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It was a bold thought experiment: that only one electron makes up the entire universe.
The most famous conversation about it took place between Richard Feynman, PhD, and John Wheeler, PhD, both American theoretical physicists, as recounted in Feynman’s 1965 Nobel acceptance speech.
Wheeler said he had a theory about why all electrons in the universe have the same charge and the same mass: “Because they are all the same electron!”
As physicists today understand it, the conversation was a thought experiment and was never meant to be that serious. But here are a couple of implications of the theory:
All fundamental particles look the same, meaning one electron cannot be distinguished from another electron. Negatively charged electrons have antiparticles, called positrons, that are identical except for their charge. Assuming an electron always moves forward in time, any electron going backward in time would be a positron. (But this creates paradoxes, as we show shortly.)
Needless to say, the theory has faced its fair-share of scrutiny from physicists. Diego Fallas Padilla, PhD, a postdoctoral physics researcher at the University of Colorado Boulder, says there are serious limitations with how the one-electron theory describes reality, so the community doesn’t take it seriously.
One of the most obvious issues is that when we observe the universe, we can see far more electrons than positrons. “That’s one reason why this theory, as a thing, is completely not feasible,” Fallas Padilla said.
Speaking to the Feynman-Wheeler thought experiment, Fallis Pallida added that the motivation behind it “is more to say that, because electrons are a fundamental particle, in our theory, you don’t expect that you measure any difference.”
There are many other issues, too, when taking into account how space-time works. Perhaps the simplest to explain is causality. If there is just one electron moving back and forth in time, then how the electron operated (and continues to operate) in the past would influence the present. That’s a paradox, similar to what you see in time travel movies when a character tries to change an event in the past using a time machine.
While our entire universe probably isn’t made up of the same incredible electron, the theory has some offshoots that are more valuable for modern scientists. The thought-experiment conversation about the one electron theory took place more than 60 years ago, reminding us that particle physics can be a counterintuitive world to play in. But Fallas Padilla says there are at least two implications with real-world applications that are better grounded in how reality works.
For starters: figuring out the ratio of matter-to-antimatter, with antimatter being matter that has the opposite charge.. The electron-positron imbalance isn’t the only universe imbalance: there also is a greater matter-to-antimatter inequity, which physicists are trying to resolve with the standard model (which, simply put, describes the world around us).
The matter-to-antimatter imbalance can be described using a concept called “CPT symmetry.” C stands for charge, P is for parity, and T is for time reversal, explained Fallas Padilla. Much like the electron reversing through time as a positron, CPT theory aims to show that, if you ran our universe’s clock backwards and replaced matter with antimatter, mathematically everything should look the same.
But real-life observations show the universe doesn’t look like this ideal construction. There is far more matter than antimatter. Why is not exactly clear, but whatever is causing this violation is a very small effect, Fallas Padilla emphasized.
He said that the answer may lie in reformulating the standard theory so that new particles with higher energies would be included. But to find these theoretical newer particles, we’ll need to build more powerful particle accelerators to search for them.
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On a related note, maybe those particle accelerators can also show us more about electrons. Electrons spin, creating magnetic moments (which refers to the strength of a magnetic field). But an eEDM—an electron electric dipole moment—can only happen when the charge of the electron is a little distorted. And if an eEDM is happening, it means that we cannot run the universe backwards in time and see no real changes.
The standard model allows for a tiny amount of variation in the eEDM, but not very much. And unfortunately, while we can theorize the variation, that variation is too small to measure with our tools by several orders of magnitude, Fallas Padilla said.
But physicists are trying to find ways to test eEDM, using molecules that could magnify its effect. One example in recent years included Nobel laureate Eric Cornell, PhD. Cornell’s team worked with hafnium fluoride, which has an internal electric field of ions that allowed researchers to test the boundaries of eEDM. While they confirmed the standard module works so far, the team is continuing to engineer new molecules to keep testing the boundaries.
As Fallas Padilla pointed out, thought experiments in general are helpful as a “self-consistency check” so that physicists can make sure they are understanding the underlying principles of the universe.
And while the single-electron theory was never meant to be a viable argument, playing with similar notions is helping physicists figure out the properties of particle physics today.
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Elizabeth Howell (Ph.D., she/her) is one of a few space journalists in Canada. She has written five books, and was Space.com’s former staff reporter in spaceflight. As a freelancer, she has written or edited articles about astronomy and space exploration for outlets such as Payload Space, Air&Space Magazine, Sky & Telescope and Salon. Elizabeth holds university degrees in journalism, science and history and also teaches an astronomy course, with Indigenous content, at Canada’s Algonquin College. Aside from watching several astronaut missions launching from Florida and Kazakhstan, Elizabeth once lived like an astronaut at the Mars Society’s Mars Desert Research Station in Utah.