what happens if you are travelling at the speed of light in a car and then you put the headlights on?:dubious:
I believe this question has been posed on the SDMB before. If I recall correctly, the answer was twofold:
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The headlights wouldn’t create beams.
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You wouldn’t be able to tell the difference, because at the speed of light, all mass is pure energy.
Anyone wish to correct me?
FoamFighter
Welcome to the boards!
This is an old question that’s been covered here plenty I’m sure (as well as in most 1st year physics courses), but here goes.
Well, you can’t travel in a car at the speed of light, but the answer you’re looking for is that the light from the headlights would travel away from you at the speed of light–just like they would if you were standing still.
You cannot travel at the speed of light. It would take an infinite amount of energy to do so. But assuming you were going even very close to the speed of light, from your point of view the light would travel at c away from your car. You might think that a stationary observer would observe the light from the headlights travelling at close to 2c, but from this perspective, light still only travels at c. To account for these differences we must invoke relativity, which is a complicated subject that I’m sure someone more knowledgeable than I can expand upon.
This one’s easy. Cars can’t go the speed of light. Only light (and stuff like light) can go the speed of light.
You can’t travel at the speed of light, it’s an invalid refernce frame full-stop.
The sum of the velocites in relativity is:
w = (u + v)/(1 + uv/c[sup]2[/sup])
So when u and v are c so is w, but you still can’t attach much meaning to a sum that describes a physically impossible situation.
Cecil knows everything.
So, what if you were going a substantial fraction of the speed of light? Well, you can’t tell the difference between traveling at a constant speed and sitting still. Not even in principle. As long as you are not accelerating*, there are no `closed-rooom’ experiments you can perform that would allow you to know the velocity you are moving at.
(*Here, acceleration means changing velocity. It can involve speeding up, slowing down, or turning.)
Why? All reference frames are equally valid. If it works going fifty-five miles per hour, it will work going nine-tenths of the speed of light in a vacuum and it will work standing stock still. Besides, velocity is a relative thing: You can only meaningfully measure it relative to another point. You can pick an arbitrary point and call that stationary, and as long as it doesn’t accelerate everthing will work out fine.
So, given the foregoing, what can you expect? Well, it will look the exact same. You will measure its velocity and you will not notice any difference from the time you measured its velocity sitting on a planet (accounting for the slowdown light experiences when it goes through an atmosphere, of course).
In fact, this can be used to explain how time slows down for those moving at speed. Imagine you have a clock built of one mirror, a laser, and a laser pulse detector. It works like this: The laser on the floor pulses briefly, the laser bounces off the ceiling and comes down to the floor, and you measure the time between when you pulse the laser and you detect the reflected pulse. Since the speed of light in a vacuum is constant, this is a rather good clock design: Your time units are of a constant length.
Now, imagine someone looking in from the outside. Relative to you, he is standing still. He sees the pulses travel from the laser up to the mirror and then down to the detector, but he sees them travel in diagonal lines to follow the movement of your craft: In each fraction of a time unit, your ship and everything in it moves forward a fraction of a unit of distance. The pulse moves to compensate, extending its path both on the way up and the way down. Since the speed of light in a vacuum is constant, the outside observer measures your time pulses as being slower than his pulses. That is, his clock pulses more often than your clock does. Remember: All frames of reference are equally valid. Therefore, you measure time more slowly than he does, so time passes more slowly for you than for him. That is what’s called time dilation.
A question for the OP. How do you know that you are not, right now, travelling at 99.9999% the speed of light?
Well, what happens when you turn on your lights at 65 mph?
Surely the headlight beams are not going at C + 65 mph.
I always toss that out to make sure everyone understands that relativity is closer to our daily lives than most think.
What about “according to General Relativity, any reference frame is physically allowed, including rotating ones, but if we use the Earth as a reference frame, then we have the nearest star rotating around the Earth in 23 hr 56min–surely it would be going faster than the speed of light?” Good question for Cecil.
The answer is that if the nearest star were rotating around the Earth that fast, spacetime would be warped so much that the speed of light in that part of spacetime would be much greater than c.
Exactly. You might as well start out with “Suppose a snail had headlights…”
Nothing to add really, except for an old joke on the subject for those who may not have seen it…
Top 20 Cool Things about a car that goes faster than c.
20 Sleep 'til noon. Still get to work by 8:00am!
19 Doppler shift makes red traffic lights look green.
18 Breaking laws of physics only a misdemeanor in most states.
17 Never in car long enough to hear an entire Madonna song.
16 Carl Sagan and Stephen Hawking keep bugging you to carpool.
15 No one can see you pick your nose while you drive.
14 Lunch breaks in Paris, circa 1792.
13 LA to Vegas in 2 nanoseconds.
12 You can stop worrying about being sucked into a black hole driving home from work.
11 You’ll be so thin while driving it you can even wear horizontal stripes.
10 That deer in your headlights is actually behind you.
9 Kid from Mentos commercial almost guaranteed to lose a limb if he tries to duck through back seat.
8 Traffic enforcement limited to cops with PhD’s in Quantum Physics.
7 Bugs never see you comin’.
6 You can get to the good hookers before Charlie Sheen.
5 Can make a fortune delivering pizza with the slogan “It’s there before you order or it’s free!”
4 Car makes it from Hollywood to London fast enough to not arouse suspicions of Elizabeth Hurley.
3 License plate: “Me=mc2”
2 Cigarette butts don’t land in the backseat – they land in last week!
And the Number 1 Cool Thing About a Car that Goes Faster than the Speed of Light…
1 Chicks dig it.
Contributed by: Aaron Walters
I know it’s a joke (and funny too) but I have to point out that light only goes one foot in a nanosecond. 
Depends on whether you’re parked in your driveway or driving on a parkway.
how was i supposed to know the answer to that, im only 14 and i won’t bother posting anymore messages if all anyones going to tell me is you can’t travel at the speed of light, because i knew that already.
I can’t find the site, but I read this bizzare theory that light actually travels at infinite speed, and that it only seems finite because locations removed from us are “distant” from us in time as well as space. In other words, it postulated that there’s a minimum non-zero difference in reference frames dependent on distance, even for observers with zero relative velocity. I don’t know enough about relativity to decide if it’s BS or not.
But you should understand that the fact that you can’t exceed c makes it impossible to talk about the possibility that you can.
You’d have to break the laws of physics to drive at the speed of light, so how are you to know what would happen under those now-broken laws?
Using what we know about the universe, but applyting it to a scenario in which you’ve broken one of the most fundamental underpinnings, is meaningless.
The problem here is that the relativity transform equations suggest that as a car approaches the speed of light, it gains in mass, eventually collapsing into a black hole, and pulling the rest of the Universe in around it.
Other than that minor difficulty…
Sorry, but I don’t think it works like that. Whether something is a black hole is a frame-invariant thing, and since you can always find a frame where you’re going 0.999999c, everything would be a black hole.
(I’m still not quite sure why it doesn’t work like that, though…)