# Why is the speed of light constant? **URL:** \ **Category:** Factual Questions\ **Created:** [August 20, 2003, 11:54pm UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554 "2003-08-20T23:54:20Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [August 20, 2003, 11:54pm UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/1 "2003-08-20T23:54:20Z")
The subject came up the other day in the breakroom. ( I am an x-ray tech. in an inner-city trauma center). We were discussing the physics of radiation protection, and some how the following question was brought up. If photons of light are particles (and supposedly waves too), then they have mass. Shouldn’t photons slow down as they pass through a medium such as the Earth’s atmosphere? ---
**Author:** ![Speaker\_for\_the\_Dead](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@Speaker\_for\_the\_Dead](https://boards.straightdope.com/u/Speaker_for_the_Dead)\ **Post date:** [August 21, 2003, 12:01am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/2 "2003-08-21T00:01:09Z")
That’s the beauty of it - photons have no mass. At least, that’s the accepted theory. They don’t accelerate to c, because they come into existance already going that fast. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [August 21, 2003, 12:03am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/3 "2003-08-21T00:03:50Z")
Photons don’t have any (rest mass), this is why they can travel at c, the reason they do is because they can’t travel at any other speed (in otherwords something travelling at lightspeed cannot accelarte or deccelrate). Light does slow down in mediums like the atmospehere bu this is to do with the absorbtion and re-emission of photons by electrons in the air molecules. ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [August 21, 2003, 12:15am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/4 "2003-08-21T00:15:22Z")
How can a photon be considered a particle if it has no mass? ---
**Author:** ![JRDelirious](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jrdelirious/32/9531_2.png) [@JRDelirious](https://boards.straightdope.com/u/JRDelirious)\ **Post date:** [August 21, 2003, 12:16am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/5 "2003-08-21T00:16:43Z")
Right. The “constant” is the speed-o-light in a vaccuum. ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [August 21, 2003, 12:19am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/6 "2003-08-21T00:19:45Z")
Follow up question: If the speed of light is constant, that means if you turn on a flashlight, the light is moving at the speed of light the moment it is created. How can it go from 0 to 186,000 miles a sec. without speeding up? ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [August 21, 2003, 12:25am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/7 "2003-08-21T00:25:36Z")
> [@](#): > > \*Originally posted by x-ray vision \* > \*\*Follow up question: > > If the speed of light is constant, that means if you turn on a flashlight, the light is moving at the speed of light the moment it is created. How can it go from 0 to 186,000 miles a sec. without speeding up? \*\* It was never going zero. The instant those photons came into being, they were going c. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [August 21, 2003, 2:50am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/8 "2003-08-21T02:50:47Z")
> [@](#): > > _x-ray vision wrote:_ > How can a photon be considered a particle if it has no mass? “Particle” means that it has the property of being approximately in one place. It doesn’t mean that it has mass. ---
**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:** [August 21, 2003, 2:58am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/9 "2003-08-21T02:58:51Z")
Think of particles as packets of energy (which, by E=MC[sup]2[/sup], is the same as mass). Particles such as the photon (and the theorized graviton) have energy - as well as momentum. It doesn’t have to be intuitive. ---
**Author:** ![Homer\_J.Simpson](https://avatars.discourse-cdn.com/v4/letter/h/a698b9/32.png) [@Homer\_J.Simpson](https://boards.straightdope.com/u/Homer_J.Simpson)\ **Post date:** [August 21, 2003, 3:02am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/10 "2003-08-21T03:02:11Z")
I believe light is also a wave. ---
**Author:** ![billy](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@billy](https://boards.straightdope.com/u/billy)\ **Post date:** [August 21, 2003, 4:16am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/11 "2003-08-21T04:16:59Z")
> [@](#): > > \*Originally posted by Homer J. Simpson \* > \*\*I believe light is also a wave. \*\* Well, not exactly a wave. We’ve all heard this before but I’ll regurgitate anyways. Photons behave in a manner similar to both the classical wave, and the classical particle. It acts according to the wave equation like any regular electromagnetic wave and can be visuallized as a wave but also acts as a particle when knocking electrons out of atoms in a photoelectric effect experiment. Having said that, I don’t think it accurate to say it is a wave, or is a particle. If think there are major areas of study today trying to figure out what it is. It just has properties of both. ---
**Author:** ![Homer\_J.Simpson](https://avatars.discourse-cdn.com/v4/letter/h/a698b9/32.png) [@Homer\_J.Simpson](https://boards.straightdope.com/u/Homer_J.Simpson)\ **Post date:** [August 21, 2003, 4:21am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/12 "2003-08-21T04:21:25Z")
[http://www.straightdope.com/mailbag/mlightwaveparticle.html](http://www.straightdope.com/mailbag/mlightwaveparticle.html) ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [August 21, 2003, 4:22am UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/13 "2003-08-21T04:22:49Z")
Excatly. Photons are neither particles nor waves. They have some properties of each, and some all of their own. ---
**Author:** ![Dogface](https://avatars.discourse-cdn.com/v4/letter/d/9d8465/32.png) [@Dogface](https://boards.straightdope.com/u/Dogface)\ **Post date:** [August 21, 2003, 12:54pm UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/14 "2003-08-21T12:54:07Z")
Just to mess with your heads, electrons are also neither particles nor waves. In a sense, they’re “localized probabilities”. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [August 21, 2003, 1:08pm UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/15 "2003-08-21T13:08:54Z")
> [@](#): > > \*Originally posted by MC Master of Ceremonies \* > \*\*Light does slow down in mediums like the atmospehere bu this is to do with the absorbtion and re-emission of photons by electrons in the air molecules. \*\* No, that is most definitely _not_ the cause. Absorption and re-emission results in the _reflection_ of the light. You need to look a lot deeper than that for the cause of light slowing in a medium. Maxwell handles it easily by dealing with it as an E-M wave phenomonon. QED is much messier, involving absortion and re-emission of _virtual_ photons, not actual photons. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [August 21, 2003, 2:44pm UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/16 "2003-08-21T14:44:24Z")
A medium that is transparent to a given frequency range of electromagnetic radiation doesn’t have any energy transitions that correspond to the frequencies of the incident light. But photons can still interact with the medium via virtual interactions in which an atom-photon energy state is formed. Since these states are not “allowed” the atom must immediately reemit the light, but these interactions still take _some_ time and therefore the light is both slowed and refracted. ---
**Author:** ![Planet\_of\_the\_Shapes](https://avatars.discourse-cdn.com/v4/letter/p/b77776/32.png) [@Planet\_of\_the\_Shapes](https://boards.straightdope.com/u/Planet_of_the_Shapes)\ **Post date:** [August 21, 2003, 2:58pm UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-constant/196554/17 "2003-08-21T14:58:49Z")
I remember a news story a while ago about some guy who had managed to slow light down until it stopped completely. Here is the story: [http://www.newscientist.com/news/news.jsp?id=ns9999340](http://www.newscientist.com/news/news.jsp?id=ns9999340) So are you saying that actually light hasn’t been stopped, it just looks like it has, as individual photons are still travelling at c? I don’t suppose you could try to explain in laymans terms how that works, or does it depend on horribly complicated maths?