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Why No One Has Measured The Speed Of Light
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- This video was sponsored by KiwiCo.
More about KiwiCo at the end of the show.
I know what you're thinking.
- Clickbait!
- No one has measured the speed of light, that's ridiculous.
The speed of light is exactly 299,792,458 meters per second.
We are so sure of it that since 1983,
we've actually used the speed of light
to define how long a meter is.
It's just the distance light travels in a vacuum
in 1/299,792,458ths of a second.
That definition ensures that the speed of light
is exactly this number, no decimals.
But hear me out, in this video,
I will prove to you
that light may never actually travel at this speed
and I can say that because no one has actually measured it.
We can't measure the speed of light
the same way we measure the speed of anything else.
Destin: I think we're recording everywhere.
What are we doing?
- This is a video about measuring the speed of stuff.
Destin: OK
Tell me about how you measured the speed of the baseball
fired out of your cannon.
- Well, to get the speed of the baseball,
you need to know two things.
You need to know the distance between two points
and you need to know the time that it takes
the baseball to travel between those points.
So basically we took distance divided by time,
and that's the speed of the baseball.
And in our case, we were shooting with a high-speed camera,
so you basically just count the frames
and then your clock is internal.
Oh, you're going relativity.
You're gonna do something weird, aren't you?
- You saw it coming.
I can't believe it.
Destin: Oh man!
The thing I want to ask you about is the speed of light.
Destin: OK
Could you measure the speed of light like this?
Imagine you have a laser that can fire a beam
through a perfect vacuum for one kilometer.
Start a timer the instant you fire the laser beam
and then exactly when it hits the end, stop the clock.
Except how do you know when light reaches one kilometer
if you and the clock are at the starting point?
Okay, so you need two clocks, one at the laser
and one at the end, which stops automatically
when it detects the laser light.
But now, how do you make sure
your two clocks are synchronized?
Well, you could connect them via a wire
and send a pulse from one to the other,
but that pulse will travel at the speed of light,
so it will arrive with a time delay.
You might think you can just subtract that time delay
but it is equal to the time it takes
for light to travel one kilometer.
That's what we don't know and are trying to measure.
Okay, new plan, start with the clocks together
and sync them up first
and then send one of the clocks down to the end.
Now, what could possibly go wrong?
Well, I'll tell you.
The clock at the finish line was moving
with respect to the one at the start
and special relativity tells us moving clocks ticks slow
relative to stationary observers.
So by the time the clock reaches one kilometer,
it will no longer be in sync with the clock at the start.
Can I tell you the only solution to this problem?
Ditch the second clock.
Put a mirror at the end to reflect the light back
and use a single clock at the start
to time the full two kilometer round trip.
- Wasn't this actually done before?
He was on a mountain and there's a wagon wheel
with a lantern and there's something like a mirror
on the other side of the mountain?
I've always wanted to do this.
- So, that sounds a little
like how the speed of light
was first experimentally measured
by Hippolyte Fizeau in 1849.
He shone a beam of light between the teeth
of a rapidly spinning gear to a mirror
up on a hill eight kilometers away.
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