 I like to answer these sort of silly or stupid questions that people write in with. So I wanted to start by talking about one of those. When I started answering questions, one of the questions someone sent in was they wanted to know if every person on Earth aimed a laser pointer at the moon, at the same time, would it change color? And this is a very simple question. I liked it a lot. I didn't know what the answer was. So I sat down to try to figure it out. The first thing I realized is that every person on Earth can't point a laser pointer at the moon at the same time, because there's no time when everyone on Earth can see the moon, because people live on different sides of the Earth. So I took a bunch of population maps and worked out that the ideal time for people to try this experiment would be when the moon was somewhere over the Arabian Sea, because at that point you get about five sevenths of the world's population able to see it all at one time, all the way from the edge of Europe around to here and Tokyo. So if you did this, the next question I'd answer was if you point a laser at the pointer at the moon, does the light even get there? And the answer is yes. I decided to use a quarter moon or a half moon so that we could see the effect on the night side and the day side. Some of it stopped by the air, but most of it goes right through to space. And they don't spread out so much that they wouldn't be able to hit the moon. The beam would mostly fall on the moon if you could aim it carefully enough. So let's assume that everyone sort of has a little stand to carefully line it up with. So then I say, OK, how much power is this? Now a standard laser pointer has, for safety reasons, they're generally limited to five milliwatts, although if you actually run tests, I think a lot of them are somewhat more powerful than that, but they all say that they are five milliwatts. So we take the 5 billion people on Earth and give them each a five milliwatt laser pointer, and we point it at the moon. Unfortunately, the answer to that question, would it change color, is no. Even on the night side of the moon, the light wouldn't be bright enough to be visible. So this is kind of disappointing. I liked the question, but the answer was not fun and not interesting. It's just no, the moon would not change color. And so we did a lot of work for nothing there. So then I thought, well, five milliwatts isn't very much. So I went online and googled brightest laser pointer you can buy. And it turns out that there are some companies that make laser pointers that are irresponsibly powerful. I found one for sale. It's about a 1.4 watt laser pointer compared to the five milliwatts. This is bright enough to pop balloons, if you pointed at them. A regular laser pointer can, in theory, damage your retina. And this one could definitely damage your retina. They sell them most places. You can just order them, although I found if you try to order them from the United States, they require you to get a letter from the FDA explaining why you need it. But you could buy them from Canada. So we can just purchase a whole bunch of them from Canada. So if we got five billion of those, and then we spread them out across Europe, Asia, and Indonesia, and all these areas, giving one to each person, pointed them at the moon. The answer is it would still have no effect. The moon is very far away, and even a 1.4 watt laser pointer is not that bright. So we could try more power. So if you've ever seen a movie where they're hunting for a fugitive, and they have helicopters, and they have the spotlight under the helicopter pointing at the forest as they're searching for the person, the brand for those, there's a specific model that's very popular of search light called a night sun. And this is very bright. And so I thought, OK, what if I got them to manufacture five billion night suns, distributed them to all these people, and pointed those at the moon? The answer is it would still be disappointingly ineffective. The great thing about math and physics is that when you're doing this kind of theoretical calculation, you can just keep going, and no one can stop you. So I decided to try IMAX projectors. And I think it depends on whether you squint or not, but you can almost convince yourself there's a little bit of a brightness there, or maybe not. These are the projectors that they used to show those big movies in Dome's, and they're some of the brightest individual projectors ever built. They project a wide beam. We would need to put on some kind of lenses, but even with that, they would not be able to light up the moon. The single most powerful search light in the world is the set of lights on the top of the Luxor Hotel in Las Vegas, which shoots a beam up into the sky. And unfortunately, because of where Las Vegas is located, this beam never hits the moon. But if we took the spotlight off of the top of the Luxor and distributed one of those to all 5 billion people, then again, it would still not be quite bright enough. To get more power, we can try using beams to focus the beams from the Luxor on the moon. And then we actually start to see a little bit of lightness on the left side of the moon. Now these would be fairly white light, but it would definitely change the appearance of the moon. And so we've answered the question, and we're done. But then I got curious. There's a Boeing project for the Department of Defense in the US to mount megawatt laser beams on aircraft to be used to shoot down missiles. And these are a lot more powerful than even the Luxor spotlight. And they can be used, in theory, to melt a missile mid-flight. And those are, by the way, I think infrared lasers. But if we shift the frequency a little bit, we can pretend they're visible light. So I thought, OK, what if we go to Boeing and the US government and say we need 5 billion of those lasers? And I'm sure they would say yes. It's for an important project. And give one to each person on Earth, which is definitely a recipe for international laser war. But what if we shine those at the moon? Now that would have an effect. It would light up the night side of the moon to be almost as bright as the day side. Of course, we can't aim that well, so it would light up the day side some too. But that would dramatically change the appearance of the moon. But I couldn't resist, so I kept going. It changed the appearance of the moon, but it didn't change it that much. So I thought, OK, well, why are we limiting them to one per person? What if you took one of these megawatt lasers and you just mounted it on a tripod, and then next to it you mounted another one, and then another one, and then you just continued that process until you had covered the entire land area of Asia, from here to, I guess, the Ural Mountains, with megawatt lasers and pointed them all at the moon at once. The first thing is that these would cause the atmosphere to experience some problems. It would superheat it, it would, the reflected radiation would be very bad for us, but let's assume the Earth is somehow going to avoid these problems and just look at what effect it would have on the moon. Now, first of all, an array of laser pointers covering all of Asia of these megawatt lasers would require a lot of power. And in fact, I worked out they would drain the Earth's entire fossil fuel reserves in a span of two minutes. But the effect on the moon would be impressive. And by the end of those two minutes, the laser light shining on the moon would have actually heated the lunar regolith to a glow. So then even when the lasers turned off, the moon would still be extremely bright in our sky. Of course, we would probably have fires all around the globe and everything, so we wouldn't be able to appreciate that. But it would definitely have an effect and definitely answers the question. But then I wanted to go one more step further. So the single most powerful laser on Earth is the lasers that are used at the National Ignition Facility in the United States. And they fire off pulses of laser light to try to confine atoms and do fusion research. Now, these pulses, and these are 500 terawatt pulses. Now, the pulses happen for only a very short time. So you can't just shine it at something. It's like nanoseconds. But if we somehow took this laser and managed to make it so it could just fire continually the 500 terawatts, and then we distributed them around to everyone across Asia, the effect would be a little bit more dramatic. First of all, we're going to have to again assume that the atmosphere somehow lets this light through without destroying us all. But if that happens, and of course, figure out we have some way to power these, the Earth would still be destroyed by the reflected light from the moon. But if we somehow shield ourselves from that, the effect on the moon would also be a lot more dramatic. Because instead of heating the lunar surface to a glow, this much power concentrated in one place would start to vaporize it at a speed that's measured in meters of lunar surface depth per second. And as they vaporized it, it would create this cloud of what would quickly become plasma that's the vaporized parts of the moon, which would then be blasted by the heat of the laser and the energy from the laser away from the moon. And this would actually function a little bit like a jet engine or like a rocket. Because as this plasma was blown away from the moon, it would push on the moon. And this is a method, this sounds kind of silly, but this method has been proposed for propelling spacecraft. Because it turns out it's a really efficient way to deliver energy to something is to vaporize the surface of it using a laser. And so this is called laser ablation propulsion. And in the case of the moon, this many completely unrealistically powerful lasers would start to push it gradually backward. And the moon is already drifting backward at the rate of an inch or two per year or century or something. But this would move a lot faster. And over the course of weeks or months, the moon would be pushed out away from the Earth's orbit and then finally leave orbit and begin circling the sun with us. Of course, the Earth would be a fiery mess at that point. But we're going to pretend that the Earth has been somehow shielded from the effects of this. And the interesting thing is that then the moon serves an important purpose. It stabilizes the Earth's axis. Our axis wobbles a little bit, but it mostly stays pointed in the same direction. Without the moon there, over millions of years, our tilt becomes chaotic. Our axis points at the sun or straight up, so it wouldn't have seasons. Or it would point toward the sun, so it would have six months of heat in the Arctic and then six months of heat in the Antarctic. And it would probably lead to the evaporation of the oceans or something. So the moon is really important. And also, as the moon is orbiting the sun, it would be in an orbit that would cross hours since it started off at ours. So at some point, probably the moon would hit the Earth and wipe out all life if it somehow had managed to survive up until then. But I think that at that point, we can all agree that we would deserve it. So I really like answering these stupid questions. And I like taking this math and making it about real things, even if they're real but somewhat unrealistic things. Because I did a physics degree. And a lot of the time, we like to reduce things and cut out the complexity and make things as abstract as possible until we're just dealing with just equations. And then we can solve the equations. And that can be really helpful. But to me, it's sometimes easier to think about them when you've added on examples from the real world. When you've brought in all of this complexity, these questions of how quickly does the lunar wriggle vaporize? And what would the reflected light be? And what would it do to the atmosphere? Instead of just thinking of equations for photons and abstract cubes getting burned away by lasers, making them about something that's more fun to think about and easier to imagine, can somehow make it more simple and more interesting? And I did a lot more laser light calculations answering this stupid question than I did in my physics degree on the same subject. My advice for everyone would just be, don't be afraid of asking stupid questions. And sometimes they can lead you to a lot of fun places. Thank you.