In February 1978, the British journal Nature published a brief letter from me, "Revised estimate of gravitational radiation from Crab and Vela pulsars". The note built upon earlier work by Kip Thorne and Bill Press, who in 1972 had surveyed possible astrophysical sources of gravitational waves. This was my first real "scientific" paper, and later formed Chapter I of my dissertation.
Quick technical background: gravitational waves are ripples in spacetime made by accelerating matter, much as electromagnetic waves (radio, light, etc.) are self-propagating ripples made by accelerating electrical charges. The more mass you take and the harder you shake it, the more gravitational waves you get. A neutron star, the remnant of a stellar collapse, is a dense ball several miles across with something close to the mass of the Sun. It can spin many times every second. (The rotation rate goes up as the star shrinks, just as an ice-skater whirls faster in the now-clichéd analogy.) Strong magnetic fields make some neutron stars give off radio waves, which we see once per turn as pulses — hence, "pulsars" — and the magnetism plus the stiffness of the neutron star's crystalline crust can make the body asymmetric. So, it spins off-axis, wobbles, precesses, nutates, quakes, shakes, rattles, rolls, and bottom-line wiggles enough to give off a goodly amount of gravitational radiation.
A major point made by my 1978 letter to Nature is that the fastest pulsars are not necessarily the strongest sources of waves. A more slowly rotating neutron star may be closer or may have a larger non-axisymmetry, for example. More specifically, my best estimates suggested that the Vela pulsar (with a period of 0.089 seconds) is likely to produce waves with amplitudes one or two orders of magnitude larger than the more-famous Crab Nebula pulsar (period 0.033 s). Although this possibility is rather obvious, it was apparently overlooked or discounted in earlier investigations. ^z's tiny contribution to the edifice of astrophysics! (^_^)
Gravitational waves from pulsars are unlikely to be much stronger than one part in a billion billion billion (10 to the -27th power). But such tiny signals, since they come at a predictable and stable frequency, might (barely!) be detectable in the laboratory using massive, supercooled, ultra-pure crystals, perhaps made of silicon or sapphire. Measurements of these waves could give new information about the structure of neutron stars.
As for me: gathering data to set up the problem, working through the equations, and coming to understand the results was a marvelous lesson in what it means to be a scientist. Even more important was the struggle to write up the results in a clear, compact, comprehensible form. I needed a lot of help, and I got it — from Carl Caves, Peter Goldreich, Kip Thorne, and David Douglass. Thanks, folks!
(see KipTheDragon, RelativityPlusAstrophysics, CherishedBeliefs, NiAndMe, SoftOutsideCrunchyCenter, and SpinningSources for other ^z thesis work)
Thursday, April 06, 2000 at 05:46:45 (EDT) = 2000-04-06
TopicPersonalHistory - TopicScience
(correlates: BowelsOfTheEarth, DannyTheDog, SpinningSources, ...)