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Comment by sghiassy | Hacker News Reader

Not really - look up “heat death” of the universe.

Heat death doesn’t mean “no heat” or that energy has depleted. It just means that energy is fully dispersed. All kg the hear exists, it’s just that no one place has any more than anywhere else, and so there is no longer any transfer of energy.

> so there is no longer any transfer of energy.

Ah, but there is.

The Second Law is a statistical law, not an absolute law. On long enough timescales, low-probability fluctuations in local entropy will allow for energy transfer. These fluctuations will also allow for the formation of structures. (Boltzmann himself was of the opinion that the low-entropy universe emerged from a higher-entropy background state. And indeed there's nothing in physics to rule out the emergence of Boltzmann Brains and even Boltzmann Galaxies from homogeneous and maximally entropic universes in "heat death." This is a philosophical problem of the highest order, because it implies that we're not necessarily going from "less likely to more likely states" as the video implies, but rather from a relatively deterministic state to a probabilistic state.)

Oh interesting, I didn’t know that

The "heat death" of the universe is a concept that deserves to die. The second principle of thermodynamics is true only if you ignore gravity. In the presence of gravity, systems tend to go towards lower entropy, just see how a planetary system can form out of a gas cloud.

> In the presence of gravity, systems tend to go towards lower entropy, just see how a planetary system can form out of a gas cloud.

This isn't correct: the entropy (and energy) of the gas cloud goes decreases as it collapses, but the entropy of its surroundings increases faster as it radiates.

Is that a fact? Or just a hypothetical way that could save the second principle?

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 rises much more rapidly, moving the universe further from the heat death with time, not closer. This results in an "entropy gap" pushing the system further away from the posited heat death equilibrium. Other complicating factors, such as the energy density of the vacuum and macroscopic quantum effects, are difficult to reconcile with thermodynamical models, making any predictions of large-scale thermodynamics extremely difficult.

[1] https://en.wikipedia.org/wiki/Entropy#Cosmology

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

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.

I think of it as a death cult. Thinking we know all that there is to know about the universe to the point where we can declare with 100% certainty that any particular thing will happen is not scientific.

Really? We are far from really understanding gravity, but I can very confidently tell you that if I kick a ball its trajectory will be a parabola (roughly, as a first order approximation and ignoring things like friction, which we can also calculate to a decent approximation). We can say where it will fall and give some confidence interval depending on the conditions and such. There is nothing unscientific about it.

Thermodynamics is not magic. In the same way that we can predict the evolution of climate without knowing where every single cloud will be, we can make statements about the evolution of large systems even though our knowledge of their state is imperfect. Again, nothing unscientific about it.

If you read what I said, I said nothing about predictions. Predictions are fine. Stating things as certainties, not so much.

It’s not a very useful point. Again, I can say with absolute certainty that a ball will follow a parabola and that an ice cube in a glass will melt.

You can say what you like. Science is not about certainties. You can only control experiments to a particular degree and have no control of confounding factors which might interfere with your experiments. Do you really want to compare the totality of all universal processes to such trivial examples? I find it absurd.

You did not qualify the complexity below which it is acceptable to be certain of something so this is unlikely to be productive. You said that certainty was impossible and I gave you examples of certain things. Besides, both a ball and a glass of water are insanely complex systems. I am not sure we have the same definition of science, to be honest.

Not my area of expertise, but in the video at least, they indicate that black holes have very high entropy. So if we imagine gravity eventually pulling all those planetary systems together into some number of black holes, isn't gravity indeed pulling the system towards a high entropy state?

The video actually directly addresses the gas cloud question, saying basically that a gas cloud is actually a highly improbable distribution of matter, whereas the eventual planetary system is much more probable. The claim being, that trend towards expected state is entropy increasing.

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