Scientists predicted this process in the 1930s, but it has
never been achieved in a single direct step. (CREDIT: Creative Commons)
Nuclear physicistshave used a powerful particle accelerator
to create matter directly from collisions of light. Researchers predicted this
process in the 1930s, but it has never been achieved in a single direct step.
The scientists accelerated two beams of gold ions to close to the speed of
light in opposite directions. At such speeds, each gold ion is surrounded by
particles of light (real photons) generated by the ion’s perpendicular magnetic
and electric fields. When the ions graze past one another without colliding,
the photons interact to produce electrons (matter) and positrons (antimatter).
The theory of creating matter from light stems from Einstein’s
famous E=mc2 equation, which states that energy and matter (mass) are
interchangeable. But using light energy to test this—and proving that the
photons are real and long-lived, not “virtual” and short-lived—is difficult.
Even today’s most powerful lasers can’t produce matter. But new information how that photons surrounding ions at the Relativistic Heavy Ion Collider
(RHIC) can. The momentum and angular distributions of the concluding electron-positron pairs indicate, within the high-precision limits of the
experiment, that these particles come from real photons. This makes the
experiment a direct demonstration of the Breit-Wheeler effect predicted in
1934.
In 1934, scientists Gregory Breit and John Wheeler predicted
that collisions of photons could create matter and antimatter, and they even
suggested doing so by accelerating heavy ions. RHIC, a DOE Office of Science
user facility that accelerates heavy ions, turned out to be the ideal facility
for testing their prediction. Accelerating heavy ions such as gold generates a
powerful magnetic field—like current flowing through a wire. At high speed, the
spiraling magnetic and perpendicular electric fields are of equal
strength—which is the definition of a photon, a quantized “particle” of light.
RHIC’s STAR collaboration searched for evidence that
collisions of photons surrounding RHIC’s ions could create matter and
antimatter. Nuclear physicists studied thousands of electron-positron pairs
produced in near-miss collisions, where only the photons interact.
Two gold ions (Au) moving in opposite directions close to
the speed of light (v≈c) are each surrounded by a cloud of real photons (γ).
When these photons collide, they create a matter-antimatter pair: an electron
(e-) and positron (e+). (CREDIT: Brookhaven National Laboratory)
Physicists measured all available motion properties of the
electron-positron pairs. By correlating the photons’ momentum, spatial
location, and polarization with experimental observables, they found that the
transverse distribution of photons was driven by the local electromagnetic
field and not by quantum uncertainty. The high-precision data were consistent
with particles being generated by real photon interactions, rather than from
virtual photons. The analysis provides solid evidence for the Breit-Wheeler
effect.
Reference:
US Dept of Energy, The Brighter Side of News.
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