# What happens once you reach light speed? **URL:** \ **Category:** Factual Questions\ **Created:** [November 20, 2003, 7:33pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351 "2003-11-20T19:33:39Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Greenhead](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/greenhead/32/5853_2.png) [@Greenhead](https://boards.straightdope.com/u/Greenhead)\ **Post date:** [November 20, 2003, 7:33pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/1 "2003-11-20T19:33:39Z")
I was talking with a friend the other day and somehow we got on the subject of time travel, light speed, and so on. Well, some would say that nothing can travel faster than the speed of light, but I remember reading an article a couple years ago stating someone had actually accomplished breaking the light speed barrier where a laser left the inside of an object filled with some sort of gas or ions or something (please forgive my vagueness) before it entered. My friend claimed it was complete balderdash and refused to believe me. I asked him: “If you were going the speed of light in a car what would happen if you turned on the headlights?” He could not answer, and I have no idea. Any ideas on the matter? ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [November 20, 2003, 7:51pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/2 "2003-11-20T19:51:20Z")
Within an inertial frame light travels at c. So if you’re traveling at .9999c inside your frame of reference (no acceleration) your headlights should travel at c. Outside of your frame a completely different state would be observed. If _you_ traveled at c then the universe would collapse on top of you and the question becomes moot. 🙂 ---
**Author:** ![Jpeg\_Jones](https://avatars.discourse-cdn.com/v4/letter/j/4bbf92/32.png) [@Jpeg\_Jones](https://boards.straightdope.com/u/Jpeg_Jones)\ **Post date:** [November 20, 2003, 7:55pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/3 "2003-11-20T19:55:35Z")
[Pulse of laser light through cesium vapor.](http://www.cbsnews.com/stories/2000/07/19/tech/main216905.shtml) [Is it possible to go faster than the speed of light?](http://www.physics.adelaide.edu.au/~dkoks/Faq/Relativity/SpeedOfLight/FTL.html) [What happens if I turn on my headlights while travelling at the speed of light?](http://www.physics.adelaide.edu.au/~dkoks/Faq/Relativity/SpeedOfLight/headlights.html) ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [November 20, 2003, 8:16pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/4 "2003-11-20T20:16:49Z")
> [@](#): > > \*Originally posted by Greenhead \* > **If you were going the speed of light…** If I started out at rest, and then I accelerated until my speed = c, wouldn’t I have infinite mass? ---
**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:** [November 20, 2003, 8:34pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/5 "2003-11-20T20:34:25Z")
Your friend is right on both counts. There have been many reports of experiments where light supposedly going faster than the speed of light, but they’re all, to put it bluntly, balderdash. Or more politely, the results are misinterpreted, either by the scientists conducting the experiments or by the journalists reporting them (in the first such result, the conclusion made by the scientists was not that light was travelling faster than c, but that a standard method for measuring the speed of a pulse was flawed). And your friend is also right in not being able to answer the question about driving at the speed of light and turning on your headlights. It’s a meaningless question. If you’re only driving at _close_ to the speed of light, then the answer is that the light travels away from you at the speed of light, and it also travels away from someone on the ground at the speed of light. But this breaks down if you try to apply it to something going _at_ the speed of light. In a sense, you can’t even have a speed that’s close to the speed of light: No matter how fast you go, the speed of light is still just as far away. ---
**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:** [November 20, 2003, 10:44pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/6 "2003-11-20T22:44:32Z")
You know, if people would just go to the library and keep up with recent issues of _New Scientist_ half of GQ would vanish. 🙂 In the 18 October 2003 issue is the article “Speed freaks,” by Valerie Johnson. She looks at a new study of these “faster-than-light” results which demonstrates seemingly conclusively that no information ever goes faster than light does. ---
**Author:** ![akrako1](https://avatars.discourse-cdn.com/v4/letter/a/c77e96/32.png) [@akrako1](https://boards.straightdope.com/u/akrako1)\ **Post date:** [November 20, 2003, 11:47pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/7 "2003-11-20T23:47:55Z")
How I understand it, time slows down as you approach the speed of light. Time will basically stop as you reach the speed of light, making it impossible to burn that last drop of fuel needed to accelerate to the speed of light. If you did get accelerated to the speed of light (possibly by an external object - if that’s possible), then you would not be able to turn on your headlights because time would stand still. ---
**Author:** ![WereOtter](https://avatars.discourse-cdn.com/v4/letter/w/ac91a4/32.png) [@WereOtter](https://boards.straightdope.com/u/WereOtter)\ **Post date:** [November 20, 2003, 11:59pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/8 "2003-11-20T23:59:52Z")
Re: the behavior of headlights when you’re travelling at 0.99c, the funny thing is, we see this happen every day. There are valid reference frames in which it appears that our galaxy is travelling at 0.99c. This means that we ourselves are being measured as travelling at 0.99c at this very moment. Does it feel like it to us? No, because we make our measurements from our own reference frame and everything looks normal to us, including the behavior of our headlights. But to this other observer, it appears that we are being subjected to relativistic effects such as increased mass, shorter length, slower time flow and doppler effects. The observer will see that our headlight beams are doppler shifted in frequency, but that the light from the headlights is only travelling at c, even though the car is travelling at 0.99c. And if you get in a car accident because you were thinking about this instead of paying attention to your driving, don’t try to blame me! ---
**Author:** ![asterion](https://avatars.discourse-cdn.com/v4/letter/a/4da419/32.png) [@asterion](https://boards.straightdope.com/u/asterion)\ **Post date:** [November 21, 2003, 1:46am UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/9 "2003-11-21T01:46:03Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\*In a sense, you can’t even have a speed that’s close to the speed of light: No matter how fast you go, the speed of light is still just as far away. \*\* True, but does it really matter? I would think this is one of those cases where the difference between _c_, 0.9_c_, and 0.99_c_ would have little meaning, much like 0.999…is equal to one. ---
**Author:** ![Truth\_Seeker](https://avatars.discourse-cdn.com/v4/letter/t/5fc32e/32.png) [@Truth\_Seeker](https://boards.straightdope.com/u/Truth_Seeker)\ **Post date:** [November 21, 2003, 2:16am UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/10 "2003-11-21T02:16:36Z")
In the interest of strict correctness (not to mention of sowing confusion ;)), I’d like to point out that c is not actually a constant. [Scharnhorst Effect](http://www.nat.vu.nl/~scharnh/paper16.htm) ---
**Author:** ![Yumblie](https://avatars.discourse-cdn.com/v4/letter/y/76d3ee/32.png) [@Yumblie](https://boards.straightdope.com/u/Yumblie)\ **Post date:** [November 21, 2003, 8:30am UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/11 "2003-11-21T08:30:59Z")
> [@](#): > > Re: the behavior of headlights when you’re travelling at 0.99c, the funny thing is, we see this happen every day. There are valid reference frames in which it appears that our galaxy is travelling at 0.99c. This means that we ourselves are being measured as travelling at 0.99c at this very moment. Does it feel like it to us? No, because we make our measurements from our own reference frame and everything looks normal to us, including the behavior of our headlights. But to this other observer, it appears that we are being subjected to relativistic effects such as increased mass, shorter length, slower time flow and doppler effects. The observer will see that our headlight beams are doppler shifted in frequency, but that the light from the headlights is only travelling at c, even though the car is travelling at 0.99c. Actually, I think a better way of putting it is that the galaxies aren’t going light speed, but that the space around them is expanding so it appears that they are. Really far away galaxies that we can’t see are actually moving away from us faster than light, which doesn’t make sense, but it’s the space between us that’s pushing us apart, so it doesn’t violate relativity. Also, if you were to drive a car at the speed of light, you better have insurance, because the friction with the atmosphere would cause a _huge_ explosion, not to mention vaporize you and anything within a few mile radius. ---
**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:** [November 21, 2003, 12:51pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/12 "2003-11-21T12:51:19Z")
> [@](#): > > _Originally posted by asterion \* > \*\*True, but does it really matter? I would think this is one of those cases where the difference between c, 0.9_c\*, and 0.99_c_ would have little meaning, much like 0.999…is equal to one. \*\* No it does matter as c is the limit, it is impossible to travel at this speed full stop. As **Wereotter** otter says there exist valid reference frames where you are currently travelling very close to the speed of light right at this moment, but there exists no valid rest frames where you are travelling at the speed of light (I would say that travelling at c is the degenrate case, but it’s not as it’s impossible). > [@](#): > > \*Originally posted by Truth Seeker \* > **In the interest of strict correctness (not to mention of sowing confusion ), I’d like to point out that c is not actually a constant.** Unless you buy into variable speed of light theories (where the value of c changes as the universe evolves), then c is always constant infact it’s often called Einstein’s constant. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [November 21, 2003, 2:50pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/13 "2003-11-21T14:50:45Z")
> [@](#): > > If you did get accelerated to the speed of light (possibly by an external object - if that’s possible) No it’s not possible. The amount of energy it takes to accelerate you goes up as your speed gets closer to c, and it would take an infinite amount of energy to reach c. ---
**Author:** ![BMalion](https://avatars.discourse-cdn.com/v4/letter/b/e274bd/32.png) [@BMalion](https://boards.straightdope.com/u/BMalion)\ **Post date:** [November 21, 2003, 6:40pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/14 "2003-11-21T18:40:03Z")
> [@](#): > > \*Originally posted by Quercus \* > \*\*…and it would take an infinite amount of energy to reach c. \*\* And an infinite amount of monkeys to type up the report for the _New Scientist_. 😃 ---
**Author:** ![Truth\_Seeker](https://avatars.discourse-cdn.com/v4/letter/t/5fc32e/32.png) [@Truth\_Seeker](https://boards.straightdope.com/u/Truth_Seeker)\ **Post date:** [November 21, 2003, 9:38pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/15 "2003-11-21T21:38:41Z")
> [@](#): > > \*Originally posted by MC Master of Ceremonies \* > \*\*Unless you buy into variable speed of light theories (where the value of c changes as the universe evolves), then c is always constant infact it’s often called Einstein’s constant. \*\* Well, to quote a phrase, you can put puppies in the oven, but that don’t make 'em biscuits. The fairly surprising fact is that QED does predict that c is not actually a constant but depends on the structure of the vacuum. Here’s a pretty readable summary from Nature [http://www.nat.vu.nl/~scharnh/m16natur.htm](http://www.nat.vu.nl/~scharnh/m16natur.htm) Discoveries like this are often the thin edge of the wedge. 1 part in 10[sup]36[/sup] doesn’t sound like much, but it’s an infinite amount more that c is _supposed_ to vary. > [@](#): > > \*\*but there exists no valid rest frames where you are travelling at the speed of light (I would say that travelling at c is the degenrate case, but it’s not as it’s impossible). \*\* This is sort of true and sort of not. Since this “acceleration” is caused by the expansion of space (in other words, it’s not really acceleration), it is possible, in theory, to have galaxies receeding from us at speeds in excess of c. I say “in theory” because these galaxies would be unobservable and, in a very real sense, permanently “outside” our universe. ---
**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:** [November 21, 2003, 10:11pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/16 "2003-11-21T22:11:18Z")
No you misunderstanfd whatever the observd velocity of light (in general relativity light can be observed to be travelling at slower speeds in a vacuum than c) c is still a constant (you’ll notice on that site they are not claiming that the value of c varies but that light can travel faster than c). c is a fundamental constant, the speed of light in a vacuum is just one of the quantites it represents. Really I was just speaking of special relativity, recession velocites are not quite the same as what we normally think of as a velocity as they are due to the expansion of space. Infact we do see far away galaxies with recession velocites greater than c in the observable universe. ---
**Author:** ![Cervaise](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cervaise/32/16693_2.png) [@Cervaise](https://boards.straightdope.com/u/Cervaise)\ **Post date:** [November 21, 2003, 10:34pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/17 "2003-11-21T22:34:51Z")
Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light. Total protonic reversal. ---
**Author:** ![g8rguy](https://avatars.discourse-cdn.com/v4/letter/g/7cd45c/32.png) [@g8rguy](https://boards.straightdope.com/u/g8rguy)\ **Post date:** [November 21, 2003, 11:45pm UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/18 "2003-11-21T23:45:45Z")
:), **Cervaise**. ---
**Author:** ![asterion](https://avatars.discourse-cdn.com/v4/letter/a/4da419/32.png) [@asterion](https://boards.straightdope.com/u/asterion)\ **Post date:** [November 22, 2003, 1:30am UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/19 "2003-11-22T01:30:22Z")
> [@](#): > > \*Originally posted by MC Master of Ceremonies \* > \*\*No it does matter as c is the limit, it is impossible to travel at this speed full stop. As **Wereotter** otter says there exist valid reference frames where you are currently travelling very close to the speed of light right at this moment, but there exists no valid rest frames where you are travelling at the speed of light (I would say that travelling at c is the degenrate case, but it’s not as it’s impossible). > \*\* That didn’t come across quite as I meant it to. What I meant was thinking in practical terms. Say you want to go somewhere 10 light-years away. If you travel at 0.9c, it’ll take you slightly over 11 years (11.111 etc, according to my calculator, if I’m doing it right). 0.99c will take 10.101…years. 0.999c will take 10.0101…years. For all practical purposes _in terms of human travel_, isn’t 0.999 (or even 0.99 or 0.9) close enough to make us happy, if lightspeed can not be broken some way? Especially with time-dilation for the people traveling? ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [November 22, 2003, 6:51am UTC](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351/20 "2003-11-22T06:51:06Z")
The folks at Rand Mc Nally will ask _YOU_ for directions. [Next page](https://boards.straightdope.com/t/what-happens-once-you-reach-light-speed/214351.md?page=2)