Materials for measuring the universe
Did wobbly neutrons give us a starry sky?
Photo: Jason Richards |
Most people have heard of the "butterfly effect," a phrase coined by mathematician and meteorologist Edward Lorenz to describe how tiny changes can have profound effects over time. Lorenz was talking about the vagaries of weather prediction, suggesting that a butterfly flapping its wings in one part of the world could affect whether a tornado forms in another.
Physicists have their own version of the butterfly effect that makes the meteorological consequences of a few wing flaps pale by comparison. Their scenario hinges on extremely precise measurements of one of the basic building blocks of the universe—the neutron.
We're all familiar with atoms from stylized images in grade school science books—a swarm of tiny electrons orbiting a big nucleus made of neutrons and protons.
For decades, physicists have been trying to accurately measure the "roundness" of the neutron—partly because of the implications of the measurement for the accuracy of models of the early universe. This measurement, also known as the neutron "electric dipole moment," was first made at ORNL's Graphite Reactor.
ORNL physicist Vince Cianciolo explains that the best measurements of the EDM to date indicate that the neutron is round— within the limits of the accuracy of those measurement techniques. However, he and his research partners at ORNL and other institutions are working to put together an experiment that would enable them to measure the EDM one hundred times more accurately—to within one quadrillionth (0.000000000000001) of the neutron's diameter.
So why all this fuss about something so small it can barely be measured?
The most widely accepted model of the early universe suggests that as the universe expanded immediately after the "big bang," there were equal amounts of matter and antimatter. When matter and antimatter meet, they annihilate each other—both cease to exist. So by rights, all the material in the universe should have been gone long ago. The fact that you're sitting here reading this, physicists theorize, suggests there was a vanishingly small imbalance in the early universe favoring the preservation of matter. That imbalance, they suggest, may be related to the EDM.
"We know that the ratio of protons and neutrons to photons in the universe today is about one part in a billion," Cianciolo explains, "This tells us that about one billionth of the matter that originally existed in the early universe escaped annihilation. In a rather complicated way, the question of why this matter was left behind is tied to the roundness, or lack thereof, of the neutron. If it's not perfectly round, that could account for the preservation of matter. If it appears to be round at that degree of accuracy, that would be a very interesting result as well."
A knotty problem
Researchers measure the EDM by polarizing a group of neutrons so they're all spinning in the same direction (a neutron spins on its axis, like the Earth), putting them in a magnetic field, and then applying an electric field—first in the same direction as the magnetic field, then in the opposite direction. If there's a difference in how fast the neutrons spin when the electric field is reversed, that's evidence the neutrons aren't perfectly round. The bigger the difference, the greater the irregularity in the neutrons' shape.
Cianciolo and his colleagues are in early stages of a years-long effort to develop an instrument called nEDM (Neutron Electric Dipole Moment) that is designed to measure the EDM with unprecedented accuracy. The nEDM will eventually be installed at ORNL's Spallation Neutron Source.
At the heart of this instrument is a particularly knotty materials science problem waiting to be solved.
The nEDM will feed neutrons into a measurement cell filled with ultra-cold liquid helium where, thanks to the super-low temperature, they can be stored while their spins are being measured. The measurement cell is surrounded by a magnetic field and high-voltage electrodes. The cell is made of clear acrylic, which is particularly good at transmitting the small bursts of light created during the measurement process and carried through the wall of the cell to a light sensor. The electrodes connected to the cells also have to be made of acrylic because the instrument will operate at a half degree above absolute zero; using different materials for the cell and electrodes would cause the instrument to fall apart as it cools and warms. Making electrodes out of clear plastic is where the materials challenge comes in.
"Obviously acrylic doesn't conduct electricity," Cianciolo explains, "so to turn it into an electrode, we have to find a way to modify its surface to make it conducting. Also, to prevent interfering with the rest of the experiment, the surface cannot be magnetic or superconducting. It can't become radioactive when it interacts with neutrons, and it has to be tough enough to endure extreme temperatures and the occasional spark."
Cianciolo's collaborators—atomic physicist Fred Meyer and polymer chemist Mark Dadmun—are leading the development of two approaches to solving this problem: implanting metal ions in the surface of the polymer or creating a carbon nanoparticle polymer composite material—an acrylic that has been seeded with conducting nanoparticles. Materials scientist Harry Meyer is providing expertise in analyzing the performance of the modified polymers.
"There are a lot of possible combinations of materials to evaluate in both of those scenarios," Cianciolo says. "Right now we're considering all of the material requirements—not magnetic, not super-conducting, etc.—and trying to match them up with the properties of the materials we're able to synthesize to determine what sort of new material might meet our needs. There is some computer simulation involved in this process, but our studies are conducted primarily through experimentation. I suppose there might be an analogy for this kind of work in Thomas Edison's search for a light bulb filament. He tested 1,000 different types before he found one that worked the way he wanted it to. We're trying to gain a similar understanding of how to make an acrylic-based material that conducts electricity."
Tweaking the standard model?
So once nEDM is up and running, what do researchers hope to learn? It all boils down to whether the results support or contradict the "standard model"—the overarching model that physicists use to explain how objects interact in the physical world. The model predicts an EDM much smaller than current measurement techniques could detect. Cianciolo finds that feature of the nEDM experiment particularly attractive.
"Because the standard model prediction is immeasurably small, any experiment that detects a non-zero EDM is exploring physics that lies beyond the standard model," he says.
Generally speaking, physicists know the standard model is incomplete, and various theories make predictions that go beyond the model. Most of those predict a measureable, non-zero EDM.
"We expect that either our experiment will see something that is not predicted by the standard model, or it will suggest that something is wrong with all of these extensions to the standard model," Cianciolo says. "Any result from this experiment will have an impact on a number of theories.
"The effort complements the work being done at the Large Hadron Collider, the world's premiere high-energy physics facility. At the LHC, which straddles the border between France and Switzerland outside Geneva, scientists are using high-energy collisions to directly produce particles that aren't explained by the standard model and are measuring their decay.
"We're trying to take the opposite approach by making a measurement that's so precise that we can see the very tiny signatures of physics that go beyond the standard model, even at the low energies we're working with," Cianciolo says.
Any results from the nEDM experiment are sure to provoke as many questions as answers. An immeasurably small EDM would leave questions about the creation of matter in the early universe unanswered. A measurable EDM would suggest that a miniscule wobble in the shape of the neutron spelled the difference between a starry sky and an empty cosmos. Either way, the universe as we know it was balanced on a knife's edge from the beginning.