Space Forum / Astronomy / March 2007

Equinox challenge to Newton's law
Zeeya Merali
31 March 2007

IF, FOR some reason, you managed to make an egg stand on its end during the
recent equinox, experimental physicists could do with your help. An
exquisitely sensitive experiment that can only be performed during equinoxes
could test some of the theories that offer an alternative to dark matter.

Stars on the outskirts of galaxies are moving much faster than can be
explained by the gravity of visible matter. To account for the extra
gravity, astronomers have proposed the existence of dark matter. In the
1980s, Mordechai Milgrom, then at Princeton University, suggested that the
observations could also be explained by tweaking Newton's law of gravity.
Milgrom's theory of modified Newtonian dynamics (MOND) has since evolved
into two forms: gravitational MOND, which modifies the inverse-square law of
gravity, and inertial MOND, which modifies Newton's second law of motion.

In both cases, when objects are moving with an acceleration above a certain
threshold, a0, the well-known laws hold true. When the acceleration falls
below a0, the laws are modified slightly to explain, for example, the speed
of stars at the edges of galaxies. The value of a0 is about a billionth of
the acceleration due to Earth's gravity, so the effects of MOND cannot
ordinarily be seen on Earth. This means astronomers have had to rely on
observations of galaxies and galaxy clusters to test MOND. Now, Alex
Ignatiev, at the Theoretical Physics Research Institute in Melbourne,
Australia, is proposing a way to test inertial MOND on Earth.

The key is to find a time and place when the acceleration felt by a test
object would be zero. Not an easy task, since besides Earth's gravity, our
planet's rotation and its motion around the sun also set up forces that
accelerate objects beyond the MOND limit. However, Ignatiev's calculations
show that along latitudes of about 80° north or south, passing through
northern Greenland and Antarctica, at two precise times of year, these
forces cancel each other out.

The window of opportunity to do any tests is extremely narrow, though. The
forces only negate each other for about a millisecond on two days close to
the vernal and autumnal equinoxes. At those precise times and places, if
Newton's second law of motion is correct at all accelerations, a mass should
feel no force. But if inertial MOND is correct, Ignatiev's calculations show
that there will be a slight residual force, causing the mass to jerk by
about 10-12 metres (Physical Review Letters, vol 98, p 101101).

Picking up such a small movement seems daunting, but Ignatiev points out
that experimentalists have already constructed gravitational-wave detectors
that can measure even smaller movements. He suggests using a similar
detector for the experiment. "It won't be simple," says Ignatiev, "but it
can be feasible."

"A positive result would have such tremendous implications for dark matter,"
says HongSheng Zhao, an astronomer at the University of St Andrews in the
UK, who works on MOND. "We should do this experiment in the near future."

A negative result would rule out inertial MOND, but would still leave open
the possibility that gravitational MOND is correct. To test this, Zhao says,
similar experiments should be carried out on a spacecraft at a point between
the Earth and the moon where the gravitational pull of each is equal.

While theoretical physicists such as Ignatiev and Zhao are enthusiastic
about the test, Eric Adelberger of the University of Washington in Seattle,
who works on high-precision tests of gravity, is less convinced. "The idea
is great in theory, but in practice it will be tough," he says. "It'll be
difficult to persuade anyone to hike around Greenland to measure it."

From issue 2597 of New Scientist magazine, 31 March 2007, page 15
http://www.newscientist.com/channel/fundamentals/mg19325974.300?DCMP=NLC-nletter
&nsref=mg19325974.300

Posted by
Robert Karl Stonjek