Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. However, in many contexts, such as black holes and Big Bang cosmology, redshifts must be calculated using general relativity. Its value is represented by the letter z.Ī special relativistic redshift formula (and its classical approximation) can be used to calculate the redshift of a nearby object when spacetime is flat. Redshifts are also seen in the spectroscopic observations of astronomical objects. Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift. ![]() Conversely, a decrease in wavelength is called blueshift and is generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field. Finally, gravitational redshift is a relativistic effect observed in electromagnetic radiation moving out of gravitational fields. A special instance of this is the cosmological redshift, which is due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase in their distance from Earth. A redshift occurs whenever a light source moves away from an observer. Some redshifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles. In general, whether or not the radiation is within the visible spectrum, “redder” means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light. In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum.
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