A tiny, artificial solar system could reveal hidden spatial dimensions and test alternative theories of gravity, a new study suggests. If the system鈥檚 鈥減lanets鈥� moved slightly differently than expected from standard gravity, it would signal the presence of new physical phenomena 鈥� 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 鈥� explaining why it is a relatively weak force in the universe we know.
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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.
鈥淒irect measurement of the gravitational force at distances smaller than a fraction of a millimetre is an extremely difficult task,鈥� 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 杏吧原创.
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 鈥渟olar system in a can鈥� 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鈥檚 radiation, which pushes gently on any objects it shines on. Any such push could change the spacecraft鈥檚 position relative to the tiny 鈥減lanets鈥� 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鈥檚 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掳 per year 鈥� 鈥渁 reasonable quantity to try and measure,鈥� 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鈥檚 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 鈥� 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鈥檚 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 鈥減lanets鈥�.
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.
鈥淪uch an experiment would be quite challenging to set up, but I don鈥檛 think it is technologically impossible,鈥� says MOND expert Stacy McGaugh of the University of Maryland, US.
He points out that other spacecraft, such as NASA鈥檚 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鈥檚 principal investigator).
