# If you were traveling at the speed of light would you be able to see someones headlights behind you?

Asked by J0E (13121) September 17th, 2009

Say your moving at the speed of light and there is another vehicle going at the speed of light behind you, and it turns on its lights, will you see the lights?

Observing members: 0 Composing members: 0

The speed of light is constant from every reference frame.

Ivan (13429)

@Ivan So you’re saying I wouldn’t be able to see the light because it wouldn’t be able to catch up with me?

J0E (13121)

Yes because of the constant state or inertia ( or at lease the same idea applies). throw a base ball out of a truck and it will go forward still.

kibaxcheza (1078)

In this case, it’s even the same as sound!

If I’m traveling at Mach 1, and someone behind me is too, and they honk at me—I don’t hear anything!

grumpyfish (6618)

Headlights? What sort of car are they driving? I want one.

DarkScribe (15450)

@J0E

No, I’m saying that no matter where you are or how fast you’re moving, light always appears to be traveling at the speed of light relative to you.

Ivan (13429)

@DarkScribe The same kind you have, all vehicles have headlights…

J0E (13121)

@J0E The same kind you have, all vehicles have headlights…

You don’t think that at the speed of light they might be redundant? I mean, if you actually could see something, what’s your stopping or turning distance?

DarkScribe (15450)

@DarkScribe I’m so confused… Headlights are just lights on the front of a vehicle, I don’t understand what you’re talking about.

J0E (13121)

I don’t really know what DarkScribe is saying, but for one, it requires an infinite amount of energy for matter to travel at the speed of light, so it’s impossible. Secondly, if you consider time dilation, going at the speed of light would mean that time would appear to stop for you, so you could essentially travel anywhere in the universe instantly.

Ivan (13429)

In short, to follow up on Ivan’s comment:

If you were going 99% the speed of light, with a car behind you at 99%, you would see the headlines turn on.

However, if you happen to figure out a way to travel AT the speed of light (thought experiment), you would not see the headlights for the reasons that @Ivan spelled out.

grumpyfish (6618)

Well obviously this wasn’t a realistic scenario, just saw the question the other day and I thought you would be able to see the light but others didn’t, I knew Fluther would come through for me.

J0E (13121)

@grumpyfish

I don’t think that’s correct. Because the speed of light is constant in all reference frames, the light still appears to be traveling at the speed of light relative to you. Time dilation allows for this. So I think that you would be able to see the light. We start our unit on Relativity next week, so I’ll be able to give you a more detailed answer then.

Ivan (13429)

@J0E I don’t understand what you’re talking about.

Seriously? Headlights are to enable you to see in the dark, to avoid crashing, running off course etc. If you are traveling at the speed of light, how much time do you really think that you would have to react to anything you managed to see? By the time your brain had registered what you had seen you would already be tens of thousands of miles past whatever it was. What be the point of headlights at that speed?

Not to worry, as they wouldn’t work anyway. Light speed is finite, regardless of the initial speed of the source.

DarkScribe (15450)

You would see the light, but the light would be from a different reference in time.
That is if the headlight went on before you got to the speed of light.

Are we talking about a light traveling with you or at a set point and you are accelerating away from it?

CMaz (26203)

I’d be too busy watching the road ahead of me and worrying about my dry cleaning bill.

mrentropy (17168)

A particle with mass cannot attain the speed of light. Period. However, inside the event horizon of a black hole, space is falling faster than light, so you’d see some interesting things – if you could possibly avoid being torn apart by tidal forces, which you could not.

When you run forwards or backwards in the aisle of a train going 60 m/p/h, your speed is dependent on your energy and not added or subtracted from that of the train.

Yet, if you fall off the train, its energy goes with you….generating a very nasty fall.

gailcalled (54403)

@DarkScribe You’re reading way to much into it, it’s just a made up scenario.

J0E (13121)

@J0E You’re reading way to much into it, it’s just a made up scenario.

You are responding far too seriously – I am just kidding you. I really didn’t think that you would be driving at the speed of light. (Well, not without the appropriate licences etc.)

DarkScribe (15450)

If there was only a way to add audible inflection.

Inflection that did not move at the speed of light.

CMaz (26203)

@DarkScribe I thought you were taking it too serious, but it’s cool, I see what you were doing.

J0E (13121)

Lurve for a great question.

Dog (24691)

It depends if both you and the vehicle are travelling in the same medium. Light travels at a different speed through matter than it does through a vacuum.

AstroChuck (37012)

If you’re traveling at the speed of light, the Lorentz-Fitzgerald contraction would make all distances parallel to the direction you’re travelling diminish to nothing. So there would be no “behind” in that direction.

When you talk about light, and about travelling close to the speed of light, the answers are very different from what you’d expect in Newtonian physics.

cwilbur (14169)

no you won’t see them because you and the light have the same speed so the distance between you and the light from behind will be constant.

Christian95 (3258)

@cwilbur If you’re traveling at the speed of light, the Lorentz-Fitzgerald contraction would make all distances parallel to the direction you’re travelling diminish to nothing. So there would be no “behind” in that direction.

Shhhh!

If Oprah hears you she’ll be up there traveling at the speed of light in order to have no behind.

DarkScribe (15450)

There are scenarios where this might work. Suppose you’re an unlucky photon and you’re not traveling in a vacuum. From time to time you hit atoms forcing you to transmute into an excited electron, though the excitement doesn’t last very long and you get your original form back and off you go, again at the speed of light. Your sister photon from the headlights behind you is more lucky missing the atoms and catching up with you so you can eventually “see” it. For example it takes photons tens of thousands of years to reach the sun’s surface. They are always traveling at the speed of light (as long as they are not in a state of excited electrons).

mattbrowne (31537)

@DarkScribe: It would take an infinite amount of energy to get Oprah to travel at the speed of light; other ways of decreasing her behind are likely to be less costly.

cwilbur (14169)

or