Shooting for the Moon

Tom Murphy, a postdoc at the University of Washington (UW), Seattle, skis "like a kamikaze." But mostly, he says, his recreational life is fairly sedate.

Murphy gets his rush from science. "LLR provides enough adrenaline to keep me happy." LLR is lunar laser ranging, which means bouncing lasers off small mirrors on the moon, then detecting the returning photons. "It [LLR] is like winning the lottery and then being told there's an equally remote chance the money will make it to your bank account."

Murphy, who earned his Ph.D. at the California Institute of Technology in Pasadena, is an astrophysicist with a knack for building scientific instruments. At Caltech he built a new kind of spectrograph for the 200-inch telescope at the Palomar Observatory and used it to study colliding galaxies, which, he learned, get hotter sooner than most people realized. "Among other things," he writes, "I learned that merging galaxies can fire up to ultraluminous status early in the encounter, against the prevailing wisdom that put such activity only late in the final merger stage."

Not satisfied with the challenge of measuring light already impinging on Earth, for his postdoc Murphy has sought out a greater challenge: He intends to shine light at the moon, bounce it off retroreflectors left by astronauts during the Apollo missions (the first retroreflector was deployed by Neil Armstrong and Buzz Aldrin in the Sea of Tranquility during Apollo 11)--then catch the photons that make it back to Earth.

The technical challenges are considerable. "First you must point the laser at the retroreflector array," Murphy wrote in a recent e-mail. "It's hard to make sure you're pointed at something you can't even see to one arcsecond precision."

Next, notes Murphy, you have to make sure your detector is looking at exactly the same spot, with the same degree of precision. "Otherwise, though you may be getting light back, you won't see it." The timing must also be precise: "The high background rate from the overwhelmingly bright moon means you have to gate the detector for periods as short as about 100 nanoseconds." 100 nanoseconds in round-trip delay time corresponds to 15 meters of one-way range. "Earth's rotation is about 400 meters/second, so the distance between your observatory and the reflector is constantly changing."

The hardest part of the experiment may be the lack of feedback; it's hit or miss, and if Murphy misses there's no way to know what went wrong. "Unless all of the conditions are satisfied, there is no return, and no indication of which element might be off. You don't know if you've got it right until you've got it right."

Big game

It's a difficult challenge, but then Murphy and his colleagues are pursuing powerful prey: They seek to expose violations of the principle of equivalence, the core concept of Einstein's General Theory of Relativity.

Einstein's theory says that gravity is a manifestation of the curvature of space-time, and that space-time's curvature is determined by the distribution of mass as well as that of energy. The General Theory has been tested extensively, and no violation has ever been observed. For example, the gravitational attraction of an atomic nucleus has been observed to depend not only on the nuclear mass but also on its binding energy--on the energy stored in the field holding the nucleons together.

But there is reason to suspect that gravitational energy is a special case, that measurements of sufficient accuracy might expose a violation of the equivalence principle when energy is stored in a gravity field. Some theories, including string theory, suggest that such a violation should occur.

A substantial portion of Earth's mass and energy (about half a part per billion) resides in its gravitational field. According to Einstein, the rate at which Earth falls toward the sun should be determined not only by its mass, but also by its gravitational energy. Put another way, Earth's gravity has gravity, and that gravity ought to affect Earth's trajectory around the sun.

The moon, in contrast, stores much less of its energy in its gravitational field, so its gravitational field will make a much smaller contribution to its motion toward the sun. Precise measurements of the relative positions of the Earth and moon make it possible to determine whether they are accelerated differently. Murphy is doing essentially the same experiment Galileo did when he mythically dropped objects off the tower of Pisa, except that Galileo's objects are now the Earth and moon, and not only the objects' mass, but also their gravity, are expected to contribute to their acceleration.

Despite its technical difficulty, lunar laser ranging is capable of exquisite precision, and Murphy thinks it's possible that his measurements--the most accurate measurements ever of the moon's path around Earth, if all goes well--may be precise enough to reveal a violation of the equivalence principle. "I should say that I believe this is only a remote chance," Murphy notes, "but you never know until you look."

Murphy is not the first to use lunar laser ranging to test the equivalence principle. Ken Nordtvedt, a member of the Seattle team, first proposed the equivalence-principle violation, known as the Nordtvedt Effect, that Murphy seeks to measure. Nordtvedt's ideas were largely responsible for NASA's decision to put retroreflectors--collections of "corner cubes," which, because of their geometry, reflect light precisely back toward its source--on the moon.

Although what Murphy is doing isn't new, he intends to do it better than it has been done before. "What we offer," says Murphy, "is a new set of glasses that will let us see potential violations an order of magnitude smaller than currently accessible." Murphy's measurements should trace the moon's orbit with millimeter precision.

Perhaps the most unusual thing about Murphy's hunt is that he's in charge. Though still officially under the wing of his advisors and unable to serve as even co-PI on a research grant, Murphy is in charge of UW's LLR project. It's rare for a postdoc to head up a research effort that involves several faculty members, but Murphy doesn't make much of this; in fact he credits his independence to his bosses. "I think my position is somewhat unique among postdocs, but to the extent that I'm simply doing something I enjoy, it's not that rare, really. I should just add that the fact I'm able to act in a leadership role in this project is a real testament to the unparalleled mentoring abilities of both [Chris] Stubbs and [Eric] Adelberger. They're placing a great deal of trust in me, and giving me more rope than I ever thought existed."

Freedom's Just Another Word ...

Murphy may not seek danger in his personal life, but his professional life is another matter. Murphy seeks out high-risk, high-payoff science projects. "Upon nearing the end of my graduate career at Caltech, I began seeking out individuals rumored to have a knack for finding interesting paths through science, largely original, off the beaten path, and instrumentation-oriented. The name Chris Stubbs came up frequently in snooping for such folks."

Stubbs and Adelberger are two of several mentors Murphy credits with affirming his desire to take scientific risks. "They are my kind of guys. They're doing this kind of research for the same reason I am. They're naturally curious and interested in bringing their tool set to bear on frontier problems in physics. Chris has reinforced my taste in scientific research (interesting vs. god-awful-boring), and Eric has shown me that natural curiosity and investigating the world we live in is an enriching experience."

... Nothin' Left to Lose

One source of resolve for Murphy is a willingness to quit. "I get some of that strength from being on the verge of leaving research and the academic environment. I've got little to loose." Murphy's disenchantment with the current conservative scientific climate has led him to make unconventional choices. "In many ways, I'm rebelling against 'the system,' which today awards short-term funding for peer-reviewed proposals that have near-guaranteed results. That, in my mind, makes for boring, predictable science, and robs the endeavor of the exploratory potential that I find so appealing."

"I figure if I can't choose the types of projects and research I want to tackle now, I never will be able to. The folks in whose hands my future career lies (e.g., faculty committees looking to hire) will either admire that quality or pass me by. If they pass me by, then it's their short-sightedness."

It's like skiing. It's impossible to know what's around the next turn, but rather than hold on tight, Murphy chooses to plunge ahead, allowing chance, gravity, and a less-than-cat-like agility to determine his fate: "I feel that I'll land feet up no matter what. As long as I'm looking out for my own satisfaction, and keep doing things I like, I'll be okay."

Follow Science Careers

Search Jobs

Enter keywords, locations or job types to start searching for your new science career.

Top articles in Careers