Gravitational redshift is one of the core predictions of Einstein’s General Relativity, but has only recently been tested directly in such a strong-field environment as our galactic center. Only if gravitation itself is linked to not only mass but energy, too, does this make sense. When a quantum of radiation leaves a gravitational field, its frequency must be redshifted to conserve energy when it falls in, it must be blueshifted. If you’re seeing a photon rising up against a gravitational field, you’ll observe it to have a longer wavelength than when it was emitted, a gravitational redshift, and if you’re seeing a photon falling down into a gravitational field, you’ll observe that it has a shorter wavelength, or a gravitational blueshift. In the former case, you’d observe a Doppler redshift in the latter, a Doppler blueshift.īy applying the equivalence principle, Einstein immediately recognized that the same shifts must apply if the acceleration were due to a gravitational field rather than a moving-and-accelerating spacecraft. Similarly, if an identical photon were emitted from “above” you instead, its wavelength would appear compressed relative to the wavelength at which it was emitted. If you were in a spacecraft (or an elevator) that was accelerating upward, then a photon that was emitted from “beneath” you would have its wavelength stretched relative to its emitted wavelength by the time the photon caught up to your eyes. In fact, it was way back in 1907, when he first thought of the equivalence principle, that Einstein published his first prediction of this new type of redshift. ( Credit: Markus Poessel/Wikimedia commons retouched by Pbroks13)īut we wouldn’t have to wait until 1915 for the Einstein shift - what we now know as gravitational redshift (or gravitational blueshift) - to arise as a robust prediction. This has been verified to ~1 part in one trillion for matter, but has never been tested for antimatter. If inertial mass and gravitational mass are identical, there will be no difference between these two scenarios. The identical behavior of a ball falling to the floor in an accelerated rocket (left) and on Earth (right) is a demonstration of Einstein’s equivalence principle.
Conversely, photons with longer wavelengths have lower amounts of energy inherent to them, with infrared, microwave, and radio waves all less energetic than visible light. Photons with shorter wavelengths have higher energies, with gamma rays, X-rays, and ultraviolet light all more energetic than visible light. Each photon, or each electromagnetic wave, has a certain amount of energy inherent to it, and the precise amount of energy it possesses is related to its wavelength.
Light isn’t just a quantum mechanical “energy packet,” but is also an electromagnetic wave. Imagine you have a photon - a single quantum of light - that’s propagating through space. ( Credit: Philip Ronan/Wikimedia Commons) The range from 0.4 to 0.7 microns, which is perceptible to human vision, is only a tiny blip compared to JWST’s wavelength range of 0.5-to-28 microns, or of the full electromagnetic spectrum, whose wavelength ranges from sizes of a subatomic particle, like a proton, up to the sizes of planets. Although visible light gives us a rich and varied view of objects in the Universe, it represents only a tiny fraction of the electromagnetic spectrum.
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