How can we test the theory of relativity?

Scientists have developed a new way to test one of the basic principles underlying Einstein’s theory of general relativity using brief blasts of rare radio signals from space called fast radio bursts.

The new method is ten- to one-hundred times better than previous testing methods that used gamma-ray bursts, according to a paper in the journal Physical Review Letters.

“This result is a significant tribute to Einstein's theory, on the hundredth anniversary of its first formulation.”

The work lines up with the 100th anniversary of Einstein’s first formulation of the equivalence principle, which is a key component of his theory of general relativity. More broadly, it also is a key component of the concept that the geometry of spacetime is curved by the mass density of individual galaxies, stars, planets, and other objects.

Fast radio bursts

Fast radio bursts are super-brief blasts of energy—lasting just a few milliseconds. Until now, only about a dozen fast radio bursts have been detected on Earth. They appear to be caused by mysterious events beyond our Milky Way Galaxy, and possibly even beyond the Local Group of galaxies that includes the Milky Way. The new technique will be important for analyzing the abundance of observations of fast radio bursts that advanced radio-signal observatories, now being planned, are expected to detect.

“With abundant observational information in the future, we can gain a better understanding of the physical nature of fast radio bursts,” says senior author Peter Mészáros, chair in astronomy and astrophysics and professor of physics at Penn State.

[Where do fast radio bursts come from?]

Like all other forms of electromagnetic radiation including visible light, fast radio bursts travel through space as waves of photon particles. The number of wave crests arriving from fast radio bursts per second—their “frequency”—is in the same range as that of radio signals.

“When more-powerful detectors provide us with more observations,” Mészáros says, “we also will be able to use fast radio bursts as a probe of their host galaxies, of the space between galaxies, of the cosmic-web structure of the universe, and as a test of fundamental physics.”

The impact of the new method using fast radio bursts is expected to increase significantly as more of the bursts are observed, and if their origin can be established more firmly.

“If fast radio bursts are proven to originate outside the Milky Way Galaxy, and if their distances can be measured accurately, they will be a new powerful tool for testing Einstein’s equivalence principle and for extending the tested energy range down to radio-band frequencies,” Mészáros says.

Incoming photons

Einstein’s equivalence principle requires that any two photons of different frequencies, emitted at the same time from the same source and traveling through the same gravitational fields, should arrive at Earth at exactly the same time.

“If Einstein’s equivalence principle is correct, any time delay that might occur between these two photons should not be due to the gravitational fields they experienced during their travels, but should be due only to other physical effects,” Mészáros says. “By measuring how closely in time the two different-frequency photons arrive, we can test how closely they obey Einstein’s equivalence principle.”

[What are theoretical physicists actually up to?]

More specifically, Mészáros says the test that he and his coauthors developed involves an analysis of how much space curvature the photons experienced due to massive objects along or near their path through space. He says, “Our test of Einstein’s equivalence principle using fast radio bursts consists of checking by how much does a parameter—the gamma parameter—differ for the two photons with different frequencies.”

Mészáros says his research team’s analysis of the less-than-a-dozen recently detected fast radio bursts “supersedes by one to two orders of magnitude the previous best limits on the accuracy of the Einstein equivalence principle,” which were based on gamma rays and other energies from a 1987 supernova explosion, supernova 1987A.

“Our analysis using radio frequencies shows that the Einstein equivalence principle is obeyed to one part in a hundred million,” Mészáros says. “This result is a significant tribute to Einstein’s theory, on the hundredth anniversary of its first formulation.”

The National Basic Research Program of China, NASA, the National Natural Science Foundation of China, and the Chinese Academy of Sciences supported the work.