# Gravitational redshift

In physics and general relativity, we have something called gravitational redshift. It's like this: Imagine you have light (photons), moving away from a place with very strong gravity, like a star or a black hole. As these photons move away, they look like they are losing energy. This makes them look like they have a lower frequency and longer wavelength, which we call a redshift because this moves light to the red part of the spectrum.[1][2]

But, if photons move towards a place with strong gravity, like towards a star or black hole, they look like they are gaining more energy. This is called gravitational blueshift, and it's like moving light towards the blue end of the spectrum.[3][4]

There are two main ways to explain gravitational redshift and blueshift. One way connects it to the equivalence principle, which says that gravity and acceleration are kinda the same thing. If you think of it this way, the redshift happens because of something called the Doppler effect.[5]

Another way to think about it involves the concept of mass-energy equivalence (E=mc²). The basic idea is that when photons move into a stronger gravitational field, they gain energy. This gain in energy makes them look bluer. But when they move away from the gravitational field, they lose energy. This loss in energy makes them look redder.[6][7]

Now, don't expect to see lots of redshift in everyday life because gravitational redshifts are very small most of the time. For example, sunlight that escapes from the Sun gets redshifted by a teeny tiny amount, and GPS signals from satellites experience a tiny blueshift. But it's still important to think about these effects for precise measurements, especially in things like GPS technology.[8]

In astronomy, when we talk about these shifts, we often describe them as equivalent velocities. So, the sunlight redshift is like saying the light looks like it is moving away at a certain speed, but it's super tiny. The redshift is only much bigger when you look at objects with strong gravity, like black holes.[9]

Scientists use these gravitational redshifts to test Einstein's theory of relativity and even look for dark matter. It's an interesting concept that helps us understand the effects of gravity on light and the universe around us.[10]

## References

1. Einstein, Albert (1907). "Relativitätsprinzip und die aus demselben gezogenen Folgerungen" [On the Relativity Principle and the Conclusions Drawn from It] (PDF). Jahrbuch der Radioaktivität (4): 411–462.
2. Valente, Mário Bacelar (2018-12-06). "Einstein's redshift derivations: its history from 1907 to 1921". Circumscribere: International Journal for the History of Science. 22: 1–16. doi:10.23925/1980-7651.2018v22;1-16. ISSN 1980-7651. S2CID 239568887.
3. Einstein, Albert (1907). "Relativitätsprinzip und die aus demselben gezogenen Folgerungen" [On the Relativity Principle and the Conclusions Drawn from It] (PDF). Jahrbuch der Radioaktivität (4): 411–462.
4. Valente, Mário Bacelar (2018-12-06). "Einstein's redshift derivations: its history from 1907 to 1921". Circumscribere: International Journal for the History of Science. 22: 1–16. doi:10.23925/1980-7651.2018v22;1-16. ISSN 1980-7651. S2CID 239568887.
5. Florides, Petros S. "Einstein's Equivalence Principle and the Gravitational Red Shift" (PDF). School of Mathematics, Trinity College, Ireland.
6. Chang, Donald C. (2018). "A quantum mechanical interpretation of gravitational redshift of electromagnetic wave". Optik. 174: 636–641. Bibcode:2018Optik.174..636C. doi:10.1016/j.ijleo.2018.08.127. S2CID 126341445.
7. Evans, R. F.; Dunning-Davies, J. (2004). "The Gravitational Red-Shift". arXiv:gr-qc/0403082.
8. Alley, Carrol Overton. "GPS Setup Showed General Relativistic Effects on Light Operate at Emission and Reception, Not In-Flight as Required by Big Bang's Friedman-Lemaitre Spacetime Expansion Paradigm" (PDF). The Orion Foundation.
9. Trimble, Virginia; Barstow, Martin (November 2020). "Gravitational redshift and White Dwarf stars". Einstein-Online. Max Planck Institute for Gravitational Physics. Retrieved 2021-01-16.
10. Alley, Carrol Overton. "GPS Setup Showed General Relativistic Effects on Light Operate at Emission and Reception, Not In-Flight as Required by Big Bang's Friedman-Lemaitre Spacetime Expansion Paradigm" (PDF). The Orion Foundation.