# Confusion about signal latency vs true relativity

#### DocZaius

I have been trying to study special relativity by looking through various websites and internet resources. Here and there I saw some vague references to the fact that there should be a distinction made by the student between signal latency and actual relativity.

I had always assumed that signal latency was the CAUSE of the relativity, but apparently they are two distinctly different concepts. Or maybe not...

So let me ask this question using a thought experiment:

Let us say that John emits a pulse of light just as Bob passes by him at half the speed of light. Let us say that there are a number of receptors along Bob's line of movement to both sides of John. These receptors trigger off a signal when the beam of light hits them. Now let us say that the pulse itself does move at the speed of light, but that for some magical reason, both Bob and John see the receptors being triggered by the pulse hitting them instantly

Would Bob and John still have different interpretations about the simultaneity of which receptors are triggered? I would think yes, but I am not 100% sure.

Related Special and General Relativity News on Phys.org

#### russ_watters

Mentor
Could you explain what you mean by "but that for some magical reason, both Bob and John see the receptors being triggered by the pulse hitting them instantly"? It doesn't sound right and I'm a little uncomfortable wit the word "magical". In asking a question about relativity, the scenario should be set up to reflect how relativity says it should work.

#### JesseM

Let us say that John emits a pulse of light just as Bob passes by him at half the speed of light. Let us say that there are a number of receptors along Bob's line of movement to both sides of John. These receptors trigger off a signal when the beam of light hits them. Now let us say that the pulse itself does move at the speed of light, but that for some magical reason, both Bob and John see the receptors being triggered by the pulse hitting them instantly
Since there is no absolute simultaneity, the concept of seeing instantly is problematic--after all, seeing an event A instantly is equivalent to the statement "the event of my seeing A happens at the same time that A actually occurs", but of course different frames disagree about whether any two events happen at the same time or different times.

The problem of signal latency can be dispensed with by having a set of clocks in different regions of space, which are synchronized in their own rest frame; then to assign an event a time-coordinate, I just look at what the reading was on a clock right next to the event at the moment it happened. This is the original suggestion given by Einstein in his 1905 paper. But then the problem is how to "synchronize" the clocks--Einstein proposed that each observer synchronize his own clocks using the assumption that light moves at the same speed in all directions, so if I set off a flash at the midpoint of two of my clocks, then I define them to be "synchronized" if they both show the same time at the moment the light from the flash hits them. But a natural consequence of this is that different observers will define simultaneity differently, so each one thinks the other one's clocks are out-of-sync; for example, if I synchronize clocks at the front and back of my ship using a flash set off at the middle, then if the ship is moving forward in your frame, in your frame the front clock is moving away from the point the flash was set off while the back clock is moving toward it, so if you assume the light from the flash moves the same speed in both directions in your frame, naturally you must conclude the light hit the back clock before it hit the front clock.

#### DocZaius

Could you explain what you mean by "but that for some magical reason, both Bob and John see the receptors being triggered by the pulse hitting them instantly"? It doesn't sound right and I'm a little uncomfortable wit the word "magical". In asking a question about relativity, the scenario should be set up to reflect how relativity says it should work.
What I am trying to do when saying "magically" is to dispense with one aspect of the problem which complicates things. In other words, it is my current assumption that not only is there a different interpretation of simultaneity between Bob and John regarding which receptors are signalled off, but there is an added degree of complication arising from the time it takes for those receptors to relay their signal to each observer.

I am trying to take out the "added" degree of complication from the problem, knowing full well that FTL signaling is impossible, but only to see if apart from signal latency there remains a simultaneity disagreement.

#### Doc Al

Mentor
...but only to see if apart from signal latency there remains a simultaneity disagreement.
All special relativistic effects--such as time dilation, length contraction, simultaneity disagreement--are effects that appear after you've already corrected raw observations for light travel time (what you are calling signal latency, I believe) as needed.

#### DocZaius

All special relativistic effects--such as time dilation, length contraction, simultaneity disagreement--are effects that appear after you've already corrected raw observations for light travel time (what you are calling signal latency, I believe) as needed.
I really wish more online resources would make that clear. The distinction can be confusing for students of SR.

Thanks!

#### A.T.

Signal latency can create a lot of weird optical effects:
http://www.spacetimetravel.org/bewegung/bewegung1.html
But they are just optical effects, which can be corrected, so it is more a technical detail of the measurement procedure. After correcting them you see what happens simultaneously in your frame, and you have only relativistic effects.

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#### pam

I have been trying to study special relativity by looking through various websites and internet resources.
That is the source of your difficulties.

#### DocZaius

That is the source of your difficulties.
What book would you recommend? I'd like to have problems to solve, like in textbooks. Is there a standard SR textbook that's deemed adequate?

#### JesseM

What book would you recommend? I'd like to have problems to solve, like in textbooks. Is there a standard SR textbook that's deemed adequate?
Spacetime Physics by Edwin Taylor and John Wheeler is good, as is Special Relativity by A.P. French.

#### Doc Al

Mentor
I recommend N. David Mermin's https://www.amazon.com/dp/0691122016/?tag=pfamazon01-20. But you'll have to supplement it with other works (such as those mentioned) if you want problems to solve. Nonetheless, Mermin is a master at explaining things. (My copy--autographed!--of his earlier pedagogical work on SR is falling apart, since I have referred to it so often.)

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#### yuiop

I hope this is not too off topic. What be a good book to take the next step, from an understanding of Special Relativity to an understanding of the tensors of General Relativity?

"Confusion about signal latency vs true relativity"

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