Hi
Hello.
This week we're asking, how many dimensions does our universe have?
and the answer is either 4, 5, 10, 11, or 26
Yeah, so the number of dimensions is simply the number of numbers that you'd need to specify
where something is.
so if you take this point on the screen for instance, you could describe where it is by
how far along the screen it is and how far up the screen it is.
Two numbers so two dimensions.
now, we're trapped in the two dimensions of your screen but you're probably more familiar
with three dimensions.
and in physics even three dimensions is often not enough.
We can measure the time that something happens as a fourth number, so we say spacetime is
four dimensional.
so it would seem our universe has four dimensions.
but theoretically there could be more hidden dimensions.
For example if you've looked into how to unify quantum theory with gravity, then you'll almost
certainly have come across this.
string theory.
Yeah, the idea is that fundamental tiny particles are no longer just dots they are replaced
by a strip of vibrational energy, or a string.
Different vibrations in the strings naturally correspond to all of the different forces
in the universe, making this an ideal candidate for a mathematical theory of everything.
But when you try and do this in a way that's consistent with Einstein's picture of space
and time, er you'll find that it's impossible.
so erm it contradicts everything we know for every experiment then...
yes, unless, we build it in 26 dimensions.
26 dimensions?
So the theory is either incompatible with everything that says that Einstein is right,
or you need 26 dimensions.
yeah, yeah, in 26 dimensions everything works perfectly.
Did you see that it's a theory that predicts how many dimensions there are.
Ok for one I mean 26 numbers to describe where something is in space and time seems a little
bit excessive, but I mean more importantly there's no fermions in here.
There's no leptons, no quarks, no fundamental constituents of matter.
no sorry, if you want er matter in your universe you will need this model.
Superstring theory.
It looks just the same.
Yeah yeah, but this model has the fermions built in, and on top of that the space time
in this model is much more stable than the other one as spacetime would tend to just
spontaneously disintegrate.
right ok, well how many dimensions does this one have.
Ah, so that one er has ten dimensions.
But I've also got M theory.
That's er, has 11 dimensions if that's more your sort of thing.
Well which one's experimentally verified?
What?
Well none of them are experimentally verified.
You can still have it on spec though.
Yep..
No, no thanks.
You see as it happens we have been looking for experimental evidence of extra dimensions
at the large Hadron Collider.
But rather than hypothising a bunch of extra dimensions to make some crazy string thing
to fit with the laws of physics, we've taken the different approach.
We've taken the four dimensions that we know and love and have carefully added one more.
So for example in the Randall-Sundrum model of spacetime, the universe has five dimensions,
but the particles that we're all made of are trapped in a four dimensional brane as in
membrane.
Much like that we're trapped in the two dimensions of your screen.
Your screen is our brane.
oh, so, that.. that's much more testable then.
well, yes it is actually.
So at the LHC we create regions of very high energy in a very small space by smashing protons
together.
The idea being that we sort of punch through to this hypothisized fifth dimension and look
for evidence that we've done so in our detectors.
ahh I get you I get it, so it's a bit like this
Well no, no, you have to reach the specific energy of the higher dimensional resonance.
So its more like this.
right yea yeah I see, I see I see so it's more like this
yea yeah, that's basically how we're looking for evidence of extra dimensions at the large
hadron collider.
By by by punching through to the fifth dimension, and looking for, for evidence of our experiments.
Right, and have you found anything?
No! which is why, for now, we're convinced that our universe has only four dimensions.
Just as I said
in the first place.