25 Days of Particles: Day 24

Graviton

Classification: boson
Fundamental: yes (we think)
Mass: Unknown, but it should be massless
Interactions: gravity
Spin: 2
Lifetime: Unknown

Today's particle was theoretically predicted because physicists like symmetries and things to behave in the same ways. All of the other forces, when crafted into modern, quantum field theory terms, have vector bosons zipping around as the force carriers. The strong force has the gluon, the weak force has the W and Z bosons, and so on. So gravity, when it finally gets made into a quantum field theory like everything else is, should have a boson, and that boson has been dubbed the graviton.

We know it should be massless, because the force of gravity travels a very long way and lighter things travel farther than heavier things. It should be spin 2, because the math says so (gravity comes from a second-rank stress-energy tensor, which provides the necessary components for a spin-2 boson). We also are not going to be directly detecting a graviton any time soon. First, the mathematical theory necessary for a quantum theory of gravity isn't behaving very well right now; our best efforts yield theories that predict infinite probabilities and other impossibilities. Second, gravitons don't interact with matter much, which is why gravity is extremely weak for particles and only really noticeable when large quantities of matter are clumped together. These means that we can't build a detector that can detect gravitons in a reasonable amount of time.

Instead, physicists hunt for gravity waves, or coherent states of many gravitons. The LIGO and VIRGO experiments are already on the hunt. While they won't detect the particles, they can possibly tell us more about how gravity behaves for quantum things, which would be awesome. Most of what we know about gravity now comes from the work of exactly two people: Isaac Newton and Albert Einstein. It isn't the easiest subject to tackle.