Just as his 1938 formula assured the world that the sun would keep shining for another 5 billion years, Hans A. Bethe, now 86, is intent upon easing our minds about the future of the universe.
While some astrophysicists argue that gravity eventually will cause the universe to collapse upon itself in a “big crunch,” and others insist the universe will keep expanding into nothingness, Bethe is convinced the universe will last forever.
His old friend the sun is whispering in his ear again at the Cornell University laboratories that he has called home since fleeing Nazi Germany in 1935.
Having once revealed to him the secret of how it shines, the sun is now giving him clues about the ultimate fate of the universe.
“If you came back to Earth 100 billion years from now, the other galaxies would hardly be visible, but the stars in our galaxy would go on just the way they are now,” he said. “It would be an infinite universe.”
Still conducting cutting-edge research, Bethe (pronounced BAY-ta) is one of the surviving “giants of physics”-including Einstein, Fermi, Bohr, Pauli, Planck-whose discoveries about the fundamental building blocks of matter revolutionized our perception of the real world.
Bethe’s sun formula, for instance, not only explained the fusion process that creates solar energy; it also opened the door to the eventual development of the hydrogen bomb and the ongoing effort to harness fusion as a limitless and cheap supply of energy for mankind.
But his new ideas about the future of the universe are bound to run into problems, just as his formula that told how the sun works took an unexpected nosedive, creating one of the biggest crises in physics this century.
Bethe received a Nobel Prize for his sun work in 1967. But almost immediately, his theory was challenged by the failure to find a key particle, the ghost-like neutrino, coming from the sun.
The neutrino is no ordinary particle. Neutrinos are zipping around everywhere, unseen and unfelt. If they could be seen, like particles of light, the universe would be ablaze in a blinding luminescence.
Not finding them pouring out of the sun’s interior caused a crisis that came to be known as “the solar neutrino problem.” It was a polite way of saying that maybe scientists really didn’t know what made the sun burn. Worst of all, it meant that there would be no assurances that the sun would keep shining.
“If we don’t understand how the sun shines, then there is reason to question everything we are doing in astronomy that depends upon understanding what stars are and how they evolve,” said astrophysicist John Bahcall of the Institute for Advanced Studies in Princeton, N.J.
After years of searching for the “ghost” particles in deep underground chambers that blocked out other cosmic particles, the physicists finally began to spot neutrinos. Last summer they found the certain types of neutrinos they were looking for.
There were enough neutrinos coming from the solar furnace to verify Bethe’s original 1938 formula-and the physics community heaved a huge sigh of relief.
“When I talk to my friends and they ask me why we’re looking for neutrinos, I tell them I want to make sure that when my grandchildren are grown up the sun will be there,” said Richard Hahn of the Brookhaven National Laboratory on Long Island, N.Y.
“Tonight everybody can rest easy because we know that when we wake up in the morning, the sun is still going to be shining,” he said.
Listed as a yellow dwarf star, the sun supplies the Earth with almost all of its energy, and it is basically responsible for almost all life.
But in the midst of counting their blessings over the discovery of solar neutrinos, scientists began to notice that something was not right. They were short some neutrinos. Bethe’s formula had called for one-third more than the scientists found.
To many scientists this was a nuisance, perhaps an equipment error. But to Bethe and some other deep thinkers, it was a clue that could push the understanding of the physical world to new heights.
In his new theory about the sun, Bethe said that the missing neutrinos were not missing at all. The “missing” ones had just changed into another type of neutrino, like a spy changing his disguise, and could not be seen.
That was a shattering concept. It meant neutrinos might have a new property that had been forbidden under the prevailing theories of how particles make up the universe. Neutrinos might have some small mass after all. It was revolutionary, like saying the world is round instead of flat.
Every second, 100 billion solar neutrinos pass through your body. They travel at nearly the speed of light and they were not supposed to have any mass, according to the “standard model,” the current theory that attempts to describe how particles interact to form our physical world.
But if neutrinos have even a slight mass, that could be the solution to another big puzzle in physics-the “missing mass” problem.
The “missing mass” or “dark matter” problem gives astrophysicists nightmares. Astronomers have been able to measure the gravitational effects of some type of matter in the universe as its invisible hands turn and twist giant galaxies in strange ways. But they can’t see what is creating the gravitational pull.
If neutrinos are the source of the missing mass, then that could dictate a fate for the universe that lies between “the big crunch” and the long dissipation into oblivion.
For Bethe, it means that there may be just enough mass to allow the universe to expand indefinitely, but at an ever decreasing rate, a cosmic cruise control.
Coming from a scientist who has a watchmaker’s gift for understanding the gears and springs that make nature work, that is a gilt-edged assurance.
The `conscience’ of science
Bethe has gone where no other human has been and he does it with a humility and concern that has made him the “conscience” of science. After serving as the director of the theoretical division of the Manhattan Project, which produced the first atomic bomb, Bethe led the campaign to prevent the proliferation of nuclear weapons.
Bethe came into his own during the golden age of physics in the ’20s and ’30s when modern atomic theory was born. Great theories like relativity, which described nature at its biggest, and quantum mechanics, which describes the smallest building blocks of matter, began to put our world into focus.
While helping to pave the road to the new kingdom of physics, Bethe built many milestones along the way. They include discoveries about how small and big stars burn; how atoms are built up from smaller particles; what makes dying stars blow up; and how the heavier elements, which make life possible, are created in the ashes of these supernovas.
“Bethe has clarified the conceptual developments of physics in the 20th Century,” said Sam Schweber, professor of physics and the history of ideas at Brandeis University in Waltham, Mass. “He showed how useful a tool physics is in explaining seemingly exotic things like stars and supernova, and more practically, he has made clear the structure of atoms, molecules and solids.
“He has clarified our understanding of these objects and, therefore, of the world.”
His ability to explain nature’s inner workings wowed the scientific world in 1938. The big puzzle of the day was the sun’s energy source. People had speculated that the sun’s fire was fueled by coal, or perhaps meteors constantly plunging into it.
Some scientists thought the sun’s heat was generated by the gravitational contraction of gasses. But when they worked out the math, they found to their dismay that the sun would have only enough fuel to burn for 10 million years.
The best scientific minds of the time had worked on the puzzle and had failed to solve it. Then, in 1938, Edward Teller, later to become known as the father of the hydrogen bomb, and U.S. astrophysicist George Gamow called a meeting of top physicists, astrophysicists and cosmologists in Washington to see if someone could figure out how the sun, the most important thing to life on Earth, generated energy.
“I decided at that point that this was a problem I could probably solve,” remembers Bethe. “So I went home and in the next few weeks, maybe five or six, I got the answer.”
The secret of the sun
Basically, his formula said that four atoms of hydrogen, which makes up 90 percent of the sun’s mass, could be squeezed together in the sun’s core to form one atom of helium. Hydrogen, the lightest atom, has one subatomic particle called the proton in its nucleus. Helium has 2 protons and 2 neutrons (made from protons) in its nucleus.
In the process of forming helium, energy is liberated. We know it as sunshine. Bethe’s formula carefully accounted for everything that happened in the fusion process except for one thing. There was a tiny bit of energy that couldn’t be accounted for except by saying it was carried away by the mysterious neutrinos. For every helium atom that was formed, Bethe said that two neutrinos were created.
When neutrinos were plugged into the formula, it worked perfectly. Bethe’s formula provided the rate at which hydrogen atoms were being fused into helium, which corresponded precisely with the sun’s mass and energy production. Once the rate was established, Bethe could figure out how long the sun has been burning-about 5 billion years-and he could predict how long it would continue to burn-another 5 billion years.
Another big win for physics! At least that’s what everyone thought. Yet there was a weak point in the formula-the neutrino.
Physicists didn’t know how to detect them. Fortunately, it turns out that even the ghost-like neutrino will bump into another particle on a very rare occasion. But zillions of target atoms have to be put in their path in hopes that one neutrino will make contact, a task that had been impossible until the mid ’60s.
Ray Davis of the Brookhaven National Laboratory was the first to try, setting up a huge vat of cleaning fluid a mile underground in the Homestake Gold Mine in South Dakota. The main component of the cleaning fluid was chlorine. When a chlorine atom absorbs a neutrino, it gets converted into an atom of argon. Argon can be separated from the chlorine; as a result, the number of neutrino hits can be counted.
But there was a drawback-Davis’ experiment could only detect high-energy neutrinos that were produced in a minor fusion process in the sun that involved the element beryllium. It couldn’t measure neutrinos from the main fusion reaction where hydrogen atoms joined to form helium.
`Did we have it all wrong?’
Nevertheless, his experiment shook the world of physics. Just about the time Bethe was receiving the Nobel Prize for describing how the sun worked, Davis was reporting that he was detecting only about a fourth of the neutrinos that were expected to come from the minor fusion process.
“That raised a lot of questions,” said Eugene Parker, a University of Chicago astrophysicist. “Did we have it all wrong about the interior of the sun? The sun is the one star you can study closely. It is the prototype of all stars. If something is flaky about the sun, then you worry about the whole works.”
An all-out effort was called for. The physics community had to know if much of what they postulated about the subatomic world was true or false.
Two years ago, 10 teams of scientists from around the world began the most ambitious effort to look for the neutrinos produced from hydrogen fusion.
Called Gallex, the experiment consisted of a huge tank filled with 100 tons of liquid, including 30 tons of gallium, located under more than 4,600 feet of rock in a mountain tunnel in the Gran Sasso range near Rome. When a gallium atom absorbs a neutrino, it is converted into an atom of germanium. Germanium is radioactive and can easily be detected.
The long wait ended last summer when the particle hunters reported that they had detected about two-thirds of the neutrinos that had been predicted from hydrogen fusion.
Embodying the ghost
To Bethe, the failure of the experiments to find all the neutrinos that had been predicted was nature’s way of telling scientists they may be in for a big surprise. The surprise is that by changing its spots, the neutrino may be able to go from being a “ghost” to being a particle of some substance.
If the neutrino does indeed have some small mass, that would be the first evidence of something out there that goes beyond the standard model. Scientists hope it will lead to new knowledge, just as Einstein’s theory of relativity lifted our understanding of the universe above the simpler one described by Newtonian physics.
“The neutrino has a slight mass,” smiles Bethe. “Many theorists are fond of the idea that the mass of the neutrino is zero. That is now wrong. The mass of the neutrino is small, very, very small, but not zero.”
“I like that,” he says, beaming.
