The Party Cools of Stoan

(Title from the wonderful book Riddley Walker, which my father insisted I read when I first decided to be a physicist.)

I have another question this week from Papou: Can you explain gravitons? Wouldn't they need to escape black holes to work?

First, let's back up a bit and talk a bit about particles in general. There's an idea called the Standard Model of Particle Physics, which is an excellent description of three out of the four fundamental forces (hey, name drop!): electromagnetism, strong nuclear force, and weak nuclear force. The particles in the Standard Model can be divided into two groups: matter particles, and interaction particles.

Wikipedia

Each of the gauge bosons (in red) mediate one of the forces: photons for the electromagnetic force, gluons for the strong nuclear force, and W/Z bosons for the weak nuclear force.

You may have heard about the discovery of the Higgs boson in the LHC a few years ago. The Higgs boson is responsible for giving some particles mass, and you may be thinking that it therefore mediates gravity, but the force of gravity is separate from the mass. I remember my middle school science teacher explaining it this way: Imagine you're floating in space with a big truck. You're both weightless, but you'd still be in trouble if the truck hit you, because it still has a lot more mass.

There's a problem when it comes to finding a particle to mediate gravity: the theories we use to describe small, slow things (like single particles) and big, fast things (like black holes) are incompatible. The areas of physics are often divided into four parts:

Each of these works very well in its own quarter, but trying to apply it to another region causes problems. Where relativity and quantum mechanics meet, quantum electrodynamics can cover some predictions, but it breaks down if you try to expand it to cover the universe.

There are various proposed theories of gravitons, including certain string theories, but since gravity is such a weak force, it's currently impossible to observe an individual graviton. Since photons are associated with electromagnetic waves, you might wonder if gravitons are behind gravitational waves. Unfortunately, this still wouldn't allow detection of a single graviton, since the waves are so weak. We can, however, hope to get more information about gravitons through gravitational waves. If the waves are observed to move slower than the speed of light, that would imply the graviton has mass. When LIGO and Virgo observed a binary neutron star merger, we were able to set limits on the speed of gravity: the difference is of order 10^-17! That suggests that the graviton is indeed massless, but as always, there's some experimental wiggle-room.

Now to the second part of the question: Do gravitons escape black holes? This is a bit difficult to answer, since we haven't observed gravitons yet. Black holes can emit energy via either gravitational waves, or Hawking radiation. That energy may take the form of gravitons, which would mediate the interactions between the black hole and surrounding bodies. Whether or not gravitons exist, and how they mesh with the rest of physics remains to be seen, but as I've said before physics is all about successive approximations: We've got a good system now, but later we'll make it a little bit better. Thanks for another great question, Papou!