Graviton is a theorised elementary particle that acts as the carrier of the force of gravitation. Since other three fundamental forces have been successfully unified in a single theory and all have their force carriers - photon for electromagnetism, gluon for the strong interaction, W and Z bosons for the weak interaction - it is necessary for gravity to have a force-carrying particle if it is to be unified with other three theories.
Since gravity has apparently unlimited range, graviton will have to be massless, just like photon for electromagnetism, which also has unlimited range. It will also have to be a boson and have integer spin of 2. This is because the source of gravitation is the stress-energy tensor, a second-rank tensor. Any field carried by a spin-2 particle would result in a force identical to gravity, because such a field must interact with the stress-energy tensor in a same way the gravitational field does.
So, if we ever discover a spin-2 particle, it must be a graviton and spin-2 is all we need for its verification. When that day comes, it will be a great success in an attempt to unify all four forces. To experimentally observe gravitons, however, an unrealistically large detector would be needed because of the almost non-existent interaction between gravitons and matter.
One hope is the recent detection of gravitational waves, which are coherent states of many gravitons. Through gravitational waves, it could be possible to reveal some properties of gravitons, although individual particles would be undetectable. If gravitational waves move at the speed of light, that implies that gravitons do not have mass.
Current theories have difficulties trying to unify gravity with other three forces. String theories, where different particles are strings that oscillate at different rates and thus have different characteristics, is a strong candidate for the so-called theory of everything that unites all four forces. In string theories, graviton is simply another oscillating string.
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