1. Gravitational Red Shift Gravitational Red Shift - The reddening of light from a very massive object caused by photons escaping and traveling away from the object's strong gravitational field. An example of gravitational redshift is light escaping from the surface of a neutron star.

2. Gravitational Red Shift Gravitational redshift -- that is, the redshifting of photons that climb out of a gravity well -- occurs for fundamentally the same reason that projectiles slow down when rising -- because they have to transfer kinetic energy (their speed) into potential energy (their height). Projectiles, such as a cannon ball, do this by slowing down.

3. Gravitational Red Shift Photons, however, cannot slow down -- they are constrained to always travel at exactly c, the speed of light, and no faster. So how does a photon shed this required kinetic energy? By lowering in energy and thus frequency. Since a lower frequency means a longer, or "redder," wavelength, this process is called gravitational redshifting. (A similar process occurs when a photon is falling into a gravitational well; it trades potential energy for kinetic energy and gains in frequency, and thus gets a shorter, "bluer," wavelength; this is called a blueshift.)

4. Gravitational Red Shift Illustration of the gravitational red-shift predicted by the General Theory of Relativity. A heavy object is denoted by a deformation of space represented by the funnel. As light leaves the vicinity of this object it is shifted towards the red: for a sufficiently compact and massive object a blue laser on the surface will be seen as red in outer space.