# Synopsis: Distant Bursts Show no Signs of Predicted Light Rotation

Cosmic gamma-ray bursts turn out to be polarized, which rules out the breaking of a fundamental symmetry down to the lowest limits ever observed.

Gamma rays from cosmic explosions travel billions of light years to reach us. This makes them excellent probes of extremely small irregularities in the fabric of spacetime, as predicted by some quantum gravity theories. Along these lines, researchers have used the detection of polarization in three distant gamma-ray bursts (GRBs) as evidence that the light did not rotate during its long journey. As described in Physical Review Letters, this lack of rotation puts the most stringent constraints yet on the violation of a fundamental symmetry.

Attempts to unify gravity with quantum mechanics often predict small variations to the laws of relativity. This can open the door to violating $C\phantom{\rule{0}{0ex}}P\phantom{\rule{0}{0ex}}T$, which is a symmetry that relates antimatter, mirror reflections, and the arrow of time. One consequence of $C\phantom{\rule{0}{0ex}}P\phantom{\rule{0}{0ex}}T$ breaking would be a speed difference between photons of one polarization and another, observable through an energy-dependent rotation of light traveling through space. Several attempts to detect this rotation have come up empty, implying that nature obeys $C\phantom{\rule{0}{0ex}}P\phantom{\rule{0}{0ex}}T$ at least to a level of one part in $10$ million.

Kenji Toma of Osaka University in Japan and his colleagues have improved on these limits using data from the Japanese IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) spacecraft. Specifically, the on-board gamma-ray burst polarimeter (GAP) has detected linear polarization in the gamma-ray emission of three GRBs. This polarization could only be observed if $C\phantom{\rule{0}{0ex}}P\phantom{\rule{0}{0ex}}T$ is respected down to a level of one part in ${10}^{15}$, an improvement of eight orders of magnitude over previous model-independent limits. – Michael Schirber

### Announcements

More Announcements »

## Previous Synopsis

Materials Science

## Next Synopsis

Biological Physics

## Related Articles

Optics

### Synopsis: Sharper Vision for Infrared Telescopes

Converting infrared light to visible light might boost the sensitivity of infrared telescope arrays. Read More »

Astrophysics

### Focus: More Hints of Exotic Cosmic-Ray Origin

New Space Station data support a straightforward model of cosmic-ray propagation through the Galaxy but also add to previous signs of undiscovered cosmic-ray sources such as dark matter. Read More »

Cosmology

### Viewpoint: Connecting the Bright and Dark Sides of Galaxies

A universal law shows that the rotation of a disk galaxy is determined entirely by the visible matter it contains, even if the disk is mostly filled with dark matter. Read More »