# The measurement of the speed of light? **URL:** \ **Category:** Factual Questions\ **Created:** [March 17, 2011, 5:33pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043 "2011-03-17T17:33:21Z")\ **Posts on this page:** 17\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 5, 2011, 1:30am UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/21 "2011-04-05T01:30:00Z")
> [@](#): > > How long? (Or, if you prefer; how short?) That’ll depend on all sorts of details of the object it’s bouncing off of, but at a rough order of magnitude, it’d be about 1/f, where f is the frequency of the photon (or, equivalently, the wavelength divided by c). ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [April 5, 2011, 3:28am UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/22 "2011-04-05T03:28:01Z")
> [@Chronos](#): > > What do you mean by “the original one”? Particles don’t have serial numbers on them. If the photon’s energy is absorbed and then re-emitted, could it not be the mirror’s electrons emitting after a photoelectric effect or something similar? That should take some serious time. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [April 5, 2011, 6:20am UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/23 "2011-04-05T06:20:19Z")
> [@Triskadecamus](#): > > How long? (Or, if you prefer; how short?) > > Tris I was going to ask this - does the experiment need to take into account a “delay” due to the number of reflections, or are those essentially instantaneous? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 5, 2011, 5:51pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/24 "2011-04-05T17:51:50Z")
> [@](#): > > If the photon’s energy is absorbed and then re-emitted, could it not be the mirror’s electrons emitting after a photoelectric effect or something similar? That should take some serious time. That could take a while, but that’s not something generally referred to as a photon “bouncing”. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [April 5, 2011, 7:13pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/25 "2011-04-05T19:13:54Z")
Speed of light in a transparent medium is somewhat slower than SoL in a vaccuum. (Ever done a Michelson-Morely interferometer experiment where one arm is through a vaccuum in a cylinder with glass ends? You can count the wavelengths of light difference in the travel path time of arm A vs. B. You could set up a similar experiment - except have the light bounce off multiple mirrors - just to prove if the path shows a slower speed of light with multiple mirror bounces than with one? Or is the theory that light only slows a bit on the first mirror bounce and does not slow after that? See, to devise an experiment you have to know what you are trying to measure. One experiment I read of involved slits on a spinning disk. A pulse of light is bounced off a mirror 10 miles away on a hillside. The light goes through the spinning disk on the way out and also on the way back. Lets build a disk with 100 evenly spaced slits and spin it at 100rpsec. Light travels about 186,000miles/sec; a 20-mile round trip should take 1/9300 of a second. Fiddle with the speed of the disk to see when the light is / is not visible; if light were instantaneous (no travel time) It should be visible no matter how fast you spin the disk. How fast at known light speed would your disk have to spin, for the light to leave through one slit and then you see it through the next slit that comes around? ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [April 5, 2011, 8:39pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/26 "2011-04-05T20:39:28Z")
> [@Chronos](#): > > Particles don’t have serial numbers on them. No hidden-variable theories for you, eh? ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [April 7, 2011, 2:40pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/27 "2011-04-07T14:40:10Z")
> [@komolono](#): > > If the photon’s energy is absorbed and then re-emitted, could it not be the mirror’s electrons emitting after a photoelectric effect or something similar? That should take some serious time. This idea was based on a false assumption, I thought that all mirrors are also good conductors: water, silver… but I can see my face in a window, too. But I still need some facts before I become a non-wave-person. What makes a good mirror? White color does not make a good mirror, although it reflects all wavelenghts. Sleek surface, but what else? I quess I could see my image in wet black paint, although it should absorb every wavelength. But then again, I might be wrong. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [April 7, 2011, 3:35pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/28 "2011-04-07T15:35:21Z")
> [@komolono](#): > > This idea was based on a false assumption, I thought that all mirrors are also good conductors: water, silver… but I can see my face in a window, too. > But I still need some facts before I become a non-wave-person. What makes a good mirror? White color does not make a good mirror, although it reflects all wavelenghts. Sleek surface, but what else? I quess I could see my image in wet black paint, although it should absorb every wavelength. But then again, I might be wrong. Two effects are at work. A typical mirror has glass that allows the light waves (I’m using waves for a reason) to pass through and a coating, say of silver, that does not, thus reflecting the waves. But you can see a reflection in a transparent, uncoated, window if it’s dark out. From _The Amazing Story of Quantum Mechanics_ by James Kakalios: > [@](#): > > Light travels slower in glass than in air and this difference in light velocities (characterized by the material’s index of refraction, for technical reasons) accounts for the reflection effect. Why is calling it a wave important for this discussion? Imagine a double-paned window in which the two sheets of glass are only a few wavelengths of light apart. > [@](#): > > It turns out that the light wave is not completely reflected at the first glass-air interface, but instead a small amount of the oscillating electric and magnetic fields leak out into the air. The small leakage is limited to a thin region very near the interface and is normally not important. But when the second sheet of glass is brought very close to the the first interface, some of the protruding light waves extend into the second piece of glass. In this case the wave is not reflected but rather propagates into the second glass. That is what creates the quantum “tunneling” effect for all matter. It’s part of the dual wave and particle nature of all matter, though the effect is normally too small for any but the tiniest bits, like photons. But if you don’t think of it as a wave, a probability wave, the tunneling gets harder to explain. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 7, 2011, 6:09pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/29 "2011-04-07T18:09:05Z")
Why should you become a non-wave-person? Light is clearly and unambiguously a wave. It’s just that it’s _also_ clearly and unambiguously a particle. ---
**Author:** ![mlees](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@mlees](https://boards.straightdope.com/u/mlees)\ **Post date:** [April 7, 2011, 6:47pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/30 "2011-04-07T18:47:26Z")
Hoo boy. Time for the Maalox. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [April 8, 2011, 3:32am UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/31 "2011-04-08T03:32:58Z")
> [@mlees](#): > > Hoo boy. Time for the Maalox. I don’t need aluminum mirrors in my brain - it makes me feel dual. You must mean Zantac. ---
**Author:** ![manila](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@manila](https://boards.straightdope.com/u/manila)\ **Post date:** [April 8, 2011, 7:50am UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/32 "2011-04-08T07:50:31Z")
> [@Chronos](#): > > Why should you become a non-wave-person? Light is clearly and unambiguously a wave. It’s just that it’s _also_ clearly and unambiguously a particle. Could you come and have a look at my cat please? Its here in this box. I think it may be dead… ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [April 8, 2011, 10:32am UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/33 "2011-04-08T10:32:53Z")
> [@manila](#): > > Could you come and have a look at my cat please? Its here in this box. I think it may be dead… Schrödinger, I presume… ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [April 10, 2011, 4:50pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/34 "2011-04-10T16:50:30Z")
> [@Chronos](#): > > Why should you become a non-wave-person? Light is clearly and unambiguously a wave. It’s just that it’s _also_ clearly and unambiguously a particle. There must be a simple explanation to this, but I just can’t figure it out: Solar sail - they get their energy from photons (and protons). After reflected the photon still “sails” at the speed of light. How can the rocket sail, too? Where did the energy come from? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 10, 2011, 7:44pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/35 "2011-04-10T19:44:19Z")
The photons that reflect off are redshifted, and therefore have less energy and momentum than they did before. We had another thread recently on exactly this sort of question. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [April 10, 2011, 8:04pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/36 "2011-04-10T20:04:28Z")
> [@Chronos](#): > > The photons that reflect off are redshifted, and therefore have less energy and momentum than they did before. We had another thread recently on exactly this sort of question. Are those redshifted photons blueshifted to any observer? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 10, 2011, 8:28pm UTC](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043/37 "2011-04-10T20:28:39Z")
Yes, but those same observers would also see the solar sail as slowing down, rather than speeding up. [Previous page](https://boards.straightdope.com/t/the-measurement-of-the-speed-of-light/575043.md?page=1)