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Wednesday
Oct122011

Hairless Black Holes

This post was chosen as an Editor's Selection for ResearchBlogging.org
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.

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 postabout 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 sharksjets 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.

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?
If the spin and the mass are the only two qualities a black hole possesses, then, by definition, they have to be able to tell everything we can know about the black hole.

A black hole's spin and mass together specify a particular warping of spacetime, and that warping of spacetime specifies a how matter and light will behave around the black hole. And the behavior scientists are most interested in is the behavior very close to the event horizon, as this is where gravity is the strongest. Extreme gravity allows tests of Einstein's General Relativity (GR), which is accurate on some scales (notable exceptions include galaxies), but still requires tests, and potential modifications, on other scales. Since we don't have multimillion solar-mass objects hanging out in labs, spaghettifying lab tables, the only way we have of testing GR at extreme scales is by observing extreme objects.

That's why we need to be able to image event horizon-scale structures: so we can see whether their orbits, and how their orbits appear from our perspective, are as expected.


The photon ring looks a bit like this, and I would have more
images if they weren't very hard to access. I'm not going to
source this, as you can see that it says Science News and a date.
With VLBI, it is possible to observe photons in close orbit around the event horizon. Photons should appear as a bright ring in the around the black hole.

If the No-Hair Theorem and GR are correct, this orbit should be defined by the black hole's bald properties. Its orbit should, once warped spacetime and thus our warped view are accounted for through GR-based calculations, be circular.

What would it mean if black holes' behaviors were not determined solely by mass and spin?
Observationally, the photon ring would be asymmetric and elliptical.

If the images showed asymmetry and ellipticity, either

a) General Relativity would not provide an accurate description of the universe in strong-field situations

or

b) black holes would not be black holes but some other kind of dark object

So what's the verdict?
So far, it appears that SagA*'s black hole's photon ring looks as expected (see the Doeleman and Johannsen articles), meaning that black holes do not have hair, which means we can all relax a little bit and tenuously pat Einstein on the back.

Astronomers, though, are working on the Event Horizon Telescope, which will develop VLBI technology so that even better images of light orbiting not-light can be made. This telescope wants to be the size of, basically, the whole world. It must be cool to be a telescope.


ResearchBlogging.org 

Tim Johannsen, & Dimitrios Psaltis (2010). Testing the No-Hair Theorem with Observations in the Electromagnetic Spectrum: II. Black-Hole Images Astrophysical Journal arXiv: 1005.1931v2 
 
Schwarzschild, B. (2008). Radio interferometry measures the black hole at the Milky Way's center Physics Today, 61 (11) DOI: 10.1063/1.3027977 

Broderick AE, & Loeb A (2009). Portrait of a black hole. Scientific American, 301 (6), 42-9 PMID: 20058635 

Doeleman (2010). Visualizing the Void: How to Capture a Black Hole Science News (October).

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Reader Comments (1)

Awesome post! Thanks you for taking the time to write this, really hope to see more post like this on my next visit.

July 13, 2014 | Unregistered CommenterWigs for Black Women

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