Posted on by John Williams

Einstein Right Again! Rapidly Spinning Pulsar Follows General Relativity

This artist’s impression shows the exotic double object that consists of a tiny, but very heavy neutron star that spins 25 times each second, orbited every two and a half hours by a white dwarf star. The neutron star is a pulsar named PSR J0348+0432 that is giving off radio waves that can be picked up on Earth by radio telescopes. Although this unusual pair is very interesting in its own right, it is also a unique laboratory for testing the limits of physical theories. This system is radiating gravitational radiation, ripples in spacetime. Although these waves (shown as the grid in this picture) cannot be yet detected directly by astronomers on Earth they can be sensed indirectly by measuring the change in the orbit of the system as it loses energy. As the pulsar is so small the relative sizes of the two objects are not drawn to scale.

A unique and exotic laboratory about 6,800 light-years from Earth is helping Earth-based astronomers test Albert Einstein’s theory of general relativity in ways not possible until now. And the observations exactly match predictions from general relativity, say scientists in a paper to be published in the April 26 issue of the journal Science.

Using ESO’s Very Large Telescope along with other radio telescopes, John Antoniadis, a PhD student at the Max Planck Institute for radio Astronomy (MPIfR) in Bonn and lead author of the paper, says the bizarre pair of stars makes for an excellent test case for physics.

“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,” says Antoniadis. “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 strange pair consists of a tiny and unusually heavy neutron star that spins 25 times per second. The pulsar, named PSR J0348+0432 is the remains of a supernova explosion. Twice as heavy as our Sun, the pulsar would fit within the confines of the Denver metropolitan area; it’s just 20 kilometers across or about 12 miles. The gravity on this strange star is more than 300 billion times stronger than on Earth. At its center, where the intense gravity squeezes matter even more tightly together, a sugar-cubed-sized block of star stuff would weight more than one billion tons. Only three other pulsars outside globular clusters spin faster and have shorter periods.

J0348+0432 could easily fit within the confines of most American cities, including Denver, Colo. Want to see how big J0348+0432 is compared to your city? Check out this map tool. Zoom into or search for your city, enter 10 km into the radius distance field, and click on a point on the map.)
J0348+0432 could easily fit within the confines of most American cities, including Denver, Colo. Want to see how big J0348+0432 is compared to your city? Check out this map tool. Zoom into or search for your city, enter 10 km into the radius distance field, and click on a point on the map. Credit: Google Maps
In addition, a much larger white dwarf, the extremely hot, burned-out core of a Sun-like star, whips around J0348+0432 every 2.5 hours.

As a consequence, radio astronomers Ryan Lynch and colleagues who discovered the pulsar in 2011, realized the pair would enable scientists to test theories of gravity that were not possible before. Einstein’s general theory of relativity describes gravity as a curvature in spacetime. Like a bowling ball nestled in a stretched bedsheet, spacetime bends and warps in the presence of mass and energy. The theory, published in 1916, has withstood all tests so far as the simplest explanation for observed astronomical phenomena. Other theories of gravity make different predictions but these differences would reveal themselves only in extremely strong gravitational fields not found within our solar system. J0348+0432 offered the opportunity to study Einstein’s theory in detail.

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This video shows an artist’s impression of the exotic double object known as PSR J0348+0432. This system is radiating gravitational radiation, or ripples, in spacetime. Although these waves cannot be yet detected directly by astronomers on Earth they can be detected indirectly by measuring the change in the orbit of the system as it loses energy. Credit: ESO/L.Calçada

Antoniadis’ team combined observations of the white dwarf from the European Southern Observatory’s Very Large Telescope with the precise timing of the pulsar from other radio telescopes, including the Green Bank Telescope in West Virginia, Effelsberg 100 meter radio telescope in Germany, and the Arecibo Observatory in Puerto Rico. Astronomers predict such close pulsar binaries radiate gravity waves and lose minute amounts of energy over time causing the orbital period of the white dwarf companion to change slightly. The astronomers found that predictions for this change closely matched those of general relativity while competing theories were different.

“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,” states Paulo Freire, another team member, in the press release.

Sources:
ESO: Einstein Was Right – So Far
Astrophysical Journal: The Green Bank Telescope 350 MHz Drift-scan Survey II: Data Analysis and the Timing of 10 New Pulsars, Including a Relativistic Binary
Aspen Center for Physics Physical Application of Millisecond Pulsars meeting January 2013: The Compact Relativistic Binary PSR J0348+0432

Posted on by John Williams

A Crinkle in the Wrinkle of Space-time

Albert Einstein’s revolutionary general theory of relativity describes gravity as a curvature in the fabric of spacetime. Mathematicians at University of California, Davis have come up with a new way to crinkle that fabric while pondering shockwaves.

“We show that spacetime cannot be locally flat at a point where two shockwaves collide,” says Blake Temple, professor of mathematics at UC Davis. “This is a new kind of singularity in general relativity.”

Temple and his collaborators study the mathematics of how shockwaves in a perfect fluid affect the curvature of spacetime. Their new models prove that singularities appear at the points where shock waves collide. Vogler’s mathematical models simulated two shockwaves colliding. Reintjes followed up with an analysis of the equations that describe what happens when the shockwaves cross. He dubbed the singularity created a “regularity singularity.”

“What is surprising,” Temple told Universe Today, “is that something as mundane as the interaction of waves could cause something as extreme as a spacetime singularity — albeit a very mild new kind of singularity. Also surprising is that they form in the most fundamental equations of Einstein’s theory of general relativity, the equations for a perfect fluid.”

The results are reported in two papers by Temple with graduate students Moritz Reintjes and Zeke Vogler in the journal Proceedings of the Royal Society A.

Einstein revolutionized modern physics with his general theory of relativity published in 1916. The theory in short describes space as a four-dimensional fabric that can be warped by energy and the flow of energy. Gravity shows itself as a curvature of this fabric. “The theory begins with the assumption that spacetime (a 4-dimensional surface, not 2 dimensional like a sphere), is also “locally flat,” Temple explains. “Reintjes’ theorem proves that at the point of shockwave interaction, it [spacetime] is too “crinkled” to be locally flat.”

We commonly think of a black hole as being a singularity which it is. But this is only part of the explanation. Inside a black hole, the curvature of spacetime becomes so steep and extreme that no energy, not even light, can escape. Temple says that a singularity can be more subtle where just a patch of spacetime cannot be made to look locally flat in any coordinate system.

“Locally flat” refers to space that appears to be flat from a certain perspective. Our view of the Earth from the surface is a good example. Earth looks flat to a sailor in the middle of the ocean. It’s only when we move far from the surface that the curvature of the Earth becomes apparent. Einstein’s theory of general relativity begins with the assumption that spacetime is also locally flat. Shockwaves create an abrupt change, or discontinuity, in the pressure and density of a fluid. This creates a jump in the curvature of spacetime but not enough to create the “crinkling” seen in the team’s models, Temple says.

The coolest part of the finding for Temple is that everything, his earlier work on shockwaves during the Big Bang and the combination of Vogler’s and Reintjes’ work, fits together.

There is so much serendipity,” says Temple. “This is really the coolest part to me.
I like that it is so subtle. And I like that the mathematical field of shockwave theory, created to address problems that had nothing to do with General Relativity, has led us to the discovery of a new kind of spacetime singularity. I think this is a very rare thing, and I’d call it a once in a generation discovery.”

While the model looks good on paper, Temple and his team wonder how the steep gradients in spacetime at a “regularity singularity” could cause larger than expected effects in the real world. General relativity predicts gravity waves might be produced by the collision of massive objects, such as black holes. “We wonder whether an exploding stellar shock wave hitting an imploding shock at the leading edge of a collapse, might stimulate stronger than expected gravity waves,” Temple says. “This cannot happen in spherical symmetry, which our theorem assumes, but in principle it could happen if the symmetry were slightly broken.”

Image caption: Artist rendition of the unfurling of spacetime at the beginning of the Big Bang. John Williams/TerraZoom