Physicists detect strongest evidence yet for matter generated by collisions of light
According to the theory, if you crush two photons strong enough, you can generate matter: an electron-positron pair, converting light to mass according to Einstein’s special theory of relativity.
It’s called the Breit-Wheeler process, first implemented by Gregory Breit and John A. Wheeler in 1934, and we have very good reason to believe it would work.
But direct observation of the pure phenomenon involving only two photons has remained elusive, mainly because the photons must be extremely energetic (i.e. gamma rays) and we do not yet have the technology to build a laser. gamma ray.
Now, physicists at the Brookhaven National Laboratory say they’ve found a way to get around this stumbling block by using the facility’s Relativistic Heavy Ion Collider (RHIC), which has made it possible to directly observe the Breit-Wheeler process in action.
“In their article, Breit and Wheeler already realized that it was almost impossible to do”, said physicist Zhangbu Xu from the Brookhaven lab.
“Lasers weren’t even there yet! But Breit and Wheeler came up with an alternative: accelerating heavy ions. And their alternative is exactly what we’re doing at RHIC.”
But what do accelerated ions have to do with photon collisions? Well we can explain.
The process involves, as the name of the collider suggests, the acceleration of ions – atomic nuclei stripped of their electrons. Because electrons have a negative charge and protons (inside the nucleus) have a positive charge, stripping leaves the nucleus with a positive charge. The heavier the element, the more protons it contains and the stronger the positive charge of the resulting ion.
The team used gold ions, which contain 79 protons, and a powerful charge. When gold ions are accelerated to very high speeds, they generate a circular magnetic field which can be as strong as the perpendicular electric field in the collider. Where they intersect, these equal fields can produce electromagnetic particles, or photons.
“So when the ions move near the speed of light, there is a bunch of photons surrounding the gold core, traveling with it like a cloud,” Xu explained.
At RHIC, ions are accelerated to relativistic speeds – those which are a large percentage of the speed of light. In this experiment, gold ions were accelerated to 99.995% of the speed of light.
This is where the magic happens: when two ions miss each other, their two clouds of photons can interact and collide. The collisions themselves cannot be detected, but the resulting electron-positron pairs can.
However, it is also not enough to detect an electron-positron pair.
This is because the photons produced by the electromagnetic interaction are virtual photons, briefly entering and exiting existence, and without the same mass as their “real” counterparts.
To be a true Breit-Wheeler process, two real photons must collide – not two virtual photons, not a virtual photon and a real photon.
At relativistic ion speeds, virtual particles can behave like real photons. Fortunately, there is a way for physicists to determine which electron-positron pairs are generated by the Breit-Wheeler process: the angles between the electron and the positron in the pair generated by the collision.
Each type of collision – virtual-virtual, virtual-real and real-real – can be identified based on the angle between the two particles produced. The researchers therefore detected and analyzed the angles of more than 6,000 electron-positron pairs generated during their experiment.
They found that the angles corresponded to collisions between real photons – the Breit-Wheeler process in action.
“We also measured all the energy, mass distributions and quantum numbers of the systems. They are consistent with theoretical calculations of what would happen with real photons,” said physicist Daniel Brandenburg from the Brookhaven lab.
“Our results provide clear evidence for the direct, one-step creation of matter-antimatter pairs from light collisions, as originally predicted by Breit and Wheeler.”
The argument could be very reasonably made that we will not have any direct first detection of the pure photon-photon Breit-Wheeler process until the collision of photons approaching the energy of gamma rays.
Nonetheless, the team’s work is very convincing – at the very least, it shows that we are barking in the right tree with Breit and Wheeler.
We will continue to watch this space eagerly.
The research was published in Physical examination letters.