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0 pointskgwgk3y ago0 comments

>>If by sheer luck all gas particles moved to the exact left side of the container the energy wouldn't change, the macrostate wouldn't change, the entropy wouldn't change - it doesn't matter how unlikely that is.

> Wrong. Energy wouldn't change. Macrostate changes. Entropy changes.

That's my thermodynamical system - that I decided to describe using the state variables N, V, E. S_kgwgk = constant

I have agreed that you can chose to define macrostates as you wish! (You cannot measure them though. You don't have access to my system. They are purely hypothetical.) And then you can say things like "If the position of the particles in your system is X, S_deltasevennine = whatever."

Hopefully you will agree that someone else could choose to define the macrostate differently and say things like "If the position of the particles in kgwgk's system is X, S_anon = somethingelse."

Let's imagine that the position of the particles is actually X. What is the entropy of the system then? S_kgwgk? S_deltasevennine? S_anon? They are all different!

If you think that you can define macrostates for my system in some arbitrary way (your own words: "I'll choose something arbitrary.") and others can't - or that yours are somehow more real - I really wonder why.

And as I said, someone could also come and say "If the position [and velocities] of the particles in kgwgk's system is X, S_vonneumann = 0." [a complete definition of the microstate requires knowing both position and momentum - there might have been some ambiguity about that before but it shouldn't distract us from the main points]

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deltasevennine3y ago· 2 in thread

I thought we agreed on my definition. If you want to make your own definitions of macrostate sure. No rule against that, I just don't see your point.

>If you think that you can define macrostates for my system in some arbitrary way (your own words: "I'll choose something arbitrary.") and others can't - or that yours are somehow more real - I really wonder why.

I really wonder why you even say this. Anyone can make up a macrostate we just choose to agree on one for discussions sake. But in the real world things like pressure and temperature are some universal agreed upon ones. I simply made one up so we can center our discussion around it and you took it into other territories.

You can switch up definitions all you want. But no matter your definition of macrostate, this remains true:

Entropy is independent of knowledge. Your initial argument was against that. I haven't seen you make any argument in your own favor. Just side discussions on who's definition of macro state to use.

kgwgkOP3y ago

  > If you want to make your own definitions of macrostate sure.

Note that I was the first one to propose a particular way to define the macrostate of the system - using energy - trying to understand what you meant. Your reply was "that's not the definition of macrostate. It's just one arbitrary choice you have chosen." And you gave your own arbitrary choice "I'll choose something arbitrary. Temperature as measured by a thermometer".

The point is that they are all arbitrary. (Energy is a conserved quantity for a closed system so it's arguably more natural - but let's say that any choice is equally arbitrary.)

  > But no matter your definition of macrostate, this remains true:
  > Entropy is independent of knowledge.

Maybe be can agree that [Gibbs] entropy [which is a property of the distribution of microstates corresponding to our description of the system based one some macroscopic properties] is independent of knowledge [other than about that particular macroscopic description].

The discussion started with me trying to understand what did you mean by "low entropy" when you said:

"Let's say those balls all have a random initial velocity at a random direction but all those balls are initially positioned near one corner in the cube. Thus the balls from a position stand point start with low entropy."

We agree that if the box is isolated and we define the state using N,V,E there is one single macrostate and one single value for the entropy - independent of the position of the particles. We agree that we can define thermodynamical systems using other variables and calculate other entropies. We agree that the entropy is not determined by the configuration of the particles alone.

(Even when a thermodynamical system is defined the entropy is not necesarily a function of the microstate when it refers to only a part of the system. The microstate may not fully determine the macroscopic description. If that box is in equilibrium in a heat bath the temperature is fixed and there is some corresponding entropy. But the same microstate can happen for equilibrium systems at different temperatures and therefore with different entropies.)

You said that you're using temperature as measured by a thermometer as the macroscopic property describing the system. And I'm still trying to understand how "higher concentration" means "lower entropy" even in the context of your own arbitrary choice.

Temperature works well as a state variable for a system in thermal equilibrium which has the same temperature everywhere. You mention the level of mercury and the way a thermometer works is by reaching thermal equilibrium between the mercury and the thing being measured. Note that if for some reason all the particles in the gas go away from the thermometer for a second the temperature of the mercury won't change (ignoring that it will radiate energy over time). The reading of the thermometer doesn't drop to zero just because there is nothing there.

Let's assume that you are somehow measuring the local temperature in a small region around some specific point. What you measure will not depend in any way on the particles that are elsewhere. The particles in the rest of the box could be well spread or all near one corner and you would have no reason to say that the latter configuration is lower entropy than the former based on your macrostate.

Another way to look at it: you reasoning seems to be that the state with the particles near one corner has low entropy because they are close to each other and there is some temperature reading in your far-away thermometer lower than the equilibrium temperature and then the entropy is low. If a similar cluster of particles was close to the thermometer rather than far from it I guess that you will tell me that the temperature reading would be higher than in equilibrium (and the entropy would also be lower).

But if you put that cluster of particles at some intermediate position the reading of the thermometer will be the same as in equilibrium! Your macrostate will be the same as in equilibrium. Your entropy will not be lower than in equilibrium. Even though you said -if I understood you correctly- that the entropy should be lower because the higher concentration has a lower probability of occurring.

deltasevennine3y ago

> Note that I was the first one to propose a particular way to define the macrostate (energy) of the system to understand what you meant. Your reply was "that's not the definition of macrostate. It's just one arbitrary choice you have chosen." And you gave your own arbitrary choice "I'll choose something arbitrary. Temperature as measured by a thermometer".

No. You explicitly ASKED for my definition, then I said that.

When I said that's not the "definition of macrostate" I did not make an arbitrary choice there. I simply stated that IF you think energy was the definition of macrostate, then you are wrong.

> Maybe be can agree that [Gibbs] entropy [which is a property of the distribution of microstates corresponding to our description of the system based one some macroscopic properties] is independent of knowledge [other than about that particular macroscopic description].

This is the point of the ENTIRE thread starting from your INITIAL reply. Technically this argument is over. You weren't in agreement with me, now you are, so you were wrong and I was right. This sentence admits that.

The rest of the stuff here is side tangents and it's hard to see the main point. I can entertain it though for a little bit but this here is essentially the end.

>The discussion started with me trying to understand what did you mean by "low entropy" when you said:

This discussion started with you saying that if I know the microstate of the system entropy is zero. If you wanted to understand What I thought about "low entropy" then this was NEVER stated. The thread is based off what is stated and what is not stated. It is not based off of what your internal thoughts and intentions are. If you want the topic to based off your own thoughts, they need to be expressed explicitly in statements. You did so just now, but way too late.

>Note that if for some reason all the particles in the gas go away from the thermometer for a second the temperature of the mercury won't change (ignoring that it will radiate energy over time). The reading of the thermometer doesn't drop to zero just because there is nothing there.

This is pedantic. Obviously. I can take it into account. Let's say the greatest average speed measured of a particle and the time it takes for a particle at this speed to travel across the box is the time it takes for the thermometer to drop to zero degrees from the maximum temperature. It's quite fast but particles within the vicinity will keep the mercury level stable but if they were concentrated in a corner, the mercury drops fast enough to change the reading of the thermometer.

>Let's assume that you are somehow measuring the temperature in a small region somewhere with a very high reaction time. What you measure will not depend in any way on the particles that are elsewhere. The particles in the rest of the box could be well spread or all near one corner and you would have no reason to say that the latter configuration is lower entropy than the former based on your macrostate.

I specifically defined it as a thermometer to make location matter. Switching the location of the thermometer is switching the definition as well.

I really don't see what your point is. You think I'm wrong about something? What am I wrong about?

>But at some intermediate point the reading of the thermometer will be the same as in equilibrium. Your macrostate will be the same as in equilibrium. Your entropy will not be lower than in equilibrium. Even though you said -if I understood you correctly- that the entropy should be lower because the higher concentration has a lower probability of occurring.

Your point seems to be buried in here somewhere and I can't parse it. There is a macrostate that is equilibrium, yes.

Are you referring to mercury level mid transition? This is pedant-ism to the max if you are. Yes the mercury level will display the WRONG temperature if it's mid transition. Not willing to constantly adjust the model to little flaws you find. Last time: Let's switch to a digital thermometer that displays temperature at time intervals that are equal to the length of time it takes for the maximum speed particle to travel across the box. There is no transition value now. All temperature readings reflect an instantaneous observed truth at an instantaneous point in time, but that temperature is displayed at non-instantaneous intervals.

It also seems to me that your definition of macrostate is meaningless. The total energy of the universe is hypothetically the same all the time. If the macrostate was just energy there's no point to it, because entropy would then be an unchanging constant.

I think we're done here. I'm just trying to guess what you're driving at. You'll need to clarify your point if you want me to continue. What exactly are you trying to say here?

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