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 tiny-to-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.
Words by Chelsea Gohd
Flammable ice, also known as methane hydrate, is created when methane gas is trapped within ice’s molecular structure. Sheets of this frozen gas and ice contain microscopic bubbles of oil and water. Scientists studying ‘flammable ice’ in the Sea of Japan found microscopic, living creatures within these tiny bubbles.
The researchers chanced upon this discovery in a unique manner. While melting hydrate to study the methane gas it contains, Glen T. Snyder, a researcher at Meiji University, noticed a powder with little microscopic spheroids in it that contained tiny spheres with dark centres in them.
“In combination with the other evidence collected by my colleagues, my results showed that even under near-freezing temperatures, at extremely high pressures, with only heavy oil and saltwater for food sources, life was flourishing and leaving its mark inside these little bubbles in the ‘flammable ice’,”said Stephen Bowden of the University of Aberdeen’s School of Geosciences in Scotland. So how does this work inform the search for extraterrestrial life? “The methane in methane hydrate is known to form as microbes degrade organic matter on the seafloor. But what we never expected to find was microbes continuing to grow and produce these spheroids, all of the time while isolated in tiny, cold, dark pockets of saltwater and oil,” Snyder said. “It certainly gives a positive spin to cold, dark places, and opens up a tantalising clue as to the existence of life on other planets.”
The dwarf planet’s famous heart-shaped feature, which NASA’s New Horizons discovered during its epic
July 2015 flyby, drives atmospheric circulation patterns on Pluto.
Most of the action comes courtesy of the heart’s left lobe, a 1,000-kilometre (600mile) wide nitrogen-ice plain called Sputnik Planitia. This exotic ice vaporises during the day and condenses into ice again at night, causing nitrogen winds to blow.
These winds carry heat, particles of haze and grains of ice westward, staining the ices there with dark streaks.
“This highlights the fact that Pluto’s atmosphere and winds – even if the density of the atmosphere is very low – can impact the surface,” said Tanguy Bertrand, an astrophysicist and planetary scientist at NASA’s Ames Research Center in California.
And that westward direction is interesting in itself, considering that
Pluto spins eastward on its axis. The dwarf planet’s atmosphere therefore exhibits an odd ‘retrorotation’.
This work revealed the likely presence of westerly winds – a high-altitude variety that races along at least four kilometres (2.5 miles) above the surface and a fast-moving type closer to the ground that follows Sputnik’s western edge.