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On the Practical Applications of Impossible Experiments
From time to time, I hear from reporters wanting me to comment on hot-off-the-presses astrophysics papers. It’s not always feasible -- sometimes for practical reasons, and sometimes because I don't have the right expertise. When I do agree to comment, it’s helpful for the reporter, but there are lots of potential downsides and not a lot of upsides for me, personally. In a quick read, I might miss something important. If I don’t like the paper, I might upset the authors and cause problems for myself. My comments could be taken out of context. Or I might just draw attention to something that doesn’t warrant more attention (especially if it’s an unpublished preprint that might not really be up to snuff). On the positive side, I might have a chance to explain something to a reporter that they (as someone outside the field) could have missed, or I might get a chance to enthuse about something really cool. But those pluses don't always outweigh the risks.
Still, when a reporter reached out a couple weeks ago to ask if I’d comment on a fairly short (10-page) research paper about black holes that had already been accepted for publication in a reputable journal (Physical Review D), I couldn’t quite come up with a reason to decline. And it sounded like a kind of fun idea: a new proposal for extracting energy from black holes.
You can find and read the paper here (or here in preprint format). The gist of it is that in principle, if you were to carefully set up some kind of apparatus to drop charged particles into tiny black holes, there is theoretically a mechanism for extracting electrical energy from it – the black hole becomes a kind of “rechargeable battery.” In theory.
Now, as a physicist, when I read something like this, my immediate interpretation is that this is one of those thought experiments that might have some value down the road in evaluating certain kinds of speculative theories (more on that in a bit). But, unfortunately, when articles about this are written in the popular press, chances are many readers will think “this is a proposal for a new energy source.” And, to be clear, it’s really, really not.
There are several reasons to conclude that this paper isn’t (and was never meant to be) a practical proposal for a new energy source. The most striking point against is that the kinds of black holes described (small ones in the asteroid-mass range) are not known to exist. Such microscopic black holes are too small to form as the result of stellar collapse, which means they would have to have been formed through some kind of exotic process in the early universe. While “primordial” black holes are certainly a neat idea (I’ve written a few papers on them myself, as possible explanations for dark matter) there’s currently no evidence for them ever existing at all. Even if primordial black holes of the appropriate mass were out there, there’s no known method to capture or even locate one, which would make it hard to build a complicated apparatus around it for the extraction of energy. And even if you did, there’s no particularly good reason to suspect that it wouldn’t be (in terms of total energy required) more trouble than it’s worth.
So what’s the point of writing a paper about an implausible, possibly massively inefficient mechanism for extracting energy from an object that probably doesn’t exist? Are these physicists just wasting everyone’s time?
When I talked to the reporter, I tried to add a bit of context to this study. You can read the article and my quotes here. There’s actually a long, celebrated history of extraordinarily impractical proposals for energy extraction from black holes. The most famous one involves something called the Penrose process: in principle, if you’re falling toward a spinning black hole on just the right trajectory, you can jettison something from your spaceship in such a way that you get more energy out than what you put in, extracting energy from the black hole’s spin to accelerate you away from the black hole out to safety. There’s an analogous process called black hole superradiance that applies to radiation, in which a cloud of particles so small that they act collectively as waves can extract energy from a spinning black hole.
Neither of these processes was described in the physics literature as a practical suggestion, but rather as a way of exploring the potential interactions and rules of an extreme physical system. And as it happens, superradiance has turned out to be immensely practical as a way of testing some theories of new particles. If certain species of light particles (like ultralight axions or “dark photons”) actually exist in the universe, the superrandiance process could create a telltale spin-down of the black hole, or perhaps a detectable gravitational wave signature. Scouring the data of black hole spins and gravitational waves can allow us to search for hints of extensions to our Standard Model of Particle Physics, and may someday illuminate the nature of dark matter.
As a theorist (especially as one interested in dark matter), I can easily see the value in coming up with an experiment that humans will, realistically, never do. Because out there in the cosmos, to a large degree, everything that can happen, will. Even the most outlandish thought experiments could very well prove to be an everyday natural occurrence somewhere out there, and looking for those signs might help us better understand the rules that govern the universe. |
