# Speed of light, how is it measured ?

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#### killium

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<p>I always wondered. Ok, i know, distance covered divided by time it took to do so. But this implies you know the exact time of departure and time of arrival, and the exact distance. This would mean that the emitter and the receiver (and the observer taking the measures) are all in the same reference frame. Now, theory says light speed is always the same even if the emitter and the receiver are moving (fast).</p><p>&nbsp;</p><p>Is it true also if the emitter and receiver are moving (fast) in respect to one other ? This triggers a series of thoughts in my mind which lead to "let's measure the speed of light that is comming at us from a distant, moving object". I bet i'm far from beeing the first one to have thought of this lol so i imagine this experiment have been done already... But then, this question arises : How do you measure this ? We don't know the (exact) distance, we don't know the (exact)&nbsp;"when" (was that light emitted). How can we physically measure the speed of a photon if the only thing we can do with it is to absorb it (in the detector) ?</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I always wondered. Ok, i know, distance covered divided by time it took to do so. But this implies you know the exact time of departure and time of arrival, and the exact distance. This would mean that the emitter and the receiver (and the observer taking the measures) are all in the same reference frame. Now, theory says light speed is always the same even if the emitter and the receiver are moving (fast).&nbsp;Is it true also if the emitter and receiver are moving (fast) in respect to one other ? This triggers a series of thoughts in my mind which lead to "let's measure the speed of light that is comming at us from a distant, moving object". I bet i'm far from beeing the first one to have thought of this lol so i imagine this experiment have been done already... But then, this question arises : How do you measure this ? We don't know the (exact) distance, we don't know the (exact)&nbsp;"when" (was that light emitted). How can we physically measure the speed of a photon if the only thing we can do with it is to absorb it (in the detector) ?&nbsp; <br />Posted by killium</DIV></p><p>You certanly can't just get out a radar gun and clock the photon.&nbsp; But you can do a few things.&nbsp; There is thing called Doppler shift that relates to the change in wavelength that occurs with a moving emitter, or reflector.&nbsp; To calculate Doppler shift you need to kow the speed of propagation of the wave.&nbsp; <br />http://en.wikipedia.org/wiki/Doppler_effect</p><p>We have Doppler radar, and it works well and provides speeds and distances for objects that are scanned by the radar.&nbsp; Those speeds and distances can be independently measured by other means, at least in controlled circumstances.&nbsp; The speeds and distances that are found are consistent with the constant speed of light.&nbsp; So that is experimental confirmation.</p><p>In addition the entire theory of electromagnetic waves is based on 4 partial differential equations called Maxwell's equations.&nbsp; From those equations one can derive a speed for electromagnetic waves based on parameters that are measurable in a laboratory.&nbsp; One finds that the speed is independent of the reference frame and one also finds tha tthe speed predicted by those equations is the same speed that is measured for light.&nbsp; Light is an electromagnetic wave.</p> <div class="Discussion_UserSignature"> </div>

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#### killium

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<p>so we use doppler shift to measure the speed of an incoming light beam ? But doppler shift's calculations assumes a speed already !?</p><p>&nbsp;</p><p>A doppler shift measures the relative speed of the emitter in respect to the observer due to wavelenght shift. But what does it says about the actual speed of the incoming photon ?</p> <div class="Discussion_UserSignature"> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>so we use doppler shift to measure the speed of an incoming light beam ? But doppler shift's calculations assumes a speed already !?&nbsp;A doppler shift measures the relative speed of the emitter in respect to the observer due to wavelenght shift. But what does it says about the actual speed of the incoming photon ? <br />Posted by killium</DIV></p><p>Read what I said.&nbsp; You can take an object of known distance and speed (known by separate means) and measure th speed and distance with Doppler radar.&nbsp; The Doppler radar measurements will be correct only if the assumed speed of light is also correct.&nbsp; You don't directly measure the speed of the photon, but you can infer it from the ability of the Doppler radar to operate as predicted by that speed. </p> <div class="Discussion_UserSignature"> </div>

A

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<p>There's&nbsp;a&nbsp;quite a few ways to measure the speed of light.&nbsp;Kepler&nbsp;argued the speed of light to be infinite in the vacuum of space as there were no obstacles. Although many (Bacon, Descartes) objected it wasn't till 1676 that R&oslash;mer made the first quantitative measurement studying the&nbsp;motions of Io with his telescope. (off by around 26% btw)</p><p>However there's also a&nbsp;more down to earth approach, maybe that's what you are after? The following is straight from Wiki and describes the somewhat simple though ingenious experiment.</p><p>&nbsp;</p><p style="margin:0px;word-spacing:0px" align="center"><font face="Verdana" size="2" color="#000080"><img src="http://sitelife.space.com/ver1.0/Forums/speedace_images/speed_of_light_fizeau.png" border="0" alt="" width="450" height="301" /></font></p><p>Diagram of the Fizeau-Foucault apparatus.</p><p>&nbsp;</p><p>The first successful measurement of the speed of light using an earthbound apparatus was carried out by&nbsp;Hippolyte Fizeau&nbsp;in 1849. (This measures the speed of light in air, which is slower than the speed of light in vacuum by a factor of the refractive index of air, about 1.0003.) Fizeau's experiment was conceptually similar to those proposed by Beeckman and Galileo. A beam of light was directed at a mirror several thousand metres away. On the way from the source to the mirror, the beam passed through a rotating cog wheel. At a certain rate of rotation, the beam could pass through one gap on the way out and another on the way back.&nbsp;But at slightly higher or lower rates, the beam would strike a tooth and not pass through the wheel. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light could be calculated. Fizeau&nbsp;reported the speed of light as 313,000 kilometres per second. Fizeau's method was later refined by&nbsp;Marie Alfred Cornu&nbsp;(1872) and&nbsp;Joseph Perrotin&nbsp;(1900).</p><p>Leon Foucault&nbsp;improved on Fizeau's method by replacing the cogwheel with a rotating mirror. Foucault's estimate, published in 1862, was 298,000 kilometres per second. Foucault's method was also used by&nbsp;Simon Newcomb&nbsp;and Albert A. Michelson. Michelson began his lengthy career by replicating and improving on Foucault's method.In 1926, Michelson used a rotating&nbsp;prism to measure the time it took light to make a round trip from&nbsp;Mount Wilson&nbsp;to&nbsp;Mount San Antonio&nbsp;in California, a distance of about 22 miles (36&nbsp;km) each way. The precise measurements yielded a speed of 186,285 miles per second (299,796 kilometres per second).</p><p>&nbsp;</p><p>*Edit: Hmm, picture won't load. Dunno if it's just me.</p> <div class="Discussion_UserSignature"> "<font color="#0000ff"><em>The choice is the Universe, or nothing</em> ... </font>" - H.G Wells </div>

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#### coeptus

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<p>His method, if used today with modern precision time keeping, and an acurate earth/sun distance, could yield a very accurate value of C.</p><p>&nbsp;</p><p>Time delay of US lunar astronauts communication with NASA gives almost anyone listening a feel for the finite nature of the C.</p><p>&nbsp;</p><p>I also note trained TV journalists still struggle with light time delay when conducting satellite interviews of distant personages.</p><p>Those 30,000 kilometer signal bounces mount up.</p><p>&nbsp;</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p><font color="#ff00ff">If not for bad Pluck, I'd have no Pluck at all . . .</font></p><p> </p><p><font color="#0000ff">This is your vogon, posting under coeptus, and trying IE and Firefox  to see if either is faster with fewer misloads.  Erf !!</font></p><p> </p><p> </p><p> </p> </div>

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#### vandivx

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I also note trained TV journalists still struggle with light time delay when conducting satellite interviews of distant personages.Those 30,000 kilometer signal bounces mount up.&nbsp;&nbsp; <br /> Posted by coeptus</DIV><br />when I switch between old analog TV reception and satelite on the same channel, there is perhaps something like one or make it two second difference (satelite broadcast is delayed) but I thought that is mainly because of the conversion from analog to digital signal (and perhaps other delays of that nature, there is also backconversion at my end to analog), it works like replay, during olympic games I could watch a key moments in disciplines twice that way, first on analog broadcast and then by pressing one button on remote on digital</p><p>but I think that light speed in that is only small factor, perhaps unnoticable in normal life 'remote TV control' experiments even though it is there, I didn't do any calculations though</p> <div class="Discussion_UserSignature"> </div>

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#### coeptus

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