Neutrons could test Newton's gravity and string theory

Neutron flux reactor (ILL) Neutrons produced at the ILL can be used to probe fundamental aspects of physics

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A pioneering technique using subatomic particles known as neutrons could give microscopic hints of extra dimensions or even dark matter, researchers say.

The idea rests on probing any minuscule variations in gravity as it acts on slow-moving neutrons in a tiny cavity.

A Nature Physics report outlines how neutrons were made to hop from one gravitational quantum state to another.

These quantum jumps can test Newton's theory of gravity - and any variations from it - with unprecedented precision.

The "quantum states" of atoms, light particles known as photons, molecules and even objects big enough to be seen have been extensively studied.

They are called quantum because it takes a packet of energy of a very specific size - a quantum - to create the states.

However, of the four fundamental forces, gravity is by far the weakest, and it took until 2002 before gravity's quantum nature was proven.

That work, by a group of researchers at the Institut Laue-Langevin (ILL) and published in a paper in Nature, used slow-moving neutrons falling due to gravity.

Now a team of researchers from the Vienna University of Technology have refined the technique at the ILL to examine and exploit gravity's quantum ways.

Before the fall

The neutrons are created in a fission reactor, and slowed to incredibly low speeds by materials known as moderators.

They are gathered up and injected into the quantum experiment at speeds of around five meters per second - just a hundredth the speed of the molecules flying around in the air.

What is useful about neutrons for these experiments is that they are electrically neutral - within the experiment, they are as isolated from all the forces of nature as they can possibly be, with only gravity to act on them.

The neutrons are shot between two parallel plates, one above another and separated by about 25 micrometres - half a hair's width. The upper plate absorbs neutrons, and the lower plate reflects them.

As they pass through, they trace out an arc, just like a thrown ball falling due to gravity. If they hit the bottom surface before passing through, they are reflected off and absorbed at the top - and thus are not detected at the other end of the plates.

The new work has added what is known as a piezoelectric resonator to the bottom plate; its purpose is to jiggle the bottom plate at a very particular frequency.

ILL at night The ILL is the most intense continuous source of neutrons in the world

The researchers found that as they changed the bottom plate's vibration frequency, there were distinct dips in the number of neutrons detected outside the plates - particular, well-spaced "resonant" frequencies that the neutrons were inclined to absorb.

These frequencies, then, are the gravitational quantum states of neutrons, essentially having energy bounced into them by the bottom plate, and the researchers were able for the first time to force the neutrons from one quantum state to another.

The differences in the frequencies - which are proportional to energy - of each of these transitions will be an incredibly sensitive test of gravity at the microscopic scale.

While it is easy to measure the effects of gravity on grander planetary or even galactic scales, the force's weakness has meant its detailed nature has been difficult to observe up until now. And any variations from the gravity that Newton's theory predicts could be a hint of some new physics.

"With theory you can assume there's only purely Newton's gravity, then to make a transition you need a certain energy," study co-author Peter Geltenbort of the ILL told BBC News.

"Now we can compare this energy with what we've measured and if there is a deviation then it would be a hint that Newton's gravity on these short distances is not 100% valid."

Any such deviations could give hints of the postulated particle known as the axion, which could in turn prove the existence and nature of dark matter.

"The experiments in astrophysics and astronomy give limits [for the axion's existence] over long distances very stringently, but not for the short distances. These are the same theories you would use to describe phenomena on a large length scale, but we have with our method the possibility to look for these axions on this short scale," Dr Geltenbort said.

The same holds true for supersymmetric particles, part of some formulations of string theory that suggest that many extra dimensions exist over tiny length scales, which would require the precision that is only now possible with the team's approach.

"We'll never be as sensitive as the methods on those astronomical scales but we can be far more sensitive on the scale between millimetres and less than micrometres," Dr Geltenbort said.

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