Talk:Arrow of time

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.

Roadrunner 06:15, 31 May 2004 (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 (http://www.google.co.uk/search?q=gravity+symmetry&ie=UTF-8&hl=en&btnG=Google+Search&meta=).  – 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...

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)

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)