Measuring The Relative Velocity Of Light

[wespe]

If you run into the light at a distance of 100,000 miles from the source, what does your speed have to do with anything as long as the experiment ended at 100,000 miles form the source. Think about it... It doesn't matter how fast you are traveling when you end the test, all you are doing is marking a specific distance from the source and measuring how long it took light to get to that spot. If you traveled for two seconds, then no matter what your speed is, you will be 372,000 miles from the source.

Tom Mattson has explained well, but just in case you are still confused..

OK, if distance is 100,000 miles, what value will you divide this by? You have to divide it by (detection time - emission time). How will you know the emission time? You can't measure it directly, because you are in the ship. The distance is also contracted due to SR effects, and it is difficult to see what the ship would directly measure as distance. These are additional complexities. To avoid them, my example did not include light emission time. It included two time values that you measured inside your ship. And the distance was again measured inside the ship. Divide and find the speed, v=dx/dt, should be simple enough.

 

[grounded]

Just so you know Tom...I really appreciate the time you have given up to respond to all my posts. Even though we don't agree, you still respond, thanks.

I do however have yet another question.

Given (x₂-x₁), there is no way to know ahead of time what (t₂-t₁) is going to be.

If I am given the location of the source (x₁), and I am given the location of the detector (x₂), isn't (t₂-t₁) equal to the amount of time it takes to travel (x₂-x₁) at the speed of light?

 

[quantumdude]

Just so you know Tom...I really appreciate the time you have given up to respond to all my posts. Even though we don't agree, you still respond, thanks.

I don't mind, because you are obviously sincere.

I do however have yet another question.

If I am given the location of the source (x₁), and I am given the location of the detector (x₂), isn't (t₂-t₁) equal to the amount of time it takes to travel (x₂-x₁) at the speed of light?

It will return the speed of light relative to me. The definition of relative speed--by itself--does not prefer the SR velocity addition formula over the Galilean velocity addition formula. If you go back to my example with the light source moving at 0.5c towards me, where I work out the prediction using both Galileo and Einstein, you'll see that the exact same definition of velocity is used in both cases. If Galileo is correct, then the time elapsed (t₂-t₁) will be such that the speed of the light in my frame is 1.5c. And if Einstein is correct, then it will be such that the speed is c.

So the experimental question is: When does the pulse arrive?

Hopefully you now see that the result is not guaranteed to be 'c' just by virtue of the definition of relative velocity.

 

[grounded]

OK, if distance is 100,000 miles, what value will you divide this by?

When I see this:

v=(x₂-x₁)/(t₂-t₁)

I see this:

v=(The distance between the source and the detector) / (The amount of time it takes light to travel from the source to the detector)

From my perspective the observers speed only changes the position of the detector, but we are still only calculating the amount of time it takes light to travel from the source to the detector.

 

[quantumdude]

When I see this:

v=(x₂-x₁)/(t₂-t₁)

I see this:

v=(The distance between the source and the detector[/color]) / (The amount of time it takes light to travel from the source to the detector)

The part in red[/color] is not quite right. Remember that Event 1 was defined to be the emission of the pulse. That means that x₁ is the location of the source when the light was emitted. The source may move after time t₁, and so x₂-x₁ is not the distance between the source and the detector. It is the distance between the emission and the detection.

In other words, it is the distance that the light travels.

 

[wespe]

All the formula says is that light travels from the source at the speed of light. So in one second the ray of light will be 186,000 miles from the source and the observers speed cannot change this.

Oh I now see why you are saying this. It's the syndrome: thinking in absolutes. You may be over this by now, but just in case..

You assumed that there is an absolute space (aether) out there, in which things can move. The speed of an object in aether (or wrt aether) can be known. Let's call this speed "absolute speed". Absolute speed of light is somehow known and is equal to c. Then of course you would say that an observer approaching an emitted light won't change light's [absolute] speed.

But, we are not talking about absolute speeds. In fact, there is no way to measure absolute speed, because there is no way to detect absolute motion. Therefore, the existence of aether has been denied. There is no evidence that aether even exists so let's forget about it, along with absolute speed.

So now we can only talk about relative speeds. Consider: when you approach something, it also approaches you with the same speed you approach it, right? That mutual speed is the relative speed. It doesn't matter who is really approaching who, relative speed between two objects has only one single value, unlike two objects having two different absolute speed values.

Still, we can measure two different values, if we consider the relative speeds of two objects wrt a third object. Then, using these two values, we can calculate relative speed of these objects wrt each other. Galilean relativity would simply add the two values as v1+v2, which is intuitive. But when the speeds are high, it turns out that the formula does not hold true.

And the rest is history.. LOL I wrote too much.

I hope this helped.

 

[reilly]

Grounded -- Not up to my challenge(#91), I guess.? Still, in preperation, I'm getting my SR lecture notes together. RA

 

[Chronos]

this has gotten totally out of hand. unless you are questioning the maxwell equations, you have no point. all einstein did was put it all together in a predictive model. is e=mc^2 a lucky guess, or is it reality? do lorentzian transforms work as a predictive model? how do you explain that future observations fit the math that predicted them? perhaps we do not have a 'perfect' model that explains all that is observed, but, it is a much better model than the kind of unsupported models proposed by quacks such as savov.

why is it such a contradiction to refine the mathematics of reality? the equations do not change without evidence in support of observation. no one is claiming we are infallible [aside from the quacks]. we adjust theory when it is supported and has the mathematical foundations to support observational evidence.

 

[grounded]

Grounded -- Not up to my challenge(#91), I guess.? Still, in preperation, I'm getting my SR lecture notes together. RA

I am willing to discuss anything, and want to, but you must realize that you all seem to be at the top of the ladder. You wrote that "I will need to master the conventional arguments", and I agree. My lack of education compared to you, Tom, Qeistkiesel, and others gives me an entirely different perspective, be it right or wrong. I wouldn't come here to waste your time teaching me something I could read in a book. In my opinion I have an extreme grasp on reality and physics. I have read many encyclopedias and books about relativity; I do admit that they are all written in laymen terms such as “Six easy pieces” and “six not so easy pieces” both by Richard P. Feynman. I understand the information I have read and responses people have given (with exception to Tom’s more educated response), but I also have to FULLY understand it in order to accept it.

Some people will call it ignorant, but before I accept something I must fully understand (for myself) why it works. Knowing the motor will start when I turn the key is not enough. I am one of those kids that just had to touch the flame to know it would burn me, knowing it would was not enough. I am not here to push my perspective; I am here more to find that missing piece that would make me understand why my perspective cannot be used, the piece that would make me say “oh, I see now!”. If that still doesn’t make sense then let me ask you this. How much respect would you have for me if I didn’t truly understand any of the information I held? If I conform without understanding, I would just be a walking textbook with a low self-esteem.

Like everyone, I am capable of understanding anything given the proper information. In order for me to understand the conventional arguments, I need to understand what the math is really saying. I have never been able to understand the purpose of using the square root. I mean I know what it does, it gives us a number that when squared equals the number we square rooted. But what does it really do?

For example: (frequency) * (length) = (speed)

This makes sense since if I am diving at 55MPH then I am in a sense driving 55 one-mile lengths per hour. If I drive 60MPH I am driving 60 one-mile lengths per hour.

Can you tell me in words what the square root does, and maybe an analogy?
This is the formula I am referring to.

(square root of –1) *( wavelength*x-frequency*t)

Also can you tell me if in the above formula x = (x₂-x₁) and if t = (t₂-t₁) as in the formula below?

v=(x₂-x₁)/(t₂-t₁)

 

[wespe]

I am willing to discuss anything, and want to, but you must realize that you all seem to be at the top of the ladder.

Grounded, I am no expert and that's why I thought I understood your view better that the experts here, I hope you were not offended by #106.

Can you tell me in words what the square root does, and maybe an analogy?

LOL, if you had a square shaped area, squareroot would convert the area value to the length of the side value (of course I know you know that). So it has something to do with dimension relations I think.

This is the formula I am referring to.
(square root of –1) *( wavelength*x-frequency*t)

I haven't seen that one before. Also there is no = sign so it's not an equation.

Also can you tell me if x = (x₂-x₁) and if t = (t₂-t₁) as in the formula below?

well, x is distance, and t is duration

Take care.

 

[grounded]

What I was originally looking for was a way to measure light while in motion and get a constant speed using traditional methods.

(LENGTH) * (NUMBER OF LENGTHS PASSED PER SECOND) = (SPEED) = (DISTANCE TRAVELED PER SECOND)

From my perspective (naive or not) SR says we can’t use this method because it will not yield the results of SR, and in order to measure it as constant, I have to measure it in a way that conforms to SR. If the speed of light is constant, shouldn’t I be able to use traditional methods to measure it? I still have not grasped the concept of having to use SR formulas to predict SR effects, which is probably why I have a problem accepting SR’s formulas as proof to SR.

The single concept that has led me to where I am today is this.

Using an interferometer we measure the wavelength of light while at rest relative to the source. If we take the same measurement while traveling towards the source and account for the distance we have traveled, we will not measure a change in the wavelength. Doing it like this we are just counting the number of lengths that pass us and multiplying it by their length. The reason I perceive this to be logical is because of the following.

When the distance between the observer and the source is decreasing, the amount of time it takes the light to complete one cycle will also be decreased, as measured by the observer compared to when the distance between the two are not changing.

An interferometer can be calibrated using a light source with a known wavelength. As long as the distance between the observer and the source are not changing, this will work. Since light travels 186,000 miles per second, the wavelength equals the amount of distance traveled, traveling at 186,000 miles per second, in the amount of time it takes the light to complete one cycle. If the distance between the observer and the source are decreasing, then the wavelength equals the amount of distance traveled, traveling at the sum of “186,000 miles per second” added to “the speed at which the distance between the source and the observer is decreasing”, in the amount of time it takes the light to complete one cycle. If we measure the wavelength in this fashion, we will not measure a change.

I realize this is the old way of calculating and it doesn’t include any of the modern formulas, but it is the only way to make the interferometer measure any ray of light and correctly measure its true wavelength, isn’t that what we want it to do? I know some of you don’t like the term true wavelength, but if I am measuring something I want to measure what it is, not what it perceives to be. I noticed that while in motion relative to the source, the change in distance between the source and observer is not being accounted for. Once accounted for, I realized that using traditional methods, the only way to measure the speed of light to be constant is to ignore the change in distance between the observer and the source.

The above is just so you can see my perspective. As of today, I don’t agree with SR, but even though, I am determined to understand it. Right now I’m going to try to make sense out of Tom’s #98 post.

 

[grounded]

I haven't seen that one before. Also there is no = sign so it's not an equation.

It refers to Reilly's post #59

The wave function is, typically, the exponential of

i *( lam*x-nu*t)

where lam is the wavelength and nu the frequency, and i is the squareroot of -1.

 

[quantumdude]

What I was originally looking for was a way to measure light while in motion and get a constant speed using traditional methods.

But that's just it: We do used traditional methods. The distance traveled divided by the time elapsed is the speed. It doesn't get any more traditional than that. What you aren't accepting is the fact that the "lengths passed" are different for different observers, in a way that has nothing to do with not including the relative motion between source and observer. The lengths are literally phsically contracted in frames other than the rest frame.

As to this:

The wave function is, typically, the exponential of

i *( lam*x-nu*t)

where lam is the wavelength and nu the frequency, and i is the squareroot of -1.

He is referring to the complex exponential function:

f(x)=e^ikx

It just so happens that this function is sinusoidal, as follows:

e^ikx=cos(kx)+isin(kx).

In other words, the function is a solution to the wave equation. But instead of using that, you could just use a combination of sines and cosines, that don't include the "i".

 

[quantumdude]

I realize this is the old way of calculating and it doesn’t include any of the modern formulas, but it is the only way to make the interferometer measure any ray of light and correctly measure its true wavelength, isn’t that what we want it to do? I know some of you don’t like the term true wavelength, but if I am measuring something I want to measure what it is, not what it perceives to be.

As long as you're still hung up on this, you aren't going to see your way through this problem. There simply is no such thing as a "true wavelength". You only believe this to be so, but in reality there is no reason to hold that position. When you make an observation, you simply cannot doctor up the measurment to fit your predisposed vision of reality. As I noted before, I appreciate your sincerity, but your biggest shortcoming as I see it is that your thinking is so rigid. If you are going to understand anything about science, then you have to be prepared to let the experimental evidence dictate your view of the universe. But what you are doing is exactly the opposite: trying to put a skewed interpretation on measured results so that they fit what you already believe. Simply put, that is anti-science, and anti-education, which is why I respond to all of your posts.

As someone who understands this problem inside and out, I can tell you this: You will be forever at odds with physicists on this "true wavelength" thing unless you can understand the more fundamental problem of the invariance of the speed of light. The difference in wavelengths in different frames are derived consequences[/color] of that fact. But I'm convinced that you do not yet understand why the speed of light postulate must be true. So in effect what you're doing with this "true wavelength" argument is arguing against a conclusion without understanding the premises used to derive that conclusion.

 

[ram1024]

when experimentation does NOT match logical and rational predictions you should look for the faults in your experimentation, not reinvent the universe to match your crazy results...

 

[quantumdude]

when experimentation does NOT match logical and rational predictions you should look for the faults in your experimentation, not reinvent the universe to match your crazy results...

And this, dear boy, is why you'll never be a scientist.

 

[ram1024]

i'm quite open to the possibility that something doesn't work the way i expected it to, but only after i rule out EVERY possible option that conforms to rational and logical approaches

 

[quantumdude]

i'm quite open to the possibility that something doesn't work the way i expected it to, but only after i rule out EVERY possible option that conforms to rational and logical approaches

Hiding behind a veil of "rationality" does not make your philosophy of science any better. The fact of the matter is that it only takes a single experimental counterexample to prove that a physical theory is false. Distorting the results in the name of logic is intellectually dishonest, and patently unscientific.

As to your prior statement:

when experimentation does NOT match logical and rational predictions you should look for the faults in your experimentation,

This is awfully presumptuous of you, and if I did not already know that it was borne of ignorance I would say that it is awfully arrogant of you.

Your method here requires that a scientist have an innate, a priori knowledge of the way the universe should[/color] work, so that he may have a "rational" standard to which he can compare his results. But who has this knowledge? Who is so all-knowing that he can tell what is "rational" apart from experimentation? You? Can you tell us the way the universe should work? Indeed, why do we need to do experimentation at all? I mean, you already know what is "rational", so why can't we just ask you?

not reinvent the universe to match your crazy results...

Your thinking is exactly backwards. Experimentation is our way of finding out how the universe really is. Twisting experimental results around so that they fit one's predisposed worldview is precisely what it means to reinvent the universe to match one's crazy ideas. I repeat: it is an intellectually dishonest, unscientific practice.

 

[ram1024]

The fact of the matter is that it only takes a single experimental counterexample to prove that a physical theory is false.

please do join me in the "Relativity dies today" thread then ;D

This is awfully presumptuous of you, and if I did not already know that it was borne of ignorance I would say that it is awfully arrogant of you.

i'm going to assume my "logic" is absolutely correct until I've exhausted every possible avenue of attack has failed. when challenging a well-known, globally accepted theory or law there is no other way to go about it. to be wishy-washy or half-assed will only lead to failure. it may be arrogant, but it doesn't make it any less "real" when something is disproven with these "tactics"

Your thinking is exactly backwards. Experimentation is our way of finding out how the universe really is. Twisting experimental results around so that they fit one's predisposed worldview is precisely what it means to reinvent the universe to match one's crazy ideas. I repeat: it is an intellectually dishonest, unscientific practice.

no one's doing any twisting... where are you reading that? :|

 

[quantumdude]

please do join me in the "Relativity dies today" thread then ;D

The others are kicking your butt just fine without me. ;D

i'm going to assume my "logic" is absolutely correct until I've exhausted every possible avenue of attack has failed.

That's a foolish choice. The only way to determine the truth or falsity of scientific propositions is by observation, not logic.

when challenging a well-known, globally accepted theory or law there is no other way to go about it.

How would you know? What scientific investigations have you ever conducted? (And no, the thread "Today SR Dies" does not count).

to be wishy-washy or half-assed will only lead to failure.

I can only assume from the context that "being wishy-washy and half-assed" means "granting epistemic privelige to empirical information over reason in questions of science".

If so, then again, your thinking is exactly backwards. Rationalism as a scientific method has done nothing but fail miserably, which is why it is dead. The empirical approach, on the other hand, has enjoyed astounding success. Indeed, the lack of a priori knowledge of the universe requires that our investigations be empirically driven.

it may be arrogant, but it doesn't make it any less "real" when something is disproven with these "tactics"

You are so mistaken. There is no way--none whatsoever--to disprove any scientific theory which is internally consistent using only logic. The only "real" way to disprove it is by experimentation.

no one's doing any twisting... where are you reading that? :|

Right here:

when experimentation does NOT match logical and rational predictions you should look for the faults in your experimentation, not reinvent the universe to match your crazy results...

 

[ram1024]

"look for faults" does not mean "stage fake experiments to produce the results desired"

having trouble with reading comprehension?

 

[quantumdude]

"look for faults" does not mean "stage fake experiments to produce the results desired"

Then what do you mean?

having trouble with reading comprehension?

Not at all. If you examine the immediate context in which you posted your comments, you'll see that the inference I made was reasonable. I was discussing the impropriety of a skewed interpretation of experimental results to fit a worldview that one subjectively deems "rational". You jumped in with your comment about looking for faults in the experiment, with no further explanation. I just took your remark in the context of the discussion. It's not my fault if you don't say what you mean.

 

[ram1024]

i said exactly what i mean.

something is wrong with your experiments. find out what it is :P

 

[quantumdude]

i said exactly what i mean.

Might I recommend a bit more exposition then? The quick two-liners simply do not convey the information you wish to convey.

something is wrong with your experiments. find out what it is :P

Here's a perfect example.

On what basis do you say that something is wrong with an experiment, as opposed to something being wrong with a theory?

 

[ram1024]

experiments can be re-done and refined, theories are set in stone

The fact of the matter is that it only takes a single experimental counterexample to prove that a physical theory is false.

 

[quantumdude]

experiments can be re-done and refined,

Correct.

theories are set in stone

That's not a very good way to look at it. If a theory is "set in stone", then it has to be discarded in total when it is falsified. But if you take the point of view that theories can also be re-done and refined (as is the point of view of real scientists, by the way), then by keeping the good and discarding the bad, you don't have to start from scratch every time.

Anyway, you did not even come close to answering my question.


Here it is again:

On what basis do you say that something is wrong with an experiment, as opposed to something being wrong with a theory?

 

[ram1024]

when an experiment using principles proven by OTHER experiments acquires data NOT equivalent to theory. duplicate the experiment a few hundred times under multiple conditions to confirm it's not a fluke. then either that experiment and all other ones supporting it are wrong OR the theory is wrong.

the more times the experiment is tried and scrutinized the MORE correct it is.

 

[quantumdude]

when an experiment using principles proven by OTHER experiments acquires data NOT equivalent to theory. duplicate the experiment a few hundred times under multiple conditions to confirm it's not a fluke. then either that experiment and all other ones supporting it are wrong OR the theory is wrong.

This I can agree with. Now we just have to get you to see that this very thing has been done many, many times with SR.

 

[ram1024]

exactly

this is such fun

 

[reilly]

Grounded -- First, please accept my apologies for being snotty and arrogant(the physicist's disease) I misjudged your zeal and passion for understanding. (Note, however, as my dear Mother told me -- You learn more with your mouth shut than with it open)

Physics is very difficult, for two primary reasons: it requires a real facility with and understanding of a lot of advanced math; the conceptual basis is highly abstract and idealized, in E&M, QM, SR, GR, ..., and is not fully consistent. So, particularly, for the last reason, most physicists are highly pragmatic -- do the best you can with what you have, don't sweat the hard stuff unless it messes up your work. As I'm sure you know, for example, there is huge controversy about the proper interpretation of QM, but many working physicists pay little attention to the controversy. The standard ways work just fine in practice.

Everyone who teaches physics, say SR, will tell you we did not understand SR until we taught it. It takes a long time, and many exposures to SR to get it right. And, most students of SR will tell you that sometimes you have to suspend your judgement to get to a good understanding -- that's the advantage of a good teacher, he/she can suggest what to sweat now, and what to defer. All told, SR is a huge subject, of which the basic space-time kinematics, the x's and t's, comprise only a very small portion. It is the totality of SR that is so compelling.

That being said, the path to a robust understanding of SR necessarily involves lot's of math -- there's no way around that fact.

Tom's explanation of the imaginary exponential is right on. The way, typically, you build up a comfortable intuition about i, the square root of -1, is to work with it, work with it a lot. It's part of the language of physics, and is used so much because it is a powerful tool in working with waves, periodic phenomena, much of QM, and many other branches of physics and engineering.

If you are serious about physics, you need to hit the books, and do the homework. Resnick and Halliday is a good place to start. And, absolutely necessary, learn calculus. Then you will understand that the x and t in the exponential are not x2-x1, t2-t1. That exponential gives you the compex value of the wave amplitude at x at time t.

Once a professor, always a professor.
Regards and good luck

Reilly Atkinson

 

[geistkiesel]

Grounded -- First, please accept my apologies for being snotty and arrogant(the physicist's disease) I misjudged your zeal and passion for understanding. (Note, however, as my dear Mother told me -- You learn more with your mouth shut than with it open)

Physics is very difficult, for two primary reasons: it requires a real facility with and understanding of a lot of advanced math; the conceptual basis is highly abstract and idealized, in E&M, QM, SR, GR, ..., and is not fully consistent. So, particularly, for the last reason, most physicists are highly pragmatic -- do the best you can with what you have, don't sweat the hard stuff unless it messes up your work. As I'm sure you know, for example, there is huge controversy about the proper interpretation of QM, but many working physicists pay little attention to the controversy. The standard ways work just fine in practice.

Everyone who teaches physics, say SR, will tell you we did not understand SR until we taught it. It takes a long time, and many exposures to SR to get it right. And, most students of SR will tell you that sometimes you have to suspend your judgement to get to a good understanding -- that's the advantage of a good teacher, he/she can suggest what to sweat now, and what to defer. All told, SR is a huge subject, of which the basic space-time kinematics, the x's and t's, comprise only a very small portion. It is the totality of SR that is so compelling.

That being said, the path to a robust understanding of SR necessarily involves lot's of math -- there's no way around that fact.

Tom's explanation of the imaginary exponential is right on. The way, typically, you build up a comfortable intuition about i, the square root of -1, is to work with it, work with it a lot. It's part of the language of physics, and is used so much because it is a powerful tool in working with waves, periodic phenomena, much of QM, and many other branches of physics and engineering.

If you are serious about physics, you need to hit the books, and do the homework. Resnick and Halliday is a good place to start. And, absolutely necessary, learn calculus. Then you will understand that the x and t in the exponential are not x2-x1, t2-t1. That exponential gives you the compex value of the wave amplitude at x at time t.

Once a professor, always a professor.
Regards and good luck

Reilly Atkinson

Really, Reilly, how dare you, but you recovered nicely. perhaps ou can elucidate us on the following:

All moving frame values are non-primed with the exception of M’, the consistent location of the observer O in the moving frame.

At no time is there an inference that M’ was at the midpoint of the A and B photons emitted in the stationary frame.

To demonstrate the following:

Einstein’s moving train calculation indicating when the oncoming B photon is detected at t1 the A photon was located at a position consistent with –t1. Said in other words, as t1 is determined from t0 which locates M’ at t0, the A and B were equidistant to M’(t0) when t = t1.

Proof:
A moving observer located at M’ on a moving frame passes through the midpoint M of photon sources located at A and B in the stationary frame just as A and B emit photons. M’ is moving along a line connecting A and B, toward B.

At this instant the moving source t = t0. Later the moving observer detects the photon from B at t1, and later the photon from A at t2. The observer has measured her velocity wrt the stationary frame as v. Determine the position of the A photon at tx in terms of t0, t1, t2, and v when the B photon was detected at t1.

The photon from A must reach the position of M’ when t = t2. Therefore, the distance traveled by the A photon during Δt = t2 – t1, is Δtc. This is equal to the distance cΔt = vΔt + vt1 + vtx . Now we rearrange somewhat to arrive at, vtx = vΔt – cΔt + –vt1. Now as vΔt - cΔt is just -vtx - vt1

vtx = -vtx - vt1 – vt1

2tx = -2t1

tx = -t1

Therefore, in the moving frame the photon from A and the photon from B were equidistant from M’(t0) at t1.

 

[geistkiesel]

But we can see that he isn't transcending anything. He is making choices that lead to the physics of the 19th century, and we know that those choices are wrong.

Tom, excuse me if I've asked this before, but show me Grounded's math in 19th Century physics, AND that 19th century physics is passe because it is so old.

 

[reilly]

geistkiesel -- I really can't figure out what's going on in your scenario. A diagram or two would be very helpful.

But I do suggest you go to any text that discusses basic SR space-time definitions and (idealized) measurements, take pencil and paper and work through, step-by-step, the arguments. Then you will be in a position to answer your own question. If not, you will have far more specific questions to ask about SR. Einstein's book Relativity is elegant and insightful, the more recent Basic Relativity by Richard Mould goes into the basic SR space time structure in enormous detail -- requires only high school algebra at that stage.

Regards,
Reilly Atkinson

 

[geistkiesel]

geistkiesel -- I really can't figure out what's going on in your scenario. A diagram or two would be very helpful.

But I do suggest you go to any text that discusses basic SR space-time definitions and (idealized) measurements, take pencil and paper and work through, step-by-step, the arguments. Then you will be in a position to answer your own question. If not, you will have far more specific questions to ask about SR. Einstein's book Relativity is elegant and insightful, the more recent Basic Relativity by Richard Mould goes into the basic SR space time structure in enormous detail -- requires only high school algebra at that stage.

Regards,
Reilly Atkinson

Reilly, I truly am grateful for your suggestions but I am a confessed SR heretic, and I am not going to indulge myself in learning something that I am working to get rid of, capice? I do not buy inro the SR story and am especially motivated since Sir Grounded came upon the scene. We should all feel rather grateful that Sir Grounded selected this forum to have his views aired. I do see a lot of denial in respoding posts, do you agree Reilly? Rather than urge Sir Grounded along with suggestions and clues, it seems the SR bushwahackers have come put of theri hiding places and are attempting a full scale assault on the citidel. Echoes, Reilly, a chorus of echoes, some on harmony some off key, but all are echoes, of SR I see.

Here is the experimental arrangement, I bet it looks familiar, doesn't it Reilly? "relativity" pages 25-27 you mean? My favorite science book of all time. Check it out.
---> motion

A________________M______|_______|______B
                 M'(t0) M'(t1)  M'(t2)

M is the midpoint of photon emitters at A and B. M'(t0) is the observer location on the moving frame that coincides with M in the stationary frame just as the photons were emitted simultaneously at A and B.
Later the Oberver detects the B photon at M'(t1) and then the photon from A at M'(t2). This is the Einstein train experiment that has been passing through the threads lately. And t'' ithe signals from A and B at the midpoint of A and B (for a long train).

IF SR predicts that the photons were not emitted simultaneously in the moving frame describe the location of the A photon when the B photon was detected at M'(t1) in terms of t's and v, the velocity known by the observer wrt to the stationary frame.

Simple high school algebra Reilly, you shouldn't have any problem, especially as I have already done the math.

Thank you for your help.

 

[geistkiesel]

Grounded -- First, please accept my apologies for being snotty and arrogant(the physicist's disease) I misjudged your zeal and passion for understanding. (Note, however, as my dear Mother told me -- You learn more with your mouth shut than with it open)

Physics is very difficult, for two primary reasons: it requires a real facility with and understanding of a lot of advanced math; the conceptual basis is highly abstract and idealized, in E&M, QM, SR, GR, ..., and is not fully consistent. So, particularly, for the last reason, most physicists are highly pragmatic -- do the best you can with what you have, don't sweat the hard stuff unless it messes up your work. As I'm sure you know, for example, there is huge controversy about the proper interpretation of QM, but many working physicists pay little attention to the controversy. The standard ways work just fine in practice.

Everyone who teaches physics, say SR, will tell you we did not understand SR until we taught it. It takes a long time, and many exposures to SR to get it right. And, most students of SR will tell you that sometimes you have to suspend your judgement to get to a good understanding -- that's the advantage of a good teacher, he/she can suggest what to sweat now, and what to defer. All told, SR is a huge subject, of which the basic space-time kinematics, the x's and t's, comprise only a very small portion. It is the totality of SR that is so compelling.

That being said, the path to a robust understanding of SR necessarily involves lot's of math -- there's no way around that fact.

Tom's explanation of the imaginary exponential is right on. The way, typically, you build up a comfortable intuition about i, the square root of -1, is to work with it, work with it a lot. It's part of the language of physics, and is used so much because it is a powerful tool in working with waves, periodic phenomena, much of QM, and many other branches of physics and engineering.

If you are serious about physics, you need to hit the books, and do the homework. Resnick and Halliday is a good place to start. And, absolutely necessary, learn calculus. Then you will understand that the x and t in the exponential are not x2-x1, t2-t1. That exponential gives you the compex value of the wave amplitude at x at time t.

Once a professor, always a professor.
Regards and good luck

Reilly Atkinson

Reilly, I don't know if you have read all of the posts Grounded has made in this thread, but a cursory examination will show you that he making a most robust effort to teach you something, but your instincts seem to be to throw the dust of whatever subtle discouragement you have to the man. Condescending, superior intelligence, education and wit you know the attitue, don't you?. When you understand what he is saying, meaning you have to read it of course, you can understand his tenacity he has built into the model he his presenting.

Again, Reilly. he is teaching you something. Physics isn't so difficult, but useless phyiscs is very difficult, don't you agree?.

 

[geistkiesel]

Might I recommend a bit more exposition then? The quick two-liners simply do not convey the information you wish to convey.



Here's a perfect example.

On what basis do you say that something is wrong with an experiment, as opposed to something being wrong with a theory?

Tom, Grounded showed by example the necessity of including the observers relative velocity less a mistake be made in the measurement of the length of the cars (wave length of light). The SR system creates a mistake in measuremment and needed to create SR to explain the error. How many times can you echo "the vast amount of experimental results" describing your collective error? The experiments that prove time dilation, contraction of matter, loss of simultaneity?

How long can you keep it up before the echo wears thin? Tom, lifting the weight of SR will be a relief, a huge relief.

They're are errors Tom, great big huge errors.

 

[geistkiesel]

What about your own faith, Geistkiesel? It must be pretty strong for you to continue to have your unwavering belief in the incorrectness of Special Relativity.

I mean, if I had a strong conviction against learning something, I would resign myself to the fact that I don't know that thing.

But here you are, steadfastly refusing to learn Special Relativity, refusing to hear evidence confirming it, and you are able to stick to your conviction that it cannot possibly be anywhere close to valid!


Wait, I'm wrong, you're not so much sticking to your convictions; you never face any scrutiny because you denounce anyone who would scrutinize you as being in a "state of blind obedient sleep-walking darkness".

You still have your blind faith, but you don't have the confidence in your faith, so you need to belittle those who might erode your faith in order to maintain it.


You liken learning SR to putting a finger in a fire. Why? Putting your finger in a fire can cause physical pain and injury; are you suggesting that you will suffer mental pain and injury if you tried to learn SR? Are you really that afraid you might be wrong?

Here is where I question your sophistication. I said I woudn't put my finger in the fire just to see how hot it was. Did you really read that as a suggestion that I would suffer mental pain and injurty learning SR?. You know what, you are absolutely correct. But you misjudge faith and conviction and are unable to distuinguish them from sheer boredom. There are millions of subject matter in the world available to me for perusal. I reject most of those subjects from scrutiny. SR happens to be one of them. All theories are temporary, SR is falling to the floor.


Why do you steer the conversation around to faith and pain and conviction? I thought you were posing as an SR theorist, here I see petty propagandist. Luring me around by your trained and practiced metaphors.

"The enemies of truth. Convictions are more dangerous enemies of truth than lies"

 

[russ_watters]

Why do you steer the conversation around to faith and pain and conviction?

Simple: your position defies rational explanation. Indeed, you actively repel any attempt at rational discussion. You ignore evidence and you ignore math. These discussions are not scientific because you refuse to be scientific in your approach to the problem.

 

[Hurkyl]

Why do you steer the conversation around to faith and pain and conviction?

If you didn't want your opinions and beliefs questioned directly, then you shouldn't state them in a discussion forum.

I've yet to understand why people who state (or imply) such things feel they should be immune from scrutiny.

 

[wespe]

An interferometer can be calibrated using a light source with a known wavelength. As long as the distance between the observer and the source are not changing, this will work. Since light travels 186,000 miles per second, the wavelength equals the amount of distance traveled, traveling at 186,000 miles per second, in the amount of time it takes the light to complete one cycle. If the distance between the observer and the source are decreasing, then the wavelength equals the amount of distance traveled, traveling at the sum of “186,000 miles per second” added to “the speed at which the distance between the source and the observer is decreasing”, in the amount of time it takes the light to complete one cycle. If we measure the wavelength in this fashion, we will not measure a change.

Are you still there, Grounded?

I see no reason to for any calibration, but apart from that, I don't think an interferometer can be calibrated in such a way that it will always measure the "true" wavelength (the wavelength in the emitter's frame). You can't even calculate the "true" wavelength by just knowing the measured relative frequency (and the speed of light, which is constant), you also have to know the relative speed [between you and the light source]. You can calibrate [at a certain relative speed], but then it will not work [when the relative speed changes]. You must constantly "calibrate" it by feeding it with the value of relative speed, then what kind of calibration is that? What do you think?

editing to add:
Am I misunderstanding? You are saying that a "true" wavelength exists independent of the relative speed. Then you should be able to measure it without taking relative speed into account. Therefore your calibration must not involve relative speed. Right? Also, the "traveled distance" is the result of relative speed. Actually, according to yourself, you aren't going anywhere. It is the other object that travels. Without relative speed, there is no meaning to traveled distance.

 

[grounded]

Whether you are using Galilean relativity or SR, the equation is the same.

I can calculate the relative velocity between a light pulse and myself by measuring the following:

Event 1: Pulse Emitted
x₁=Location of emitter on x-axis at time t₁.
t₁=Time of emission.

Event 2: Pulse Detected
x₂=Location of detector on x-axis at time t₂.
t₂=Time of detection.

The speed of the pulse relative to me is then:

v=(x₂-x₁)/(t₂-t₁).

For light, this will always come out to be c. And as you can see, that result is not "built in" to the way we calculate relative speed. It is a simple, undeniable experimental fact.

Tom, you say it is not "built in", but consider the following;

Can you calculate the relative velocity between a moving car and yourself by measuring the following?

Event 1: Car Accelerated (Assume instantaneous acceleration to 60 Miles Per Hour)
x₁=Location of car on x-axis at time t₁.
t₁=Time of acceleration.

Event 2: Car Detected
x₂=Location of car on x-axis at time t₂.
t₂=Time of detection.

The speed of the car relative to you is then:

v=(x₂-x₁)/(t₂-t₁).

Can you alter the relative speed of the car while using this formula?
If not, then how is this formula the very definition of relative speed?

 

[grounded]

Are you still there, Grounded?

I see no reason to for any calibration, but apart from that, I don't think an interferometer can be calibrated in such a way that it will always measure the "true" wavelength (the wavelength in the emitter's frame). You can't even calculate the "true" wavelength by just knowing the measured relative frequency (and the speed of light, which is constant), you also have to know the relative speed [between you and the light source].

I agree...partly.

The relative speed IS the distance the observer travels towards (or away from) the source in one second, which IS the same thing as the change in distance between the observer and the source per second.

This is why I say we must include the distance the observer has traveled relative to the source in order to accurately measure the wavelength.

The speed of light is calculated to be constant because we use an incorrectly measured wavelength.

You can calibrate [at a certain relative speed], but then it will not work [when the relative speed changes]. You must constantly "calibrate" it by feeding it with the value of relative speed.

If the change in distance between the observer and the source is not at a constant velocity, then yes, it must be constantly calibrated.

 

[wespe]

This is why I say we must include the distance the observer has traveled relative to the source in order to accurately measure the wavelength.

The speed of light is calculated to be constant because we use an incorrectly measured wavelength.

OK, suppose I claim the following:

"There exists a true frequency and we are measuring it wrong. Because we must include relative speed [or, the distance the observer has traveled relative to the source] in order to accurately measure the frequency. We must calibrate our equipments constantly according to the relative speed."

Do you see? If you do such a correction on wavelength, you must also do it for frequency. And with both wavelength and frequency "corrected", you get constant c when you multiply them. That's what I meant by "mixing frames" in an earlier post.

 

[grounded]

Am I misunderstanding? You are saying that a "true" wavelength exists independent of the relative speed. Then you should be able to measure it without taking relative speed into account.

I am saying that if you account for the change in distance between the observer and the source, you can always measure the true wavelength.

From my original post:

Traveling towards the train will increase the number of boxcars that are passed and it will increase the relative speed between the observer and the train, but it will not change the length of the boxcars. If the observer plotted the number of boxcars that passed in one minute on a four-inch line, and then did the same thing after increasing speed towards the train, the second experiment would have more marks on the four-inch line and they would be closer together. This does not mean the length of the boxcars have gotten shorter, it means that the four-inch line represents a greater distance while traveling towards the source than it does when not moving relative to the source.

 

[jcsd]

I am saying that if you account for the change in distance between the observer and the source, you can always measure the true wavelength.

From my original post:

But the problem is the light is not atatched to the refernce frame of the source, what happens when the source is accelarting? what happens when the source is unknown?

 

[grounded]

OK, suppose I claim the following:

"There exists a true frequency and we are measuring it wrong. Because we must include relative speed [or, the distance the observer has traveled relative to the source] in order to accurately measure the frequency. We must calibrate our equipments constantly according to the relative speed."

Do you see? If you do such a correction on wavelength, you must also do it for frequency. And with both wavelength and frequency "corrected", you get constant c when you multiply them. That's what I meant by "mixing frames" in an earlier post.

Current measurments of frequency already include the distance the observer has traveled relative to the source. The change in distance between the source and the observer in one second, divided by the true wavelength, equals the change in frequency.

We include the distance the observer has traveled relative to the source when measuring frequency, but we do not include it when measuring wavelength, which is why the relative speed can never change.

The frequency wrt the observer equals the number of cycles that pass by the observer due to the velocity of the light, added to the number of cycles passed caused by the observers speed towards the source.

Like you said, what we do to one, we must do to the other.

 

[wespe]

Current measurments of frequency already include the distance the observer has traveled relative to the source. The change in distance between the source and the observer in one second, divided by the true wavelength, equals the change in frequency.

We include the distance the observer has traveled relative to the source when measuring frequency, but we do not include it when measuring wavelength, which is why the relative speed can never change.

The frequency wrt the observer equals the number of cycles that pass by the observer due to the velocity of the light, added to the number of cycles passed caused by the observers speed towards the source.

Like you said, what we do to one, we must do to the other.

OK

Let's say, according to light source: the frequency is 300,000 Hz and wavelength is 1 km.

Due to some relative speed..
The observer measures: frequency is 600,000 Hz, wavelength is 0,5 km
The observer calculates: relative speed = relative frequency x relative wavelength = 300,000 km/sec
The observer can also directly measure relative speed like I described in an earlier post. This measurement verifies the calculated relative speed: 300,000 km/sec

According yo you: true wavelength is 1 km, but true frequency is still 600,000 Hz
So observer must calculate: relative speed = 600,000 km/sec
This result does not match the directly measured relative speed. What do you say about that?

 

[grounded]

But the problem is the light is not atatched to the refernce frame of the source, what happens when the source is accelarting? what happens when the source is unknown?

To say the source is traveling towards the observer at a faster rate, is the same thing as saying the observer is traveling towards the source at a faster rate, assuming the observer was already traveling towards the source.

Like Tom has said, it is not the speed of the observer or the speed of the source that is important, it is the amount of change in distance between the source and the observer in one second that matters.

We can use a spectrometer and a spectrograph to analyze and discover what the source is made of and what its true wavelength is. The amount of shift in the spectrum will tell you the relative speed. This shift is caused by not changing the scale.

 

[jcsd]

To say the source is traveling towards the observer is the same thing as saying the observer is traveling towards the source at a faster rate, assuming the observer was already traveling towards the source.

Like Tom has said, it is not the speed of the observer or the speed of the source that is important, it is the amount of change in distance between the source and the observer in one second that matters.

We can use a spectrometer and a spectrograph to analyze and discover what the source is made of and what its true wavelength is. The amount of shift in the spectrum will tell you the relative speed. This shift is caused by not changing the scale.

No it is not because accelartion is npot relative in the same way asvelcotiy is relative you can't transform an accelarated frame into an inertail frame in special relatvity.

 

[grounded]

What do you say about that?

I say I am extremely interested in learning about a real world experiment such as you described, minus the spaceship. Remember, it must be done while in motion relative to the source. A link describing the details of the experiment would be appreciated.