# Why does light move at the speed of light? **URL:** \ **Category:** Factual Questions\ **Created:** [May 9, 2001, 10:28pm UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916 "2001-05-09T22:28:55Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![Punoqllads](https://avatars.discourse-cdn.com/v4/letter/p/d2c977/32.png) [@Punoqllads](https://boards.straightdope.com/u/Punoqllads)\ **Post date:** [May 10, 2001, 10:59pm UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916/21 "2001-05-10T22:59:29Z")
Libertarian, I cannot make head or tail of your first paragraph, so I can’t answer that. As for the second paragraph, no, you do not necessarily see the two ends at the same time. The light needs time to travel from the ends to your eyes. Because you are so close to the table, it seems like it is instantaneous, but it is not. Now, if you measured the distance from your eyes to the two ends, and divided that by the speed of light in air (pretty close to 3.0 \* 10[sup]8[/sup]m/s, IIRC) you would get the amount of time it takes for the light to propagate to your eyes from each end. So, if you saw something happened, and noted exactly when you saw it, you could subtract off that propagation time and tell when that event happened. Now, suppose there was another observer of this same coffee table, but moving very fast with relation to the table. Suppose this person saw two things happen on the two ends of the table. Now, let’s say this person could also measure their distance from each end of the table, and, consequently, could tell at what point in time each event happened. Let’s say that, in their frame of reference, the two events happened at exactly the same time. Relativity says that you, the observer not moving with respect to the table, observing the exact same events, would calculate that the two events did **not** happen at the same time. ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [May 10, 2001, 11:21pm UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916/22 "2001-05-10T23:21:46Z")
I’m all for continuing the hijack on light moving through media and virtual particles or whatever slowing it down. I asked a similar question several months back but didn’t get a clear answer [Why does light slow down?](http://boards.straightdope.com/sdmb/showthread.php?threadid=47008) In that thread I was (maybe) confusing the issue with the slowing-down & stopping light experiments with Bose-Einstein condensates. As I understand it now, after you stop the photons, you don’t ‘really’ get the same photons out, you just get photons that look and act exactly like the ones you put in (i.e. carry the same information). Is this the same thing as light refracting? One objection to my belief was that someone said if you shine a laser through glass it all comes out coherent, so the individual light particles couldn’t have been interfered with on their trip through. But if they were all interfered with the same way …? anyone here able to resolve this? ---
**Author:** ![Silo](https://avatars.discourse-cdn.com/v4/letter/s/ac91a4/32.png) [@Silo](https://boards.straightdope.com/u/Silo)\ **Post date:** [May 11, 2001, 12:33am UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916/23 "2001-05-11T00:33:13Z")
> [@](#): > > \*Originally posted by Ring \* > Pardon me?? > > Photons are absorbed and reemitted by **virtual particles???** > > I’m sorry Silo, but I’m afraid you have no idea of what you speak. On the other hand I could be wrong and you have actually devised a new and wonderful form of quantum physics. Could you perhaps post a cite or further explain your Earth shattering theory? Darn, I read medium as vacuum. Yeah, charged particles. Doh, sorry, that was dumb. ::hangs head in shame:: Interchange the two and I stand by absorption and reemission by some virtual particles popping out of the vacuum. However, that’s getting into some weird new string theory stuff envolving “string fragments” within the ether forming very temporary and unstable charged particles. Oh nevermind… > [@](#): > > The mechanism which releases a photon has nothing to do with the speed of light in a vacuum. Consider an electron a string. There’s a specific vibration which causes the release of a photon. This “specific vibration” dictates why “c” _in most part,_ carries the value it has. Silo - you probably think I’m in left field, but that’s okay. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 11, 2001, 1:47am UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916/24 "2001-05-11T01:47:02Z")
> [@](#): > > What about a photon that has come through a medium that has slowed it down such that it is moving at c - x, where x is the amount of retardation throught that medium. here’s my stab at this one: when the photon comes out of the media it again goes to a velocity of c. The reason this is possible is because the rest mass is zero. F=ma, Ke=1/2mv^2, Momentum=mv all these laws (which I understand fall apart for light anyway) have mass which for a photon is zero. So the photon can instantly accelerate up to c and has no problem changing it’s velocity. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 11, 2001, 2:01am UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916/25 "2001-05-11T02:01:52Z")
The below has a nice GIF on the slowing of a photon [http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/em.html](http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/em.html) ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 11, 2001, 2:05am UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916/26 "2001-05-11T02:05:25Z")
Excuse me that should have read “absorbion and reemission of a photon” not “the slowing of a photon” ---
**Author:** ![Snetho](https://avatars.discourse-cdn.com/v4/letter/s/a4c791/32.png) [@Snetho](https://boards.straightdope.com/u/Snetho)\ **Post date:** [May 11, 2001, 11:20am UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916/27 "2001-05-11T11:20:27Z")
Here is an article that talks about how light can be stopped! [http://www.cnn.com/2001/US/01/25/delaney.debrief/](http://www.cnn.com/2001/US/01/25/delaney.debrief/) ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [May 11, 2001, 4:45pm UTC](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916/28 "2001-05-11T16:45:44Z")
Chronos: If you’re still around and find the time, would you address my questions above? Thanks. [Previous page](https://boards.straightdope.com/t/why-does-light-move-at-the-speed-of-light/64916.md?page=1)