Scientists Use Extreme Stars to Test the Limits of Albert Einstein's Greatest Theory

Einstein's theory of general relativity may be over 100 years old, but a 16-year-long investigation of extreme stars has shown that it is still the best description of gravity we have.

An international team of researchers, including scientists from the University of East Anglia (UEA) and the University of Manchester in the U.K., studied a pair of pulsars with seven radio telescopes looking for behavior not covered by Einstein's crowning achievement.

And while they discovered relativistic effects that have never been witnessed before, the new phenomena were hypothesized in the theory, also known as the geometric theory of gravity, published in 1915.

"As spectacularly successful as Einstein's theory of general relativity has proven to be, we know that is not the final word in gravitational theory," UEA School of Physics professor, Robert Ferdman, said. "More than 100 years later, scientists around the world continue their efforts to find flaws in his theory."

The team chose to search for these flaws using pulsars, which Ferdman, the lead author of a paper describing the research published in the journal Physical Review X, describes as a highly magnetized rotating compact star that emits beams of electromagnetic radiation out of its magnetic poles.

"They weigh more than our sun but they are only about 15 miles across, so they are incredibly dense objects that produce radio beams that sweep the sky like a lighthouse," Ferdman continued.

The researchers chose to observe a double-pulsar which was discovered by members of the team in 2003 and presents what Ferdman calls "the most precise laboratory we currently have to test Einstein's theory."

The double pulsar observed by the team consists of two pulsars orbiting each other in just 147 minutes with a speed as great as 620,000 miles per hour.

One pulsar is spinning rapidly at about 44 times a second, while its younger companion has a rotation period of 2.8 seconds. It is the pulsars' motion around each other and their incredibly powerful gravitational fields that provide a near-perfect gravity laboratory.

General relativity suggests that the mass of an object warps the very fabric of spacetime, and the greater the mass, the more extreme the curvature. A common analogy physicists use to describe this is that of placing a marble and a bowling ball on a trampoline.

The bowling ball clear causes the larger dent, and when the marble rolls past it the path it takes is curved to follow this dent. Light does the same when it passes a massive cosmic object like a black hole or these pulsars.

University of British Columbia at Vancouver professor, Ingrid Stairs, explained what observing light emitted from the pulsar showed her and her fellow researchers. She said: "We see for the first time how the light is not only delayed due to a strong curvature of spacetime around the companion but also that the light is deflected by a small angle of 0.04 degrees that we can detect."

In addition to this, the team of scientists were able to test a series of other predictions made by general relativity, including the effect of time dilation—the stretching of time caused by tremendous gravitational fields—and the change in the orientation of the orbit previously seen around Mercury, but 140,000 times stronger around these pulsars.

"Never before has such an experiment been conducted at such a high spacetime curvature," Stairs added.

Physicists continue to search for physical characteristics of the Universe not described by general relativity because, despite how successful the theory has been in describing gravity and the physics of truly massive bodies in space, it can't be united with quantum physics, the best description physicist have of the sub-atomic.

This means that gravity, one of the Universe's four fundamental forces, stands apart from the other three—electromagnetism and the strong and weak nuclear forces—all of which are united in quantum physics.

"It is therefore important to continue to place the most stringent tests upon general relativity as possible, to discover how and when the theory breaks down," Ferdman continued.

"Finding any deviation from general relativity would constitute a major discovery that would open a window on new physics beyond our current theoretical understanding of the Universe."

Ferdman points out, this could eventually lead to a "unified theory" of physics, that offers a complete description of the forces of the Universe, and something that physicists like Stephen Hawking have spent a lifetime searching for.

Max Planck Institute for Radio Astronomy researcher Michael Kramer said: "Our work has shown the way such experiments need to be conducted and which subtle effects now need to be taken into account.

"And, maybe, we will find a deviation from general relativity one day."

albert einstein pulsars
An illustration of two pulsars, extreme stars orbiting around each other. Researchers have conducted a 16-year experiment around such a double pulsar to put Einstein's theory of general relativity to the test. (Right) Theoretical physicist Albert Einstein. MAX PLANCK INSTITUTE FOR RADIO ASTRONOMY /Getty Images

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