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Scientists ponder the problem with gravity
IMAGINE the weight of a nagging suspicion that what held your world together, a constant and consistent presence you had come to understand and rely on, wasn’t what it seemed. That’s how scientists feel when they ponder gravity these days.
For more than three centuries, the basics of gravity were pretty well understood.
Newton described the force as depending on an object’s mass. Though it extends infinitely, gravity weakens with distance (specifically, by the inverse square of the distance). Einstein built on these givens in developing his theory of relativity.
Then more than a decade ago a researcher noticed something funny about two Pioneer spacecraft that were streaming toward the edge of the solar system. They weren’t where they should have been.
Something was holding the probes back, according to calculations of their paths, speed and how the gravity of all the objects in the solar system — and even a tiny push provided by sunlight — ought to act on them. Now scientists have proposed a new mission to figure out what’s up with gravity.
Pioneer 10 and 11 launched in 1972 and 1973. Today each is several billion miles away, heading in opposite directions out of the solar system. The discrepancy caused by the anomaly amounts to about 400,000 kilometers, or roughly the distance between Earth and the moon. That’s how much farther the probes should have traveled in their 34 years, if our understanding of gravity is correct. (The distance figure is an oversimplification of the actual measurements, but more on that in a moment.)
Scientists are quick to suggest the Pioneer anomaly, as they call it, is probably caused by the space probes themselves, perhaps emitting heat or gas. But the possibilities have been tested and modeled and penciled out, and so far they don’t add up.
Which leaves open staggering possibilities that would force wholesale reprinting of all physics books:
— Invisible dark matter is tugging at the probes
— Other dimensions create small forces we don’t understand
— Gravity works differently than we think
Slava Turyshev at Nasa’s Jet Propulsion Laboratory is one of a handful of scientists who wrestle mentally with the Pioneer anomaly every day. He is not paid to work specifically on the problem, so he has to juggle the disturbing thought with his regular research, which involves other aspects of gravity and, significantly, whether theories that explain the glue of the whole universe might one day match neatly with those describing the invisible, subatomic world.
Galileo was crashed into Jupiter last year, and Ulysses will never go farther than it has.That leaves two data points — one from each Pioneer craft. Turyshev pointedly considers the pair as one data point, so as not to inflate the case for strange new physics.
Unraveling the enigma will require a new mission, the researchers say. Nasa, however, doesn’t have such a project on its agenda and has not expressed much interest in one. Europeans, for reasons both historic and having to do with a current strong desire to better grasp gravity, seem more interested in investigating the problem. So Anderson’s team recently proposed to the European Space Agency a “mission to explore the Pioneer anomaly” using the latest accelerometers and advanced navigation methods. All possible sources of onboard radiation would be eliminated in “the most precisely tracked spacecraft ever to go into deep space,” the group writes in the September issue of Physics World magazine. — Sci-tech World Report
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