For the first time, a research team from Brookhaven National Laboratory – one of the largest research laboratories in the United States of America – has been able to accurately monitor how light can transform into particles of matter.
Although this type of interaction was predicted by Albert Einstein’s theory of relativity more than 100 years ago, it was impossible with the technology of mankind at the time.
According to their research paper, which was published in the journal “Physical Review Letters” and announced by the laboratory in a press release issued on July 28 last, the team used a relatively recent particle collider, called “RHIC”, for this task.
The primary finding is that pairs of electrons and positrons—particles of matter and antimatter—can be created directly by colliding very energetic photons, which are quantum “packets” of light.
This conversion of energetic light into matter is a direct consequence of Einstein’s famous E=mc2 equation, which states that energy and matter (or mass) are interchangeable. Nuclear reactions in the sun and at nuclear power plants regularly convert matter into energy. Now scientists have converted light energy directly into matter in a single step.
The second result shows that the path of light traveling through a magnetic field in a vacuum bends differently depending on how that light is polarized.
Such polarization-dependent deflection (known as birefringence) occurs when light travels through certain materials. (This effect is similar to the way wavelength-dependent deflection splits white light into rainbows.) But this is the first demonstration of polarization-dependent light-bending in a vacuum.
Both results depend on the ability of RHIC’s STAR detector—the Solenoid Tracker at RHIC—to measure the angular distribution of particles produced in glancing collisions of gold ions moving at nearly the speed of light.
|Making matter from light: Two gold (Au) ions (red) move in opposite direction at 99.995% of the speed of light|
Colliding clouds of photons
Einstein’s theory of special relativity states that energy and matter are two sides of the same coin, one of which can be converted into the other.
In 1934, the American scientists Gregory Breit and John A. Wheeler first described the hypothetical possibility of colliding light particles to create pairs of electrons and their antimatter counterparts, known as positrons.
“In their paper, Breit and Wheeler already realized this is almost impossible to do,” said Brookhaven Lab physicist Zhangbu Xu, a member of RHIC’s STAR Collaboration. “Lasers didn’t even exist yet! But Breit and Wheeler proposed an alternative: accelerating heavy ions. And their alternative is exactly what we are doing at RHIC.”
An ion is essentially a naked atom, stripped of its electrons. A gold ion, with 79 protons, carries a powerful positive charge. Accelerating such a charged heavy ion to very high speeds generates a powerful magnetic field that spirals around the speeding particle as it travels—like current flowing through a wire.
“If the speed is high enough, the strength of the circular magnetic field can be equal to the strength of the perpendicular electric field,” Xu said.
And that arrangement of perpendicular electric and magnetic fields of equal strength is exactly what a photon is—a quantized “particle” of light. “So, when the ions are moving close to the speed of light, there are a bunch of photons surrounding the gold nucleus, traveling with it like a cloud.”
At RHIC, scientists accelerate gold ions to 99.995% of the speed of light in two accelerator rings.
“We have two clouds of photons moving in opposite directions with enough energy and intensity that when the two ions graze past each other without colliding, those photon fields can interact,” Xu said.
STAR physicists tracked the interactions and looked for the predicted electron-positron pairs.
But such particle pairs can be created by a range of processes at RHIC, including through “virtual” photons, a state of photon that exists briefly and carries an effective mass. To be sure the matter-antimatter pairs were coming from real photons, scientists have to demonstrate that the contribution of “virtual” photons does not change the outcome of the experiment.
To do that, the STAR scientists analyzed the angular distribution patterns of each electron relative to its partner positron. These patterns differ for pairs produced by real photon interactions versus virtual photons.
“We also measured all the energy, mass distributions, and quantum numbers of the systems. They are consistent with theory calculations for what would happen with real photons,” said Daniel Brandenburg, a Goldhaber Fellow at Brookhaven Lab, who analyzed the STAR data on this discovery.
Other scientists have tried to create electron-positron pairs from collisions of light using powerful lasers—focused beams of intense light. But the individual photons within those intense beams don’t have enough energy yet, Brandenburg said.
One experiment at the SLAC National Accelerator Laboratory in 1997 succeeded by using a nonlinear process. Scientists there first had to boost the energy of the photons in one laser beam by colliding it with a powerful electron beam.
Collisions of the boosted photons with multiple photons simultaneously in an enormous electromagnetic field created by another laser produced matter and antimatter.
“Our results provide clear evidence of direct, one-step creation of matter-antimatter pairs from collisions of light as originally predicted by Breit and Wheeler,” Brandenburg said.
“Thanks to RHIC’s high-energy heavy ion beam and the STAR detector’s large acceptance and precision measurements, we are able to analyze all the kinematic distributions with high statistics to determine that the experimental results are indeed consistent with real photon collisions.”
It took years of testing, more than 6,000 collisions to spot the resulting electrons, and the use of sophisticated computational mechanisms to prove that they did indeed emerge from this collision of photons.