Light did move at a constant, wavelength-independent speed through the vacuum of space, with no need for a medium based on experiment and observation. Light that arrives appears to have less energy than the light that must've been emitted, but the lack of increased blurring at greater distances ruled that out. One alternative was the tired light scenario, which predicted that light would lose energy as it traveled through the medium of space. Did light always move at a constant speed through the Universe? Was there really no need for a medium for light to travel through? Did the fabric of space really curve and distort due to the presence of matter and energy? And was the Universe actually expanding? LucasVB / Wikimedia Commonsīack in the early days of relativity, there were a large number of challenges to Einstein's theories and the predictions they made.
The speed of light is constant in a vacuum, but different wavelengths of light travel at different speeds through a medium. and infrared eyes, you'd be able to see that ultraviolet light bends even more than the violet/blue light, while the infrared light would remain less bent than the red light does. Schematic animation of a continuous beam of light being dispersed by a prism. The only difference is that the light, having passed through a medium, is now blurred. Once it goes back into a vacuum, however, it resumes moving at the speed of light. This slowing affects different frequencies (or colors) of light by different amounts, just as how white light passing through a prism will split into different colors at different angles, because the amount that light slows down is dependent on the individual energy of the photons. Only when it's not in a vacuum, i.e., when it passes through a matter-containing medium, does light slow down. Whereas an object with mass will always move slower than the speed of light - since accelerating it to the speed of light would require an infinite amount of energy - light itself must always travel at the same speed: c, or the speed of light in a vacuum. Light, by virtue of having no rest mass but still carrying both energy and momentum, can never slow down as it travels through the Universe it can only travel at the speed of light. But we can only understand redshifts and blueshifts if we attribute them to a combination of motion (special relativistic) and the expanding fabric of space (general relativistic) contributions both. A galaxy moving with the expanding Universe will be even a greater number of light years away, today, than the number of years (multiplied by the speed of light) that it took the light emitted from it to reach us. The faster it goes, the farther the spectral lines are from their normal position in the spectrum.The farther a galaxy is, the faster it expands away from us and the more its light appears. From that, they can tell whether it is moving toward us or away from us, and also how fast it is going. Once they know that, they check to see the difference between where the spectral lines are compared to where they normally are. Astronomers use spectroscopy to analyse the light from an object (galaxy or star). The reason astronomers can tell how far the light gets shifted is because chemical elements, like hydrogen and oxygen, have unique fingerprints of light that no other element has. This is where red shift got its name, since the colours are shifted towards the red end of the spectrum. A star or galaxy moving away from us will look more red than it would if the source were not moving in our frame of reference. An object, like a star or a galaxy that is far away and moving toward us, will look more blue than it normally does.
The same happens with light when an object that emits light moves very fast. As you fill, the sound of the filling gets higher and higher, for the same reason that the noise the train makes gets higher as it gets closer to you. This is also the same as filling a water bottle under a tap. As the train speeds away, the sound gets stretched out, and sounds lower in tone. As the train moves towards a person, the sound it makes as it comes towards them sounds like it has a higher tone, since the frequency of the sound is squeezed together a little bit.
The easiest way to experience the Doppler effect is to listen to a moving train. The red shift is one example of the Doppler effect. Red shift is a method astronomers use to tell the speed of any object that is very far away in the Universe. As you can see, all the lines shift towards the red end of the spectrum due to red shift. On the left is a ray of light from the Sun, and on the right one from a far off galaxy.
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