Last December, the Nobel Prize in Physics was awarded for experimental evidence of a quantum phenomenon that has been known for more than 80 years: entanglement. As envisioned by Albert Einstein and his collaborators in 1935, quantum objects can be mysteriously correlated even when separated by great distances. But as strange as the phenomenon may seem, why is such an old idea still worthy of the most prestigious award in physics?
Coincidentally, just weeks before the new Nobel laureates were honored in Stockholm, another team of respected scientists from Harvard, MIT, Caltech, Fermilab and Google reported that they ran a process on Google’s quantum computer that could be interpreted as a wormhole. Wormholes are tunnels through the universe that can function as a shortcut through space and time and are loved by science fiction fans, and although the tunnel realized in this latest experiment only exists in a two-dimensional toy universe, it could be a breakthrough for futurology at the forefront of physics.
But why does entanglement have to do with space and time? And how can it be important for future breakthroughs in physics? Properly understood, entanglement means that the universe is what philosophers call “monistic,” that is, at the most fundamental level, everything in the universe is part of a single, unified whole. It is a defining property of quantum mechanics that its underlying reality is described in terms of waves, and a monistic universe would require universal functioning. Decades ago, researchers such as Hugh Everett and Dieter Zeh showed how our everyday reality can emerge from such a universal quantum mechanical description. But it is only now that researchers such as Leonard Susskind and Sean Carroll are developing ideas as to how this hidden quantum reality could explain not only matter but also the structure of space and time.
Entanglement is much more than just another weird quantum phenomenon. It is the working principle why quantum mechanics merges the world into one and why we experience this basic unity as many separate objects. At the same time, entanglement is why we seem to be living in a classic reality. It is – in the truest sense of the word – the glue and creator of worlds. Entanglement refers to objects composed of two or more components and describes what happens when the quantum principle that “anything that can happen, happens, will” is applied to such composite objects. Accordingly, an entangled state is the superposition of all possible combinations that the components of a composite object can be in to produce the same overall result. Again, it is the rippled nature of the quantum domain that can help illustrate how entanglement actually works.
Imagine a perfectly calm, crystal clear sea on a windless day. Now you are wondering, how can such a plane be created by superimposing two single wave patterns? One possibility is that superimposing two completely flat surfaces will again result in a completely flat result. However, another way of creating a flat surface would be to superimpose two identical wave patterns shifted by half an oscillation period, so that the crests of one pattern cancel out the troughs of the other and vice versa. If we looked at the glassy ocean as just the result of two waves combining, there would be no way for us to find out about the patterns of each wave. What sounds completely ordinary when we talk about waves has the most bizarre consequences when applied to competing realities. If your neighbor told you that she has two cats, one alive and one dead, that would mean that either the first or second cat is dead and the remaining cat is alive – it would be an odd and morbid way of describing your pets , and you may not know which of them is the lucky one, but you would understand the neighbor’s drift. Not so in the quantum world. In quantum mechanics, the same statement implies that the two cats have merged in a superposition of cases, including the fact that the first cat is alive and the second is dead and the first cat is dead while the second is alive, but also possibilities at where both cats exist are half-alive and half-dead, or the first cat is one-third alive while the second cat adds the missing two-thirds of life. In a quantum pair of cats, the fates and states of each animal completely resolve into the state of the whole. Likewise, there are no single objects in a quantum universe. All that exists is merged into a single “One”.
“I’m almost certain that space and time are illusions. These are primitive notions being replaced with something more sophisticated.“
— Nathan Seiberg, Princeton University
Quantum entanglement reveals a vast and entirely new area to explore. It defines a new foundation of science and turns our quest for a theory of everything on its head – to build on quantum cosmology rather than particle physics or string theory. But how realistic is it for physicists to take such an approach? Surprisingly, it’s not only realistic—in fact, they’re already doing it. Researchers at the forefront of quantum gravity have begun to reconsider spacetime as a result of entanglement. More and more scientists justify their research with the inseparability of the universe. The hopes are high that with this approach they can finally understand what space and time really are.
Whether space is stitched together by entanglement, physics is described by abstract objects beyond space and time, or the space of possibility represented by Everett’s universal wave function, or everything in the universe is reduced to a single quantum object – all of these have ideas somewhat common monistic taste. It is currently difficult to judge which of these ideas will shape the future of physics and which will eventually disappear. It is interesting that while the original ideas were often developed in the context of string theory, these seem to have outgrown string theory and strings no longer play a role in recent research. A common thread now seems to be that space and time are no longer considered fundamental. Contemporary physics does not begin with space and time to proceed with things placed in this already existing background. Instead, space and time are themselves viewed as products of a more fundamental projection reality. Nathan Seiberg, a leading string theorist at the Institute for Advanced Study in Princeton, is not alone in his opinion when he says, “I’m almost certain that space and time are illusions.” These are primitive notions replaced by something more sophisticated Moreover, entanglement plays the fundamental role in most of the scenarios proposed by emerging spacetimes. As the philosopher of science Rasmus Jaksland points out, this ultimately implies that there are no longer discrete objects in the universe; that everything is interconnected: ” Adopting entanglement as a world-forming relationship comes at a price of giving up separability, but perhaps those willing to take that step should look to entanglement for the fundamental relationship that this world (and perhaps all other possible So when space and time disappear, a unified one emerges.
Conversely, such startling consequences of quantum gravity are not far off from the perspective of quantum monism. Even in Einstein’s general theory of relativity, space is no longer a static stage; rather it is generated by the mass and energy of matter. Similar to the view of the German philosopher Gottfried W. Leibniz, it describes the relative order of things. If now after quantum monism there is only one thing left, then there is nothing left to order or order and eventually the concept of space at this most basic level of description is no longer needed. It is “the One,” a single quantum universe that creates space, time, and matter.
“GR=QM,” Leonard Susskind boldly asserted in an open letter to quantum computing researchers: General relativity is nothing more than quantum mechanics—a hundred-year-old theory that has been applied with great success to everything imaginable, but never really fully understood. Sean Carroll has pointed out, “Perhaps quantifying gravity was a mistake, and space-time has been lurking in quantum mechanics all along.” For the future, “perhaps instead of quantifying gravity, we should try gravitating quantum mechanics.” Or, more accurately but less evocatively, ‘find gravity in quantum mechanics,’” Carroll suggests on his blog. Indeed, it seems that if quantum mechanics had been taken seriously from the start, if understood as a theory that does not take place in space and time but within a more fundamental projection reality, many of the impasses in the study of quantum gravity would be broken could have been avoided. If we had already acknowledged the monistic implications of quantum mechanics—the legacy of a three-thousand-year-old philosophy adopted in antiquity, pursued in the Middle Ages, revived in the Renaissance, and manipulated in the Romantic era—as Everett and Zeh had pointed out, rather than at To adhere to the pragmatic interpretation of the influential quantum pioneer Niels Bohr, who reduced quantum mechanics to a tool, we would be on the way to demystifying the very foundations of reality.
Adapted from The One: How an ancient idea holds the future of physics by Heinrich Pas. Copyright © 2023. Available from Basic Books, an imprint of Hachette Book Group, Inc.
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