# Isn't light speed faster than c? **URL:** \ **Category:** Factual Questions\ **Created:** [September 10, 2011, 2:10pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821 "2011-09-10T14:10:26Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![Chipacabra](https://avatars.discourse-cdn.com/v4/letter/c/e19b73/32.png) [@Chipacabra](https://boards.straightdope.com/u/Chipacabra)\ **Post date:** [September 11, 2011, 12:13am UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/21 "2011-09-11T00:13:34Z")
> [@Skald\_the\_Rhymer](#): > > In that case, why didn’t you say 186,282 (and change) miles per second)? Do the last 282 (and change) miles per second mean nothing to you? I find it’s simpler to use a slightly larger mile, so it comes out to exactly 186,000. Adjust your odometers accordingly. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [September 13, 2011, 6:00pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/22 "2011-09-13T18:00:24Z")
> [@Chipacabra](#): > > I find it’s simpler to use a slightly larger mile, so it comes out to exactly 186,000. Adjust your odometers accordingly. I actually often wondered as a kid why they didn’t define a meter as being exactly 1/300 000th of the speed light goes in a second. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [September 13, 2011, 6:12pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/23 "2011-09-13T18:12:04Z")
> [@BigT](#): > > I actually often wondered as a kid why they didn’t define a meter as being exactly 1/300 000th of the speed light goes in a second. The meter is defined in reference to the earth, not to the speed of light. It originally was one ten-millionth of the distance from the Earth’s equator to the North Pole but has been refined since then. ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [September 13, 2011, 6:19pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/24 "2011-09-13T18:19:32Z")
> [@Telemark](#): > > The meter is defined in reference to the earth, not to the speed of light. It originally was one ten-millionth of the distance from the Earth’s equator to the North Pole but has been refined since then. The meter [_is_ defined](http://lamar.colostate.edu/~hillger/unit-definitions.html#meter) in reference to _c_, not the Earth (any longer). ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [September 13, 2011, 6:28pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/25 "2011-09-13T18:28:16Z")
> [@Kinthalis](#): > > Hopefully someone will correct me if I’m wrong, but when we say light is a “wave” we mean that it can be described using the mathematical model of a wave. Even more precisely, I’d say that a mathematical description of light is in some ways like the math for an idealized perfect wave (and is in other ways like the math for an idealized perfect point particle (and in other ways like neither)). Don’t know if that’s any clearer, though. > [@AaronX](#): > > The speed of a wave is not measured by “squiggles”, any more than the speed of sound depends on the speed of air molecules. Actually, the speed of sound _does_ depend on the speed of air molecules. That’s really what sound is: air molecules banging into other molecules next to them, which knocks those molecules through space to bang into more molecules a little farther along, etc. [Light of course is very different from sound] ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [September 13, 2011, 6:41pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/26 "2011-09-13T18:41:02Z")
Maybe the light isn’t traveling in a wave. Maybe space is bent. _tokes_ ---
**Author:** ![drewtwo99](https://avatars.discourse-cdn.com/v4/letter/d/d07c76/32.png) [@drewtwo99](https://boards.straightdope.com/u/drewtwo99)\ **Post date:** [September 13, 2011, 6:46pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/27 "2011-09-13T18:46:23Z")
Nothing can travel faster than c! Farnsworth: Of course not. That’s why scientists increased the speed of light in 2208 ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [September 13, 2011, 8:22pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/28 "2011-09-13T20:22:25Z")
> [@Kinthalis](#): > > Hopefully someone will correct me if I’m wrong, but when we say light is a “wave” we mean that it can be described using the mathematical model of a wave. > > It’s not _actually_ a bunch of squiggly lines undulating about. The double-slit experiment would beg to differ. When you do the experiment the resulting picture on the screen behind the double-slits is exactly what you would expect of waves and not particles (you get an interference pattern). However, if you mess with the experiment in certain ways (e.g. try to determine which slit the photon passes through) the waveform collapses and the light behaves as a particle and the results on the screen will reflect that (no interference pattern). ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [September 13, 2011, 10:15pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/29 "2011-09-13T22:15:44Z")
> [@Bytegeist](#): > > The meter [_is_ defined](http://lamar.colostate.edu/~hillger/unit-definitions.html#meter) in reference to _c_, not the Earth (any longer). That’s true, I was imprecise. The original definition of the meter was based on the size of the earth. And that is the answer to the question of why it wasn’t chosen to be a round number in relation to light speed which is what I was answering. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [September 14, 2011, 3:43am UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/30 "2011-09-14T03:43:20Z")
> [@Quercus](#): > > Actually, the speed of sound _does_ depend on the speed of air molecules. That’s really what sound is: air molecules banging into other molecules next to them, which knocks those molecules through space to bang into more molecules a little farther along, etc. [Light of course is very different from sound] Ok it does. I meant “equal”. ---
**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:** [September 14, 2011, 8:57pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/31 "2011-09-14T20:57:33Z")
> [@](#): > > The double-slit experiment would beg to differ. > > When you do the experiment the resulting picture on the screen behind the double-slits is exactly what you would expect of waves and not particles (you get an interference pattern). Light _really is_ a wave (among other things). It _really is not_, however, a squiggly line. ---
**Author:** ![ModernPrimate](https://avatars.discourse-cdn.com/v4/letter/m/85e7bf/32.png) [@ModernPrimate](https://boards.straightdope.com/u/ModernPrimate)\ **Post date:** [September 14, 2011, 9:27pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/32 "2011-09-14T21:27:47Z")
> [@Whack-a-Mole](#): > > The double-slit experiment would beg to differ. > > When you do the experiment the resulting picture on the screen behind the double-slits is exactly what you would expect of waves and not particles (you get an interference pattern). It doesn’t “beg to differ”. The light is still travelling in a straight line, it’s just been interfered with along the interference pattern, cancelled out by the other light that happens to have its inverse wave intensity along those points. The squiggly bits don’t really move, they are just there with the light. If you measured them, obviously yes they are longer than the unidirectional light. But they are not a “thing” that’s travelling, you could consider them an electromagnetic field given off by the light. It’s the same with all electromagnetic waves. ---
**Author:** ![DHMO](https://avatars.discourse-cdn.com/v4/letter/d/ecc23a/32.png) [@DHMO](https://boards.straightdope.com/u/DHMO)\ **Post date:** [September 15, 2011, 7:21pm UTC](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821/33 "2011-09-15T19:21:00Z")
> [@CanTak3](#): > > After reading my own subject, I know how insane that sounds. But here’s my thought: Light behaves as a wave, and as a particle. We know the speed as measured from point A to point B is _c_. However, the light didn’t take the shortest route! It went up and down (or side to side) in its wave form. So didn’t it travel farther than we measured, and hence travel faster than _c_? > > I know I’m not understanding quantum mechanics correctly. I’ll just wait to see why! Light (or, more generally, electromagnetic radiation,) as it propagates from point A to point B at a speed of _c_, _ **does** _, in fact, travel in a straight line. The method of propagation involves Maxwell’s equations relating electric fields and magnetic fields. When a stationary electric field is formed, the field, itself, will [diverge](http://en.wikipedia.org/wiki/Divergence). Any charged particle will be affected by the field, in direct proportion to the strength of the field at that point. The rate at which the electric field diverges is proportional to the[permittivity of free space](http://en.wikipedia.org/wiki/Permittivity). Nothing physical is actually _moving_ as the electric field grows. A consequence of a changing electric field is that it causes a magnetic field. The magnetic field grows and diminishes as a direct consequence of the change in the strength of the electric field. The rate of change in the strength of the magnetic field is proportional to the [permeability of free space](http://en.wikipedia.org/wiki/Permeability_%28electromagnetism%29). The _existence_ of a changing electric field causes a magnetic field, and the _existence_ of a changing magnetic field causes an electric field. It turns out that the rate of change of these fields causes a wave-like change in field strength to propagate in a direction at right angles to the field, at a velocity determined by the values of the permittivity and the permeability of free space. This velocity turns out to be equal to the [speed of light](http://en.wikipedia.org/wiki/Speed_of_light)in a vacuum. Note carefully that nothing physical is moving in a transverse sine wave. The amplitude of the wave-like change in field strength may be sinusoidal, but no physical particle is moving “faster than light” along that path. [Previous page](https://boards.straightdope.com/t/isnt-light-speed-faster-than-c/595821.md?page=1)