Gravitational Waves and Gold

One of the more interesting things to be announced recently was the detection of gravitational waves that were generated by the collision of two neutron stars. What was really interesting to me was that the event was also seen by telescopes, so we know what actually caused the gravitational waves. Originally it was thought that gravitational waves would be generated by colliding black holes, but I found that to be disturbing because I thought the frequency of detection should be rather low. The reason is that I thought black holes themselves would be rather rare. As far as we can tell most galaxies contain one in their centre, but the evidence for more is rather sparse. True, they are not easy to see, but if they come into contact with gas, which is present within galaxies, the gas falling into them will give out distinct Xray signals and further, they would perturb the paths of close stars, which means, while we cannot see the black hole, if they were common, there should be some signs.

There are some signs. The star Cygnus X-1 is apparently shedding material into some unseen companion, and giving off X-rays as well as light. A similar situation occurs for the star M33-X-7, which is in the galaxy Messier 33, which in turn is 2.7 million light years away. Now obviously there will be more that we cannot see because they are not tearing stars apart, but they are still rare. Obviously, collisions would be rarer still. With all the stars in the Universe, how often do we see a collision between stars? I am unaware of any in my life. Nevertheless, there have been estimates that there are about a billion moderately sized black holes (i.e. about 15- 20 times the size of our sun) in our galaxy. However, when we probe to see how they came up with this figure, it turned out that it arose because it was obvious that you needed them to be this common to get the frequency of the detection of gravitational waves. That reasoning is somewhat circular.

How would they collide? Like other stars within the galaxy, they would travel in orbit around a galactic centre, in which case the chances of meeting are rather low. And even if they did approach, why would they collide? The problem involves the conservation of angular momentum (the same sort of thing that the skater uses to slow down the spin by sticking her arms out). When one body approaches another, assuming they are not going to directly collide (in which case there is no angular momentum in their joint system) then they follow a hyperbolic orbit around each other and end up going away from each other in the reverse of however they approached. For one to capture the other, either the systems have to spin up to conserve angular momentum, or they have to throw something out and whichever they do, orbital decay requires a mechanism to get rid of a lot of energy. Further, they cannot spin up, which is an exchange of angular momentum, without tidal interactions forcing it. Now tidal interactions work by part of one body “flowing” in response the gravity of the other. The material does not have to move a lot, but it has to be able to move, and the black hole is so dense I don’t see it is very likely, although admittedly we know nothing about what goes on inside a black hole. We do know that nothing can escape from a black hole, so the mechanism of losing energy and angular momentum by ejecting something is out.

Now that such an event has been properly assigned to the collision of two neutron stars it makes me, at least, feel that everything is far more likely to be correct. Neutron stars are what is left over from a supernova, and it is easier to see neutron stars capture each other. First, neutron stars are made from very large stars, and while these are rarer than most other types of star, sometimes they come as double stars. That would make neutron stars close to each other, which is a start. Further, neutron stars are more likely to be deformable, and most certainly are more able to eject material into space. Neutron stars are really only held together by their intense gravity, and as they approach each other, the gravity tends to cancel, as the approaching object pulls against the pre-existing force. If the force needed to hold the neutrons becomes insufficient, the ejection of a significant amount of material is possible. After all, nuclei with a significant number more neutrons than protons are quite unstable, and each decaying neutron gives off over 1 MeV of energy. The neutron star is effectively a huge nucleus, but is held together by intense gravity rather than the strong force.

Apparently in this collision, a huge amount of material was ejected into space, and it is argued that this sort of event is what caused the formation of the metals heavier than iron, including gold and platinum. Such atoms then get mixed with the ejecta from supernovae and the hydrogen and helium from the start of the Universe, and here we are. It is a good story. However, I wonder if it is true that that is where all the gold etc. comes from? What bothers me about that explanation is that it is argued that atoms up to iron are made in stars and supernovae, but heavier ones are not because in the clouds of the ejecta, there isn’t time for the processes we know about. In my opinion, the intense pressures of the supernova that forms the neutron star would also form heavier elements. They don’t have to be made by adding protons and neutrons, after all, the synthetic elements we make, such as element number 118, are made by colliding big nuclei together. In this context, if you see a graph of the relative occurrence of the various elements, the curve is more or less smooth. Yes, there is a bit of a peak for elements around iron, but it then decays smoothly, and I would have expected that if some were made by a totally different procedure, then their relative concentrations would lie on different curves. I guess I shall never know. I can’t see anyone taking samples from the core of a supernova any time soon.

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