Seven thousand light years away a recently discovered super dense binary star system is proving itself to be the ideal test case for verifying a prediction made by Albert Einstein in his General Theory of Relativity (GR) back in 1915.
The binary system is composed of a white dwarf star encircling a fast-spinning type of neutron star known as a pulsar.
Pulsars are super-fast rotating neutron stars that emit regular on/off bursts of radio transmission beams (hence the root term ‘pulse’) as they spin. This particular pulsar (designated pulsar J0348+0432) is considered quite massive at twice the mass of our sun (2.01 ± 0.04 solar mass (M☉)) but it’s packed into a spheroid stellar object just 12 miles (19 kilometers) in diameter — and it spins at a dizzying 25 rotations per second. The pulsar was most likely the result of a supernova explosion in which the protons and electrons collapsed in on themselves to form the dense neutron matter that gives it its name.
Describing his observations at a press conference, John Antoniadis, a Ph.D. student at the Max Planck Institute for Radio Astronomy and lead author of a paper reporting the find, stated:
I was observing the system with ESO’s [the European Southern Observatory’s] Very Large Telescope, looking for changes in the light emitted from the white dwarf caused by its motion around the pulsar. 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.
The orbiting white dwarf star is much more compact (0.172 ± 0.003 M☉) than the neutron star (but is also “super dense”) and was formed from an ageing star that had lost its outer atmospheric layer. The gravitational attraction of the pulsar is so intense that it has pulled the white dwarf into its gravitational field in a rapid orbit (once every 2.46 hours).
Just how intense? Astronomers estimate that the force of gravity at the surface of the central star is 300 billion times that on Earth.
So, What’s the Deal with Einstein’s Theory of Gravity?
As remarkably predictive and accurate as GR has been in describing large-scale features of the universe, it does not do so well with describing the phenomena on the very small scale of the universe — in the realm of the quantum. On this scale, the Theory of Quantum Mechanics (QT) rules — a complex theory that is also remarkably accurate in making predictions about events on the sub-atomic level. But neither theory offers a complete picture of things and, when it comes to explaining or describing objects that are both super small and super massive — such as black holes — the two theories actually contradict each other.
Such rare systems like this binary one offer scientists an equally rare chance to test basic tenets of both GR and QT — and possibly find data that might reconcile the two, somehow.
Quoting from the paper’s abstract:
Neutron stars with masses above 1.8 solar masses (M☉), possess extreme gravitational fields, which may give rise to phenomena outside general relativity. These strong-field deviations lack experimental confrontation, because they become observable only in tight binaries containing a high-mass pulsar and where orbital decay resulting from emission of gravitational waves can be tested.
According to GR, massive objects actually warp the ‘space-time fabric’ surrounding them. This warping or curvature causes other objects (and even light waves) to follow the curved space as they approach the more massive object. General Relativity makes very precise predictions about how much an approaching object will curve due to this warping of space.
However, the gravitational field around this particular pulsar is so strong that scientists originally thought they might observe deviations in the motion of the white dwarf that were out of line with the predictions of GR.
“We thought this system might be extreme enough to show a breakdown in general relativity, but instead, Einstein’s predictions held up quite well,” said Paulo Freire, an astronomer at the Max Planck Institute for Radio Astronomy in Germany, in a press statement.
More (Tentative) Confirmation of Einstein’s Theory — ‘Gravity Waves’
GR also states that a close-orbiting binary system should give off gravitational energy (note: gravity is one of the four known forces of Nature and the only force that has no verified particle to carry it) in the form of “ripples” in the space-time fabric — gravitational waves. And due to this loss of energy GR predicts that any orbiting object (like the white dwarf in this system) would experience an alteration (“decay”) in its orbital period over time.
Now, there are a few alternative gravitational theories out there (like quantum gravity) that make different predictions about the orbiting object’s motion in response to this loss of energy. But, as it turns out, it’s Einstein’s theory that takes the prize for predictive accuracy.
“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 — exactly what Einstein’s theory predicts,” said Paulo Freire, at the Max Planck Institute.
Once again, the predictions of Einstein’s General Theory of Relativity have been confirmed — this time through observational (telescopic) measurements. The earlier — and longer term — space probe B experiment confirmed another of Einstein’s predictions concerning “frame dragging”. To date, there is but one discovery that would appear to violate a tenet of GR: the recent discovery of a quasar super cluster.
Going Forward with Gravity Research
The confirmation of GR’s predictive power does not solve the basic question as to the nature of gravity (so far, no actual ‘gravitation waves’ have been detected or verified). However, it does give scientists a shot of confidence that they are on the right track with their current gravitational waves research (guided by Einstein’s theoretical predictions).
Again, quoting from the paper’s abstract:
“The resulting constraints on deviations support the use of GR-based templates for ground-based gravitational wave detectors. Additionally, the system strengthens recent constraints on the properties of dense matter and provides insight to binary stellar astrophysics and pulsar recycling.”
Gravitational waves passing through our solar system should cause measurable perturbations in space-time distances. Several experiments are under way to detect these perturbations using filtering techniques to eliminate interference from other sources.
The pulsar/white dwarf system was first detected using the National Science Foundation’s Green Bank Telescope in West Virginia, followed by validating observations made by the Apache Point telescope in New Mexico, the Very Large Telescope in Chile, the William Herschel Telescope in Spain, the Arecibo telescope in Puerto Rico and the Effelsberg telescope in Germany.
The findings were published in the April 26 issue of the journal Science, under the title: ‘A Massive Pulsar in a Compact Relativistic Binary’ (Antoniadis et al)
Some source material (including additional quotes and images) for this post came from the Yahoo News / Space.com article: ‘Einstein’s Gravity Theory Passes Toughest Test Yet’ by Clara Moskowitz