Red/blue shift question.

[Daecon]

Sorry if I sound stupid, but it only just occured to me that I didn't know:

 

If light is constantly moving away/towards you at light speed, and relatively speaking ALWAYS at light speed - how can you create an effect of red/blue shift?

 

Surely no matter what speed you're travelling at either away from or towards the source, the light will STILL hit you at exactly 300'000 kms...?

[swansont]

The wavelength and frequency change, but their product (the speed) remains fixed.

[danny8522003]

This might be a load of garbage...

 

But what if light was at rest wrt time, and it is us that is moving through time at c?

[timo]

It´s no piece of garbage so far: The magnitude of 4-velocity is always 1 (or whatever you norm it to - c for example) for massive particles and 0 (regardless of the norm, of course) for massless ones as light.

I just don´t undertand what "at rest with respect to time" and "moving throgh time at c" means. And I could bet you don´t know that either.

[Daecon]

The wavelength and frequency change, but their product (the speed) remains fixed.

 

How can a product remain the same if the the things that cause it change?

[timo]

v = k*f

k-> k'= C*k

f -> f' = f/C

v -> v' = k'*f' = C*k*f/C = k*f

[mezarashi]

This might be a load of garbage...

 

But what if light was at rest wrt time' date=' and it is us that is moving through time at c?[/quote']

 

I think this has been thought about. Everything tends to choose the shortest path to transverse in the space-time continuum, and everything also transverses this continuum at the speed of light. For light, it is entirely in 3-dimensional space. For non-zero mass objects like us, partially through time and space.

[Saint]

Sorry if I sound stupid' date=' but it only just occured to me that I didn't know:

 

If light is constantly moving away/towards you at light speed, and relatively speaking ALWAYS at light speed - how can you create an effect of red/blue shift?

 

Surely no matter what speed you're travelling at either away from or towards the source, the light will STILL hit you at exactly 300'000 kms...?[/quote']

 

Remember, your velocity doesn't change wrt the light beam. Your velocity wrt something else (say, a light source) alters both space and time to provide the illusion that the frequency/wavelength has changed. Even though that alteration of space and time isn't supposed to effect the light beam, since there is no physical space for light to travel through. See, even though the MM experiment was supposed to separate light from any connection to space (aether), relativity likes to use the alteration of space and time to effect light. See how clear it is?

 

I hope you caught the sarcasm there. I think the only way you can get red/blue shift is to accept the fact that you are moving toward/away from a light beam at a faster/slower rate. I.E, additive velocities. Why make it harder than it has to be?

 

Or, maybe the redshift we see isn't motion-based at all. Maybe light simply loses energy as it interacts with space over long distances. That would fit nicely with the fact that the farther away things appear to be, the more redshifted they become.

[danny8522003]

It´s no piece of garbage so far: The magnitude of 4-velocity is always 1 (or whatever you norm it to - c for example) for massive particles and 0 (regardless of the norm' date=' of course) for massless ones as light.

I just don´t undertand what "at rest with respect to time" and "moving throgh time at c" means. And I could bet you don´t know that either.[/quote']

I meant that time is a 4-dimensional object and light is attached to this object, like flys stuck on paper. The universe being 3-dimensional travels 'accross' or 'through' time and photons interact with it as we pass over them. Photons appear to travel at c because we move through time, which the photons are attached to, at c.

 

I thought maybe a property of a 4-dimensional object would allow for something like this, maybe not but i just thought id throw something in the air and see where it lands.

[V-Man]

According to Einstein, light travels the same no matter which frame of reference you have, so in the case of what you are talking about, that would actually make a lot of sense. Hence the warping of space the nearer one gets to the speed of light. In your scenario, it is time which warps space. Also, it may be that the light is attached to an additional dimension of space, or time, or something in between(!), and it may be this which is warping time AND three-dimensional space.

(Should I have posted that in pseudo-science? They certainly don't teach that in high school.)

[danny8522003]

Damn, i didnt think that last one posted cos my net went down.

 

Can a mod delete this please?

[Royston]

How can a product remain the same if the the things that cause it change?

 

Speed of light is constant...space-time is equated using the speed of light.

[Daecon]

Speed of light is constant...space-time is equated using the speed of light.

 

That's my point. If the speed of light is constant, then whether you're moving away from the sun at a million miles an hour, or towards it at thousand - wouldn't the light still reach you at c and therefore how can you catch upto it or move away from it at a different speed, to give red or blue shift?

 

I understansd how it works with sound waves and sonic booms because you *can* go faster of slower than sound does. But you can't go faster or slower than light does, can you? Even if you're totally stationary, light is still approaching you at light speed.

[mezarashi]

I understansd how it works with sound waves and sonic booms because you *can* go faster of slower than sound does. But you can't go faster or slower than light does' date=' can you? Even if you're totally stationary, light is still approaching you at light speed.[/quote']

 

Actually, if you think about it carefully, even with sound in air, sound still travels at the speed of sound. The speed of sound is a function of the composition and temperature of the medium in which it is propagating. The moving source doesn't make sound go faster or slower. Neither does moving towards the sound source. You are simply running faster into the waves. Because your ears are trained to understand that when there are more waves hitting it in a certain period compared to another, then that sound must be of higher pitch or frequency.

[Daecon]

Actually, if you think about it carefully, even with sound in air, sound still travels at the speed of sound. The speed of sound is a function of the composition and temperature of the medium in which it is propagating. The moving source doesn't make sound go faster or slower. Neither does moving towards the sound source. You are simply running faster into the waves. Because your ears are trained to understand that when there are more waves hitting it in a certain period compared to another, then that sound must be of higher pitch or frequency.

 

*has headache*

 

But the speed of sound waves is changable, and it *is* physically possible to run faster into them.

 

What I don't get: Is it physically impossible to run faster into light waves because light speed is constant no matter how fast you move?

 

*finds a corner to sulk in*

[mezarashi]

*has headache*

 

But the speed of sound waves is changable' date=' and it *is* physically possible to run faster into them.

 

What I don't get: Is it physically [b']impossible[/b] to run faster into light waves because light speed is constant no matter how fast you move?

 

*finds a corner to sulk in*

 

I feel for you

The thing is, the doppler effect for sound and light are actually very similar. Actually you can use the same equations if you are not considering sources that are moving at relativistic speeds or close to the speed of sound (in the sound case). For relativistic calculations, you add in a correcting factor.

 

Don't get confused. The fact that you can change the speed of sound in air because you can change the properties of air, has nothing to do with the Doppler effect. I think this is the point you want to make clear to yourself. The ability to move faster than the speed of sound means that you will create sonic booms, which obviously is not related to the doppler effect... only that now you know that there won't be an "electromagnetic booms".

[swansont]

*has headache*

 

But the speed of sound waves is changable' date=' and it *is* physically possible to run faster into them.

 

What I don't get: Is it physically [b']impossible[/b] to run faster into light waves because light speed is constant no matter how fast you move?

 

*finds a corner to sulk in*

 

Yes, that's right. Welcome to the wonderful world of relativity. The implications of this reality are that length and time measurements depend on your reference frame, and that simultaneity is not an absolute phenomenon.

 

It is situations like this that drive some of our visitors to declare that relativity is wrong, because it makes no sense that it could work this way.

[Janus]

*has headache*

 

But the speed of sound waves is changable' date=' and it *is* physically possible to run faster into them.

 

What I don't get: Is it physically [b']impossible[/b] to run faster into light waves because light speed is constant no matter how fast you move?

 

*finds a corner to sulk in*

 

But you don't need to run into faster light waves to see a doppler effect any more that you need to run into faster sound waves to hear a doppler shift.

 

If I'm standing on the train platform on a windless day and someone on the platform yells at me, the sound travels at the speed of sound in air towards me. If a train approaches and blows its whistle the instant it passes that person, the sound from it will travel at the same speed as the yell (they will both reach me at the same time). The train whistle will be doppler shifted however.

[Saint]

But you don't need to run into faster light waves to see a doppler effect any more that you need to run into faster sound waves to hear a doppler shift.

 

If I'm standing on the train platform on a windless day and someone on the platform yells at me' date=' the sound travels at the speed of sound in air towards me. If a train approaches and blows its whistle the instant it passes that person, the sound from it will travel at the same speed as the yell (they will both reach me at the same time). The train whistle will be doppler shifted however.[/quote']

 

Actually, there isn't a good correlation between the dopple shift between light and sound. Sound travels through air. All that matters is the velocity of the source and sink wrt to the medium (air).

 

So, in your example, the train moving through the air would generate a higher frequency because the train itself is compressing the wavelength. In that case, it's the movement of the source generating the shift. But remember, the doppler shift in sound, which everyone is so eager to provide as an example for the doppler shift in light, depends on a common medium; air. That would be analogous to the mysterious aether. Since light has no such medium, or aether, you should not try to use examples from the world of sound.

[Daecon]

Yes' date=' that's right. Welcome to the wonderful world of relativity. The implications of this reality are that length and time measurements depend on your reference frame, and that simultaneity is not an absolute phenomenon.

 

It is situations like this that drive some of our visitors to declare that relativity is wrong, because it makes no sense that it could work this way.[/quote']

Well I wouldn't go as far as to say relativity is wrong. I like relativity.

 

But I'm doubting that you can measure the speed of a distant star by studying it's light spectrum, as light can't be subject to a doppler effect as it will always leave is source and reach it's destination at a set speed irrelevant of the motion and position of each of the the two bodies...?

 

Tell me if I'm being stupid or just ignorant.

[mezarashi]

Since light has no such medium, or aether, you should not try to use examples from the world of sound.

 

Right you are. That's the way it is, but sometimes, sometimes, just for the sake of clarity to a struggling learner, it is helpful to use analogies we can visualize or are familiar with.

 

When I was learning kinematics in 10th grade, my physics teacher never told me to keep in mind that time wasn't universal. He never told me that we couldn't travel faster than the speed of light. He never told me that there's no way we could make perfectly precise measurements. But yet I was able to do classical mechanics and predict (to his satisfaction), where a ball falling off a ramp would land.

[swansont]

Well I wouldn't go as far as to say relativity is wrong. I like relativity.

 

But I'm doubting that you can measure the speed of a distant star by studying it's light spectrum' date=' as light can't be subject to a doppler effect as it will always leave is source and reach it's destination at a set speed irrelevant of the motion and position of each of the the two bodies...?

 

Tell me if I'm being stupid or just ignorant.[/quote']

 

You're being wrong. The Doppler shift happens with light. An atom that absorbs light resonantly at rest will not absorb it with the same probability if moving, because it "sees" a different, Doppler-shifted, frequency when it's moving, and the absorption probability depends on the frequency of the light. You can move it significantly off-resonance for kv (the shift) being several times larger than the linewidth of the transition (k is the wavenumber, v is the speed of the atom). For alkali atoms, with linewidths of around 5MHz, this amounts to only a few tens of m/s of motion as the absorption varies as the square of the detuning.

[Saint]

You're being wrong. The Doppler shift happens with light. An atom that absorbs light resonantly at rest will not absorb it with the same probability if moving, because it "sees" a different, Doppler-shifted, frequency when it's moving, and the absorption probability depends on the frequency of the light. You can move it significantly off-resonance for kv (the shift) being several times larger than the linewidth of the transition (k is the wavenumber, v is the speed of the atom). For alkali atoms, with linewidths of around 5MHz, this amounts to only a few tens of m/s of motion as the absorption varies as the square of the detuning.

 

What is the absorbant atom moving wrt in order to see a change in frequency? I would think it would be the beam of light. But how can you change your velocity wrt to a beam of light? Isn't that relative velocity always constant?

[swansont]

What is the absorbant atom moving wrt in order to see a change in frequency? I would think it would be the beam of light. But how can you change your velocity wrt to a beam of light? Isn't that relative velocity always constant?

 

The velocity is wrt the lab frame. An atom at rest in the lab frame has a resonance at one frequency. An atom moving wrt the lab frame has a resonance that is shifted by kv, because there is motion relative to the light source. It doesn't matter if the source is moving or the receiver is.

[Janus]

Well I wouldn't go as far as to say relativity is wrong. I like relativity.

 

But I'm doubting that you can measure the speed of a distant star by studying it's light spectrum' date=' as light can't be subject to a doppler effect as it will always leave is source and reach it's destination at a set speed irrelevant of the motion and position of each of the the two bodies...?

 

Tell me if I'm being stupid or just ignorant.[/quote']

 

Let's see if we can clear iy up for you. light is subject to Doppler shift for the very reason that its speed is invarient for all observers.

 

First consider the following animation. It shows a source located between two observers (the red and blue dots) The expanding green rings represent lightwaves being emitted by the source. Note that the rings expand in perfect circles at c from the point of emission. The first light wave hits both the red and blue dots at the same time.

 

 

Now consider the second animation which shows the situation when the source has a relative velocity with respect to the observers. The first part of the wave is emitted. It expands out as a circle at c and hits the red and blue dots at the same time (as it did in the above animation). An instant later, the next part of the wave is emitted and also expands out as a circle at c. But in the intervening time, the source has moved with respect to the the observers, it is closer to the blue observer than the red observer. Thus this partr of the wave will take less time to reach the blue observer than the red observer (the blue observer will see these parts of the wave arrive closer together than the red obsever will). The pattern repeats for each successive part of the wave.( note that each successive part of the wave also expands in a perfect circle at c from its emission point relative to the blue and red dots.) As a result, the blue observer measures the light at a shorter wavelength and higher frequency and the red observer measures a longer wavelength and lower frequency.

 

 

Note that all this requires is for the source to have a velocity with respect to the observers and that the observers measure the speed of light as a constant with respect to themselves.