Author Topic: Entropy  (Read 8291 times)

Offline bn

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Re: Entropy
« Reply #15 on: July 11, 2012, 04:27:13 PM »
Thanks for your reply! What you wrote is pretty much what I understood from other sources. I'd still like to hear more about this (hence this being in "topic suggestions"), especially "why" entropy has a preferred direction in time. I hear that's one of the big unknowns in modern physics, but I wonder if there are any theories...

yeah, it's a great topic. I'll definitely do a show on it.
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Offline Lynx Cat

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Re: Entropy
« Reply #16 on: July 11, 2012, 05:44:31 PM »
Since about last year, I'm trying to teach myself more advanced physics (special & general relativity and quantum physics), but it's sometimes hard to find the time to do that, so I'm still struggling with the mathematical foundations.

Wow that is impressive! I imagine this to be some pretty hard work. Most of the stuff is difficult to understand even if you are in a university with teachers and students around you who do nothing else all day... You Sir or Madam have my respect
Uhh... thanks  :P It's not easy, but I'm trusting the principle that patience goes a long way. That's something I usually have ample stores of. And since you're wondering, it's "sir"...

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especially "why" entropy has a preferred direction in time

I am not sure I completely understand what you mean by that. Do you want to understand why processes only happen when the entropy increases? Or do you want to know why the entropy increases always in the direction of positive time?
The latter. What is the relationship between entropy and time that gives the statistical processes represented by entropy this "slant"?

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yeah, it's a great topic. I'll definitely do a show on it.
Wonderful!  ;D
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Offline stephako

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Re: Entropy
« Reply #17 on: July 16, 2012, 03:55:53 AM »
The latter. What is the relationship between entropy and time that gives the statistical processes represented by entropy this "slant"?

The second law of TD actually doesn't involve time at all.
<specualting> So there is nothing in there that says processes only happen in +t direction. It just gives processes a preferred direction in the space of the state of the systems. We just call that direction in time in which systems go to this state positive</spaculating>

Although the more I think about it the less it makes sense to me...The reason is: While the TD laws do not usually have time dependences the processes in pratice of course do and I don't quite know how to bring all that stuff together...
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Offline Lynx Cat

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Re: Entropy
« Reply #18 on: July 16, 2012, 05:09:56 AM »
Therein lies the rub.
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Offline bn

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Re: Entropy
« Reply #19 on: July 16, 2012, 06:46:32 AM »
i think there's an interesting thing when it comes to thermodynamics and the problem of time.
specifically, we use the law of increasing entropy to power all of our clocks!

I mean, there are natural rates of change in the world, from the slow dissipation of energy away from a coiled spring in a pocket watch, to the decay of radioactive isotopes; which we use to accurately measure the march of time.

so is our perception of time anything OTHER than the laws of thermodynamics?
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Offline stephako

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Re: Entropy
« Reply #20 on: July 16, 2012, 06:50:55 AM »
You just brought up an interesting point Ben: The radioactive decay is not entropic is it? That is just the particles trying to get to the state with the lowest energy. Furthermore the expansion of the universe is not driven by entropy, yet both of those things could be used to measure time, or at least give it some direction.
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Offline bn

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Re: Entropy
« Reply #21 on: July 16, 2012, 06:55:45 AM »
sure the radioactive decay of a single particle is not entropic, but the radioactive decay of a collection of particles will be. you can imagine that you are packing in protons and neutrons and photons into a very special state (using a particle accelerator or a supernova), like an ice cube in a glass of water; and then as time goes on, those protons, neutrons, electrons and photons re-organize themselves in the general vicinity.

this process can be reversed, and there's a finite probability that two crappy particles and some energy might combine to re-make a uranium atom. it's just... low.
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Offline bn

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Re: Entropy
« Reply #22 on: July 16, 2012, 06:58:29 AM »
as for the expansion of the universe... that's a mystery.
one could argue that the super-early universe was in a state of very low entropy, since gravitational collapse messes up the even-ness of the distribution.

black hole geometries, on the other hand, have a ton of entropy. at least, as far as the laws of "black hole thermodynamics" are concerned.

so the evolution of the universe could be thought of as a move from "low entropy" to "high entropy" as the universe moves from a state of super even-ness, to a state which is full of only black holes.

but. people are still working on that.
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