If you are traveling at the speed of light and you turn a flashlight on, what happens?
Even to this day, when I think about Einsteinien Special Relativity, I am amazed, both at how a single man could come up with such a theorem, and also the purely bizarre nature of it. Fortunately, Special Relativity is the answer to this question.
Before even thinking about answering the question, we must throw out all of our common sense. It is very dangerous if we extrapolate our common sense logic to science, so as of right now, we know nothing.
Our common sense would tell us that turning on a flashlight moving at light speed would not work, since the photons cannot escape the bulb because it is already traveling at c, or the speed of light (300 000 000 meters/second, or 186 000 miles/second). Unfortunately, this is not the case.
That’s why common sense is a waste, people.
To begin with, this is a purely hypothetical scenario, since objects with mass can’t travel at light speed. Only massless particles, like photons, can. It’s okay though, because we can still imagine what would happen.
First of all, according to Special Relativity, time will stop for the flashlight since it is moving at c (I’ll use c for light speed from now on, since it’s less tedious to write), a phenomenon called time dilation.
Also, the flashlight would shrink, a phenomenon called length contraction or Lorentz contraction.
Why does the universe slow down time and shrink objects as they get faster? Well, that’s a mystery that we’ll probably never know. But we do know what happens because of it. So, what’s the answer?
The flashlight will still project a light beam at 300 000 000 meters/s, or 186 000 miles/s relative to any and all frames of reference.
What?!
Yes, it’s true. If you turned that flashlight on anywhere in the universe (assuming that it has batteries and is fully functioning), it’s beam of light will still travel at the same speed as anywhere else in the universe.
It makes sense too, if you think about it. Answer me this:
How fast are you moving right now?
*If you are a scientist, I understand with Galilean Relativity, we must specify what we are moving relative to, but I’m using the colloquial sense of the question, so don’t hurt me. But to be specific, the light will travel at c (300 000 000 meters/second) relative to all frames of reference.
Well, let’s see:
- Earth is rotating on its axis.
- Earth is orbiting the sun
- The sun is orbiting Sagittarius A, the black hole at the center of the Milky Way galaxy
- The Milky Way galaxy is an insignificant constituent of the gargantuan number of galaxies that orbit some point within the Virgo Supercluster.
- We have not explored and know everything there is to know, so the Virgo Supercluster may even be orbiting other supermassive things.
Wow! You’d think that with how incomprehensibly fast you are currently moving right now, that light on planet Earth would be shockingly slow.
But it’s not.
So that means that time on Earth moves slower than that of Sagittarius A
*If you are a scientist, we are not taking General Relativity into account. I am aware that time would slow near black holes but it would be very inconvenient including it in this example. After all, it is a hypothetical scenario that we’re talking about.
In conclusion, turning on a flashlight moving at the speed of light, would not be possible. However, moving to the hypothetical realm, the headlights would project a beam of light that travels at the same exact speed that it would project on planet Earth, or anywhere else in the universe.
Light’s speed never changes.