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Hank: Do you think you could you make a black hole out of light? That might sound like a trick question, or like something straight out of the “showerthoughts” subreddit.

But physicists say that it should be possible to create a black hole out of nothing but light -- called a kugelblitz, after the German word for “ball lightning. ” Even though they have no idea what would actually happen if you tried to do it. Before we talk about that though, let’s talk about the more standard black holes.

Usually, a black hole forms when a lot of matter is packed into very little space, which increases its density. If the matter is dense enough, the gravity at its surface becomes so strong that nothing, not even light, can escape its clutches. So to make the Earth into a black hole, you would have to compress its 6 trillion trillion kilograms of rock and water and metal down into a sphere with a diameter of just 16 millimeters.

That’s how dense I’m talking about. But we’re just getting started here. Einstein taught us that E=mc^2. In other words, that the energy stored in a bit of matter is equal to its mass times the speed of light squared. This means that there are huge amounts of energy in tiny bits of matter, like atoms -- which is the basic idea behind nuclear power and nuclear weapons. It also means that energy can sometimes act like matter.

Now, light doesn’t have any mass, but it does have energy. And, thanks to E=mc^2, gravity can’t tell the difference between energy and matter -- and pulls on them both. And this raises an interesting possibility.

All it takes to make a black hole is to pack a bunch of matter in one place, and gravity treats light just like it treats matter. So you could theoretically make a black hole out of nothing but light -- a kugelblitz. Creating a kugelblitz would take a really, really incredible amount of light, though -- because now we’re working with Einstein’s famous equation in the opposite direction. Before, we had a tiny amount of matter producing a huge amount of energy. Now, we have to supply a ton of energy -- in the form of light -- for it to act like tiny bits of matter.

So let’s go back to that hypothetical black hole with the mass of the Earth. If we wanted to make that same black hole out of light, how much light would would we need? All of the lights on Earth? All of the sunlight that hits the Earth in a day? In a year? What about all of the sunlight put out by the Sun over the course of an entire year? Not even close. To get an Earth-mass kugelblitz, we would need all of the light put out over 10 years by all of the stars within 350 light-years of Earth – all squished into a bit of space the size of a mosquito. Okay, that’s a lot of light.

But what if we wanted to make a black hole that was way, way smaller? Like, I don’t know... the mass of a big cruise ship, like the Titanic ... about 50 million kilograms? To make a kugelblitz that small, you’d need to squeeze all of the sunlight that hit Earth last year into a volume 8,000 times smaller than a proton. It’s hard to even imagine how so much light could get shoved into one place -- or any sort of technology that could accomplish even a fraction of that in the foreseeable future.

But even if it could happen, there’s another problem: making a kugelblitz means that you have to pump so much light-energy into such a tiny region of space that it would get really hot, since heat is just a form of energy. We're talking hotter than the universe was a hundredth of a millionth of a trillionth of a trillionth of a trillionth of a second after the Big Bang. And at that extreme temperature, physics as we know it just sort of stops working: space turns soupy, gravity is nonsense, and we have no idea how to describe anything.

So if space gets that hot, gravity might go haywire and not be able to form black holes at all any more. Or maybe gravity would get super strong and form a black hole with a million times less energy than we think it should take right now. We just don’t know, we might never know.

For now, it seems like kugelblitzes might be theoretically possible, based on Einstein’s basic equations relating energy and mass. It’s a cool idea: using lots of highly focused light to create a black hole, something that even light can’t escape from. But we have no idea how we -- or the universe -- would make one, or even if there’s some undiscovered law of nature that forbids it from existing.

Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who let us talk about these weird things, thank you so much for that. If you want to help us keep making episodes like this, you can go to Patreon.com/SciShow to learn more, and don’t forget to go to YouTube.com/SciShowSpace and subscribe!