How can a Doppler effect be attributed to light?

[ynot]

If the speed of light is constant, regardless of whether it is being emitted from an object that is moving toward or away from an observer, I don‘t see how motion can have a Doppler effect on light as in the expanding universe theory. In other words, how can something that is not effected by motion, be effected by motion?

 

[AnotherSillyAlias]

If the speed of light is constant, regardless of whether it is being emitted from an object that is moving toward or away from an observer, I don‘t see how motion can have a Doppler effect on light as in the expanding universe theory. In other words, how can something that is not effected by motion, be effected by motion?

Maybe this will help answer your question.

 

[epepke]

If the speed of light is constant, regardless of whether it is being emitted from an object that is moving toward or away from an observer, I don‘t see how motion can have a Doppler effect on light as in the expanding universe theory. In other words, how can something that is not effected by motion, be effected by motion?

It isn't the Doppler effect. It's certainly not at all like the Doppler effect from a wave in a medium. However, it's called the Doppler effect, for some strange reason that I can't make out. It sorta kinda resembles the Doppler effect, to a zeroth approximation.

Basically, receeding from a photon has to result in a decrease in energy measured from the photon, to conserve energy. The only way for a photon to do this is to have a lower measured frequency.

 

[ynot]

Maybe this will help answer your question.

Thanks - I have already read this (many times) along with other similar articles but am still having problems understand and/or accepting the concept.


So motion doesn't effect the speed of light, but motion effects the wavelength and frequency of light? Seems to me that the motion of light is completely free of inertia so motion wouldn’t stretch or compress the wavelength and frequency light. Perhaps the “penny will drop” if I gain a better understanding of the Special Theory of Relativity.

 

[ynot]

It isn't the Doppler effect. It's certainly not at all like the Doppler effect from a wave in a medium. However, it's called the Doppler effect, for some strange reason that I can't make out. It sorta kinda resembles the Doppler effect, to a zeroth approximation.

Basically, receeding from a photon has to result in a decrease in energy measured from the photon, to conserve energy. The only way for a photon to do this is to have a lower measured frequency.

What do you mean when you say "receding from a photon"?

 

[epepke]

What do you mean when you say "receding from a photon"?

In the simple case, meaning, when you and the source of a photon are going away from each other at uniform speed, as in the case of a red-shifted star. Obviously, the photon doesn't care about speed, because it's always going at the same speed. However, the photon carries kinetic energy, and your kinetic energy relative to the source has to be taken into account for conserving energy.

The only way that the photon can do that in your frame of reference is by going redder.

Now, when it's more complex than the simple case, it gets really complex, which is why there are only a handful of solutions for GR.

 

[AmateurScientist]

It is the Doppler effect as applied to electromagnetic radiation. It has to do with how the observer perceives the transmitted wave or radiation, be it sound or electromagnetic radiation, moving towards or away from him.

It is the observed frequency of the wave or radiation that changes, not the frequency emitted from the source itself. "Observed" is the key word here. It refers to how the observer experiences relative motion with respect to the source of the wave or the radiation, and how he perceives the frequency of the wave or radiation.

In the case of a source of sound waves transmitted through a medium, if the source is moving away from the observer, the observed frequency of the waves will be diminished (lower than it was at an earlier time t). In the case of the source of electromagnetic radiation, whether it is in the spectrum of observable light or at a higher or lower frequency (e.g., gamma rays or microwaves), if the source of the radiation is moving away from the observer, then the observer will observe a similar decrease in the frequency of the radiation. In visible light radiation, red has a lower frequency than all other colors in the visible spectrum (and consequently a longer wavelength), so the light observed by the observer will be red-shifted. This does not mean he or she will see nothing but red light, or even that he will see red light at all. It simply means the observed frequency will be lower than it was at an earlier time t. In fact, if the electromagnetic radiation is outside the visible light spectrum, then we probably won't "see" it at all, but we can still detect it with radio or microwave detectors or gamma ray detectors. In that case, the same principle would apply, and the observer would perceive a dinimished frequency of the radiation from the source. Red shift merely means having an observed diminished frequency of the radiation emitted from the source due to the source's moving away from the observer. That's it.

AS

 

[epepke]

It is the Doppler effect as applied to electromagnetic radiation. It has to do with how the observer perceives the transmitted wave or radiation, be it sound or electromagnetic radiation, moving towards or away from him.

No; it isn't, not at all, because with respect to waves transmitted in a medium, the effect of a moving source is different from the effect of a moving observer. The difference is small when the speed is small with respect to the speed of the wave in the medium, but it is different. And that famous experiment with the guys with their trumpets on the railway car was all about changing the speed of the source.

Without going beyond the fringe of audience comprehension, the difference can be grokked by noticing that a moving source can produce a sonic boom, whereas a moving observer cannot result in a sonic boom.

The similar effect for light is called the Doppler effect, but that's mostly because people are imprecise and probably stupid.

Now, there is something like a sonic boom that happens when there is a disturbance that goes faster than the nominal speed of light in a medium, but I fear that is beyond the fringe of audience comprehension.

 

[ynot]

In the simple case, meaning, when you and the source of a photon are going away from each other at uniform speed, as in the case of a red-shifted star. Obviously, the photon doesn't care about speed, because it's always going at the same speed. However, the photon carries kinetic energy, and your kinetic energy relative to the source has to be taken into account for conserving energy.

The only way that the photon can do that in your frame of reference is by going redder.

Now, when it's more complex than the simple case, it gets really complex, which is why there are only a handful of solutions for GR.

If a proton carries kinetic energy, doesn’t it follow that it also has inertia? If a proton has inertia, doesn’t it follow that the speed of a proton would be effected by motion?

 

[AmateurScientist]

Without going beyond the fringe of audience comprehension, the difference can be grokked by noticing that a moving source can produce a sonic boom, whereas a moving observer cannot result in a sonic boom.

A sonic boom isn't caused by the Doppler effect. It's caused by compression of the medium itself -- air -- directly in front of the moving aircraft. The aircraft creates waves moving at the speed of sound in front of and behind the aircraft, much like a moving ship creates waves at the bow and the stern. Once the moving aircraft reaches the speed of sound in air at that altitude, however, the waves at the front and the back can no longer get out of each other's way and they merge into a single wave moving at the speed of sound. The sonic boom is created as the nose of the aircraft moves through that merged wave, and again when the tail passes through it and the pressure returns to normal. This results in a characteristic double boom which can often be heard on the ground nearby.

It has nothing to do with the Doppler effect, but you are right; it would be impossible for a non-moving observer to create a sonic boom. He or she isn't moving air fast enough to create the famous N-wave which results in the pressure differential and the boom.

AS

 

[ynot]

the effect of a moving source is different from the effect of a moving observer.

Why?

a moving observer cannot result in a sonic boom.

Surely (no not Shirley) a moving anything can create a sonic boom if it travels beyond the speed of sound.

 

[epepke]

A sonic boom isn't caused by the Doppler effect. It's caused by compression of the medium itself -- air -- directly in front of the moving aircraft.

No; it results from exceeding the speed of sound in the medium. That there is a compression is secondary. The equations for a moving source (i.e. the Doppler effect) work neatly up to exceeding the speed of sound in the medium, at which point one can expect something funny to happen.

It has nothing to do with the Doppler effect, but you are right; it would be impossible for a non-moving observer to create a sonic boom. He or she isn't moving air fast enough to create the famous N-wave which results in the pressure differential and the boom.

Of course.

 

[epepke]

If a proton carries kinetic energy, doesn’t it follow that it also has inertia? If a proton has inertia, doesn’t it follow that the speed of a proton would be effected by motion?

I won't answer this until you elucidate: are we talking protons or photons?

 

[ynot]

Of course.

But you said a moving observer

 

[ynot]

I won't answer this until you elucidate: are we talking protons or photons?

Sorry - I meant Photons - We are talking about light - not food

 

[epepke]

Why?

Because, as has been pointed out already, a moving source in, say air, will reach a point where the wavelength of waves propagating to the front will become zero. Whereas, a moving observer will never approach a point where the wavelength is zero.

Whereas, for light, it does not matter whether the source or the observer is moving, because it's all the same, according to relativity.

 

[ynot]

Sorry - I meant Photons - We are talking about light - not food

That was meant to be a joke.

If any component of light has kinetic energy, wouldn't it follow that light has inertia and be effected by motion?

 

[epepke]

Sorry - I meant Photons - We are talking about light - not food

OK. Well, a photon doesn't exhibit the same kind of inertia as something moving subluminally, but it does exhibit something very much like inertia, not because it has energy, but because it has momentum.

And the velocity of light can and does change, in a fairly obvious way. Momentum is the 3-dimensional component of the energy/momentum vector. The speed, however, doesn't change.

This is why if you have a really fast spaceship, all the stars are going to appear to be scrunched up in front of you. And for light at an angle, momentum will be transformed into energy or vice versa.

But if you're moving straight up toward or directly away from a star, the energy has nowhere else to go, see, because it's straight on, and so the wavelength of the light will change.

 

[I less than three logic]

If you truly need convincing that the Doppler Effect can apply to electromagnetic energy, you could always contest in court that the police officer had no way of knowing how fast you were going. This is the same principle that allows radar and laser guns to clock your speed.
(You can’t trust these guys talking science and such, but courts, laws, and politicians are never wrong. )

http://www.straightdope.com/mailbag/mradar.html
http://auto.howstuffworks.com/question396.htm
http://en.wikipedia.org/wiki/Radar_gun

Had a hard time finding links to relevant information. Search for radar guns and you get 10,000 links to radar jammers.

 

[ynot]

If you truly need convincing that the Doppler Effect can apply to electromagnetic energy, you could always contest in court that the police officer had no way of knowing how fast you were going. This is the same principle that allows radar and laser guns to clock your speed.
(You can’t trust these guys talking science and such, but courts, laws, and politicians are never wrong. )

http://www.straightdope.com/mailbag/mradar.html
http://auto.howstuffworks.com/question396.htm
http://en.wikipedia.org/wiki/Radar_gun

Had a hard time finding links to relevant information. Search for radar guns and you get 10,000 links to radar jammers.

Thanks - I like a good "real world" argument. Unfortunately I have to leave the forum now so will give it some thought and respond another day.