Why does light not reach any distance instantly?

[chillpenguin]

After learning about special relativity, and the behavior of light, I do not understand why light is not able to reach any distance instantly. Doesn't time stop at the speed of light? So if no time is passing, yet light is moving, wouldn't that mean that light is traveling infinitely fast? Obviously light doesn't travel that fast, but I can't see what stops it from going that fast if it is able to move without the passing of time.

 

[PeterDonis]

Doesn't time stop at the speed of light?

No. A better way of describing what SR says about lightlike objects (things that move at the speed of light) is that the concept of "time" (more precisely, "proper time") does not apply to them. That is, the question "how much time passes for a light ray?" is not well-defined.

 

[chillpenguin]

Oh I see... The more you learn about physics, the more you realize you don't understand the half of it!

 

[ghwellsjr]

After learning about special relativity, and the behavior of light, I do not understand why light is not able to reach any distance instantly.

How would you know if it did or it didn't? Have you ever thought about how you would determine how long it took for light to get somewhere? You can't watch it like you can watch a baseball being thrown, or even with the aid of high-speed photography, you can't capture the motion of light, can you? For other things, we illuminate them with light so we can tell where they are, but what are you going to do with light? Have you considered that as light travels from the sun to the moon, you can't see it? Even the light that is reflected off the moon coming back to the Earth you can't see while it is in transit. All you can see is when it finally gets to your eyes. And that's the problem. We cannot determine how fast light is traveling but we can measure how long it takes to make a round trip from your laser to a remote target and back to you. In fact, that's how a laser rangefinder works that you can buy at a hardware store; it measures how long it takes for the light to make a round trip between you and a target and then using the defined value for the speed of light, it can calculate the distance. So what we do in Special Relativity, is we say that the light took the same amount of time to get to the target as it did to get back. That's Einstein's second postulate.

Doesn't time stop at the speed of light?

No, this is a common misconception. Einstein said that time is what a clock measures and since a clock has to be made out of some material substances and since material substances cannot travel at the speed of light, we cannot talk about time slowing down for light like we can for material objects.

So if no time is passing, yet light is moving, wouldn't that mean that light is traveling infinitely fast? Obviously light doesn't travel that fast, but I can't see what stops it from going that fast if it is able to move without the passing of time.

Well, since your assumptions are wrong, then there is no quandary for you, is there?

 

[berkeman]

How would you know if it did or it didn't? Have you ever thought about how you would determine how long it took for light to get somewhere? You can't watch it like you can watch a baseball being thrown, or even with the aid of high-speed photography, you can't capture the motion of light, can you?

Of course we can. The velocity of a foot per nanosecond is not hard to measure in the lab...

 

[WannabeNewton]

Of course we can. The velocity of a foot per nanosecond is not hard to measure in the lab...

There's a difference between measuring the one-way speed of light and measuring the two-way speed of light. The latter only requires one clock so there's no issue but the former requires at least two synchronized distant clocks; in other words you need a notion of distant simultaneity. But how do you go about synchronizing the two clocks in the first place? If you use light signals to synchronize the clocks, as is usually done, then you need to know something about the one-way speed of light so we're back to square one. This is why in Einstein's original paper he established by definition that the one-way speed of light is isotropic in inertial frames. Using this we can then synchronize distant clocks. So it's important to distinguish between the two-way speed of light (which can be measured using a single clock) and the one-way speed of light (which requires a synchronization convention such as Einstein's).

 

[ghwellsjr]

How would you know if it did or it didn't? Have you ever thought about how you would determine how long it took for light to get somewhere? You can't watch it like you can watch a baseball being thrown, or even with the aid of high-speed photography, you can't capture the motion of light, can you?

Of course we can. The velocity of a foot per nanosecond is not hard to measure in the lab...

I don't know how, please tell me.

 

[berkeman]

I don't know how, please tell me.

You use techniques similar to optical TDRs: http://en.wikipedia.org/wiki/Optical_time-domain_reflectometer

 

[ghwellsjr]

I don't know how, please tell me.

You use techniques similar to optical TDRs: http://en.wikipedia.org/wiki/Optical_time-domain_reflectometer

I read the article. I saw where the OTDR can measure fiber length, similar to what I said about a laser rangefinder, but I didn't see any claim or mention that it can measure how long it takes for a signal to get from one end of the fiber optic cable to the other.

So I still don't know how. Can you please point me to the place in the article where I misunderstood or overlooked or whatever your understanding is that I don't have.

 

[berkeman]

"How long" is just the horizontal axis on the oscilloscope.

 

[ghwellsjr]

"How long" is just the horizontal axis on the oscilloscope.

The article says the horizontal axis on the oscilloscope is length, not time:

The strength of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length.

 

[berkeman]

The article says the horizontal axis on the oscilloscope is length, not time:

That is for the cable tester. Length is time -- about a foot per nanosecond in free space, less in a dielectric like a cable. It is definitely a "time of flight" measurement, which is what you were asking about. It can be done with multiple photodetectors along the path of flight of a short laser pulse in air as well.

 

[ghwellsjr]

The article says the horizontal axis on the oscilloscope is length, not time:

That is for the cable tester. Length is time -- about a foot per nanosecond in free space, less in a dielectric like a cable. It is definitely a "time of flight" measurement, which is what you were asking about.

Are you saying that if the clock in the instrument measures 10 nanoseconds from the time the pulse was emitted to the time when it was detected, then the length is about 10 feet?

It can be done with multiple photodetectors along the path of flight of a short laser pulse in air as well.

Can we deal with this more complicated case later?

 

[AlephZero]

How would you know if it did or it didn't? Have you ever thought about how you would determine how long it took for light to get somewhere?

You can do it with a "simple" mechanical device, to within a few percent:
http://en.wikipedia.org/wiki/Fizeau–Foucault_apparatus

even with the aid of high-speed photography, you can't capture the motion of light, can you?

Yes you can. http://web.media.mit.edu/~raskar/trillionfps/

 

[ghwellsjr]

How would you know if it did or it didn't? Have you ever thought about how you would determine how long it took for light to get somewhere?

You can do it with a "simple" mechanical device, to within a few percent:
http://en.wikipedia.org/wiki/Fizeau–Foucault_apparatus

That's measuring how long it takes for light to get there and back. Do you have a way to measure how long it takes for the light to get there?

even with the aid of high-speed photography, you can't capture the motion of light, can you?

Yes you can. http://web.media.mit.edu/~raskar/trillionfps/

You are again seeing light that has gone from a source to different places on the objects and back to the camera. This does not measure how long it takes for the light to get just from the source to the objects, does it?

 

[berkeman]

Are you saying that if the clock in the instrument measures 10 nanoseconds from the time the pulse was emitted to the time when it was detected, then the length is about 10 feet?

Yes. Are you really not understanding what Aleph and I are saying? The only practical issue for electrical detection is to use a fast laser pulse as the source, and have high bandwidth detectors that can see nanosecond light pulses. The rest is pretty trivial.

 

[ghwellsjr]

Are you saying that if the clock in the instrument measures 10 nanoseconds from the time the pulse was emitted to the time when it was detected, then the length is about 10 feet?

Yes.

Are you sure? I would have thought that if the OTDR measured 10 nsecs, it would have reported 5 feet for the length of the fiber. Please help me understand your answer.

Are you really not understanding what Aleph and I are saying?

I really don't understand how what you are saying is related to measuring how long it takes for light to reach a distance, which is the subject of this thread.

The only practical issue for electrical detection is to use a fast laser pulse as the source, and have high bandwidth detectors that can see nanosecond light pulses. The rest is pretty trivial.

Can we please finish with your first answer regarding the OTDR before going on to other issues?

 

[m4r35n357]

you can't capture the motion of light, can you?

Not to diminish your point in the slightest, but there was an example of just this on UK TV recently (using a state of the art high speed video camera), I think Brian Cox introduced it in one of his pop science programmes but I haven't managed to find it online yet. If anyone can provide a link I'd be interested in seeing it again ;)

EDIT: here's the link
http://www.ted.com/talks/ramesh_raskar_a_camera_that_takes_one_trillion_frames_per_second.html

 

[DrGreg]

To ghwellsjr and berkeman

It seems that neither of you is understanding what the other is saying.

I think ghwellsjr's point is that you can't directly measure the 1-way speed of light; all you can do is measure the 2-way speed. The 1-way speed is set by convention rather than by experiment.

I think berkeman is using the convention that the 1-way and 2-way speeds are equal to each other so there's no need to make any distinction between them. I think all the methods proposed are 2-way measurements.

Have I summarised each of your postions correctly, and does that help you understand each other?

 

[berkeman]

Are you sure? I would have thought that if the OTDR measured 10 nsecs, it would have reported 5 feet for the length of the fiber. Please help me understand your answer.

Yes, 10 feet in free space, and more like 5 feet in fiber.

I really don't understand how what you are saying is related to measuring how long it takes for light to reach a distance, which is the subject of this thread.

If you space out high-speed photodetectors (with beam splitters) and use equal length coax cables to get the detected signals to a high-speed oscilloscope, you can see the optical pulse as it hits each photodetector, with the pulses spaced out in time. That's the way I read the OP's question.

Can we please finish with your first answer regarding the OTDR before going on to other issues?

 

[ghwellsjr]

Not to diminish your point in the slightest, but there was an example of just this on UK TV recently (using a state of the art high speed video camera), I think Brian Cox introduced it in one of his pop science programmes but I haven't managed to find it online yet. If anyone can provide a link I'd be interested in seeing it again ;)

EDIT: here's the link
http://www.ted.com/talks/ramesh_raskar_a_camera_that_takes_one_trillion_frames_per_second.html

This was already asked in post #14 and answered in post #15.

 

[stevmg]

How about this? To the traveler near c, time is going infinitely slowly. To the stationary observer, time proceeds normally. i.e., one can traverse the "entire" universe consuming virtually no time on board the spacecraft while to the observer at one "fixed" point, time goes normally.

P.S., there is NO central or fixed point in the universe.

 

[ghwellsjr]

How about this? To the traveler near c, time is going infinitely slowly.

No, not infinitely slowly. It's finitely slowly according to the gamma function.

To the stationary observer, time proceeds normally.

And to the traveler near c, time proceeds normally. And it's the "stationary" observer whose time is going slowly. More precisely, we should be talking about the rest frame of the "stationary" observer and the rest frame of the traveler because neither one can observe the Time Dilation of the other one.

i.e., one can traverse the "entire" universe consuming virtually no time on board the spacecraft while to the observer at one "fixed" point, time goes normally.

Not "no time" but as small a time as you want. Again, pay attention to the frames.

P.S., there is NO central or fixed point in the universe.

True.

However, this discussion is off topic. This thread is about how long it takes for light to traverse a distance, not a spacecraft .

 

[stevmg]

Correction! One doesn't experience "time dilation" and time doesn't go infinitely slowly.

But, you get what I mean!

 

[stevmg]

All I know about the gamma function is it's relation to factorials.

 

[ghwellsjr]

Are you sure? I would have thought that if the OTDR measured 10 nsecs, it would have reported 5 feet for the length of the fiber. Please help me understand your answer.

Yes, 10 feet in free space, and more like 5 feet in fiber.

Now this is what I don't understand. We're talking about the OTDR that you said could measure how long it takes light to travel a certain distance. The OTDR has only one detector located at the emitter. It can only measure the round trip time it takes for light to go down a fiber optic cable and return. Even if we were talking about free space, the distance would be 5 feet, not 10 feet, correct? For a fiber optic cable it would be something less than 5 feet but in either case, the OTDR does not report anything according to time, it reports information according to length along the fiber optic cable, doesn't it? That's what the article you pointed to says.

I really don't understand how what you are saying is related to measuring how long it takes for light to reach a distance, which is the subject of this thread.

If you space out high-speed photodetectors (with beam splitters) and use equal length coax cables to get the detected signals to a high-speed oscilloscope, you can see the optical pulse as it hits each photodetector, with the pulses spaced out in time. That's the way I read the OP's question.

Can we please finish with your first answer regarding the OTDR before going on to other issues?

I guess we are just about finished with the OTDR.

We'll go on to other schemes. Let's assume that instead of coax cables with dielectrics which add the complication of speeds less than that of light, let's use waveguides that don't have that issue, OK? And let's stipulate that the measured value of the speed of light is one foot per nanosecond, OK?

Since you haven't provided any specifics, I'm going to propose a scenario that I think is in agreement with your proposal. Let me know if it isn't.

We start with a photo source that we can switch on at will. We pass the light through a beam splitter that includes a fast photodetector connected by a five-foot straight waveguide to one input of our high-speed oscilloscope. The other beam continues on along a straight path parallel to the waveguide and on to another high-speed photodetector, ten feet away from the beam splitter. This photodetector is connected to a second straight waveguide, also five feet long going back to the other input of the oscilloscope.

Now we switch on the light and we measure a difference in the arrival times of the two pulses of 10 nanoseconds. But have we measured how long it took for the light to get from one photodetector to the other?

I think not. As the light propagates from the beam splitter and the electrical signal propagates in parallel, they travel in tandem for a distance of 5 feet. Then the waveguide signal goes into the oscilloscope while the light continues parallel to the other waveguide for the remaining 5-foot distance to the second photodetector and then the electrical signal travels back on that second waveguide to the oscilloscope. So all we have measured is the roundtrip time for the light to traverse 5 feet in one direction and the electrical signal to traverse 5 feet back the same distance in the opposite direction and we're calling it the one-way time for the light to traverse the 10 foot distance.

I think it is easy to note that we could have made that first straight waveguide any length we wanted and moved the beam splitter an appropriate distance and we would still get a measurement of 10 nanoseconds. At this stage, I think we can see a similarity to the OTDR example if we just eliminate that first waveguide and put the beam splitter right at the input to the oscilloscope with the first photodetector connected directly into the oscilloscope, don't you concur?

All we have done by this scheme is say that the time it takes for the signal to propagate down the waveguide in one direction is identical to the time for the signal to propagate up in the other direction but we haven't measured it to make sure that the statement is true. In fact there is no way to know the answer to this question. That's why Einstein said that we are free to stipulate that the two times are equal, thus creating the concept of relative time.

Lest anyone think I'm nitpicking, let me remind you that in one reference frame, where we define light to propagate at c in all directions, we do not claim that the signal takes the same amount of time to propagate in both directions in a moving waveguide.

 

[ghwellsjr]

All I know about the gamma function is it's relation to factorials.

The gamma function that I was talking about is:

γ = 1/√(1-β²)

where β is the speed as a fraction of the speed of light.

 

[stevmg]

Start over again -

Emitted light travels from point A to point B (no matter how far) relative to its own frame instantly - zero time.

To observer in another frame, the time consumed is in its own frame relative to the frame that the light is in.

Nowhere in space is a physical frame moving "at the speed of light". Every where else, other than on the light beam, any physical frame is moving at some finite speed below c. Hence, from the aspect of any other frame in the in the universe there will be consumed time from its travels from point A to point B. What is that equation 1/[sqrt(1 - v^2/c^2)]?

Hell, I don't even understand the gamma function, only as it relates to factorials, and even then I don't understand it. Someone try to put the gamma function into something meaningful for me?

Oops! Sorry George. Didn't see your post. Your gamma function is the same as the Lorentz equation I mentioned above.

I am 71 years old, a retired physician (MD) and my expertise with the nuances of relativity and Lorentz is between slim and none, but I find the Lorentz equations fascinating.

 

[ghwellsjr]

Start over again -

Emitted light travels from point A to point B (no matter how far) relative to its own frame instantly - zero time.

That doesn't make any sense. In Special Relativity, the speed of light is defined to be c in any Inertial Reference Frame (IRF). That's Einstein's second postulate. When we use the expression "its own frame", we mean the IRF in which it is at rest. How can there be an IRF in which light can both be at rest and traveling at c? It's a contradiction of well-accepted terms.

To observer in another frame, the time consumed is in its own frame relative to the frame that the light is in.

In all IRF's, light is defined to travel in all directions at c. So if we know the distance, we know the time according to SR. It's real simple. Just definitions.

Nowhere in space is a physical frame moving "at the speed of light".

Let's be clear, frames are not physical, they're man-made constructs. But, yes, no IRF moves at c relative to any other IRF.

Every where else, other than on the light beam, any physical frame is moving at some finite speed below c. Hence, from the aspect of any other frame in the in the universe there will be consumed time from its travels from point A to point B. What is that equation 1/[sqrt(1 - v^2/c^2)]?

Hell, I don't even understand the gamma function, only as it relates to factorials, and even then I don't understand it. Someone try to put the gamma function into something meaningful for me?

Oops! Sorry George. Didn't see your post. Your gamma function is the same as the Lorentz equation I mentioned above.

I am 71 years old, a retired physician (MD) and my expertise with the nuances of relativity and Lorentz is between slim and none, but I find the Lorentz equations fascinating.

What you should do is devise a scenario using the coordinates of an IRF. Then use the Lorentz Transformation process (which includes gamma) to transform to the coordinates of another IRF moving at some speed (less than c) relative to your defining IRF. That's the quickest, easiest, and least confusing way to learn the important aspects of SR.

 

[berkeman]

Now this is what I don't understand. We're talking about the OTDR that you said could measure how long it takes light to travel a certain distance. The OTDR has only one detector located at the emitter. It can only measure the round trip time it takes for light to go down a fiber optic cable and return. Even if we were talking about free space, the distance would be 5 feet, not 10 feet, correct? For a fiber optic cable it would be something less than 5 feet but in either case, the OTDR does not report anything according to time, it reports information according to length along the fiber optic cable, doesn't it? That's what the article you pointed to says.

Sorry if I confused the issue by mentioning the OTDR. I was mainly trying to reference the kind of emitter and detectors that would be used to maesure the time of flight (and hence the distance) of light.

 

[stevmg]

OK -

Light always moves at c = 300,000 km/sec in a total vacuum devoid of mass or other energy in the same space. It always moves at c no matter what the IFR is. Light doesn't have "it's own IFR."

So, light does move instantaneously with respect to itself from any point A to a second point B. It is in any other IFR that time goes by depending on that IFR in relation to an IFR containing point A to point B.

This doesn't seem hard to understand.

Where is a reference about the gamma function and 1/[sqrt(1 - v^2/c^2)]?

I know about the gamma function from calculus and probability (the Student-t distribution for small samples) but beyond that I haven't a clue.

 

[stevmg]

γ function

γ(n) = (n - 1)!

 

[ghwellsjr]

OK -

Light always moves at c = 300,000 km/sec in a total vacuum devoid of mass or other energy in the same space. It always moves at c no matter what the IFR is. Light doesn't have "it's own IFR."

So, light does move instantaneously with respect to itself from any point A to a second point B.

Light doesn't have a "respect to itself". It's a meaningless term. Terms need definitions. There is no definition that you can come up with that would give meaning to the term. You need to stop trying to assign any concept of time with respect to light.

It is in any other IFR that time goes by depending on that IFR in relation to an IFR containing point A to point B.

This doesn't seem hard to understand.

It's not hard--it's impossible for me to understand what you are talking about. You should think about just one IRF at a time. It has coordinates and we assign coordinates to all events. Then we transform to another IRF moving with respect to the first one and we get a new set of coordinates for all the events. Time has meaning in each IRF, both the Coordinate Time and the Proper Time of material objects or clocks moving in that IRF. But there is no Proper Time for the light signals. If we have two events in one IRF that are separated so that light travels from one to the other at c according to that IRF, then when we transform to another IRF, even though the Coordinate Times and Distances are different, the light will still travel at c between the two events but we don't apply Proper Time along the path of the light. That only works for material objects that are traveling at less than c. In this case, the Proper Time will be the same between any two events even though the Coordinate Times and Distances are different.

Where is a reference about the gamma function and 1/[sqrt(1 - v^2/c^2)]?

I know about the gamma function from calculus and probability (the Student-t distribution for small samples) but beyond that I haven't a clue.

γ(n) = (n - 1)!

I'm sorry, I shouldn't have called it the gamma function. I should have called it the Lorentz Factor which is assigned the greek letter gamma in relativity. You can read about it here just to the left of the first diagram.

 

[pervect]

OK -

Light always moves at c = 300,000 km/sec in a total vacuum devoid of mass or other energy in the same space. It always moves at c no matter what the IFR is. Light doesn't have "it's own IFR."

This is good, and correct so far.

So, light does move instantaneously with respect to itself from any point A to a second point B. It is in any other IFR that time goes by depending on that IFR in relation to an IFR containing point A to point B.

This doesn't seem hard to understand.

If you are trying to say that light moves along a null worldline, so that the Lorentz interval between any two events on the worldline of a light beam is zero, you'd be correct. You might even add that the Lorentz interval is zero in any reference frame (though this would be redundant, because the Lorentz interval is independent of reference frame, so if it's zero in one, it's zero in all).

What you actually said may seem clear to yourself, but it's not going to be generally understood :-(. I was going to suggest the above as being what you were trying to say, but I'm not really sure it is!

The problem is that "moves with respect to itself" is problematic. In order for something to "move", it must experience time. But the whole point is that light can't be said to "experience time". Your phrasing implicitly assumes it can, as nearly as I can tell. It also seems to assume that there is such a thing as a "reference frame" for light (if I'm understanding it correctly). This is widely known to be false, if the usual definition of reference frame is used. There's a FAQ article on this that shouldn't be too hard to find.

As a practical matter, people have different understandings of words and what they mean. The problem is particularly acute in technical fields, technical "jargon" has a very precise meaning. A general procedure for making sure communication is happening is to try to find a phrasing that's acceptable to everyone, so that one is sure that one communicates what was intended.

If I had a penny for every time I completely misunderstood something written in popular language, and answered a completely different question than what they were intended to ask, I'd be rich.

 

[stevmg]

Thus, by what pervect states, is that light is independent of IFRs. Thus, from any point A in the universe to a second point B, no matter what IFR is chosen, there will be a time difference between emission and reception.

It would also seem that if light speed, c, is independent of IFRs, then the time difference will always be (distance between A and B))/c no matter what IFR is chosen.

Am I on the correct path here? You have so clearly illustrated the concept that c is always c (in a total vacuum) no matter what IFR is chosen and there is no standard IFR for light, just as the universe has no built in clock and no center.

Pervect, or whoever, please address the issue of SR when light travels through a medium, energy, or subject to gravity which "slows light speed down". Where do SR or even GR rules fit?

 

[ghwellsjr]

OK -

Light always moves at c = 300,000 km/sec in a total vacuum devoid of mass or other energy in the same space. It always moves at c no matter what the IFR is. Light doesn't have "it's own IFR."

This is good, and correct so far.

You agree that light moves.

So, light does move instantaneously with respect to itself from any point A to a second point B. It is in any other IFR that time goes by depending on that IFR in relation to an IFR containing point A to point B.

This doesn't seem hard to understand.

If you are trying to say that light moves along a null worldline, so that the Lorentz interval between any two events on the worldline of a light beam is zero, you'd be correct. You might even add that the Lorentz interval is zero in any reference frame (though this would be redundant, because the Lorentz interval is independent of reference frame, so if it's zero in one, it's zero in all).

Again, you agree that light moves.

What you actually said may seem clear to yourself, but it's not going to be generally understood :-(. I was going to suggest the above as being what you were trying to say, but I'm not really sure it is!

The problem is that "moves with respect to itself" is problematic. In order for something to "move", it must experience time. But the whole point is that light can't be said to "experience time".

Now it seems to me, following your line of reasoning, you are saying that light can't move. But since you twice agreed it does move, I must be misunderstanding what you are saying. Can you please clarify?

Your phrasing implicitly assumes it can, as nearly as I can tell. It also seems to assume that there is such a thing as a "reference frame" for light (if I'm understanding it correctly). This is widely known to be false, if the usual definition of reference frame is used. There's a FAQ article on this that shouldn't be too hard to find.

As a practical matter, people have different understandings of words and what they mean. The problem is particularly acute in technical fields, technical "jargon" has a very precise meaning. A general procedure for making sure communication is happening is to try to find a phrasing that's acceptable to everyone, so that one is sure that one communicates what was intended.

If I had a penny for every time I completely misunderstood something written in popular language, and answered a completely different question than what they were intended to ask, I'd be rich.

It would appear that somebody deserves another penny here, just not sure who.

 

[ghwellsjr]

Thus, by what pervect states, is that light is independent of IFRs. Thus, from any point A in the universe to a second point B, no matter what IFR is chosen, there will be a time difference between emission and reception.

It would also seem that if light speed, c, is independent of IFRs, then the time difference will always be (distance between A and B))/c no matter what IFR is chosen.

Am I on the correct path here?

No, you've got the cart before the horse. An IRF is defined or created or fabricated or established by using the definition or postulate or stipulation that light propagates at c in all of them. This then establishes what time and distance mean. This is why time and distance are relative to the IRF chosen. It's not the other way around.

You have so clearly illustrated the concept that c is always c (in a total vacuum) no matter what IFR is chosen and there is no standard IFR for light, just as the universe has no built in clock and no center.

Pervect, or whoever, please address the issue of SR when light travels through a medium, energy, or subject to gravity which "slows light speed down". Where do SR or even GR rules fit?

That's a different subject outside the scope of this thread and should be brought up in another thread.

 

[stevmg]

You say "tomato", I say "tomahto", you say "potato", I say "potahto", "potato, potahto, tomato, tomahto, let's call the whole thing off."

Not necessary to call the whole thing off but agree on zero time for light internally. Also agree on no matter what IFR chosen, time from A to B is (distance)/c. But distance AB will vary by what IFR chosen. That's the old flashing light in the front and back of a moving train paradigm.

I will take that penny, thank you.

 

[ghwellsjr]

You say "tomato", I say "tomahto", you say "potato", I say "potahto", "potato, potahto, tomato, tomahto, let's call the whole thing off."

Not necessary to call the whole thing off but agree on zero time for light internally. Also agree on no matter what IFR chosen, time from A to B is (distance)/c.

I will take that penny, thank you.

Again, it's not zero time for light internally. The concept of time doesn't apply to light internally. It's not like the number of pennies in your piggy bank when it is empty, it's like there's no piggy bank. So please don't continue to say that time for light is zero or instantaneous or anything equivalent.

Of course, after you define an IRF by using the definition that light propagates at c in all of them, then you can also say that the distance that light travels between A and B is time multiplied by c.

Are you aware that we define distance by how far light travels in a given time?

 

[stevmg]

-ghwellsjr

Thanks for the clarification. I am learning the jargon word-by-word. Yes, I am now quite aware that in space-time, distance is measured by c*t (such as a light-year). Hence, because there is no time in a light beam, no distance is appreciable. I appreciate that in a light beam, one cannot talk about zero time. I got it.

I LOVE learning this stuff but am at a very primitive point.

Still have to find out the meaning of the gamma and beta functions, though they are not germane to this forum.

 

[pervect]

Thus, by what pervect states, is that light is independent of IFRs. Thus, from any point A in the universe to a second point B, no matter what IFR is chosen, there will be a time difference between emission and reception.

It would also seem that if light speed, c, is independent of IFRs, then the time difference will always be (distance between A and B))/c no matter what IFR is chosen.

Yes. Note that the distance between A and B will depend on which IFR you measure it in.

Pervect, or whoever, please address the issue of SR when light travels through a medium, energy, or subject to gravity which "slows light speed down". Where do SR or even GR rules fit?

As far as SR goes (this means no gravity):

The "c" in the SR formula s basically means the speed of light in a vacuum, though sometimes people skip over this part for whatever reason (my opinion is that it's just too long to keep repeating).

The speed of electromagnetic radiation, including light, will depend on the medium it travels through, and the motion of the medium. See for instance the Fizeau experiment, http://en.wikipedia.org/w/index.php?title=Fizeau_experiment&oldid=578154337 for experiments of the speed of light in moving water.

When gravity enters the picture one has to deal with the effects that are called "gravitational time dilation" due to general relativity.

In that case, it becomes necessary to decide which clock to use to measure the speed of light, since all clocks do not run at the same rate due to the effects of gravitational time dilation.

If you set up a local frame of reference by using a local clock and a local meter stick, in the small region of space where the speed of the light is to be measured, you will find that the speed of light in a vacuum does not change, and is always "c".

If you set up a global frame of reference, usually using some centralized "master clock", you will find that the speed of light in a vacuum, measured in this manner, is not always equal to "c".

For instance on the Earth, the usual global frame of reference is TAI time, (international atomic time), a coordinate time standard.

http://en.wikipedia.org/w/index.php?title=International_Atomic_Time&oldid=581789205

Actual physical clocks need to be adjusted to keep TAI time.

In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation, and the combined TAI scale therefore corresponded to an average of the altitudes of the various clocks. Starting from Julian Date 2443144.5 (1 January 1977 00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at mean sea level (the geoid).

Thus, if you measure the speed of light using the time scale defined by the TAI standard (which requires adjusting the readings of clocks depending on their altitude), you'll find that the speed of light in a vacuum is not c. If you don't adjust the clocks for TAI time, and use a clock at the same height above sea level as your light beam is to measure the speed of light in a vacuum, you'll find that the speed of light in a vacuum is always "c".

 

[stevmg]

DQ = Dumb Question:

As light travels through total vacuum space, does the presence of any other electromagnetic energy have a GR time dilating effect? (Assume that the mass equivalent of the "other electromagnetic energy" would be E/c^2)

Dumb Question, no?