Dreaming The Dream
[...] What worries cosmologists is not so much the possibility that we might all be Boltzmann brains dreaming unreal dreams about the world around us. Nobody seriously believes that. Cosmologists build theories about the Universe and how it evolves. In other areas of science, when you have come up with a theory (say about the behaviour of mice) you then set up an experiment (observe many mice) to see if your theory fits with observations. If it does, good, if not, you need to improve the theory, or throw it out. But since it's impossible to replicate our Universe in a lab, cosmologists can't do this. They need to come up with other ways of finding out whether a theory is plausible.
This is where Boltzmann brains come in. Once you have constructed a theory about the Universe you can calculate the probability that certain types of observers (including Boltzmann brains) come into existence. If the calculation suggests that we are overwhelmingly likely to be Boltzmann brains, rather than the good and honest observers we know we are, then that's an indication that something about the theory, or the assumptions you made in your probability calculation, is wrong. [...]
[...] What worries cosmologists is not so much the possibility that we might all be Boltzmann brains dreaming unreal dreams about the world around us. Nobody seriously believes that. Cosmologists build theories about the Universe and how it evolves. In other areas of science, when you have come up with a theory (say about the behaviour of mice) you then set up an experiment (observe many mice) to see if your theory fits with observations. If it does, good, if not, you need to improve the theory, or throw it out. But since it's impossible to replicate our Universe in a lab, cosmologists can't do this. They need to come up with other ways of finding out whether a theory is plausible.
This is where Boltzmann brains come in. Once you have constructed a theory about the Universe you can calculate the probability that certain types of observers (including Boltzmann brains) come into existence. If the calculation suggests that we are overwhelmingly likely to be Boltzmann brains, rather than the good and honest observers we know we are, then that's an indication that something about the theory, or the assumptions you made in your probability calculation, is wrong. [...]