Dumb Name; Awesome Concept

So, as I mentioned in my first post a few days ago, I am (among other things) a Master’s student in theoretical physics, specializing in black hole entropy. I admit, I have been fascinated by black holes for as long as I can remember (no doubt a side-effect of my adolescent continuing obsessing with Star Trek). Initially, I imagined them as hungry monsters, inevitably sucking everything up into their maw (I was not as bad about this as some of my classmates in elementary though; I remember one boy who started crying in what I can only assume to be Lovecraftian horror when someone told him that there was probably a giant black hole in the centre of the galaxy). After a while (specifically, after reading a picture book on astronomy when I was 8 years old which explained to me why the “hungry monster” image was stupid) I started imagining them instead as being, essentially, a pallette swap of what I saw whenever I flushed the toilet; a vortex of colour swirling-in towards a inescapable black abyss.

So, basically something like this.

It is a useful mnemonic, to be sure; infalling matter really does “swirl,” although there ought to be some jets of superheated plasma being ejected in a perpendicular direction (best not to consider what the precise analogue of that would be in terms of our toilet metaphor), but of course it has a major problem. Obviously, black holes are gravitational entities, whereas vortices arrise from fluid mechanics. Obviously, gravity and fluid mechanics are two very different things.

And yet…

A great deal of modern theoretical physics research suggests that there exists a surprising and profound duality between the two things. Personally, I’m currently studying the AdS-CFT correspondence, which the idea (based on a little bit of string theoretic legerdemain) that the theory of gravity in an enclosed (“Ant-de Sitter”) universe in a certain number of spacetime dimensions is equivalent to a theory governing the behaviour of subatomic particles which exists on the boundary of that universe. In certain limits, these subatomic particles behave like a fluid (one might think of them, in terms of quantum chromodynamics, as being like a soup of loose quarks and gluons- the quark-gluon plasma). In these situations, it actually is useful to think of a black hole as being based on fluid dynamics: or, more accurately from a functional perspective, to think of fluids as being based on gravity.

But there are much less, shall we say, airy-fairy ways in which black holes can be linked to fluid dynamics: namely, there are the so-called “dumb holes,” which, in addition to being easily the most unfortunately-named entities physics, are the sonic analogues of black holes; namely, just as there is a certain point on the inward approach to a black hole after which not even light may escape, there is a certain point on the approach to a dumb hole after which not even sound may escape.

The basic idea is this: a fluid speeds-up as it flows through a bottleneck. It is possible for it to be accelerated to the point that it is travelling faster than sound can propagate through this medium; if that happens, it has formed a dumb hole. Outside of a dumb hole, sound waves can propagate in both inwards and outwards directions. But once it’s passed the “sonic horizon,” there can be no more outgoing sound waves, as they will be swept inwards faster than they may propagate outwards. This analogy is strong enough (light, like sound, is ultimately behaves, for our purposes, like a wave) that it raises the prospect that alot of the conjectured behaviour black holes can be studied experimentally in the lab using dumb hole analogues.

By why stop there? Light may travel faster than anything else in the universe*, but that it is only through a vacuum. When it is travelling through matter, it slows down, often quite considerably- to the point that it is possible for the fluid velocity to travel faster than the speed of light in the medium. Moreover, light is bent by travelling through matter by refraction. Analagously, light is bent by gravity in general relativity; in that case, it is because spacetime itself is bent by gravity (and light is simply taking the shortest route between two points, regardless of the local geometry), but, if we ignore the movement of the fluid in this case and pretend that light is moving in a vacuum, it allows us to consider an “effective” geometry. Naturally, you can see where I am going with this: an “effective” black hole.

So yes; the next time you flush the toilet, remember: it really is like a black hole. And there’s nothing “dumb” about it.


*Or not, depending on how that thing with that neutrino pans out.

About thevenerablecorvex

I have the heart of a poet, the brain of a theoretical physicist, and the wingspan of an albatross. I am also notable for my humility.
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