# Colloquium Detail

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*
Mass, Photons, Gravity*

### Date: 12/9/1999

**
Lev Okun
**
Professor, ITEP Moscow

The concept of mass is often misinterpreted in the literature. One can find a host of various masses: relativistic mass, rest mass,inertial mass, gravitational mass, active mass, passive mass, etc., while in fact there exists only one mass. Photon has vanishing mass. The behaviour of a photon in a given gravitational field is determined by its energy-momentum tensor. The correct explanation of the gravitational redshift is not the loss of photon's energy as it "climbs out" of gravitational potential, but the fact that clocks (atoms , nuclei) run faster at higher altitudes (at larger distances from gravitating bodies).