The Doppler Effect (in general)

[Capt. McCoy]

Again, it's the speed of the source that matters, not the speed of the light. Same thing with sound: In still air, the speed of sound with respect to a stationary observer will be the same, regardless of the speed of the source. (In fact, that was the example that started this thread.)

I think what doc as is trying to say here is there that in a doppler effect, there are various movements, (1) movement of the source of the energy (light/sound), (2) the movement of the energy itself.

 

[vin300]

I think what doc as is trying to say here is there that in a doppler effect, there are various movements, (1) movement of the source of the energy (light/sound), (2) the movement of the energy itself.

That's alright, but doc said the velocity at the closest point is not zero but as I see it there is no change in the direction in which the components point so no contribution of transverse velocity anywhere along the process (always perp to long vel)

 

[Doc Al]

As Doc Al stated,there is still a frequency shift when the source and observer at the point of closest aproach(their relative velocity is transverse in this case) ,due to time dilation----is this the so called 'tansverse doppler effect'?

That is correct.

That's alright, but doc said the velocity at the closest point is not zero but as I see it there is no change in the direction in which the components point so no contribution of transverse velocity anywhere along the process (always perp to long vel)

Answer this: At the point of closest approach, is the source moving with respect to the observer? If so, time dilation (and the associated "transverse Doppler shift") applies.

 

[Capt. McCoy]

That's alright, but doc said the velocity at the closest point is not zero but as I see it there is no change in the direction in which the components point so no contribution of transverse velocity anywhere along the process (always perp to long vel)

it's not zero because basically (from what you said also) it's just the closest point...not the point itself...

 

[Urmi Roy]

it's not zero because basically (from what you said also) it's just the closest point...not the point itself...

At the point,then,there is no doppler effect at all,right?

This must be the same for sound waves also.

 

[Urmi Roy]

I was going through the wikipedia article about the doppler effect and it said that "The Doppler effect is time-dependent in general (thus we need to know not only the source and receivers' velocities, but also their positions at a given time."

What does this mean?

 

[Doc Al]

I was going through the wikipedia article about the doppler effect and it said that "The Doppler effect is time-dependent in general (thus we need to know not only the source and receivers' velocities, but also their positions at a given time."

What does this mean?

If you are at rest and all you know is that the source is moving at a given velocity (10 m/s north, say), how can you determine the expected Doppler shift? You need to know whether the source is north, south, east, or west of you. In general, position matters.

 

[Urmi Roy]

You need to know whether the source is north, south, east, or west of you. In general, position matters.

Direction does come in when we're just trying to get the details of the situation under dicussion,but it doesn't affect the results.

Whether we have our source moving away at 10m/s north,or south ,or in any direction,the observation is the same.

Also,in regard to the position,besides the fact that objects farther away look smaller,it doesn't affect the doppler effect---its theory and observations.

Is that right?

 

[Doc Al]

Whether we have our source moving away at 10m/s north,or south ,or in any direction,the observation is the same.

You're the one who read the Wiki article. Did they meaning radial velocity with respect to the observer or just velocity. (Let's not go another 10 pages based on ambiguity.)

If you to discuss the statement in the wiki article, then post a reference.

 

[vin300]

Direction does come in when we're just trying to get the details of the situation under dicussion,but it doesn't affect the results.

Whether we have our source moving away at 10m/s north,or south ,or in any direction,the observation is the same.

So you didn't understand all the concept of radial velocity
It's the component of velocity towards you(vcos(angle)) that is added to c and causes the traditional doppler effect, and the actual relative velocity appears in the lorentz factor causing relativistic effect
Derive, you get the result as fo={1-[vcos(angle)/c]}*f*gamma
Now this formula can be put in another way, using the aberration formula
It happens because at high speeds the angle at which the wave is sent to the velocity decreases, can be well explained by bending of grids
If you study the grids, the components vcos(angle) and vsin(angle) which were mutually perp decrease inclination but this does not mean they contribute to each other because the grids are themselves bent

 

[Urmi Roy]

I wasn't talking about the 'radial' or 'transverse' direction----we can have these components of relative velocity whatever direction(north,south,east) the motion is in.

I was actually reffering to the importance of 'position' of source and observer in the analysis.Besides,as long as we know the relative velocity,and the direction of it,it doesn't matter whether it's north or south,what matters is the direction w.r.t each other (moving towards or moving away)--we can choose north-south directions as we please.

The line I'm referring to is on the page given below,in the third last paragraph of the section 'Analysis'.

http://en.wikipedia.org/wiki/Doppler_effect

 

[vin300]

Time reversal?Iwould wwant to know more on it

 

[Doc Al]

I wasn't talking about the 'radial' or 'transverse' direction----we can have these components of relative velocity whatever direction(north,south,east) the motion is in.

I was actually reffering to the importance of 'position' of source and observer in the analysis.Besides,as long as we know the relative velocity,and the direction of it,it doesn't matter whether it's north or south,what matters is the direction w.r.t each other (moving towards or moving away)--we can choose north-south directions as we please.

The line I'm referring to is on the page given below,in the third last paragraph of the section 'Analysis'.

http://en.wikipedia.org/wiki/Doppler_effect

That wiki paragraph is referring to the fact that in a more complete analysis one must take the travel time of the signal into account (from source to observer) in order to determine what the observer actually observes at any given time. To do that requires knowing the position of the source at the time of emission, not just its direction of motion.

 

[Urmi Roy]

Thanks,Doc Al,I understand now.

 

[vin300]

Waves farther from the source lag behind those closer to it.So if you have positive radial velocity greater than the speed of sound you hear the song played backwards
That's time reversal