Optical Black Holes in the Lab

spaceorb writes "According to researchers ... it may be possible to create black holes by creating a vortex of fluid that swirls at velocities comparable to the speed of light. Follow the above link for the theoretical discussion or here for the story on unisci.com." These are optical analogues of black holes, not really gravity wells, but they may advance our understanding of the real thing.

Comments

Authors hung up on "not even light can escape"

[Robert Link]

The authors appear to think that the salient property of a black hole is the pop science description that "not even light can escape its pull." Thus, they reason, if light cannot escape, it must be a black hole. This just isn't the case. Indeed, the authors' own text hints at this: "The concerned reader should note that optical black holes are safe. They would attract only light..." This property is, of course, fundamentally different from what you expect of a true (that is, astrophysical) black hole, and so we should be skeptical of any further analogies between these "light traps" and black holes. In particular, I see no reason that these things should emit Hawking radiation. Certainly the "virtual particle trapped inside the event horizon" interpretation of Hawking radiation fails because there is no event horizon.

What's more, from the standpoint of General Relativity these constructs don't look anything like a black hole. The stress-energy tensor (the relativistic analog of mass density) is virtually unchanged by the modest rotational flows light traps made from Bose-Einstein condensate would require, meaning that these constructs should have exactly the gravitational properties you would expect of a static body of liquid in the laboratory (i.e. none to speak of). That means no space-time curvature, no ergosphere or frame dragging, no gravitational redshift, and no time dilation. For example, if they directed a stream of muons through these things they should find the muons' decay lifetime basically unchanged from what it would be if they sent the muons through the same liquid while it wasn't rotating.

I believe the authors make an important mistake when they say "... a moving dielectric medium acts on light as an effective gravitational field." That is clearly not true because this putative "gravitational field" does not obey the equivalence principle; viz. it accelerates light but not matter. The mistake is comparable to saying, "A charged pith ball in an accelerating train car will experience an `effective electric field' which will tend to accelerate it." and proceeding to compute the electromagnetic properties of this moving field. The analogy falls flat because the acceleration is not caused by an electric field, and so it can't be expected to act like one when you study its influence on anything else in the train car. Similarly, although you can compute a gravitational field that would trap light in the same way as these constructs, that doesn't mean that there is actually a gravitational field present, nor does it mean that other effects that would be present for the gravitational field you calculated will actually show up in your apparatus.

None of that means that this isn't interesting research, of course, but as far as I can tell the connection to black holes and astrophysics is nonexistent.

-r