A Black Hole Death Ray?

by Clifford Johnson     Department: Physics & Chemistry

Well, it has been all over the news, so you might have already heard about it and seen the image. It's such a nice result, however, I can't resist pointing it out just in case you've not seen it. It is a jet or ray of material being ejected from the core of a galaxy and being sprayed onto another galaxy. What's powering this cosmic ray-gun? A black hole! The offending galaxy has a supermassive black hole at its core (as do most galaxies), and this one is active indeed, using its intense gravitational fields to squirt material away. The other galaxy is sort of an innocent  passer-by. There's more information,  background on the image to the left, and discussion at the Chandra Observatory site, for example. Extract:

In the image, data from several wavelengths have been combined. X-rays from Chandra (colored purple), optical and ultraviolet (UV) data from Hubble (red and orange), and radio emission from the Very Large Array (VLA) and MERLIN (blue) show how the jet from the main galaxy on the lower left is striking its companion galaxy to the upper right. The jet impacts the companion galaxy at its edge and is then disrupted and deflected, much like how a stream of water from a hose will splay out after hitting a wall at an angle.

There's an excellent chat on NPR on this that you can listen to. It is Melissa Block  talking with Daniel Evans of the Harvard-Smithsonian Center for Astrophysics about the work (he's part of the team of scientists who presented the image recently):

Striking pictures combine images from across the electromagnetic spectrum -- from radio, to optical, all the way up to X-ray wavelengths -- to show the never-before-witnessed galactic act of violence, Evans says.

"What we're looking at is a really extraordinary act of violence by a black hole," he says. "We can see that a very powerful jet of particles is being ejected from a supermassive black hole in the center of a distant galaxy, and there's an unfortunate neighboring galaxy that has moved directly into its line of fire.

Is this really a death ray though? It is all about destruction? Well, maybe not. As Evans (and others in the context of colliding galaxies, for example) have pointed out, this sort of supposedly violent activity is often the place where new possibilities emerge.  New regions of star-formation appear along striking fault lines in Astrophysics - this is where material can get crunched together enough to  seed the  birthing process. This event may well be the source of such opportunities to create anew.

The most common question at this point is something like "if a black hole is black and so can't be seen, how do we know it is a black hole doing this?". A subsequent question is "isn't a black hole supposed to be sucking stuff in? How come it is spewing stuff out?" Both excellent questions -  thanks for asking. Well, both questions are addressed by pointing out that we know of them by their effects on the stuff around them. Black hole are regions of intense gravitational fields, due to there being a high concentration of matter in a small space ("small" here depends upon how much matter "mass" we're talking about   - see below.) Typically, there's ordinary matter in circulating around the hole, some spiraling into it, forming a region of intense activity coming from the interaction of lot of such matter in a small space, together with magnetic fields associated to that matter. These interactions can give rise to jets of radiation and matter and being ejected from the region near the hole.  The black hole provides the enormous energy required to give these jets a huge signature that can be seen with the right equipment here at earth. The matter is not really escaping from inside the hole itself -  nothing can do that (see below for more, and a caveat). There's a lot about this all over the web, so I won't try to reproduce it here. I found a nice set of wiki pages as the University of Tennessee for example (see here), and of course there's NASA's site, and that of the Harvard-Smithsonian, and several others.

There are several other ways that black holes can reveal themselves as well, depending upon the situation. Key to almost all methods to date is to look for highly energetic events generated in a very very compact region. A black hole is almost certainly to blame, often being the only thing that even comes close to being able to power the event. Sometimes, as with the most dramatic and convincing of the supermassive black hole observations, you can directly see  that objects are orbiting   compact mass in a space that is so small it has to be a black hole (given our understanding of the physics of gravity). Have a look at the website of Andrea Ghez' Galactic Center Group at UCLA   for more on this.

It is fun to reflect from time to time on how far we've come in various areas of scientific endeavor. This could easily be cast as one of those "science fiction becomes reality" stories, starring black holes. My point here is that we're now casually talking about black holes as responsible for some of the remarkable images we see in space whereas not too long ago they were still thought of as somewhat fanciful objects, more fiction than fact - rather strange solutions of Einstein's equations of General Relativity.

Hmm. Normally, at this stage I'd move right on to the next sentence without saying anything further about equations and solutions, but just for a bit of fun (so you can see what we actually deal with in our notebooks) I'm going to write down a rather compressed form of Einstein's (ten) "field equations" of 1915:

and the simplest solution for a black hole, the "Schwarzschild solution" of the same year:

Welcome back. There's probably about a quarter of you returning of those who started reading the post. Thanks for coming back. I did not expect you to get a lot out of that except a snapshot of what working equations look like. What does the squiggly blue stuff tell us? Well, you've heard about warped spacetime - gravity, according to GR, is the warping or bending of space and time (better put - "spacetime"), and that is what the left hand side of Einstein's equations describe. What causes it? That's what the right hand side describes. Matter, energy, pressures, stresses of various sorts... that's what's in the big  "T" thing. The capital G is Newton's constant, that sets the strength of the effect generated by those causes. The Schwarzschild solution in the second line, is a particular example of this business with special properties. It is a recipe for reading off how spacetime is curved and in what way: the precise nature of the "gravitational field" if you prefer to put it in other terms. It actually represents the "fields" associated to any spherical object of mass M (it can't be spinning or have any electric charge, etc - just a simple sphere). Even if not exactly spherical, this will describe things pretty well. It describes the fields outside the mass - the fields within the mass depends upon the details of the mass and how it's made up. This is not our concern. That Schwarzschild describes the fields around a mass - any mass - regardless of what it is made up of is what makes it so powerful and special since this includes our very own sun, or the earth, for example, or a bowling ball, or...

What does this have to do with black holes? Well, it all depends upon how compactly that mass is distributed, and this can be read off from the equation too. A given amount of mass sets a special radius, given by 2GM (you also need to divide by the square of the speed of light to get the result in regular units like meters), called the "Schwarzschild radius". If the  massive object is spread out over distances greater than this, then it is  indeed just a regular object, like the sun, or the earth, or a bowling ball... If the mass is compressed into a region smaller than 2GM though, then it is a black hole. Why is that a special situation? Well, for a start, ignoring quantum mechanics for now (which we can do for all the black holes under discussion in the astrophysics context above)  it turns out that nothing (including light) is fast enough to get away from any part of the object that is inside this  radius. Everything gets trapped inside, and not even light can escape - the "black" in "black  hole". This all happens because the mass M is compact enough that the equation above is relevant right up to and including the region where the radial distance, r, becomes equal to 2GM - the famous "event horizon". There's a lot more to say here, but I think I'm going to far afield, and to read off more from the equation would require more care.

If you were to plug the numbers into the radius 2GM in an idle moment, you'd see why it is not surprising that the earth is not a black hole, and even the sun. Newton's constant G is a tiny number, and so even a large amount of mass packed together with the sort of densities we are familiar with here on earth would fill up a sphere way bigger than radius 2GM (this is made worse by the fact that, as I said before, I have to also divide by the square of the speed of light, which is a huge number). For the mass of the earth you'd have to compress it to a sphere whose radius was about the size of a dime - for the sun, about 3 kilometers. Either way, that's pretty densely packed.

So it takes rather exotic situations to drive matter to such extremes of density that it can form black holes. This took a lot of time to work out by astrophysicists in the middle of the 20th century (see Kip Thorne's wonderful book "Black Holes and Time Warps" for an excellent account), and eventually it became clear that "exotic" is in the eye of the beholder. For example, the ordinary life cycle of a large enough star (ours is too small) will almost inevitably  lead to the formation of a black hole after it runs out of fuel to support itself from collapsing under its own gravity. There are a lot of stars out there - this happens a lot! As I said above, we (the physics and astrophysics community) think that black holes are also at the core of most galaxies (it's the best explanation for a dazzling variety of observed phenomena), including our own galaxy! These "supermassive" ones are millions to billions the mass of our sun.  A computation of the size of such a hole gives you a 2GM of order the size of our solar system, which is still pretty tiny by galactic standards.

How fast we've come from seemingly strange solutions of some strange equations to realizing that they play a vital role in our universe.

-cvj

P.S. If you're wondering "Are there other weird solutions of the equations that might be for real one day - and if not why not?" See my earlier post about the show I contributed to on the History Channel. There's a lot about black holes, white holes, and wormholes (including time machines). Called "Cosmic Holes", it's a fun episode, and I think they'll be repeating it on TV next week. (See also this post about a joke sort of hidden in the show.)

P.P.S. Astrophysics black holes (the ones that form "naturally" from stellar collapse and other aggregation of matter) are almost always rotating as well (are "spinning"). In such cases, the Schwarzschild solution above is generalized to the "Kerr solution", which is much more complicated to write and I dare not lose all my readers in one post, so I won't.

Tags: astronomy, black holes, physics

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