We still don't know what produces fast radio bursts (FRBs), but that hasn't prevented scientists from using them to confirm Einstein's theory of general relativity. Indeed, the authors of a new paper claim that FRBs are many times better for this purpose than gamma-ray bursts or supernovae.
As their name suggests, FRBs are short radio pulses, detected by some of the largest telescopes on Earth. Originally found in archived Parkes Radio Telescope data, FRBs have now been detected from the giant Arecibo radio telescope. More importantly, one has now been caught in real time, allowing 12 other telescopes to immediately focus on the source, providing data across the electromagnetic spectrum.
Although debate continues as to the bursts' origins, the much lower frequency components of FRBs arrive after high frequencies provide evidence that the sources are billions of light-years away, since this behavior is indicative of having passed through fields of low-density free electrons.
The fact that FRBs come from outside the galaxy makes them a useful tool for testing general relativity. One aspect of Einstein's theory, known as Einstein's equivalence principle, holds that gravitational fields do not differentiate between photons of different frequencies, be they X-rays, visible light, or radio waves. The path of light can be distorted by the presence of a gravitational field, but this distortion will apply equally to high and low frequencies.
"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," said senior author Professor Peter Mészáros of Penn State University in a statement. "By measuring how closely in time the two different-frequency photons arrive, we can test how closely they obey Einstein's Equivalence Principle."
Such tests require confidence that the photons were emitted at the same time, which is not possible with long-lasting objects. FRBs, which last a few milliseconds, can safely be assumed to release their high- and low-frequency photons simultaneously.
In Physical Review Letters, Mészáros outlines a test for the value of the gamma parameter, which measures how much mass curves space. According to general relativity, gamma is 1 – any other value for gamma would indicate that gravity is being influenced by other fields, in contradiction to Einstein's expectation.
Although general relativity has survived a century of testing, physicists continue to devise new trials, in part because alternative theories exist that are expected to give the same results but to incredibly fine levels of precision. Moreover, our inability to reconcile general relativity with quantum theory suggests that Einstein's work may need modification. However, Mészáros' says his work demonstrates that if there are any deviations from the equivalence principle, they must be tiny – less than one part in 100 million. The figure is 10 to 100 times smaller than the minimum allowed by previous studies on gamma-ray bursts. As more FRBs are discovered, Mészáros expects these tests can become even more rigorous.