# If you're in a vehicle going the speed of light, what happens when you turn on the he **URL:** \ **Category:** Factual Questions\ **Created:** [February 26, 2006, 6:52am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161 "2006-02-26T06:52:21Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![littlegoetjen](https://avatars.discourse-cdn.com/v4/letter/l/47e85d/32.png) [@littlegoetjen](https://boards.straightdope.com/u/littlegoetjen)\ **Post date:** [February 26, 2006, 6:52am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/1 "2006-02-26T06:52:21Z")
If you’re in a vehicle going the speed of light, what happens when you turn on the headlights? ---
**Author:** ![garygnu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garygnu/32/2924_2.png) [@garygnu](https://boards.straightdope.com/u/garygnu)\ **Post date:** [February 26, 2006, 7:15am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/2 "2006-02-26T07:15:24Z")
So many jokes, so little time. Answer: nothing. ---
**Author:** ![Fish\_Cheer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fish_cheer/32/17179_2.png) [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)\ **Post date:** [February 26, 2006, 7:45am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/3 "2006-02-26T07:45:52Z")
[Old](http://boards.straightdope.com/sdmb/showthread.php?t=52802) thread on subject, somewhat [newer](http://boards.straightdope.com/sdmb/showthread.php?t=246102) thread and some words from the [Master](http://www.straightdope.com/classics/a3_143.html). ---
**Author:** ![ParentalAdvisory](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)\ **Post date:** [February 26, 2006, 10:07am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/4 "2006-02-26T10:07:21Z")
You and the light particles from the headlights travel at the same speed, if you’re traveling at THEE speed of light. Right? ---
**Author:** ![Electronic\_Chaos](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@Electronic\_Chaos](https://boards.straightdope.com/u/Electronic_Chaos)\ **Post date:** [February 26, 2006, 11:15am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/5 "2006-02-26T11:15:30Z")
> [@ParentalAdvisory](#): > > You and the light particles from the headlights travel at the same speed, if you’re traveling at THEE speed of light. Right? Amazingly, no. The speed of light is relative to the observer. While it may look like both are traveling at the same speed to an outside observer, to you (the person in the vehicle), the light is travelling at your speed plus the speed of light. At least, as far as I understand Einstein. ---
**Author:** ![MsMitey](https://avatars.discourse-cdn.com/v4/letter/m/f19dbf/32.png) [@MsMitey](https://boards.straightdope.com/u/MsMitey)\ **Post date:** [February 26, 2006, 1:05pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/6 "2006-02-26T13:05:27Z")
> [@Electronic Chaos](#): > > to you (the person in the vehicle), the light is travelling at your speed plus the speed of light. At least, as far as I understand Einstein. Then I think you misunderstand him a bit. That’s the old Galilean transformation, where you just add velocities, which works fine at speeds well below c. But when you get faster, you have to whip out Lorentz transformations. If you use the relatavistic velocity addition formula, which is (u+v)/(1+uv/c^2), with u being your speed (.999c, in this case), and v being the speed of the headlights ©, then you (effectively) get 2c/2, which is c. Speeds add funny in relativity. ---
**Author:** ![Otanx](https://avatars.discourse-cdn.com/v4/letter/o/c5a1d2/32.png) [@Otanx](https://boards.straightdope.com/u/Otanx)\ **Post date:** [February 26, 2006, 2:18pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/7 "2006-02-26T14:18:14Z")
Forget the headlights. You are missing the important question here. What happens when I turn on the windshield wipers. Which leads to another question. What the $%\* am I doing driving that fast in the rain? -Otanx ---
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [February 26, 2006, 2:26pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/8 "2006-02-26T14:26:15Z")
If you are moving forward at the speed of light, what would you see in the rearview mirror? ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [February 26, 2006, 2:33pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/9 "2006-02-26T14:33:00Z")
> [@Mr. Blue Sky](#): > > If you are moving forward at the speed of light, what would you see in the rearview mirror? The wavering incorporeal husk of the cat you just ran over. Dr. Schrodinger is gonna be plenty miffed! ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [February 26, 2006, 2:39pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/10 "2006-02-26T14:39:02Z")
> [@Otanx](#): > > Which leads to another question. What the $%\* am I doing driving that fast in the rain? It is well known that more crashes happen in rainy conditions than in dry conditions. So the less time you spend driving in the rain the safer you are. The faster you drive, the less time you are driving in the rain. Therefore when it rains you should drive as fast as possible. 😃 ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [February 26, 2006, 3:03pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/11 "2006-02-26T15:03:50Z")
I’ll just add that as I understand it, it’s impossible for a vehicle, (or one made of matter at any rate) to be ‘going at the speed of light.’ It can approach the speed of light, but all kinds of gradual things reach infinity or singularity at that point, including time dilation, (no time passes at all for anything travelling at light speed, so you wouldn’t see anything at all,) foreshortening (squished flat anyone,) and possibly even mass magnification. (So you’d be squished flat and infinitely heavy at the same time.) for that matter, it would require infinite propulsion to reach the speed of light, because the closer you approach it, the more your mass increases, and nearly all the kinetic energy you acquire goes towards increasing your mass instead of increasing velocity. (At normal, non-relativisitic speeds, the more you push the faster you go. At relativistic speeds, you can push and mostly get heavier at about the same speed.) ain’t science funky? ---
**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:** [February 26, 2006, 4:24pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/12 "2006-02-26T16:24:37Z")
> [@Mr. Blue Sky](#): > > If you are moving forward at the speed of light, what would you see in the rearview mirror? This has been answered. [Scientifically](http://www.sff.net/people/rothman/cplus.htm). ---
**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:** [February 26, 2006, 4:49pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/13 "2006-02-26T16:49:41Z")
> [@MsMitey](#): > > Then I think you misunderstand him a bit. That’s the old Galilean transformation, where you just add velocities, which works fine at speeds well below c. But when you get faster, you have to whip out Lorentz transformations. I think he has it right. The speed of light is a constant for _all_ observers. No mater how fast you are going you would measure the speed of light to be the same as anyone else measuring it. Speed is a measurment of distance over time so if the same answer pops out for two different observers going different speeds then the other side of the equation has to adjust itself. Time and distance alter themselves to allow the same answer to pop out. That said **chrisk** is correct. The person travelling at the speed of light wouldn’t see anything when turning on their headlights because they couldn’t turn on their headlights. Actually reaching the speed of light is impossible bbut for the sake of argument if you could time would stop for you. The Universe would end before you got the headlights on. If you are going slightly less than the speed of light and turned on your headlights it would seem just like doing it here on earth at more mundane speeds. To the observer all would seem “normal”. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [February 26, 2006, 6:41pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/14 "2006-02-26T18:41:50Z")
> [@Whack-a-Mole](#): > > I think he has it right. The speed of light is a constant for _all_ observers. No mater how fast you are going you would measure the speed of light to be the same as anyone else measuring it. Speed is a measurment of distance over time so if the same answer pops out for two different observers going different speeds then the other side of the equation has to adjust itself. Time and distance alter themselves to allow the same answer to pop out. > > That said **chrisk** is correct. The person travelling at the speed of light wouldn’t see anything when turning on their headlights because they couldn’t turn on their headlights. Actually reaching the speed of light is impossible bbut for the sake of argument if you could time would stop for you. The Universe would end before you got the headlights on. > > If you are going slightly less than the speed of light and turned on your headlights it would seem just like doing it here on earth at more mundane speeds. To the observer all would seem “normal”. But what if you turn on the high beams? ---
**Author:** ![Fish\_Cheer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fish_cheer/32/17179_2.png) [@Fish\_Cheer](https://boards.straightdope.com/u/Fish_Cheer)\ **Post date:** [February 26, 2006, 6:56pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/15 "2006-02-26T18:56:56Z")
> [@Johnny L.A.](#): > > So the less time you spend driving in the rain the safer you are. The faster you drive, the less time you are driving in the rain. Therefore when it rains you should drive as fast as possible. 😃 For safety and [comfort](http://www.straightdope.com/classics/a3_395.html). ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [February 26, 2006, 7:28pm UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/16 "2006-02-26T19:28:43Z")
> [@Johnny L.A.](#): > > It is well known that more crashes happen in rainy conditions than in dry conditions. So the less time you spend driving in the rain the safer you are. The faster you drive, the less time you are driving in the rain. Therefore when it rains you should drive as fast as possible. 😃 Your logic is impeccible. 😉 As for the OP’s question, let’s provide a more illustrative…er, illustration. Okay, so anyway, as various people have noted, no material object can equal or exceed the speed of light; if it did, it would become a supermassive singularity onto which the rest of the universe would crush. The benefit, of course, is that it would make the question posed to be a moot one (plus it would take care of the “security situation” which has caused me to be at work on a loverly Sunday afternoon), but since that interferes with my lunch plans we won’t go there, at least not until I’ve had a couple of beers. So you’re in a car going, let us say, 99% of **c** (the accepted symbol for speed of light.) Owing to vB code’s inability to cope with simple symbols or LaTex coding I’m just going to refer you to this site for the equations on [Lorentz contraction](http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Eddie_Trochim/Lorentztransform.htm), but suffice it to say that weird things happen when you are moving at this velocity–even weirder than the things that happen in a David Lynch movie, though typically with less blood. [Here](http://www.adamauton.com/warp/)’s a simulator that will show you the magnificent weirdness of what occurs. Anyway, getting away from the math, here’s what happens: When you’re moving faster (relative to another locus…let us say a photon emitted by your supercharged spacecar) time compresses, as do lengths (in your direction of motion), while lengths behind you stretch out. So the light in front of you (and behind you, and beside you) all appears to be moving at **c** , but the rest of the universe is aging rapidly. From an “objective” POV–someone “at rest” relative to you and your photons–you are aging very slowly, and the dash clock is counting seconds as if they are eons. The faster you go, the worse it gets, until your great-great-great-great-great-great-great-great-great-great grandchildren are having kids who don’t even look vaguely like you and sure as hell aren’t going to visit you at the hospital after you slam into a mislaid asteroid. So, if light appears to be moving the same speed regardless of the relative velocity of the observer…WTF? You have to account for differential energies (that is to say, the difference in momentum between an observer chasing his headlights and one content to let them fly away in front of him). How do you cope with this conundrum? You do it by accepting a change in the frequency (or reciprical wavelength) of light; when moving at relativistic speeds the light in front of you becomes (from your POV) a higher frequency, or “blueshifts”. Go fast enough and even visible light will shift into gamma ray range, which would be bad for you unless you are a cartoon superhero. The light behind you becomes “redshifted”; i.e. lower frequency (or longer wavelength), until it redshifts below visible, infrared, radio, and into invisibility. Light to the side makes a starbow, a spectrum of visible colors. That should keep the kids quietly entertained for at least a few minutes. If you could somehow manage to achieve **c** the movement of time, from your POV, would stop; you’d be unable to differentiate between tomorrow and next century. You couldn’t slow down (how’d you measure time rate of change without time?), and of course, you can’t speed up. A photon, like Paris Hilton, has no sense of its past, future, or surroundings; everything in its lifespan occurs as one long, drug-hazed blur between inception and terminus. In a sense, all particles are moving at **c** , but most devote their velocity along the time axis (which is a one-way street AFAWK), while photons move exclusively through space and only shift “sideways” by varying frequency. Trippy shit, eh? Now, let’s go catch _2001_ down at the Arclight, and lie down underneath the screen during the wormhole sequence. I think, with the assistance of a couple ounces of Jamacan Gold, that I might just be able to figure it all out this time. Stranger ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [February 27, 2006, 12:48am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/17 "2006-02-27T00:48:03Z")
> [@Johnny L.A.](#): > > It is well known that more crashes happen in rainy conditions than in dry conditions. So the less time you spend driving in the rain the safer you are. The faster you drive, the less time you are driving in the rain. Therefore when it rains you should drive as fast as possible. 😃 By a strange coincidence that is the very same argument I use in favor of driving drunk, fast. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [February 27, 2006, 2:28am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/18 "2006-02-27T02:28:48Z")
> [@](#): > > if it did, it would become a supermassive singularity onto which the rest of the universe would crush. No that’s not true. Velocity, no matter how high, has nothing to do with creating gravitating mass or the formation of a black hole. (This is one of the main reasons physicists dislike the concept of relativistic mass- it causes too much confusion.) All inertial observers are free to consider themselves at rest no matter how fast they’re moving in someone else’s frame. Gravitomagnetism is the only gravitational effect due to velocity (momentum). ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [February 27, 2006, 4:39am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/19 "2006-02-27T04:39:12Z")
> [@Ring](#): > > Gravitomagnetism is the only gravitational effect due to velocity (momentum). Gravitomagnetism … Never heard of that one. I can almost see how it works, though. You don’t happen to have a link to someplace that explains the math behind it, for someone whose tensor calculus is really rusty? (Or is that too much to hope for?) Just curious … ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [February 27, 2006, 5:33am UTC](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161/20 "2006-02-27T05:33:01Z")
> [@SCSimmons](#): > > Gravitomagnetism … Never heard of that one. I can almost see how it works, though. You don’t happen to have a link to someplace that explains the math behind it, for someone whose tensor calculus is really rusty? (Or is that too much to hope for?) > > Just curious … I’m sorry **SCSimmons** , but, no, I don’t have a link, I know it strictly from textbooks. However it’s very similar to regular magnetism in that it can pretty much be explained as a special relativistic phenomenon. Just substitute a moving line of mass for a moving line of charge and you’ve pretty much got it. (Sort of) [Next page](https://boards.straightdope.com/t/if-youre-in-a-vehicle-going-the-speed-of-light-what-happens-when-you-turn-on-the-he/346161.md?page=2)