“To probe the multiverse more deeply, we must learn how to characterize observers in entirely different regions. Despite the great variability of local physical laws, there are a few laws that operate everywhere. Gravity is a universal force. All other forces and particles, provincial details aside, fit into the general framework of quantum mechanics. The laws of thermodynamics rule the whole multiverse. The challenge is to phrase the conditions for life in this universal language.
“A central tenet of thermodynamics is the “second law,” which states that a quantity called entropy cannot decrease. Entropy reflects the number of different microscopic configurations that are available to a system. For example, there are only a few ways to stack up a brick wall. But there are many ways to arrange the same bricks into a messy pile, so the pile has more entropy. In other words, entropy measures disorder. All the second law says, then, is that things don’t tend to order themselves when left alone: Walls collapse into piles, but piles don’t assemble themselves into walls.
Some structures actually do assemble themselves, though the second law still holds. A bucket of water left outside on a cold night will contain a crystal the next morning. Diffuse particles condense into spiral galaxies. Hydrogen clouds collapse to form stars. Dust coalesces into planets. On a few planets, self-replicating organisms arise from organic molecules. But in every one of these processes, the overall entropy grows. As a system becomes ordered, radiation escapes into its surroundings where it vastly increases the overall disorder.
The second law in no way forbids orderly structures from forming so long as enough disorder is created elsewhere. But conversely, if no disorder is produced, then no order can form. Taking this one step further, I recently proposed that the production of disorder could be used as a kind of cosmic life-detector. The more entropy is created in a given region, the more likely it is that complex structures such as life are forming in it. To a physicist this idea is attractive: Unlike life, entropy is a number, and one that can be defined in every region, whatever its local laws of physics.
Sometimes, of course, a mess is just a mess. Not every entropy increase is accompanied by the formation of an ordered structure. Our life-detector, in other words, is susceptible to false alarms. To check that this does not spoil its usefulness, Roni Harnik, Graham Kribs, Gilad Perez, and I decided to identify the chief sources of entropy production in the visible universe. Amazingly, almost every one of the biggest entropy-producing processes turned out to be essential to the development of life: the formation of galaxies and the burning of stars; supernova explosions, which forged the elements we are made from; large molecules that scatter starlight. Most remarkably, the last process produced more entropy than all the others put together. This was good news. The digestion of solar power into messy thermal radiation is precisely what allowed planet Earth to play host to increasingly sophisticated life forms. Without complex molecules, entropy production would drop sharply — and human life would be impossible.
Just because our local test run went well does not mean that entropy production alone can reliably determine whether observers are present in a given region of the multiverse. But remember that a one-by-one approach would be futile; the multiverse is so large that statistical methods are more powerful. What we needed was a criterion that told us which physical laws, on average, tend to be associated with the presence of complex structures like observers. Estimating entropy production may prove to be a first, crude technique that will help us step back and study the vast canvas of the multiverse — and, ultimately, to learn whether we can discern in its intricate patterns the tiny, billion-light-year brushstroke that fills our night sky.”