LibriumArcana.Games

arstechnica.com wrote:In 1934, two physicists came up with a theory that describes how to create matter from pure light. But they dismissed the idea of ever observing this effect in the laboratory because of the difficulties involved in setting up such an experiment.

Now, Oliver Pike of Imperial College London and his colleagues have found a way to achieve this dream 80 years after US physicists Gregory Breit and John Wheeler explained their theory. This group hopes to use high-energy lasers aimed at a specially designed gold vessel to convert photons into matter/antimatter particle pairs, recreating what happens in some exceptional stellar explosions.

Pike, who led the research published in the journal Nature Photonics, said, "The idea is that light goes in and matter comes out." To be sure, the matter created won't be everyday objects; instead the process will produce sub-atomic particles. "To start with, the matter will consist of electrons and its antimatter equivalent positrons," Pike said. "But with higher energy input in the lasers, we should be able to create heavier particles."

Pike concedes that this won't be the first time light has been converted into matter. In 1997, US researchers at the Stanford Linear Accelerator Centre (SLAC) were able to able to do so, albeit in a different way. The SLAC experiment used electrons to first create high-energy light particles, which then underwent multiple collisions to produce electrons and positrons, all within same chamber. This is called the multi-photon Breit-Wheeler process, named after the two physicists who came up with the theory in 1934.

"The key difference in the SLAC experiment and the one we propose is that our process will be more straightforward," Pike said. In the new proposal, the laser beam will still be generated using free electrons, but it will be separated from the electrons.

Why create light using matter and then convert it back? Apart from showing that the Breit-Wheeler process can happen without the multiple photons the SLAC experiment needed, Pike thinks their process provides a clean way of doing particle physics experiments.

Current particle-physics experiments involve smashing subatomic particles at great speeds and sorting through the mess of new particles that are created in the explosion. This is how the Higgs boson was found in the Large Hadron Collider.

The new experimental design will be similar but simpler. Rather than involving a complicated mix of particles and photons, the laser beam will be sent into a small gold hohlraum (German for "empty room"). There, individual photons can interact with the radiation field that is generated when the hohlraum is excited by a laser, creating the electron/positron pairs.

"While physicists have excellent methods to sift through such data, our process has the advantage that it will be easier to analyze," Pike said. "Light will go in from one end of the hohlraum and the particles created will come out from the other end."

Pike and colleagues are now working to secure time on high-energy laser beams to carry out the experiment. The two likely candidates are Aldermaston, Berkshire in the UK or Rochester, New York in the US.

Andrei Seryi at the University of Oxford found the work interesting but warned that it's still too far away from being used in particle-physics experiments. "Theoretically, however, it would be great if we are able to create particles from only light."

"With such high energy lasers, we may not need to build big particle colliders, such as the Large Hadron Collider, which is a 22 km underground tunnel," Seryi said.

Even if we do manage to create a photon collider, we’d only be catching up with the natural world, where a specific type of supernova, called “pair instability,” involves the creation proton/antiproton pairs. If Pike is able to achieve this phenomenon, he will essentially be creating a supernova in a bottle—or at least an empty room.

phys.org wrote:Imperial College London physicists have discovered how to create matter from light - a feat thought impossible when the idea was first theorised 80 years ago.

In just one day over several cups of coffee in a tiny office in Imperial's Blackett Physics Laboratory, three physicists worked out a relatively simple way to physically prove a theory first devised by scientists Breit and Wheeler in 1934.

Breit and Wheeler suggested that it should be possible to turn light into matter by smashing together only two particles of light (photons), to create an electron and a positron – the simplest method of turning light into matter ever predicted. The calculation was found to be theoretically sound but Breit and Wheeler said that they never expected anybody to physically demonstrate their prediction. It has never been observed in the laboratory and past experiments to test it have required the addition of massive high-energy particles.

The new research, published in Nature Photonics, shows for the first time how Breit and Wheeler's theory could be proven in practice. This 'photon-photon collider', which would convert light directly into matter using technology that is already available, would be a new type of high-energy physics experiment. This experiment would recreate a process that was important in the first 100 seconds of the universe and that is also seen in gamma ray bursts, which are the biggest explosions in the universe and one of physics' greatest unsolved mysteries.

The scientists had been investigating unrelated problems in fusion energy when they realised what they were working on could be applied to the Breit-Wheeler theory. The breakthrough was achieved in collaboration with a fellow theoretical physicist from the Max Planck Institute for Nuclear Physics, who happened to be visiting Imperial.

Demonstrating the Breit-Wheeler theory would provide the final jigsaw piece of a physics puzzle which describes the simplest ways in which light and matter interact (see image in notes to editors). The six other pieces in that puzzle, including Dirac's 1930 theory on the annihilation of electrons and positrons and Einstein's 1905 theory on the photoelectric effect, are all associated with Nobel Prize-winning research (see image).
Professor Steve Rose from the Department of Physics at Imperial College London said: "Despite all physicists accepting the theory to be true, when Breit and Wheeler first proposed the theory, they said that they never expected it be shown in the laboratory. Today, nearly 80 years later, we prove them wrong. What was so surprising to us was the discovery of how we can create matter directly from light using the technology that we have today in the UK. As we are theorists we are now talking to others who can use our ideas to undertake this landmark experiment."

The collider experiment that the scientists have proposed involves two key steps. First, the scientists would use an extremely powerful high-intensity laser to speed up electrons to just below the speed of light. They would then fire these electrons into a slab of gold to create a beam of photons a billion times more energetic than visible light.

The next stage of the experiment involves a tiny gold can called a hohlraum (German for 'empty room'). Scientists would fire a high-energy laser at the inner surface of this gold can, to create a thermal radiation field, generating light similar to the light emitted by stars.

They would then direct the photon beam from the first stage of the experiment through the centre of the can, causing the photons from the two sources to collide and form electrons and positrons. It would then be possible to detect the formation of the electrons and positrons when they exited the can.

Lead researcher Oliver Pike who is currently completing his PhD in plasma physics, said: "Although the theory is conceptually simple, it has been very difficult to verify experimentally. We were able to develop the idea for the collider very quickly, but the experimental design we propose can be carried out with relative ease and with existing technology. Within a few hours of looking for applications of hohlraums outside their traditional role in fusion energy research, we were astonished to find they provided the perfect conditions for creating a photon collider. The race to carry out and complete the experiment is on!"

Christian Science Monitor wrote:Converting light into matter may sound like alchemy, but it's a natural outcome of physics – one that scientists have been demonstrating to varying degrees for decades. Now, a team of European physicists is proposing a way to do it much more simply.

If the approach works as the researcher's initial calculations suggest, the results are unlikely immediately answer any vexing question, physicists say. The fundamental science behind the process of turning light to matter is already well understood. But it would be a new tool in physicists' toolkit.

Currently, the process of getting packets of light, known as photons, to collide and make particles can be a complicated business, requiring a few tricks. But the new technology might enable a range of new experiments, which could lead to unexpected answers or uses.

"Often when people first get an idea for some big thing, it turns out to be fantastically useful for something else," says Chris Quigg, a senior theorist at the Fermi National Accelerator Laboratory in Batavia, Ill., who was not involved in the study.

The tool is a collider for photons, the subatomic particles associated with visible light and other forms of electromagnetic radiation, such as radio waves and gamma rays. By crashing them head-on, the collider would turn the photons into electrons and their anti-matter counterparts, positrons. Calculations suggesting how this might work appear in the current issue of the journal Nature Photonics.

The theory behind this was first proposed in 1934 by two American physicists, Gregory Breit and John Wheeler. But it wasn't until 1997 that scientists working at the Stanford Linear Accelerator Center (SLAC) in Stanford, Calif., were able to successfully carry out the first true photon-to-photon collision and create the two particles.

They did it by using electrons to ricochet light back into itself. The team accelerated a beam of electrons to high energies, then blasted the beam with a laser. Some of the photons in the laser scattered off these electrons, traveling back toward the laser beam and picking up energy in the process. When these higher-energy photons collided with additional photons the laser was sending out, the collisions produced pairs of electrons and their antimatter counterparts, positrons.

While the research team saw positrons, as Drs. Breit and Wheeler predicted, it had to fall back on modeling to piece together the events that led to the generation of the positrons.

With their new plans, a quartet of physicists from Britain and Germany are proposing a collider that performs the experiment in a simpler, more direct fashion and lays bare the entire process.

"The experimental design we propose can be carried out with relative ease and with existing technology," said Oliver Pike, a physicist at Imperial College in London who led the team, in a prepared statement.

An lower-energy electron beam would smack into a gold target, generating a beam that includes electron-positron pairs as well as high-energy gamma ray photons. Magnets would deflect the electrically-charged electrons and positrons, leaving only gamma ray beam. That beam would head into a tube smaller than a thimble and kept in a high vacuum. Another laser would heat the tube's interior, generating additional photons. The gamma rays would hit photons the heated tube emits, generating electron-positron pairs, which would be detected as they escape from the other end of the tube.

The hotter the tube, the more electron-positron pairs the collisions would create, the researchers estimate. By boosting the energy of the electron beam, the team suggests that the collisions could produce heavier particles than electrons or positrons. These heavier particles are known collectively as hadrons, and are built from combinations of smaller particles known as quarks. Hadrons include the familiar proton and neutron, as well as a range of other particles from the particle-physics zoo.

Indeed, high-energy photon-to-photon colliders could help scientists study some of these particles. Researchers have been exploring ways to use photon-to-photon colliders to create vast numbers of Higgs bosons in a next-generation linear accelerator. The Higgs boson, whose discovery was announced in 2012, is a particle associated with a quantum field that imparts mass to other subatomic particles. But to study the particle's properties in detail, researchers must generate large quantities of them.

The immediate scientific payoff for using the newly proposed approach to make matter from light could well be small, suggests Dr. Quigg at Fermilab.

Out of the seven interactions between matter and light physicists have identified and verified, making two beams of light collide "has not been done," he says. "On the other hand, the reverse reaction – electrons and positrons annihilating to make two photons – is done every day at every laboratory. We know all about the fundamental science about that."

That said, if researchers can produce a sufficiently dense collection of photons in the tube to make them an easy target to hit, some interesting physics might emerge form such experiments, Quigg says. "You could imagine shooting all sorts of other particles at them."