Sunday, September 18, 2016

Where Are All the Black Holes?

In a previous post it was noted that there seems to be a lot of undiscovered black holes around. If you go back and figure out how many O and heavier B class stars there were, all of which produce black holes when they die, it turns out there should be numbers comparable with the number of stars in the galaxy. Is it remotely possible that the Milky Way is holding, say, ten billion black holes with masses ten or more times that of the sun?

Everyone seems to already know about the discovery of missing mass in galaxies, and how stars out in the disk are rotating faster than they should, if all the mass in the galaxy was in visible stars. They act as if something else is pulling on them gravitationally, but no matter how carefully stars are counted up, they don’t match what is needed. So the problem of missing mass came up and has been around for the most part of a century. No solution yet.

Having ten billion black holes with ten solar masses each would be a good start toward filling that gap, but there doesn’t seem to be any way to see most of these. A few have been noticed because they are binary with something else, and that makes some X-rays or some gravitational waves, and these are detectable and traced to being a likely black hole. But, is it possible that such a huge number of them might be present here in our own galaxy?

Throwing in another ten percent of the current population of stars isn’t going to change the fact that stars in the disk are widely separated and don’t have close encounters very frequently. If the black hole population is distributed like the stellar population, and why wouldn’t it be, the black holes are just spread out here and there doing nothing at all, not emitting any radiation or interacting with anything. They are too small to occlude anything. They likely blew away their planets when they went supernova to form the black hole. If they did have planets, in anything but very close in orbits, no one would notice as they do not radiate. The planets would have cooled down from whatever heating the supernova explosion gave them.

In a phrase, they are very quiet and tidy neighbors. What difference would it make to an alien civilization to realize that there were many black holes spread all over the galaxy? Sending out an interstellar probe to another star would mean there is a chance of the probe passing near an undetected black hole and being deflected, but the chance of this happening is negligible.

A black hole would make a fine target for a gravitational slingshot velocity assist, except that all the stars, and presumably black holes, in the vicinity are going around the same speed in orbit around the galactic center. This velocity is nowhere near fractional light speed, even if there was some geometry that could take advantage of it. So, velocity augmentation is likely a dead end.

If there was some gas around it, and it were possible to put a satellite there and direct some of the gas into the event horizon, there would be a source of X-rays. What could an alien civilization do with this? A galactic equivalent of a GPS satellite system? Granted that black holes are small, there might be a bit more precision available from this than with simple star-tracking, but stellar navigation is likely to be excellent using only star locations. Once measurements are made of all the proper motion of the largest and brightest stars in a neighborhood, traveling around that area can be done with elementary sighting apparatus.

Communication would not be a good idea, either, as the information carried by a system like this would be so small as to be useless. Material deposited on one side of a black hole would be only visible from receivers in that direction, so the idea of a beacon with signaling doesn’t work either. In short, there doesn’t seem to be any benefit that an alien civilization would have for discovering a black hole near them, and perhaps going out to it and establishing an orbiting station there, any more than there would be if they did this around some O class star with no planets.

One possible exception comes from the fact that about half the stars in the galaxy are binaries or higher multiples, and therefore it might be that significant numbers of black hole binaries exist. Black hole binaries have been successfully modeled, and it turns out they generate huge amounts of energy as the binary black holes come closer after having lost angular momentum and energy to gravitational waves. We just barely know how to detect gravitational waves, but could an alien civilization be able to develop some unimaginable-to-us apparatus to absorb the energy of the two black holes, for a few million years as they grow slowly closer together?

Everyone talks about how fusion energy is the source of everything in the universe, as it is the source of energy for every star that exists. While this is true, it is also true that gravitational attraction is what makes fusion energy generation possible, and the amount of gravitational energy in a binary black hole is so much more than the energy an alien civilization needs to operate that, if there is any way to tap it, it would provide the energy source they need. If fusion fails, does going out twenty light years to the nearest black hole binary and capturing its energy make even the slightest amount of sense? Is there any way this amount of energy could be beamed back to the home planet, to avoid the difficulties involved with capturing it, converting it, storing it in some materials, shipping the materials through immense distances, and then doing the opposite to make it available? If beaming were possible over a distance like twenty light years, this would eliminate the delays involved in moving matter around in the galaxy.

So, for now, a substantial black hole population appears to be useless, with some very remote chance that a power source could be developed from them. This clearly appears to be a very interesting topic to delve into.

No comments:

Post a Comment