Testing Einstein With a Box Left on the Moon

Thursday, July 16, 2009 | Forty years ago today, as the folks at NASA were making final preparations for the Apollo 11 launch, they made sure that a suitcase sized box of reflectors was securely onboard the rocket ship headed for the moon.

Four days later, Buzz Aldrin placed the box on the moon’s surface.

University of California, San Diego physicist Tom Murphy wasn’t alive for the famous moon landing. But these days he is one of the greatest beneficiaries of the achievement. Murphy is the latest in a line of scientists to use the box to push Albert Einstein’s theory of relativity to its limits.

Since the Apollo 11 crew left the moon, researchers have been shooting lasers via telescope at the 100 one-and-a-half inch diameter reflectors situated on the box to measure the Earth’s and moon’s orbit of the sun.

The essence of the 40-year effort has been to challenge the world’s most famous scientific theory. And since 2005, the 39-year-old Murphy has been leading the charge.

“It is going for the jugular of relativity,” he said.

Relativity, or general relativity as it’s also known, is the most accurate theory of gravity that man has ever come up with. Developed by Einstein between 1907 and 1915, the theory holds that gravity is the result of mass and energy curving spacetime. Relativity disproved Isaac Newton’s theory of gravity, which had been the dominant theory since 1687.

However, as Einstein acknowledged before his death, while relativity may be the best gravitational theory we’ve got, it is quite possibly wrong. In short, it has “character flaws,” Murphy said.

The big problem with relativity is that it doesn’t square with quantum mechanics, which is the set of scientific laws that describe physical systems at the microscopic — or atomic — level. When the theory is applied at these levels, it comes up with an answer that physicists know is almost certainly wrong.

But knowing that it isn’t so, and proving that it isn’t so are two entirely different things. That is where the reflectors Aldrin left on the moon come in to play.

Murphy is trying to prove the belief that the theory of relativity will break down at some point by measuring the time it takes for a laser pulse to go from a telescope in southern New Mexico, bounce off the reflectors and come back to the telescope. By taking that measurement over and over again, he is able to map out the difference of the orbits of the Earth and the moon around the sun.

He then compares his measurements to models, based on relativity, which predicts the difference in the two orbits out to infinity. So what Murphy is doing is proving relativity at levels that are more and more precise until he reaches a point where his measurements don’t match the models based on relativity, or until he can’t prove it anymore.

So far, Murphy has shown that relativity holds up down to the scale of a centimeter. This represents huge progress from the first measurements in 1969 when the best scientists could do is show overlapping orbits down to the 25-centimeter level.

Now, Murphy said, it gets interesting. And he plans to keep at it until the millimeter level. If it breaks down at that point it would be a “revolution for the physics of gravity,” he said. Then the race will be on for the next theory.

Or relativity will hold up. Either way, Murphy said, we will know something new about the universe.

Murphy has been developing and implementing this experiment since 2000, when he was a post-doctoral student at the University of Washington. And it has been the focus of his research since he came to UCSD in 2003. He is guessing that it will take another year and a half to accurately test relativity at the millimeter level. Once he gets there, regardless of the outcome, he said he’ll probably be ready to move on to another of the universe’s mysteries.

Murphy is confident that, when the time comes, someone else will gladly take his place shooting lasers at Aldrin’s box. But they probably won’t have another 40 years. The reflectors have already degraded significantly since 1969, and they are becoming less and less useful by the year, he said.

Knowing that the reflectors won’t last forever adds meaning to the connection Murphy has with the historic achievement, he said.

“I feel privileged for the opportunity to touch those reflectors with my laser beam.”

Please contact David Washburn directly at david.washburn@voiceofsandiego.org with your thoughts, ideas, personal stories or tips. Or set the tone of the debate with a letter to the editor.