Andrew Strominger

And soft is the word that is used in physics for things which have very low energy.

And these things actually carry no energy.

There are things in the universe which carry no energy.

Yeah, so the soft gravitons and photons have been known about since the 60s.

But exactly what we're supposed to do with them or how we're supposed to think about them, I think has been well understood only recently.

And in quantum mechanics, the energy of a particle is proportional to Planck's constant times its wavelength.

So when the energy goes to zero, the wavelength goes to infinity.

Now, if something has zero energy and it's spread all over the universe, in what sense is it actually there?

That's been the confusing thing, to make a precise statement about when something is and isn't there.

Now, the simplest way of seeing, so people might have taken the point of view

that if it has zero energy and is spread all over the universe, it's not there, we can ignore it.

But if you do this, you'll get into trouble.

And one of the ways that you'll get into trouble is that even though it has zero energy, it doesn't have zero angular momentum.

If it's a photon, it always has angular momentum one.

If it's a graviton, it's angular momentum two.

So you can't say that the state of the system with the zero energy photon should be identified with the one without the zero energy photon, that we can just ignore them, because then you will conclude that angular momentum is not conserved.

And if angular momentum is not conserved, things won't be consistent.

And of course, you can have a lot of these things, and typically you do get a lot of them.

And you can actually do a calculation that shows that every time you scatter two particles, you create an infinite number of them.

And they're able to store information.