Transcript: If you throw an object up into the air it will eventually slow down and fall back to Earth. The object is losing kinetic energy by trying to climb out through the gravitational field of the Earth. Photons also lose energy as they climb out of the pit of gravity. This effect is called the gravitational redshift. It’s a very subtle effect. For the Sun the gravitational redshift is only 0.0002 percent, but it has been measured. In a situation of more intense gravitational field, the gravitational redshift is proportionally larger, and of course at the event horizon of a black hole, the gravitational redshift is infinite. The infinite redshifting or loss of energy corresponds to the trapping of radiation at the event horizon.