nakayama
In outer space, an observer observes a star. When observer jets out gas and moves along the light path (in different uniform motions), frequency of star light changes.
However, if light speed is constant (to observer), wave length of the light path (from observer to the star) changes also (from the formula : light speed = frequency x wave length). And Number of waves changes also. Impossible !! Note : Wave length is inverse number of wave number.
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Welcome aboard.
Is this the so called red or blue shift? AFAIK that is emmission lines in the spectrum shifted to one end or other? Don't know if it is related to frequency.Originally Posted by nakakayama
Cuddles...help..
However, if light speed is constant (to observer), wave length of the light path (from observer to the star) changes also (from the formula : light speed = frequency x wave length). And Number of waves changes also. Impossible !! Note : Wave length is inverse number of wave number.
http://www.geocities.co.jp/Technopolis/2561/eng.html
P.S. i can't receive E-mail. i don't have PC.
Last edited by ZERO; 11th December 2007 at 06:18 AM.
I'm not sure what you're having problems with, since this is just the Doppler shift, which is exactly the same for light as it is for sound, like when an ambulance siren goes past you and changes pitch. The only place I think you may be misunderstanding is when you say "wave length of the light path (from observer to star)". I'm not sure exactly what you mean by this, but it looks like you might be saying that wavelength is the distance from the observer to the star. This is not what wavelength means, it is simply the distance between two peaks in the wavetrain.
It's really very simple when you think about it. Imagine you are stationary and there are waves coming towards you. Any waves, it doesn't have to be light. A certain number of peaks will reach you each second, which is the definition of frequency. Now imagine you are moving towards the source of the waves. Each peak will reach you slightly sooner after the previous one than if you weren't moving, so more peaks will reach you every second, which means the frequency is higher. It also means that each peak appears closer to the previous one, since they are traveling at the same speed, but reaching you slightly sooner, so the wavelength is shorter. Which is exactly what the formula tells you. Hooray.
Allow me to explain the problem once more, please.???
Problem is about red-shift (blue-shift) or Doppler effect (caused by observer's motion. not relative but observer's motion !!). If following both are right, what does happen on the light path ??
* Constancy of light speed (to observer)
* light speed = frequency x wave length
Wave length may change. Wave number may change also. Density of waves (of the light path that leads to the star ??) may be changed by observer's motion. Unimaginable !! (our understandings must be wrong !!).
I'm confused. Where does it say that the observers motion will change the number of waves?
Also, you are moving relative to the light wave, so the doppler shift will still have an effect; there is no absolute rest frame to compare your motion to, so you re moving relative to the light, not to an absolute rest frame.
I think the problem is to reconcile the Doppler effect with special relativity. I understood this several decades ago, but please remind me - when you say Any waves, it doesn't have to be light you seem to be ignoring a vital difference between light and sound, that light has no medium, and its speed is the same in any frame of reference. I can see the Doppler effect for the ambulance, but the explanation for light is not so obvious (to me). Can you shed some light on this (any frequency)?
The medium is irrelevant, and the explanation is exactly the same for all waves. The point is, it is not the wave itself that changes, it is the observation of it. Frequency is just the number of waves you see per unit time. If you move towards the source of the wave, you will see more waves in a given time, and therefore see the frequency as higher. Another observer moving in the opposite direction will see less waves and measure a lower frequency. The wave itself is still exactly the same, regardless of what the observers are doing.I think the problem is to reconcile the Doppler effect with special relativity. I understood this several decades ago, but please remind me - when you say Any waves, it doesn't have to be light you seem to be ignoring a vital difference between light and sound, that light has no medium, and its speed is the same in any frame of reference. I can see the Doppler effect for the ambulance, but the explanation for light is not so obvious (to me). Can you shed some light on this (any frequency
The exact formulae are different when you involve relativity, as they are with everything, but the principle behind it is exactly the same.
I thought I used to understand this, but now I don't. Suppose you are travelling past a stationary light source at a relativistic speed, and the light source emits a burst of light with, say, identifiable spectral line in it. The source is the centre of a sphere of light with a radius expanding at the rate of c, but in your frame of reference, you are also the centre of a sphere of light. Why should your spectral lines be displaced when an observer stationary to the source would see no displacement? ???
I'm not sure I understand what you're asking. If you're moving and another observer is stationary, you can't both be at the centre of the same sphere of light, so there is no reason you should both see the same thing.
I think the problem with your question is Lorentz contraction. When you're moving at a relativistic velocity, length is shortened in the direction of travel. The light appears to be traveling at the same speed for both you and the stationary person, but you will measure the time and distance differently, and will therefore get different answers for the frequency. Unless you were asking something completely different, in which case I'm confused as well.
I'm sure you are right, but I shall have to go away and think about this. I took mathematical physics and relativity as a special subject in my physics degree, but it was so long ago I've forgotten even the basics. I'll be back.
To Mr. vblock,
Sorry to confuse you. When space ship emits gas, frequency of star light (at this space ship) changes (value of change corresponds to the value of jet emission). In this problem, absolute rest frame doesn't appear (i think).
P.S. About rest frame, see my web-site, if you please.
As you wrote, we are moving relative to the light wave (may be) !! Then light speed (it's wave's speed) is changeable !!
I think I understand what nakakayama is saying.
The speed of light is supposed to be a universal constant. How can there be a doppler change. Frequency should be the same for all observers.
???
To Mr.Cuddles.
You wrote, "The wave itself is still exactly the same, regardless of what observers are doing". Common sense !!
Star light is coming. Can we to make (exert) any effect on this coming light path ?? No we can't !! Wave length of star light may be changed only by lens or the like (or by reflection).
To Mr.vbloke,
Sorry, i forgot to write following. Change of wave length (number of waves) is result from 2 lines (marked *). It's inevitable (then, one of 2 lines must be wrong).
* Constancy of light speed (to observer)
* light speed = frequency x wave length
To Mr.ZERO,
Yes, should be the same.
No it shouldn't. Since both time and distance are different for different observers, why should something which depends on both not be different as well?
I don't know....isn't light a constant though? Isn't it the same speed everywhere, all the time?
I understand Doppler shift with sound waves but this light thing is
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