Tired light effect
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The tired light effect is a hypothetical effect of frequency of light diminishing as it travels as if photons have been losing their energy along their way.
Since according to Planck's formula energy of a photon is equal <math>E = h\nu<math>, where h is Planck's constant and <math>\nu<math> is frequency of light, the lower the energy of the photon the lower its frequency and longer its wavelength. For that reason and because of the existence of Hubble redshift in the universe it has been postulated that photons encounter on their way certain resistance to their movement so that the energy of photons drops along distance x as <math>E(x) = E(0)e^{-x/R}<math>, where <math>E(x)<math> is energy of the photon at distance x from the source of light, <math>E(0)<math> is energy of the photon at the source of light, and R is some constant characterizing the "resistance of the space", the distance along which the energy of the photon drops by e.
There is an important reason for which "tired light effect" is not acceptable as the reason for the Hubble redshift: The observations of Hubble redshift show that together with this redshift there is also a proportional to it time dilation i.e. the time at the source of light runs slower proportionally to the lower energy of the photons. This is not accounted for by the "tired light effect" hypothesis.
The above tells that photons deliver all of their energy from the source to the detector and the observed redshift is simply a way of interpreting the lower frequency of photons caused by the time running slower at the source of light than at the detector. It is also consistent with the gravitational redshift (or blueshift) in which redshift (or blueshift) is also caused by the time running slower (or faster) at the source of light with photons delivering all of their energy from the source to the detector.