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The Question

(Submitted June 08, 1997)

Force carrying particles for electromagnetic radiation, the weak/strong nuclear forces and gravity have been either discovered or postulated. What are the experimental difficulties in "discovering" gravitons?

The Answer

There is a strong analogy between the photon and the graviton. EM forces are propagated by virtual photons, and of course we detect 'real' photons in the form of (classical) EM radiation. Similarly, since the graviton must be massless (since gravity has an infinite range), the graviton can only be 'detected' analogously to the way that the photon is detected, i.e., in the detection of 'classical' gravity waves. Part of the idea of unification theory is that at high enough energies all four forces 'merge', and can be propagated by the same particle (i.e., this has been shown to be the case for the EM and weak interaction, and hence they are often referred to collectively as the electro-weak interaction). But I think you would need a particle accelerator bigger than the size of the Earth to get to energies high enough for all of the forces to be unified.

Andy Ptak
for Ask an Astrophysicist

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