Physicists Detect Strongest Evidence Yet of Matter Generated by Collisions of Light


In keeping with concept, if you happen to smash two photons collectively arduous sufficient, you may generate matter: an electron-positron pair, the conversion of sunshine to mass as per Einstein’s concept of particular relativity.


It is known as the Breit-Wheeler course of, first laid out by Gregory Breit and John A. Wheeler in 1934, and we’ve got superb cause to imagine it might work.

However direct remark of the pure phenomenon involving simply two photons has remained elusive, primarily as a result of the photons have to be extraordinarily energetic (i.e. gamma rays) and we do not have the know-how but to construct a gamma-ray laser.

Now, physicists at Brookhaven Nationwide Laboratory say they’ve discovered a method round this stumbling block utilizing the ability’s Relativistic Heavy Ion Collider (RHIC) – leading to a direct remark of the Breit-Wheeler course of in motion.

“Of their paper, Breit and Wheeler already realized that is virtually unimaginable to do,” mentioned physicist Zhangbu Xu of Brookhaven Lab.

“Lasers did not even exist but! However Breit and Wheeler proposed an alternate: accelerating heavy ions. And their different is precisely what we’re doing at RHIC.”

However what do accelerated ions need to do with photon collisions? Effectively, we are able to clarify.

The method entails, because the collider’s title suggests, accelerating ions – atomic nuclei stripped of their electrons. As a result of electrons have a unfavourable cost and protons (throughout the nucleus) have a optimistic one, stripping it leaves the nucleus with a optimistic cost. The heavier the aspect, the extra protons it has, and the stronger the optimistic cost of the ensuing ion.


The workforce used gold ions, which comprise 79 protons, and a strong cost. When gold ions are accelerated to very excessive speeds, they generate a round magnetic subject that may be as highly effective because the perpendicular electrical subject within the collider. The place they intersect, these equal fields can produce electromagnetic particles, or photons.

“So, when the ions are transferring near the pace of sunshine, there are a bunch of photons surrounding the gold nucleus, touring with it like a cloud,” Xu defined.

On the RHIC, ions are accelerated to relativistic speeds – these which are a big share of the pace of sunshine. On this experiment, the gold ions have been accelerated to 99.995 p.c of sunshine pace.

That is the place the magic occurs: When two ions simply miss one another, their two clouds of photons can work together, and collide. The collisions themselves cannot be detected, however the electron-positron pairs that end result can.

Nevertheless, it is not sufficient to simply detect an electron-positron pair, both.

colliderDiagram displaying how the near-miss of gold ions produces photon collisions. (Brookhaven Lab)

That is as a result of the photons produced by the electromagnetic interplay are digital photons, popping briefly out and in of existence, and with out the identical mass as their ‘actual’ counterparts.

To be a real Breit-Wheeler course of, two actual photons have to collide – not two digital photons, nor a digital and an actual photon.


On the ions’ relativistic speeds, the digital particles can behave like actual photons. Fortunately, there is a method physicists can inform which electron-positron pairs are generated by the Breit-Wheeler course of: the angles between the electron and the positron within the pair generated by the collision.

Every sort of collision – virtual-virtual, virtual-real and real-real – may be recognized based mostly on the angle between the 2 particles produced. So the researchers detected and analyzed the angles of over 6,000 electron-positron pairs generated throughout their experiment.

They discovered that the angles have been in keeping with collisions between actual photons – the Breit-Wheeler course of in motion.

“We additionally measured all of the power, mass distributions, and quantum numbers of the methods. They’re in keeping with concept calculations for what would occur with actual photons,” mentioned physicist Daniel Brandenburg of Brookhaven Lab.

“Our outcomes present clear proof of direct, one-step creation of matter-antimatter pairs from collisions of sunshine as initially predicted by Breit and Wheeler.”

The argument could possibly be very fairly made that we can’t have a direct first detection of the pure, single photon-photon Breit-Wheeler course of till we collide photons approaching the power of gamma rays.

Nonetheless, the workforce’s work is extremely compelling stuff – on the very least, it reveals that we’re barking up the proper tree with Breit and Wheeler.

We’ll be persevering with to observe this house, avidly.

The analysis has been printed in Bodily Assessment Letters.



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