Einstein's Gravity Theory Passes Toughest Test Yet

Clara Moskowitz, SPACE.com, 04/25/13

An extreme pair of superdense stars orbiting each other has put Einstein's general theory of relativity to its toughest test yet, and the crazy-haired physicist still comes out on top.

About 7,000 light-years from Earth, an exceptionally massive neutron star that spins around 25 times a second is orbited by a compact, white dwarf star. The gravity of this system is so intense that it offers an unprecedented testing ground for theories of gravity.

Scientists know general relativity, proposed by Albert Einstein in 1915, isn't the complete story. While it does very well describing large, massive systems, it's incompatible with quantum mechanics, which governs the physics of the very small. For something extremely small, yet extremely massive — such as a black hole — the two theories contradict each other, and scientists are left without a physical description. [6 Weird Facts About Gravity]

Rare systems like this binary star pair offer a chance to probe the boundary between the two theories, and search for possible openings toward new physics that could reconcile them.

"We thought this system might be extreme enough to show a breakdown in general relativity, but instead, Einstein's predictions held up quite well," Paulo Freire, an astronomer at the Max Planck Institute for Radio Astronomy in Germany, said in a statement.

Extreme pair

The white dwarf in the pair represents the remains of an aging star that lost its atmosphere and is gradually cooling down. Its companion, the neutron star, is much more massive and is so dense that the protons and electrons in its atoms have collapsed to form neutrons. Created in a supernova explosion that marked the death of a star, the neutron star contains twice the mass of the sun packed into an area just 12 miles (19 kilometers) wide. On the surface of this star, gravity is 300 billion times stronger than on Earth.

The neutron star is what's called a pulsar, because it gives off a beam of light that appears to pulse on and off as it rotates past Earth.

"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," said John Antoniadis, a Ph.D. student at the Max Planck Institute for Radio Astronomy and lead author of a paper reporting the find, published in the April 26 issue of the journal Science. "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."

Gravitational waves

The gravitational field created by the pulsar is so strong that the scientists suspected they might notice deviations from the predictions of general relativity in the motions of the white dwarf around it. General relativity posits that massive objects warp the space and time around them, causing other objects, and even light, to travel along curved paths when they pass nearby.

General relativity also predicts that a close binary system such as this one will radiate gravitational energy in the form of ripples in space-time called gravitational waves. This loss of energy would cause the orbital period of the system to change slightly over time. Alternative theories of gravity offer slightly different predictions for the white dwarf's motions.

"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, another team member at the Max Planck Institute.

Though the results don't help physicists solve the fundamental gravity quandary, they do confirm that current efforts to detect gravitational waves, based on Einstein's predictions, are on the right track. Several Earth-based tests are underway to look for perturbations in space-time distances caused by passing gravitational waves.

"Our results indicate that the filtering techniques planned for these advanced instruments remain valid," said Ryan Lynch, a physicist at McGill University in Montreal.

The pulsar system was first discovered using the National Science Foundation's Green Bank Telescope in West Virginia, and follow-up observations were made at 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.

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http://www.space.com/20826-einstein-gravity-theory-toughest-test.html

NASA Gravity Probe Confirms Two Einstein Theories

SPACE.com Staff

A NASA probe orbiting Earth has confirmed two key predictions of Albert Einstein's general theory of relativity, which describes how gravity causes masses to warp space-time around them.  

The Gravity Probe B (GP-B) mission was launched in 2004 to study two aspects of Einstein's theory about gravity: the geodetic effect, or the warping of space and time around a gravitational body, and frame-dragging, which describes the amount of space and time a spinning objects pulls with it as it rotates.

"Imagine the Earth as if it were immersed in honey," Francis Everitt, GP-B principal investigator at Stanford University in Palo Alto, Calif., said in a statement. "As the planet rotates, the honey around it would swirl, and it's the same with space and time. GP-B confirmed two of the most profound predictions of Einstein's universe, having far-reaching implications across astrophysics research." [6 Weird Facts About Gravity]

Gravity Probe B used four ultra-precise gyroscopes to measure the two gravitational hypotheses. The probe confirmed both effects with unprecedented precision by pointing its instruments at a single star called IM Pegasi.

If gravity did not affect space and time, GP-B's gyroscopes would always point in the same direction while the probe was in polar orbit around Earth. However, the gyroscopes experienced small but measurable changes in the direction of their spin while Earth's gravity pulled at them, thereby confirming Einstein's theories.

"The mission results will have a long-term impact on the work of theoretical physicists," said Bill Danchi, senior astrophysicist and program scientist at NASA Headquarters in Washington, D.C. "Every future challenge to Einstein's theories of general relativity will have to seek more precise measurements than the remarkable work GP-B accomplished." [Top 10 Strangest Things in Space]

A long time coming

These results conclude one of the longest-running projects in NASA history. The space agency became involved in the development of a relativity gyroscope experiment in 1963.

Decades of research and testing led to groundbreaking technologies to control environmental disturbances that could affect the spacecraft, such as aerodynamic drag, magnetic fields and thermal variations. Furthermore, the mission's star tracker and gyroscopes were the most precise ever designed and produced.

The GP-B project has led to advancements in GPS technologies that help guide airplanes to landings. Additional innovations were applied to NASA's Cosmic Background Explorer mission, which accurately determined the universe's background radiation left over from shortly after the Big Bang.

The drag-free satellite concept pioneered by GP-B made a number of Earth-observing satellites possible, including NASA's Gravity Recovery and Climate Experiment. These satellites provide the most precise measurements of the shape of the Earth, which are critical for navigation on land and sea, and understanding the relationship between ocean circulation and climate patterns.

Gravity Probe B's wide reach

The GP-B mission also acted as a training ground for students across the United States, from candidates for doctorates and master's degrees to undergraduates and high school students. In fact, one undergraduate who worked on GP-B went on to become the first female astronaut in space, Sally Ride.

"GP-B adds to the knowledge base on relativity in important ways and its positive impact will be felt in the careers of students whose educations were enriched by the project," said Ed Weiler, associate administrator for the science mission directorate at NASA headquarters.

GP-B completed its data collection operations and was decommissioned in December 2010. The probe's findings were published online in the journal Physical Review Letters.

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Top 10 Strangest Things in Space 

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