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  The simulation concept has attracted an enthusiastic set of fans – perhaps unsurprisingly seduced by the lure of the anything-goes physics. The tech mogul Elon Musk has declared himself a believer, and the New Yorker magazine reported that two Silicon Valley billionaires are so convinced it’s true, they’ve been funding an effort to find a way to break out into the real world.

  Many of these supporters reckon the odds our world is computer generated are far higher than Bostrom himself estimated. Some regard it as a near certainty, arguing that it could explain many lingering mysteries, such as claims about supernatural phenomena, or why the universe appears to be curiously fine-tuned to support our existence.

  Of course, as the simulation hypothesis has grown in popularity, it has simultaneously generated a backlash from scientists who feel that enough is enough: that, at the end of the day, it’s an absurd concept and we should accept the reality of our universe.

  Part of the reason they feel this way is because they believe that Bostrom and his supporters are vastly overestimating the chances that such convincing simulated worlds will ever be built. The technical challenge alone, they argue, would be daunting, and possibly insurmountable. Creating lifelike graphics in a computer game is one thing, but generating the data necessary to create an entire world, down to the quantum level, is another thing altogether. Even futuristic supercomputers might not be up to the task.

  And, really, why would any advanced civilization create such a thing? The physicist Sabine Hossenfelder has made the case that anyone capable of creating artificial intelligences would surely want to put those intelligences to work solving real-world problems, not trap them in a simulated environment.

  The larger complaint, though, is that the entire simulation debate seems frivolous and pointless. After all, there’s nothing we can do with the information that the universe might be a simulation. It offers no obvious implications about how we should live our lives. Nor does it increase our knowledge in any way, because there’s no way to prove or disprove the hypothesis. The whole idea, they insist, should be relegated to the category of pseudoscientific nonsense.

  Bostrom himself agrees that we shouldn’t alter our behaviour in any way just because we might be living in a simulation. In that respect, it’s true that the hypothesis is completely irrelevant to our everyday lives.

  But a line of thinking in favour of the hypothesis could be that it draws attention to one of the underlying assumptions upon which scientific knowledge is built. This is the assumption that the universe is, in the words of the astronomer William Keel, playing fair with us.

  Scientists take it for granted that nature is following certain rules and isn’t breaking them. One of these is that the laws of nature, such as gravity and electromagnetism, are universal and apply uniformly. But there’s no way to definitively test that this is true. All we can say is that, so far, we’ve never observed a case where the laws of nature are broken.

  Another assumed rule – and this is more directly relevant to the simulation hypothesis – is that the parts of the universe we can’t see are similar to the parts that we can. When astronomers look out from the Earth, they can peer approximately forty-six billion light years in any one direction, which is the furthest light has been able to travel since the birth of the universe. Since they suspect the universe may be infinite in size, this bubble of observable space around us represents a tiny fraction of the whole universe.

  Nevertheless, cosmologists routinely make conclusions about the entire universe. They can only do this because they assume the universe as a whole closely resembles the part of it we live in and can see. They assume our local cosmic neighbourhood is a representative sample of the whole thing. They refer to this assumption as the cosmological principle. But, again, there’s no way to test if this is true. For all we know, once you venture past the edge of the observable universe, space might be made of mozzarella cheese.

  A mozzarella universe would violate every known law of physics, but a simulated universe wouldn’t break any laws. There’s nothing inherently impossible about it. It’s unlikely, perhaps, but not impossible. So, if we were to imagine a plausible way in which the universe wasn’t playing fair with us, this might be it. Which is to say that, if we were somehow able to zoom out and see the big-picture view of the entire cosmos, beyond the observable universe, we wouldn’t find galaxy upon galaxy stretching away to infinity. Instead, we’d encounter the surface of a computer hard drive.

  What if there’s only one electron in the universe?

  In the sixth century BC, the Greek philosopher Thales declared that everything was made from water. What exactly did he mean by this? Did he really think everything was made from water, or was he speaking metaphorically? Unfortunately, we’re not sure, because none of his writings have survived. We only know he said this because Aristotle, writing some 250 years later, briefly mentioned that he did. The cryptic statement has nevertheless earned Thales credit for being the first person to ever utter a recognizably scientific statement, because it seems he was attempting to explain the world naturalistically rather than by appealing to mythology, which is what everyone else had done up until then. He also seemed to be suggesting that the apparent complexity of nature, all the bewildering diversity of its forms, might be constructed out of some more basic type of material.

  This latter idea lies at the heart of what it means to do science. Scientists strive to understand nature by discovering the simpler patterns and structures that underlie its outward complexity, and this quest, first articulated by Thales, has yielded stunning results. Biologists have discovered that the vast assortment of living organisms on the Earth all acquire their incredible variety of forms from cellular instructions written in a genetic code that consists of only four letters: A, C, G and T. From these four letters, arranged in different ways, nature produces species as widely varied as bacteria, fungi, oak trees, polar bears and blue whales.

  On an even grander scale, physicists have demonstrated that all the different substances in the universe – diamonds, granite, iron, air and so on – are made out of atoms, which, in turn, are made out of just a few types of subatomic particles, including electrons, protons and neutrons. Thales would have been impressed!

  But what if this search for the hidden building blocks of nature could be taken a step further? Scientists may have identified the fundamental types of particles that all things are made out of, but what if there’s actually only one thing, in a literal, quantitative sense? What if everything in existence is made from a single subatomic particle?

  This is the premise of the one-electron universe hypothesis. It imagines that, instead of there being an infinite number of particles in the universe, there’s only one that appears in an infinite number of places simultaneously. It achieves this trick by constantly travelling back and forth through time.

  The one-electron universe hypothesis sprang from the imagination of Princeton professor John Wheeler, who was one of the most respected physicists of the twentieth century. He came up with the idea in 1940, while sitting at home one evening, puzzling over the mystery of antimatter, which, at the time, had only recently been discovered.

  Antimatter is like the bizarre evil twin of matter. It presumably looks just the same as matter – no one knows for sure, because no one has ever seen a chunk of it – but it has an opposite electric charge, which means that, if the two ever come into contact, they instantly annihilate, transforming into pure energy. In fact, the mutual destruction of matter and antimatter is the most efficient way known to exist in nature of releasing energy from matter. This has apparently attracted the interest of the US military. It’s long been rumoured that Department of Defense researchers have been trying to figure out how to build an antimatter bomb, the power of which would dwarf any nuclear bomb.

  The existence of antimatter was first predicted in 1928 by the British physicist Paul Dirac. He had been trying to come up with an equation to describe the behaviour of electron
s, but his calculations kept yielding two answers, a positive and a negative one, which struck him as curious. Most physicists probably would have ignored the negative number, assuming that it couldn’t correspond to anything in real life, but Dirac believed that mathematics provided a window onto a deeper reality, even when it gave seemingly illogical answers. So, he eventually concluded that electrons must possess some kind of mirror-image subatomic doppelgänger.

  It turned out he was right. The physicist Carl Anderson experimentally confirmed this in 1932 by finding evidence of exactly such an antimatter particle in a cloud-chamber experiment. The particle that Anderson detected was, in most respects, identical to an electron. It possessed the same mass and spin. However, it had a positive charge, whereas an electron has a negative charge. For this reason, Anderson called this anti-electron a ‘positron’.

  Anderson had confirmed that antimatter existed, but it wasn’t clear what brought it into being or how it fitted into the larger picture that was emerging of the subatomic world. These were the questions John Wheeler was thinking about on that night in 1940 when he came up with his odd idea. It suddenly occurred to him that perhaps positrons were simply electrons travelling backwards in time. After all, positrons and electrons seemed to be identical in all ways except for having an opposite charge, and moving backwards in time could reverse the charge of an electron.

  Wheeler imagined an electron going forwards in time, and then reversing course and coming back as a positron. This made him realize that, from the point of view of someone such as ourselves, stuck in a particular instant, it would seem as if the electron and positron were two separate particles, whereas, in fact, they were actually the same particle at different stages of its journey through time.

  Wheeler then envisioned this process continuing. The electron would travel forwards until it reached the very end of time, at which point it would reverse course, travelling backwards as a positron until it bumped up against the beginning of the universe, and then it would reverse its course again. If this back-and-forth time travel continued to infinity, zig-zagging between the beginning and end of time, that single electron could conceivably become every electron in the universe. What we in the present perceive to be an enormous number of different electrons might actually be the same one repeatedly passing through our moment in time.

  It occurred to Wheeler that, if this really was true, it would solve another mystery: why electrons are indistinguishable from one another. Because they are, indeed, all exactly alike. There’s no possible way to tell two of them apart.

  The story goes that, at this moment of epiphany, Wheeler excitedly telephoned his brilliant young graduate student, Richard Feynman, to share his revelation. ‘Feynman,’ he triumphantly declared, ‘I know why all electrons have the same charge and the same mass.’

  ‘Why?’ replied Feynman.

  ‘Because they are all the same electron!’

  Although Wheeler had envisioned a single time-travelling electron, you can’t build a universe out of just electrons. What about all the other particles, such as protons and neutrons? Are there lots of them, but only one electron?

  The answer is that Wheeler’s hypothesis could indeed be extended to include the whole suite of other particles. His focus on electrons alone was just an accident of history, because, in 1940, positrons were the only form of antimatter whose existence had been confirmed. It wasn’t until the 1950s that researchers proved that other particles also have antimatter counterparts.

  However, after sharing his weird idea with Feynman, Wheeler didn’t actually try to develop his concept further. He considered it to be little more than idle speculation, and so the one-electron universe hypothesis might have faded away. That didn’t happen, though, because Feynman kept it alive.

  Feynman was doubtful about the hypothesis as a whole. He didn’t think there could really be just a single electron in the entire universe, but he was extremely intrigued by Wheeler’s idea of time-travelling electrons. He proceeded to develop this concept further, and in doing so he laid the foundations for the work that eventually made him one of the most famous physicists of the twentieth century.

  What he was able to demonstrate, by the end of the 1940s, was that thinking of antimatter as time-reversed matter wasn’t crazy at all. In fact, it provided a powerful way of understanding the behaviour of subatomic particles. If you envision the subatomic world in this way, then, when a particle of matter and antimatter collide, they don’t actually annihilate each other in a burst of energy. Instead, the apparent collision represents the moment the particle has changed the direction of its travel through time.

  This is now called the Feynman–Stueckelberg interpretation of antiparticles, and it’s considered to be a perfectly valid, and widely used, way of conceptualizing antimatter. Which isn’t to say that physicists believe that antimatter really is time-reversed matter, only that this is a useful way of modelling its behaviour. From a mathematical perspective, a positron is, in fact, the same thing as a time-reversed electron.

  This lends some credibility to Wheeler’s one-electron universe hypothesis, because it means that there’s a nugget of valid insight at its core. Thinking of antimatter as time-reversed matter isn’t some kind of crackpot idea; it’s a concept that physicists take quite seriously.

  Feynman received a Nobel Prize in 1965, and during his acceptance speech he told the story of Wheeler’s late-night phone call to him in 1940. It was because of this speech that the one-electron universe hypothesis finally gained a wider audience.

  If Feynman liked Wheeler’s time-travelling electrons so much, however, why didn’t he buy into the rest of the one-electron universe hypothesis? It was because he immediately recognized there was a big problem with the general concept of a single electron zig-zagging back and forth between the beginning and end of time. If that were true, half the universe should consist of antimatter, because this one particle would have to spend half its time going backwards (as antimatter), and half going forwards (as matter). But, as far as researchers can tell, there’s almost no antimatter in the universe. Whenever antimatter is created, whether in nature or the lab, it almost immediately gets destroyed when it promptly collides with matter.

  From our perspective, it’s a good thing there isn’t more antimatter around, as it means we don’t have to worry about being instantly obliterated by stumbling upon random chunks of it. But the lack of antimatter is actually one of the great mysteries in science, because physicists believe that an equal amount of matter and antimatter should have been created during the Big Bang. If this happened, though, where did all the antimatter go? Scientists aren’t sure. The current thinking is that, for various reasons, slightly more matter than antimatter must have been created during the initial moments of the Big Bang. Everything then collided in a cataclysmic event known as the Great Annihilation. After the smoke had cleared (metaphorically speaking), all the antimatter was gone, but, because of that slight initial imbalance, some matter remained, and that surviving amount represented all the matter that now exists in the universe.

  This explanation, however, doesn’t work for the one-electron universe, because the hypothesis suggests that the amount of matter and antimatter should be evenly distributed throughout time. During Wheeler’s phone call about the hypothesis to Feynman, back in 1940, Feynman had actually pointed out the missing antimatter problem, which prompted Wheeler to suggest a possible solution: perhaps all the missing antimatter was hidden inside protons! After all, protons have a positive charge, just as positrons do. But Wheeler quickly dropped this idea as he realized that protons are about 2,000 times bigger than electrons. The size mismatch just wasn’t plausible. Also, if protons were really the antimatter form of electrons, you’d expect atoms to constantly be self-destructing as their protons and electrons collided.

  Fans of the hypothesis have subsequently tried to think up other solutions to the missing-antimatter puzzle. For instance, if it isn’t in protons, perhaps it’s h
idden somewhere else, like in a distant part of the universe. Perhaps faraway stars and galaxies are actually made out of antimatter.

  The idea that vast swathes of the universe might be made out of antimatter has intrigued many scientists, and astronomers have been keeping their eyes open for any evidence that this might be the case, such as cosmic fireworks where an antimatter zone is butting up against a matter-filled zone. So far, they’ve seen no sign of such a thing.

  But there are even more exotic ways antimatter might be hiding. What if Wheeler’s single particle doesn’t return by the same route? In the 1950s, the physicist Maurice Goldhaber suggested that an antimatter universe might have formed alongside our matter-based universe. After all, if subatomic particles come in matter/antimatter pairs, why shouldn’t this also be true of the universe as a whole? If such an anti-universe exists, perhaps the particle returns to the beginning of time via this route.

  Or perhaps time loops. Instead of having to go back the way it came, perhaps, when an electron reaches the end of time, it instantly finds itself returned to the beginning.

  Critics of Wheeler’s hypothesis complain that speculations of this kind roam pretty far from any kind of verifiable evidence, into the realm of pure whimsy. They also ask, ‘So what?’ Even if the hypothesis were true, what new knowledge would we gain? What new possibilities for research or understanding would it open? Seemingly none, because, whether the universe is filled with an infinite number of particles or one particle in an infinite number of places simultaneously, the two amount to the same thing. The physics is the same.