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Entropy: A little understood concept in physics [video]

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Re: Entropy: A little understood concept in physics [video]

#131

Post by cubefox »

lll-o-lll wrote: post #48
szundi wrote: post #6

Just after Sabine’s

Yes, and the Sabine video contained far more information (lower entropy?) and one genuinely interesting idea I’d never heard before. The idea that heat death may not be the end of intelligent life! The concept this relies on is the idea that macro states are actually just combinations of micro states all of which have the same probability. E.g. the sequence 1, 2, 3, 4, 5 and 3, 1, 4, 2, 5 are equally likely if you ar…

I suspect that "macro state that is more important to us" really is special in some objective way, not just subjectively.

Re: Entropy: A little understood concept in physics [video]

#132

Post by kergonath »

javajosh wrote: post #68
dekhn wrote: post #54

Earlier quoted context omitted.

I took a stat thermo class and it was basically all about entropy, which was expresed as ln W- the log of the number of ways (permutations) that a system can be ordered, which gives a convenient denominator when calculating the probability of a specific permutation. Here's the professor's book, which was still only in latex form when we took the class: https://www.amazon.com/Molecular-Driving-Forces-Statistical-...

yes, there are lots of quantitative details. I wanted to emphasize the key qualitative concept, from which the others can derive. In a similar way you can derive all of special relativity, and approach an intuition about the strangeness of spacetime, starting with only two ideas: the laws of physics are the same in all reference frames; the speed of light is constant. I prefer to start there and derive e.g. Lorentz f…

> starting with only two ideas: the laws of physics are the same in all reference frames; the speed of light is constant.

Isn’t this redundant, though? The constant velocity for light in a vacuum comes directly from the laws of (classical) electromagnetism in the form of Maxwell’s equations. So “the laws of Physics are the same in all reference frames” implies “Maxwell’s equations are valid in all reference frames”, which in turn implies “the velocity of light in vacuum is the same in all reference frames”. That’s what I understood reading Einstein himself.

I think it’s much stronger that way. Otherwise we get to why light should be a special case, which is difficult to defend. The constant velocity of light (in vacuum) being an unavoidable consequence of the laws of Physics makes it much stronger.

> I prefer to start there and derive e.g. Lorentz factors than start with the mathy stuff.

That’s how Einstein himself explained it (with trains and stuff, but still) and it makes a lot of sense to me. Much more than the professor who did start with the Lorentz transform and then lost everyone after 20 minutes of maths.

Re: Entropy: A little understood concept in physics [video]

#133

Post by kuchenbecker »

guga42k wrote: post #98
javajosh wrote: post #38

Entropy only made sense when I learned it from the perspective of statistical thermodynamics. It's a very programmerly understanding, IMHO, and it's quite intuitive. EXCEPT that the language used is ridiculous: grand canonical ensemble indeed! Anyway, the idea that a system can be in some number of specific states, and that equilibrium is that unique situation where the number of possible specific states is at its ma…

If somebody needs to build an intuition about entropy he could think about simple problem. You are given insulated cylinder with a barrier in the middle. Left side of the cylinder filled with ideal gas A, and the right side filled with gas B. If given a particle one can distinguish A from B. The pressure and temperature on both sides are the same. Then you remove the barrier and gases mix. Question: how much work you…

Enter Maxwell's demon as a completely valid solution to this problem showing you can decrease entropy within that system (but you need to exclude the demon from the system).

Re: Entropy: A little understood concept in physics [video]

#134

Post by cubefox »

rssoconnor wrote: post #61

While this is a reasonable historical explanation of entropy, and explains that we don't gain net energy from the sun, it still misses the mark on what entropy is now known to be. Entropy isn't a property of an object, or a system or things in physics. Entropy is a property of our _description_ of systems. More precisely it is a measure of how poorly a given specification of a physical system is, i.e. given descripti…

Hossenfelder says the same in her entropy video. A really interesting hypothesis.

Re: Entropy: A little understood concept in physics [video]

#135

Post by kergonath »

cubefox wrote: post #109
Balgair wrote: post #87

Earlier quoted context omitted.

To be suuuuuuper pedantic here: Relativity tells us that time is a dimension, one that is a bit unique. In that it has a constant attached to it. So, the 3 dimensions you're used to are just normal, they have no constants. (x,y,z) Meters of x are meters of z and meters of y. Relativity (and I'm really simplifying a lot by just saying 'relativity'), well relativity comes along as says that time is also a dimension, ju…

The last sentence surprised me.

And yet… If you want to get properly depressed, there is a lot we don’t understand about gravity, either.

Re: Entropy: A little understood concept in physics [video]

#136

Post by kergonath »

TexanFeller wrote: post #19

I recently enjoyed this presentation by Sean Carroll that touches on definitional and philosophical issues with entropy. The talk made me feel less stupid for not feeling entirely comfortable with how entropy was explained to me before. Turns out there are a few different ways to define and quantify entropy that are used in different contexts and they each have some unresolved philosophical issues. "Can you get time…

I would recommend reading some Carlo Rovelli, it sounds like something you might like.

Re: Entropy: A little understood concept in physics [video]

#138

Post by nobody9999 »

Timon3 wrote: post #128
spuz wrote: post #124

Earlier quoted context omitted.

When you say "how much entropy is left?" you make it sound like a quantity that is decreasing, not increasing. That seems incorrect, no?

That's why I specified that it's meant as "how much can entropy still increase between the current state and the highest entropy state". If you can't read it in that sense, either ignore the shorter question or read it as "how much increase of entropy left". Or maybe "how much entropy left until max".

>"how much entropy left until max".

10^106 years[0] worth or so.

[0] https://en.wikipedia.org/wiki/Heat_death_of_the_universe

Re: Entropy: A little understood concept in physics [video]

#139

Post by kergonath »

credit_guy wrote: post #77
consilient wrote: post #69

Earlier quoted context omitted.

It's a fact. See for instance https://arxiv.org/pdf/0907.0659.pdf

It's more like an opinion. Of one particular guy who has a Ph.D. in Physics. But there are many, and there is no consensus overall. Here's the relevant quote from wikipedia [1] Recent work has cast some doubt on the heat death hypothesis and the applicability of any simple thermodynamic model to the universe in general. Although entropy does increase in the model of an expanding universe, the maximum possible entropy…

This is quite different from what you were saying about gravity reducing entropy, though. And I know quite well that having a PhD in Physics does not make someone right, but then quoting Wikipedia in such an argument is really not great.
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