Bradley Carroll, a professor of physics at Weber State University in Ogden, Utah, responds:
"According to modern physics field theories, each of the four basic interactions (a better term than 'force') is mediated by a type of particle:
"The strong (nuclear) interaction is carried by gluons. (This is the interaction that holds together the particles in the nuclei of atoms.)
"The electromagnetic interaction is carried by photons. (This is the interaction responsible for all electrical and magnetic phenomena.)
"The weak (nuclear) interaction is carried by weak bosons. (This is the interaction that governs certain radioactive decays, such as beta decay.)
"The gravitational interaction is carried by gravitons. (This, of course, is the interaction that gives rise to the familiar pull of gravity.)
"Although the graviton has yet to be observed, some of its hypothesized properties are known. It is a massless particle having no electrical charge. Its spin (a property of subatomic particles that is not directly analogous to the rotation of a macroscopic object like a top) is twice that of the other field particles listed above; in technical terms, its spin is 2 hbar instead of 1 hbar, where hbar is Planck's constant divided by 2 pi.
"Two masses attract each other gravitationally because they are constantly exchanging virtual gravitons, just as two electrically charged particles are drawn together--or repelled apart--by the exchange of virtual photons. (A 'virtual particle' is one that cannot be directly detected.) This exchange happens at all times. Gravitational waves, in contrast, can arise when an object undergoes an acceleration. Asymmetric supernova explosions or collisions between neutron stars are the kinds of events that could produce powerful blasts of gravitational waves. Gravitational waves have been indirectly detected in certain binary neutron star systems, in which the energy carried off by those waves causes observable changes in the stars' orbits.
"Virtual gravitons pass between two objects even when there are no gravitational waves present (for instance, when the masses are at rest), so it really isn't correct to say that gravity is a wave.
"An analogy with an electrically charged particle might help clarify the situation. When a charged particle is at rest, it is surrounded by a static electric field (no waves). If another charged particle encounters this field, it experiences a force. The quantum view would describe this in terms of an exchange of virtual photons by the two particles. On the other hand, if a charged particle is accelerated, its electric field is ' shaken' to produce an electromagnetic (light) wave that spreads out from the particle. In this case, the energy and momentum of the light wave are carried by real, detectable photons.
"In a similar manner, when a massive particle is at rest, it is surrounded by a static gravitational field (a static curvature of spacetime, no waves) . If another massive particle encounters this field, it experiences a force that can be described in quantum terms as an exchange of virtual gravitons by the two masses. On the other hand, if a massive particle is accelerated, its gravitational field is 'shaken' to produce a gravitational wave that spreads out through spacetime from the particle. The energy and momentum of that gravitational wave are carried by real gravitons."