When you grow up as a child that does not have to worry about being fed and housed and all that, you get to think about all kinds of awesome and fantastic things: Dinosaurs! Space ships! Black holes! Lasers!
What if you could combine all of those? Would that create the ultimate awe-pocalypse? Dinosaurs flying into black holes in spaceships powered by lasers? Obviously we can't pull this off because dinosaurs are extinct, we have a hard time building spaceships (though Elon is working hard to change that) and we certainly can't fly into black holes, even though movies such as Interstellar have teased that. But where do Lasers fit in? Well, hold on to your seats: it appears that (at least theoretically) black holes can be turned into Lasers, or maybe some already are, but we don't know it yet!
If you are reading this post you probably already know what Lasers are, but in order to make the case for BHLs (black hole lasers), let me reintroduce you to them briefly. `Laser' is an acronym for "Light Amplification by Stimulated Emission of Radiation" (be honest, it's one of the greatest Science Acronyms of all time, because it is not tortured at all). The phenomenon relies entirely on Einstein's work concerning the probability of absorption and emission of light from atoms or molecules. Here's the title of this groundbreaking (and, as a matter of fact, easy to read) paper:
3. If an atom is in the excited state and absorbs a quantum at the same time, the quantum stimulates the emission of another quantum just like it:
Wikipedia's depiction of a Schwarzschild wormhole. |
Basically, it is two black holes connected by a "throat". We don't know if it's traversable for people, but you certainly can imagine that a particle thrown into one of the black holes might come out at the other end.
Come out? But nothing can come out of a perfectly absorbing black hole, right? Well, this is both right and wrong. Physical particles cannot come out, but you can clone those particles, so copies can come out, which after all is just as good.
Let's see how this would look like. We now take the black hole horizon from Fig. 1, and add another one like so:
Fig. 2: Two connected black holes with horizons \(H_1\) and \(H_2\). The anti-clone that travels towards \(H_2\) is reflected there, and stimulates another pair. |
What happens now? Well it's clear. The absorbed particle and one of the anti-clones are on a collision course leading to annihilation, but the other anti-clone will reflect on \(H_1\) and stimulate another pair. As if by magic, there are now two clones outside horizon \(H_1\), and two outside horizon \(H_2\). But the anti-clones inside the wormhole keep reflecting between the horizons just as in the optical Laser described above. Except that we don't need semi-transparent mirrors: the "Laser beams" will emanate from the horizon in a coherent manner as long as the inside of the wormhole is coherent!
Fig. 3: The wormhole Laser. Anti-clones that are reflected from the inside of the black holes stimulate emission of clones outside the respective horizons. |
Now to the last question: what would a BHL look like? First of all, it is clear that whatever radiation emanates from the black hole, it will look like it is coming from a disk surrounding the black hole. We also know from explicit calculations that the stimulated emission in response to absorbed material is not red-shifted (because this is "late-time" absorption). However, what happens to material that reaches the second horizon I can't say without a calculation. The important distinction for Laser light, however, is that it is coherent. If the stuff that is stimulated outside the horizon is similarly coherent, we might be able to detect this using typical Hong-Ou-Mandel interferometry of light coming from such a black hole. We've just learned how to look at light emitted from black holes using telescopes like the Event Horizon Telescope, so it might be some time before we can check if that light is really BHL light. We don't know how many black holes are actually connected to others making BHLs possible, but at least there is a chance to find out!
[1] The reason Hawing ignored stimulated emission in his calculation of radiation coming from a black hole is that he thought that it would require energy from a black hole's rotation (the rotational energy would provide the "pump energy"). Because he treated a non-rotating black hole, he decided he could ignore the effect. It turns out that stimulated emission does not require black hole rotation.