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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]

#81

Post by pcwelder »

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 all the particles' velocities reversed. Then this universe's entropy would decrease.

So you are equally like to select both the universe and hence the original assumption of increasing entropy is wrong.

Discarding quantum properties of the particles, is it then fair to say that time's direction is unrelated to whether entropy increases or decreases?

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

#82

Post by [deleted] »

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…

[deleted]

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

#83

Post by dist-epoch »

evouga wrote: post #66
dist-epoch wrote: post #45

Sabine Hossenfelder also had a video recently on entropy: > I don't believe the 2nd law of thermodynamics. https://www.youtube.com/watch?v=89Mq6gmPo0s

What I really like about this explanation is that it highlights the fact that entropy is not a natural property of the physics system: entropy is only defined with respect to some coarse-graining operation applied by an imperfect observer of the system. So as Sabine points out it seems we should really be talking about multiple different entropies, each of which corresponds to a different mechanism for coarse-grainin…

> mass-spring system is periodic

I don't pretend to understand this stuff, but wouldn't a real mass-spring system slowly stop, due to friction, air resistance, heat dissipation, ...? So a real system wouldn't be periodic.

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

#84

Post by canjobear »

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 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; if it has high entropy then the entropy will probably fluctuate up and down statistically.

> Let's assume that universe's entropy would increase. Consider another universe exactly the same as current universe, but all the particles' velocities reversed. Then this universe's entropy would decrease.

The key is that you're exponentially unlikely to find yourself in a universe where all the particles' velocities are reversed. See this: https://en.wikipedia.org/wiki/Fluctuation_theorem

The probability that a system randomly evolves in a way that reduces entropy is very very small.

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

#85

Post by rixed »

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…

If by "random universe" you mean a universe in which all states of every particule are random, then my understanding is that we would probably conclude that entropy is neither increasing nor decreasing. Our universe is not random. We could spot local phenomenons where entropy is clearly going in one direction. We assume everywhere the entropy would go in the same direction (increasing), and deduce from this hypothesis that the universe started with a very low entropy.

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

#87

Post by Balgair »

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.

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, just with the constant of 'c' attached (the speed of light). That way you can convert seconds into meters.

(x,y,z,ct) not just (x,y,z,t).

So now the time dimension is much larger than the spatial dimensions. About 300,000,000 times larger, a third-ish of a billion. So a meter of x is ~1/3 of a billion meters of time.

Now, there is a lot more about relativity, like, just tons. And I skipped most of it. And trying to just say that time is a simple little conversion away from meters is just wrong. And how that all relates to entropy is a mess that we really haven't figured out yet.

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

#88

Post by danq__ »

quickthrower2 wrote: post #62
floatrock wrote: post #58

The part that was new to me was the bit about how a space full of life tends toward more entropy faster than the same amount of space without life. Like the best ideas, it’s simple and makes sense if you think about it, but it’s still a really interesting framing that the complex machinery of life is really just the most efficient “entropy converter”. If there’s something about the arrow of time that speeds towards t…

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?

Entropy is an orthogonal to Einstein's laws of physics.

Both newtons and Einstein's description of the universe are time reversable. The physics are valid independent of the direction of time.

Entropy is literally a separate observable phenomenon with it's own set of axioms.

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

#89

Post by Balgair »

SanderNL wrote: post #56
hospitalJail wrote: post #17

Last night me and my wife were deciding if we should watch The Witcher or this video. I decided I didn't have the brainpower/mental capacity to think about The Witcher and that this video on Entropy would be easier to digest.

Not sure if you are humble bragging, but I agree The Witcher is more demanding. Layers of meaning, emotions, allegory, subtext. It’s no Shakespeare, but physics and math are simple in comparison especially in the wonderfully produced and easily digestible format of Veritasium.

I mean, the author has a doctoral degree in science communication. It's his job and the point of the channel to try to make things easy to understand.

The opposite is true with fiction. You're intentionally trying to have the audience make connections themselves, like a Sherlock story or the Great Gatsby. The point is in the discovery by the viewer.

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

#90

Post by consilient »

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…

That's discussing the effects of metric expansion, which are not relevant for gravitationally bound systems. It also doesn't claim the second law of thermodynamics fails. On the contrary,

> entropy does increase in the model of an expanding universe

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