Why Astronauts left a reflector on the Moon! (ft. MinutePhysics) - Smarter Every Day 73

Hey, it's me, Destin. Welcome back to Smarter Every Day! So, did you know that in the 60s and 70s, when the astronauts went to the moon in the Saturn V rocket, they actually left experiments on the surface of the moon? In fact, there's one that's still there today and it's still operating. We're still getting data out of it!

But before I explain that, I kind of need to go to the bathroom. All right, here. I went to grad school; we shared this building with NASA, which is interesting because every time I go wash my hands after using the bathroom, I think about the moon! Check this out: on one wall, we have a mirror, and on the other wall, we have a mirror, and they're lined up.

Now, all this makes for some pretty interesting fun stuff, but what I want to talk about today is right here. Look at that! No matter where you go in the room, you always see the camera lens right in the corner of the mirror. This has pretty serious implications for NASA science on the moon. Now, it's so important that I've asked Henry from MinutePhysics to explain it to you.

Let's check it out! A corner reflector like this one is an ingenious and beautifully simple device. It takes advantage of the fact that a mirror mirrors one dimension of the world. If you have a second mirror, you can reverse back to how you started. But if the second mirror is at 90 degrees to the first, now, when you mirror back, the world will have rotated 180 degrees.

So, if you send a beam of light into a corner reflector, regardless of what direction you send it in or what direction the corner reflector is oriented, it'll come back rotated 180 degrees into a parallel return path to its source. If you look here at the top of the camera, you can see that my name is mirrored, but if I bring it over here to the side in the corner, look at that! You can read it.

You can probably see a little bit better with my shirt; can't read it? And now you can! That's because you're seeing a reflection of a reflection. But look at this: if I look back at the corner and then I tilt the camera up, I'm no longer aligned. I can't see the lens anymore. Now that tells me that I'm not in a true return path like you tried to explain to me.

That's because those mirrors are only reflecting in the two horizontal directions. There's no mirror reflecting back to you in the vertical direction. If you have a third mirror at a right angle, everything will be mirrored, reversed, and mirrored again, so your name will be backwards. But at least now you can move your camera or eyes around however you want, and as long as you can still see all three mirrors, you'll see a reflection of yourself in the middle.

Mathematically, if you want to reverse a vector, you need to reverse each of its components, so you need three mirrors to get full retroreflection in 3D—one mirror for each dimension. Oh, this is just like the reflectors on the road! If you look real close at this one that my wife hit with a lawnmower, you can see that they're actually just little bitty corner reflectors.

Exactly! But those are imperfect on purpose because a high precision corner reflector would bounce the light back to your car's headlights and not your eyes. Oh, well, I just happen to have an array of high-precision corner reflectors here with me in Germany. And what's that? It's Johan, the laser guy from the Netherlands! You remember him, right?

This corner reflector is so precise, I can set it here on this bridge, go all the way upriver to the next bridge, hit it with a laser, and I'll still get a reflection even though it's snowing. Check it out! What are you doing, Johan?

I'm warming up my laser.

All right, and why are you doing that?

Together, the right temperature because it's very temperature sensitive.

Okay. All right, can you ping the reflector?

There it is! Clearly, you're getting your return. That's what—like five hundred meters away, would you say?

Three hundred fifty.

How do you know that?

I counted the deficit.

Okay, so here's the question: if Johan is able to get a really good reflection at 350 meters away in the snow with a handheld laser that he's having to blow on to keep people alive, then what are we able to do if we put a high precision laser reflector on the moon? In fact, that's what Apollo did!

What was it? Apollo... what?

Apollo 11, 14, and 15. They put retroreflectors on the moon and we're still using it for experiments to this day, right? Experiments like monitoring the distance to the moon. If you count how long it takes a photon to get to the moon and back, you can just multiply by the speed of light, divided by two, and figure out the distance to the moon in centimeters!

Exactly, Henry! So if you shoot the moon with the laser, two point five six seconds later, divided by T, one point two eight light seconds to the moon. So if you do the math, that ends up to about this far from the earth.

Anyway, this is useful for understanding the detailed nuances of the moon's rotation and orbit, measuring movement of tectonic plates here on Earth, and even testing the fundamental principles of general relativity. Corner reflectors are not just used in optics; you can also use it for any type of reflected electromagnetic radiation.

Here in Frankfurt, Germany, there's a bridge with long posts coming off the big pylons. There, under the bridge, we've got a big boat about to go under the bridge. If you look at the very end, there are corner reflectors. Only, they're not mirrors; they're just metal. He's using a radar to guide off of those corner reflectors so he doesn't hit the bridge. It's basically a passive lighthouse—only it's not light, it's radar!

Cool! So corner reflectors are like a lighthouse that tells you its distance, except better because they don't need their own source of power. That's why they were a good experiment to leave on the moon.

Actually, the Apollo astronauts left other junk on the moon. Come over to my channel, MinutePhysics, and I'll tell you all about it! That's how the Apollo guys went to the bathroom in the command module. How'd they do it on the lunar lander?

Do you realize that there might be... there might be poop on the moon?

I didn't think about that! We always talked about the other stuff they sent. Did they leave their poop on the moon?

I imagine they must have because, like, why would you bring that back? When I go hiking, I try to find the perfect spot. That guy has won that game! That's like the most distant portage on the planet.

I'm just looking at the other lunar landers on the moon. [Music]