Impossible !! : Constancy of Light Speed (to observer)

[nakakayama]

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.

http://www.geocities.co.jp/Technopolis/2561/eng.html

P.S. i can't receive E-mail. i don't have PC.

[ZERO]

Welcome aboard.

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.
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.
Cuddles...help..:eek3:


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.

[Cuddles]

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.

http://www.geocities.co.jp/Technopolis/2561/eng.html

P.S. i can't receive E-mail. i don't have PC.

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.

[nakakayama]

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 !!).

[vbloke]

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.

[Janot]

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. ...

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. 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:cheesy: )?

[Cuddles]

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:cheesy: )?

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.

The exact formulae are different when you involve relativity, as they are with everything, but the principle behind it is exactly the same.

[Janot]

The medium is irrelevant, and the explanation is exactly the same for all waves. ....
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? ???

[Cuddles]

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.

[Janot]

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. 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. :cheesy:

[nakakayama]

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 !!

[ZERO]

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.

???

[nakakayama]

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.

[Cuddles]

Frequency should be the same for all observers.

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?

[ZERO]

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 >:-)

[Cuddles]

I don't know....isn't light a constant though? Isn't it the same speed everywhere, all the time?

Yes, the speed of light is constant. But as I keep saying, time and space are not. If you are moving, you measure time and space differently from someone who is not, so light will have a different frequency even though it's speed is exactly the same.


I understand Doppler shift with sound waves but this light thing is >:-)

But it's exactly the same. Think of waves in the sea. You're standing stationary on the beach and count a certain number of waves every minute. Now run further out into the sea. Since you are moving towards the waves, you will count more of them, ie. the frequency is higher. Now, have the waves changed in any way? No. They are still moving in exactly the same way at exactly the same speed, but since you are moving as well, you observe them differently. It's exactly the same for light. No matter how you move, you don't change the light at all, but you do change how you observe the light.

I have to say, I honestly don't understand the confusion here. There are plenty of things with light and relativity that really are confusing and counter-intuitive, but this really isn't one of them. The ideas behind the Doppler shift are identical, no matter what kind of waves you are looking at, relativistic or not. The exact equations change since you have to take into account time dilation and so on, but if you can understand the Doppler shift with sound, then you understand it with light because it's exactly the same thing.

[Janot]

But it's exactly the same. Think of waves in the sea. You're standing stationary on the beach and count a certain number of waves every minute. Now run further out into the sea. Since you are moving towards the waves, you will count more of them, ie. the frequency is higher. Now, have the waves changed in any way? No. They are still moving in exactly the same way at exactly the same speed, but since you are moving as well, you observe them differently. It's exactly the same for light. No matter how you move, you don't change the light at all, but you do change how you observe the light.....

I have to say, I honestly don't understand the confusion here. .And I have to say, I am puzzled that you can't see the room for confusion. The wave in the sea comparison is unhelpful, because when you move out to sea, you are moving relative to the medium, water. This is obvious. Light is not exactly the same. There is no medium. :'(

[ZERO]

Yes, the speed of light is constant. But as I keep saying, time and space are not. If you are moving, you measure time and space differently from someone who is not, so light will have a different frequency even though it's speed is exactly the same.



But it's exactly the same. Think of waves in the sea. You're standing stationary on the beach and count a certain number of waves every minute. Now run further out into the sea. Since you are moving towards the waves, you will count more of them, ie. the frequency is higher. Now, have the waves changed in any way? No. They are still moving in exactly the same way at exactly the same speed, but since you are moving as well, you observe them differently. It's exactly the same for light. No matter how you move, you don't change the light at all, but you do change how you observe the light.

I have to say, I honestly don't understand the confusion here. There are plenty of things with light and relativity that really are confusing and counter-intuitive, but this really isn't one of them. The ideas behind the Doppler shift are identical, no matter what kind of waves you are looking at, relativistic or not. The exact equations change since you have to take into account time dilation and so on, but if you can understand the Doppler shift with sound, then you understand it with light because it's exactly the same thing.

If its the same as sound waves, I understand it.
What confuses me is, if you have a wave moving through water at 10 MPH and you sail a boat in the same direction at 5 MPH, then the wave appears to be travelling at 5 MPH and not 10.
The wave keeps a constant speed but it appears to change according to the observers motion.
Is this the same with light?
I thought light was the same speed to all observers all the time.

[Cuddles]

OK, look at it a different way. Forget Doppler shifts, forget everything to do with any other waves. You have a pulse of light 1m long with 10 wavelengths in it. When you're stationary, you wil measure this to have a certain frequency. Now you move towards it at a relativistic speed. The Loretz contraction means that it is now shorter than 1m long. Since it is traveling at the same speed, those 10 wavelengths pass you faster and it therefore has a higher frequency.

Of course, in reality it's a little more complicated since you also have to take time dilation into account, which has the opposite effect. The effect is still qualitatively the same though.

[Janot]

You have a pulse of light 1m long with 10 wavelengths in it. When you're stationary, you wil measure this to have a certain frequency. But what do you mean? How can you NOT be stationary relative to an electromagnetic wave?

[Cuddles]

But what do you mean? How can you NOT be stationary relative to an electromagnetic wave?

It doesn't matter. First you're stationary relative to something, then you're moving relative to something. The source of the radiation is the usual point of reference, but as long as your motion is different in the two cases, you will measure distance and time differently and will therefore measure the frequency differently.

[nakakayama]

Allow me to add the following, please.???

Two light paths are coming from a star. These are parallel (and close to). One is coming to observer A (moving on the light path at high speed), one is coming to observer B (moving on the light path at low speed). Wave length/wave number of two lights may be the same, and light speed may not be the same (then frequency is not the same. Note : light speed = frequency x wave length), i think.

[Admin]

light speed = frequency x wave length)

Yes, but the frequency is inversely proportional to the wavelength. As the frequency increases the wavelength decreases.

Multiplying the two will always give the same answer for the speed of light.

I think that's where the confusion lies here.

If not, we'll need a restatement of the problem as I too can't see what the problem is if it's not confusing a change in observed frequency with speed.

[Cuddles]

Wave length/wave number of two lights may be the same

No


and light speed may not be the same

No.

Light speed will always be the same, but frequency and wavelength will both be different for observers undergoing different motion.