What would happen if you tried to hit a baseball pitched at 90 percent the speed of light? If every person on Earth aimed a laser pointer at the Moon at the same time, would it change color? Is it possible to build a jetpack using downward-firing machine guns? If an asteroid was very small but supermassive, could you really live on it like the Little Prince? What would happen if you tried to fly a normal Earth airplane above different solar system bodies?
I had never wondered about any of those questions, but clearly someone had. And all of those various someones wrote in to Randall Munroe, creator of the webcomic xkcd, who had launched a sideline in “serious scientific answers to absurd hypothetical questions.” In the book version of What If? Munroe not only delights in answering questions like these (more than half of which have not previously appeared online), he often extends them, taking the situation as posed and then asking “But what if we tried more power?”
As with the question about laser pointers and the Moon.
[Cartoon of unchanged moon]
Well, that’s disappointing.
It makes sense, though. Sunlight bathes the Moon in a bit over a kilowatt of energy per square meter. Since the Moon’s cross-sectional area is around 10^^13 square meters, it’s bathed in about 10^^16 watts of sunlight—10 petawatts, or 2 megawatts per person—far outshining our 5-milliwatt laser pointers.
[Cartoon figure asks] What if we tried more power?
A 1-watt laser is an extremely dangerous thing. It’s not just powerful enough to blind you—it’s capable of burning skin and setting things on fire. Obviously, they’re not legal for consumer purchase in the US.
Just kidding! You can pick one up for $300. Just do a search for “1-watt hand-held laster.”
So, suppose we spend the $2 trillion to buy 1-watt green lasers for everyone. (Memo to presidential candidates: This policy would win my vote.) In addition to being more powerful, green laser light is nearer to the middle of the visible spectrum, so the eye is more sensitive to it and it seems brighter.
Here’s the effect.
[Cartoon of unchanged moon]
The cartoon figure keeps asking “What if we tried more power?” until Monroe finds a laser power level at which he can write both “Under those circumstances, it turns out Earth would still catch fire” and “This flow of material effectively turns the entire surface of the Moon into a rocket engine—and a surprisingly efficient one, too.”
There are nearly 300 pages devoted to the questions and answers, along with several sections of short answer (more than one Q&A per page) and 12 pages of “Weird (and worrying) questions from the What If? inbox.” I was interested and amused all the way through. What If? hit the sweet spot of serious science mixed up with deadpan presentation, and proved a (periodically dangerous) garden of delights.
By the way, the answers to the other questions noted above are “an explosion leveling everything within about a mile of the baseball field,” “yes, especially with Russian weapons,” “sort of but it would be really weird,” and a chart showing seriously short life expectancies for the Cessna pretty much everywhere. I hope there will, eventually, be a second volume.