DescentBB

If the speed of light is always observed to be a constant, then why does light have a Doppler effect?

Your observed doppler effect is caused by the velocity (relative to you) of the object emitting the wave, but the effect itself changes the frequency of the wave, not its velocity.

Think about a sonic boom, which is an extreme example of the doppler effect when an object is moving toward you at the speed of sound. As the object moving toward you increases in speed, you will observe the frequency of the sound increase, but even when the object reaches the speed of sound, the sonic boom itself is still traveling toward you at the plain old speed of sound.

If the object can be observed to have a different speed relative to me, then why can't sound? Doesn't air act like this object? Why not?

Sound does, that's why police sirens coming towards you sound higher pitched than police sirens going away. Higher Frequency.
And why redshift is in-front of you. Higher frequency.

Light shifts towards red as the source is moving away from you, it shifts towards blue when the object is moving closer to you.

If sound does have a different velocity, then why can't light? What does the object moving have do to microwaves? Aren't microwaves always microwaves? (within a certain frequency range)

Both the speed of sound and the speed of light are a function of what kind of medium they are passing through, not a function of the velocity of whatever is emitting them. The speed of light in a vacuum is constant.

If I may also science for a sec: That's why a fart sounds lower leaving you than when it comes towards you; there's higher frequency of fartting

How can the speed of light be a function of the properties of the medium when it's a constant? If the speed of sound is variable with relative motion then why not light?

Jeff250 wrote:...The speed of light in a vacuum is constant.

That's more or less the clue to all your questions.
That's a very important point some people (me included in the past) usually tends to overlook.

Burlyman wrote:If the speed of sound is variable with relative motion then why not light?

The speed of sound doesn't vary with relative motion; it's the frequency and wavelength of the sound that do. The speed of a waveform is equal to multiplying its frequency (how many waves per unit time, usually per second) by its wavelength (how long each individual wave is). Those two are inversely related: as the wavelength increases, the frequency decreases, and vice-versa. However, so long as the wave stays in the same medium, the product of those two, the speed, stays the same.

What if light is staying still and we're the ones moving around it?

A vacuum just implies that the propagation of light isn't impeded by a material medium.

Top Gun wrote:

Burlyman wrote:If the speed of sound is variable with relative motion then why not light?

The speed of sound doesn't vary with relative motion; it's the frequency and wavelength of the sound that do. The speed of a waveform is equal to multiplying its frequency (how many waves per unit time, usually per second) by its wavelength (how long each individual wave is). Those two are inversely related: as the wavelength increases, the frequency decreases, and vice-versa. However, so long as the wave stays in the same medium, the product of those two, the speed, stays the same.

Speed appears to vary with relative motion, but you guys are saying it doesn't with light. The product of frequency and wavelength represent the speed relative to the medium, but not necessarily to the observer. My question is about velocities that are relative to observers, not velocities relative to a medium, or a vacuum.

Ah, okay, now I understand what you were asking. For "normal" types of waves, including sound, you're correct that the speed of the wave relative to the observer changes depending on the motion of the wave's source; it's pretty easy to see just from looking at ocean waves. The fun thing about light is that it doesn't follow these rules of relative motion. Say a source of light is moving towards or away from me at some very high speed. The light's speed through whatever medium we're in will remain constant, as we'd expect...but no matter how fast the light source is moving, I as an observer will always receive the light in the same amount of time. The principle of the Doppler effect still applies, because the moving source creates the changes in my perception of the wave's frequency and wavelength, but the speed I observe will always remain at that same constant value. As it turns out, the speed of light is constant across all inertial (that is, not accelerating) frames of reference. This was one of the core points of Einstein's seminal paper in 1905.

Um... may I disagree on a (fairly) minor point?

I believe that "Normal" waves propagate at some constant speed in a certain medium, but can propagate at different speeds to the outside world according to the movement of the medium.

In other words: sonic booms happen because the speed of sound in air is a constant.... but:

If you toss a pebble in a stream, the waves caused by the pebble will travel out in a circle relative to the stream (a constant), but will travel downstream and out in a circle relative to the shore (variable, depending on the speed of the stream).

Light, however, would travel upstream and downstream at the same speed relative to the shore.

An interesting thought I just had about Doppler shift: If you have a source that is travelling at ~c, the "downstream" spectrum would approach only a wavelength of 0; and the "upstream" spectrum would approach only a frequency of 0, huh?

snoopy wrote:Um... may I disagree on a (fairly) minor point?

I believe that "Normal" waves propagate at some constant speed in a certain medium, but can propagate at different speeds to the outside world according to the movement of the medium.

In other words: sonic booms happen because the speed of sound in air is a constant.... but:

If you toss a pebble in a stream, the waves caused by the pebble will travel out in a circle relative to the stream (a constant), but will travel downstream and out in a circle relative to the shore (variable, depending on the speed of the stream).

Light, however, would travel upstream and downstream at the same speed relative to the shore.

Yeah, that falls under what I said about the wave's source being in motion. The source point of the waves generated by tossing the pebble is in the stream, so that source point moves at the stream's speed, and an observer standing on the stream's bank will view the wave's speed as changing as the source point moves past them. As you said, if you extend the analogy to light, it wouldn't matter how fast the stream was going, since the observer would always see the same constant speed.

An interesting thought I just had about Doppler shift: If you have a source that is travelling at ~c, the "downstream" spectrum would approach only a wavelength of 0; and the "upstream" spectrum would approach only a frequency of 0, huh?

Pretty much, to the point where something moving at a very large fraction of c in relation to us would wind up being difficult to properly observe. You also run into the issue that it would require infinite energy to accelerate an object with mass to the speed of light, which is why we refer to it as the "universal speed limit."

On a related note, because the actual "fabric" of spacetime is expanding, beyond a certain distance from us, objects are essentially moving away from us "faster" than the speed of light: the objects themselves don't exceed c, but because the universe is undergoing accelerating expansion, their increasing distance from us winds up outpacing the speed of light. The concept of the Hubble volume describes this; it's basically the limits of the universe that we can actually observe, because anything further out is receding too fast for us to be able to observe it.

Photon is Pho and Wanton soup, combined.

Burlyman wrote:A vacuum just implies that the propagation of light isn't impeded by a material medium.

Top Gun wrote:

Burlyman wrote:If the speed of sound is variable with relative motion then why not light?

The speed of sound doesn't vary with relative motion; it's the frequency and wavelength of the sound that do. The speed of a waveform is equal to multiplying its frequency (how many waves per unit time, usually per second) by its wavelength (how long each individual wave is). Those two are inversely related: as the wavelength increases, the frequency decreases, and vice-versa. However, so long as the wave stays in the same medium, the product of those two, the speed, stays the same.

Speed appears to vary with relative motion, but you guys are saying it doesn't with light. The product of frequency and wavelength represent the speed relative to the medium, but not necessarily to the observer. My question is about velocities that are relative to observers, not velocities relative to a medium, or a vacuum.

This infact is what got Einstein thinking about such things and eventually developed the theory of relativity. He concluded that light waves reflecting from a horse drawn wagon do not combine with the speed of the horse and carriage or the horse and carriage would appear in the nearby intersection (that he was looking at) before it actually did. however, we no know that light is radically effected by space-time distortion, but that's a different principle.

I think you missed the point of what I just said. You guys are trying to say that light is somehow different than sound just because "Einstein said this and this." I haven't seen an explanation here as to why there should be a difference in principle.

Photons can behave like insects and all must return back to the sun, where the queen lives.

Top Gun wrote:For "normal" types of waves, including sound, you're correct that the speed of the wave relative to the observer changes depending on the motion of the wave's source; it's pretty easy to see just from looking at ocean waves.

This is *not* true for sound waves. When you are moving at mach 0, the sound you emit travels at mach 1. When you are moving at mach 1, the sound you emit still travels at mach 1. If the speed of sound were additive to your speed, then you wouldn't be able to create a sonic boom at mach 1 (or at any speed)!

Futurama wrote:Fry: Go after him, Leela!

[Leela pushes the throttle forward. The engines struggle.]

Leela: It's no use. We were going full speed when we fired him so he's going even faster than that.

Oops I was thinking of the wrong Leela.
A very interesting discussion that makes me feel like a total animal. My lack of knowledge is astounding.

Jeff - I think he was referring to the motion of the medium relative to the observer... I think. If it were the observer that were moving, the effect would be the same - depending on the direction of movement within that medium, the velocity of sound/other waves could appear to change.

If I understand correctly, this phenomenon doesn't happen with light. I was never really able to follow why not though - Maxwell and Einstein kind of confused me.

Burlyman wrote:I think you missed the point of what I just said. You guys are trying to say that light is somehow different than sound just because "Einstein said this and this." I haven't seen an explanation here as to why there should be a difference in principle.

Einstein postulated based on earlier evidence that the speed of light didn't vary based on the frame of reference, and described a relation for showing how that affects what we observe, but we're not just basing it on his word: since the, we've experimentally confirmed many times over that what Einstein said is true. As for the why...well, you might as well ask why the universe exists in the first place. The invariance of the speed of light is a fundamental property of the space-time of the universe, so there isn't a real "why" we can assign to it.

Jeff250 wrote:

Top Gun wrote:For "normal" types of waves, including sound, you're correct that the speed of the wave relative to the observer changes depending on the motion of the wave's source; it's pretty easy to see just from looking at ocean waves.

This is *not* true for sound waves. When you are moving at mach 0, the sound you emit travels at mach 1. When you are moving at mach 1, the sound you emit still travels at mach 1. If the speed of sound were additive to your speed, then you wouldn't be able to create a sonic boom at mach 1 (or at any speed)!

Yeah, I kind of mucked up that initial explanation, but I tried to clarify things in my next post. You're right that the speed of sound in air is constant at a certain altitude, so no matter what speed a jet is moving, the sound waves will propagate from it at that set speed. However, like that river example, if the air itself was moving, then we'd observe a different relative speed taking that into account: if a sound wave is moving through air, and that air is also moving in the form of wind, then we'll observe the sound at an equivalent speed to the speed of sound plus the speed of the wind. As we know, that isn't true when it comes to light waves.

Just to muddy the water, gravity can have a profound effect on the speed of light as evidenced by black holes. So Burly is right when he talks about mediums affecting light. Gravity is one of those "mediums"

Sirius wrote:Jeff - I think he was referring to the motion of the medium relative to the observer... I think. If it were the observer that were moving, the effect would be the same - depending on the direction of movement within that medium, the velocity of sound/other waves could appear to change.

If I understand correctly, this phenomenon doesn't happen with light. I was never really able to follow why not though - Maxwell and Einstein kind of confused me.

I think the key to this is that light doesn't really have a medium like sound. Sound is the compression of the air it's traveling in, whereas for light, air is just stuff in the way that may slow it down somewhat. For light moving in a vacuum, what would it even mean to say that you are moving at a certain speed relative to its medium?

woodchip wrote:Just to muddy the water, gravity can have a profound effect on the speed of light as evidenced by black holes. So Burly is right when he talks about mediums affecting light. Gravity is one of those "mediums"

Well, I wouldn't say that gravity is a "medium"...rather, we can view gravity as a "bending" of the fabric of space-time, which in sufficient amounts can actually "bend" the path that light travels, as well; a black hole is basically the most dramatic result of that bending, to the point where light cannot escape from it. This all gets into general relativity, which I'd consider beyond my ability to completely wrap my head around and explain. As noteworthy as Einstein's formulation of special relativity was, it was something that other people probably would have come up with not too long afterwards as well, but general relativity was a massive leap forward.

Jeff250 wrote:

Top Gun wrote:For "normal" types of waves, including sound, you're correct that the speed of the wave relative to the observer changes depending on the motion of the wave's source; it's pretty easy to see just from looking at ocean waves.

This is *not* true for sound waves. When you are moving at mach 0, the sound you emit travels at mach 1. When you are moving at mach 1, the sound you emit still travels at mach 1. If the speed of sound were additive to your speed, then you wouldn't be able to create a sonic boom at mach 1 (or at any speed)!

This is the closest thing I've found here to a logical answer. Several of you are just repeating what Einstein said while insisting that it's been experimentally verified. Jeff just showed that the speed of the sound wave from the source moving at mach 1 relative to the ground would actually be mach 2. So why should this be different than light?

No, quite the opposite. I tried to provide intuition as to why the speed of sound is *not* additive (otherwise sonic booms would be impossible).

Burlyman wrote:

Jeff250 wrote:

Top Gun wrote:For "normal" types of waves, including sound, you're correct that the speed of the wave relative to the observer changes depending on the motion of the wave's source; it's pretty easy to see just from looking at ocean waves.

This is *not* true for sound waves. When you are moving at mach 0, the sound you emit travels at mach 1. When you are moving at mach 1, the sound you emit still travels at mach 1. If the speed of sound were additive to your speed, then you wouldn't be able to create a sonic boom at mach 1 (or at any speed)!

This is the closest thing I've found here to a logical answer. Several of you are just repeating what Einstein said while insisting that it's been experimentally verified. Jeff just showed that the speed of the sound wave from the source moving at mach 1 relative to the ground would actually be mach 2. So why should this be different than light?

Well, "mach" is an expression of energy transferred over particles of a given medium. In this case air. Mach is not constant. Pressure levels matter. As density and material change, so does the speed at which sound travels. So, if you had a submarine that was stupid fast, you could also have a "sonic" boom under water. Light does not cohere like air or water so there would be no "boom" necessarily. I'm sure things would change though if a material object were to approach light speed. ... assuming you didn't get thrashed by dust or a gravity well or something larger than dust.

Light is the fastest particle/substance/medium that was known by Einstein. Since then, we've found other "particles" that are faster. the problem is that they don't exactly adhere to regular physics. Neither does light. All of these types of particles are more energy than solid matter. I suspect that in the next couple of decades, some new ideas and theories will pop up about Quantum physics and beyond as well as FTL particles. Also, I kinda doubt we'll understand it or perhaps have more quantifiable evidence for such things once we start building in space; things like colliders and such.