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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]

#111

Post by cubefox »

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…

Another surprising thing is that physicists have not yet succeeded in reducing the ordinary notion of cause and effect to fundamental physics. Carroll has also worked on this issue.

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

#112

Post by lisper »

JumpCrisscross wrote: post #103
dahart wrote: post #57

Earlier quoted context omitted.

Why is this pithier than the video? I’m not entirely sure I see added pedagogical value. Asking the rhetorical question how can there be a shortage of energy sounds a little like someone sort-of intentionally misunderstanding what that phrase “energy shortage” means in any practical economic context. “Energy shortage” is an economics phrase, not a physics phrase. The first law of thermodynamics doesn’t suggest there…

It’s pithy, but in the way of word play. Energy, colloquially, means useful energy. The question collides the conventional and technical definitions to create the illusion of profundity.

Pithy != profound. The intent was to get people to think about the fact that the word "energy" means different things in different contexts, and that the thing that actually has value is not energy but the absense of entropy.

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

#113

Post by klabb3 »

semi-extrinsic wrote: post #75
quickthrower2 wrote: post #62

Earlier quoted context omitted.

Does Einstein’s model of time care about entropy. In other words if there are 2 regions, one where entropy is increasing at that time and one where it isn’t as much, does it affect time?

No. Entropy can be used to explain the direction of time, as a kind of symmetry breaking of all the microscopic laws that are symmetric in time. But it does not say anything about the "speed of time". Relativity does tell us the speed of time - it's the speed of light.

> Relativity does tell us the speed of time - it's the speed of light.

I forget the name of the book, but it was trying to convey intuitions about relativity (first special, then general).

It’s very easy to make the mistake of trying to understand space and time first, concepts we think we intuit, but in relativity these are somewhat higher-level concepts. Instead, start with what the most fundamental part of the theory and go from there: the speed of light is constant. Accept that first. It’s the comfort zone. You can always return safely to this point.

So, when moving to space and time, the book explained it like this: everything moves at the speed of light, at all times. It’s just that instead of x,y,z – we add t, time, as well. So for an object that’s still, all it’s movement is through the time dimension. Conversely, an object that moves incredibly fast, like a photon, already “used” it’s speed in the spatial dimensions, so it doesn’t “age” in terms of time.

This is just special relativity, but I liked this approach. It’s basically embracing the theory first instead of trying to shoehorn it into the world we have so many misconceptions about.

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

#114

Post by ko27 »

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…

Entropy is very much "real" and it exists outside of our mind. The resolution to Maxwell's demon is that knowledge of every particle's state is not free, you need to increase the system's entropy by obtaining knowledge more than you can ever eliminate by opening chamber doors.

If it only existed in our minds and not in physical reality that would mean it would be possible to construct a device that decreases global entropy on average.

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

#115

Post by Lichtso »

I think the most unintuitive even unsettling aspect of entropy is that the entropy of black holes is proportional to their surface area, not their volume [0]. That is only briefly mentioned in the video and not discussed any further.

[0] https://en.wikipedia.org/wiki/Holographic_principle#Black_ho...

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

#116

Post by winwang »

MaxRegret wrote: post #32
spuz wrote: post #27

I think the concept would be easier for me to understand if we talked about the inverse of entropy - i.e. some kind of measurement for the concentration of useful energy or "order". I think it would then be more intuitive to say that this measurement always decreases. Do we even have a word for the opposite of entropy?

Negentropy? This is a concept in information theory, but maybe also in physics.

And it's closely related to the Gibbs free energy (available energy), which decreases with increasing entropy, all other things equal.

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

#117

Post by guga42k »

ithkuil wrote: post #107
guga42k wrote: post #98

Earlier quoted context omitted.

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…

If you need to do work in order to revert to the previous state, does it imply you can extract work when going to the first to the second state? Given the scenario you just laid out it seems no work can be extracted just by letting mix two substances that are at the same temperature and pressure. But there is something about it that doesn't quite add up to my intuition of symmetry and conservation laws. Could you ple…

>If you need to do work in order to revert to the previous state, does it imply you can extract work when going to the first to the second state?

Nope. The work comes from the system coming from ordered state into unordered. Why the problem above is good for intuition because you can work out how to reverse the state. You invent semi-magical barrier which is fully transparent for particles A and reflects particles B, then you start to push such barrier from left to right up to the middle, compressing gas B (and making work!) and leave left part with gas A only, then repeat similar exercise on the right side.

>Given the scenario you just laid out it seems no work can be extracted just by letting mix two substances that are at the same temperature and pressure. But there is something about it that doesn't quite add up to my intuition of symmetry and conservation laws. Could you please elaborate more on that?

As far as I understand this asymmetry was the exact reason why entropy was introduced. Then later explained by Boltzmann via a measure of number of microscopic states.

Naturally second law of thermodynamics forbids perpetual engines.

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

#118

Post by cubefox »

canjobear wrote: post #84
pcwelder wrote: post #81

So the law of increasing entropy is not a fundamental law of the reality because it can be derived from other fundamental equations. Suppose I show you a snapshot of a random universe, would you be able to tell if the entropy of the universe is going to increase or decrease as the time progresses? Let's assume that universe's entropy would increase. Consider another universe exactly the same as current universe, but…

The expected increase in entropy can be derived from laws of mechanics plus the critical stipulation that, in the past, entropy was very low. Essentially, physical systems want to be in high-entropy states. So if you observe one to be in a very low-entropy state, then you can conclude that with high probability the future of that system will go to higher-entropy states. > Suppose I show you a snapshot of a random uni…

> > Suppose I show you a snapshot of a random universe, would you be able to tell if the entropy of the universe is going to increase or decrease as the time progresses?

> Yes, if it has low entropy then entropy will probably increase

The problem is that it probably increases in both time directions, such that the state of minimum entropy is now. As you said, we have to stipulate that the entropy in the past is low, we can't (yet?) infer it from observation. Which raises the question what justifies us making this assumption in the first place.

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

#119

Post by Lichtso »

ithkuil wrote: post #107
guga42k wrote: post #98

Earlier quoted context omitted.

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…

If you need to do work in order to revert to the previous state, does it imply you can extract work when going to the first to the second state? Given the scenario you just laid out it seems no work can be extracted just by letting mix two substances that are at the same temperature and pressure. But there is something about it that doesn't quite add up to my intuition of symmetry and conservation laws. Could you ple…

I think you can very well extract work from having a membrane and selectively let one substance mix into the other but not the other in the first [0]. It is called Osmosis [1].

[0]: https://en.wikipedia.org/wiki/Semipermeable_membrane [1]: https://en.wikipedia.org/wiki/Osmosis

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

#120

Post by guga42k »

antimora wrote: post #92

The video did not explain why the sun is a low entropy source. I found this explaining what I am sharing with you: So, the sun is a low-entropy source of energy, and Earth (and everything on it) increases that entropy as it uses and then reradiates that energy. This process is entirely consistent with the second law of thermodynamics. The relationship between light frequency and entropy comes from the fact that entro…

>The video did not explain why the sun is a low entropy source. I found this explaining what I am sharing with you:

to my best understanding, to go from high entropy state to low entropy state you need work to do. The sun is a source of energy to do the work

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