## Universe Has 100x More Entropy Than We Thought 304

Posted
by
kdawson

from the clean-up-your-room-that'll-help dept.

from the clean-up-your-room-that'll-help dept.

eldavojohn writes

*"Previous estimates are now thought to skimp on the entropy of the observable universe. The researchers contend that super-massive black holes are the largest contributor of entropy. Since they contribute two orders of magnitude more than previously thought, the total of all the observable universe is correspondingly higher. The paper highlights (in gruesome detail) new issues that arise with these new calculations — like estimating us a little bit closer to heat death (moving entropy totals from 10^102 to 10^104 out of a maximum of 10^122)."*
## Heat Death (Score:5, Informative)

Here's a link [wikipedia.org] for anyone curious about the Heat Death of the Universe concept

## Re:Excellent! (Score:2, Informative)

Construction of the cannon ain't helping the situation any!

## Re:Fourth Law (Score:2, Informative)

Fourth Law of Thermdynamics [wikipedia.org]: There's always more entropy then you think there is, even when you take into account the Fourth Law of Thermodynamics.

I still wouldn't worry about the heat death of the universe [wikia.com], though, unlike those in the aforementioned link.

## Re:Black holes contribute to entropy ? (Score:5, Informative)

## Re:Black holes contribute to entropy ? (Score:5, Informative)

"Order" and "disorder" are human perceptions, not states of matter and energy. Sometimes we perceive more order when there are clear differences in energy states, sometimes we perceive less

To you, which is more ordered: a bowl of cherries next to a glass of water, or a completely smooth blend of all of them? The latter is more entropic. In the case of the room, replace the garbage bin with an incinerator, and the "empty" room (plus the stuff that used to be in it) is now in a more entropic state. The fact that you personally find it tidier isn't relevant. Assuming that you might have actually needed some of the stuff that we just burned, too, you might find it a rather poor solution to the problem of a messy room.

## Re:Black holes contribute to entropy ? (Score:3, Informative)

Three very separate questions

Could this Hawking Radiation be harnessed?

Sure, although there are some minor engineering challenges.

That is, can it be considered as free energy

Not "free energy" because you're converting mass into energy.

as opposed to entropy?

On an entire system wide basis, entropy times temperature equals energy, so "as opposed to" is a weird phrase to use. Lots of energy emitted, at a low enough temperature, means the entropy must be high.

http://en.wikipedia.org/wiki/Entropy [wikipedia.org]

## Re:Black holes contribute to entropy ? (Score:3, Informative)

Actually, the Hawking radiation doesn't come from inside the black hole.

When a particle/anti-particle pair tunnels into existence at the edge of a black hole, for some reason, the anti-particle tunnels into existence inside the black hole and is immediately annihilated by an already existing particle in the black hole (thus reducing the mass of the black hole by one particle's worth). The particle that tunneled into existence outside the black hole spins off and is referred to as "Hawking Radiation".

The wikipedia article is excellent.

## Re:Black holes contribute to entropy ? (Score:3, Informative)

Everything that comes into a black hole comes back out eventually via Hawking Radiation. It goes in as a star or a chicken or a pistachio and comes out as random energy, which is a pretty clear increase in entropy.

Exactly. Another way to think about it is this: the speed of light is the maximum speed information can travel. Since the escape velocity of a black hole is greater than the speed of light, information cannot escape either.

Because information cannot escape, it means you cannot infer what went into a black hole by looking at what comes out. (Note: information being "destroyed" in a black hole doesn't prevent you from e.g. keeping a record of what you tossed into a black hole. Once it goes inside, it won't somehow destroy your records, as I used to believe was the implication.)

So the Hawking radiation must be completely uncorrelated with what went into the black hole, except that the rate of radiation emission must depend on the temperature of the black hole (which would increase from turning objects into heat). This in turn means that to give the least information possible, Hawking radiation must be the lowest quality (highest entropy) form of energy in the universe.

And, if that doesn't make sense, just keep in mind that destruction of information is thermodynamically irreversible, so a black hole must increase entropy that way.

## Re:Heat Death (Score:3, Informative)

They are related but not the same, they are describing different facets of the same issue. "Heat Death" refers to the ultimate state of entropy - that all the energy in the universe had moved from a high concentration to a low concentration, which results in a perfect equilibrium.

"Cold Death" happens long before heat death, it referse to the point when the expansion of the universe spreads the stars so far apart that new stars can not be re-formed from the gasses of dead stars/systems. Once all stars have consumed all their fuel and no new stars can be formed, life in the universe is no longer possible. That is the cold death of the universe.

Basically, heat death will happen hundreds of billions of years after cold death happens - the Universe will have reached a completely uniform 0 degrees kelvin and it will be completely dead.

## Re:Black holes contribute to entropy ? (Score:3, Informative)

Interestingly, this is wrong, and Stephen Hawking lost a bet over it.

The Second Law is actually very tightly coupled to Shannon-style Information Entropy [lesswrong.com]: if you knew enough information (as a

fait accompli) about the current state of a system at equilibrium, you could successfully build a Maxwell's Demon that used that information to separate the system into hot and cold reservoirs, allowing you to cancel out the entropy of the system with your information's "negentropy" (as it's called).Learningthe information in the first place causes your information entropy (i.e. correlation with the system, negentropy) to increase, whichby physical necessityalso causes your thermodynamic entropy to increase in tandem. This is why a Maxwell's Demon doesn't work: the entropy undone in using the information is always less than (or, in a perfect system, equal to) the entropy done while learning it. (If blind faith provided non-tautological and accurate information about the universe with better-than-random chance, then you could build a Maxwell's Demon that broke the Second Law -- and since the Second Law is inviolable, it must be the case that blind faith tells you nothing... except possibly tautologies if your brain uses reversible computing. If you think hard enough about it, it also disproves substance dualism.)In the specific case of information entropy and black holes, it turns out that the information never crosses the event horizon, and thus never has to break the speed of light limits when leaving it. As modern physics and Stephen Hawking have both discovered, all the entropy of a black hole's formative mass/energy is encoded in two dimensions as ripples in the event horizon of the black hole. Black holes have also been discovered to be maximum-entropy objects in modern physics, containing the largest amount of entropy physically possible for the volume of space enclosed by the event horizon. (This has interesting implications on the nature of reality -- look up the anti-de Sitter/CFT correspondence for all sorts of 2D/3D weirdness, like the universe being equivalent to a 2D hologram.) When a quantum of Hawking Radiation emerges from the event horizon, it carries off precisely the amount of entropy equal to the entropy carried by the change in the surface area of the event horizon when expelling the quantum, thus maintaining the invariant that the black hole is a maximum-entropy object. Because "information entropy" is another way of saying "too random to predict ahead of time" -- that's what information is, by Shannon's definition of it -- the radiation looks quite random indeed. But that doesn't mean it's uncorrelated with the history of the black hole.