source NewScientistSpace
by David Shiga
<The mini solar system's spheres would need to be very precisely shaped,
like this one used for NASA's Gravity Probe B (Image: NASA)
A tiny, artificial solar system could reveal hidden spatial
dimensions and test alternative theories of gravity, a new study
suggests. If the system's "planets" moved slightly differently than
expected from standard gravity, it would signal the presence of new
physical phenomena – which have proven very difficult to test.
Numerous
theories that attempt to unify all the forces of physics into one
cohesive model call for hidden spatial dimensions in addition to the
three we can sense. In some of these theories, gravity would leak into
the extra dimensions – explaining why it is a relatively weak force in
the universe we know.
<The 'solar system in a can' would reside in a space craft 'parked' at the L2 Lagrange point (Image: NASA)
This leakage would dilute its power and cause deviations from the
standard law of gravity, which would be especially noticeable at very
small scales. But scientists have not been able to measure the force of
gravity between closely spaced objects in the lab with enough accuracy
to test these theories.
"Direct
measurement of the gravitational force at distances smaller than a
fraction of a millimetre is an extremely difficult task," says Varun
Sahni of the Inter-University Centre for Astronomy and Astrophysics
(IUCAA) in Pune, India. Stray electromagnetic forces tend to overwhelm
gravity in experiments at this scale, he told New Scientist.
Fixed in space
Sahni
proposes an alternative way to measure gravity on small scales. He and
colleague Yuri Shtanov at the Bogolyubov Institute for Theoretical
Physics (BITP) in Kiev, Ukraine, say it could be done by sending a
"solar system in a can" into space.
This
artificial system would reside inside a spacecraft that would be sent
to the L2 Lagrange point (see image, below right). That point lies
about four times as far away from Earth as the Moon does.
A
spacecraft placed there would stay fixed in space, relative to Earth,
making it easier to monitor. The Earth would also shield it from the
Sun's radiation, which pushes gently on any objects it shines on. Any
such push could change the spacecraft's position relative to the tiny
"planets" held inside it.
Once
at the Lagrange point, the artificial solar system would be set in
motion inside the spacecraft. An 8-centimetre-wide sphere of tungsten
would act as an artificial sun, while a smaller test sphere would be
launched 10 cm away into an oval-shaped orbit. The miniscule planet
would orbit its tungsten sun 3,000 times per year.
Higher dimensions
If
gravity is leaking into extra dimensions, the slight change in its
force should cause the planet's oval-shaped orbit to rotate, or
precess, slowly. Sahni and Shtanov calculated the effect for a theory
called the Randall-Sundrum model, which says that our universe is a 3D
slice of a bigger, higher dimensional universe. They find the orbit
would precess by 1/3600° per year – "a reasonable quantity to try and
measure," they say.
The
artificial solar system could also be used to test an alternative
theory of gravity, called Modified Newtonian Dynamics (MOND). It posits
that gravity is stronger than expected across larger distances than
predicted by Einstein's theory of general relativity (see Gravity: Were Newton and Einstein wrong? ).
MOND
was devised to explain the motions of stars in galaxies without
invoking dark matter – an unknown substance that appears to outweigh
visible matter in the universe by a ratio of six to one, and whose
presence is only detected through its gravitational effect on visible
matter. MOND could also explain why the Pioneer 10 and 11 space probes
are slowing down more than expected as they coast away from the Sun
(see 13 things that do not make sense ).
According
to MOND, gravity starts diverging from Einstein's theory below a
certain acceleration. And the team says that threshold could be met by
placing one or more planets in orbits larger than the one at 10 cm. The
slight extra strength of gravity at those larger orbits, as predicted
by MOND, would make the planets there move faster than predicted under
general relativity.
Attractive symmetry
But
there are major practical hurdles to overcome before such a mission
could be launched. Static electricity from charged particles in space
called cosmic rays could alter the course of the tiny "planets".
And
the spacecraft components themselves would exert gravitational forces
on the spheres. These forces could be minimised by making the
spacecraft as symmetrical as possible and putting its heaviest
components as far from the artificial solar system as possible.
"Such
an experiment would be quite challenging to set up, but I don't think
it is technologically impossible," says MOND expert Stacy McGaugh of
the University of Maryland, US.
He
points out that other spacecraft, such as NASA's Gravity Probe B, have
already been built to specifications of incredibly high accuracy.
Gravity Probe B, designed to test the theory of general relativity,
contains the most perfect spheres ever made (read an interview with the mission's principal investigator - "My life with Einstein ) .
source NewScientistSpace
See also:
|
