what is the Boltzmann brain?

Question

I read the wikipedia article, but I just don’t get it…

In: 37

Answer 1

A Boltzmann brain is a hypothetical brain-in-a-vat, floating in the vast emptyness of space, that came about by sheer chance. A bunch of atoms were just flying around, randomly collided and formed this brain-in-a-vat. It formed spontaneously only 1 minute ago, and in only 10s it will break apart as tenuously as it was formed. This brain contains ‘memories’ that never actually happened. This brain believes it lives in a world called earth, and that it’s currently reading something called a ‘comment’ on ‘reddit’. It has formed a theory about the universe. But all it’s theories and beliefs are wrong, as the universe it sees is fictitious. None of it is real.

The Boltzmann brain limits how much you can appeal to chance in your cosomological model. Over-relying on chance is tempting. If the universe is eternal, even the most unlikely events are bound to occur eventually. For example, suppose you have a theory for what caused the big bang. It goes something like this:

* The universe, on average, tends to towards higher entropy (more spread out and colder)
* However, changes towards lower entropy states are not impossible, just unlikely.
* The universe is eternal, so even the most unlikely events will happen eventually
* Eventually, the universe will experience another unlikely fluctuation into a low entropy state (smaller and hotter).
* Following this sudden contraction, the universe will evolve toward higher entropy again. And the cycle repeats.
* Conclusion: The big bang was a momentary fluctuation of an existing universe to a lower entropy state.

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Ok, so the big bang was a chance fluctuation of entropy? This model has a problem: Boltzmann brains.

In such a model, a Boltzmann brain would be a FAR more common occurence than an entropic fluctuation the size of the big bang. So if this model is true, then it also follows that you are most likely a Boltzmann brain. The world you see is completely fictitious, any conclusions you derived from it are pointless, and you’re about to non-exist in 3…2….1…..

So, there seems to be a limit to how much random luck you can put into your model of the universe. If you rely on it too much, you would get Boltzmann brains. And then the most likely situation is that the universe you see is not even real so any model you have is bogus.