AN INTERESTING 1980 FOR JOHN ANDERSON'S:

Gravity May Lose Its Pull: When conventional physics couldn't explain why space probes were acting strangely, one JPL scientist was determined to find the answer. (John Johnson, December 21, 2004, LA Times)

It was in 1980 that John Anderson first wondered if something funny was going on with gravity.

The Jet Propulsion Laboratory physicist was looking over data from two Pioneer spacecraft that had been speeding through the solar system for nearly a decade.

Only something was off base. The craft weren't where they were supposed to be.

Rather than traveling at a constant velocity of more than 25,000 mph toward the edge of the solar system, Pioneers 10 and 11 were inexplicably slowing down. Even factoring in the gravitational pull of the sun and its other planets couldn't explain what he was seeing.

How could that be?

At first, Anderson figured there must be a simple explanation. Maybe there was a malfunction on board the spacecraft. Maybe his calculations were wrong.

Shy, bookish and soft-spoken, Anderson was not the type to call a news conference to announce that two U.S. spacecraft appeared to be disobeying the physical laws of the universe. [...]

"I started plotting this anomalous acceleration toward the sun," Anderson said. In space science-speak, that meant the spacecraft were improbably slowing down.

To be sure, the anomaly was small, just 8 X 10--8 centimeters/second2. That amounted to about 8,000 miles a year, a tiny fraction of the 219 million miles the spacecraft covered annually. The anomaly is about 10 billion times weaker than the Earth's gravity.

But over time, even inches and meters add up.

Today, after three decades, the difference is about 248,000 miles, the distance from Earth to the moon.

Anderson, ever the cautious scientist, didn't tell anyone what he was seeing for a decade. Early on, the probes were still so close to the sun that he reasoned radiation and solar wind — streams of ionized gas spewing forth through the solar system — could be affecting them.

The other possibility was a spacecraft "systematic" — an onboard mechanical problem. Prime suspects were gas leaks, along with releases of energy by the plutonium-powered radioisotope thermoelectric generators that provided electric power to the instruments.

None of these candidates seemed capable of producing errors as large as Anderson was charting.

There was one piece of evidence that seemed to support the idea that the anomaly could be real: It was almost exactly the same on both spacecraft. On the other hand, both Pioneers were built by the same company to identical specifications, so why shouldn't the same problem show up on both?

As years passed, and the Pioneer probes moved away from the sun's influence, the anomaly didn't disappear — or change even one iota.

Anderson was stumped. Unable to get the problem out of his head, he began spending his own time burrowing deeper into the numbers streaming back from space.

He was still scratching his head when physicist Michael Martin Nieto at the Los Alamos National Laboratory in New Mexico called up one day in 1994 looking for material for an upcoming speech about new developments in physics. Anything interesting going on?

"Well, I've got this thing with Pioneer," Anderson said.

"I almost fell off my chair," Nieto said. [...]

"There are two possible explanations," Turyshev said. "The most plausible is systematics."

The second possibility is new physics.

"If it's new physics, the implications are truly tremendous," he said.

So what would be the implications?

One possibility is that invisible, so-called dark matter is holding the spacecraft back. Some cosmologists believe that dark matter exists because only 10% of the expected mass of the universe has been found. If 90% of the universe's mass and energy is invisible, maybe it could exert gravitational pull on spacecraft.

Another possibility, even more fanciful, is that invisible dimensions of space are tugging at the Pioneers. This idea has its origin in string theory, an idea that suggests we are surrounded by far more than the three dimensions we know about. Some versions of string theory suggest there may be as many as 11 dimensions, most of which are curled up and hidden from us.

As with dark matter, no hard evidence has been found proving the existence of vibrating strings far tinier than the smallest known particles.

A third possibility is that gravity has been hiding secrets that three centuries of research have failed to uncover.

Anderson and his colleagues have known for some time that the only way to prove the anomaly is to duplicate it with another spacecraft.

So all we know is that gravity is fine-tuned here, at the center of the Universe.

Posted by Orrin Judd at December 21, 2004 9:22 AM