Why do we have consciousness? There are a lot of ideas, including a number of theories related to the highly speculative hypothesis of quantum consciousness, but the short answer is that we don't know yet.
But when physicists try to consider our conscious experience from the perspective of thermodynamics, they come to an intriguing and perplexing conclusion: that the most likely way to explain our consciousness is as a consciousness that spontaneously manifests, with a complete set of memories, surrounded by disorder, and then it pops out of existence. This bizarre and depression situation is called a Boltzmann brain, and physicists occasionally try to spend a bit of time trying to explain why this isn't actually the case.
One reason that this matters is that an often-unspoken assumption of physics is that we humans are what is known as "typical observers" of the universe. If we are somehow not typical - that is to say, if we are vastly outnumbered in the universe by disembodied consciousnesses - then we would likely have to begin questioning pretty much all other assumptions.
No scientist (that I know of) seriously suggests that we are Boltzmann brains, or that Boltzmann brains actually fill the universe, but it's still something that would be better if it could be explained ... or explained away. And now, perhaps, it can, thanks to string theory. Basically, the argument put forth is that when you consider the laws governing the region of the multiverse that we live in, the fact that it has a very clear thermodynamic arrow of time would suggest that Boltzmann brains are unlikely to form, according to research recently published in the journal Physics Review D.
It should be obvious, but this is a purely theoretical conclusion, based on a mathematical analysis using models from string theory. There's no experimental or observational evidence to suggest that Boltzmann brains actually exist as real entities (whatever "real" means in this context), or that string theory is a correct theory to describe our universe for that matter. (Although have already suggested that string theory's ability to explain away Boltzmann brains counts as a success of the theory.)
However, for those who are deeply concerned about the puzzle presented by the unconfirmed Boltzmann brains, this provides an equally unconfirmed reason for hope. So sleep well, citizens of Earth, because the Boltzmann brains are not coming for us.
- Physics Review D - Multivacuum initial conditions and the arrow of time
- New Scientist - String theory may limit space brain threat
- i09 - Humanity Will Never Be Overrun By Spontaneously Forming Space Brains
- Peter Woit's Not Even Wrong blog - Hard Evidence for the Multiverse Found, but String Theory Limits the Space Brain Threat