous states with new states which do not contain enough information to unambiguously recover the previous states.

192.139.122.42 (talk) 01:39, 20 November 2012 (UTC)

Is there a specific suggestion to add something to the article, and do you have any sources to support this? a13ean (talk) 23:16, 22 November 2012 (UTC)

## Blacklisted Links Found on the Main Page

Cyberbot II has detected that page contains external links that have either been globally or locally blacklisted. Links tend to be blacklisted because they have a history of being spammed, or are highly innappropriate for Wikipedia. This, however, doesn't necessarily mean it's spam, or not a good link. If the link is a good link, you may wish to request whitelisting by going to the request page for whitelisting. If you feel the link being caught by the blacklist is a false positive, or no longer needed on the blacklist, you may request the regex be removed or altered at the blacklist request page. If the link is blacklisted globally and you feel the above applies you may request to whitelist it using the before mentioned request page, or request its removal, or alteration, at the request page on meta. When requesting whitelisting, be sure to supply the link to be whitelisted and wrap the link in nowiki tags. The whitelisting process can take its time so once a request has been filled out, you may set the invisible parameter on the tag to true. Please be aware that the bot will replace removed tags, and will remove misplaced tags regularly.

Below is a list of links that were found on the main page:

• http://preposterousuniverse.com/eternitytohere/faq.html
Triggered by `\bpreposterousuniverse\.com\b` on the local blacklist