By Zeeya Merali
Rearrange the following words to tell a coherent life story: A man dies, later he gets married, and finally he is born. Thanks to our built-in temporal sense, it’s pretty straightforward: Tomb always follows womb, it’s never the other way around.
Yet at a fundamental level, time’s origin remains a mystery. “It’s one of the deepest questions at the forefront of science, but when we ask, ‘What is time? Where does it come from?’ it’s not even clear the words make any sense,” says Nima Arkani Hamed, a physicist at the Institute of Advanced Studies (IAS) in Princeton, N.J. “We can barely articulate what a world without time, or physics without time, means.”
Confusing as the absence of time would be, there is mounting evidence that at the most basic level of reality, time is an illusion. Stranger still, laboratory tests with laser lights and advances in our understanding of string theory—the proposed framework positing that particles are composed of small threads of energy—independently point to the idea that time doesn’t really exist.
Little more than a century ago, our picture of time and space was far less complicated. Physicists happily tracked objects across a fixed background set by our three spatial dimensions and marked how fast they moved against a single clock—God’s proverbial stopwatch, that they believed ticked at the same rate no matter where you were in the cosmos. But in the early 20th century, two revolutions in physics disrupted this view.
In the first revolution, Einstein’s theory of relativity wove together time and space into a flexible four-dimensional fabric. That fabric, which Einstein called “spacetime,” could mold itself around massive objects, creating a curvature. Smaller objects could roll down those curves toward the larger masses, investing the universe with a force called gravity. In this new theory of the universe, time was no longer an immutable bystander, but an interconnected dimension enmeshed with space itself. Instead of being that unambiguous dimension against which others could be measured, time was now relative. Einstein’s relativity showed that clocks would tick at different rates depending on their motion through space and their proximity to massive objects that pulled them in with gravitational force.
The second development disrupting our view of time was quantum mechanics, the physics of the subatomic realm. Quantum mechanics revealed that on the smallest scales, reality was strange indeed. For instance, two particles can become “entangled” in such a way that they always act in tandem. An experiment carried out on one will immediately influence its partner, no matter how distant it may be. In other words, the distant particles communicate instantly, apparently defying the rule that nothing can travel faster than the speed of light and the very concept of time itself.
But the real “problem of time,” as it has become known, arose in the 1960s as physicists struggled to combine these two frameworks—each successful at describing its own realm of the universe, either the very tiny or the large. The search for an overarching “theory of everything,” a set of rules that governed objects of all sizes, was on. One of the most famous but controversial hypotheses came from two New Jersey physicists: John Wheeler of Princeton University and Bryce DeWitt of the IAS. Wheeler and DeWitt tried to describe the whole universe through quantum mechanics—that is, they attempted to apply the physics of the very small to planets, galaxies, and other cosmic structures on a mass scale. Many questioned whether their tactics would work, because there had been no evidence to suggest that quantum laws held sway over cosmic distances, notes Marco Genovese, a quantum physicist at Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, Italy. But it seemed reasonable to at least try to unite the mathematics of the two theories and see what would happen.
When the two physicists tried to combine Einstein’s equations of relativity with quantum physics, they came up with a surprise. Both sets of laws independently featured time as a variable against which events evolved. But when the theories were combined into one, the time variable was literally cancelled out of the mathematical equation. The duo had derived a new equation for how the universe behaved, yet there was no longer a quantity in their mathematical description that could be used to mark out change or the passage of time. “The Wheeler-DeWitt equation says that the universe is stationary and that nothing evolves,” says Genovese. “But, of course, we all experience time and change.”
The conclusion that the universe never changes was clearly wrong. Yet physicists could not find anything wrong with the mathematical steps that Wheeler and DeWitt had taken. At first, it seemed that the pair must have been mistaken to think that the whole cosmos could be described in quantum terms. But there was another intriguing possibility, proposed in the 1980s by physicists Don Page, now at the University of Alberta, in Edmonton, Canada, and William Wooters, at Williams College in Williamstown, Mass.
Page and Wooters decided to apply the controversial concept that the universe as a whole could be treated as a giant quantum object—subject to the same physical laws as electrons, protons, and other tiny particles of the subatomic world. They imagined splicing the contents of the cosmos into two pieces. Because quantum laws prevailed, the pieces would be entangled. Scientists have found that two entangled particles measured in the lab can have equal but opposite values. If one is spinning clockwise, for instance, the other will be spinning counter-clockwise so that, when summed together, the properties cancel each other out. Page and Wooters argued that in similar fashion, each section of their divided cosmos could independently evolve, but because they were entangled, the changes in one would be counter-balanced by the changes in the other. To someone inside one of the sections, time would appear to pass. But to the outside observer, the overall universe would appear static.
While Page and Wooters had offered a theoretical sketch, based on quantum entanglement, for how the cosmos might appear to be stationary to someone peering in from the outside, there seemed to be no way to confirm or rule out their idea. But, in 2013, Genovese and his colleagues performed an experiment to test whether—at least in the lab—it is possible to create a model of the universe in miniature, with just two particles of light, or photons, generated from a laser. The aim of the experiment was to prove that it is possible to create a situation in which a quantum system, when viewed from outside, appeared unchanging, but when observed from within appeared to evolve.
To do the experiment, Genovese set out to monitor the photons’ polarizations—the directions in which they vibrated. If a polarized particle could be made to rotate at a constant rate, then its position at any moment could be used to mark out intervals in time, just like a second-hand on a clock. The team entangled the two photons together, in such a way that their polarizations took on opposing traits. For instance, if the polarization of one was measured to move up and down, the other would vibrate from side-to-side.
In order to set their photons’ “second-hands” in motion, the team passed both particles through quartz plates, causing their polarizations to rotate. The amount of rotation was related to the actual time spent within the plates, giving physicists a means of measuring the passage of time. They carried out their experiment repeatedly and in each run they stopped at a different moment and measured the polarization of one of the photons. “By measuring the first clock photon, we became entangled with it,” says Genovese. “That means we became part of that universe and can register the evolution of the second photon against our clock photon.” Vested with this ability, the team confirmed that one photon appeared to change when measured against its partner, in the same way that Wooters and Page believed one part of the universe could be seen to evolve if measured against another portion of the cosmos.
From Nautilus magazine. Part 2 to follow.
Zeeya Merali is a freelance science writer based in London, and editor for the Foundational Questions Institute, based in the U.S.
