Is the brightness of light invariant?

QQ, you know that the speed of the beam relative to the observer is only c in the observer's rest frame, right?

If you're thinking about the speed difference between the beam and the observer in the source rest frame, then it's no wonder you're stuck. Yes, in the source's rest frame, the speed difference between the beam and the observer is more than c if the observer is approaching the source, and less than c if the observer is departing from the source.

Is that an issue, or are my suspicions unfounded?
 
so you would agree that a ship traveling towards a stationary light source would experience and record light speed in excess of 'c'?
 
We are not actually discussing SRT here Pete but merely doppler shift and the speed of light being recorded in excess of 'c'.....
 
Quantum Quack said:
so you would agree that a ship traveling towards a stationary light source would experience and record light speed in excess of 'c'?
Click to expand...
Not if they're using rulers and clocks in the ship's rest frame.

If the ship is using clocks on the ship and a ruler at rest with respect to the light souce or vice-versa, then yes.
 
Pete said:
If you're thinking about the speed difference between the beam and the observer in the source rest frame, then it's no wonder you're stuck.
Click to expand...

No, we're not considering that. It's the speed of the beam relative to the observer.
 
Pete:
We have two lightspeed measuring devices LSMD's on platforms that are capable of running on a rail at the same velocity towards a fixed light source at the center of the rail between the two LSMD's.

All LSMD's and the light source are connected to this rail. Only the light source is fixed.
The LSMD's are stationary and measure the frequency and light speed of the light emmissions of the fixed source. It is determined that 'c' is corrrect. The whole shebang is in a vacuum.

At a given time both LSMD's start to move towards the light source from opposite directions and when they are both at a certain velocity they again measure frequency and light speed of the light coming from the fixed light source.

The LSMD's both record the same record a small doppler shift.

Would they still record 'c' at the usual constant or would they record 'c' as being in excess of that constant?

Now before you throw time and length issues at the problem earlier it was clearly stated that doppler effects are not greatly effected by time dilation and length contractions.

So they are not relevant concerns. [ the speeds of the LSMD's are not relativistic any way]
The source is clearly stationary and the LSMD's have all the velocity.

Do the LSMD's record Light speed to be in excess of 'c'?
 
Last edited:
Hi dav,

Relativity puts the reason for the Doppler shift on the source because that's where it belongs. It is the source that determines the color of the light in a particular frame, and you merely observe what the source has emitted. As I explained earlier, what the source is doing now has got nothing to do with it. What matters is what the source was doing when it emitted the light. Your talk of the source vanishing has got nothing to do with it; the source had to be around when it was emitting the light, and that is when the source matters. You had this straight a while back when we were talking about the light from ancient stars, so what confused you?

Also, what makes you think you can "ignore relativity?" Light is inherently a relativistic thing! You must understand that it is not a sound wave or water wave or whatever. The equations which describe light, Maxwell's equations, have relativity built into them. Now, my answer was accurate because the source does matter, so what you should do is go solve the Maxwell equations for a source moving with respect to you. The change of wavelength comes right out.
 
Quantum Quack said:
`Now before you throw time and length issues at the problem earlier it was clearly stated that doppler effects are not greatly effected by time dilation and length contractions.
Click to expand...
Perhaps not (I haven't checked. Have you?), but the measured speed of light certainly is.
Why don't you run the numbers and see for yourself?

do the LSMD's record Light speed to be in excess of 'c'?
Click to expand...
No. Assuming, of course, that each LMSD has a ruler and clocks built in... which means that for that measurement, each LMSD considers itself to be stationary (it measures its own speed to be zero), right?
 
Do you agree that the LSMD's are able to record a doppler shift that relates to their own velocity and not that of the light source?

Please bare with me....if you can.....I just want to be absoilutely clear about what the LSMD's are recording.

So if they slow down and speed up teh doppler shift they record will be changing. They know how fast they are going according to the rail thay are running on.
 
dav57 said:
If you can't move relative to light then how has science derived a figure for our velocity relative to it?

I guess you're glad I'm not one of your students...ahhem :rolleyes:
Click to expand...

I don't see the point of this comment, I thought we were having a polite discussion.

I already explained that what we have measured is the speed of light relative to inertial observers , we have never measured the speed of anything relative to light because that doesn't make sense. To repeat, light doesn't have a point of view.

Also, I would be happy to have you as one of my students since it never hurts to ask questions. What I would expect from you is that you should think hard about what I say and be willing to learn from it. My past students seemed to think this was a good arrangement
 
Last edited:
Again, you are correct, time dilation and length contraction do negate the Doppler effect, just not completely. In other words, say you start with the 1Hz signal you were discussing previously and say that the observer is moving at .5c towards the source. Then the Doppler effect shifts it to 2Hz, but time dilation and length contraction bring it down to 1.73Hz, not all the way back to 1Hz.
Click to expand...

Just quoting an earlier post from Dale
 
Quantum Quack said:
Do you agree that the LSMD's are able to record a doppler shift that relates to their own velocity and not that of the light source?
Click to expand...
The LSMD's can't tell the difference. How do they know that the rail+source isn't moving? In fact, if that were the case they'd record exactly the same thing.

And they know the light source is stationary because they both record the same doppler shift.
Click to expand...
Nope, doesn't follow. If one LSMD is stationary, the rail and source are moving, and the other LSMD is moving even faster (at the right speed), then both will record the same doppler shift.
 
Pete said:
The LSMD's can't tell the difference. How do they know that the rail+source isn't moving? In fact, if that were the case they'd record exactly the same thing.
Click to expand...

Both LSMD's record the same doppler shift, as they are heading towards the light source from opposite directions simultaneously.

If both LSMD's record the same doppler shift then the light source must be stationary...yes? [ Their velocities are proven to be also the same relative to the rail, which is common to both LSMD's]

LSMD----->-------->--------Lightsource---------<-------<------LSMD
----------------------------------[]----------------------------------

Maybe thinking of it in purely mechanistic terms might help.
The light source is attached to the rail. The rail is used to not only to provide a track for the LSMD's but also provide velocity data.

The rail and the light source are the one frame of reference.
 
Last edited:
Pete said:
Nope, doesn't follow. If one LSMD is stationary, the rail and source are moving, and the other LSMD is moving even faster (at the right speed), then both will record the same doppler shift.
Click to expand...
OK. I see.....how ever if both LSMD are being dragged to the center of the rail by cables driven by the same motor situated and fixed under the light source. WE woul dstill have the same outcome yes? That no matter how the gedanken is formulated The SRT prefered reference fram concept will pushthe light source into a moving state and the LSMD's will be either stationary or chasing the moving light source...Hmmmmmm......

Maybe if we throw in a third observer as part of the light source frame watching all this from a distance and taking the recordings of both LSMD's

Would that change anythng?
 
ACtually an amusing arguemnent comes to mind.

If the LSMD is considered as stationary how is the energy it is expending to stay stationary conserved unless we consider that the energy is somehow pulling the light source and other LSMD towards him with out actually having any ability to do so.

Say we assume that the light source has enourmous inertia......

For the LSMD to be stationary yet expending energy...doesn't make much sense hey?

And

Why would I take the position that the energy expended by the LSMD is somehow enough to pulll the lightsource and the other LSMD towards it?

Why would I take such an irrational position as declaring that the LSMD is at rest and everything is moving towards it with out having the energy output to create such a thing. Considering that the energy is applied to move the LSMD's and not the rail....

And why is it not easy to see that by simply using a little imagination and incorporating a cable and winch set up at the light source we can elliminate this preferred frame regarding this gedanken and place the light source as stationary and giving the LSMD's all the velocity?
 
dav57 said:
Why are you talking about the EMITTER changing the wavelengh all the time?

If you approach an already emitted sound wave then you as an observer will measure an increased frequency and yet the wavelengh remains unchanged.

Think about the observer moving, NOT the emitter.
Click to expand...
Since you apparently missed it the first time I will repeat myself:
DaleSpam said:
For a detector moving relative to the air the process is simply the (time) reverse of the emission.
Click to expand...
I don't know why this is such a problem for you. You have had dozens of perfectly good explanations of the Doppler effect.


dav57 said:
Measuring sound in this case is different to light because light "appears" to change its wavelength as you the observer accelerate towards it.
Click to expand...
The only difference between sound and light is that with light you can use any reference frame to do the analysis and with sound you must use the reference frame where the air is stationary. You really should sit down and do the math. It is painfully obvious that you are not understanding the explanations in English.

-Dale
 
Excuse me while I vent a littel fristration.

It is surprising that when dealing with most SRT gedankens that we can so easilly swap velocities from one frame to another with out considerin gthe energy that is involved.

For example a rocket expends a very small amount of energy yet it can be considered enough energy to pull the entire universe twoards it when we swap frames.

I was under the impression that SRT is also about energy. And to simply place the velocity on a universe instead of the object expending the energy seems ridiculous. Given the inertia of that universe and the miniscule amounts of energy being expended. Is it little wonder that SRT is the focus of such incredulous reactions.

regariding the above gedanken:

How can it be decared that the light source and other LSMD are moving to wards the stationary LSMD when no energy has been expended to create such a thing?

If energy has been applied that I don't see please point it out?
 
You must log in or register to reply here.
Back
Top