What Would Happen If You Drove Your Car Close To The Speed Of Light And Turned On The Headlights

Though it may seem like an unusual question, what would happen if you were driving your car at nearly the speed of light and you turned on your headlights. It’s not likely something that will ever happen, but what if? 

Light, travels at the speed of light, obviously. But what if you were to turn on a light that is already traveling close to the speed of light? What then? Does it speed up? Does it travel slower? Or, does it stay the same? 

We want to answer this question for you. 

What happens if you turn on your headlights, in a car traveling close to the speed of light?

What would happen if you drove your car close to the speed of light and turned on the headlights

What is the speed of light?

The speed of light is 299,792,458 m/s. 

The speed of light in a vacuum is commonly denoted ‘c’. It is a universal physical constant that is important in many areas of physics.

Before we get into the niggling facts of this question, let’s give you an idea of what the speed of light is. We already know that it is 299,792,458 m/s, this is approximately 1,080,000,000 kilometers per hour, or 671,000,000 miles per hour. 

Light travels one foot in 1.0 nanosecond, and one meter in 3.3 nanoseconds. So, it’s pretty fast.

Nothing manmade can travel at the speed of light, and it’s probably a good thing, as the speed of light can affect the physical properties of an object. This is where we turn to talk about what Einstein had to say about this. 

Einsteins Theory of Relativity

In 1905, Albert Einstein determined that the laws of physics were the same for all non-accelerating observers, that includes us, and that the speed of light in a vacuum was independent of the motion of all observers, again, that includes us.

This was the theory of special relativity. 

There are two main concepts of his theory, first that the natural world allows no ‘privileged’ frames of reference. As long as an object is moving in a straight line at a constant speed with no acceleration, the laws of physics are the same for all.

Much like when you are in a car and you see an adjacent car moving at the same speed, are you moving, or is it? He recognized that if motion is uniform it is near impossible to tell. 

His second concept was the speed of light. And, no matter how fast an observer is moving or how fast a light-emitting object is moving, the measurement of the speed of light always yields the same result. In short, the speed of light never changes. 

Traveling near the speed of light 

So, what if you had a super-duper-magic car, and you got in and drove nearly at the speed of light? 

If you were in a car traveling at the speed of light, you would also be traveling at the speed of light. This may seem strange when you think about it, but for you traveling at this speed, time would be passing more slowly relative to those not in the car with you.

A good way to think of this would be if there was an observer on the side of the road watching you in your car going past. 

For a start though, driving any vehicle at the speed of light is pretty much impossible. As objects get faster they gain mass. So accelerating faster demands more energy as the mass increases.

To hit this speed an object would need infinite energy.

However, if this seems like a weird concept, it is also worth noting that if you were in a car and you reached c (speed of light) then while in motion a few weird things would happen; time would stop, under the circumstances of the speed of light, you couldn’t do anything else, turn up the high beams, whatever. 

But… are things the same just below the speed of light, at perhaps 98% the speed of light?

If you turned on your headlights 

To answer the last question, yes you can. Of course, the headlights would still work.

Just below the speed of light, the light is still traveling at c, and you are just below c, meaning that the light is just faster than you and is still able to race ahead of you and your car. 

An observer outside your car as you travel would measure the light coming from your headlights at c. Even if you are traveling near the speed of light, any light you omit is always traveling at c.

While this may seem confusing and weird, it is a natural phenomenon. 

Time and speed, two things we think of as fixed concepts are both variable at high speeds.

So, if you were to be the observer of a car traveling close to c, then its length in the direction of travel would appear much shorter and the passage of time onboard the car would appear much shorter too. 

This is because time and speed are both relative. Light however is persistently traveling at c. 

At the speed of light, however, you wouldn’t be able to turn on your headlights, time would effectively stop. 

As we have said, traveling near the speed of light makes time slow down if you travel at the speed of light, it stops entirely. The light would match the speed you are traveling at and therefore, your headlights wouldn’t work. 


Light travels at a speed of 299,792,458 m/s, if you were somehow able to drive at that speed you wouldn’t be able to activate your headlights or do anything else.

But if you traveled a bit slower than the speed of light, you could indeed switch on the headlights and get illumination as the light would still be faster than you. 

So, if we do ever end up with vehicles that fast, although very unlikely, if you want to be able to use your high beams, low beams, or fog lights, maybe go a bit slower than a speed of light so you can use them. 

Gordon Watts
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