# Fermilab

## Inquiring Minds

Photons and the Higgs field

If all particles except photons and gluons acquire "mass" by theoretically interacting with the Higgs field and mass is felt by the gravitation field. Then why do photons curve around large bodies and can not leave a black hole?

Don

Dear Don:

It is true that photons and gluons are massless, and that to the best of our theoretical knowledge, the other particles acquire mass by interacting with the Higgs field.

But to answer your question, we need only general relativity. Since before most of modern particle physics (including the idea of a Higgs field) was developed, it was known that photons would curve around large bodies and be trapped inside black holes. Both of these things were predicted by general relativity, which was presented in 1916 by Einstein. The prediction of photons being curved around large bodies was in fact one of the dramatic early experimental results in agreement with general relativity that helped bring the theory to the attention of the general public. The experiment was done by waiting for a solar eclipse and then photographing the stars right around the blocked-out sun. It was first done by Eddington and collaborators in 1919. Because of the deflection of the light by the sun's gravity, the stars appeared to be in slightly different positions than they really were.

So why does this happen? Basically gravity is a deformation of space-time. General relativity allows you to calculate how space-time is deformed for a given set of gravitating masses. In flat space-time, objects that have no forces acting on them just travel in straight lines. If space-time is curved (by the presence of some masses) then the "straight lines" are bent: objects with no forces acting on them (treating gravity now not as a force but as the curvature of space-time) travel along curved paths, which are specified by the "shape" of space-time in that region (which is in turn calculable from general relativity). This is why photons, or any particle regardless of its mass, will curve around large bodies. In black holes, the curvature of space-time is so severe that, at the horizon of the black hole, the path that an object with no forces acting on it takes is bent all the way around in a circle! Thus if you have some light travelling by a black hole right at the black hole's horizon, it will get deflected so far that it will just orbit the black hole and never escape. If light travels closer than the horizon of the black hole, it will just go in and never come out.

Heather

Back to Questions About Physics Main Page