what is red shift and how do we know about it?
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Think of the difference in a train horn or police siren when it is heading towards you compared to driving away. The sound waves stretch out or compress depending on direction. It’s called the Doppler Effect. Same concept with light. The light shifts towards the blue or red side of the spectrum depending on if it’s moving towards our away.
Light from distant stars is shifted in the red direction (lower frequency = longer wavelength, as opposed to blue shift to a higher frequency). We know this by observing fine grain structures in the light they emit. For example, sodium radiates at two specific frequencies which are close to each other (588.9950 and 589.5924 nanometers). When you observe a pair of identical magnitude at 600nm and 600.5974nm you know you’re seeing sodium, but red shifted about 11nm.
Red-shift has to do with the wavelength of light.
Light, while moving at a constant speed in a vacuum, propogates (moves) from one place to another in a wave-like pattern (not literally in waves, that’s just how it is expressed in the electromagnetic spectrum, but since this is ELI5 I’m going to keep it simple). Those waves can be measured, which is called the wavelength. The number of times a full wavelength passes a fixed point is called the frequency. In short, the frequency/wavelength of light determines the color.
So you’ve got light coming from a source. As you move toward the light source, the waves are going to get to you a bit faster. This increases the frequency of the wave, which means the light is going to appear more blue. Move away from the source, and the waves take a little longer which lowers the frequency, which means the light is going to appear a little more red. Go faster toward it, even more blue. Go faster away from it, even more red. Stop moving, and the light appears to be it’s original color again.
That’s all red-shift and blue-shift means, really. There is an impressive amount of gloriously complicated answers to the original question (there’s an awful lot of math involved), but that’s the simplest way I can break it down for you.
Say you have a tennis-ball gun. It first one tennis ball every second. You fire it at me. I will get hit by one tennis ball every second.
But what if you start walking away from me (or me from you – relativity says it doesn’t matter). Each tennis ball now has further to travel than the previous one. So it will take a bit longer to hit me. While you are still firing one a second, I will get hit by one at a slightly slower rate (or frequency). If I know how often you are shooting them, and how often they are hitting me I can figure out how fast you are moving.
The same happens with light (and sound). Light has a frequency associated with colour (for example red light is about 405-480 THz, blue being 580-675 THz, so 600,000,000,000,000 ‘things’ per second). So if the thing emitted the light is moving, like with the tennis balls, the frequency it emits will be different to the frequency received; the colour of the light will change – if the thing is moving away the frequency will get lower (fewer hits per second) – so more towards the red (or towards blue if the thing is getting closer).
In cosmology this can be really useful, as if we know what colours light received from a distant galaxy *should* be, we can compare what it is and figure out how fast it is moving away. And that can help us figure out how far away it is.
Certain atoms/compounds make very distinct spectral lines, some like hydrogen and helium are present in any star, when those lines are shifted taped the red part of the visual spectrum we can deduce that that star must be moving away form is very fast, this is because the wavelength of light will get stretched out if something is moving away really fast, it is analogous to the Doppler shift you hear when a truck/motorcycle passes you.
I see alot of ***how*** explanations but no ***why***
If you detonate a nuclear bomb in space, you would see the explosion expand outwards equally in all directions. Just like throwing a rock in the water, you’ll see the explosion wave large at first then dissipate the farther and farther away it goes from the source. Like sound the closer/farther you get from the source, the louder/quieter it is.
**But Why**
The reason for this is the nature of energy waves in our universe. In space as your explosion expands from its origin the shell of the explosion sphere keeps increasing, so the energy of the explosion must keep covering a larger and larger area over time. Like those expanding ball toys. To compensate for this and obey conservation of energy, the intensity/amplitude of the energy wave decreases as the area it covers increases.
**Le Shift But How**
Now a star is that explosion, but its moving and constantly exploding every second. So as the distance between you changes you will be getting either bigger or smaller waves over time, if we see the wave energy dissipate with distance we call it red shift or increase as the source gets closer blue shift. We also call this the doppler effect. As others have said you can experience this with sound by standing next to a busy road.
This also means a star with no frequency shift is moving in parallel with us, for a moment at least
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