[John Chapman (OP)]

The speed of light is always constant, regardless of the position and relative speed of the observer.

For example, apparently if you were in a spaceship travelling incredibly fast and you were passed by a ship travelling at a similar speed in the opposite direction and at the same time you both passed a third stationary ship then the light from your headlights would be travelling at exactly the same speed relative to all three ships. Because speed is equal to distance divided by time then, to make this impossible thing happen, time and space will both become distorted to accommodate it. So, the speed of light is ALWAYS constant.

In that case how come light is subject to the Doppler effect and gives us Red Shift?

[Vern]

According to Einstein's theory space and time are both distorted for objects moving relative to each other. There is another way to look at it that I like better. The Lorentz Aether Theory contended that all things were distorted when they moved. It did not warp space and time. All the distortion was in the material things that moved. Space and time remained constant. Things that moved experienced dimensional and time distortion. This allowed H. Ziegler to assign a cause for relativity phenomena.

The Lorentz transformations are still used to calculate the distortions even in Einstein's theory. But the Lorentz theory is largely forgotten as is H. Ziegler.

Doppler shifting happens because motion tends to shrink and stretch waves whether it is sound or light. The constant speed of light doesn't effect the Doppler shift.

[John Chapman (OP)]

Hi Vern

Thanks for that interesting reply. I've not come across Lorentz and H Ziegler before, although I am not a physicist. I shall have a go at reading the information you linked to - though I may not understand it.

I also didn't understand your last sentence

Doppler shifting happens because motion tends to shrink and stretch waves whether it is sound or light. The constant speed of light doesn't effect the Doppler shift.

That seems counterintuative. But then so does most of quantum physics to me! How can the waves be stretched by motion without the speed being effected?

[lyner]

That isn't a QM thing. It's what happens with sound waves as well. If you are approaching the source of some waves then you bump into the peaks of the waves at a higher rate than the source was emitting them - giving you a higher frequency. The same sort of argument  applies when either source or detector moving towards or away. Once launched, the waves travel at their normal wave speed.

[Vern]

That seems counterintuative. But then so does most of quantum physics to me! How can the waves be stretched by motion without the speed being effected?

Think about if you were walking along a path and dropping a cookie every second so you could follow the return path. If you walk slow, the cookies will be closer together than if you walk fast. If you run along, still dropping a cookie every second, the cookies will be further apart still; they will in effect be red shifted.

[John Chapman (OP)]

Thanks for entering the discussion, sophiecentaur

Don't the waves only travel at their normal speed relative to their source? Surely the sound of a police siren is approaching your ears faster as the car travels towards you than it does when the car has passed and is travelling away?

Isn't this the same as saying that if you shoot a tree from a train that's travelling at 200 mph it doesn't matter if you shoot it as the train approaches the tree or as the tree disappears into the distance? The bullet will hit it at the same speed!

[John Chapman (OP)]

Hi Vern

If you run along dropping the cookies doesn't that mean that the leading edge of the line of biscuits is approaching you at a faster speed?

[Vern]

It isn't the leading edge that makes the distortion; you need the whole distance between biscuits [] The wave length of light determines its colour; the wavelength is the distance from crest to crest, or as in the analogy, from cookie to cookie.

[John Chapman (OP)]

Hold on. Let me think about this. This is beginning to make my head hurt!

[justaskin]

But the thing is that light is supposed to travel at the one speed.
Didn't Einstein say that two light beams traveling toward one would only appear to an observer to be approaching one another at the speed of light.So here is were I get confused with red shift if I as the observer only observe light traveling at the one speed how can I observe a red shift.This would suggest to me that I am seeing light at a speed different to the speed of light and if I understand Einstein that is not possible.

Cheers
justaskin

[lightarrow]

The speed of light is always constant, regardless of the position and relative speed of the observer.

For example, apparently if you were in a spaceship travelling incredibly fast and you were passed by a ship travelling at a similar speed in the opposite direction and at the same time you both passed a third stationary ship then the light from your headlights would be travelling at exactly the same speed relative to all three ships. Because speed is equal to distance divided by time then, to make this impossible thing happen, time and space will both become distorted to accommodate it. So, the speed of light is ALWAYS constant.

In that case how come light is subject to the Doppler effect and gives us Red Shift?

λ = c/ν
Since c is constant and invariant, if the frequency ν is lower, wavelenght λ is greater and you have red shift.

[John Chapman (OP)]

λ = c/ν
Since c is constant and invariant, if the frequency ν is lower, wavelenght λ is greater and you have red shift.

Eh? Is it possible to convert that answer into everyday language?

[lyner]

Hi Vern

If you run along dropping the cookies doesn't that mean that the leading edge of the line of biscuits is approaching you at a faster speed?

It's not the right analogy, though.
The waves travel through the air (let's talk about sound because it's more familiar) at he same speed, however fast you are going - why? Because, once you have 'let go of them' you can have no more influence on them.

Treat it like a very long train, going slowly through a station. Stand still and they will go past you at, say, on every second. If you run along beside the train, in the same direction, they will pass you at a slower rate - which would be zero if you ran at the speed of the train.
Running against the direction of the train will speed up the rate that you pass coaches.
The sound waves are the coaches and the frequency is how often they pass by you.

The formula
 λ = c/f
can be rewritten as
c = λf
c is the speed of the waves
f is the frequency ('real' Physicists tend to use the symbol ν - the greek letter "nu" but that's just likely to scare you off even more. f stands for frequency for most mortals)
λ is the wavelength.
So speed = wavelength times frequency
Why?
Back to the train: If the coaches are going past you (frequency) at 2 per second and they are 20m long (wavelength) then the speed of the train could be calculated by multiplying the number of coaches going pas per second by the length of a coach (=40m/s). So the wave speed is, similarly, frequency times wavelength.

To appreciate the red shift effect, it is not necessary to get relativistic. Just treat the light waves as traveling through space at speed c and you still have your train going through the station and the 'doppler shift'.
You will observe the speed of sound (and the train) to be different, according to which direction and how fast you, as observer go.  As it happens, relativity tells us that, however fast you are going, you will always measure the speed of light as the same value - but this is a bit of a red (shifted?) herring as far as getting an understanding of the basic red-shift mechanism. The sums come out a bit different but get used to the classical world first - then get relativistic.

[John Chapman (OP)]

I’ve just had a eureka moment about the Doppler effect. I think I’ve got it.

Instead of a police siren consider an approaching speaker. As the speaker produces sound the diaphragm vibrates. The faster the vibration, the higher the pitch. As the diaphragm vibrates, it’s surface is moving towards you creating a pressure wave, albeit a small movement. If the whole speaker is moving towards you then, when it vibrates, the diaphragm is moving towards you faster than when it is stationary. Therefore the pitch is higher. The actual sound isn’t traveling any faster as it’s speed is determined by the medium through which it is traveling, in this case air. Sound must always travel at the same speed in air or it wouldn’t be possible to overtake your own sound if you were traveling supersonically.

Therefore, as the police car travels towards you the siren has a higher pitch even through the sound is not actually reaching your ears any faster. I think.

Maybe.

[Vern]

I think you're associating wave length with the speed of the wave and there is no association there. The wave length is independent of the propagation of the sound; or light. The speed of propagation is constant, the wave length is variable and depends upon the relative speeds of source and observer. If you're going away from the wave, it will seem stretched out. If you're going toward the wave, it will seem squeezed together.

In the case of sound, you could measure a different speed of propagation if you were moving through the medium. In the case of light; its speed is constant no matter your movement through space.

According to Einstein, this constant speed is the result of a distortion of space and time.

According to Lorentz, this constant speed is the result of distortion of measuring sticks and clocks due to their material composition consisting only of a most fundamental substance that must always move at the speed of light.

[John Chapman (OP)]

Well, I only just sort of had a bit of a grasp of what I possibly thought might be happening, maybe. Now that's trashed.

I'll give it some more thought and try to get my head round it.

Thanks for your continued attempts to try and put me right on this.

[Madidus_Scientia]

Not trashed, I think you had it. Whether you're talking about the pitch of sound or the colour of light, it's solely to do with frequency, not speed.

And the reason frequency changes is because the source of the waves moves, like you say with your siren example.

[John Chapman (OP)]

When I said

I only just sort of had a bit of a grasp of what I possibly thought might be happening, maybe.

what I meant was

I'm completely confident I've got this speed of light and Dopplar thing nailed! The only thing that's separating me from Einstein is that he has neater hair.

Actually, I think I was 90% there before Vern's last comment but now I've got my head round it more completely. But just to make sure, can one of you tell me this:

If the source of the sound is travelling towards me then the wavelength is shortened but the speed the sound travels at is unchanged. But if I am travelling towards the source of the sound then it's the other way round. The relative speed of the sound is increased (relative to my ears, that is) and the wavelength is unchanged but the peaks of the wave reach your ears in closer succession which has the same end result as the shorter wavelength scenario.
 

[Vern]

Yes; I think you've got it right.

[Madidus_Scientia]

Correct