Speed of light / redshift

G

GetLastError

Registered Member
Please refer to Redshift on Wikipedia (can't post urls yet...)

Isn't this a proof that the speed of light isn't fixed?
I'm a bit confused, can anyone clarify this?

Is it the same as the doppler effect for sound waves?
 
GetLastError said:
Please refer to Redshift on Wikipedia (can't post urls yet...)

Isn't this a proof that the speed of light isn't fixed?
I'm a bit confused, can anyone clarify this?

Is it the same as the doppler effect for sound waves?
Click to expand...


The speed of light is a constant for any and all FoRs.
We have three kinds of "shifts "
[1] Doppler red or blue shift:
Is the change in frequency of a wave [mainly sound] when the emitter or the receiver is moving, resulting in a change in frequency

[2] Cosmological red or blue shift:
This occurs when there is an expansion of space between the emitter and receiver rather then either the emitter or receiver moving.
Again as happens with Doppler, it results in a change of frequency, and occurs when we view very distant objects in the Universe

[3] Gravitational red shift:
This occurs when light is emitted from a dense gravitational source resulting in a reduction in frequency and subsequently "time dilation "
 
GetLastError said:
Please refer to Redshift on Wikipedia (can't post urls yet...)

Isn't this a proof that the speed of light isn't fixed?
I'm a bit confused, can anyone clarify this?

Is it the same as the doppler effect for sound waves?
Click to expand...

Red/Blue shifting for light is similar to Doppler for sound...yes. As you know when a train goes by, the speed is (usually) constant but the frequency changes from your perspective. It's the same for light. Blue is toward the short end of the spectrum and Red is at the long end of the visible spectrum.

Anything moving closer (Andromeda Galaxy) is blue shifted and everything not "local" is red shifted due to the expansion of the universe.
 
GetLastError said:
Please refer to Redshift on Wikipedia (can't post urls yet...)

Isn't this a proof that the speed of light isn't fixed?
I'm a bit confused, can anyone clarify this?

Is it the same as the doppler effect for sound waves?
Click to expand...

The Doppler effect for sound can be found by

$$f = \frac{c+v_r}{c+v_s} f_0$$
where.
c is the velocity of waves in the medium;
vr is the velocity of the receiver relative to the medium; positive if the receiver is moving towards the source (and negative in the other direction);
vs is the velocity of the source relative to the medium; positive if the source is moving away from the receiver (and negative in the other direction).

The simplest examples are when the source is motionless with respect to the medium and the receiver is moving, or the receiver is stationary and the source is moving.

The first case simplifies to

$$f = \frac{c+v_r}{c} f_0$$

and the second to

$$f = \frac{c}{c+v_s} f_0$$

Now for light, it is a little different because there is no medium, and both the source and receiver measure the speed of light relative to themselves as being the same.

So in a way it is like the situation where the source moves and receiver stands still, and you always use

$$f = \frac{c}{c+v_s} f_0$$

Where c is the speed of light and
vs is the speed of the source relative to the receiver.

However, the real equation has to take the effects of relativity into account, so the relativistic Doppler shift formula is:

$$f=f_o \sqrt{\frac{1-\frac{v}{c}}{1+\frac{v}{c}} $$

where v is the relative velocity between receiver and source and is positive when they are approaching each other.
 
Allright, so is it that the "expansion of space" is accounted for the shifted frequencies? and not actually the speed?

Janus58, the relativistic formula still consider the relative velocity, but is it just a representation of the expansion of space? I'm even more confused now :)

To get this right, scientists measure light frequencies and detect a deviation in their observation (from what they think it should be), so they use the numbers and use the relativistic formula and deduce the relative velocity? which is actually an indication for the space expansion as well???
 
GetLastError said:
Allright, so is it that the "expansion of space" is accounted for the shifted frequencies? and not actually the speed?

Janus58, the relativistic formula still consider the relative velocity, but is it just a representation of the expansion of space? I'm even more confused now :)

To get this right, scientists measure light frequencies and detect a deviation in their observation (from what they think it should be), so they use the numbers and use the relativistic formula and deduce the relative velocity? which is actually an indication for the space expansion as well???
Click to expand...

My post addressed the Doppler shift aspect of red-shift. Cosmological redshift is somewhat different. Doppler shift gives you a redshift based on the relative velocity of the source at the moment the light left the source. Cosmological redshift gives you an answer based on how much space has expanded since the light was emitted.

Let's use the elastic sheet analogy. Let's say you say have a ball on this sheet, which at the moment is static. The ball is rolling away from you. You would would measure a Doppler shift from the ball that relate to the speed of the ball at the moment the light left the ball. For example if the light left the ball 1 sec ago, the red shift you see is due to the speed of the ball one second ago. Any changes in the ball's speed during that time will not effect it.

Now assume that the ball is not rolling, but the elastic sheet itself is stretching, Keeping in mind that the elastic sheet represents space itself. As the light travels from the ball to you, it is also stretched, and you will see a redshift. The difference being that this redshift will depend on how much the elastic sheet stretched between the time the light left the ball and you saw it. So unlike the Doppler shift example, where a change in speed of the ball after emission has no effect on the redshift, with Cosmological redshift a change in the expansion rate of space after emission does change the redshift.

There is another difference. With Doppler shift, you can't have velocities greater than c, however this limit does not apply for recessional velocities that are due to the expansion of space.
 
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