Einstein's dream come true
Earth & Sky has more with astronomer Maura McLaughlin.
Read or listen: Pulsar-pair-confirms Einstein's-theory-of-gravity
Read or listen: Scale-of-cosmos-gives astronomer-perspective
Maura McLaughlin’s team makes a discovery that helps confirm theory of gravity.
Maura McLaughlin is an astronomer at West Virginia University. She’s part of a team that discovered a “double pulsar system,” the first such system known. Pulsars are exploded stars that have become so compressed and so dense that they can form ripples in the fabric of space and time itself. Earth & Sky’s Jorge Salazar spoke with McLaughlin about her work.
Salazar: Tell me about the recent work done with the double pulsar system in confirming Einstein’s theory of gravity. I understand that’s the subject of a September 14, 2006 article in the journal Science.
McLaughlin: Our recent article is about a double pulsar system. It’s a system of two pulsars that are orbiting each other.
A pulsar is a neutron star, which is a very, very dense star. It’s about the mass of our sun with a radius about the size of a city. So it’s a really, really dense object, about a 20-kilometer (12.5 miles) in radius. And these pulsars spin very, very quickly, some of them up to hundreds of times a second. They’re very bright radio sources.
And they’re kind of like lighthouses. Every time the pulsar spins by us, we see this bright burst of radio emissions as its beam crosses our line of sight.
Gases and dust swirl around the Crab Nebula pulsar, made colorful here by the Hubble telescope through combining optical and x-ray images. McLaughlin describes pulsars as “lighthouses,” which beam radio waves like a beacon to observers on Earth.
This is the only system known where we have two radio pulsars in orbit around each other, that we can detect both pulsars. There may be more out there in the galaxy, we think there are, but this is the only system which we’ve actually detected so far.
The recent paper that we’ve just published in Science is not so much about the discovery of the pulsars. They were discovered a couple of years ago. It was about the applications of the pulsars to general relativity.
Gases and dust swirl around the Crab Nebula pulsar, made colorful here by the Hubble telescope through combining optical and x-ray images. McLaughlin describes pulsars as "lighthouses," which beam radio waves like a beacon to observers on Earth.
It turns out that systems like this are kind of like Einstein’s dream come true, because we have these two extremely massive objects in orbit around each other, and we can measure all of these really cool relativistic effects that we can’t measure on Earth, and we can’t measure with any other astronomical system.
Pulsars act like clocks. So, we can predict with really incredible accuracy exactly what time we expect the next pulse from the pulsar to arrive. And we can see tiny little perturbations on the arrival times of the pulses due to effects predicted by Einstein, such as the warping of space-time. We can actually see that space-time is curved in the presence of these really massive objects.
Salazar: How would you describe a pulsar?
McLaughlin: Pulsars, in general, are formed when a star undergoes a supernova. So, a normal star reaches the end of its life, and it can’t support itself against gravity anymore, so, it explodes as a supernova. And, all of the outer bits of the star make this beaurtiful remnant called a supernova remnant. You can see pictures of those online. They’re gorgeous, there are all of these bright filiments.
In the center of these remnants, though, is left a very compact core of material. And that’s what the pulsar is. It’s the very compact core leftover when a star reaches the end of its life.
The double pulsar system is "kind of like Einstein's dream come true," McLaughlin told Earth & Sky, "because we have these two extremely massive objects in orbit around each other, and we can measure all of these really cool relativistic effects that we can't measure on Earth, and we can't measure with any other astronomical system."
This double pulsar system, what we think happened is that it was a binary star system at first. One of the stars underwent a supernova, and then another star underwent a supernova as well. And these systems are very, very rare.
Usually, when one of the stars explodes, the orbit is disrupted, and they can’t remain in a binary anymore. So we’re very lucky to find this system. There aren’t that many of them where both stars undergo supernova and actually stay in their orbit.
But this system is one that happened to survive these two explosions.
A second reason that we’re very lucky to detect this system is that pulsar emission is beamed, like lighthouses, and you have to be right on the proper line of sight so that you can actually see this beam. We’re very lucky to have a system where both stars have survived the explosion, and we happen to see both beams pointing towards us.
Salazar: What inspired you to become an astronomer?
McLaughlin: I guess I’ve always been interested in astronomy from when I was very young, from reading Issac Asimov and other science fiction books.
I remember from high school reading Stephen Hawking’s book, “A Brief History of Time,” I mean I think everyone read it. It was a very popular book. But somehow it just really got to me, and I was so interested. That was what sort of made me think, “Wow, I really might want to be an astronomer.”
I also read lots of Carl Sagan’s books, like “Contact,” and things like that, thinking about other worlds out there. Mostly just lots of reading.
Maura McLaughlin and her husband Duncan Lorimer, pictured here next to the radio telescope at West Virginia University.
Salazar: How does your work as an astronomer translate to your day-to-day life?
McLaughlin: I think that you lose track of the scale, really. Most of the time I’m just sitting here in my office with my computer, writing computer programs to do this or that, debugging code and reading data off of tapes. I forget about how amazing it is that we are actually looking at things that are thousands of light-years away. So I think that for the most part it doesn’t have that much of an effect on my life, because I really do forget how amazing it is.
But then sometimes, I’ll have a good day at work, and you find something really big like this, and it does put things in perspective. And when you’re worrying about trying to get the garbage out on time, and you think, “Wow, I’ve just been looking at things that are thousands of light-years away.” How important, really, is it, that I’m holding on to my recycling for one more week because I forgot it?
Little problems on Earth do seem more insignificant when you try to put them in perspective. For the most part, though, it’s very easy to forget about the scale of things and how awesome it is to be studying this stuff. I have to kind of remind myself most of the time.
In their report “Tests of General Relativity from Timing the Double Pulsar” (Science, 6 October 2006, p. 97), Kramer et al. claim they have verified Einstein’s General Relativity Theory to an accuracy of 0.05% by four independent tests. See also Cho’s news story in Science, 15 September 2006, p.1556.
The data present pulse timing measurements of the binary radio pulsar system PSR J0737-3039A/B for 2.5 years. In this paper the “masses” of the two pulsars were not determined directly (What is meant by the word “mass” used here and in most of current literature is “gravitational force,” see P. Spolter, “New Concepts in Gravitation,” Physics Essays 18, 37-50, 2005). The authors have assumed that Einstein’s equation for the advance of the perihelion, or periastron here is correct and they have calculated the presumed parameters. Furthermore, to fit the data with the theory they have accepted 68% confidence ranges, which is statistically not significant.
Data presented in this paper do not support the authors’ claim. Observations of the rate of orbital period change of the 11-minute X-ray binary 4U 1820-30 for more than 13 years do not support loss of energy through gravitational radiation (J. Tan et al., Astrophys. J. 374, 291, 1991). No gravitational wave signals have been detected by the two 4 km long laser interferometers LIGO (B. Abbott et al., Class. Quantum Grav. 23, S29, 2006).