All About Space – February 2020
English | 99 pages | pdf | 50.48 MB

Photons could uncover ‘massive gravity’,brand new theory suggests

Gravitational waves, ripples in spacetime, slip through Earth all the time, carrying secrets about the universe. Until a few years ago we couldn’t detect these at all, and even now we have only a basic ability to detect the stretching and squeezing of the cosmos. A proposed new gravitational-wave hunter, which would measure how particles of light and gravity interact, could change that. In the process it could answer big questions about dark energy and the universe’s expansion.
The detectors on Earth today, called the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo, operate according to the same principle: as a gravitational wave moves through the Earth, it faintly stretches and squeezes space-time. By measuring how long a laser light takes to travel over long distances, the detectors notice when the size of that space-time changes. But the changes are minute, requiring extraordinarily sensitive equipment and statistical methods to detect.
A team led by Subhashish Banerjee, a physicist at the Indian Institute of Technology in Jodhpur,India, proposed a radical new method: hunting gravitational waves by looking for effects of direct interactions between gravitons – theoretical particles that carry gravitational force – and photons, the particles that make up light. By studying those photons after they’ve interacted with gravitons, you should be able to reconstruct properties of a gravitational wave. Such a detector would be much cheaper and easier to build than existing detectors, Banerjee said. “Measuring photons is something which people know very well,” said Banerjee. “It’s extremely well-studied, and definitely it is less challenging than a LIGO kind of set-up.”
No one knows exactly how gravitons and photons would interact, largely because gravitons are still entirely theoretical. But the researchers made a series of theoretical predictions. When a stream of gravitons hits a stream of photons, those photons should scatter. That scattering would produce a faint, predictable pattern, which physicists could amplify and study using techniques developed by quantum physicists who study light.
Linking the physics of the tiny quantum world with the large-scale physics of gravity and relativity has been a goal of scientists since Albert Einstein’s time. But even though the newly suggested approach to studying gravitational waves would use quantum methods, it wouldn’t fully bridge that tinyto- large-scale gap on its own, Banerjee said. Probing the direct interactions of gravitons might solve some other deep mysteries about the universe, though, he added.
Banerjee and his team showed that the way the light scatters would depend on the specific physical properties of gravitons. According to Einstein’s theory of general relativity, gravitons are massless and travel at the speed of light. But according to a collection of theories, together known as ‘massive gravity’, gravitons have mass and move slower than the speed of light. These ideas, some researchers think, could resolve problems such as dark energy and the expansion of the universe. Detecting gravitational waves using photon scattering,
Banerjee said, could have the side-effect of telling physicists whether massive gravity is correct.

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