What is the speed of light?
The speed of light in a vacuum always travels at exactly 299, 792, 458 meters per second. This is the unit c in equations. This is the case from the moment of its creation, light does not need to accelerate to this speed, it is already going this speed from the moment of creation.
So, what happens when you turn your headlights on?
Just like when you turn your headlights on if you are travelling 30 miles per hour, the light from your headlights will still travel at c, the speed of light.
This will not change no matter how fast you a travelling in your car. If you managed to get your car to travel close to the speed of light, the light from the headlights would simply still travel at the speed of light, c.
How would it look to the person in the car?
Since the light from the headlights is travelling at the speed of light from the moment of creation, this would simply look like the lights were turning on as usual and the light would travel at the speed of light away from the driver and the car itself, just lie it always does.
How would it look to people stood at the side of the road?
Well, if your car is travelling at a speed close to the speed of light, they probably would not see anything.
But if, hypothetically, this person had incredible sight and could detect objects moving at the speed of light, they would also see that the light from the headlights would travel at the speed of light when turned on.
How is this possible?
When a frame of reference, in this case the car, is travelling extremely fast, close to the speed of light, in relation to a rest frame, its time will slow down.
This will only be observable to someone in the rest frame, in this case someone stood at the side of the road. This effect is known as time dilation.
The other effect present known as length contraction occurs when the space of a moving reference frame and the object within that frame are contracted in the way of motion.
Contraction? Does that affect the driver?
This all means that the car will actually contract: the front and back of the car will be squashed. This does not affect the driver because it is not the car that is the only thing affected, it is actually space itself that is being contracted.
The car, therefore, does not experience anything different than the space around it. So, the driver will also not be affected by this.
The way this works is that the driver will see the car as if it is at rest, so it will always look normal. To the driver, the person on the side of the road is the one that is moving.
Meaning that the driver will actually see the person on the side of the road length-contracting as opposed to the car. This is because all of these effects are reliant on relative motion, that is to say movement relative to rest.
How do we know this is the case?
The reason we are so confident that this is the way that the speed of light and acceleration works, is because of Einstein’s theory of Special Relativity.
This is the mainstream science that we have been following for over a century. This is the mathematical formula for length contraction and time dilation and even the speed of light.
The speed of light will always be the same no matter what frame it is being viewed in, this is thanks to the time dilation and the length contraction.
This means that the light coming out of the headlights would look like c to the person standing at the side of the road, and the car would be travelling close to c.
This means that the light would appear to be struggling to get away from the car and would seem almost frozen in relation to the car itself. All this means is that the car is doing well at keeping up with the light being observed from the rest frame.
That’s if the car is nearly travelling at the speed of light, what about if it is travelling at the speed of light?
The answer to this is that an object with mass can never actually travel at the speed of light in a vacuum, c. Since a car does not have zero mass, it cannot reach the speed of light.
On the opposite side of the scale, objects that have no mass will always travel at the speed of light in a vacuum and cannot travel any other speed.
If you wanted to get the car to travel at exactly the speed of light in a vacuum, you would have to input infinite energy, and this is not possible.
Some final notes
There are some practicalities that need to be addressed. Obviously, in real life a car would never get anywhere near the speed of light. This is because there is too much air friction in the atmosphere of earth would inhibit the car from reaching such speeds.
The main reason that the speed of light exists in a vacuum is because there is nothing to inhibit it in a vacuum. This intense air friction would burn up the car before it came near to the speed of light.
The fastest object to travel in earth’s atmosphere was the Stardust sample return capsule. This only reached 0.004% the speed of light at a speed of 28, 000 mph.
This still caused a temperature of 4700-degress Fahrenheit. This means that getting a space ship to travel close to the speed of light would be more feasible in the near vacuum that space is, since there are less obstacles.