So. We know that light bue- and redshifts, we know this because the galaxies speeding towards us are blue and the one speeding away are red. However, we also know that the speed of light is a universal constant, so the velocity of those galaxies cannot be added or subtracted from the speed of light to change its frequency and therefore wavelength. So how is this happening? Is it because of time dilation? We know that time does dilate when moving at speed, so that does make sense and would explain the change in wavelength for the observer. But, if that is so, what happens to the wavelength of light if two galaxies are zooming towards each other at equal speed? Do they each see a blueshift, even though time for each galaxy would be passing by equal amounts? Or is the light just white, which would make sense re time but makes no sense re two objects hurtling towards each other with a combined speed that is double their individual speed and not having any shift in wavelength. So confused!
Ps I’m shit at maths and don’t have a great background in physics so if you know the answer, please show me a little mercy and explain in terms of the real world rather than an equation 😀👍

You need to watch interstellar another 3 times

But, if that is so, what happens to the wavelength of light if two galaxies are zooming towards each other at equal speed?

They will both see the same shift in wavelength. If there’s a third galaxy involved, the two original galaxies will see a different shift in wavelength for that third galaxy if it is moving at different relative speeds for the first two.

You cannot just linearly add the velocities. For example, if A and B accelerate away from C in exactly the same direction but A accelerates twice as hard as B, when B reaches half the speed of light relative to C, A will not be travelling away from C at the speed of light. No object with mass can be accelerated to the speed of light relative to any other object.

You need to watch interstellar another 3 times

Happy to, I love that film!

However, we also know that the speed of light is a universal constant, so the velocity of those galaxies cannot be added or subtracted from the speed of light to change its frequency and therefore wavelength.

You’re not changing the speed of light, you’re changing the time it takes to get to you by moving the emitter – effectively changing the wavelength.

Maybe think of it like this; you and a friend are in a vacuum, your friend has a marshmallow machine gun and shoots at you. You get a stream of marshmallows hitting you at a constant frequency. If your friend starts moving away from you, that frequency will drop as it takes each subsequent marshmallow a bit longer to cover the increased distance.

I’m not a physicist, but this is my understanding. The speed of light is constant for any observer, and that can happen because time speeds up to allow for the relative movement. There is still a redshift/blueshift effect but it’s adjusted by the time dilation.

Redshift is also due to the expansion of the universe. Imagine all the galaxies drawn on a huge sheet of rubber, which is stretched so that they move apart. As the sheet is stretched, the light waves are stretched and the wavelength increases, hence red.

They will both see the same shift in wavelength

But what is the shift?

1) Blue
2) Red (admittedly unlikely)
3) White (as in no shift)?

The speed of light is constant for any observer, and that can happen because time speeds up to allow for the relative movement. There is still a redshift/blueshift effect but it’s adjusted by the time dilation.

This is my understanding also

Redshift is also due to the expansion of the universe. 

This was my initial understanding, but it doesn’t explain how we can see blue shift galaxies as the universe is expanding universally- none of it is contracting. Which is why your first explanation works better for me

If something is moving away from you, it is redshifted. If it’s moving towards you, it’s blue shifted.

It’s all a hologram. Aliens are laughing at us trying to work it all out.

If something is moving away from you, it is redshifted. If it’s moving towards you, it’s blue shifted

That’s what I thought. But how is this possible if time is passing at the same rate for both galaxies? Which of the three variable that I am aware of (distance, time and velocity) is giving way to allow the frequency to change? We already know it isn’t velocity as c is a constant. It doesn’t appear to be time as that will be passing at the same rate for both observers as they are travelling at the same speed. There’s no reason for distance to change- that will also be passing at the same rate as c is constant etc. So how can the light blue shift?
If there is another variable I haven’t thought of let me know 👍 Energy maybe? But how would that work?

it doesn’t explain how we can see blue shift galaxies as the universe is expanding universally

If you blow a balloon up – they are the things you use to tell everyone it’s a party in case you didn’t know, it gets bigger but it doesn’t mean all the molecules of air in it are moving away from each other. My guess is it’s something like that. but it doesn’t matter how many RI lectures I watch on YouTube I generally remain happily baffled

If you blow a balloon up – they are the things you use to tell everyone it’s a party in case you didn’t know, it gets bigger but it doesn’t mean all the molecules of air in it are moving away from each other

Quite

I guess my question is how is light able to blue or red shift if it’s speed- and therefore the frequency at which the waves of light hit your eyeballs- is constant. What mechanism is allowing for this?

You need to watch interstellar another 3 times

This, but while drunk

The speed at which galaxies are “moving” isn’t constant – they’re accelerating. This explains the shift as it’s caused by the Doppler effect.
(I say moving but in the case of very distant galaxies it’s actually more to do with the space between the galaxies expanding as per the balloon analogy)

However galaxies such as our one & Andromeda are, relatively, very close together so gravitational attraction is causing them to accelerate towards each other! (And ultimately they will collide together)

Daniel and Jorge Explain the Universe is a great podcast that covers this kind of stuff in a somewhat-easy-to-understand way!

If something is moving away from you, it is redshifted. If it’s moving towards you, it’s blue shifted

That’s what I thought. But how is this possible if time is passing at the same rate for both galaxies?

I think this was the question Einstein asked himself ^^^

Space is not flat, it’s curved. It has deep hollows in it where stars and galaxies are, and the bits in between are stretching all the time. Because it’s stretching the other galaxies are mostly moving away from us which means whilst the waves are travelling at the same speed they are taking longer to reach us. Similar to how a police siren gets lower in pitch as it moves away from you.

But how is this possible if time is passing at the same rate for both galaxies?

Within each inertial frame, time will appear to pass at the same rate. However, if you accelerate you change your inertial frame and time will pass at different rates for different inertial frames. If you accelerate way from earth, your watch will appear to run slower to someone still on earth, but you won’t notice any difference with your own watch. This is a problem for things like GPS, where the speed of orbiting satellites is enough to cause a detectible difference in the speed of atomic clocks.

Frequency is proportional to the energy of the photons, not the speed/velocity.

Frequency is proportional to the energy of the photons, not the speed/velocity.

Ok, that makes more sense

My head hurts just thinking about this stuff.  then what it is with some tiny bits of stuff acting like waves in one way and particles in another?  Wavicles?  FFS make your mind up!

What if the Galaxy concerned is on a conveyor belt?

What if the Galaxy concerned is on a conveyor belt?

Then better chocolate is available.

You’re confusing two different things, relativity, and the doppler effect.

Relativity says that the speed of light will always look the same (which is needed for the doppler effect to be consistent, but you get the same effect with sound waves which are less consistent so you can explain one without the other).

The Doppler effect is what makes a car go:

NNNNNNNNNEEEEEEE(high pitch)EEEEEEEEEEEEE <observer> OOOOOOOOOOWWWWWWW(low pitch)WWWWWWWWW as it passes you, but when your sat in the car it just goes EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE (just watch some F1 with the onboard Vs pit wall cameras).

Explanation based on a fast car and a stationary observer (the one you can relate to in every day life but is harder to explain):

Each oscillation of the sound wave is being transferred to the air at the same frequency, but as it travels towards you it’s transferring them into the air at different locations. Now those waves travel out from the car at a constant speed in every direction. So the ones going in front of the car have been created at the same frequency, but because the car was moving the wavelength is shorter, but they’re traveling at the speed of sound (assumed constant) so they can only actually be one of those things, so the observer hears the shorter wavelength/higher frequency. Reverse that as the car passes.

Explanation based on a fast observer and a stationary generator (the one you cant really relate to in everyday life but is easier to explain):

Imagine you’re on a boat at sea. The waves (which are behaving exactly like sound or light, just at a really slow speed) are traveling in one direction. You can either point your boat towards them, and it will feel really choppy and you hit a lot of waves (you observe a high frequency, perceive a short wavelength), or you turn your boat around and go with them, at this point you perceive a much lower frequency.

How that relates to relativity:
Both cases are the same, you can’t say the boat is moving except it’s moving relative to the sea, but for all you know you’re battling against a tide. You get exactly this effect in a Channel (English or Bristol work for this example), if the swell is coming from the soutwest, and the tide is rushing out (heading west) then you get (to an observer on a boat, also affected by tides) waves closer together and it feels rough. On the other hand, a bouy monitoring the waves would see a different frequency, because it thinks it’s moving relative to the water, but isn’t actually moving.

This is why relativity becomes important, because you can think up a thought experiment with a wave tank on a conveyor belt and use it to demonstrate what you would see if you broke the laws of relativity. E,g if the waves speed and the tide oppose each other exactly you end up with what appears to be a stationary wave to an external observer. But a boat being dragged by the tide sees something completely different. We know that if you observe light from something you still observe it traveling at the same speed, it doesn’t stop just because you change your velocity relative to the emitter or the wave.

How this relates to relativity and the speed of light:
If you know the speed of light should be constant, and you know what color the object should be (because you know it’s got certain elements in it and therefore certain absorption lines) then you can calculate how fast it is moving relative to the speed of light (a 1% shift is 1% the speed of light). And that gives you it’s speed relative to you.

The fact that everything in the sky looks “red” means it’s all moving away from us.
The fact that everything looks redder the further away it is (i.e. it kinda looks like we’re the center of the universe), proves both that the universe is expanding in every direction, and that light must travel at a constant speed relative to the observer (because we’re not the center, but I’ll be honest I stopped being able to follow the maths at that point which is why I went into engineering, not Physics).

Which of the three variable that I am aware of (distance, time and velocity) is giving way to allow the frequency to change?

I think the answer to that is that the relationship between those variables, in Newtonian physics, is something most of us understand. But this isn’t Newtonian physics. In Special Relativity they don’t behave the same way, so the frequency can change. Special Relativity is is maths and relates to the curvature of space-time; I don’t pretend to understand it but I believe it’s the explanation.

I don’t believe in special relativity or quantum physics.  I think they are made up excuses to try to account for the gods being mischievous and winding physicists up.

what happens to the wavelength of light if two galaxies are zooming towards each other at equal speed?

Are any galaxies moving towards each other, or are they all moving away from the centre of the universe, post-Big Bang?

Are any galaxies moving towards each other

Yes. For example the Andromeda Galaxy is moving toward the Milky Way (our galaxy) and a collision is predicted in about 4.5 billion years.

Some good detailed answers there… but the marshmallow gun is my favourite.

Wavelength is effected by the changing distance between emitter and receiver. “Stretching” or “compressing” the waves (yes, I know, it isn’t, but it has that effect on what you can observe).

Are any galaxies moving towards each other, or are they all moving away from the centre of the universe, post-Big Bang?

It’s not either/or… the distance between both and the centre of the universe can be increasing, but if that’s not at the same rate, then they can be moving towards each other (while also both “moving” away from the centre of the universe, they’re not moving away, but… we can observe things that tell us they are both increasingly far from the centre).

@thisisnotaspoon, the speed of light is a fundamental constant whereas the speed of sound isn’t, so the analogy doesn’t quite translate

I thought the speed of light does change depending what the material it is going thru?  ( diffraction etc?) so speed of light in a vacuum is a constant but then there is no true vacuum anywhere?

I thought the speed of light does change depending what the material it is going thru? ( diffraction etc?) so speed of light in a vacuum is a constant but then there is no true vacuum anywhere?

Yes, speed of light in a vacuum. Deep space is so close to a vacuum that it doesn’t matter. If you get into quantum physics, even a vacuum is a roiling cauldron of virtual particles (if I understand it correctly). However, relativity and quantum physics operate at vastly different scales and the virtual particles will just wash out as random noise, I assume. I’m sure there have been Ph.D. theses on that.

@thisisnotaspoon, the speed of light is a fundamental constant whereas the speed of sound isn’t, so the analogy doesn’t quite translate

It does and it doesn’t.

The speed of sound is constant if you define the medium, it’s just that in the real world “air” is far from homogenous. Ditto the speed of light is only constant if you define the medium also. As soon as you put something in the way it appears to slow down. It doesn’t actually slow though, it just hits a particle, gets absorbed, then re-emitted. You can even bring “light” to a complete standstill! Well you can’t, but you can create a material that at very cold temperatures will absorb the light, sore it as an excited state within the atoms, then emit it again. But visually it’s been stopped, and it’s an extreme example of the same process as occurs when the light goes from air to water and loses about 1/4 of it’s speed.

Sounds like we’re all going to end up perishing in a ball of blue flame.

Probably on a Friday, with a packed weekend ahead.

The speed of sound is constant if you define the medium

Fair point!

I think whwre I’ve been going wrong is it’s all about the point at which the light or sound wave was propagated, rather than the speed of the waves

while also both “moving” away from the centre of the universe

The universe doesn’t have a centre.

Imagine a football, but only the surface. It’s two dimensional, you can only go left, right, forward and backwards, but it’s not flat in the third dimension. It has a finite area but no edges, and no centre. The universe is like that but in 3 dimensions.

Drifting a bit from the OP’s questions, but the scale of this puts my daughter’s complaints about my roast onion farts into perspective.