Pulsars, the subject of Ransom’s research, are neutron stars. Neutron stars are the collapsed cores of exploded stars that, when they were still shining, were several times more massive than our Sun. The violent gravitational collapse that forms a neutron star ends with typically twice the mass of the Sun crushed into a sphere just six to eight miles wide. An average neutron star’s surface gravity is a hundred billion times stronger than Earth’s. If one were to hit Earth (impossible since we know there are none within several light years: they would be easily visible on our X-ray sky maps), the strong rock and iron of which our lovely planet is made would be like cotton candy. Earth would be effortlessly crushed and devoured by the neutron star’s relentless gravity, until it was reduced to a hot, ultra-dense layer roughly half an inch thick coating the surface of the neutron star.
But that’s all old hat for an astronomer. What we especially like is spinning neutron stars: pulsars. These emit radio beams out their north and south magnetic poles (no one is quite sure how). If the neutron star’s rotation axis is not aligned with its magnetic axis (quite common in the Universe: even Earth’s are somewhat misaligned, which is why a magnetic compass doesn’t quite give true North), the radio beams sweep through a cone of space something like the beam from a lighthouse. Every time one of the beams sweeps across earth, we see a flash (that is, a pulse) of radio emission. Since nothing can speed up or slow down the rotation of a neutron star very quickly, the interval between pulses is exquisitely consistent for any given pulsar. For a typical pulsar, this interval is a few seconds.
Consider what that means: an object twice the mass of our Sun is spinning several times a minute.
But even neutron stars like that have become comfortable, familiar objects in astronomy. What Scott Ransom and his research group study are objects called millisecond pulsars, which rotate many times per second. There is one – a neutron star more massive than the sun – that goes through 716 rotations every second.
These are really pretty fantastic objects. As far as we can tell, they can only get spinning so fast by gravitationally devouring the outer layers of an orbiting companion star over a period of some millions of years. The infalling gas from the companion star forms a rotating accretion disk that transfers angular momentum to the pulsar and makes it spin faster. So millisecond pulsars are usually found close to (and in a binary orbit with) the dead core of a companion star whose outer layers they have eaten (this dead core is usually a kind of object called a white dwarf). Ransom and other radio astronomers can measure the pulse times with exquisite accuracy – among the most accurate measurements in all of science – and use them to precisely calculate the orbit of the pulsar and its white dwarf companion. In at least two cases they have seen an additional delay in the pulse times, which only happens when the pulsar is passing almost behind the white dwarf. This delay is caused by the pulses taking longer to reach earth because they are traversing the curved space caused by the white dwarf’s gravity: a prediction of Einstein’s theory of General Relativity which has been precisely verified by observations of millisecond pulsars.
OK, millisecond pulsars are fine. All the ones we know are in our own Milky Way Galaxy: they are too faint for us to see in other galaxies because of the huge intergalactic distances. But Ransom had something to say about supermassive black holes in the centers of other galaxies. Most galaxies apparently have one: our Milky Way’s is several million times the mass of the Sun. It’s a wimp: some galaxies have a black hole tens of thousands of times bigger even than that. So supermassive black holes are a normal part of galaxies. Also, galaxies are colliding all the time (not to worry: the collisions take hundreds of millions of years, are usually harmless to the galaxies’ constituent stars and planets, and our Milky Way isn’t due for one until it hits the Andromeda Galaxy about 5 billion years from now). Anyway, a frequent result of a collision between two galaxies is that they both merge into one bigger galaxy. If both galaxies had supermassive black holes (which they usually do, as far as we can tell), the two supermassive black holes will both sink into the center of the new, merged galaxy, and eventually they will collide and merge. In the last few centuries before they merge, they will be orbiting each other in a tight inward spiral and emitting gravitational waves (as predicted by Einstein).
Such gravitational waves haven’t yet been directly observed (they are really hard to detect), and the idea that black holes collide in merged galaxies hasn’t been proven, it’s just quite likely based on what we do know. But Scott Ransom and his colleagues have determined that (with very careful observations) it should be possible to detect the signature of passing gravitational waves by monitoring the timing deviations they should cause in millisecond pulsars. Basically, the gravitational waves from the colliding black holes will periodically stretch and compress the space between Earth and the millisecond pulsar, causing tiny variations in exactly when the pulses reach us.
So, astronomers get together and what do they talk about? How to detect colliding supermassive black holes in distant galaxies by measuring the mysterious pulses of spinning neutron stars. Even though our work is hard and our wild ideas don’t always work, I feel enormously blessed just to be able to talk and dream about stuff like that. Yes, science is fun.