Matteo Ianeselli / Wikimedia Commons
Physicists have always hoped that once we understood the fundamental laws of physics, they would make unambiguous predictions for physical quantities. We imagined that the underlying physical laws would explain why the mass of the Higgs particle must be 125 gigaelectron-volts, as was recently discovered, and not any other value, and also make predictions for new particles that are yet to be discovered. For example, we would like to predict what kind of particles make up the dark matter.
These hopes now appear to have been hopelessly naïve. Our most promising fundamental theory, string theory, does not make unique predictions. It seems to contain a vast landscape of solutions, or “vacua,” each with its own values of the observable physical constants. The vacua are all physically realized within an enormous eternally inflating multiverse.
Our problems arise because the multiverse is an infinite expanse of space and time.
Has the theory lost its mooring to observation? If the multiverse is large and diverse enough to contain some regions where dark matter is made out of light particles and other regions where dark matter is made out of heavy particles, how could we possibly predict which one we should see in our own region? And indeed many people have criticized the multiverse concept on just these grounds. If a theory makes no predictions, it ceases to be physics.
But an important issue tends to go unnoticed in debates over the multiverse. Cosmology has always faced a problem of making predictions. The reason is that all our theories in physics are dynamical: The fundamental physical laws describe what will happen, given what already is. So, whenever we make a prediction in physics, we need to specify what the initial conditions are. How do we do that for the entire universe? What sets the initial initial conditions? This is science’s version of the old philosophical question of First Cause.
The multiverse offers an answer. It is not the enemy of prediction, but its friend.
The main idea is to make probabilistic predictions. By calculating what happens frequently and what happens rarely in the multiverse, we can make statistical predictions for what we will observe. This is not a new situation in physics. We understand an ordinary box of gas in the same way. Although we cannot possibly keep track of the motion of all the individual molecules, we can make extremely precise predictions for how the gas as a whole will behave. Our job is to develop a similar statistical understanding of events in the multiverse.
This understanding could take one of three forms. First, the multiverse, though very large, might be able to explore only a finite number of different states, just like an ordinary box of gas. In this case we know how to make predictions, because after a while the multiverse forgets about the unknown initial conditions. Second, perhaps the multiverse is able to explore an infinite number of different states, in which case it never forgets its initial conditions, and we cannot make predictions unless we know what those conditions are. Finally, the multiverse might explore an infinite number of different states, but the exponential expansion of space effectively erases the initial conditions.
In many ways, the first option is the most agreeable to physicists, because it extends our well-established statistical techniques. Unfortunately, the predictions we arrive at disagree violently with observations. The second option is very troubling, because our existing laws are incapable of providing the requisite initial conditions. It is the third possibility that holds the most promise for yielding sensible predictions.
But this program has encountered severe conceptual obstacles. At root, our problems arise because the multiverse is an infinite expanse of space and time. These infinities lead to paradoxes and puzzles wherever we turn. We will need a revolution in our understanding of physics in order to make sense of the multiverse…