By Robert Roy Britt
Senior Science Writer
Researchers have measured the quantum effects of
gravity for the first time, a significant breakthrough in the
understanding of an enigmatic force at tiny scales.
The work is reported in the Jan. 17 issue of the journal
Nature.
Gravity is relatively easy to observe in the everyday world
of orbiting planets and falling apples. Yet even the
smartest physicists don't know where gravity actually
comes from. And on very small scales, in the so-called
quantum realm of subatomic particles, the effect of
gravity is so weak that its effects have never been seen.
Theory says gravity should be at work there, nonetheless.
Quantum mechanics lays out rules for how electrons and
other particles inside atoms (the quantum world) must
behave. For example, an electron can only move from
one position to another -- changing quantum states -- by
jumping; it cannot slide smoothly from one position to
another.
In theory, this rule applies to all matter under the influence
of nature's four fundamental forces: electromagnetism,
the so-called strong and weak nuclear forces, and gravity.
But with gravity, it's hard to tell, because things at the
subatomic level are in constant motion. It's a frenetic
place, really, full of what scientists call kinetic energy --
not unlike a very, very small version of a typical first-grade
class just before recess.
So the researchers, led by Valery Nesvizhevsky at the
Laue-Langevin Institute in Grenoble, France, isolated
hundreds of neutrons from all major effects except
gravity, then watched them in a special detector as gravity
pulled them down.
It was not a smooth fall. As expected, the neutrons fell in
quantum jumps.
"The work of Nesvizhevsky and colleagues could provide
physicists with a new probe of the fundamental properties
of matter," writes Thomas Bowles of the Los Alamos
National Laboratory in an accompanying analysis in
Nature.
Bowles said the new observational technique might allow
scientists to figure out why quantum mechanics is at
odds with Einstein's theory of general relativity, which
describes how gravity treats large objects in the universe.
It might even solve a most elusive goal in helping
researchers understand out what actually creates gravity,
he said.
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