by David Pesci in The University of Connecticut Traditions Alumni Newsletter, Fall 1996
How hot is the center of the sun?
Finding a correct answer to this question has led to one of the most
controversial and important issues in science today: the solar neutrino
problem.
The solar neutrino problem has become a point of contention in part
because of one solution being championed by many scientists would mean a
radical shift in fundamental physics. This is the stuff of Nobel Prizes,
prestigious appointments to lucrative university chairs and other appointments
of the academic acropolis. Curiously, the other solution being offered is
somewhat less dramatic, but no less emotional. It states that past measurements
of the neutrino have simply not been accurate.
Or, in other words, one side is proclaiming that it is on a
genius-level breakthrough while the other side is saying the devil is in the
details.
Sitting squarely between the warring parties in this scientific melee
is Moshe Gai, a UConn professor of physics. In 1992, Gai pioneered a new
measurement method that will help provide a definitive answer to some of those
details. At the very least, this solution will give us a better understanding
of how the sun and the other stars in the universe work. It could also make or
break the solar neutrino problem.
It's a tremendous opportunity for Gai and his team, but one that
carries a considerable amount of responsibility, although some would call it
pressure. After all, virtually every physicist in the world, including Nobel
laureates and internationally renowned researchers, are looking over his
shoulder waiting for the results. But Gai has remained cool and detached from
the controversy.
"In all honesty I do not have a preference or an opinion on which
way this turns out," he says. "I am just trying to get the most
accurate measurements possible. I am interested to see what the data reveal,
but I do not have a stake in either side. My interest lies in the process, in
good science."
The controversy surrounding the solar neutrino problem can be traced back to a fundamental theory about the sun, stars and nuclear fusion developed during this century known as the standard solar model of physics. In the early 1960s, physicists sought to prove the solar model by determining the core temperature of the sun. Among other things, knowing that temperature would give scientists accurate information about the sun's fusion rate, the amount of energy it generates. But since an actual core reading could not be made, scientists needed an indicator. The solar neutrino seemed like a natural candidate.
Solar neutrinos are subatomic particles produced by the constant
series of nuclear reactions occurring at the sun's center. They are the only
byproducts of the sun's core that actually reach the earth. And they are
plentiful. As you read this, about 10 billion solar neutrinos will pass through
your body. To get a clear idea about the workings of the solar core, scientists
need to measure, within a 5 percent accuracy, how many neutrinos reached a
given spot of the earth over a specific period of time.
But counting neutrinos is not easy. They are massless, smaller than
protons, and move at the speed of light. Still, scientists devised a method
using giant "detectors." These use underground tanks more than 10
stories tall filled with thousands of tons of such liquids as heavy water,
carbon tetrachloride (which is also used for dry cleaning) and gallium. These
tanks can detect the presence of the rarer, highly energized radioactive
neutrinos spun off from solar fusion. The early detectors were able to identify
about one such neutrino every three days. The newer facilities are more sensitive
and have increased this rate to about one every three hours.
However, in both cases, this number of neutrinos was far less than
what the standard model predicted - anywhere from 50 percent to 70 percent
less. This meant one of two things: either the standard model of particle
physics was wrong and scientists were on the verge of discovering a completely
new explanation for fundamental physics in the universe, or the estimated
neutrino number was inadequate and needed to be adjusted. The debate over which
side is right has raged one for more than 30 years.
But is "raged" the right word? After all this is only a
physics problem.
"It's very emotional," Gai says. "People have built
their whole careers on this for 30 years. Some are anticipating acclaim and
prizes. Former colleagues now don't speak to each other except with insults.
The debate is as bitter as any you can think of."
This was the environment that Gai stepped into in 1992 while an
associate professor at Yale University. He had a proposal for a new way to
measure the elusive radioactive solar neutrinos that hinged on another basic
tenet of physics: every reaction can be reversed. Knowing this, Gai reasoned
that he could design an experiment where the process for producing the
neutrinos went backward, decomposing instead of fusing. The analysis of the
break-up would give scientists the information on what it took to create the
particle in the first place.
"It was like running a movie in reverse," he says.
"Only we would be starting at the end and watching it go back so we could
observe what created it."
Some colleagues saw genius in Gai's idea, but, to put it politely,
many others thought he was wasting time and money.
"I had people tell me I was crazy and being foolish," he
says. "But I said nothing to them. I decided to pursue it and let the data
we got show whether my idea was sound or not."
The experiment Gai was proposing required an atomic accelerator, a
sophisticated facility that cost tens of millions of dollars to construct.
Opportunity was with him, though. The Japanese had just built an atomic
accelerator at the Institute of Physical nd Chemical Research of Japan, better
known as RIKEN.
The experiment, which takes weeks to prepare, is over in the blink of an eye, and must be performed many times over to obtain the required level of accuracy.
Gai was able to successfully conduct the experiment at RIKEN. Word
spread, and soon the "crazy idea" was accepted as standard.
In 1994, Gai, now at Uconn, presented his results at a major
conference in Israel that attracted more than 600 scientists, including three
Nobel Prize winners. What he found was that the neutrino number was still at
odds with the standard solar model. Gai was then asked by a member of the
audience on which side of the solar neutrino problem he stood.
"I stated my interest was only in the accuracy of my
measurements," he said. "This produced a tremendous round of applause
from the audience. That is something that isn't really done at these
conferences. It was very flattering."
Unfortunately, Gai's announcement did nothing to cool the debate. If
anything, it heightened the emotions involved. But the debate will soon be
coming to a close.
Gai has secured $1.2 million in grants to continue his work and gather
more data. This information will be combined with data collected by two new
solar neutrino detectors, one in Japan and the other in Canada. These are the
most advanced yet, and, according to Gai, will collect the rest of the
information needed to end the controversy.
"I fully expect for us to have an answer within the next five
years at the very latest," he says. "Perhaps even within two
years."
But when they have the answer, what will it really mean?
"Every culture has a story of creation, and this is ours,"
he says. "Unlike the old stories of creation, though, ours is based in
science, not folklore. If we can understand the sun, then we can understand the
other stars and perhaps other solar systems, or even how life began here and
how it begins elsewhere. It is the most basic of questions, and we are very
close to now producing a definitive answer. I think that is very
exciting."
Originally published by the University of Connecticut Fall 1996.
Converted to HTML by the Laboratory for Nuclear Sciences 22 April 1997.