# Why does light not reach any distance instantly?

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.

Related Special and General Relativity News on Phys.org
PeterDonis
Mentor
2019 Award
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.

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

ghwellsjr
Gold Member
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?

Last edited:
berkeman
Mentor
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).

1 person
ghwellsjr
Gold Member
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.

ghwellsjr
Gold Member
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.

Last edited:
berkeman
Mentor
"How long" is just the horizontal axis on the oscilloscope.

ghwellsjr
Gold Member
"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
Mentor
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
Gold Member
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?

ghwellsjr
Gold Member
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?
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
Mentor
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
Gold Member
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?

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 ;)

Last edited:
DrGreg
Gold Member
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.

berkeman
Mentor
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
Gold Member
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 ;)

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
Gold Member
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.

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

But, you get what I mean!

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