when the astronauts went to the moon in the Saturn 5 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. [Toilet Flush]

Alright. Here where 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.

(Henry) 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. (Destin) 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 it a little bit better with my shirt.

Can't read it.. 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 any more. Now that tells me that I'm not

in a true return path like he tried to explain to me. (Henry) 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,

and so you need three mirrors to get full retroreflection in 3D.

One mirror for each dimension. (Destin) Oh. This is just like the reflectors on the road.

If you look real close at this one that my wife hit with the lawn mower you can see that they're actually

just little bitty corner reflectors. (Henry) 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. (Destin) Ohh! 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 up river 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?!

(Johan) I'm warming up my laser. (Destin) [laughs] Alright.

And ah.. Why are you doing that? - To get it to the right temperature.

Because it's very temperature sensitive. - Oh OK. Alright can you ping

the reflector? There it is. Clearly you're

getting a return. That's what? Like 500 meters away would you say? - 350

- How do you know that? - I counted steps. [laugh]

- OK so here's the question. If Johan is able to get a really good reflection

at 350 metres away, in the snow, with a hand-held laser

that he's having to blow on to keep 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? - 11, 14 and 15.

- Apollo 11, 14 and 15. They put retro-reflectors on the moon

and we're still using it for experiments to this day.

(Henry) 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, divide by two

and figure out the distance to the moon, within centimeters. (Destin) Exactly Henry.

So if you shoot the moon with a laser...

2.56 seconds later, divided by two, 1.28 light

seconds to the moon. So if you do the math that ends up to about... this far from the Earth.

(Henry) 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. (Destin) 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. So if you look at the very end, there's 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.

(Henry) 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 too.

Come over to my channel, MinutePhysics, and I’ll tell you all about it.

[Music]

That's how the Apollo guys went to the bathroom, in the command module.

How'd they do it on the lunar module? (Henry) Imagine it's fairly similar.

They probably left it on the moon though. Do you realise there might be.. - There might be poop on the moon. We gotta think this through! [laugh]

I didn't even think about that. We always talk about the other

stuff that they sent. (Henry) Did they leave their poop on the moon? [laughs]

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. [laugh]

(Henry) The most distant porta-john. [laugh] The most distant porta-john is the

on the lunar landers on the moon.

[ Captions by Andrew Jackson ] captionsbyandrew.wordpress.com

Captioning in different languages welcome. Please contact Destin if you can help.