Chapter 1.8 Gravitation Redshift

According to General Relativity, the wavelength of light (or any other form of electromagnetic radiation) passing through a gravitational field will be shifted towards redder regions of the spectrum. To understand this gravitational redshift, think of a baseball hit high into the air, slowing as it climbs. Einstein's theory says that as a photon fights its way out of a gravitational field, it loses energy and its color reddens. Gravitational redshifts have been observed in diverse settings.

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Earthbound Redshift

In 1960, Robert V. Pound and Glen A. Rebka demonstrated that a beam of very high energy gamma rays was ever so slightly redshifted as it climbed out of Earth's gravity and up an elevator shaft in the Jefferson Tower physics building at Harvard University. The redshift predicted by Einstein's Field Equations for the 74 ft. tall tower was but two parts in a thousand trillion. The gravitational redshift detected came within ten percent of the computed value. Quite a feat!

Solar Redshift

In the 1960s, a team at Princeton University measured the redshift of sunlight. Though small, given the Sun's mass and density, the redshift matched Einstein's prediction very closely.

White Dwarf Redshift

Take a star like the white dwarf star, Sirius B that is 61,000 times denser than the Sun. Its gravitational field is correspondingly much stronger and so is the redshift for the light it emits: 30 times greater, ac cording to earlier observations from the Mount Wilson Observatory taken by W.S. Adams way back in 1924. Still larger redshifts have more recently been detected in studies of so-called neutron stars -- collapsed stars that are even denser. What about the redshift caused by a black hole? It can be thought of as infinite. In other words, photons inside the hole are so redshifted they can never get out!

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