# How is the speed of light being measured?

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1. Jun 10, 2014

### undertakerph

It is very important for me to know how scientists detect the speed of light in details...because there is something confuses me ....einstein concluded the dilatation of time on basis of that the speed of light is constant even if it comes from a mobile source ... but this fact is supposed to be hidden from us forever because the instruments which measure the speed of light have nothing to do with the dilatation of time .... this matter would be true if the instrument that measures the speed of light is being installed on the light beam ... so , the dilatation of time that happens ... happens to the instrument , hence its scale of time changes .. so the bigger distance per time on a dilated scale of time will give us a constant speed ..... but is that how we measure the speed of light ?????!!! Are scientists install the detector on the moving light beam ?????!!!!!!! i think they detect the speed with instruments ""outside"" the world of light .... so how is it possible that scientists detect a constant speed of light ....the instrument which is outside the world of light has its scale of time which hasn't been altered ... hasn't been dilated ... so they are supposed to detect higher speed of light coming from a mobile source ... and the fact of dilatation would be hidden from us ....so i want to know ..... how the scientists in details detect the speed of light ... may be they have a way of installing the detector on the light beam i don't know about ....knowing that i've read about machelson experiment and his detector as i expected was something in the lab meaning outside the world of light meaning its scale of time is the same not dilated ... so the result of speed was supposed to be higher as for the light from a mobile source ....is there anyone following me ?????

i want to understand this dilemma

2. Jun 10, 2014

### Staff: Mentor

This is not correct. The instruments that measure the speed of light are affected by time dilation if they are in motion. They are also affected by length contraction; so an instrument that is moving is time dilated and length contracted in such a way that it measures the same speed of light as an instrument that is not moving.

3. Jun 10, 2014

### pervect

Staff Emeritus
I don't understand your question, sorry.

One way of measuring the speed of light would be as follows. One would take a copy of the standard meter bar in Paris, and put a mirror on one end of the meter bar(we'll call that end the right end). Then one would shine a light beam from the left end, have it reflected from the mirror on the right end, and return to the left end. One would measure the time this process takes by a clock placed on the left end of the bar. The total path length the light travelled (2 meters) divided by the time reading on the clock would be the "round trip" speed of light.

On an advaned note, note that the idea of using the meter bar as a length standard is a bit outdated, however using such outdated ideas is necessary to make the speed of light an experimentally measurable quantity rather than a defined constant.

4. Jun 10, 2014

### Hernik

It might help you to think of it this way: The speed of light in vacuum always is the same for any observer. This is not dependant of the way you measure it. It is simply the way nature is. Now - because of this strange and very counter intuitive natural constant you must conclude that time is relative and space also.

5. Jun 10, 2014

### phinds

undertakerph, something that no one has bothered to point out to you is that you can rule out the concept of having detectors travel along with the light beam. That would be impossible. Nothing material can ever travel at the speed of light.

6. Jun 10, 2014

### undertakerph

ok ok you are right i don't disagree with you ... but as you said ..The instruments that measure the speed of light are affected by time dilatation if they are in motion....if they are in motion ...but when scientists do that they don't use instruments in motion .... that's what i am talking about ... revise machelson experiment for example ... he measured the speed of light with an instrument observing the light .... observing the light ... so the instrument is outside the world of light ... are you following me ? ...you know that the dilatation of time happens to anything moves with high speed ....but anything else outside this small moving world...isn't affected by dilatation of time ... so when we want to measure speed of something ...what we do exactly ? ... we just calculate ... distance over time ...our time ... so if lamb moves at speed of light and emits light ....of course this emitted light's speed is supposed to be the double .... why .... because we calculate the bigger distance per the same scale of time ...our time .. and the same scale of time of the instrument ...so how it could be constant speed ???

it would be a constant speed if ....if the instrument was on the moving lamb....so ...the dilatation of time happens to the lamb....happens to the instrument ... so the bigger distance per time the emitted light travels would be per a dilated scale of time ... so the result of speed would be the same

but are the scientists measure speed like that ??? ... no ..... so haaaaaoowwww is that ????

7. Jun 10, 2014

### undertakerph

and that's what i am talking about ... in this this method we calculate per our not dilates scale of time .... so when we apply the same technique to detect the speed of light from a mobile source ... why we detect a constant speed as long as the method is mathematical and the instrument is outside the moving source of light ... it would be supposed to be a higher speed the truth would be hidden

do you understand me ???

or

Do I understand the theory wrong ???

8. Jun 10, 2014

### undertakerph

That's what i am talking about ....as long as it is impossible the detectors travel along the light beam or the moving source of the light beam .... how our outsider detectors detect a constant speed ... as they not affected by dilatation of time ... when we calculate ... we calculate per its still scale of time ... so bigger distance per the same scale should give us higher speed !!!!

what do you think ???

9. Jun 10, 2014

### undertakerph

ok i will explain what i got from this theory ...the speed of light in vacuum always the same ... ok ... but how can we get to this fact ...by measurement of course ...ok ? ..now you said ... the speed of light is constant to any observer ...and that's is very important ..to any observer ...that's mean that speed of light is not constant ... but it is just a game of time ...because ....photons of light travels 300,000 km/s .... ok .... what about a lamb travels 300,000 km/s emit a beam of light ... what the speed of this light ???? .... plz focus with me .... it is impossible for the beam of light with this condition to travel the same distance per second ... it is impossible ...of course it travels 600,000 km /s .... but it is our second .... our scale of time ....but it's not the fact ... the fact is ...the time of the moving lamb is dilated ...so if there is a measuring instrument on this lamb ...its time would be dilated ... so ...it measures 600,000 km per a dilated second ...which values 2 seconds of our second ...so the speed would be the same 300 ,000 km /s ... just game of time and measures ....but as i said ...if the instrument would be on the moving source ... because in that case it is not as a observer .... but in real measurements ...we use instruments as observers ... they are outside the small world of moving beam ...they are not affected by the dilatation of time ...so we should notice bigger distance per our scale of time ...how could be moving photons emitted from a moving source travel the same distance per the same scale of time ...impossible ... they travel bigger distance per a dilated scale of time ...but outsider instrument should calculate false results with higher speed ....so that 's my question ........how is that ????????!!!!!!!!!

10. Jun 10, 2014

### Staff: Mentor

undertakerph, before I respond, a general note: it's a lot easier to read someone's posts if they use ordinary sentences and capitalization instead of all lower case and a lot of ...'s.

Motion is relative. If scientists in Australia are doing an experiment to measure the speed of light, their instruments aren't in motion relative to them, but they are in motion relative to scientists in the United States, and vice versa. Yet both sets of scientists get the same answer.

11. Jun 10, 2014

### Staff: Mentor

As phinds already pointed out, this is impossible. No light source or measuring instrument can travel at the speed of light.

12. Jun 10, 2014

### undertakerph

Dear Peter , you left the concept and criticized the example .....ok ... let's talk as real ....please Peter stand me .. Go with me to the end ... because I really want to understand ...First question .. How could the scientists know that the speed of light in vacuum is 300,000 km/s ??? ... Knowing that any Use of any source of light would be considered as a moving Source....Because we are on a moving Planet .

Last edited: Jun 10, 2014
13. Jun 10, 2014

### Vanadium 50

Staff Emeritus
It would be helpful and courteous, not to mention easier to understand, if you used proper English. I don't understand the point you are trying to make.

14. Jun 10, 2014

### undertakerph

What is not proper in my english ???

ok .. read that ... I hope it would clear my Point to you

ok i will explain what i got from this theory ...the speed of light in vacuum always the same ... ok ... but how can we get to this fact ...by measurement of course ...ok ? ..now you said ... the speed of light is constant to any observer ...and that's is very important ..to any observer ...that's mean that speed of light is not constant ... but it is just a game of time ...because ....photons of light travels 300,000 km/s .... ok .... what about a lamb travels 300,000 km/s emit a beam of light ... what the speed of this light ???? .... plz focus with me .... it is impossible for the beam of light with this condition to travel the same distance per second ... it is impossible ...of course it travels 600,000 km /s .... but it is our second .... our scale of time ....but it's not the fact ... the fact is ...the time of the moving lamb is dilated ...so if there is a measuring instrument on this lamb ...its time would be dilated ... so ...it measures 600,000 km per a dilated second ...which values 2 seconds of our second ...so the speed would be the same 300 ,000 km /s ... just game of time and measures ....but as i said ...if the instrument would be on the moving source ... because in that case it is not as a observer .... but in real measurements ...we use instruments as observers ... they are outside the small world of moving beam ...they are not affected by the dilatation of time ...so we should notice bigger distance per our scale of time ...how could be moving photons emitted from a moving source travel the same distance per the same scale of time ...impossible ... they travel bigger distance per a dilated scale of time ...but outsider instrument should calculate false results with higher speed ....so that 's my question ........how are the instruments detecting a constant speed ????????!!!!!!!!!

15. Jun 10, 2014

### Staff: Mentor

Because they've measured it.

Which doesn't matter, because they've measured it in many *different* states of motion. They've measured it at different places on Earth, which, as I pointed out already, must always be in motion relative to one another. They've measured it at different times of the year, so the Earth's motion relative to the Sun and other planets is different. They've measured it in space, by timing radar signals to and from spacecraft and various planets. They've measured it every which way, and it always comes out the same. And all of this is in perfect accord with the predictions of the theory of relativity.

These are the facts. Do you agree that these are the facts? If you don't, then that's your problem right there: you have a mistaken belief about what the facts are.

If you do agree that these are the facts, then what, exactly, are you questioning? Are you questioning whether the theory of relativity can explain these facts? It can. Are you asking *how* the theory of relativity explains these facts? Then don't mix in your own personal (and mistaken) ideas about what you think the theory says. *Ask* what the theory actually says. Propose a scenario--*without* bringing in your own personal (and mistaken) ideas about the scenario--and *ask* what the theory actually predicts about it.

Until you do this, I'm afraid this discussion will be pointless. And you haven't done it yet; every post you make is full of your personal (and mistaken) ideas. You need to let go of these ideas and confine yourself to asking a simple question about a simple scenario.

16. Jun 10, 2014

### undertakerph

God bless you .....God bless you ..... haaaaaahahaha .... thank you very very very much Peter....Finally now I understand ...and You were right .. the Problem is my personal mistaken ideas... I really thank you very much :) :) .... so , what I was talking about wasn't fully wrong ...it is just I didn't get the whole picture .... so , the instruments on our moving Planet in its different motion cases are considered as if I installed the measuring instrument on the beam of light :) ...and now we calculate per the dilated scale of time

hahahahaha

I am speechless to you Peter , thank you again ... your simple words have enlightened the way ... I've finally ended this Dilemma :) :) , thanks to you :)

Last edited: Jun 10, 2014
17. Jun 10, 2014

### Staff: Mentor

I'm not sure this makes sense, but if what I wrote has answered your question, I guess that's good.

18. Jun 11, 2014

### undertakerph

First of all I should apologize about some sentences like " Focus with me " and so on.

I really didn't mean something bad , I was just talking , that's all , but the Admin drew my attention to that point, so I am sorry again :)

As for you my dear PeterDonis , I'll try to make my point clear but , as you said , the important thing is that I got it, even if I couldn't make my point clear to you .

Your words made me notice that I wasn't aware of that any source of light we use , and the measuring instrument we use, are both on the same moving Planet, which means they are both on the same phase, which means that the dilatation of time that happens to the source of light , happens also to the instrument , so we actually calculate distance per a changed scale of time in each case of the different cases of motion of the Planet .

Remember my problem .. I wanted to install the instrument on the moving lamb which emits the light in the example I imagined ... I didn't notice that it is what actually happens because the measuring instrument and the source of light both are on the same moving Planet , so the instrument is affected by dilatation of time ..... My problem was that I thought the instrument is an observer, and it is outside the world of moving source of light, but I was wrong ... Actually it is inside the world of the moving source of light as they are both on the same moving Planet :)

So , We can explain the Theory of Relativity as following : When Scientists tried to measure the speed of light , they weren't aiming to detect the speed of light in vacuum , because they can't .

They were aiming to detect the difference in light velocities coming from different moving sources ... Now I understood the experiment of " Machelson " ... He tried to detect the difference in velocities of light coming from different moving sources .

As " PeterDonis " in the previous comment clarified , Using a source of light in experiments during different cases of motion of the Planet actually is considered as a moving source of light with different speed in each time , and " Machelson " was supposed to detect a difference in the speed of light in each time, but the surprise was he didn't find any difference .

He found the speed constant each time ... Being constant form different sources moving at different velocities means axiomatically that the detected value of speed is also the speed of light in vacuum ... It's constant in every case .

" Machelson " stopped at that point of conclusion ....Speed of light is constant in vacuum, but " Einstein " didn't ... He was right not buying this conclusion ...How is that possible ??? ... Light in vacuum coming from different moving sources at different velocities should travel different distances per time, but he realized that the speed is a relative Quantity ... It's something per something else ... It's distance per time ..It is not an absolute Quantity .

So , what if that the bigger distance happened , was happening per a bigger time also ... In this case the result of speed would be constant, and he came out with his theory to us .

The time is relative, and the time of anything moving with higher speed will be dilated .

Our time on our moving Planet changed in its different cases of motion with different speed, meaning the scale of time of our measuring instruments changed also , that's why when we calculate the speed of light we find it constant each time .

We can't feel these changes of time because we are in the world of the moving Planet .. The Effect happens to every thing in this small world, so , the relativity between things is still the same , so we feel nothing .

The constant speed of Light is considered as the Static Station we wished for, so that we can observe and detect our relativity ... But for that , we never could .

Last edited: Jun 11, 2014
19. Jun 11, 2014

### Staff: Mentor

For measurements done in a single lab on Earth, yes, the source and the measuring instrument are both in the same state of motion. But there are other measurements of the speed of light in which the source and the measuring instrument are *not* in the same state of motion.

No, this is not correct. They can, and they did. Michelson and Morley may not have done their experiment in a vacuum (as far as I can tell they didn't), but other scientists certainly have measured the speed of light in a vacuum.

That was part of his experiment, because he ran it at different times of the year. But the main point was to try to detect the difference in velocities of light emitted by the same source, at the same time, but in different directions. He detected no difference, and *that* was the big surprise.

20. Jun 11, 2014

### undertakerph

Can you tell me an example for such an experiment or give me a link to it , sorry Peter for bothering you , so sorry

Same request please

Why did he predict a difference in speed of light in different directions ???

I got confused again :(

21. Jun 11, 2014

### Staff: Mentor

Because he believed that light was a wave in a material medium called "ether", and that this ether was in a particular state of motion, which was not the same as the Earth's state of motion. (Even if the Earth happened to be at rest relative to the ether at some particular instant, it wouldn't stay that way very long, since the Earth changes direction as it moves around the Sun.) And if the Earth was in motion relative to the ether, this would (according to the theory Michelson was using) mean the speed of the light waves in the ether, as measured by instruments moving with the Earth, would be different along the direction of motion vs. perpendicular to the direction of motion.

Of course, the theory Michelson was using was wrong, as we now know; but it was a common theory at the time, which is why the null result of the Michelson-Morley experiment was so surprising.

22. Jun 11, 2014

### undertakerph

OK , I thank you very very much Peter for your great help .

I need to revise myself again and again , there are missing items I should know about and link between them all .

23. Jun 11, 2014

### Staff: Mentor

See here:

http://en.wikipedia.org/wiki/Speed_of_light#Astronomical_measurements

The astronomical bodies used for these measurements are in relative motion.

The astronomical measurements are measurements of the speed of light in vacuum, by the way; but there are also Earth-bound laboratory measurements done in vacuum; see, for example, the section headed "Interferometry" on the Wikipedia page I linked to above.

24. Jun 11, 2014

### D H

Staff Emeritus
Sound waves are variations in pressure. The medium that carries these sound waves is the air. Ocean waves are variations in the height of the water. Here the medium is water. Grab one end of a long, suspended rope and give it pull to the side. A neat wave travels up and down the rope. The medium is the rope. And so on and so on.

Every wave phenomenon known at Michelson's time required a medium of some sort. It made sense at the time to think that the electromagnetic waves described by Maxwell also required a medium of some sort. A very weird medium, but how else could those electromagnetic waves travel? That's the reason this was such a common theory.

25. Jun 11, 2014

### Staff: Mentor

Another important point is that at the time the aether was assumed to be some extremely rigid and extremely low density solid medium. The rigidity and solidity were important to explain the high value for the speed of light as well as the fact that light waves are transverse (can be polarized).

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