The equation at the heart of Einstein’s theory of relativity, E=mc2, is surely the most familiar equation in the world. It expresses that matter and energy are interchangeable, a fact that explains how the sun can shine for billions of years — and that led to the development of nuclear energy.
We usually think of this process as a way to convert a small amount of matter into a lot of energy. The very form of the equation, “solving for E” as the mathematicians put it, underlines this capability. But the theory’s famous equation actually works in both directions. If you write the equation m=E/c2, it is exactly equivalent to the familiar version and still expresses the theory of relativity.
We usually write the equation the familiar way for a few reasons. One is that people have typically been more interested in obtaining a lot of energy rather than a little bit of matter, which we already have in piles all around us. Another reason, though, is that creating matter from energy may be good Einstein theory but is very difficult to do in practice.
Going Uphill Is Harder
A procedure for creating matter (specifically an electron-positron pair) from colliding light beams was outlined as early as 1934 by a pair of physicists named Gregory Breit and John A. Wheeler. But as Brookhaven National Laboratory physicist Zhangbu Xu explains, “Breit and Wheeler already realized this is almost impossible to do.” The process would require a collision of extremely high-energy photons — in fact, a collision of gamma ray beams.
Breit and Wheeler did offer a theoretical alternative involving acceleration of heavy ions. And, as Xu explains, “their alternative is exactly what we are doing” at the Brookhaven laboratory’s Relativistic Heavy Ion Collider (RHIC).
As IFL Science reports, in an enchanting twist, the ions used for the RHIC experiments were gold ions — just the thing for a whiff of alchemy. More remarkable is the speed to which those ions are accelerated: 99.995% of the speed of light — barely under the universe’s ultimate speed limit, itself another consequence of the theory of relativity.
Two streams of gold ions were driven to this enormous speed, in opposite directions and arranged so that they just barely missed a head-on collision. But according to Live Science, when charged particles such as ions (which have their own magnetic fields) are fired through another magnetic field at very high speed, they produce a wake of so-called virtual photons that, in Xu’s words, are “traveling with [the ion] like a cloud.”
These photons do collide, and their collisions produce electron-positron pairs — exactly as the Einstein theory of relativity says should happen. (As a bonus, Brookhaven reports that the experiment also demonstrates another previously elusive effect of relativity: a vacuum polarized by a powerful magnetic field, affecting light passing through it, similar to what sunglasses do.)
But Is It Real?
There is a complication, though. These virtual photons are not quite real — they exist only for profoundly small fractions of a second, and for that brief time they have mass, which “real” photons do not.
The electron-positron pairs produced when these photons collide are definitely real, but if they result from virtual-photon collisions, do they truly represent energy converted to matter or merely some other effect of very high-energy physics?
As Science News reports, the researchers tested this question by using Brookhaven’s STAR (Solenoid Tracker at RHIC) detector to make extremely precise measurements of the electron-positron pairs’ trajectories. STAR showed that, thanks to the enormous speed of the gold ions that triggers the whole process, the trajectories of these created particles match exactly what would be expected from collisions of “real” photons.
So are we seeing the “real” Breit-Wheeler effect? For now, the question is almost philosophical. But as other research teams ramp up the power of experimental lasers, we may soon be able to test the effect with indisputably real laser beams.
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