This past December, the physics Nobel Prize was awarded for the experimental confirmation of a quantum phenomenon known for more than 80 years: entanglement. As envisioned by Albert Einstein and his collaborators in 1935, quantum objects can be mysteriously correlated even if they are separated by large distances. But as weird as the phenomenon appears, why is such an old idea still worth the most prestigious prize in physics?
Coincidentally, just a few weeks before the new Nobel laureates were honored in Stockholm, a different team of distinguished scientists from Harvard, MIT, Caltech, Fermilab and Google reported that they had run a process on Google's quantum computer that could be interpreted as a wormhole. Wormholes are tunnels through the universe that can work like a shortcut through space and time and are loved by science fiction fans, and although the tunnel realized in this recent experiment exists only in a 2-dimensional toy universe, it could constitute a breakthrough for future research at the forefront of physics.
But why is entanglement related to space and time? And how can it be important for future physics breakthroughs? Properly understood, entanglement implies that the universe is "monistic", as philosophers call it, that on 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 a universal function. Already decades ago, researchers such as Hugh Everett and Dieter Zeh showed how our daily-life reality can emerge out of such a universal quantum-mechanical description. But only now are researchers such as Leonard Susskind or Sean Carroll developing ideas on how this hidden quantum reality might explain not only matter but also the fabric of space and time.
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Entanglement is much more than just another weird quantum phenomenon. It is the acting principle behind both why quantum mechanics merges the world into one and why we experience this fundamental unity as many separate objects. At the same time, entanglement is the reason why we seem to live in a classical reality. It is-quite literally-the glue and creator of worlds. Entanglement applies to objects comprising two or more components and describes what happens when the quantum principle that "everything that can happen actually happens" is applied to such composed objects. Accordingly, an entangled state is the superposition of all possible combinations that the components of a composed object can be in to produce the same overall result. It is again the wavy nature of the quantum domain that can help to illustrate how entanglement actually works.
Picture a perfectly calm, glassy sea on a windless day. Now ask yourself, how can such a plane be produced by overlaying two individual wave patterns? One possibility is that superimposing two completely flat surfaces results again in a completely level outcome. But another possibility that might produce a flat surface is if two identical wave patterns shifted by half an oscillation cycle were to be superimposed on one another, so that the wave crests of one pattern annihilate the wave troughs of the other one and vice versa. If we just observed the glassy ocean, regarding it as the result of two swells combined, there would be no way for us to find out about the patterns of the individual swells. What sounds perfectly ordinary when we talk about waves has the most bizarre consequences when applied to competing realities. If your neighbor told you she had two cats, one live cat and a dead one, this would imply that either the first cat or the second one is dead and that the remaining cat, respectively, is alive-it would be a strange and morbid way of describing one's pets, and you may not know which one of them is the lucky one, but you would get the neighbor's drift. Not so in the quantum world. In quantum mechanics, the very same statement implies that the two cats are merged in a superposition of cases, including the first cat being alive and the second one dead and the first cat being dead while the second one lives, but also possibilities where both cats are half alive and half dead, or the first cat is one-third alive, while the second feline adds the missing two-thirds of life. In a quantum pair of cats, the fates and conditions of the individual animals get dissolved entirely in the state of the whole. Likewise, in a quantum universe, there are no individual objects. All that exists is merged into a single "One."
Quantum entanglement reveals to us a vast and entirely new territory to explore. It defines a new foundation of science and turns our quest for a theory of everything upside down-to build on quantum cosmology rather than on particle physics or string theory. But how realistic is it for physicists to pursue such an approach? Surprisingly, it is not just realistic-they are actually doing it already. Researchers at the forefront of quantum gravity have started to rethink space-time as a consequence of entanglement. An increasing number of scientists have come to ground their research in the nonseparability of the universe. Hopes are high that by following this approach they may finally come to grasp what space and time, deep down at the foundation, really are.
Whether space is stitched together by entanglement, physics is described by abstract objects beyond space and time or the space of possibilities represented by Everett's universal wave function, or everything in the universe is traced back to a single quantum object-all these ideas share a distinct monistic flavor. At present it is hard to judge which of these ideas will inform the future of physics and which will eventually disappear. What's interesting is that while originally ideas were often developed in the context of string theory, they seem to have outgrown string theory, and strings play no role anymore in the most recent research. A common thread now seems to be that space and time are not considered fundamental anymore. Contemporary physics doesn't start with space and time to continue with things placed in this preexisting background. Instead, space and time themselves are considered products of a more fundamental projector reality. Nathan Seiberg, a leading string theorist at the Institute for Advanced Study at Princeton, is not alone in his sentiment when he states, "I'm almost certain that space and time are illusions. These are primitive notions that will be replaced by something more sophisticated." Moreover, in most scenarios proposing emergent space-times, entanglement plays the fundamental role. As philosopher of science Rasmus Jaksland points out, this eventually implies that there are no individual objects in the universe anymore; that everything is connected with everything else: "Adopting entanglement as the world making relation comes at the price of giving up separability. But those who are ready to take this step should perhaps look to entanglement for the fundamental relation with which to constitute this world (and perhaps all the other possible ones)." Thus, when space and time disappear, a unified One emerges.
Conversely, from the perspective of quantum monism, such mind-boggling consequences of quantum gravity are not far off. Already in Einstein's theory of general relativity, space is no static stage anymore; rather it is sourced by matter's masses and energy. Much like the German philosopher Gottfried W. Leibniz's view, it describes the relative order of things. If now, according to quantum monism, there is only one thing left, there is nothing left to arrange or order and eventually no longer a need for the concept of space on this most fundamental level of description. It is "the One," a single quantum universe that gives rise to space, time, and matter.
"GR=QM," Leonard Susskind claimed boldly in an open letter to researchers in quantum information science: general relativity is nothing but quantum mechanics-a hundred-year-old theory that has been applied extremely successfully to all sorts of things but never really entirely understood. As Sean Carroll has pointed out, "Maybe it was a mistake to quantize gravity, and space-time was lurking in quantum mechanics all along." For the future, "rather than quantizing gravity, maybe we should try to gravitize quantum mechanics. Or, more accurately but less evocatively, 'find gravity inside quantum mechanics,'" Carroll suggests on his blog. Indeed, it seems that if quantum mechanics had been taken seriously from the beginning, if it had been understood as a theory that isn't happening in space and time but within a more fundamental projector reality, many of the dead ends in the exploration of quantum gravity could have been avoided. If we had approved the monistic implications of quantum mechanics-the heritage of a three-thousand-year-old philosophy that was embraced in antiquity, persecuted in the Middle Ages, revived in the Renaissance, and tampered with in Romanticism-as early as Everett and Zeh had pointed them out rather than sticking to the influential quantum pioneer Niels Bohr's pragmatic interpretation that reduced quantum mechanics to a tool, we would be further on the way to demystifying the foundations of reality.
Adapted from The One: How an Ancient Idea Holds the Future of Physics by Heinrich Päs. Copyright © 2023. Available from Basic Books, an imprint of Hachette Book Group, Inc.
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