Tuesday, July 11, 2006

Searching for Gravity

Research is what I'm doing when I don't know what I'm doing. ~Wernher Von Braun

In the realm of the physicist, few things are so aggravating as gravity. Science was happy to have Newton's laws of gravity and motion because they worked so well. But as we looked at the very small, the very large, and the very fast, there were issues with Newton's formulas. As we learned to compute orbits more precisely, we could see that there were niggling inaccuracies that kept creeping in. So, physicists were relieved when Einstein came along and gave us General Relativity which seemed to answer these concerns.

But, there was always the nagging issue: Why is gravity so weak? You might question whether gravity is weak based on your most recent pratfall, but it really is. To use a common example, when you pick up a book from a table you are overcoming the full gravitational force of the Earth pulling back on that book. That makes gravity pretty puny. In fact, it takes an immense object (like Jupiter or a star) or an immensely dense object (like a neutron star or a black hole) to really make gravity seem a significant force. Because gravity is so weak it has been pretty much impossible to detect its propagating wave/particle (remember, everything has the properties of both a wave and a particle). Everyone feels that there are gravitons out there, but no one can find one.

It is probably gravity that has spoiled the Grand Unified Theory (GUT) attempts more than any other factor. Compared to gravity, the subatomic electroweak and strong forces are well understood. Weak as gravity is, though, there's so much of it pulling at galaxies in the universe that the known amount of matter in the universe won't account for it. So, not only do we have a force that we can only feel but not detect, we have matter causing it that we can't see, dark matter.

We also have dark energy, but I'm not going there today.

There has been a lot of indirect evidence for dark matter based on interactions between galaxies and their satellite dwarf galaxies and globular clusters. Some astronomers think they may finally have found some in the form of a “glowing blob.” Don't you love it when they talk technical? At any rate, this sort of phenomenon has been observed before, in something called “star burst” galaxies.

Basically, the galaxy is obscured by ordinary dark clouds of gas. The galaxy is undergoing massive star formation, though, and the release of energy heats up the surrounding hydrogen and makes it glow. Or it could be a supermassive black hole, always a popular candidate for anything weird going on in the universe. But, evidently, there are behaviors involved in the current observations that make these alternatives less tenable.

Of course, it could simply be a new phenomenon altogether, having nothing to do with dark matter at all, but dark matter, exotic as it is, is evidently a simpler explanation for the “blob” because, in this case, a star burst galaxy or a black hole doesn't fit the bill as well as either might.

Meanwhile, back on the gravity front, the hot theory these days seems to stem from brane theory (string theory's big brother). It seems that with all those extra dimensions lying around, the possibility exists that gravity is actually leaking into them. In other words, gravity is actually very strong, but since it bleeds into other dimensions (and maybe into parallel universes) and gets diffused, it seems weak. Some scientists are looking at the GLAST mission to shed some light on these hidden dimensions. But another group has a rather clever way of trying to impute those hidden dimensions.

The idea is to make a teeny little solar system (an 8 cm ball of tungsten being orbited at 10 cm by a smaller tungsten ball) and put it into orbit at one of the Earth's Lagrange points. Lagrange points are areas of gravitational balance. A probe plunked out to Lagrange point stays put, making it possible to study gravitational effects on a small scale like this. What they're looking for a minuscule precession of the smaller ball's orbit around the larger one. The precession may be minuscule, but it is measurable.

Of course, doing this from a Lagrange point is not like trying to do it in a lab. It's a lot more difficult, but in a lab, unless you've got the mythical antigravity chamber of sci-fi, you're not going to maintain a tiny planet's orbit around a tiny sun for very long.

Aside from looking for multiple dimensions, this little experiment would be used to test MOND, or Modified Newtonian Dynamics, an alternative to both Newton and Einstein. MOND posits that gravity is stronger than expected over large distances than predicted by General Relativity. Interestingly, MOND was created as an alternative to – wait for it – dark matter.

I hope that GLAST is launched successfully next year and that the teeny solar system gets off the ground. It's not that I expect them to disprove or prove General Relativity, but I do think that they have the potential to return interesting results, the kind of results that can spur some imaginative thinking. One way or the other, they might break the logjam that string theory seems to have wrought in the physics community, where no one really has anywhere to take the theory until some sort of predictions are tested. Well, multiple dimensions are a necessity to string theory, and if neither experiment returns evidence of them, then string theory is done.

On the other hand, if the experiments do show the possible existence of such dimensions, it still doesn't necessarily validate the theory. It will, however, finally give the string section something physical to work with.

And, who knows, the experiments could just indicate that Einstein was right – again. That wouldn't be a bad thing, but it would still leave us confused about the weakness of gravity and the nature of dark matter. It would also make a GUT look very far away again.

But, then, a person's reach should exceed one's grasp, shouldn't it?

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