# if traveling in a car at the speed of light, and you turn on your headlights, what happens?

### 32 Answers

- dawgdaysLv 73 years ago
While a car can't go the speed of light, we'll treat this as a thought experiment in special relativity.

Let's imagine the person in the car has a meter stick pointing in the direction of travel, and is also holding a clock.

The person in the car would see the light streaming away from the car at the speed of light. (This is because light always travels the speed of light in any inertial reference frame.) That person would also see a stick that is one meter long, and will see the clock ticking normally.

An external observer that has the car passing by at the speed of light would see the following. The light from the headlights is traveling at the speed of light, even though it's the same light seen in a different reference frame. However, the car is also traveling at the speed of light, so the observer "sees" light that never leaves the headlights. Meanwhile, he observer sees a meter stick that has zero length, and the clock is at a standstill. This is a result of length contraction and time dilation, in other words, space and time adjusting to keep the light traveling at the same speed, even when it is seen from different reference frames.

And here's an example if the car is moving at half the speed of light.

Again, the person in the car sees the same as above.

Now, the observer sees the car passing at half the speed of light, a meter stick that is 0.866 meters in length, and a clock running at one tick every 1.155 seconds.

The conversions come from the application of the Lorentz factor, which is sqrt(1 - (v^2/c^2)).

[I don't get why people offer an answer if they don't understand this classic thought experiment in special relativity.]

- AmyLv 73 years ago
A lot of people are saying "cars (mass) cannot travel at the speed of light," and that's true. But the bigger problem is that time doesn't pass and space doesn't exist while you're traveling at the speed of light.

Speed flattens your perception of the universe. At the speed of light, spacetime is two-dimensional: you are traveling zero distance in zero time. (the dimensions other than the two you are moving in still exist, but without time you can't learn anything about them.)

At any other speed, everything seems normal. When you turn on your headlights the light travels away from the car, bounces off something, and returns to you at the speed of light.

What do you mean by flatten? Is that literal or figurative? Is this why obects in lightyear distances appear to be in the past?

- 3 years ago
I'm guessing not a lot. You're already reached the speed limit. The light from the headlights can't travel faster than the car.

- JackolanternLv 73 years ago
It depends on other objects in that vicinity that it will act upon. Now you have an object traveling at the speed of light with objects on board that produce light that travels at the speed of light faster than you. No one knows for sure what it will do as related to other objects. But if you had no other object around and you were the only thing in the universe, then it would be the same as standing still with your lights turned on. You have to have at least 2 objects in the universe to produce time and distance. The universe is a giant time machine. The only place time stands still is at the entrance of a 'black hole'.

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- 3 years ago
The light and the car would move at a constant speed in relation to each other.

You would overtake said light.

Light would just glow around bulbs.

Nothing would happen at all.

The entirety of the universe would collapse.

Cthulu will rise from the ocean.

You would get blinded and serious car sickness.

Pointless using them- won't be able to see out and will crash into something causing kaboomy on whatever you hit.

- TroyLv 63 years ago
Two objects in space can be closing at faster than the speed of light. However, neither can see the other because of the speed limit of light.

the cars on each of the two objects can see their immediate surroundings just as if they were on a stationary object.

Assuming objects are not on a collision course, then just as the closing velocity reduces to that of the speed of light, each car will see a brilliant flash of light.

Thereafter, each car can see the others headlights but the electromagnetic frequency will be much reduced ( I e below the visible range) and the cars will appear to be traveling backwards along the path from which they came.

- Mr. Un-couthLv 73 years ago
The car's speed increases because the car's net weight decreased when the light beam exited the headlights.

Hope this helps.

- Captor of sinLv 43 years ago
You'll see where you're going dumbass. That's what headlights do. But not for long. It takes a lot of energy to go the speed of ligh so you'll have to go the speed of light to every gas station or you'll run out of gas. Also you need to communicate with others like me because I don't want to be driving my car at the speed of light and run into some dumbass that doesn't have his lights on

- Andrew SmithLv 73 years ago
So many times the same question.

I will repeat. "The definition of madness is asking the same question repeatedly expecting that THIS TIME the answer will be different".

a) there is no such thing as " travelling at the speed of light" Speeds are not absolute but only relative. (Einstein special relativity)

b) the speed of light relative to you is always c. So if you were "Travelling at the speed of light" then you would be no closer to the speed of light than you are at present. ( Michelson Morley at al )

c) Related to b) if you accelerate by adding energy then you are no closer to the speed of light.

If you add more energy you are still no closer to the speed of light.

So it takes an infinite amount of energy and you have still not gained on bit upon the light.

Hence it is impossible to reach the speed of light.( Lorentz transforms )

But if you were ALMOST at the speed of light relative to the earth, the speed of light is still c and when you turned on the headlights it would do exactly what it does at any other speed in the vacuum of space.

The light moves forwards and, without hitting anything, moves off into the inky blackness giving absolutely no indication that the lights were on at all. (Isaac Newton "Optiks" )

Photons are all at the speed of light. No matter how much or how little energy they have. Damage is caused if each one has sufficient energy to change a cell. Otherwise they simply reflect or pass through.

- BigBadSteveLv 53 years ago
According to the laws of relativity, the car could not reach the speed of light.

So take the size of the universe. Apply the lorentz conversion.

L' = L * sqrt( 1-(v^2/c^2) ) =L* sqrt( 1 - (c/c)^2) =l* sqrt( 1-1) = l* sqrt( 0)=0

T' = T * sqrt( 1/( v^2/c^2) = T* sqrt( 0) = 0

For any length L and time T. No time, no distance = no universe.

Hmmmm.