Last December the Nobel Prize in Physics was awarded for the experimental confirmation of a quantum phenomenon known for more than 80 years: entanglement. As Albert Einstein and his collaborators imagined in 1935, quantum objects can be mysteriously correlated even if they are separated by great distances. But strange as the phenomenon may seem, why is such an ancient idea still worth the most prestigious award in physics?
Coincidentally, just weeks before the new Nobel laureates were to be 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 function as a shortcut through space and time and are beloved by science fiction fans, and while the tunnel made in this recent experiment exists only in a toy universe two-dimensional, 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 advances in physics? Properly understood, entanglement implies that the universe is “monistic,” as philosophers call it, that 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 a universal function. Decades ago, researchers such as Hugh Everett and Dieter Zeh demonstrated how the reality of our everyday lives can emerge from such a universal quantum mechanical description. But only now are researchers like Leonard Susskind or Sean Carroll developing ideas about 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 strange quantum phenomenon. It is the principle behind 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 literally the glue and creator of worlds. Entanglement applies to objects that comprise two or more components, and describes what happens when the quantum principle that “everything that can happen really does” is applied to such composite objects. Consequently, an entangled state is the superposition of all the possible combinations that the components of a composite object can be in to produce the same overall result. Again, it is the wavy nature of the quantum domain that may help to illustrate how entanglement actually works.
Imagine a perfectly calm, glassy sea on a windless day. Now ask yourself, how can such a plane be produced by superimposing two individual wave patterns? One possibility is that overlapping two completely flat surfaces will again result in a completely level result. But another possibility that a flat surface could produce is if two identical wave patterns displaced by half an oscillation cycle were to overlap each other, so that the wave crests of one pattern annihilate the wave troughs of the other, and vice versa. If we just looked at the crystal clear ocean, considering it to be the result of two combined swells, there would be no way we could figure out the patterns of the individual swells. What sounds perfectly ordinary when we talk about waves has the strangest consequences when applied to competing realities. If your neighbor told you that she has two cats, one cat alive and one dead, this would imply that either the first cat or the second cat is dead and the remaining cat, respectively, is alive, which would be a weird and morbid way to describe pets. of one, and you may not know which one is the lucky one, but you would get the neighbor’s idea. Not so in the quantum world. In quantum mechanics, the same statement implies that the two cats merge in a superposition of cases, including the first cat alive and the second dead and the first cat dead while the second lives, but also possibilities where both cats 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 couple of cats, the fates and conditions of individual animals completely dissolve into the state of the whole. Likewise, in a quantum universe, there are no individual objects. Everything that exists merges into a single “One”.
“I am almost certain that space and time are illusions. These are primitive notions that will be replaced by something more sophisticated.“
— Nathan Seiberg, Princeton University
Quantum entanglement reveals vast and entirely new territory to explore. It defines a new foundation for science and turns our search for a theory of everything upside down, to build on quantum cosmology rather than particle physics or string theory. But how realistic is it for physicists to follow this approach? Surprisingly, it’s not only realistic, they’re actually already doing it. Researchers at the forefront of quantum gravity have begun to rethink spacetime as a consequence of entanglement. An increasing number of scientists have come to base their research on the non-separability of the universe. There are great hopes that by following this approach, you will finally be able to understand what space and time really are, deep down at their core.
Whether space is bound together by entanglement, physics is described by abstract objects beyond space and time or the space of possibilities represented by the universal Everett wave function, or everything in the universe goes back to a single quantum object, all these ideas share a monistic flavor. It is currently difficult to judge which of these ideas will inform the future of physics and which will eventually disappear. What is interesting is that while the ideas were often originally developed in the context of string theory, they seem to have outgrown string theory, and strings no longer play a role in more recent research. A common thread now seems to be that space and time are no longer considered fundamental. Contemporary physics does not start from space and time to continue with things placed on this pre-existing background. Instead, space and time themselves are seen as products of a more fundamental projector reality. Nathan Seiberg, a leading string theorist at the Princeton Institute for Advanced Study, is not alone in the same view when he states: “I am almost certain that space and time are illusions. These are primitive notions that will be replaced by something more sophisticated. Furthermore, in most of the scenarios that propose emergent space-times, entanglement plays a fundamental role. As the philosopher of science Rasmus Jaksland points out, this eventually implies that there are no longer individual objects in the universe; that everything is connected to everything else: “Adopting entanglement as the relationship that makes the world comes at the price of giving up separability. But those who are willing to take this step should perhaps seek in entanglement the fundamental relationship with which to constitute this world (and perhaps all other possible ones).” Thus, when space and time disappear, a unified One emerges.
On the contrary, from the perspective of quantum monism, these mind-bending consequences of quantum gravity are not far off. Already in Einstein’s theory of general relativity, space is no longer a static setting; rather it originates from the masses and energy of matter. Like the opinion of the German philosopher Gottfried W. Leibniz, he describes the relative order of things. If now, according to quantum monism, there is only one thing left, there is nothing left to fix or order and finally the concept of space is no longer needed at this fundamental level of description. It is “the One”, a single quantum universe that gives rise to space, time and matter.
“GR=QM,” Leonard Susskind boldly asserted in an open letter to researchers in quantum information science: general relativity is nothing more than quantum mechanics, a centuries-old theory that has been applied with great success to all sorts of things. , but never really. fully understood. As Sean Carroll has pointed out, “Maybe it was a mistake to quantize gravity, and spacetime lurked in quantum mechanics all along.” For the future, “instead of quantizing gravity, maybe we should try to gravitate quantum mechanics. Or, more accurately but less evocatively, ‘finding gravity within quantum mechanics,’” Carroll suggests on his blog. In fact, it seems that if quantum mechanics had been taken seriously from the start, if it had been understood as a theory that is not occurring in space and time but within a more fundamental reality of the projector, many of the dead ends departure in the exploration of quantum gravity could have been avoided. If we had approved of 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 manipulated in Romanticism, Everett and Zeh had already As noted, rather than sticking to the pragmatic interpretation of the influential quantum pioneer Niels Bohr that reduced quantum mechanics to a tool, we would be more on the road to demystifying the 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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