Forgetted term from physics/thermodynamics

August 18, 2007 10:29 PM

There's a name for the principle stating that the bulk properties of a substance are a consequence of the ensemble average of the behaviors of the individual molecules comprising that substance. What is it?

I cannot for the life of me remember this word, and I might be off a little bit on the concept, too. I cannot figure out a way to search for this. Trust me, I've tried.

posted by mr_roboto to science & nature (21 comments total)

Oh, and a typo in the title, too. Heh.
posted by mr_roboto at 10:30 PM on August 18, 2007

Its not the equipartition theorem is it?
posted by vacapinta at 10:43 PM on August 18, 2007

Nope. It's called the "_______ principle". I think it starts with the letter "t".
posted by mr_roboto at 10:47 PM on August 18, 2007

Superposition?
posted by JackFlash at 11:12 PM on August 18, 2007

you're not thinking of the correspondence principle?

the thing you describe is basically the central tenet of statistical mechanics, and i dont know that it has a name. here's what my book (mcquarrie) says:

"It is at this point that we appeal to the work of Maxwell, Boltzmann, and particularly Gibbs. The modern (postquantum) version of their approach is that in order to calculate the value of amny mechanical thermodynamic property (say, the pressure), one calculates the value of that mechanical property in each and every one of the quantum states that is consistent with the few parameters necessary to specify the system in a macroscopic sense. The average of these mechanical properties is then taken, giving each possible quantum state the same weight. We then postulate that this average mechanical property corresponds to a parallel thermodynamic property."

thats all he says. he does then go on to talk about the principle of equal a priori probabilities, which is about how each microstate is equally likely to exist. but that's not really what you're describing.
posted by sergeant sandwich at 11:38 PM on August 18, 2007 [1 favorite has favorites]

Maybe the thermodynamic limit?
posted by you're a kitty! at 11:39 PM on August 18, 2007

sergeant sandwich writes "the thing you describe is basically the central tenet of statistical mechanics, and i dont know that it has a name."

It does, though! It's very fundamental. I'm not sure if stat mech is the place to look for it, even. It might be more of a philosophy of science concept.
posted by mr_roboto at 11:44 PM on August 18, 2007

oh, in the index it's listed under the "postulate of Gibbs".
posted by sergeant sandwich at 11:46 PM on August 18, 2007

I'm a physicist: you're talking about statistical mechanics.

Therefore, if that's not the term you're looking for then your definition (i.e. your question) is "wrong". Can you give any more information?

Could you be looking for the partition function? (See also).
posted by alby at 1:24 AM on August 19, 2007 [1 favorite has favorites]

alby writes "Can you give any more information?"

It comes up every once in a while when people are discussing single-molecule experiments.
posted by mr_roboto at 2:03 AM on August 19, 2007

methological reductionism? or just reductionism - In philosophy, reductionism is a theory that asserts that the nature of complex things is reduced to the nature of sums of simpler or more fundamental things. This can be said of objects, phenomena, explanations, theories, and meanings.
posted by andrew cooke at 5:45 AM on August 19, 2007

the only thing i can remember from statistical mechanics is "an ensemble will always evolve in the direction of the greatest number of microstates". not sure if it has a name.
posted by bruce at 7:27 AM on August 19, 2007

You're a kitty is correct, it's the thermodynamic limit.
posted by Mapes at 7:50 AM on August 19, 2007

Boltzmann's principle?
posted by tra at 7:52 AM on August 19, 2007

or statistical limit?
posted by andrew cooke at 8:12 AM on August 19, 2007

I think you might be talking about the Ergodic hypothesis, which states that the ensemble average of a system at any instant is equivalent to the average behavior of a single component of the system if you look at it for a sufficient period of time. That one is pretty important for single molecule work.
posted by overhauser at 12:35 PM on August 19, 2007 [1 favorite has favorites]

Yes! Thank you, overhauser! This was driving me nuts. And, clearly, my memory was a bit off. I'm glad someone was able to pull it out of the mishmash I was spouting.
posted by mr_roboto at 12:59 PM on August 19, 2007

Oddly, I considered the ergodic hypothesis next but refrained from posting because it is not a principle nor does it begin with t... :)
posted by vacapinta at 3:51 PM on August 19, 2007

That'll teach you to trust my memory...
posted by mr_roboto at 5:26 PM on August 19, 2007

Glad to help.

I have a friend who is trying to prove that this isn't true in the case of membrane proteins, so it is a topic that comes up frequently in my world.
posted by overhauser at 6:29 PM on August 19, 2007

That's the angle I'm coming at, too.
posted by mr_roboto at 6:36 PM on August 19, 2007