Why more physicists are starting to think that space and time are ‘illusions’

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 envisioned by Albert Einstein and his collaborators in 1935, quantum objects can be mysteriously associated even when they are separated by great distances. But as strange as the phenomenon may seem, why is such an old idea still worthy of the most prestigious prize in physics?

Coincidentally, just a few weeks before the new Nobel laureates were honored in Stockholm, another team of leading scientists from Harvard, MIT, Caltech, Fermilab and Google reported running a process on Google’s quantum computer that could be interpreted as a wormhole. . . Wormholes are tunnels through the universe that can act as shortcuts through space and time and are loved by science fiction fans. Although the tunnel realized in this recent experiment only exists in a two-dimensional toy universe, it could represent a breakthrough for the future. research at the forefront of physics.

But why is entanglement related to space and time? And how might it be important for future physics breakthroughs? Properly understood, entanglement implies that the universe is “monistic,” as philosophers call it, that at its most fundamental level, everything in the universe is part of a single, unified whole. It is a defining property of quantum mechanics that the 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 already showed how our everyday reality can arise from such a universal quantum mechanical description. But only now are researchers such as 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 a strange quantum phenomenon. It is the operating principle behind both why quantum mechanics assembles 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 consisting of two or more components and describes what happens when the quantum principle that “everything that can happen actually happens” 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 undulating nature of the quantum domain that can help illustrate how entanglement really 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 laying two completely flat surfaces on top of each other again results in a completely flat result. But another possibility that a flat surface could yield is if two identical wave patterns shifted by half an oscillation cycle were superimposed so that the wave crests of one pattern destroy the wave troughs of the other, and vice versa. If we merely observed the glassy ocean and viewed it as the result of two combined swells, there would be no way for us to learn more about the patterns of the individual swells. What sounds perfectly normal 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 one dead, it would imply that either the first cat or the second is dead and the remaining cat is alive respectively – it would be a strange and morbid situation. way of describing someone’s pets, and you may not know which of them is the lucky one, but you would understand the neighbors’ opinion. Not so in the quantum world. In quantum mechanics, the same statement that the two cats are merged in a superposition of cases, including the first cat alive and the second dead and the first cat dead while the second is alive, also implies 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 fate and circumstances of the individual animals completely dissolve into the state of the whole. Similarly, in a quantum universe there are no individual objects. Everything that exists is merged into a single “One”.

Quantum entanglement reveals a vast and entirely new area for us to explore. It defines a new foundation of science and turns our search 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 pursue such an approach? Surprisingly, it’s not just realistic — they’re actually already doing it. Researchers at the forefront of quantum gravity have begun to rethink space-time as a result of entanglement. An increasing number of scientists have begun to base their research on the inseparability of the universe. The hope is high that by taking this approach they will finally understand what space and time, deep down, really are.

Whether space is sewn 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 reduced to a single quantum object – all of these ideas have a distinctly monistic flavour. At the moment it is difficult to judge which of these ideas will shape the future of physics and which will eventually disappear. What is interesting is that while ideas were originally often developed in the context of string theory, they seem to have outgrown string theory and strings no longer play a role in most recent research. A common thread now seems to be that space and time are no longer regarded as fundamental. Contemporary physics does not start with space and time to move on with things placed in this pre-existing background. Instead, space and time are themselves viewed as products of a more fundamental projector reality. Nathan Seiberg, a leading string theorist at Princeton’s Institute for Advanced Study, is not alone in his opinion when he says, “I am almost certain that space and time are illusions. These are primitive concepts that will be replaced by something more advanced.” Moreover, entanglement plays the fundamental role in most of the scenarios proposing emergent space ages. Ultimately, as philosopher of science Rasmus Jaksland points out, this implies that there are no longer any individual objects in the universe; that everything is connected to everything else: “Adopting entanglement as the world that makes relationships comes at the cost of giving up separability. But those ready to take this step may need to look to entanglement for the fundamental relationship by which they can form this world (and perhaps every other possible one). So when space and time disappear, a unified One comes into being.

Conversely, from the perspective of quantum monism, such mind-boggling consequences of quantum gravity are not far off. Already in Einstein’s general theory of relativity, space is no longer a static stage; rather, it comes from the masses and energy of matter. Like the German philosopher Gottfried W. Leibniz, it describes the relative order of things. Now, according to quantum monism, if there is only one thing left, there is nothing left to order or order, and ultimately there is no need for the concept of space at this most fundamental level of description. It is ‘the One’, a single quantum universe giving 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 but quantum mechanics – a hundred-year-old theory that has been extraordinarily successfully applied to all sorts of things, but has never really been fully understood. As Sean Carroll has pointed out, “Maybe it was a mistake to quantize gravity, and space-time had been lurking in quantum mechanics all along.” For the future: “rather than quantizing gravity, maybe we should try to attract quantum mechanics. Or, more accurately but less evocatively, “find gravity in quantum mechanics,” Carroll argues on his blog. Indeed, it seems that if quantum mechanics had been taken seriously from the start, if it had been understood as a theory set not 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 already approved the monistic implications of quantum mechanics—the legacy 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—in Everett and Zeh had she pointed out, instead of adhering to the pragmatic interpretation of the influential quantum pioneer Niels Bohr that reduced quantum mechanics to a tool, we would be further along the path of demystifying the foundations of reality.

Adapted from The One: How an Ancient Idea Shapes the Future of Physics by Henrich Pas. Copyright © 2023. Available from Basic Books, an imprint of Hachette Book Group, Inc.