Black HoleA black hole is an object so massive that even light cannot escape from it. This has been approached with the idea of an effective gravitational mass for a photon, which then allows the calculation of an escape energy for an object of that mass. When the escape energy is equal to the photon energy, the implication is that the object is a "black hole". 
Index General relativity ideas Black hole concepts  

Go Back 
Gravity and the PhotonThe relativistic energy expression attributes a mass to any energetic particle, and if applied to the photon The gravitational potential energy* is then When the photon escapes the gravity field, it will have a different frequency Since it is reduced in frequency, this is called the gravitational red shift or the Einstein red shift. *The idea of "gravitational potential energy" for a photon is currently being challenged with the point of view that the observed clock rate discrepancies are due to the local curvature of the space time. This gives the frequency differences without postulating gravitational properties for the photon.

Index General relativity ideas  

Go Back 
Escape Energy for PhotonIf the gravitational potential energy* of the photon is exactly equal to the photon energy thenNote that this condition is independent of the frequency, and for a given mass M establishes a critical radius. Actually, Schwarzchilds's calculated gravitational radius differs from this result by a factor of 2 and is coincidently equal to the nonrelativistic escape velocity expression *The idea of "gravitational potential energy" for a photon is currently being challenged with the point of view that the observed clock rate discrepancies are due to the local curvature of the space time. This gives the frequency differences without postulating gravitational properties for the photon.

Index General relativity ideas  

Go Back 