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.
Physical Theories,
Eternal Inflation, and Quantum Universe By Yasunori Nomura
The Multiverse
Interpretation of Quantum Mechanics By Raphael Bousso and Leonard Susskind
AR Good idea to try and merge the
multiverse and quantum branching. Hope it works.
