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0xDEADBEEF
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Could someone give me a quick explanation what Boltzmann brains are? Does it have something to do with the landscape?
Well, I think what we're missing is something interesting about the physics that produces universes like our own that causes real brains to vastly outnumber such "boltmann brains". One proposal for how this may be so is given by this paper by Sean Carroll and Jennifer Chen:Haelfix said:By assumption, if the universe is infinitely large and with infinite time (like in some versions of the multiverse scenario) you are guarenteed to produce exactly those types of fluctuations. Worse, the observed low entropy of the universe is vanishingly small in the set of random ensembles of possible universes relative to having a single entity spawned with fake memories.
ANyway, one of the problems with the Boltzmann brain argument isn't that we haven't observed nonsense *yet*, its that it makes a prediction for what we should see identically one unit of time in the future. Each additional second we observe the world, adds astronomical probabilities against the hypothesis. But that's the problem in the first place -- statistics and how to make sense of them in these infinite random cases.
Except that one can use the same derivation of the second law of thermodynamics that proves that entropy must increase in the future to equally prove that entropy must increase in the past. You just can't derive a time-asymmetric law from underlying laws of physics that are perfectly symmetric in time.Haelfix said:I didn't like that paper, as its far fetched and going on about a nonissue imo.
Actually, I really don't understand the conceptual problem in the first place or why people are still harping about the arrow of time and what it can possibly solve. As far as I can see, this has been settled for nearly a hundred years and the answer was given by the second law - period! This is the unique arrow of time and we aren't reversible.
You can't derive an arrow of time by putting it in by hand! The only reason why we derive the second law of thermodynamics going forward in time is that that is the direction that accords with observation. That assumption is still put in by hand, though, and does not stem from the underlying laws.Haelfix said:I don't agree at all. This is explained at length in the Feynman lectures.
It is true, that if you naively send t --> -t, you will still see an entropy increase into the past, but the point is you cannot do this operation b/c of friction and dissapative effects.
It's not crazy at all. It just means that the arrow of time is dependent upon the initial conditions. It's counterintuitive, to be sure. But the physics work out identically for an arrow of time like the one we experience, or one in the opposite direction.Haelfix said:The obvious problem with that idea is the following.
If there was some special property of the early universe/cosmology that allowed us to evolve from a low entropy state to a large entropy state, then there should be another universe that has the opposite behaviour, at least in principle. So they would by hypothesis go from a high entropy state to a low entropy state. Or in other words, an observer in that universe would see humpty dumpty spontaneously coming back together again. He would see large scale structures flying apart, rather than being clumped by gravity.
But that's crazy!
That would require that the arrow of time be different in different regions of the universe, or different on different size scales. And that does appear to be disallowed.Haelfix said:The large scale behaviour of spacetime absolutely cannot change the local laws of physics (say solar system physics). Ergo the perpetual rise in entropy and the irreversability that it entails (even on human scales) cannot possibly be explained by cosmological initial conditions.
The only thing that cosmology can tell us, is in principle why is the initial condition so low relative to what it could be (eg not why z1 < z2 for any universe, but why z1 was not ~ z avg).
You still haven't grasped the fundamental point: from the way we have derived the second law of thermodynamics, watching a liquid go from blue and red to a mixed state is exactly as weird as watching it go from that mixed state back to blue and red.Haelfix said:The point is changing cosmological initial conditions or models won't change the nature of the 2nd law, nor can it define it. It simply cannot, by locality.
Even if you start with a very high entropy initial condition, whatever universe (and by universe I mean the whole thing) you are in, will simply proceed to an even higher entropy (you'll get more and more black holes) state. Thats what the 2nd law tells us. Eg, the one thing that is explicitly forbidden is to go from high to low entropy globally. It could be that we are a quasi local fluctuation in the multiverse but that same multiverse is still monotonically increasing its entropy.
Any observer, regardless of which cosmology they are in will almost never see a liquid spontaneously going from a mixed state into blue and red, whether they are in a fluctuation or not.. This perfectly local arrow of time will not change directions depending on location either, why would it.. Its identically the same everywhere we look, contrary to what a cosmological origin would indicate (where you might have various parts of the universe changing their cosmological arrows willy nilly).
No, it wasn't. As far as I can tell, Boltmann's solution depends critically upon the assumption that the correlations between velocities are uninteresting. But even if we assume that, there's the problem that if you take the "initial point" and then go backward in time, statistical mechanics says you'll get an increase in entropy backward in time.Haelfix said:Thats just a statement of Loschmidt's paradox, which was succesfully resolved eons ago by Boltzman himself (and proved conclusively in the 50s).
Because it is a fundamental impossibility to derive time-asymmetric behavior from underlying laws which are symmetric in time. People keep trying, but nobody has succeeded. The arrow of time is always put in by hand.Haelfix said:Its wrong to argue that the 2nd law is reversable and time symmetric (you can't replace t-->-t and expect things to be the same b/c the probabilities and the volume of the phase spaces are not symmetrical). So the 2nd law, together with the initial condition of the universe, defines past and future for all observers uniquely. The past is the low entropy state, and the future is the high entropy state and it can't be any other way, even in principle! I don't see why you think this is put in by hand, as its a derivable consequence and I still fail to see what the fuss is about.
Well, that's why a "big crunch" in the most naive sense, where such a reversal is required, can be considered a fundamental impossibility. Apparently the ekpyrotic scenario claims to get around this, but I am as yet unclear how this is accomplished.Haelfix said:Hawking himself made this error at one point during his career, when he incorrectly argued that a big crunch in his no boundary proposal would lead to a reversal of the thermodynamic arrow. He correctly changed that position shortly thereafter.
The Boltzmann Brain theory is a proposed explanation for the existence of intelligent conscious beings in the universe. It suggests that rather than being the product of a long and complex evolutionary process, our consciousness may be the result of a random fluctuation in the universe's energy, known as a Boltzmann Brain. This theory is highly controversial and has not been proven.
The landscape theory is a concept in theoretical physics that suggests the existence of multiple possible universes, each with its own set of physical laws and constants. In the context of Boltzmann Brains, the landscape theory suggests that there may be an infinite number of universes, some of which may randomly produce Boltzmann Brains with no need for a long evolutionary process.
No, the Boltzmann Brain theory does not provide a satisfactory explanation for the fine-tuning of the universe. The fine-tuning of physical constants and laws is still a mystery and remains a topic of debate among scientists. The existence of Boltzmann Brains does not negate the need for an explanation for the fine-tuning of the universe.
If the Boltzmann Brain theory is true, it would have significant implications for our understanding of the universe and our place in it. It would challenge many scientific theories and raise questions about the purpose and meaning of our existence. However, until more evidence is gathered, the Boltzmann Brain theory remains a speculative concept.
Currently, there is no way to conclusively test the validity of the Boltzmann Brain theory. It is a highly speculative concept that is difficult to prove or disprove. Some scientists suggest that further research into the landscape theory and studying the distribution of matter and energy in the universe could provide evidence for or against the existence of Boltzmann Brains.