In a neat
demonstration of E=mc2, physicists believe they can create electrons and
positrons from colliding photons
The Guardian, Ian Sample, science correspondent, Sunday 18 May 2014
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| In theory light and matter are interchangeable, but a practical demonstration was thought to be impossible. Photograph: Lawrence Manning/Corbis |
Researchers
have worked out how to make matter from pure light and are drawing up plans to
demonstrate the feat within the next 12 months.
The theory
underpinning the idea was first described 80 years ago by two physicists who
later worked on the first atomic bomb. At the time they considered the
conversion of light into matter impossible in a laboratory.
But in a
report published on Sunday, physicists at Imperial College London claim to have
cracked the problem using high-powered lasers and other equipment now available
to scientists.
"We
have shown in principle how you can make matter from light," said Steven
Rose at Imperial. "If you do this experiment, you will be taking light and
turning it into matter."
The
scientists are not on the verge of a machine that can create everyday objects
from a sudden blast of laser energy. The kind of matter they aim to make comes
in the form of subatomic particles invisible to the naked eye.
The
original idea was written down by two US physicists, Gregory Breit and John
Wheeler, in 1934. They worked out that – very rarely – two particles of light,
or photons, could combine to produce an electron and its antimatter equivalent,
a positron. Electrons are particles of matter that form the outer shells of
atoms in the everyday objects around us.
But Breit
and Wheeler had no expectations that their theory would be proved any time
soon. In their study, the physicists noted that the process was so rare and
hard to produce that it would be "hopeless to try to observe the pair
formation in laboratory experiments".
Oliver
Pike, the lead researcher on the study, said the process was one of the most
elegant demonstrations of Einstein's famous relationship that shows matter and
energy are interchangeable currencies. "The Breit-Wheeler process is the
simplest way matter can be made from light and one of the purest demonstrations
of E=mc2," he said.
Writing in
the journal Nature Photonics, the scientists describe how they could turn light
into matter through a number of separate steps. The first step fires electrons
at a slab of gold to produce a beam of high-energy photons. Next, they fire a
high-energy laser into a tiny gold capsule called a hohlraum, from the German
for "empty room". This produces light as bright as that emitted from
stars. In the final stage, they send the first beam of photons into the
hohlraum where the two streams of photons collide.
The
scientists' calculations show that the setup squeezes enough particles of light
with high enough energies into a small enough volume to create around 100,000
electron-positron pairs.
The process
is one of the most spectacular predictions of a theory called quantum
electrodynamics (QED) that was developed in the run up to the second world war.
"You might call it the most dramatic consequence of QED and it clearly
shows that light and matter are interchangeable," Rose told the Guardian.
The scientists
hope to demonstrate the process in the next 12 months. There are a number of
sites around the world that have the technology. One is the huge Omega laser in
Rochester, New York. But another is the Orion laser at Aldermaston, the atomic
weapons facility in Berkshire.
A
successful demonstration will encourage physicists who have been eyeing the
prospect of a photon-photon collider as a tool to study how subatomic particles
behave. "Such a collider could be used to study fundamental physics with a
very clean experimental setup: pure light goes in, matter comes out. The
experiment would be the first demonstration of this," Pike said.
Andrei
Seryi, director of the John Adams Institute at Oxford University, said:
"It's breathtaking to think that things we thought are not connected, can
in fact be converted to each other: matter and energy, particles and light.
Would we be able in the future to convert energy into time and vice
versa?"
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