This week's astronomy article was written at the request of a colleague. Last week, Mark and I sat alone in a room and watched a colloquium via videoconference. This is kind of awkward because then everyone at the location of actual colloquium can watch you watching them. I always feel obliged to sit up straight and not stare at the corner of the room.





Anyway, this colloquium was by Dr. Dimitrios Psaltis and was about a method for proving the No-Hair Theorem of black holes. Videoconferencing: It's good for

creating solutions, leveraging investments,

synergizing, networking, pushing paperwork through,

thinking "outside" the "box."

Source. Mark said, basically, "Dude, you're going to write about this aren't you? Black holes plus hair. You can't not." So here I am. Mark said, basically, "Dude, you're going to write about this aren't you? Black holes plus hair. You can't not." So here I am.





What traditionally allows us to see black holes?

In isolation, black holes look as their name suggests: black, hole-like. Which means that, for the most part, unless they're gravitationally interacting with another object, we won't know they're there. Stars can orbit black holes, and then we can detect the black holes by the fact that we see stars orbiting blackness. Material, from these orbiting stars or from objects falling into the black hole, accelerates as it spirals toward the event horizon, forming a hot, energetic accretion disk and collimated jets. For more about this, see my previous post about black hole basics.

What's missing from this picture?

The actual black hole! The stars, accretion disks, and jets are all large-scale compared to the actual black hole itself. And here, when I say "black hole," I mean the area encircled by the event horizon, or the distance from the singularity where the escape velocity is equal to the speed of light, or the actual black part of the black hole. We have inferred the existence of black holes from the jets and the sharks jets and the disks, but making an image of the actual black hole at the place where light and stars and horses disappear into it--that's difficult.





What's the problem? And why is that important?

Black holes are tiny! The biggest one we've got close-by, the one at the center of our galaxy--Sagittarius A*--would have an angular size of 55 microarcseconds.





It's difficult to resolve something of this size. To make a useful image, you need a telescope whose resolution is smaller than the black hole is.





The problem is that resolving power is proportional to the size of the telescope and inversely proportional to the wavelength you're trying to see, which means that youe need a really big telescope.





How big does that telescope have to be?

The size of a continent, or, ideally, much larger.





It's a good thing we have the money and space to do that. We can all just live under its surface like little science hobbits.





Doesn't that sound great?

I'm sorry to hear that you don't want to live under the same roof. But it's okay, for the black hole observations, at least, because of interferometry . Specifically, a technique called Very Long Baseline Interferometry (VLBI). By placing telescopes across over the world, pointing them at the same thing, and combining their signals, we can make them act like a telescope that is as big as they are far apart.





Source.

The array that first made images of smaller-scale structure around SagA*. We're given another boost by a thing called "gravitational lensing," which means that light must travel around the warped spacetime of massive objects, and these massive objects thus act like lenses, essentially magnifying for us. In the case of SagA*, the ring around the event horizon will appear about five times bigger than it actually is. We're given another boost by a thing called "gravitational lensing," which means that light must travel around the warped spacetime of massive objects, and these massive objects thus act like lenses, essentially magnifying for us. In the case of SagA*, the ring around the event horizon will appear about five times bigger than it actually is.





What other things can we find out from an image of the black hole, besides that it's a black hole?

That it has no hair.







What is the No-Hair Theorem?

The No-Hair Theorem says that black holes have only three properties: mass, spin, and charge (and for the observations we're talking about, the holes are neutral).





Here, as in life, hair is a stand-in for personality, or distinguishing characteristics.





Everything that disappears into a black hole becomes part of the black hole, but, at least from outside the event horizon, loses all characteristics except the mass and the angular momentum that it adds to the black hole. Did a star fall in? Did your spinning, stellar-mass dog? Doesn't matter. They're the same once they cross the event horizon. All distinguishing information about things that go beyond the event horizon is lost. So is all information about the star/s that originally formed the black hole. Mass and spin are not considered "hair" because

a) then we couldn't have the No-Hair Theorem

b) they don't distinguish one object from another. Lots of things spin. Lots of things have mass. If I said, "I have something that rotates once every five seconds and weighs 5 pounds on Earth," you wouldn't be able to tell me what it was.





So what can measure to see if the No-Hair Theorem is correct?