# What happens to a spaceship, travaling at the speed of light turns on there headlight **URL:** \ **Category:** Factual Questions\ **Created:** [December 27, 2000, 4:16pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386 "2000-12-27T16:16:25Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![Dubland](https://avatars.discourse-cdn.com/v4/letter/d/2bfe46/32.png) [@Dubland](https://boards.straightdope.com/u/Dubland)\ **Post date:** [December 27, 2000, 4:16pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/1 "2000-12-27T16:16:25Z")
What happens when a space ship traveling away from you at the speed of light turns on the headlights? I Would Like to know this… ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [December 27, 2000, 4:18pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/2 "2000-12-27T16:18:26Z")
Here’s a good article on the question… [http://math.ucr.edu/home/baez/physics/headlights.html](http://math.ucr.edu/home/baez/physics/headlights.html) ---
**Author:** ![manhattan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manhattan/32/7_2.png) [@manhattan](https://boards.straightdope.com/u/manhattan)\ **Post date:** [December 27, 2000, 4:19pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/3 "2000-12-27T16:19:22Z")
**Dub** , I think you are going to want to spend some time searching the archives of this forum and of **Cecil** ’s columns before you ask further questions. Really. ---
**Author:** ![August\_West](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/august_west/32/18069_2.png) [@August\_West](https://boards.straightdope.com/u/August_West)\ **Post date:** [December 27, 2000, 7:22pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/4 "2000-12-27T19:22:26Z")
> [@](#): > > \*\*What happens when a space ship traveling away from you at the speed of light turns on the headlights? \*\* The person inside the spaceship flips the switch for the headlights, allowing electricity to flow to the headlight bulbs. The bulbs contain a Tungsten filament that, when heated by the current, produces light. Bingo, the headlights are on! That was easy! ---
**Author:** ![darik](https://avatars.discourse-cdn.com/v4/letter/d/3be4f8/32.png) [@darik](https://boards.straightdope.com/u/darik)\ **Post date:** [December 27, 2000, 9:01pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/5 "2000-12-27T21:01:25Z")
> [@](#): > > The person inside the spaceship flips the switch for the headlights, allowing electricity to flow to the headlight bulbs. The bulbs contain a Tungsten filament that, when heated by the current, produces light. Bingo, the headlights are on! > > That was easy! This hypothetical ship can go faster than light, and there’s a _switch?_ C’mon, we’re fighting ignorance here. The pilot says, “Lights” and **then** electricity flows to the headlights, etc. ---
**Author:** ![QuantumGrid](https://avatars.discourse-cdn.com/v4/letter/q/c6cbf5/32.png) [@QuantumGrid](https://boards.straightdope.com/u/QuantumGrid)\ **Post date:** [December 27, 2000, 9:30pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/6 "2000-12-27T21:30:34Z")
I was under the impression the human flying the space ship would have evolved to develop psychic powers. So… He thinks, “Gosh, it sure is dark out there.” Mental computer senses his brain waves and boom, turns on the lights. Which it doesn’t want to do anyway because it is flying itself and doesn’t want to waste its own power on something silly like lights. Then it asks the pilot Dave if he wants to play a nice game of chess… ---
**Author:** ![AVSC916](https://avatars.discourse-cdn.com/v4/letter/a/f0a364/32.png) [@AVSC916](https://boards.straightdope.com/u/AVSC916)\ **Post date:** [December 27, 2000, 9:34pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/7 "2000-12-27T21:34:35Z")
I my opinion, and I actually have given it much thought, but not being a mathematical genius nor knowing Quantum physics, I figured that the bean emitted from the headlight would travel at lightx2, since it is being emitted from a source already up to the speed of light. Nobody favoring Einstein agrees with me. I keep telling them to go test it out for themselves, which gives them headaches. It is similar to a question asking how fast is the Earth moving in space in comparison to something at an absolute stop. There actually is an answer, but I disagree because no one has been able to find anything at an absolute stop in the universe. We are in motion, the solar system is in motion, the galaxy is moving through space and our attendant galaxies are also moving. We can determine our relative speed in comparison to the other galaxies, but without being able to find a reference point at an absolute stop, in my opinion, we cannot determine the speed of the known universe. Spatial theorists ignore me, but then, they keep saying they can see the edges of the universe, when in actuality they are seeing only to the distance which our current technology allows. Kind of comparing the first telescope’s range with that of the Hubbell. I keep disagreeing. They don’t let me in their offices anymore. Ignoring all of what you might see traveling at the speed of light when you turn on your headlight and what light turns into in comparison, I still say your headlight beam will go twice the speed of light. Like being in a train traveling at 30 mph, and firing off a bullet along the cars that travels at a maximum of 100 mph, total velocity in relation to anyone standing still alongside the track would be 130 mph. Want me to explain why a fly traveling at a normal speed of 4 mph can, when buzzing around the inside of a car doing 60 mph, is actually doing 64 mph in relation to stationary outside objects? No. I didn’t think so. ---
**Author:** ![JeffB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeffb/32/188_2.png) [@JeffB](https://boards.straightdope.com/u/JeffB)\ **Post date:** [December 27, 2000, 9:38pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/8 "2000-12-27T21:38:58Z")
Yeah, but what happens if you switch to high beams? ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [December 27, 2000, 9:49pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/9 "2000-12-27T21:49:37Z")
And would your tail lights glow in the infrared? ---
**Author:** ![Rysdad](https://avatars.discourse-cdn.com/v4/letter/r/7cd45c/32.png) [@Rysdad](https://boards.straightdope.com/u/Rysdad)\ **Post date:** [December 27, 2000, 10:54pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/10 "2000-12-27T22:54:39Z")
> [@](#): > > \*Originally posted by AVSC916 \* > \*\*I figured that the bean emitted from the headlight would travel at lightx2… \*\* That’s one freakin’ fast _frijole!_ ---
**Author:** ![Skelji](https://avatars.discourse-cdn.com/v4/letter/s/b38774/32.png) [@Skelji](https://boards.straightdope.com/u/Skelji)\ **Post date:** [December 27, 2000, 11:38pm UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/11 "2000-12-27T23:38:34Z")
If the ship were travelling _away_ from you, would this be from a standstill, or as it passes you? If it were from a standstill, I’m guessing the backwash would vaporize you. If it were passing you, that would be some sonic boom you’d hear, huh? Now, if you’re the one travelling at the speed of light, or - let’s be real here…_faster_ than the speed of light (you’re flying that fast and you’re going to pay attention to the speed limit? Ha!) - then you would be in some type of time-shift-warp-continuum deal, right? Everything that has ever happened in time up until that point will be visible before you. The frequency of the headlights would be such that a flicker effect was produced, illuminating the images with that old-timey projector feel. As darik and QuantumGrid pointed out, you would only need to think or say “Popcorn”, then kick back and enjoy. ---
**Author:** ![manhattan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manhattan/32/7_2.png) [@manhattan](https://boards.straightdope.com/u/manhattan)\ **Post date:** [December 28, 2000, 12:02am UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/12 "2000-12-28T00:02:46Z")
> [@](#): > > \*Originally posted by Smeghead \* > \*\*And would your tail lights glow in the infrared? \*\* Saves a lot of money, that does. When a tail light cover gets broken, you don’t have to cough up $300 to the dealer for a replacement. Just let the light shine and let the redshift do the work for you. Reverse lights, though, are another problem entirely. ---
**Author:** ![handy](https://avatars.discourse-cdn.com/v4/letter/h/b5a626/32.png) [@handy](https://boards.straightdope.com/u/handy)\ **Post date:** [December 28, 2000, 12:04am UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/13 "2000-12-28T00:04:20Z")
More to the point, why would anyone want to turn on your lights if you are going that fast? duh ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [December 28, 2000, 12:37am UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/14 "2000-12-28T00:37:47Z")
No, no, no. All wrong. The equation for adding veolocities is: ```auto v' + v r v = -------------- 1 + v' v / c^2 r ``` where v’ = the speed of the man on the spacecraft. v\_r = the speed of the spacecraft. v = the speed of the man as seen from outside the spacecraft. c = the speed of light in a vacuum (186,282.3970512 miles per second). So using that equation, if v’ = 186,282.3970512 miles per second (the speed of the photons coming out of the headlight on the spacecraft) and v\_r = 186,282.3970512 miles per second then v = 186,282.3970512 miles per second Ah, work it out for yourself using a calculator. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [December 28, 2000, 1:18am UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/15 "2000-12-28T01:18:40Z")
basically, the headlights will ALWAYS travel at the speed of light no matter where your going and how fast. If you are on the spaceship the headlights will travel at the speed of light, on the space station, the headlights of the passing starship will travel at the speed of light, in another spaceship passing the 1st spaceship, the headlights of the 1st spaceship will be traveling at the speed of light. Time and distance will change for each observer to allow this. ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [December 28, 2000, 1:21am UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/16 "2000-12-28T01:21:06Z")
And, BTW, in case anyone thinks Special Relativity has not been proven, all you have to do is look at the Global Positioning System. If Einstein were wrong, it wouldn’t be able to pinpoint a location. So the answer to the question is that any spaceship made out of matter can’t be traveling at the speed of light, since there isn’t enough energy in the universe to get it going that fast. The question is meaningless. ---
**Author:** ![MrWhy](https://avatars.discourse-cdn.com/v4/letter/m/9f8e36/32.png) [@MrWhy](https://boards.straightdope.com/u/MrWhy)\ **Post date:** [December 28, 2000, 2:03am UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/17 "2000-12-28T02:03:05Z")
> [@](#): > > \*Originally posted by QuantumGrid \* > \*\* > He thinks, “Gosh, it sure is dark out there.” > \*\* But he would have to think it _in Russian_. 😃 ---
**Author:** ![nineiron](https://avatars.discourse-cdn.com/v4/letter/n/e56c9b/32.png) [@nineiron](https://boards.straightdope.com/u/nineiron)\ **Post date:** [December 28, 2000, 3:48am UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/18 "2000-12-28T03:48:42Z")
Hmm, I forget which comedian this 10-year-old line is lifted from. Maybe Stephen Wright? ---
**Author:** ![Upham](https://avatars.discourse-cdn.com/v4/letter/u/e9bcb4/32.png) [@Upham](https://boards.straightdope.com/u/Upham)\ **Post date:** [December 28, 2000, 4:39am UTC](https://boards.straightdope.com/t/what-happens-to-a-spaceship-travaling-at-the-speed-of-light-turns-on-there-headlight/47386/19 "2000-12-28T04:39:23Z")
What would happen? Well, there’d be a sudden, piercing flash of light, only for a second. Then, a massive explosion, twice as large as the Big Bang herself (i like to call her Jen). The stars would become dark, Space would become heavy and everything of mass would re construct it’s self into one gigantic, universal sign saying “NO HIGH BEAMS” And that’s how come Santa doesn’t carry a flashlight anymore Upham