The above prediction , that all forms of energy interact gravitationally , has been subject to experimental tests . The first observation testing this prediction was made in 1919 . During a solar eclipse , Arthur Eddington observed that the light from stars passing close to the Sun was bent . The effect is due to the gravitational attraction of light by the Sun . The observation confirmed that the energy carried by light indeed is equivalent to a gravitational mass . Another seminal experiment , the Pound – Rebka experiment , was performed in 1960 . In this test a beam of light was emitted from the top of a tower and detected at the bottom . The frequency of the light detected was higher than the light emitted . This result confirms that the energy of photons increases when they fall in the gravitational field of the Earth . The energy , and therefore the gravitational mass , of photons is proportional to their frequency as stated by the Planck 's relation
solar eclipse
Solar eclipse of May 29, 1919
Mass–energy equivalence