Red/blue shift question.

[Saint]

Good picture. That was nicely illustrated. Of course, I still have some questions. When you described the scenario above, you included both the source and sink for the light.

 

How about this:

A very distant star (at rest wrt to us for purposes of example) generates light for a million years. Then it dies.

 

We begin to recieve it's light after it is already dead.

 

We begin to move toward the location of the now dead star at a high rate of speed.

 

Can that light be blue-shifted?

[danny8522003]

Yes, as you'd still simply just be moving through the waves and creating a blue shift because the peaks would hit you more often.

[Mowgli]

[...]

light can't be subject to a doppler effect as it will always leave is source and reach it's destination at a set speed

[...]

You can have a Doppler effect even when the speed of the waves involved is not changed. Janus gave you an example: you are standing at a railway station, a train passes by, whistling. The tone of the whistle is changed by the Doppler effect. You can calculate the speed of the train from the Doppler shift.

 

Mowgli

[Mowgli]

The ability to move faster than the speed of sound means that you will create sonic booms, which obviously is not related to the doppler effect... only that now you know that there won't be an "electromagnetic booms".

Actually, we don't know that there won't be an electromagnetic boom. If there is one, we may very possibly mistake it for something else. Let me explain it this way: imagine human-beings were all blind and we heard a sonic boom. Would we think of it as something moving faster than sound or an explosion of some sort? We would have to think of it as a strange explosion because that is what we hear. In an exact analogy, because we don't have a stronger sense to "see" what is going on in our sense of sight, we may mistake a "luminal" boom for something else -- a gamma ray burst, for instance.

 

Mowgli

[MetaFrizzics]

Note that all this requires is for the source to have a velocity with respect to the observers and that the observers measure the speed of light as a constant with respect to themselves.

In this case, it seems to me that you have not distinguished between Special Relativity (SRT) which postulates that the speed of light is constant, versus say sound waves, which travel through air also at a constant speed but change in frequency if the source is moving.

 

For completeness you should explain the difference between SRT and sound waves, as well as the similarities!

[Thomas Kirby]

The redshift/blueshift of light is not due to the Doppler effect.

[swansont]

The redshift/blueshift of light is not due to the Doppler effect.

 

 

Why on earth would you think that?

[timo]

Cause it´s a way to get attention. If he really had another opinion he had added an "...it´s actually due to <cosmological epansion/time dilatation/temperature/magical elves/...>", I think.

[MetaFrizzics]

I was hoping someone *else* would explain the real difference between SRT/QM and Newtonian versions of waves and redshifts. I suppose I'll have to do it myself. But I thought you guys would comb through it like Rapunzel's hair.

 

I still think it is the real question in this thread: or at least a good one:

What is the difference between the SRT/QM treatment of waves versus the classical treatment in plain English?

Still, what do you think?

[Severian]

In this case' date=' it seems to me that you have not distinguished between Special Relativity (SRT) which postulates that the speed of light is constant, versus say sound waves, which travel through air also at a constant speed but change in frequency if the source is moving.

[/quote']

 

There is no real difference. As long as the speed of sound didn't vary in between you and the source (ie. it was a constant medium), then sound and light behave exactly the same way. The only possible difference is that you can move to a frame where you can overtake sound, but not to a frame where you can overtake light. In other words, the transformation laws from one frame to the other are different, but since the observer isn't changing his frame of reference, this is immaterial.

[Daecon]

"The speed of light is constant."

 

Is that meant in an absolute reference or relative?

 

I feel that there's an inherent difference between the two (when regarding light) that isn't made clear here. Which one is being talked about specifically?

[swansont]

"The speed of light is constant."

 

Is that meant in an absolute reference or relative?

 

I feel that there's an inherent difference between the two (when regarding light) that isn't made clear here. Which one is being talked about specifically?

 

The speed of light is the same in all inertial frames.

[Thomas Kirby]

 

Why on earth would you think that?

 

Because the "wavelength" of the photon is the physical size of the packet. Redshift and blueshift are due to the change in the energy carried by the photon, not the change in "frequency" such as in Doppler shift. The Doppler shift is about overtaking or moving away from wavefronts, so that the observer intercepts fewer or more wavefronts depending on the direction of motion.

[MetaFrizzics]

Yes, it helps if you make plain whether you are looking at the problem from classical EM theory or SRT, or from the QED viewpoint..

[Thomas Kirby]

This is more like I'm explaining it from the observed reality viewpoint.

[timo]

Because the "wavelength" of the photon is the physical size of the packet.

No. Wavelength is the wavelength of a photonic state contributing to a packet. A packet with only a single definite photonic state contributing has infinite size as the probability density for such a state is constant with regard to position.

 

Redshift and blueshift are due to the change in the energy carried by the photon, not the change in "frequency" such as in Doppler shift.

Frequency and energy of a photon is the same; it´s just expressed in different units.

[Thomas Kirby]

I'm not going any further than the Einstein-Planck relationship.

[Mowgli]

This is more like I'm explaining it from the observed reality viewpoint.

I'm intrigued...

Did you mean "observed reality" as opposed to the absolute, physical reality?

 

Mowgli

[Thomas Kirby]

I'm intrigued...

Did you mean "observed reality" as opposed to the absolute' date=' physical reality?

 

Mowgli[/quote']

 

The size and energy of a given photon can be measured.

[Saint]

There is[/b'] no real difference. As long as the speed of sound didn't vary in between you and the source (ie. it was a constant medium), then sound and light behave exactly the same way. The only possible difference is that you can move to a frame where you can overtake sound, but not to a frame where you can overtake light. In other words, the transformation laws from one frame to the other are different, but since the observer isn't changing his frame of reference, this is immaterial.

 

There is a huge difference. The doppler shift for sound is based only on the speed of the source or receiver relative to the medium, not to each other. When a source moves wrt the medium, it changes the frequency of the wave that travels through that medium. If a receiver is moving wrt to the medium, it changes the frequency of the wave it receives. And the reason for this?? The velocity of the sound wave in the medium is constant (pretty much), but the velocity of the sound wave wrt the source or receiver is not constant. The mechanism for altering the frequency is the additive velocity relationship which you are not supposed to apply to light.

 

So this goes back to my question for Janus; once a beam of light has been emitted, and the source has stopped emitting (the beam is simply travelling through space), what is the mechanism for red/blue shifting that beam? If moving toward the dead source of the beam will blueshift the frequency, what are you moving wrt to create that shift? Once the source has died, you are basically moving toward a light beam. How can motion wrt a beam of light create any shift in the frequency?

[Daecon]

There is[/b'] no real difference. As long as the speed of sound didn't vary in between you and the source (ie. it was a constant medium), then sound and light behave exactly the same way. The only possible difference is that you can move to a frame where you can overtake sound, but not to a frame where you can overtake light. In other words, the transformation laws from one frame to the other are different, but since the observer isn't changing his frame of reference, this is immaterial.

Using another anaolgy, imagine "i" is a photon and "." is a period of time between one photon and the next:

 

Moving away from the light source photons would be i...i...i...i...i...i...i

and moving towards the light source would be i.i.i.i.i.i.i.i.i

where if you were stationary in relation to the light source it would be i..i..i..i..i..i..i

 

Would that be an accurate anaolgy?

 

With time dilation always occuring, no matter your movments either towards or away from the source, dosn't that make "time" remain constant with respect to the speed of light?

If the speed of light never changes, how can the time interval between photons ever change? It shouldn't be different from i..i..i..i..i..i..i because they'll always move at c, therefore the 'next' photon will be emmited by the source at c after the standard ".." period - which would reach you at c after the ".." amount of time?

 

I hope what I'm trying to say makes sense. I hate not being able to convey an idea very well.

[Severian]

The mechanism for altering the frequency is the additive velocity relationship which you are not supposed to apply to light.

 

But the important velocity difference is that between the source and the observer - not the signal propagation speed. As long as the observer isn't travelling at relativistic speeds relative to the source, it is exactly the same phenomena as it is for sound.

[Daecon]

But the important velocity difference is that between the source and the observer - not the signal propagation speed. As long as the observer isn't travelling at relativistic speeds relative to the source, it is exactly the same phenomena as it is for sound.

Well that just begs the question, "What if the observer is travelling at relativistic speeds?"

 

I don't mean to sound desperate but I NEED TO KNOW!!1!one!!

[Severian]

Well that just begs the question' date=' "What if the observer is travelling at relativistic speeds?"

[/quote']

 

Then you will get time dilation of the source, decreasing the frequency.

[Daecon]

Then you will get time dilation of the source, decreasing the frequency.

Would that negate the shifting effect in light then?