Saint

Severian - this might be a clearer way to ask the question. If are in a space ship located 10 light-years from a star, and stationary wrt the star, when we start moving toward the star, will we immediately notice the bluse shift (asuming we are moving fast enough to produce an appreciable shift)? Or will we have to wait until the light from the star being emitted right now reaches us in order to notice the shift.

Severian - in your last example, say your friend throws a string of balls at time interval delta t while you are both standing still wrt one another and you are separated by some large distance. He then stops throwing the balls. You then start moving toward him. Will you see an increase in the frequency of the balls you receive? That is, in the frequency of the balls he has already thrown and are currently in transit? Basically, I want to know if you can influence the frequency of an existing beam of light (string of balls), or if the frequency shift can only be generated based on changing the relative distance from the source to the receiver. Sorry if this is unclear. Hopefully you see what I'm getting at.

No, that's not the important velocity difference. The differences that matter are the difference between the source and the medium, or the observer and the medium. If you choose to model the doppler shift of sound in a laboratory setting, with no medium velocity, that's great, but it doesn't change the reality of the shifting mechanism. Why are you holding onto a model for doppler shifting that relies entirely on additive velocities and propogation mediums to explain doppler shift in a theory that doesn't allow either?

There is a huge difference. The doppler shift for sound is based only on the speed of the source or receiver relative to the medium, not to each other. When a source moves wrt the medium, it changes the frequency of the wave that travels through that medium. If a receiver is moving wrt to the medium, it changes the frequency of the wave it receives. And the reason for this?? The velocity of the sound wave in the medium is constant (pretty much), but the velocity of the sound wave wrt the source or receiver is not constant. The mechanism for altering the frequency is the additive velocity relationship which you are not supposed to apply to light. So this goes back to my question for Janus; once a beam of light has been emitted, and the source has stopped emitting (the beam is simply travelling through space), what is the mechanism for red/blue shifting that beam? If moving toward the dead source of the beam will blueshift the frequency, what are you moving wrt to create that shift? Once the source has died, you are basically moving toward a light beam. How can motion wrt a beam of light create any shift in the frequency?

When you say that once something undergoes an acceleration, it can be determined to be moving, wrt what is it "moving". It already has a velocity wrt the other something. What does acceleration do for you? It sounds like you're trying to define absolute motion in a round about way.

Good picture. That was nicely illustrated. Of course, I still have some questions. When you described the scenario above, you included both the source and sink for the light. How about this: A very distant star (at rest wrt to us for purposes of example) generates light for a million years. Then it dies. We begin to recieve it's light after it is already dead. We begin to move toward the location of the now dead star at a high rate of speed. Can that light be blue-shifted?

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?

In your example, what is realy happening is that you are running through a string of balls at a given velocity (your velocity wrt the source), and the string of balls is also moving toward you at some velocity (the velocity of the thrown balls). That should look like an old familiar idea to you. Additive velocities. We're not allowed to have that with a beam of light. So that goes back to the question. Without additive velocities, what is the mechanism for changing the frequency/wavelength of a beam of light due to velocity toward or away from the source?

Thanks, and I agree that's the standard response to the question. What's the squashing mechanism. That is, the beam of light has been emitted. Once you start moving toward the source, what is the mechanism that changes the wavelength/frequency. Are you simply moving through the wave peaks faster?

What would cause the blue-shift to occur? Are you travelling faster wrt to the beam of light coming toward you?

you paraphrased (although you highlighted it correctly)- I said it was the defined upper limit for real answers. If you input a FTL speed, you get an imaginary number. That is, non-real. So c is the upper bound for real answers. And I agree with what Tom said in the last post. Feel free to build a better mousetrap, but don't expect science to discard a process that works just because it doesn't give you a warm fuzzy feeling. As I said, this post isn't about bashing relativity. It's about the misuse, in my opinion, of relativistic equations. That's all. And, I'm not trying to build a better mousetrap here. I'm trying to tell people not to misuse the one they have. It's a mousetrap, not a catch-all.

Nevermind. I'm wrong, and you are all correct. When something travels FTL wrt something else, it's time and length both become imaginary. Of course it makes no physical sense, but that's just the way it is becuase the SR equations say so. And, since SR was apparently constructed to deal with every possible speed, all numbers (useful or otherwise) produced by an equation with a defined upper limit for real answers should also simply be accepted. Beyond that, we should accept that reality will follow that equation into the imagination of the mathematician. Solid reasoning, folks.

Swansont/Tom - I never said that the speed limit it was a postulate. Why do you keep telling me I did? The equations in SR use c as the thing against which some relative velocity is compared (or divided). And the equations cease to provide usable information for relative velocities greater than c. You're acting as though I'm bashing relativity here. While I have done that before, I'm not doing it here. I'm just saying that you can't take the equations past c and get anything useful. Those equations are designed to provide imaginary answers for FTL speeds. Tom - Do you truly believe that SR predicts imaginary time? Is any misuse of an equation to be called a prediction of the base theory?

I would disagree that it's entirely semantic. It's an important point. That equation was not developed to provide answers (or predictions) for FTL speeds. Period. To better illustrate this point you need to look no further than the equation itself. It uses c as it's upper limit for providing real answers. If it had been developed for FST speeds, it would not use c as it's upper limit. The backbone of SR is not simply the constancy of c, but also that it is the universal "speed limit". Everything was developed based on that idea. Since it was clearly developed with the assumption that FTL speeds are impossible to achieve, it cannot provide useable answers to inputs beyond c. Janus even alluded to this in his response.

I should definitely take back my statement about the options one will have. I agree with all of you that there are more possible answers than the three I posted. Sorry for that one. Swansont, Tom, et al - Time does not become imaginary. The equation YIELDS an imaginary result. As I said, it would be more useful to simply say that we don't know. Janus did not say that. He said that time would become imaginary. I think that's a dangerous way to look at the world. And, for Swansont, I think it shows that Jauns did not acknowledge the assumptions that went into the formulation of the equation.

I'm just trying to make sure that when someone asks a valid question about reality, they are not told that space and time become imaginary numbers based on an equation. Or, if that represents a reasonable model of reality, could you please tell me what imaginary time is.

As much as you'd like it to be, math is not a justification. It's tool for modelling. If GR makes no sense to you conceptually (without math), that should tell you something.

OK - Does the expansion of space itself provide the same time dilation as moving through space? I mean that if you have spaceships at points A and B, and the "space" between them expands at such a rate as to increase their distance from one another at a rate close to c, do relativistic effects apply?

I always thought that this was brought up to illustrate a particular point of view. Kind of a Copenhagen-ish view. That is - knowledge of reality collapses the wave function, reality alone does not.

Well, they are redshifted to the degree that would suggest FTL speeds. There's a difference. Basically, we need to start accepting that physical objects can reach FLT speeds, or find alternate explanantions for the extreme redshift. Up to this point, most relativists have had to stick to the idea that space itself is expanding. Although I don't know how they can claim that "nothingness" is expanding. Where is the extra "nothing" coming from?