Artificial Solar System Could Search for Extra Dimensions

June 23rd, 2006


The concept of extra dimensions, that there are whole other regions of reality that we can’t perceive, has tantalized physicists for years. Although the math looks good (if you like that kind of thing), scientists want physical evidence they can measure. And how do you test for 4 dimensions of space when you’ve only got a 3-dimensional ruler? One idea is to use gravity, a force that might actually reach across an extra dimension and give scientists the evidence they’re looking for. In order to run this experiment, a pair of physicists think the best strategy is to start from scratch and build a whole new solar system… in miniature.

With powerful new equipment and sensitive instruments, astronomers are turning up mysteries faster than they can resolve them. We’ve got a handle on the 4% of the Universe that’s regular matter - while the other 96% is made up of dark matter and dark energy. One possible explanation for dark energy is that it’s not a mystery at all, it’s just regular, familiar gravity acting strangely over large distances. It just takes an extra dimension of space; one that we can’t see with our existing instruments.

A strange coincidence for this “gravity at large scales” theory is hinted at by the Pioneer spacecraft. Astronomers noticed in the 1980s that Pioneer 10 and 11 weren’t exactly where they were supposed to be. Some force is slowing them down, more than can be explained by gravity from the Sun. Unfortunately, the Pioneers are affected by gravity by many different objects, buffeted by the solar wind, blasted by cosmic radiation, and encountering interstellar particles.

Scientists need a better instrument to test their theories; a way to test gravity without outside interference. In a new paper called APSIS - an Artificial Planetary System in Space to probe extra-dimensional gravity and MOND, physicists Varun Sahnia and Yuri Shtanov propose an unusual spacecraft - a miniature solar system.

They believe a spacecraft carrying a miniature solar system could launched to the L2 Lagrange Point of the Earth-Sun system. This is a spot along the Earth’s orbit where the forces of gravity from the Earth and the Sun cancel out. Spacecraft stationed there are very stable (WMAP’s there already). This artificial solar system would be enclosed within the larger spacecraft that would protect it from cosmic rays, dust, solar wind, and anything else that could interact with the orbiting “planets”. Even the spacecraft’s fuel tank, which will decrease in mass over time would need to be positioned as far away from the mini-planets as possible so they don’t experience changing gravity over time.

Once at the L2 Lagrange Point, the spacecraft would release the mini-planets into elliptical orbits inside its protective shell. Even if the orbits aren’t perfect, the spacecraft would have lasers it could use to make tiny changes with light pressure. The positions of the orbiting mini-planets could be tracked with tremendous precision over the course of several years. Any gravitational anomalies would compound over time, giving cosmologists mountains of data to test for the impact of an extra dimension.

In addition to its main function, the artificial solar system could help cosmologists test other theories of gravity, extra dimensions, dark energy and dark matter.

Written by Fraser Cain

Mini solar system could reveal hidden dimensions
13:41 07 July 2006
NewScientist.com news service
David Shiga

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.

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