Universe Today
This is Part 2 of a series on large extra dimensions. Read Part 1.
Oh yes they can, which is why I’m devoting an entire series just to this topic. The problem that large extra dimensions just might solve is called the hierarchy problem, and it’s one of the nastiest outstanding problems in modern physics.
Here’s the thing. We’ve got four forces of nature. Three of them – electromagnetism, strong nuclear, and weak nuclear – are all roughly the same strength. Not really, they’re actually many orders of magnitude different from each other. But kinda sorta close. And then there’s gravity. Gravity is so weak that it could be billions upon billions and then throw in some trillions on top of that strong and it would STILL be the weakest of the forces.
Here's another way to put it. We discovered just how out of whack the forces of nature are compared to each other when we discovered in the mid-20th century that at high energies we could merge some of them together.
At around 246 GeV, the electromagnetic and weak nuclear forces combine into a single force, called appropriately enough the electroweak force. So that’s how much energy you need to bring these two forces into alignment. If you want gravity to join the party, however, then you need to go all the way up to the Planck energy, which is ten thousand trillion times stronger.
Yes, there’s also the strong force and GUT unification scale, which isn’t much smaller than the Planck scale, but we’re not talking about that right now. We’re really really interested in the difference between the weak force and gravity because we know what’s responsible for splitting the electromagnetic and weak forces, and that’s the Higgs boson. We don’t know how or why the strong force got split off, so we’ll save that as a homework problem for later generations.
All of this is a roundabout way of repeating that gravity is way weaker than it should be. Okay, great, how do large extra dimensions solve this problem?
Well, what if gravity wasn’t actually so weak? Or put another way, what if the Planck-scale unification energy wasn’t really that high? What if they only APPEARED that way, to us, from our perspective?
What if there was more to the universe than just the universe? What if what we called the universe, with its stars and galaxies and all sorts of nonsense, and its forces of nature to govern and guide all that nonsense, existed in our usual three spatial dimensions? But what if there was another dimension? Well normally we wouldn’t notice, because all the “stuff” of the universe was stuck to the normal three. Like imagine we’re ants on the floor. We have everything we need in two dimensions: scent trails, bits of dropped food, a colony to call home. All of our material existence is contained within two dimensions, we would have no awareness, and not even a way to access, a third dimension.
But maybe gravity is different. Maybe gravity can spill out into the fourth dimension. Maybe for some special reason the gravitational force gets to spread out through all the dimensions of the universe, while everything else is stuck to the floor.
It’s a crazy idea and we LOVE crazy ideas in physics. But we’re always going to take them seriously. This idea would explain why gravity is so weak. It gets diluted amongst all the extra dimensions, so only some of it stretches out in our normal three (sorry, temporal dimension, you’re not going to be a big part of this story). Like not enough olive oil to dip your bread in, we’re left a little dry and crusty when it comes to the gravitational strength. Most of gravity is getting wasted blasting out into hidden, inaccessible dimensions.
Another way to put this is that maybe the Planck energy scale is much more normal than it appears. But it’s only that scale across the FULL universe, with all of its multidimensional glory. And to us, because we only get to see and experience a thin cross-section of the complete cosmos, does the Planck scale appear so big.
Why gravity, and not the other forces? Well, for one, that’s the only way this idea can work, because if the other forces also experienced the extra dimensions, then we would’ve literally seen it by now. But gravity is special. We know that gravity is a response of spacetime to the presence of matter and energy (thanks Albert!), and so maybe it gets some superpowers that set it apart from the other forces of nature.
And exactly how many dimensions are we talking about? Kaluza and Klein set the stage by introducing one extra dimension, then string theorists had a lot of fun introducing many more. So maybe there’s just one extra dimension. Or maybe there are a dozen. I dunno, we’re going to figure out how to test it.
And just how large are these “large” extra dimensions, anyway? They can’t be big enough that we would’ve seen them in our experiments already. So they have to be at least small enough to escape detection. To put the size of the dimension within range of the next generation of particle colliders, they have to be about a tenth of a millimeter across.
A tenth of a millimeter! That’s not much thinner than the hair on your head! Yeah, that’s tiny, but that’s way, way bigger than the Planck scale. And the only reason we haven’t noticed this compact, curled-up dimension existing everywhere throughout the universe at the scale of a millimeter is that only gravity gets to experience it.
But we’re going to take this wild idea seriously. We’ve seen how it can fix a problem. We’ve seen how it connects to the rest of our understanding of the world. Now we need to figure out how to test it.
To be continued…
