Many Worlds and the Multiverse

By Sean Carroll
Cosmic Variance, May 26, 2011

Edited by Andy Ross

Physicists talk about the multiverse of inflationary cosmology and the many worlds or branches of the wave function in quantum mechanics.

In inflationary cosmology, different regions of spacetime can be pocket universes, as Alan Guth calls them. When you combine this with string theory, the emergent local laws of physics in the different pocket universes can be very different. So we can think about them as separate universes in a multiverse.

In quantum mechanics, wave functions assign amplitudes to all the various possibilities for what we can see when we make an observation. In the many worlds or Everett interpretation, the possibilities continue to exist, but we observers branch or split into each one.

These two ideas might be the same. Two ideas make this proposal seem sensible. The first is quantum vacuum decay.

For particle physicists, a vacuum is a field state that has the lowest energy of all similar states. Imagine a scalar field filling the universe that can take on different values, and each different value has a different potential energy associated with it. In the course of normal evolution the field settles down to a minimum of its potential energy a vacuum. But there can be the true vacuum, where the energy is really the lowest, and all sorts of false vacua, in local minima.

Fields are subject to quantum fluctuations. So if you observe a scalar field, you might find it straying away from its false vacuum state. Eventually it strays so far that it climbs right over the barrier in the direction of the true vacuum. That happens in a bubble. But the bubble grows. Other bubbles form elsewhere and also grow. Eventually all the bubbles crash into each other, and you complete a transition from the false vacuum to the true one.

Quantum fields don't really fluctuate. Our observations fluctuate. We can look at the same field multiple times and measure different values. Likewise, bubbles don't really form and grow. What really happens is that there is a quantum amplitude for a bubble to exist, and that amplitude grows with time. When we look at the field, we see a bubble or we don't. A quantum wave function describes all the possibilities at once.

Horizon complementarity is a generalization of the idea of black hole complementarity. For black holes, complementarity says you can talk about what's going on inside the black hole, or outside, but not both at once. It is a way of escaping the paradox of information loss as black holes evaporate.

If you throw a book into a black hole and if information is not lost, you should be able to reconstruct what was in the book from the Hawking radiation. Complementarity says everything that is happening inside the black hole can be encoded as information on the event horizon itself. This idea works very well with holography, and the fact that the entropy of the black hole is proportional to the area of the horizon rather than the volume inside.

We are inside the universe. The cosmological horizon is a sphere that surrounds us. You can talk about what's inside your cosmological horizon, but not what's outside. Everything that you think might be going on outside can be encoded in the form of information on the horizon itself. This becomes a fairly sharp and believable statement in empty space with a cosmological constant. Horizon complementarity says that it's true more generally.

Let's put together horizon complementarity and quantum vacuum decay. Every question that it is sensible to ask can be answered in terms of what's happening inside a single horizon. But a complete description of what's actually inside our observable universe includes an amplitude for being in various possible states. We replace the cosmological multiverse, where different states are located in widely separated regions of spacetime, with a localized multiverse, where the different states are all right here, just in different branches of the wave function.


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The Multiverse Interpretation of Quantum Mechanics
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AR  Good idea to try and merge the multiverse and quantum branching. Hope it works.