Extreme pair of stars is no match for Einstein’s genius

The story of Einstein's general theory of relativity is starting to read like the ultimate 'David and Goliath' play, but in this case the hero keeps facing bigger and bigger opponents. Now the hero of the story has faced the biggest, or at least the most extreme opponent yet, and continues to come out on top.

The latest challenger for Einstein is actually a binary pair of stars — a white dwarf orbiting a pulsar. The white dwarf seems to be a fairly typical one — the leftover core of a dead star, with the mass of the Sun in an object only the size of the Earth. The pulsar, on the other hand, is the most massive one that astronomers have been able to confirm so far — the dense, crushed core of a huge star that compacts twice the mass of the Sun into a sphere just 20 kilometres across, that is spinning at the incredible rate of 25 times a second! Spinning that quickly, it throws off beams of radiation from its poles, which act like spotlights through space.

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"I was observing the system with ESO’s Very Large Telescope, looking for changes in the light emitted from the white dwarf caused by its motion around the pulsar," said John Antoniadis, the lead author of the study, who is a PhD student at the Max Planck Institute for Radio Astronomy. "A quick on-the-spot analysis made me realize that the pulsar was quite a heavyweight. It is twice the mass of the Sun, making it the most massive neutron star that we know of and also an excellent laboratory for fundamental physics."

Here's an artist's impression of what the pair looks like, as the white dwarf whips around the pulsar every two and a half hours.

The spiraling waves around the stars show how the gravity of this pair is causing ripples in the fabric of spacetime around them. The astronomers can't see the ripples directly, but they can measure how the orbit of the pair loses energy and slows down over time. The energy lost goes into 'gravitational radiation', which creates those ripples.

The equations of Einstein's general theory of relativity are designed to show how things behave as they move through space and time. Scientists believe that there will come a point when we observe something that these equations can't handle. According to the authors of the study, this binary pair was a good possibility for a system that could violate those equations, because of their extremely powerful gravity, but Einstein's theory held up.

"Our radio observations were so precise that we have already been able to measure a change in the orbital period of 8 millionths of a second per year," said co-author Paulo Freire, "exactly what Einstein's theory predicts."

The study, titled A Massive Pulsar in a Compact Relativistic Binary was published today, April 26th, in the journal Science. It included work from 22 astronomers from the around the world, including three Canadian astronomers: Ryan Lynch, a post-doc scholar at McGill University, Marten van Kerkwijk, a professor of astronomy at the University of Toronto, and Ingrid Stairs, who is a professor of astronomy with the University of British Columbia.

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It's amazing that Einstein's equations, even though they were written down over 100 years ago, are still standing up to the most rigorous tests we can throw at them. We already know they don't play well with the stuff that's really, really small, but the quantum world forced physicists to come up with an entirely new branch of the science to deal with it, so that's understandable. However, there's long been the idea that we'd eventually find some kind of system at the opposite end of the spectrum (very large, very massive, very complex) that would cause Einstein's equations to break down.

Well, I guess we just have to wait to see what opponent steps up next!

(Image and video courtesy: ESO/L. Calçada)

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