Red shift. How ??

[mistermack]

Is there anyone out there who understands red shift?
I'm talking about the doppler effect, that says that the farthest galaxies are moving away from us fastest? I know it happens, I just can't see the mechanism.

The doppler effect on sound waves I find quite easy. Motion by the sound source relative to the ambient air medium effectively changes the frequency.

But with light in space, there is no ambient medium, and the speed of light relative to the source is always c, no matter what the speed of the source is. Which is totally different to sound, when the speed of the source can vary hugely, relative to the air medium which the sound wave travels through.

To such an extent that the source can outrun the soundwaves. Whereas light always travels at c relative to the source, no matter what.

So, the vibrating source of a sound wave squashes up the peaks and troughs of the wave, if it's travelling in the same direction. But how can light do that, if the peaks and troughs are always travelling at c relative to the source?

I'm sure the answer is out there, I just can't picture it.

[Xamonas Chegwé]

One way of understanding it is to think of light as the transfer of energy. When a photon is emitted from a source that is moving away from you, it uses some of its energy overcoming its original momentum, hence its frequency decreases. The opposite is true when the source is travelling towards you, where the initial momentum adds to the energy of the photon, increasing the frequency.

[Jason]

Light is a sine wave/particle yes? So think of it as a sinusoidal function. When the source is moving away the period of the function increases (or the frequency decreases) as the distance from peak to peak, x axis on the graph, increases. When the source is moving toward you the opposite is true as the length of the period decreases (or the frequency increases).

I picture the graphs of the functions when I think of it so that might help make sense of it.

[mistermack]

One way of understanding it is to think of light as the transfer of energy. When a photon is emitted from a source that is moving away from you, it uses some of its energy overcoming its original momentum, hence its frequency decreases. The opposite is true when the source is travelling towards you, where the initial momentum adds to the energy of the photon, increasing the frequency.

That's a good way of looking at it.
It seems to be a different phenomenon to the doppler effect of sound waves then? And just a coincidence that the result is similar?

From what you said, I'm guessing it's down to frames of reference again. In the frame of reference where the source is stationary, there is no red shift. But in all the frames of reference moving towards or away from the source, the photon has higher or lower energy, expressed by higher or lower frequency.

[Xamonas Chegwé]

One way of understanding it is to think of light as the transfer of energy. When a photon is emitted from a source that is moving away from you, it uses some of its energy overcoming its original momentum, hence its frequency decreases. The opposite is true when the source is travelling towards you, where the initial momentum adds to the energy of the photon, increasing the frequency.

That's a good way of looking at it.
It seems to be a different phenomenon to the doppler effect of sound waves then? And just a coincidence that the result is similar?

No. Not a coincidence at all! Think about your next statement below - there is the key to understanding red-shift from a "wave" perspective.

From what you said, I'm guessing it's down to frames of reference again. In the frame of reference where the source is stationary, there is no red shift. But in all the frames of reference moving towards or away from the source, the photon has higher or lower energy, expressed by higher or lower frequency.

Think about the General Relativity thought-experiments using a light-beam reflected between two mirrors on a moving train.

To those on board the train, the light moves back and forth and each reflection travels the distance between the mirrors.

To those watching from the station as the train roars past, the light travels in a diagonal zigzag due to the mirrors moving relative to them.

But it is the same light - with the same number of peaks and troughs in its wave. So, since the light in the second example travels further, it has fewer peaks per metre and hence a longer wavelength.

Not only are time and space relative - energy is too! And mass, for that matter.


You can also think about a brick that is stationary in space, relative to you. It is stationary and has 0 potential energy. To someone travelling towards you at 500mph, that brick is moving at 500mph and has fucking tons of PE - enough to rearrange his face quite spectacularly!

[mistermack]

The difference with sound though, is that the speed of sound is different, depending on your motion.
If I'm travelling towards the sound at 200 mph, I'm experiencing sound that is travelling at about 900 mph. And the reverse, if I'm going away from the sound source, the speed of sound is 500 mph relative to me. And the air medium itself then has lots of kinetic energy, relative to me, which doesn't come into it in space.

I'm sure you meant kinetic energy, not potential, of the brick. But yes, I don't have any problem with the concept of kinetic energy being relative.
I was just trying to get a mental picture of the red shift happening.

So if you could get on a space ship, travelling at the same speed and direction as the light source was travelling , you would see no red shift. Which is how they calculate the speed and direction of the distant galaxies I guess.

[Xamonas Chegwé]

You're dead right I meant kinetic energy - blame the wine!

They measure redshift by comparing the absorption lines of Hydrogen in light coming from different sources. As light passes through Hydrogen, certain, specific wavelengths are absorbed (those that are at precisely the energies needed to raise the electrons of the atom to the next (or greater) levels.) Light coming from distant sources can be seen to have the same absorption spectrum - only shifted towards lower energies - hence red-shifted.

The Doppler effect is affected by three things: The motion of the wave-source, the motion of the observer and the motion of the medium. The change in frequency of an approaching and retreating siren is different on a windy day!

With light waves, there is no medium, so only the relative velocities of the source and receiver are relevant.

The classical mathematical formula of the Doppler effect is

Where f is the observed frequency, f0 is the emitted frequency, c is the speed of the wave through the medium, vr is the velocity of the receiver relative to the medium and vs is the velocity of the source relative to the medium. This is perfectly applicable to the changes in sound waves in air.

However, when taking relativistic effects into account, things get a little more complicated. It is impossible to ignore time dilation which has an effect on the maths involved. It is explained here but I doubt that will help much without some background study.

Suffice to say that the absence of a medium does not negate the Doppler effect for light. However, it does affect the observed frequencies.

Oh, and there are 3 kinds of redshift - just to muddle things a little more!

The one we have been discussing is the Relativistic Doppler Effect, caused by the relative motion of two objects. There is also Cosmological Redshift, caused by the expansion of the universe by inflation and the Gravitational Redshift, caused by light from a distant galaxy expending energy to escape from its gravity field that is different to the energy it acquires by being drawn into our gravity field!

[mistermack]

I think I can picture it better now. For some reason, it's much harder to picture what's happening, if you imagine yourself in the line of sight of the light beam, meeting the light head-on.
As an observer from the side, it seems to be much easier.

If the original light source and the receiver of the light are moving towards each other at 2000 mph, an observer on the receiver seeing the light head-on won't be aware of that fact, because of time dilation. He will still measure the relative speed of the light and himself as c.

But any external observer will measure varying relative speeds, between the receiver and the light.

If the external observer is moving at the same speed and direction as the original emitter of the light, he will measure the light speed at c, and the receiver speed as 2000 mph, so that from his viewpoint, the light and the receiver are meeting at c + 2000 mph. And he sees no red shift, as he's stationary, relative to the emitter.

But if he is travelling at the same speed and direction as the receiver, ie 2,000 mph towards the original emitter, he sees the receiver as stationary, and the light he still measures at c. So he observes the light meeting the receiver 2,000 mph slower than the first case, (ie at c) because of time dilation, but because he's moving himself relative to the original emitter, he observes an apparent shift in the frequency of the light. Or I should say a real shift, from his point of view.
And because he's stationary in the same frame of reference as the receiver, that's what the receiver experiences too.

So every observer sees the same light at different frequencies, and the only one who sees it's original frequency, is one who is stationary in the same frame as the emitter was stationary.

[Xamonas Chegwé]

Sort of true. However, bear in mind that you can only ever "observe" light by standing in the line of sight! An observer to one side has no way of observing the photons travelling towards the receiver at all - only those coming straight at him. Your explanation works as a hypothetical thought-experiment but could never be observed experimentally.

[mistermack]

Could it be that all photons are the same, and the only difference is the relative motion of emitter to the receiver? That high energy photons are simply ones that were emitted by a source travelling at a phenomenal rate in our direction, and vice versa?
It could be momentary motion, like in something rotating very fast, or explosions, or very hot gases.

[JimC]

Could it be that all photons are the same, and the only difference is the relative motion of emitter to the receiver? That high energy photons are simply ones that were emitted by a source travelling at a phenomenal rate in our direction, and vice versa?
It could be momentary motion, like in something rotating very fast, or explosions, or very hot gases.

No, because a source stationary with respect to an observer can produce a variety of different energy (& thus frequency) photons depending on the energy released when an electron transits various energy levels.

[mistermack]

Could it be that all photons are the same, and the only difference is the relative motion of emitter to the receiver? That high energy photons are simply ones that were emitted by a source travelling at a phenomenal rate in our direction, and vice versa?
It could be momentary motion, like in something rotating very fast, or explosions, or very hot gases.

No, because a source stationary with respect to an observer can produce a variety of different energy (& thus frequency) photons depending on the energy released when an electron transits various energy levels.

Depends what you mean by source though. A star, or a planet, or a molecule, or an electron?
Could be that within the molecule, the momentary motion that produces the photon varies in speed relative to the observer.

[JimC]

Could it be that all photons are the same, and the only difference is the relative motion of emitter to the receiver? That high energy photons are simply ones that were emitted by a source travelling at a phenomenal rate in our direction, and vice versa?
It could be momentary motion, like in something rotating very fast, or explosions, or very hot gases.

No, because a source stationary with respect to an observer can produce a variety of different energy (& thus frequency) photons depending on the energy released when an electron transits various energy levels.

Depends what you mean by source though. A star, or a planet, or a molecule, or an electron?
Could be that within the molecule, the momentary motion that produces the photon varies in speed relative to the observer.

No. For any given photon released, its intrinsic energy (and therefore frequency) depends on the energy difference between the initial energy level of the charged particle involved (often electrons, but also protons) and the final energy level. Within an atom's electron shell, these differences in energy levels are quantised.

Any red shift phenomena are superimposed upon the initial frequency of the photon involved.

[mistermack]

Could it be that all photons are the same, and the only difference is the relative motion of emitter to the receiver? That high energy photons are simply ones that were emitted by a source travelling at a phenomenal rate in our direction, and vice versa?
It could be momentary motion, like in something rotating very fast, or explosions, or very hot gases.

No, because a source stationary with respect to an observer can produce a variety of different energy (& thus frequency) photons depending on the energy released when an electron transits various energy levels.

Depends what you mean by source though. A star, or a planet, or a molecule, or an electron?
Could be that within the molecule, the momentary motion that produces the photon varies in speed relative to the observer.

No. For any given photon released, its intrinsic energy (and therefore frequency) depends on the energy difference between the initial energy level of the charged particle involved (often electrons, but also protons) and the final energy level. Within an atom's electron shell, these differences in energy levels are quantised.

Any red shift phenomena are superimposed upon the initial frequency of the photon involved.

That wouldn't negate the possibility. If the difference in energy states is quantised, then the speed of the change is quantised too.
Like a rifle shooting a bullet. The powder charge is quantised, and the muzzle velocity of the bullet is a quantum value, giving a quantum energy for the bullet.
The speed of motion from one state to another, could give the photon it's frequency.

So for a source moving away from you, that quantised speed is slightly less than a source that isn't.

[JimC]

When you are talking about "speed of change", you really mean acceleration. In classical physics, it is accelerating charges which generate electromagnetic radiation. In a wire carrying AC current, for example, the oscillation involves regular backwards and forwards acceleration, which produces RF waves of 50 Hz. In a classic x-ray machine, the stream of electrons hits a barrier, and undergoes violent deceleration, producing high frequency X-rays.

But in an electron shell, it makes no sense to separate the energy levels and rates. The energy, & thus the frequency of the emitted photon is entirely fixed by the difference in energy levels as the electron makes its quantum jump downwards.