Tom
Murphy got jazzed about astronomy in high school when Halley’s
Comet streaked by in 1986. He read about the five spacecraft from
the USSR, Japan, and the European Community that visited the comet,
and th en went on to study the history of the Apollo lunar missions.
But he never imagined he’d be shooting lasers at the moon.
An assistant professor of physics at UCSD’s Center for Astrophysics
and Space Sciences, Murphy’s love “for all things astronomy” led
him to Caltech, where he obtained his doctorate and built a novel
infrared spectrograph for the Palomar Observatory. Then, he “switched
gears” as a postdoctoral fellow at the University of Washington,
where he got interested in problems of fundamental physics. Now
he’s back in Southern California, spending nights on the
fourth floor of the Science and Engineering Research Facility directing
a New Mexico telescope via computer to shoot pulses of laser light
at the moon.
The reason? To determine if Galileo or Einstein may have been
wrong.
More than 400 years ago, as the apocryphal story goes, Galileo
dropped cannon balls, musket balls, wood and other objects from
the Leaning Tower of Pisa and discovered that lighter objects hit
the ground at the same time as the denser, more massive ones. Galileo
concluded that gravity accelerates all objects equally, regardless
of their mass or composition. It’s a cornerstone of modern
physics that physicists call the “equivalence principle.”
Einstein constructed his theory of gravity—the general theory
of relativity—based on that principle. And physicists since
then have shown Einstein’s theory and the equivalence principle
to be largely correct.
But what if the objects of different mass and composition Galileo
dropped were as big as the Earth and moon? And what if they fell
not from the 183-foot-high Tower of Pisa, but from the 93 million
mile distance between the Earth and the sun? Do the Earth and moon
fall toward the sun at exactly the same rate? Or is the moon’s
orbit slightly skewed, either toward or away from the sun, suggesting
that something is wrong with Einstein’s theory?
That, in a nutshell, is what Murphy wants to find out. His modern
day version of Galileo’s experiment, financed by NASA and
the National Science Foundation, is no easy task. To find any deviations
from the predicted orbit of the moon, he must measure the distance
from the Earth to the moon to within the accuracy of a millimeter,
or about the thickness of a paperclip.
Decades of lunar-ranging experiments by NASA’s Jet Propulsion
Laboratory in Pasadena have allowed physicists to measure the distance
from Earth to moon to within 1.7 centimeters. (It is 238,700 miles
on average, varying by plus or minus six percent as the moon goes
around the Earth). Murphy’s goal is to do 10 times better.
 To accomplish that task, he and UCSD graduate student Eric Michelsen
have, for the past year, been sending pulses of laser light from
the Apache Point Observatory, near White Sands, New Mexico, to
suitcase-sized reflectors left on the moon by the Apollo astronauts.
Their project, appropriately dubbed APOLLO for Apache Point Observatory
Lunar-Laser-ranging Operation, is fairly straightforward: Collect
photons of light that return from those reflectors and measure
the time it takes light to travel to the moon and back. Since they
know the speed of light, the one-way travel time gives them the
precise distance from Earth to moon.
FITFUL PHOTONS
The difficulty with this technique, called lunar-ranging, is getting
enough photons. Since the pulse of laser light spreads outward
like a flashlight beam, only a tiny fraction of the photons ever
hits the reflectors on the moon. An even smaller fraction returns
to the telescope. The process is statistically akin to tossing
a 100-meter cube of beach at the moon, then waiting for a single
grain of sand to return.
“Only one out of 30 million photons hit the reflector,” says
Murphy. “And of those lucky ones that do, only one out of
30 million return to the telescope.”
To measure the distance from Earth to moon with one millimeter
precision, the physicists confront another seemingly impossible
challenge: They must measure the time for the photons to make the
roundtrip to within a trillionth of a second.
“We send 20 pulses per second and it takes two and a half
seconds for each pulse to make its round trip, so the net effect
is that we have 50 pulses that are out at any given time,” Murphy
says. “Since we have 50 balls in the air at any one time,
we can’t drop any of them or we’d get confused over
which one is which.”
Add to this mix the cloudy nights, rain and high winds that prevent
the physicists from using their telescope on this remote, wind-swept,
bitterly cold mountain peak in New Mexico’s Sacramento Mountains
on about half of their 10 scheduled observing nights a month, and
Murphy admits the experiment has become much more difficult than
first envisioned. “This whole business can be very frustrating,” he
says. “We had a few runs in the last lunar month where we
didn’t see any signals at all.”
THE SEARCH GOES ON
“
I wanted to get into this project because it was fun, I could build
this apparatus, shoot lasers at the moon, measure something about
general relativity,” Murphy explains. “But when I started
this project in September 2000 as a postdoc at the University of
Washington, I didn’t have any idea it would take this long.
I thought that by the time I left I’d be shooting lasers
at the moon, but it was actually 5 years later that we were finally
doing this and getting returns. And it was another year before
we got into this steady campaign. I’ve never worked on one
thing for so long.”
Some people ask him why he would devote himself to a single experiment
that takes so much time, so much detail and probably has very little
chance of overturning Einstein or Galileo. Murphy nods in partial
agreement.
“We’re dealing with things in basic physics that are
very hard to do, because all of the easy things have been done.
The safe bet is that we won’t find anything wrong with general
relativity. And the reason for that is that already at a part in
one thousand, the lunar orbit shows that Einstein’s theory
is correct. If we push it another order of magnitude, to a part
in ten thousand, it’s an incremental improvement. But you
never know until you check. I’d rather do the experiment
and know that general relativity is good to another order of magnitude
than just assume it is. That’s not the way science is done.”
Aside from testing the equivalence principle, Murphy points out
that APOLLO has other practical benefits. The precise lunar orbit
that he and his team develop should provide planetary scientists
with more information about the moon itself. “It will help
us understand more about the lunar interior,” he says. “How
does the moon react to torques? Does the moon have a solid or liquid
core? All of this may tell us something generally about the formation
of solid bodies.”
The past year of regular measurements by APOLLO, in fact, has
already improved the existing information about the moon’s
orbit. “It’s confirmed that we are roughly at the half
centimeter error level. After a year’s worth of one to three
millimeter data, we should have a better understanding of the lunar
orbit and gravitational physics.”
By 2008, Murphy says APOLLO should allow his team to take a first
stab at answering the question of whether Einstein and Galileo
were right. And that’s given him the optimism and confidence
to continue for the long haul.
“I feel lucky to be doing something that people can understand,
and that relates to our legacy and space exploration. Also I feel
that I’ve been able to personally experience the Apollo lunar
landings, because now I’m finally seeing photons come back
from those reflectors.” 
Kim McDonald is director of science communications at UCSD. |
