Speed of Light: Measuring Meters and Seconds

[Gadhav]

TL;DR Summary

SR theory

When we say speed of light = c m/s. Who is measuring meters and second. From SR perspective, that photon is moving towards you at speed of light so there should be time dilation, correct?

 

[Ibix]

When we say speed of light = c m/s. Who is measuring meters and second.

Anybody, so long as their clocks and rulers are moving at the same constant velocity.

From SR perspective, that photon is moving towards you at speed of light so there should be time dilation, correct?

Why would what the light pulse is doing affect your clocks and rulers? Also, the time dilation equations cannot be applied to things moving at the speed of light - it turns out to be self-contradictory to try to do so.

 

[Gadhav]

Anybody, so long as their clocks and rulers are moving at the same constant velocity.

Why would what the light pulse is doing affect your clocks and rulers? Also, the time dilation equations cannot be applied to things moving at the speed of light - it turns out to be self-contradictory to try to do so.

Why not. If we assume that photon is a train traveling towards you or away from you, the time from that should appear to dilate and length contract from my reference frame, right? Whether it is moving at speed of light or not can be moot. Laws of physics if give you results at 99.999999% speed of light ,that should be valid at c too. So I am still confused on who is measuring the time and distance to measure speed of photon.

 

[PeroK]

If you know this.

Laws of physics if give you results at 99.999999% speed of light ,that should be valid at c too.

Then, why can't you answer your own question?

So I am still confused on who is measuring the time and distance to measure speed of photon.

How can you be so sure about one thing when you are confused about the another?

 

[Janus]

Summary:: SR theory

When we say speed of light = c m/s. Who is measuring meters and second. From SR perspective, that photon is moving towards you at speed of light so there should be time dilation, correct?

Time dilation has no effect on the speed we measure for objects moving relative to ourselves. It would effect how fast we would determine that a clock moving relative to ourselves ticks.
If you have a clock and you measure an identical clock moving at 0.886c relative to you, you would determine that the other clock ticks ~1/2 as fast as your own. Someone with the other clock would, in turn, measure you and your clock as having a relative velocity of 0.866c relative to him, and would determine that your clock ticks ~1/2 as fast as his own.
As to what Ibix said about time dilation not applying to light, if you try to put c in for v in the time dilation equation, you end up with an equation of t = `t/0. Division by 0 is undefined and has no answer.

 

[Ibix]

If we assume that photon is a train traveling towards you or away from you, the time from that should appear to dilate and length contract from my reference frame, right?

A photon is not a train. Clocks on the train would, indeed, be slowed down as measured by my clocks. But my clocks and rulers are not affected by that, and it is my clocks and rulers that I use to measure speed.

Whether it is moving at speed of light or not can be moot. Laws of physics if give you results at 99.999999% speed of light ,that should be valid at c too.

Why? You can't make a clock that travels at the speed of light, so why must we be able to describe what would happen to something impossible? And I'm not sure time dilation is really a "law of physics" anyway. It's a coordinate effect - so perhaps a decent analogy is you asking which way is north from the north pole, me saying there's no such thing, and you insisting that you can go north from everywhere else, so you must be able to go north from the north pole. That isn't remotely analogous to the physics, but pretty good for the sense of the question.

So I am still confused on who is measuring the time and distance to measure speed of photon.

You are, with your clocks and rulers. Or I am. Or a guy in the train you mentioned. As long as we're all using our own clocks and rulers, anybody can do it.

 

[Janus]

So I am still confused on who is measuring the time and distance to measure speed of photon.

Isn't it a bit self evident that it is whoever is doing the measuring that is measuring the time and distance?
I want to measure the speed of a photon. I have a clock to measure the time, and a meter stick to measure the distance.( both at rest with respect to myself). I note how much time ticks on my clock for the photon to travel a distance measured by my measuring stick and get the speed of the photon relative to me.

 

[PeterDonis]

If we assume that photon is a train traveling towards you or away from you, the time from that should appear to dilate and length contract from my reference frame, right?

This has nothing whatever to do with measuring the speed of the train relative to you. When you measure the speed of the train relative to you, you don't care how the train's clocks are ticking or how long its rulers are compared to yours, because you're not using the train's clocks and rulers to measure its speed, you're using your own. Your own clocks and rulers are at rest relative to you, so they aren't dilated or contracted at all.

 

[ZapperZ]

Why not. If we assume that photon is a train traveling towards you or away from you, the time from that should appear to dilate and length contract from my reference frame, right? Whether it is moving at speed of light or not can be moot. Laws of physics if give you results at 99.999999% speed of light ,that should be valid at c too. So I am still confused on who is measuring the time and distance to measure speed of photon.

"c" is a constant in any inertial frame, which is why Galilean transformation doesn't work.

You attempted to make one serious flaw here, in which it appears that you are attempting to transform to the light's reference frame. This will violate all postulates of SR (and thus, you can no longer use any of SR's equations and conclusions, meaning that the concept of "time dilation" and "length contraction" are no longer valid).

No matter which inertial reference frame you measure c, there is no ambiguity in its value, because SR has indicated that you will always measure the same value, and thus, the consequences of SR (which are time dilation and length contraction, among others).

And yes, there is a HUGE difference between 0.9999c and c.

Zz.

 

[jbriggs444]

When we say speed of light = c m/s.

Minor quibble, but the speed of light is c. Full stop. c is a speed, not a number.

If you want to express the speed of light in meters per second, it is about 300,000,000 m/s. The 300,000,000 there is a number not a speed.

 

[Mister T]

Laws of physics if give you results at 99.999999% speed of light ,that should be valid at c too.

Which laws are you referring to? People routinely accelerate particles to speeds that are 99.99999% of the speed of light. They have little trouble measuring their speed or the speed of light, either. And they use the same laws of physics to do both.

 

[Janus]

Laws of physics if give you results at 99.999999% speed of light ,that should be valid at c too.

It equates to the difference between dividing 1 by 0.0000000...1 and dividing 1 by zero.
No matter how many zeros the ... represents, as long as you end with a final 1, you will get a finite answer. A number that you can multiply by 0.0000000...1 and get a result of 1. 1/0 doesn't yield such an answer. As there is no number that can be multiplied by 0 and give a result of 1.
So no matter how close to zero the number you divide zero by is, there is a world of difference between that and dividing by 0 itself,