MadSci Network: Physics

Re: If only energy can travel at the speed of light ...

Area: Physics
Posted By: Georg Hager, Grad student Theorie III
Date: Fri Apr 4 05:02:32 1997
Message ID: 859563747.Ph

Dear Steve!

It is not true that only massive objects can be deflected by a gravitational field. In fact, everything that has energy feels the warping of spacetime. For example, an excited nucleus is `heavier' than the same nucleus in the ground state: the additional energy contributes to its mass and inertia. Photons do not have rest mass, all right. But they do have energy according to E=hbar x omega!

According to General Relativity, objects in free fall subject to a gravitational field follow a path which is called a geodesic. A geodesic is (locally) the shortest connection between two points in spacetime (not space!). A photon is such an object, and thus moves on a geodesic.

According to the conservation of momentum, a massive body must also feel a recoil (or drag) as a photon whizzes by, but that effect is certainly much too small to be detectable.

Hope that helps,

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