Wishing on a ( dead) star
Two dead stars, in fact, that may unlock the secrets of relativity
VANCOUVER
ngrid Stairs dotes on
a double- double. But
her passion has nothing to do with coffee and everything to do with the ideal laboratory for testing Einstein’s general theory of relativity.
This double- double is two neutron stars that are rapidly orbiting one another about 2,000 light years from the nearest Tim Hortons. Neutron stars are the dense remains of a supernova explosion; some neutron stars, like these two, are also pulsars, flashing beams of radio waves that reach the Earth as pulses.
Stars that come in pairs, called binary stars, are not rare — there’s one visible on dark nights in the handle of the Big Dipper, and astronomers recently identified a handful of binary neutron stars, some of which even boast a single pulsar — but this is the first binary system discovered where both are neutron stars and both also pulsars, thus a double- double.
Officially, says Stairs, this cosmic relativity lab is known as J0737-3039AB, a designation that indicates its celestial co- ordinates just off the tail of the constellation Canis Minor. A pause and a grin. “ We call it 0737 for short.” The astronomy professor at the University of British Columbia smiles a lot and has a ready laugh as she explains the exotic workings of pulsars to a visitor. And why not? At age 33, a stripling in the astronomical community, Stairs is part of an international team investigating the most exciting discovery in astronomy in decades.
Equally satisfying is that Stairs was invited to join the team because of her expertise, honed at the Green Bank radio telescope in West Virginia, in precisely measuring celestial events such as pulsar flashes.
For an observational radio astronomer who got turned on to pulsars by attending a talk as an undergrad at McGill University, this investigation is a dream come true.
“ I’m not going out stamp- collecting to try to find new things,” she says. “ What I want to do is investigate the physical properties of objects, understand either the evolution or do the tests of general relativity.” The double- double is the ideal relativity lab because its life promises to be — in Thomas Hobbes’s famous phrase about human existence — “ nasty, brutish and short.” Short in astronomical terms is an estimated lifespan of 85 million years.
That early demise is dictated by 0737’ s unique physical makeup. Consider that the two pulsars are both 30 per cent heavier than our sun, yet only 20 kilometres across. They’re closer together than any other binary neutron stars, separated only by about twice the distance between the Earth and the moon. And they’re really moving, at about 300 kilometres a second, or one- thousandth the speed of light. At that velocity, the two stars orbit each other in less than 2.5 hours. As well, both stars are spinning like hyperactive children’s tops: the older A pulsar rotating 44 times a second, the younger B pulsar rotating more sedately once every 2.8 seconds. Each rotation sends a radio pulse toward Earth, like the mirror turning around the lamp on top of a lighthouse.
All this frenetic activity is like sticking a huge Mixmaster into space-time around 0737, producing intense gravitational upheavals and putting the predictions of Einstein’s general relativity to their toughest test yet, says Stairs.
“ One of the reasons we like to do pulsar tests of general relativity is that there are very different gravitational regimes there — the gravitational fields are much stronger than you would ever get in the solar system. So that makes doing these tests worthwhile.”
IEinstein’s theory is holding up well in this demanding celestial laboratory. The international team of astronomers has witnessed four general relativity effects for sure and possibly a fifth. Their measurements of these phenomena agree with the theoretical predictions ( see box above). Such agreement doesn’t “prove” Einstein’s theory, because another finding could always come along that is at odds with general relativity. Real scientific theories always come with the prospect of being “ falsified” — exposed as false because they can’t account for some new observation.
inding this ideal cosmic
laboratory for testing general relativity turns out to have been part determined slogging and part happy fluke.
FIn 1997, astronomers in the U. K., Australia and Italy launched a collaborative search for the two cosmic “ holy grails” that could provide the sternest test of general relativity: a double pulsar or a pulsar orbiting a black hole. To accelerate the search, the researchers developed a new multibeam receiver that looked at 13 different areas of the sky simultaneously, probing longer, and therefore deeper, into space. They installed this super gizmo on Australia’s Parkes radio telescope, the remote facility credited with relaying pictures of the first moon landing, and which won fame in the quirky Australian movie The Dish. In April 2003, a Ph. D. student from Italy’s University of Bologna, Marta Burgay, found the signal from the older A pulsar buried in the torrent of data from Parkes. Although the nature of the signal indicated that A should have a neutron star companion, six months of further looking turned up nothing.
Then, that October, an astronomer from Britain’s Jodrell Bank Observatory was testing a new search technique for binary pulsars on some old data sets at Parkes, including the earlier recordings from 0737.
Unexpectedly, he uncovered a second radio signal, meaning the suspected companion star was also a pulsar. The pulsar was missed originally because it is only strong enough to be detected for about 20 minutes in each 2.5- hour orbit.
“ With this discovery, a race was suddenly on,” Jodrell astronomers Andrew Lyne and Michael Kramer wrote in the March issue of Physics Today.
“ It was clear to us that as soon as our colleagues elsewhere in the world read our paper announcing the discovery of the first pulsar, they would use all available telescopes to observe it. The question was, how long would it take them to discover the signals from the second pulsar?” Wanting to learn as much as possible about the double- double before other researchers got into the act is understandable. Lurking in the background is the knowledge that the two astronomers who discovered the first neutron binary star system in 1974 ( which supported general relativity effects) were awarded the 1993 Nobel Prize for Physics. As Stairs says, however, 0737 doesn’t reveal its secrets easily.
“ I’m helping work on the timing of the pulsars. That’s the procedure where we count literally every single pulse that arrives from the pulsars and try to develop a model that explains all of it. That’s how we measure the parameters that let us make the comparisons to the predictions of general relativity.” The measurements need to be precise to a matter of millionths of a second for the quick- flashing A pulsar and thousandths of a second for the B pulsar.
At this point in the discussion, Stairs begins manoeuvring her hands like a Battle of Britain pilot describing a dogfight, trying to evoke the spinning, orbiting, wobbling and Doppler shifting of the two pulsars in mid- air.
“ One way to think about it is that what we’re really doing is measuring the changes in the pulsar’s spin period as it goes around its orbit. Something called the Shapiro delay will make the pulsar briefly look like it’s rotating a little bit slower because the signal will take longer to get to you. Then it will ‘ speed up’ back to its previous spin rate after it comes out from behind the companion.” She produces a diagram on which the general relativity parameters being measured at 0737 are plotted against the possible masses of each star. To a non- astronomer’s eye it looks like a Gary Larson Far Side cartoon, with different animal tracks converging at a wilderness bar. To astronomers, however, the fact that these tracks all meet at a single point means more support for general relativity.
This is truly exciting frontier science. And there should be even more ahead, including probing the magnetosphere draping each pulsar, the envelopes of hot, magnetized plasma also found around the Earth. As well, small variations in the signals from a whole bunch of pulsars might provide the first direct detection of gravitational waves supposedly rippling through space.
“ What appeals to me about doing pulsars is there is a whole set of different physics problems that are accessible through these observations,” Stairs says.
“ You can start looking for other irregularities in the timing that might tell you something about the structure of the neutron stars themselves. You can use the statistics of pulsars and the distributions to look at some populations in the galaxy and even some components of the structure of the galaxies themselves.” There’s obviously enough challenges concerning 0737 to consume several lifetimes for any astronomer, even a young one. But there’s a possible glitch — what the astronomy gods give, they can also take away.
General relativity predicts the two stars will slowly wobble like spinning tops, altering the shape of the radio pulses recorded on Earth. Stairs compares this to the roof of a lighthouse shifting its position relative to the viewer. The team has already found evidence for this strange relativity effect — called geodetic precession — for the slower B pulsar and some hints of such variations in the faster A pulsar. On the surface, that may sound like good news. But that geodetic precession could also nudge the pulsar beams out of the line of sight from Earth. This might happen in months — or in decades. The astronomers don’t know what point the two pulsars have already reached in precession cycles estimated to be 70 years long.
Stairs, as is her nature, sees a bright side. At least the 0737 double-double doesn’t have a precession cycle of hundreds of years. That’s the case for two other well- studied binary neutron star systems, meaning they might not return for several generations after they fade.
If 0737 does wink out some day, that might explain why it wasn’t spotted in earlier radio telescope surveys of that same patch of sky: It simply wasn’t beaming toward Earth.
That would solve yet another cosmic mystery — and provide still more support for Einstein’s general theory of relativity. 4ENDNOTE