Archive for the Science! Category

The Alternative to Many Worlds

Posted in Science! with tags , on April 18, 2012 by Alex

This could be my engineering training talking, but the “Many Worlds” interpretation of quantum mechanics has never rung true for me. For those who are unfamiliar with this, here’s a brief summary:
Very small particles appear to behave probabilistically, satisfying something called the “Born Rule.” This means that it is impossible to determine the exact position (or momentum, or energy) of a particle, it is only possible to determine a set of possible positions (or momenta, or energies) and the probability of the particle having each one.
Ok, so far we are still in the land of verified facts, with no mention of parallel worlds containing (possibly) evil goatee sporting versions of ourselves.

The Many Worlds interpretation of QM is an attempt to grapple with the question: Why does nature choose this particular path for the particle rather than another one? Many Worlders would answer this question with “it doesn’t! The particle takes every possible path, each one creating a universe of its own, and we happen to live in the universe where it took this one.”

This all sounds very cool and science fiction-y, but it does have some problems.

1. We cannot ever observe these other universes, so this interpretation will always be speculative.

2. Unless I’m mistaken (which is always possible), the creation of a kajillion universes at every moment in time would grossly violate conservation of energy.

3. It doesn’t explain why the paths of particles obey the particular statistics that they do, the Born Rule, the thing that makes particles look like waves when there are a lot of them together. Granted, some very smart people (actual experts in this sort of thing) have come up with solutions to this third problem. See here.

So what’s the alternative?
Let’s look at the facts:

Particles do take actual paths through space.

For some reason, the set of possible paths a particle can take behaves like a wave.

The probability that a particle takes a particular path is the square of the amplitude of that wave.

When the possible paths of one particle collide with the possible paths of another, the waves interfere with one another and the particles become “entangled.”

If enough particles become entangled, the set of possible paths loses its wavy nature and things start to behave like classical objects such as baseballs and W-2 forms.

The mysterious question is: what exactly is that wave? And what determines the particular path of a particle? I should note that any interpretations beyond this point do not change any of the above facts (Many Worlds included.) They are all attempts to answer a question for which nobody has an answer.

The alternative to Many Worlds is something called “Hidden Variable Theory,” or sometimes “Bohmian Mechanics.” The idea is that there is some as yet undiscovered (possibly undiscoverable in principle) mechanism that tells the particle which of the possible paths to take. For the purposes of making observations and predictions, this interpretation of QM is exactly the same as Many Worlds. It also has the burden of possibly being unverifiable. It doesn’t violate conservation of energy, however. And it sidesteps the issue of why the set of possible paths acts like a wave. In the end, it is an alternate speculation, but at least it doesn’t make my head explode by trying to think about an extremely large number of universes being created at every moment in time.

One universe is mysterious enough.