Gravity's quantum leaps detected

New Scientist ^ | 19:00 16 January 02 | Hazel Muir

[Ernest_at_the_Beach]

Gravity's quantum leaps detected

19:00 16 January 02

Hazel Muir

Gravity's subtle influence in the quantum world has been directly observed for the first time.

On tiny scales, nature makes particles behave according to curiously rigid rules. For instance, negatively charged electrons trapped around a positive nucleus under the pull of the electromagnetic force cannot have any energy they want -they have to fall into a set of distinct energy levels.

In the same way, the pull of gravity should make particles fall into discrete energy levels. But because gravity is extremely weak on small scales, the effect has been impossible to spot. "To be able to measure it, you need to suppress interference from all the other fields," says Valery Nesvizhevsky of the Laue-Langevin Institute in Grenoble, France.

Now Nesvizhevsky and his colleagues have achieved the feat using a beam of neutrons. Neutrons were ideal because they're neutral, so they don't feel the electromagnetic force and can ignore its quantum rules.

Experts say it is a convincing result from an extremely tricky experiment. "The difficulty of this measurement should not be underestimated," says Thomas Bowles of Los Alamos National Laboratory in New Mexico. "In the quantum realm, the gravitational force is so weak that it is difficult to observe quantum effects."

Bouncing neutrons

Nesvizhevsky's team took a beam of ultracold neutrons with tiny energies, moving from left to right at less than eight metres per second. Under the force of gravity, the neutrons fell down onto a reflecting mirror and bounced off it before arriving at a detector.

The team could limit the energies of the neutrons arriving at the detector by placing an absorbing material at different heights above the mirror. The material mopped up all the neutrons that bounced too high.

Forgetting quantum mechanics, you would expect neutrons with any energy to arrive at the detector. But no neutrons appeared unless the neutron-mop was at least 15 micrometres above the mirror. This means the neutrons have to have a certain, minimum energy (equal to 1.41 x 10^-12 electronvolts) in the Earth's gravitational field.

Step up

There were also hints that neutron transmission took little leaps at different, higher energies, corresponding to higher quantum levels. However, the team has still to confirm this.

Nesvizhevsky says the technology is exciting because it could test some other key ideas in physics - for instance, whether or not the neutron carries some minuscule amount of electric charge. "If it's there, it's very, very small," says Nesvizhevsky.

It could also put on trial the equivalence principle, a famous concept of Einstein's. It says that all particles, regardless of their mass or composition, should fall with the same acceleration in a uniform gravitational field.

Journal reference: Nature (vol 415, p 297)

19:00 16 January 02

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[G32]

[Southack]

"Each falling object is not only attracted to the earth in proportion to its mass, but it [the falling object] also has inertia that resists the motion, so the falling object's mass cancels itself out."

You're using the "Gravity equals Inertia" argument?!

[Southack]

"For the speed of gravity determination they are using massive but highly sensitive devices. If something heavy moves, like a supernova, they might be able to get a measurement. I think they are waiting for a suitable event so they can get some numbers. Until then they are detecting neutrinos and doing Aetvos experiments with artificial satellites. Maybe we will be the generation that gets a clue."

The Speed of Gravity may not be found by pursuing General Relativity experiments with satellites in Space.

http://www.nasa.gov/centers/marshall/news/news/releases/2005/05-066.html

After a year in orbit, Stanford University and NASA's Gravity Probe B has detected no distortion of Space/Time. Results will be released in late 2006.

If you're a big fan of Einstein, better hold your breath.

[Mind-numbed Robot]

Thanks. (For a person so smart you must be a very slow reader. You are responding to something I posted two and a half years ago. :-)) My answer was intended as jest but I do appreciate your technical explanation. I was not the one saying things which are heavier should fall faster or harder, I was reponding to that.

Regardless, my jest was intended to point out that we are discussing definitions and you provided one of those definitions. We define weight as gravity acting on mass but we have no definition of gravity other than an observation of its effect. Just as "I think, therefore I am." gravity exists only through the observation and measurement of other actions.