# Speed of light is hard... **URL:** \ **Category:** Factual Questions\ **Created:** [November 23, 2011, 4:33pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880 "2011-11-23T16:33:50Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [November 23, 2011, 4:33pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/1 "2011-11-23T16:33:50Z")
Ever since the speed of light became the universal speed limit, there has been the counter argument: if I’m taveling at _almost_ the speed of light, and I turn on my flashlight do the photons emit at less than light speed? I know this argument is almost as viable as the airplane on the treamill one, but it escapes me why that’s the case. I guess my question is, is the speed limit relative to a starting point? or some other fixed point in space? or am I really not thinking about it in the right way? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [November 23, 2011, 4:38pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/2 "2011-11-23T16:38:05Z")
> [@Inigo\_Montoya](#): > > Ever since the speed of light became the universal speed limit, there has been the counter argument: if I’m taveling at _almost_ the speed of light, and I turn on my flashlight do the photons emit at less than light speed? I know this argument is almost as viable as the airplane on the treamill one, but it escapes me why that’s the case. > > I guess my question is, is the speed limit relative to a starting point? or some other fixed point in space? or am I really not thinking about it in the right way? The speed limit is the only absolute thing in a universe where everything else is relative. If you drive by me in your car at almost the speed of light, we’ll both measure the photons your flash-light emits as moving at the speed of light. To me that will be because your time is slowed down by special relativity, to you that will be because you have an equal claim that I’m the one moving at almost the speed of light and that I’m the one whose clock is running slow. Special relativity all the way! ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [November 23, 2011, 4:38pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/3 "2011-11-23T16:38:16Z")
In **your** frame of reference, they appear to be traveling at “the speed of light” - 186,000 mi/sec. In a stationary frame of reference, they also appear to be traveling at the same speed. Which is why the speed of light is [invariant](http://en.wikipedia.org/wiki/Invariant_speed). ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [November 23, 2011, 4:46pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/4 "2011-11-23T16:46:09Z")
The point is that the statement, “if A sees B moving at velocity U, and B sees C moving at velocity V, then A sees C moving at velocity U + V”, is true by every intuition we have, but nevertheless is wrong. In actuality, A sees C moving at (U + V)/(1 + U_V/c_c). ---
**Author:** ![Washoe](https://avatars.discourse-cdn.com/v4/letter/w/ecae2f/32.png) [@Washoe](https://boards.straightdope.com/u/Washoe)\ **Post date:** [November 23, 2011, 5:02pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/5 "2011-11-23T17:02:00Z")
> [@beowulff](#): > > Which is why the speed of light is [invariant](http://en.wikipedia.org/wiki/Invariant_speed). > [@wiki](#): > > The invariance of the speed of light is a **consequence of the postulates of special relativity** , and the terms speed of light and invariant speed are often considered synonymous. Bolding mine. I thought this was demonstrated by Maxwell’s equations. Wasn’t this established prior to the publication of Einstein’s paper? 😕 ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 23, 2011, 5:06pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/6 "2011-11-23T17:06:50Z")
> [@Inigo\_Montoya](#): > > Ever since the speed of light became the universal speed limit, there has been the counter argument: if I’m taveling at _almost_ the speed of light, and I turn on my flashlight do the photons emit at less than light speed? I know this argument is almost as viable as the airplane on the treamill one, but it escapes me why that’s the case. > > I guess my question is, is the speed limit relative to a starting point? or some other fixed point in space? or am I really not thinking about it in the right way? It doesn’t matter if you are traveling at .000000000001c or .9999999999999c. The speed of the photons will be c. Think about it. If it\* doesn’t happen at low speeds, what would be the “magic” speed at which it suddenly started happening? \*“It” being the speed of the photon affected by the speed of the source. ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [November 23, 2011, 5:15pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/7 "2011-11-23T17:15:20Z")
> [@Inigo\_Montoya](#): > > Ever since the speed of light became the universal speed limit, there has been the counter argument: if I’m taveling at _almost_ the speed of light, and I turn on my flashlight do the photons emit at less than light speed? I know this argument is almost as viable as the airplane on the treamill one, but it escapes me why that’s the case. It’s not exactly a “counter argument” (it would require decades of experimental proof to gain that status, and there is none, given that it’s not true). It’s a question, though, for sure, and a common one. The answer is that - and this has been proven time and time again both mathematically and in practice for over a hundred years; there’s no dispute that this is the case - the speed of light is the same for all observers whatever speed they’re moving at. This is known as “relativity” and, yes, it’s counter-intuitive, like much of our universe is. It “gets around” that by doing wacky things like actually slowing time down and stuff. The speed of light is the same relative to all observers. ---
**Author:** ![tdn](https://avatars.discourse-cdn.com/v4/letter/t/94ad74/32.png) [@tdn](https://boards.straightdope.com/u/tdn)\ **Post date:** [November 23, 2011, 5:29pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/8 "2011-11-23T17:29:16Z")
I still don’t get it. If **Inigo** is driving past me at nearly the speed of light, am I going to see his high beams all squished just in front of his car? ---
**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\ **Post date:** [November 23, 2011, 5:46pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/9 "2011-11-23T17:46:25Z")
> [@tdn](#): > > I still don’t get it. > > If **Inigo** is driving past me at nearly the speed of light, am I going to see his high beams all squished just in front of his car? I believe the: (1) High beams would be blueshifted considerably (you though HID headlights were annoying? How about X-ray headlights?), (2) **Inigo** ’s entire car would be squished (in the direction of travel) quite a bit due to length contraction, from your perspective. (3) From your perspective (at least for the fraction of a second it takes him to whiz by) **Inigo** will also appear to have time slowed down to a crawl. ---
**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\ **Post date:** [November 23, 2011, 6:01pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/10 "2011-11-23T18:01:53Z")
whoops, nevermind about the X-ray thing. They’d still be blueshifted, but not to that degree. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 23, 2011, 6:20pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/11 "2011-11-23T18:20:54Z")
> [@tdn](#): > > I still don’t get it. > > If **Inigo** is driving past me at nearly the speed of light, am I going to see his high beams all squished just in front of his car? Yes, but the speed of propagation is independent of the wavelength. ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [November 23, 2011, 6:23pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/12 "2011-11-23T18:23:56Z")
I still don’t get it either. Explain it to a four year old. ---
**Author:** ![Punoqllads](https://avatars.discourse-cdn.com/v4/letter/p/d2c977/32.png) [@Punoqllads](https://boards.straightdope.com/u/Punoqllads)\ **Post date:** [November 23, 2011, 6:28pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/13 "2011-11-23T18:28:09Z")
Time passes at different rates and distances are warped for observers moving with a nonzero velocity with respect to each other such that they both observe a given photon move at exactly the speed of light through a vacuum. ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [November 23, 2011, 6:29pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/14 "2011-11-23T18:29:13Z")
> [@Leaffan](#): > > I still don’t get it either. > > Explain it to a four year old. Can’t. Relativity ain’t an easy concept. Some of the popular science books do a good job of it. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 23, 2011, 6:30pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/15 "2011-11-23T18:30:00Z")
> [@Leaffan](#): > > I still don’t get it either. > > Explain it to a four year old. What, exactly, don’t you get? ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [November 23, 2011, 6:52pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/16 "2011-11-23T18:52:06Z")
> [@John\_Mace](#): > > What, exactly, don’t you get? So, I’m parked and light from my headlights is coming out of the bulbs at C. Now I’m traveling at 100 KPH and light is still coming out of my headlights at C, because it can’t travel at C + 100 KPH. Now I’m travelling at 0.5 C and light is still coming out at C. Now I’m travelling at C, therefore no light can be emitted. But I know that’s not correct. Please correct what actually happens. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [November 23, 2011, 6:54pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/17 "2011-11-23T18:54:44Z")
> [@Leaffan](#): > > So, I’m parked and light from my headlights is coming out of the bulbs at C. > > Now I’m traveling at 100 KPH and light is still coming out of my headlights at C, because it can’t travel at C + 100 KPH. > > Now I’m travelling at 0.5 C and light is still coming out at C. > > Now I’m travelling at C, therefore no light can be emitted. > > But I know that’s not correct. > > Please correct what actually happens. You can’t travel at C, but you can get arbitrarily close. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 23, 2011, 6:55pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/18 "2011-11-23T18:55:49Z")
> [@Leaffan](#): > > So, I’m parked and light from my headlights is coming out of the bulbs at C. > > Now I’m traveling at 100 KPH and light is still coming out of my headlights at C, because it can’t travel at C + 100 KPH. > > Now I’m travelling at 0.5 C and light is still coming out at C. > > Now I’m travelling at C, therefore no light can be emitted. > > But I know that’s not correct. > > Please correct what actually happens. Everything is correct except the last part-- you can’t travel at c unless you are light. Are you light? 🙂 ---
**Author:** ![Tom\_Scud](https://avatars.discourse-cdn.com/v4/letter/t/f08c70/32.png) [@Tom\_Scud](https://boards.straightdope.com/u/Tom_Scud)\ **Post date:** [November 23, 2011, 6:57pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/19 "2011-11-23T18:57:57Z")
You can’t actually get to C, because it would require an infinite amount of energy to accelerate you to C, and there isn’t an infinite amount of energy. If you are at .99999999 C, you will still see your light traveling away from you at C. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [November 23, 2011, 6:58pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/20 "2011-11-23T18:58:16Z")
> [@Candyman74](#): > > Some of the popular science books do a good job of it. I recommend [One, Two, Three, Infinity](http://www.amazon.com/One-Two-Three-Infinity-Speculations/dp/0486256642/ref=sr_1_1?s=books&ie=UTF8&qid=1322074521&sr=1-1) by George Gamow. Written 60 years ago, fun and easy to read. Explains relativity to a child (a pretty sophisticated child). Hasn’t lost its relevance, as long as the speed of light hasn’t changed. [Next page](https://boards.straightdope.com/t/speed-of-light-is-hard/603880.md?page=2)