In 1905 Albert Einstein wrote four groundbreaking papers on quantum theory and relativity. It came to be known as Einstein’s annus mirabilis, or miraculous year. One was on Brownian movement, one brought him the Nobel Prize in 1921, and one outlined the fundamentals of the special theory of relativity. But it is Einstein’s last paper from 1905 that is the most unexpected.
The paper is only two pages long and outlines how special relativity could explain a strange aspect of radioactive decay. As Marie Curie best demonstrated, some materials, such as radium salts, can emit particles with much more energy than would be possible with simple chemistry. Einstein’s little paper speculated that the excess energy could be compensated for by a loss of mass of the core particles. This idea eventually led to Einstein’s most famous equation, E = mc2.
Pierre and Marie Curie in their laboratory. 1904, author unknown.
This equation is often understood to mean that matter and energy are two sides of the same coin. It actually means that the apparent mass and energy of an object depend on the relative motion of an observer, and for this reason the two are intertwined, much like the connection between space and time. But a consequence of this relationship is that, under the right circumstances, objects should be able to produce energy by losing mass.
Today we know that this is exactly what happens with radioactive decay. The effect is also how stars create energy in their cores through nuclear fusion. Of course, if matter can become energy, it should also be possible for energy to become matter. This trick is a little trickier, and it took particle accelerators to pull it off. Nowadays we do that all the time. Accelerate particles to near the speed of light and slam them together. The large apparent mass of the particles releases enormous energy, and part of this energy is converted back into particles. All of modern particle physics can trace its history back to Einstein’s two-sided paper.
Two gamma ray photons can become matter. Photo credit: Mathieu Michel Lobet
But the laws of physics not only say that energy can be produced from matter and vice versa, it also imposes specific restrictions on the nature of the matter and energy produced. One of the simplest examples of this is electron-positron annihilation. This happens when an electron collides with its antimatter twin. The two particles have the same mass but opposite charge, so that when they collide they generate two high-energy photons. The mass of the electron and positron is completely converted into energy. This experiment was first proposed in the 1930s but was not carried out until 1970.
If you can completely convert matter into energy, you should be able to do the opposite. It’s known as the Breit-Wheeler process and involves the collision of two photons to create an electron-positron pair. We have used light several times to create matter, but it is very difficult to convert two photons directly into matter. But a recent experiment shows that it is possible.
The team used data from the Relativistic Heavy Ion Collider (RHIC) and examined more than 6,000 events that created electron-positron pairs. Instead of simply beaming two lasers together, they used high-energy particle collisions to generate intense bursts of photons. In some cases, these photons collided to create an electron-positron pair. From the data they could show when a couple was created directly from light.
Because these pair productions occurred in the intense magnetic field, the team also demonstrated another interesting effect known as vacuum birefringence. Normal birefringence occurs when light is split into two rays of different polarization. This effect occurs naturally in materials like Iceland spar. With vacuum birefringence, light that passes through an intense magnetic field is split into two polarizations, with each polarization taking a slightly different path. It’s an amazing effect when you think about it because it means that you can only change the light path in vacuum with a magnetic field. Vacuum birefringence has been observed in light from a neutron star, but this is the first time it has been observed in the laboratory.
Relation: Einstein, Albert. “Does the inertia of a body depend on its energy content?” Annals of Physics 323.13 (1905): 639-641.
Relation: Sodickson, L., et al. “One-quantum annihilation of positrons.” Physical review 124.6 (1961): 1851.
Relation: Breit, Gregory, and John A. Wheeler. “Collision of two light quanta.” Physical Review 46.12 (1934): 1087.
Relation: Adam, Jaroslav, et al. “Measurement of e + e? Momentum and angular distributions of linearly polarized photon collisions. “Physical Review Letters 127.5 (2021): 052302.