# Extra Dimensions' dimensions

1. Jun 8, 2014

### ChrisVer

I've heard that extra dimensions can be (due to gravitational experiments) of dimensions ~some millimeters...
I'm quiet skeptical about this issue... Why would someone choose gravity (such a weak force for those experiments) to determine the scale? why not use Electromagnetism , that has allowed us to see up to ~some Armstrong scale (seen the atoms)? with the last, we know that we haven't seen any extra dimensions up to that scale...
Moreover on that, if the extra dimensions were really of order of some millimeters, then a dust of dirt could be completely erased from our world - being able to travel within those dimensions. It doesn't sound nice....

I am sorry if this belongs to Beyond SM, but it's not about "research".

2. Jun 8, 2014

### kurros

Well there are various models where different things happen, but if the extra dimensions are 'large' then one might constrain the Standard Model particles and forces to live only on some hypersurface of the higher-dimensional space, i.e. on a brane. Stuff will therefore not leave the brane and apparently "vanish" from our world. Usually, though, gravity is not constrained to live on the brane, and can propagate in the bulk spacetime, which is then an explanation for why it appears so weak compared to the other forces (it is "diluted" by more dimensions). I don't know exactly what experiments you refer to, but that might be one reason why gravity is the relevant probe of the new physics.

This lets the "real" scale of gravity, i.e. the real Planck scale, be much lower than we usually expect, and can be a way to solve the hierarchy problem. But as I said it depends on some model details.

3. Jun 9, 2014

### dauto

In this kind of model where the extra dimensions are large (mm instead of Planck scale), only gravity can see the extra dimensions. There are other kinds of models where all particles see the extra dimensions but then the dimensions won't be large. They will be in the Planck scale.

4. Jun 9, 2014

### Staff: Mentor

Right. That's why we know if there are large extra dimensions, the electromagnetic force does not extend into them, and we need gravity to search for them.

There is nothing wrong with a model of large extra dimensions that includes the electromagnetic interaction in them, but we already know we do not live in such a universe.