Why No One Has Measured The Speed Of Light

- 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 are so sure of it that since 1983, we've 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.

is exactly this number, no decimals. But hear me out, in this video, I will prove to you that light may never travel at this speed and I can say that because no one has measured it. We can't measure the speed of light the same way we measure the speed of anything else.

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

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 we took distance divided by time, and that's the speed of the baseball.

So 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 just count the frames and then your clock is internal. , you're going relativity. You're gonna do something weird, aren't you? - You saw it coming. I can't believe it. Destin: man! The thing I want to ask you about is the speed of light.

Destin: 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

Start a timer the instant you fire the laser beam and then exactly when it hits the end, stop the clock. Except how do 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

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.

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

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

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.

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 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?

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

He shone a beam of light between the teeth of a rapidly spinning gear to a mirror up on a hill eight kilometers away. And then by increasing the speed of the gear, he reached a point where the reflected light passed through the next gap on the gear and so it was observed. So he measured the speed of light to be 313,000 kilometers per second,

So he measured the speed of light to be 313,000 kilometers per second, which is within 5% of the presently accepted value. So someone has measured the speed of light? or have they? What has been measured is the round trip or two-way speed of light, but no one has measured the one-way speed of light.

but no one has measured the one-way speed of light. One thing I'm gonna throw at you. And I'm just gonna come out and tell you. It's like what if the speed of light in this direction is different from the speed of light in this direction? - Then that sounds like a Veritasium video.

- Then that sounds like a Veritasium video. (both laughing) - The question is could you figure it out? The crux of this problem is that the only way people have managed to measure the speed of light is for a round trip. No one's ever managed to measure the speed of light in just one direction. It's possible that the speed of light

to measure the speed of light in just one direction. It's possible that the speed of light is half of 'c' in one direction, and then instantaneous on the return journey. Destin: What?! That's possible. Destin: Are you serious? - Think about communicating with an astronaut stranded on Mars. Let's call him...

- Think about communicating with an astronaut stranded on Mars. Let's call him... Mark. We send out a signal and get a response 20 minutes later. So we imagine our signal takes 10 minutes to get there and the reply takes 10 minutes to come back, but it's possible that our message took all 20 minutes to get there

but it's possible that our message took all 20 minutes to get there and the reply came back instantaneously. There's no way we could tell the difference between these two scenarios. But why would the speed of light be different? Well, it's possible that there is some preferred direction through spacetime.

Well, it's possible that there is some preferred direction through spacetime. , our universe has a lot of symmetries, but there is also some asymmetry. For example, why is there so much matter relative to anti-matter? And physicists have worked out internally consistent theories of physics

And physicists have worked out internally consistent theories of physics in which the speed of light is different forwards and in reverse. The speed of light could vary by just a few percent up to at the extreme, going 'c' over two in one direction and infinitely fast in the other direction. Destin: Okay, so let me figure this out.