Gravitons are tiny particles that carry the "force" of gravity. They are what brings you back down to Earth when you jump. So why have we never seen them, and why are they so impossibly complicated we need string theory to figure them out? Find out here!
Photons, Gluons, and Gravitons
Light comes streaming at us in waves. People have known that for quite some time. The fact that light is also a particle gave everyone something of a shock, but we adjusted. What's often left out of the story is the fact that light is not just a way of giving our eyes a good time. Photons aren't just particles hitting our retinas. They are the (massless) heavies doing the work that gives us another well-studied force - electromagnetism.
Many of you know that, when a photon of a precise energy hits an atom, that atom's outer electron jumps up a level. When the electron comes back down, it emits that photon again. This process describes why things glow. It also describes why they glow only certain colors, as the photon has to be of a precise energy to make the electron jump, which translates to a precise wavelength, which translates to a precise color. When we look more deeply at this process, we see photons for what they are. They are the particles that push electrons around. They carry the "force" that is electromagnetism.
Photons are not the only particles that carry force. The strong nuclear force, which binds together protons in the atomic nucleus, is carried by the aptly-named gluon.
What we call "force" at the macro level seems to be conveyed by particles at the micro level. The graviton should be one of these particles. The trouble with gravitons - or, more precisely, the first of many troubles with gravitons - is that gravity isn't supposed to be a force at all. General relativity indicates that gravity is a warp in spacetime. General relativity does allow for gravitational waves, though. It's possible that these waves could come in certain precise wavelengths the way photons do, and that these can be gravitons.
Gravitons and String Theory
Even without observing gravitons, scientists know a few things about them. They know, because gravity is a force with an infinite reach, that gravitons would have to be massless. This technically makes them "gauge bosons," and puts them in the company of photons and gluons. Scientists also know that gravitons have a spin of two, which makes them unique among particles. The combined properties mean that, if scientists were able to pin down an event involving a mysterious particle with no mass and a spin of two, they would know they were looking at a graviton.
There is, however, a major problem. To understand it, let's go back to photons and electrons. When an electron falls from one level to another, out pops a photon. When that photons falls, or otherwise moves, it produces no second photon. Electron movement produces photons. Photon movement does not produce more photons. There are occasional times when photons can do odd things. They can split into electron and positron pairs, which can produce more photons, and which then recombine into a photon again. Although this burst of particles may get hectic, it doesn't produce an endless branching chain of photons. Because of this, photons and electron interactions are said to be renormalizable. They can get weird, but they can't become endless.
Gravitons are not so tame. While photons are spawned by movement in electrons, gravitons are whelped by energy and mass. Gravitons are massless, but they do carry energy. This means a graviton can create more gravitons.
Like other quantum particles, gravitons can carry a lot of energy, or momentum, when confined to a small space. A graviton is confined to a small space when one graviton is popping out another graviton. At that moment, two gravitons are in a tiny space, one right next to the other. That huge amount of energy causes the newly-created graviton to create yet another graviton. This endless cycle of graviton production makes gravitons nonrenormalizable.
String theory is invoked in these situations part because nonrenormalizable gravitons are points. Strings are longer than points, and so the creation of the stringy graviton isn't so confined in time and space. That bit of wiggle room keeps the creation of a graviton from being so energetic that it necessitates the creation of yet another graviton, and makes the theory renormalizable.
How Can We Find Gravitons?
There are facilities that look for gravity waves, but the most promising place to find a graviton is as the Large Hadron Collider. Primarily, CERN would be looking for something that is not there. Collisions between particles need to be balanced. When examining a collision and its results, all the momentum, mass, energy, and spin need to be accounted for. There can't be mass going one direction without an equivalent amount of mass going in another direction.
Complicating the matter is the possibility that gravitons might be rolled up into other dimensions. This may be why we haven't been able to see them up until now. What the scientists at CERN would be looking for would be a hole, rather than an actual graviton. There would have to be a strange imbalance in energy in and momentum after an event. The energy and momentum would amount to that of an escaped graviton - a graviton that flickered briefly her before heading into other dimensions. Granted, the fact that there are other dimensions might overshadow the discovery of gravitons, but they've never wanted glory, anyway.
Top Image: Paul Bica
[Via Gravity and Strings, Extra Dimensions, Gravitons, and Tiny Black Holes, What are Gravitons, How to See Quantum Gravity in Big Bang Traces, The Little Book of String Theory.]