E = mc2
You most likely already know what is perhaps Einstein’s most famous equation. You might not know what it stands for, but you remember how it goes. You might not have even cemented Pythagoras’ famous theorem into your head yet—probably not for a year or two—and you might have already known how it’s written out: E = mc2.
The famous equation, known formally as the mass-energy equivalence, describes how mass and energy are essentially the same physical entity, and can thus be converted from one to the other. Detailed in Albert Einstein’s theory of special relativity, the energy of a body at rest, described as E, is equivalent to its mass at rest m multiplied with the square of the speed of light in a vacuum c (299,792,458 m/s, ~671 million miles per hour). It’s the very equation that describes why a molecule of water (H2O) weighs less than two atoms of free hydrogen (H) and one atom of free oxygen (O); the difference in mass between the two is equivalent to the energy needed to “break apart” the water molecule into its three constituent atoms.
The Breit-Wheeler Process
In principle, there are various special considerations and conditions that must be taken into account when playing around with this equation. However, demonstrations and examples, much like the water molecule example above, are out there; one of which is the Breit-Wheeler process, described by physicists Gregory Breit and John A. Wheeler back in 1934. In it, they describe that two photons colliding into each other will create a pair of particles: an electron and its antimatter equivalent, known as an antielectron or positron. The process conforms with Einstein’s famous equation, thus making it the simplest way of transforming pure light into matter.
The two physicists, however, did note that observing the process would be one of the most difficult to do in physics. For starters, the photons headed for a mutual crash must be high-energy gamma ray photons (the most energetic type of photon; you usually see this stuff during radioactive decay, or around quasars and black holes); scientists didn’t have gamma ray lasers then, and they still don’t have them now. This, among other hindrances, makes it difficult to demonstrate Einstein’s equation at the simplest level; if you ask the physicists over at Brookhaven National Laboratory over in New York, however, they might have a workaround for it.
The Experiment
Detailed in a paper published in the journal Physical Review Letters, Brookhaven physicists set about to perform a workaround prescribed by Breit and Wheeler themselves all those years ago: instead of impacting two gamma ray photons, they instead impacted ions accelerated at 99.995% the speed of light.
Ions are atoms whose number of protons are unequal to the number of electrons, disrupting their usual neutral charge. Atoms, in essence, are electrically neutral—that is, the number of positively-charged protons in their nucleus exactly matches the number of negatively-charged electrons around them. Should it have one of its electrons taken away, the atom will find itself with more protons than electrons. As a result, the entire atom gains a net positive charge—a cation. Should it get one more electron instead, there will be more electrons than protons, and the atom gains a net negative charge—an anion.
This time around, the physicists used gold cations, and proceeded to mash them together at the facility’s Relativistic Heavy Ion Collider (RHIC). As the positively-charged gold cations are accelerated, they produce their own magnetic field; this interacts with the RHIC’s own electrical field, creating electromagnetic particles: photons. As Brookhaven physicist Zhangbu Xu put it: “[…]When the ions are moving close to the speed of light, there are a bunch of photons surrounding the gold nucleus, traveling with it like a cloud.”
Thing is, the two gold cations don’t necessarily hit each other; they really just barely miss each other, leaving their photon “clouds” to hit each other instead. These photon “cloud” collisions can be an obstacle; that’s because instead of the usual photons, they instead produce ”virtual” photons—photons that, simply put, just pop in and out of existence. Accelerating the gold cations to 99.995% the speed of light was a way to get around this; at relativistic speeds (speeds close to the speed of light), these “virtual” photons behave like their real counterparts.
Now, the results from the Breit-Wheeler process differ when considering real-to-real, real-to-virtual, and virtual-to-virtual photon collisions. Physicists can identify which is happening from the angles between the electron-positron pair generated by the collision. In the case of Brookhaven’s recent study, the angles produced by their gold cation experiments remained consistent with those from real-to-real collisions. Brookhaven physicist Daniel Bradenburg also mentioned that they measured “all the energy, mass distributions, and quantum numbers of the systems,” saying that “[t]hey are consistent with theory calculations for what would happen with real photons.” Said Bradenburg: “Our results provide clear evidence of direct, one-step creation of matter-antimatter pairs from collisions of light as originally predicted by Breit and Wheeler.”
As it stands, scientists have yet to devise a way to directly detect real-to-real Breit-Wheeler process collisions; the scientists at Brookhaven National Laboratory, however, have made considerable progress in the pursuit, and show that science is, at the very least, going down the right path towards this ideal.
Bibliography
- Brookhaven National Laboratory. (2021, July 28). Collisions of Light Produce Matter/Antimatter from Pure Energy. Brookhaven National Laboratory. Retrieved September 5, 2021, from https://www.bnl.gov/newsroom/news.php?a=119023
- Perkowitz, S. (2021, July 22). E = mc2. Encyclopedia Brittanica. Retrieved September 5, 2021, from https://www.britannica.com/science/E-mc2-equation
- Pile, D. (2014, June). Light into Matter. Nature Photonics. https://www.nature.com/articles/nphoton.2014.115.pdf
- Starr, M. (2021, August 10). Physicists Detect Strongest Evidence Yet of Matter Generated by Collisions of Light. ScienceAlert. Retrieved September 5, 2021, from https://www.sciencealert.com/physicists-claim-they-ve-finally-observed-matter-being-made-out-of-colliding-light
- Turner, B. (2021, August 17). Famous Einstein equation used to create matter from light for first time. LiveScience. Retrieved September 5, 2021, from https://www.livescience.com/einstein-equation-matter-from-light
- STAR Collaboration, Adam, J., Adamczyk, L., Adams, J. R., Adkins, J. K., Agakishiev, G., Aggarwal, M. M., Ahammed, Z., Alekseev, I., Anderson, D. M., Aparin, A., Aschenauer, E. C., Ashraf, M. U., Atetalla, F. G., Attri, A., Averichev, G. S., Bairathi, V., Barish, K., Behera, A., … Zyzak, M. (2021). Measurement of ${e}^{+}{e}^{\ensuremath{-}}$ momentum and angular distributions from linearly polarized photon collisions. Physical Review Letters, 127(5), 052302. https://doi.org/10.1103/PhysRevLett.127.052302
