Posted on Sep 17, 2018 in Physics, Science
“I knew, of course, that, at least in our corner of the universe, there are lots more electrons than positrons, but I still found it an exciting idea to think of trajectories in spacetime that could go unrestricted in any direction — forward in time, backward in time, up, down, left, or right.”
The one electron universe is one of the very few attempts to explain why all electrons in the universe are identical. It has its roots in an entirely different form of symmetry between particles, that of an electron and its antimatter counterpart, the positron. The two particles have the same mass, the same spin, the same everything except for its charge. Setting the the charge aside, the electron and the positron are indistinguishable, and in 1940 that gave John Archibald Wheeler, Princeton quantum physicist (and coiner of the term “black hole” and the quote above) an idea.
Here’s the story behind about “the idea” –Wheeler’s idea is often associated with his doctoral student, the legendary physicist Richard Feynman, who gave the one electron universe idea its most famous boost in his 1965 Nobel Prize Lecture:
I received a telephone call one day at the graduate college at Princeton from Professor Wheeler, in which he said, “Feynman, I know why all electrons have the same charge and the same mass.” “Why?” “Because, they are all the same electron!” And, then he explained on the telephone, “suppose that the world lines which we were ordinarily considering before in time and space – instead of only going up in time were a tremendous knot, and then, when we cut through the knot, by the plane corresponding to a fixed time, we would see many, many world lines and that would represent many electrons, except for one thing. If in one section this is an ordinary electron world line, in the section in which it reversed itself and is coming back from the future we have the wrong sign to the proper time – to the proper four velocities – and that’s equivalent to changing the sign of the charge, and, therefore, that part of a path would act like a positron.”
Wheeler had keyed into a basic if bizarre point of particle physics, continues Alasdair Wilkins in Gizmodo, the direction in which time flowed doesn’t seem to matter much at all, and the arrow of time is, in most cases, completely reversible. The upshot is that, with a few simple equations, Wheeler could transform an electron moving forward in time to one traveling backwards, and the only observable change would be the particle’s charge, which would flip from negative to positive. In other words, an electron would become a positron.
As Wheeler pointed out, each electron traces out a unique path through spacetime, which is its world line. He simply connected all the forward-traveling electrons and backwards-traveling positrons into a single gigantic world line, imagining a particle tracing its way back and forth through the history of the universe to become every electron and positron we had ever observed. And that was why all electrons were indistinguishable.
We’ll let Joe Scott and PBS Spacetime take up the story from here.
The single electron universe falls down on experimental grounds, writes Alasdair Wilkins, not theoretical ones. You might have spotted the problem – for a single electron to account for all electrons in the universe, it needs to travel backwards through the universe exactly as many times as it travels forwards. That means, in this model, that there should be exactly as many positrons as there are electrons. We know that that simply isn’t the case, and that matter completely and utterly dominates antimatter, which means the one electron universe can’t be true.
To be clear, Wheeler never pretended otherwise. As Feynman recalled in his lecture, Wheeler was aware of this problem from the outset and, probably half-joking, offered a rather unlikely way to explain the positron shortage:
“But, Professor”, I said, “there aren’t as many positrons as electrons.” “Well, maybe they are hidden in the protons or something”, he said.
In his memoir, Geons, Black Holes & Quantum Foam, Wheeler made it clear that this “positrons in protons” idea was not meant to be taken seriously:
I knew, of course, that, at least in our corner of the universe, there are lots more electrons than positrons, but I still found it an exciting idea to think of trajectories in spacetime that could go unrestricted in any direction — forward in time, backward in time, up, down, left, or right.
Wheeler, observes Wilkins, did leave himself a tiny loophole there, pointing out that we know electrons far outnumber positrons “at least in our corner of the universe”, which leaves open the theoretical possibility that elsewhere in the cosmos there might be all the positrons needed to make up this vast local discrepancy, which however, goes against the cosmological principle –the general assumption that the universe is essentially the same all over and we don’t occupy a special or unusual position in it. Technically, that doesn’t have to be true, but there’s 500 years of physics backing it up, and there would need to be some truly extraordinary reasons for physicists to consider abandoning it.
The image at the top of the page shows dark matter particles as gray spheres attached to shaded trails representing their motion. NASA Goddard’s Scientific Visualization Studio and NASA Goddard/Jeremy Schnittman
The Daily Galaxy via Gizmodo and PBS
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