A black hole in the galaxy Centaurus A gobbles up dust, heavy elements and even light. Submillimeter wavelengths are orange, and x-ray data is blue. Image Credit: NASA/CXC/CfA/R.Kraft et al.; Submillimeter: MPIfR/ESO/APEX/A.Weiss et al.; Optical: ESO/WFI
Although photons have no mass, they do have energy, and the gravity of a black hole (or any other gravitating body) will attract them. According to Einstein's theory of gravity, it is energy, not mass, that feels the gravitational force. That's the short answer.
You might think that Einstein's famous equation, E=mc2, says that energy and mass are essentially the same thing, so how does gravity tell the difference? The true meaning of the equation, however, is that a body with "rest mass" m will have an associated energy mc2. But the body can have other forms of energy as well, such as kinetic energy — energy of motion. Although photons have no rest mass, they're always moving at the speed of light; their energy is all kinetic energy and not rest-mass energy.
The fact that gravity attracts light was corroborated in a famous experiment in 1919, less than 4 years after Einstein formulated his theory of gravity, which predicted by how much a given body should bend a beam of light. The effect was too small to be observed on Earth. However, a beam of light just grazing the edge of the sun would be bent by a tiny — yet detectable — amount.
How do you observe a beam of light just grazing the edge of the sun? You wait for a solar eclipse, and then you very carefully measure the location of a star that appears near the sun in the sky (obviously, without the eclipse, the sky would be too bright for the star to be seen). If Einstein is right, the position of the star should be slightly different in the sky from where it was, relative to other stars in the night sky when the sun is not present.
In 1919, teams of astronomers went to two locations: one in Brazil and the other on an island in the Atlantic. They intercepted the path of the total solar eclipse, and carefully photographed the positions of nearby stars. Several months later, they announced their result: Einstein was right! Before this event, Einstein was well-known inside the scientific community, but his fame soon spread worldwide.
A black hole is much, much denser than the sun, and the amount of bending is correspondingly much greater. This force can be so great, in fact, that the path of a light ray will curve back on itself and never escape from the black hole.
Alan Chodos, PhD
Associate Executive Officer
American Physical Society
Kynan from the U.S.