Sean Carroll

The singularities are not a major motivation for quantum gravity.

Sometimes they're presented that way.

But I think that a more honest presentation would be to say that one of the nice things about quantum gravity is it should—

get rid of singularities because you can't have singularities in quantum mechanics because the Schrodinger equation is linear.

That's just how it works.

Singularities are inherently nonlinear kind of phenomenon.

But you can certainly imagine getting rid of the singularities in a classical theory.

People have tried to do that.

The reason you need quantum gravity is because gravity exists and quantum mechanics exists.

That's it.

That's the total sum of the argument.

People have also tried in respectable ways to see whether or not you could make a theory where gravity is itself classical and matter fields and energy fields are quantum mechanical.

I think that the chances of something like that working are essentially zero, pretty, pretty close to zero.

So I think that which is good because it gives us room to invent quantum gravity.

We're not done doing that yet.

And it's fun to think about it.

Gary says, fermions are things that take up space, and bosons are things that don't.

Therefore, helium is a fermion, right?

Yes, please?

No, helium is not a fermion, but it's a composite particle with fermions inside.