Pulsar Timing and Gravitational Waves

Greetings en route from State College, Pennsylvania, home of Penn State University, the only University of which I am aware with a library named after its football coach, Joe Paterno. More relevant to me, Penn State is also the home of the Center for Gravitational Wave Physics, which has been hosting a workshop, “The Pulsar Timing Array — A Nanohertz Gravitational Wave Telescope“. Since I’ve just been accused of not having enough science in this blog:

First, “pulsars” are thought to be spinning neutron stars (a super-dense object that a star can become after it’s run out of fuel), with a lighthouse-like beam that we can detect using radio telescopes — when it happens to be pointed at the earth. The fastest of these rotate almost 1,000 times a second — the millisecond pulsars, discovered in the early 80s by Don Backer and colleagues. Amazingly, the rotation rate is so stable that pulsars can be better timekeepers than the best atomic clocks on the earth.

Next, “gravitational waves,” predicted by Einstein’s General Relativity, are ripples in space and time, actually stretching or compressing the distance between objects as they propagate through the Universe. Gravitational waves are created by the movement of very massive objects like Black Holes (or, indeed, neutron stars: the best indirect evidence for the existence of gravitational waves comes from a pair of pulsars in orbit around one another, speeding up in their orbits as they emit gravitational waves just as predicted by Einstein’s theory).

So, what do these things have to do with one another? If a gravitational wave traverses the space between our radio telescope and a pulsar, it changes the amount of space along the way — so the arrival of the pulsar’s signal is accelerated or delayed by just the amount of time given by the speed of light multiplied by the change in distance — which we can observe through these very small changes in the pulses’ arrival times. The challenge is doing this measurement well enough — we expect the delay to be a few nanoseconds (one billionth of a second), compared to the milliseconds (one thousandth of a second) between pulses.

For three days, we talked about ways of generating gravitational waves, about atomic clocks, about all the other physical processes that can distort a pulsar’s signal, and all the great new technology that we hope to be able to use to observe these objects.

Let me also thank the locals for the fact that State College is also the home of lots of free WiFi access points…