Only the Boring Kind of Parallel Universes


No. No. Bad journalist. See what happens when you…

Mir Faizal, one of the three-strong team of physicists behind the experiment, said: “Just as many parallel sheets of paper, which are two dimensional objects [breadth and length] can exist in a third dimension [height], parallel universes can also exist in higher dimensions.

Bad physicist, bad! No biscuit for you!

Not nice at all!

For the technically-minded, Sabine Hossenfelder goes into thorough detail about what went wrong here. Not only do parallel universes have nothing to do with what Mir Faizal and collaborators have been studying, but the actual paper they’re hyping here is apparently riddled with holes.

BLACK holes! …no, actually, just logic holes.

But why did parallel universes even come up? If they have nothing to do with Faizal’s work, why did he mention them? Do parallel universes ever come up in real physics at all?

The answer to this last question is yes. There are real, viable ideas in physics that involve parallel universes. The universes involved, however, are usually boring ones.

The ideas are generally referred to as brane-world theories. If you’ve heard of string theory, you’ve probably heard that it proposes that the world is made of tiny strings. That’s all well and good, but it’s not the whole story. String theory has other sorts of objects in it too: higher dimensional generalizations of strings called membranes, branes for short. In fact, M theory, the theory of which every string theory is some low-energy limit, has no strings at all, just branes.

When these branes are one-dimensional, they’re strings. When they’re two-dimensional, they’re what you would normally picture as a membrane, a vibrating sheet, potentially infinite in size. When they’re three-dimensional, they fill three-dimensional space, again potentially up to infinity.

Filling three dimensional space, out to infinity…well that sure sounds a whole lot like what we’d normally call a universe.

In brane-world constructions, what we call our universe is precisely this sort of three-dimensional brane. It then lives in a higher-dimensional space, where its position in this space influences things like the strength of gravity, or the speed at which the universe expands.

Sometimes (not all the time!) these sorts of constructions include other branes, besides the one that contains our universe. These other branes behave in a similar way, and can have very important effects on our universe. They, if anything, are the parallel universes of theoretical physics.

It’s important to point out, though that these aren’t the sort of sci-fi parallel universes you might imagine! You aren’t going to find a world where everyone has a goatee, or even a world with an empty earth full of teleporting apes.

Pratchett reference!

That’s because, in order for these extra branes to do useful physical work, they generally have to be very different from our world. They’re worlds where gravity is very strong, or world with dramatically different densities of energy and matter. In the end, this means they’re not even the sort of universes that produce interesting aliens, or where we could send an astronaut, or really anything that lends itself well to (non-mathematical) imagination. From a sci-fi perspective, they’re as boring as can be.

Faizal’s idea, though, doesn’t even involve the boring kind of parallel universe!

His idea involves extra dimensions, specifically what physicists refer to as “large” extra dimensions, in contrast with the small extra dimensions of string theory. Large extra dimensions can explain the weakness of gravity, and theories that use them often predict that it’s much easier to create microscopic black holes than it otherwise would be. So far, these models haven’t had much luck at the LHC, and while I get the impression that they haven’t been completely ruled out, they aren’t very popular anymore.

The thing is, extra dimensions don’t mean parallel universes.

In fiction, the two get used interchangeably a lot. People go to “another dimension”, vaguely described as traveling along another dimension of space, and find themselves in a strange new world. In reality, though, there’s no reason to think that traveling along an extra dimension would put you in any sort of “strange new world”. The whole reason that our world is limited to three dimensions is because it’s “bound” to something: a brane, in the string theory picture. If there’s not another brane to bind things to, traveling in an extra dimension won’t put you in a new universe, it will just put you in an empty space where none of the types of matter you’re made of even exist.

It’s really tempting, when talking to laypeople, to fall back on stories. If you mention parallel universes, their faces light up with the idea that this is something they get, if only from imaginary examples. It gives you that same sense of accomplishment as if you had actually taught them something real. But you haven’t. It’s wrong, and Mir Faizal shouldn’t have stooped to doing it.

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