By Zeeya Merali
However, Genovese still had to confirm the second part of the hypothesis: that when the entire entangled system was monitored as a whole, from the outside, it would appear static. In this part of the experiment, the team took the point of view of a “super observer” standing outside the universe. This external watcher could never look at the individual state of either photon because by doing so he would become entangled with them, becoming an internal observer. Instead, the observer could only measure the joint state of the pair of photons. The team ran the test many times, stopping at different points. They looked at the two photons as a combined whole and measured their joint polarization. Each time, they ascertained that the two entangled photons were polarized in equal but opposite ways. No matter how much time passed, the two photons were always poised in exactly the same “embrace.” The mini-universe appeared to be static from the outside and completely unchanged. It turns out the so-called “problem of time,” discovered by Wheeler and DeWitt, can be resolved if time is an artifact of quantum entanglement.
Over the past few decades, support for the illusory nature of time has also emerged from string theory, developed in the 1960s to help describe the strong nuclear force that binds elementary particles together within atoms. As they studied the strong force, physicists came up with the idea that subatomic particles, then thought to be the smallest objects in the universe, were in fact themselves composed of tiny vibrating strings.
This new way of perceiving the basic objects in nature had far-reaching consequences. It turned out that string theory was extremely helpful for those like Wheeler and DeWitt, who wanted to unite general relativity with quantum mechanics. Such a unifying framework is needed to explain what the universe was like soon after the Big Bang, when all cosmic matter was squashed into a tiny volume. A unified theory could also reveal what happens at the cores of black holes—the corpses of stars that have collapsed under the force of gravity, compressing matter into a small central point.
Before the discovery of string theory, physicists ran into trouble whenever they tried to combine the equations of general relativity with those of quantum mechanics. The combined mathematics appeared to tell them that infinitely small points in space all around us should contain infinitely large amounts of energy—essentially predicting that we are surrounded by black holes everywhere we turn, which is not true. String theory sidestepped this problem, however, by positing that nothing can be smaller than the size of a string. That meant that its equations never had to worry about regions of space that were smaller than this fundamental limit, eliminating the messy math with its predictions of infinite energies and other impossible results. With string theory, the physics of the very large and the very small appeared as if they could coexist—at least once string theory was finessed.
Yet string size raised new questions about the reality of space, and, in turn, of time itself. This is because string theory says that no experiment, no matter how elaborate, will ever be able to show us what happens at distances smaller than the size of a single string. “What happens at short distances,” explains IAS string theorist Nathan Seiberg, “is an ill-defined concept—maybe space exists, but we can’t measure it, or perhaps there is nothing there to measure at all.” That meant that space may simply not exist below a certain limit. Since Einstein had already shown with his theory of relativity that time is just another dimension, like space, then “if space becomes ambiguous, time must do so too,” says Seiberg. “People often ask: ‘What happened before the Big Bang?’ But what we are seeing is that at the start of the universe, the notion of time ceases to make sense.”
This ambiguity gave string theorists their first inkling that time might not exist at a fundamental level, notes Seiberg. Instead, our experience of time might be constructed from underlying building blocks, much like temperature, which arises from the motion of a collection of atoms. An individual atom does not have a temperature; the concept of hot or cold only has meaning when you measure the average speed of a large number of atoms: Fast moving particles have a higher temperature than slow atoms. In a similar way, there may be fundamental grains that together generate our experience of time. But just what those grains may be, well, “that’s the $64,000 question,” says Seiberg.
Stranger still, later advances in string theory suggest that time’s seeds are sown at the very edges of reality. This idea has its roots in an odd model of a hypothetical universe devised in the late 1990s by string theorist Juan Maldacena, then at Harvard University, who was searching for a mathematical relationship that might connect quantum mechanics and general relativity. He decided that he could get there by using strings.
Maldacena’s imaginary cosmos was shaped like a soup can, but with walls that are infinitely far away. Inside his can, he placed strings and black holes, whose behavior was governed by gravity. On the surface of the can he placed normal subatomic particles that interacted through the laws of quantum mechanics. Although Maldacena’s soup-can universe did not sound much like ours, it helped him visualize how the deepest laws of nature could be connected.
In the model, general relativity held sway in the vast three-dimensional space within the can, while quantum mechanics ruled the particles lining the two-dimensional surface. Maldacena’s hunch was that the two sets of laws were somehow equivalent, and that gravitational events unfolding inside the can would correspond to quantum processes on the surface, like a shadow projected onto the can’s walls. Using this mathematical model, Maldacena indeed found that for every quantum process on the surface, an equivalent event unfolded within the can. Theoretical models developed by Maldacena and others indicate that quantum particles entangled on the surface of the soup can rewrite their patterns by creating tunnels, or “wormholes,” within the inner realm. This suggests that entanglement itself is the fundamental cosmic process generating the emergent properties of space and time.
The idea that both space and time are created by quantum entanglement has been independently bolstered by string theorist Mark van Raamsdonk at the University of British Columbia in Vancouver, who also investigated Maldacena’s soup can model. Using a mathematical model, he found that by gradually eroding particle entanglement on the can’s surface, the spacetime fabric within the can starts to disintegrate as well. This implies that quantum entanglement somehow plays a role in tying the threads of space and time together; without it, the fabric of spacetime itself could not exist.
Maldacena’s model provides more support than ever for the claim that, when it comes to cosmic ingredients, entanglement is more fundamental than space and time. Time, it turns out, is not present at the most basic layer of reality; it springs from fundamental seeds. But while emerging physics suggests time is an illusion, the forces that conjure it remain at large. “My intuition is that it will take more than just a re-working of quantum physics, it will require a breakthrough that will come totally out of left field,” Seiberg says. “Only time will tell what that revolution will be.”
Zeeya Merali is a freelance science writer based in London, and editor for the Foundational Questions Institute, based in the U.S.
This magazine article was first posted on-line on Nautilus.
