
Entropy in the Universe - chmaynard
https://johncarlosbaez.wordpress.com/2020/01/25/entropy-in-the-universe/
======
foxes
Entropy is not disorder. It is

S ~ log #{microstates}

For hn readers, microstates you can think of as a configuration of a system,
so its counting number of configurations. Disorder is not well defined. Stop
using the word disorder with entropy. A bunch of more complicated elements
have more configurations than just pure H.

~~~
spectramax
This may be correct but its useless to make any engineering sense out of it.
For engineering and practical intuition, use the following definition that we
used at an Ivy-league college in US:

[https://en.wikipedia.org/wiki/Entropy_(energy_dispersal)](https://en.wikipedia.org/wiki/Entropy_\(energy_dispersal\))

Quoting the article:

> Even though courses emphasised microstates and energy levels, most students
> could not get beyond simplistic notions of randomness or disorder. Many of
> those who learned by practising calculations did not understand well the
> intrinsic meanings of equations, and there was a need for qualitative
> explanations of thermodynamic relationships.

~~~
NotSammyHagar
Trust the formula you used at an ivy league college? Come on, that's a
laughable call to some unknown expert! I don't care if your ideas come from an
ivy league college. I care about the ideas and their quality.

I distrust people that think because something came from a prestigious school
I should trust them.

~~~
spectramax
That's extreme and I am sorry you feel that way. I didn't realize that
mentioning Ivy-League college would be seen as ego-inflating thing. I simply
wanted to say that it was used in a college level class. I definitely agree,
truth matters and not the source.

~~~
NotSammyHagar
Thanks for your civilized reply to my semi-rant ;-)

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_Microft
Since this is interesting and nobody has commented yet, let me say as a side
note that I like how Mr. Baez emphasized the difference in orders of
magnitudes here:

 _The neutrinos from the Big Bang also carry about 10^90 bits — a bit less
than the photons. The gravitons carry_ much less, _about 10^88 bits._

A difference in exponents of 2 might not look like a lot but one can almost
ignore any of the earlier numbers once a larger exponent appears.

------
acqq
From the comments of the author:

"You can puzzle your group by asking whether entropy increases as clouds of
gas and dust in the early Universe collapse and form stars and planets. If it
doesn’t increase, this process violates the Second Law. So _it must increase.
But why is it increasing? It seems that order is increasing… so doesn’t that
mean entropy is decreasing?_

If someone can’t answer this puzzle correctly, they don’t understand how
thermodynamics works in our Universe."

So, who knows the right answer to the puzzle: where and how the increase of
the entropy happens? (I also suggest that those who really know should wait
for some hours... I'd first actually like to see some wrong answers which try
to be right but don't manage to be -- just to get some better picture of the
readers here -- I don't know if this will work).

~~~
c1ccccc1
According to Baez, all the matter fields and radiation fields were in
equilibrium at the big bang, so by elimination the answer has to have
something to do with gravity (which is, after all, the force driving the
formation of the gas clouds, stars, etc). So here's my guess: Evenly
distributed matter has gravitational potential energy. By collapsing, a cloud
of gas can convert this into thermal energy, raising its entropy. This
compensates for the fact that the cloud's volume has reduced, which would
reduce entropy on its own.

How did I do? :)

~~~
c1ccccc1
Here's another thing: I remember learning that all the nice usable energy we
get from the sun is due to the fact that the big bang produced lots of
hydrogen, but fusing four hydrogens into a helium yields a bunch of energy.
(And you can still get some energy out by fusing further, all the way down to
iron.) So if matter and radiation were at equilibrium at the big bang like
Baez says, why wasn't more helium (and heavier elements down to iron) formed,
leaving nothing for the stars to burn?

I think the answer has to be that the temperature was incredibly high at the
time of the big bang: At really high temperatures, it's more favourable for
nucleons to be flying around separate from each other as protons. At lower
temperatures, it starts being better for them to be things like helium and
carbon, and at lower temperatures still, iron. So if the universe's
temperature had lowered very very gradually, we would be stuck with nothing
but iron at this point, leaving nothing for the stars to burn. So it seems we
owe our sunlight, in some sense, to the fact that the universe cooled fairly
quickly?

~~~
acqq
> At lower temperatures, it starts being better for them to be things like
> helium and carbon, and at lower temperatures still, iron.

It actually doesn’t work that way: we don’t expect that the whole universe
will ever be all iron.

------
sq_
I find it interesting that he mentions gravitons as a definite piece of
particle physics, on the same order as photons, etc.

I was under the impression that gravitons were simply one part of one possible
conception of quantum gravity, and that we were definitely unsure about
whether or not they exist.

~~~
auntienomen
Gravitons are speculative in that they haven't been observed, but they're
almost certainly the correct language for describing mild quantum corrections
to general relativity. Simple scaling arguments show that _any_ theory of
quantum gravity that reduces to general relativity at low energies passes
through a regime where it can be described by gravitons. Whether gravitons are
fundamental objects in quantum gravity is irrelevant for the kind of state
counting Baez is talking about.

~~~
sq_
Thanks for the explanation!

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gab007
That zoom-able image is amazing. Sometimes we forget how big the universe is.
And that's just an image of the Milky Way.

~~~
monkeydust
Yea that also amazed me, aside from the science a great way to put things into
perspective when you are struggling with something.

~~~
K0SM0S
I honestly think this is what I love in sci-fi, some space opera most notably:
whereas most stories are usually told in terms of space and time, sci-fi
sometimes speaks in terms of scale, of orders of magnitudes — because space (I
mean the cosmos, not the dimensions) is a natural playground for that.

Sometimes, when I ponder the discrepancy between quantum, or whatever's
smallest, perhaps strings 'below' (in scale) or 'within' (physically), and the
largest relativistic objects like "dark attractors" and meta-galactic
structures (streams of 'dark matter', light nodes, etc)...

 _It almost seems like scale is but another kind of dimension in and of
itself._

Like you've got spacetime at our human-ish scale (roughly 10 orders of
magnitude smaller to larger around our size), and that spacetime behaves in a
certain way, that probably Einstein describes accurately. Then you've got
spacetime at smaller scales, below "quantum uncertainty" if we must place a
limit, and there spacetime behaves in a much different way, the picture is
_very, very different_.

And so there could be yet another spacetime at a higher threshold of scale,
and intuition tells me it may be at the galacatic level already, because dark
matter, rotation discrepancy, and those weird supermassive blackholes; the
flow of galaxies, the meta-structure we see. Dark energy, if it's real, also
begins at that scale (doesn't break galaxies apart, but makes holes bigger
between them, the non-correlated enough sets).

Going from this idea to postulating disjointed (1+3) spacetimes forming, in
effect in this example, 3x4=12 dimensions to explore mathematically, let alone
physically, is a stretch which I'm certainly not willing to make (although I
think that's what some string theories must reduce to, somehow, maybe reduced
to a single unified dimension of time maybe?); no really this doesn't make
sense in my head. It's pretty but nonsensical, like art I guess. Worth the
sci-fi, not the studies.

But the feeling, the intuition really is that, and conveyed by these extreme
zooming animations: a different scale means totally different phenomena, and
no macro-system can be reduced to its parts in that view, nor can it be
deduced from its parts (I mean, we can't even solve the 3-body problem, that's
harsh on "scaling continuity" or smoothness, on the "linearity" of scales so
to speak). A "basic" intuition I guess is that the 4 fundamental interactions
have ranges, and kick in or out depending on a particular scale of the system.

Scales are obsessing me.

------
Zamicol
The total entropy of all the stars, 10^82, is 2^270

I love this, because the useful 256 bit string (2^256) is only 43 characters
long in base64, and to get to 2^270, it is only 45 base64 characters.

I was just playing with this on Thursday, January 9th 2020 because I wanted to
predict how many bits I needed to address every atom in the observable
universe, which also just so happens to be 10^82. I use this example to depict
the power of cryptography.

------
badrabbit
I thought you 0 kelvin was the lowest possible value on the kelvin scale. How
come this post mention negative kelvin values? Is it not temperature?

~~~
c1ccccc1
Interestingly, there actually is such a concept as negative temperature. It
applies to systems that have a maximum allowable energy as well as a minimum
one. Such systems must be isolated from the outside world, and set up
carefully, but it is possible to actually get negative temperatures.
[https://en.wikipedia.org/wiki/Negative_temperature](https://en.wikipedia.org/wiki/Negative_temperature)

~~~
mnemonicsloth
I remember that from a class on lasers I took when I was an undergrad. All the
physicists thought it was really cool and significant and argued about who was
the first to see it coming. The engineers were totally unfazed. Whatever it
takes to get the project working. And I was a would-be mathematician, so I
asked whether temperature could be imaginary.

~~~
Stupulous
Did you get a no or something more interesting?

~~~
Stupulous
(I am asking for elaboration on what physicists and engineers have to say
about imaginary temperatures if they didn't flatly dismiss the concept. I
recognize it may read as snarky, but I really was just aiming for clever.)

~~~
mnemonicsloth
No problem.

Basically the physicists thought I was winding them up and the engineers,
notably including the prof, thought my priorities were all screwed up. I
really did want to know the answer, but in retrospect they were both a little
bit right.

------
ShamelessC
The article got me thinking about the universe's mass distribution.

Is the number of galaxies at any edge of our observable universe similar to
the number of galaxies in our observable universe?

tl;dr does the (entire) universe have a uniform distribution of mass?

Now, I'm aware that the universe has always been expanding from every point in
all directions since the singularity before the big bang. This means that
effectively 1.) every point in the universe can make the claim of being "at
the center" 2.) none of them can 3.) the question is meaningless

Which of those answers you find satisfying is more of a philosophical question
which I'm not concerned with for this question.

My question is this - if we were to somehow reach the edge of our universe
instantaneously, simply to observe what things looked like from that point of
view, would we find that there's roughly the same distribution of
stars/galaxies/matter in all directions?

I'm aware that questions with the premise that FTL/instantaneous travel is
possible tend to be "unknowable" to a degree.

Perhaps another way of wording it would be - do we expect the amount of matter
to be of a uniform distribution in every spot in the universe? Is it
impossible to know? Or can we use models of the big bang theory to predict
that the distribution is roughly equal everywhere?

 __Some philosophical implications that are curious but are probably
scientifically meaningless: __

If there 's reason to believe that there is a uniform distribution of mass in
all observable universe, and the universe is infinite, does that imply there
are an infinite number of galaxies, all of which are inaccessible if they're
outside our observable universe?

If so, is out ever appropriate to consider these infinite observable universes
as essentially a multi-verse, containing many combinations of observable
universes, one of which is statistically likely to be similar to our own
observable universe?

As always with questions regarding the size of the universe, it's center, etc.
I suspect the answer will be "it's a meaningless question because we can never
know", but I'm hoping that research into the big bang itself could provide
some evidence.

Please let me know if I haven't used proper terms or am making a critical
mistake in my current understanding of astronomy.

~~~
aeternum
The way I understand it is that there is not really an edge. Spacetime itself
is expanding, the best analogy I've heard is consider a spherical balloon that
is being inflated. We exist as two dimensional beings on that balloon. No
matter where you put two dots on that balloon, they will move further apart.

You could also draw a circle on the surface of that balloon which represents
the furthest points that we can reach if we traveled at the speed of light.
Nothing outside that circle would be observable to us, so it could be argued
that it doesn't exist.

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drskaugen
The image is awesome!

