Max Tegmark publishes a preprint of a paper arguing from physical principles that consciousness is “what information processing feels like from the inside,” a position I've previously articulated on lesswrong. It's a very physics-rich paper, but here's the most accessable description I was able to find within it:
If we understood consciousness as a physical phenomenon, we could in principle answer all of these questions [about consciousness] by studying the equations of physics: we could identify all conscious entities in any physical system, and calculate what they would perceive. However, this approach is typically not pursued by physicists, with the argument that we do not understand consciousness well enough.
In this paper, I argue that recent progress in neuroscience has fundamentally changed this situation, and that we physicists can no longer blame neuroscientists for our own lack of progress. I have long contended that consciousness is the way information feels when being processed in certain complex ways, i.e., that it corresponds to certain complex patterns in spacetime that obey the same laws of physics as other complex systems, with no "secret sauce" required.
The whole paper is very rich, and worth a read.
Because it does. Why do charged particles attract or repel? Why do some particles experience mass? At some point the answer is simply "because that's how the universe works."
We know consciousness exists, as we each have first hand evidence. If we want to believe that we live in a reducible universe, then there must be some reduction bringing consciousness down to a most basic physical process. At some point that reductive explanation ends with a very unsatisfying "because that's just how the universe works."
But I would be very suspicious of any model which reached that level before arriving at the level of fundamental particles and their interactions. Why? Because every other phenomenon in the universe also reduces down to that level, so why should we expect the explanation of consciousness to be different?