Testing the multiverse

Given the strange implications of parallel universes, you might be forgiven some skepticism about whether they exist.

But who are we to judge what is weird and what is not? Scientific ideas stand or fall, not by how they "feel" to us, but by experimental testing.

And that is the problem. An alternative universe is separate from our own. By definition, it is beyond reach and out of sight. On the whole, multiverse theories cannot be tested by looking for those other worlds.

Yet even if other universes cannot be experienced directly, it might be possible to find evidence to support the reasoning behind them.

For example, we could find strong evidence for the inflationary theory of the Big Bang. That would strengthen, but not prove, the case for an inflationary multiverse.

Some cosmologists have proposed that an inflationary multiverse might be more directly tested. A collision between our expanding bubble universe and another one should leave detectable traces in the cosmic microwave background – if we were close enough to see them.

Similarly, experiments envisaged for the Large Hadron Collider could search for evidence of the additional dimensions and particles implied by the braneworld theory.

Some argue that experimental verification is over-rated anyway. They say we can gauge the validity of a scientific idea by other means, such as whether it rests on sound logic spun from premises that do have observational support.

Finally, we might make statistical predictions.

For example, we could use the inflationary multiverse theory to predict which values of the physical constants would be expected in most universes, and then see whether these are close to the ones we see – on the basis that there is no reason to expect us to be anywhere special in the multiverse.

At any rate, it does seem odd that the multiverse keeps cropping up wherever we look. "It's proven remarkably hard to write down a theory which produces exactly the universe we see and nothing more," says physicist Max Tegmark.

Even so, it is not clear that newspaper headlines will announce the discovery of another universe any time soon. Right now, these ideas lie on the border of physics and metaphysics.

In the absence of any evidence, then, here is a rough-and-ready – and frankly subjective – ranking of the probabilities of the various multiverses, the most likely first.

The patchwork multiverse is hard to avoid – if our Universe really is infinite and uniform.

The inflationary multiverse is rather likely if inflationary theory is true, and right now inflation is our best explanation for the Big Bang.

Cosmic natural selection is an ingenious idea but involves speculative physics, and there are a lot of unanswered questions.

Brane worlds arefar more speculative, because they can only exist if all those extra dimensions do, and there is no direct evidence of that.

The quantum multiverse is arguably the simplest interpretation of quantum theory, but it is also vaguely defined and leads to an incoherent view of selfhood.

Date: 2016-04-22; view: 753