Rare space radio signals to better test Einstein's theory

Scientists have developed a new way to test one of the basic principles of Einstein's theory of General Relativity using brief blasts of rare radio signals from space.
The new method using radio waves, called Fast Radio Bursts, is ten to hundred times better than previous testing methods that used gamma-ray bursts, researchers said.
The method is considered to be a significant tribute to Albert Einstein on the 100th anniversary of his first formulation of the Equivalence Principle, which is a key component of the theory of General Relativity.
It also is a key component of the concept that geometry of spacetime is curved by the mass density of individual galaxies, stars, planets and other objects.

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.

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, 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," said senior author Peter Meszaros, professor at Pennsylvania State University in US.

"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," Meszaros said.
Einstein's Equivalence Principle requires that any two photons of different frequencies, emitted at the same time from the same source and travelling 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," Meszaros said.

"By measuring how closely in time the two different-frequency photons arrive, we can test how closely they obey Einstein's Equivalence Principle," Meszaros said.
Meszaros said that the test involves an analysis of how much space curvature the photons experienced due to massive objects along or near their path through space.
"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," he said.
The research was published in the journal Physical Review Letters.