# Relativistic Doppler effect and relativity principle

1. Feb 21, 2010

### E=mc^84

Let's say i am in a car travelling at 0.5c relative to a truck- which is at rest relative to the ground- approaching me from the opposite direction. Relative to me, the truck is going at 0.5c. Both, truck and car, frames of references are inertial frames of reference. Therefore, the principle of relativity applies to both frames. I am also imagining that the air has been sucked out, and the car and truck are travelling in a vacuum. Therefore, light is measured to be a constant c in both frames of reference. Now, both the car and the truck will both agree that each others relative velocity is 0.5c and that the speed of light c is constant. Now, if the air were to be put back in all of sudden and the Doppler Effect were to take place, then the light approaching the car from the side would be redshifted relative to the car. The car would be receeding away from the light and the car would see darkness infront of him and some light at the side of the car. So, the car woud not see the truck infront of him? But, relative to the truck, the car would still be approaching? What i am trying to ask is if the relativity principle is broken between the car and the truck if the Doppler Effect is introduced?

Can anyone helpme with this one, its been knawwing at my brain for awhile, lol

2. Feb 21, 2010

### Staff: Mentor

Air has nothing to do with the Doppler effect in light, unlike with sound. The Doppler effect already exists for the light in vacuum, before you put the air back in.

3. Feb 21, 2010

### ssope

Light is blueshifted when moving toward a source, not redshifted.

NOTE: Neither me nor the one before me have even attempted to answer your question.

I'm drunk and can only process bad information at the moment

4. Feb 21, 2010

### E=mc^84

Assume that the observer is moving away from the light source.

5. Feb 21, 2010

### E=mc^84

How is the Doppler effect still present in a vacuum if the speed of light is to remain constant? I mean, in a medium the speed of light changes therefore it is not constant. If the relativity principle is to be used so that c is constant in a vacuum, does that not mean that the freaquency of light shud not change for c to be constant? Is not the dopplar effect in contradiction with the principle of relativity?

I am trying to see this problem from different angles, so let me know what u think, Thanks.

6. Feb 21, 2010

### JesseM

Like jtbell said, the Doppler effect for light doesn't depend on air.
Why darkness in front? Why light to the side? The Doppler effect doesn't deflect the angle you see the light coming from...
No, both see each other redshifted by the same amount.

7. Feb 21, 2010

### E=mc^84

Thanks for your reply.

I am assuming that the observer in the car is moving away from the light source causing a redshift. And to repeat: How is the Doppler effect still present in a vacuum if the speed of light is to remain constant? I mean, in a medium the speed of light changes therefore it is not constant. If the relativity principle is to be used so that c is constant in a vacuum, does that not mean that the freaquency of light shud not change for c to be constant? Is not the dopplar effect in contradiction with the principle of relativity?

I am just using a different perspective to see this problem, so i chose to add a medium to the picture. Thanks. Let me know wat you guys think.

8. Feb 22, 2010

### E=mc^84

TIc toc, Tic toc. Damn Light clock! lol

9. Feb 22, 2010

### JesseM

Doppler effect doesn't change the speed, just the frequency. Suppose the source is sending out pulses once every 20 seconds in its own frame, and it's moving away from you at 0.6c. Then since its clock is slowed down by a factor of squareroot(1 - 0.6^2) = 0.8 in your frame due to time dilation, it will only be sending out pulses once every 20/0.8 = 25 seconds in your frame. That's just time dilation though, not the Doppler effect. The Doppler effect comes about because the source has moved farther away from you between each pulse, so the light from each pulse has a longer distance to travel to reach your eyes. For example, suppose at t=30 seconds in your frame the source was 100 light-seconds away from you and sent pulse #1, then 25 seconds later at t=55 seconds the source had moved 25*0.6c=15 light-seconds farther away from you so it was now 115 light-seconds away from you when it sent pulse #2. Since the first pulse takes 100 seconds between being emitted and reaching your eyes, it reaches your eyes at t=30+100=130 seconds; since the second pulse takes 115 seconds between being emitted and reaching your eyes, it reaches your eyes at t=55+115=170 seconds. So in spite of the fact that it's sending a pulse once every 25 seconds in your frame, the time between your seeing successive pulses is 170-130=40 seconds; that extra delay is the Doppler effect.

Last edited: Feb 22, 2010
10. Feb 22, 2010

### E=mc^84

I understand what you are saying, but you did answer my question fully. Applying the relativity principle, how would i see the truck approaching me and how would the truck see me if the light around me is doppler shifted away from my line of sight?

11. Feb 22, 2010

### JesseM

I did address this in an earlier post when I said:
I don't know where you get the idea that light should be "doppler shifted away from my line of sight"--it doesn't work that way (at least not for an object moving directly away from you along a line drawn between you and it...if the object is moving along a line that never crosses the point where you're sitting it's more complicated).

Last edited: Feb 22, 2010
12. Feb 22, 2010

### E=mc^84

Here is the question in simple terms: Would i still see the truck approaching me and would the truck see me? Also, how would the truck appear to me and the truck of me?

13. Feb 22, 2010

### JesseM

I thought in post #4 you changed it so the two observers were moving away from each other? But if the truck was approaching, then yes, you'd see it approaching and the truck would see you approaching.
Both of you would see the other one redshifted (or blueshifted, if you were approaching one another) by the same amount.

Last edited: Feb 22, 2010
14. Feb 22, 2010

### TcheQ

Your assumption is wrong, and hence any other questions you ask based on this incorrect assumption are invalid/meaningless.

Light is redshifted ALWAYS (by varying amounts, dependent on velocity of an object). The density of atmosphere between a light source(truck) and the detector(car) has no affect on the visibility or character of the redshift (<insert semantic clauses here>).

In other words, it doesn't matter if air is between the truck and car or not, the light is always going to be redshifted!

15. Feb 23, 2010

### E=mc^84

You dont understand why i used that logic, i know that, read on and you will understand why i added a medium to the problem.

16. Feb 23, 2010

### JesseM

Did my argument in post #9 convince you that your original reason for adding a medium was based on a misconception about the Doppler effect?

17. Feb 23, 2010

### E=mc^84

It did not convince me because i already knew that this was true. I was looking at the problem from a different perspective. Also, i wanted to know how the relativity principle ties into the doppler effect. For this, i would love to witness an experiment one day to see what really happens and what i will "really" see.

Thanks for your help though:)

18. Feb 23, 2010

### Staff: Mentor

Then why did you write
and
and
I'm sorry, but it is not helpful to the discussion to say that you already understood something that you clearly did not understand.

The Doppler effect is compatible with the principle of relativity, however it has a slightly different form. Basically, the relativistic Doppler effect is the same as the classical Doppler shift with an additional redshift due to time dilation. My favorite page on the subject is this one:
http://www.mathpages.com/rr/s2-04/2-04.htm

You also mentioned a desire for experimental confirmation. This was first performed by Ives and Stilwell in 1938, and some of the results of that experiment are shown in the page I just linked to, confirming the relativistic formula. It has since been followed-up by many other more precise experiments:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#Tests_of_time_dilation

19. Feb 23, 2010

### TcheQ

You seem to be having trouble reconciling the Doppler effect in light. It is not the same Doppler effect as for mechanical waves (sound), even though it has the same name.

I did. None of it made sense because your initial assumption is wrong.

You can't look at a problem from a different (wrong) perspective, state an incorrect assumption (correct assumptions have been proved to the limits of current science, such as c=~3*10^8m) and then say "hey, based on these WRONG assumptions, for which I have NO evidence to support, why does it work like this".

Here are your answers (that you didn't ask because you started with a wrong assumption, but you expected other people to translate it, and also seem to be refusing all the answers to the initial question because they refused to incorporate your wrong assumption)

Putting the air back in has no measurable efffect,
and
there will be no "redshifted relative to the car" due to this air put in
and
the car would not see darkness infront of him
and
the car sees the truck infront of him
and
the car is still approaching
and
the relativity principle is not broken

20. Feb 23, 2010

### Matterwave

From $$\lambda\nu=c$$ we can see that as $$\lambda\rightarrow\lambda '$$; $$\nu\rightarrow\nu '$$ such that $$\lambda '\nu ' = c$$

The Doppler effect affects the wavelength, and frequency, but not the speed of light.

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