Opinion

Decades later, string theory continues its march toward Einstein’s dream

Four decades and tens of thousands of research papers later, where do we stand? The answer lies not only in assessing scientific progress but also in understanding the profound influence of human nature, even in the ostensibly objective realm of science.

Scientists assess the validity of proposed theories by testing their predictions. The challenge for string theory is that it has yet to produce any definitive, testable predictions. This isn’t surprising. String theory diverges from conventional theories only under extreme conditions: where distances are unimaginably small and masses are extraordinarily large, such as in the core of a black hole or in the instant of the big bang. Unfortunately, exploring these realms is beyond our capabilities.

Critics argue that the situation is untenable, noting, “If you can’t test a theory, it’s not scientific.” Adherents counter, “String theory is a work in progress; it’s simply too early to pass judgment.” The critics retort, “Forty years is too early?” To which the adherents respond, “We’re developing what could be the most profound physical theory of all time — you can’t seriously cross your arms, tap your foot and suggest that time’s up.”

And so the debate continues, with stakes for science that couldn’t be higher. Two generations of some of the world’s most talented physicists — occupying coveted university research positions and supported by limited government funding — have spent their careers on a theory whose validity remains uncertain.

Does this make sense?

Here’s where human nature stakes its ground. There’s no one-size-fits-all answer because it comes down to individual scientific taste, one’s tolerance for risk and the extent to which one is willing to defer experimental evidence in favor of mathematical progress. To be sure, I would readily abandon string theory — and I’m confident my colleagues would as well — if experimental evidence undercut it or if a mathematical inconsistency were uncovered. I’m an advocate for truth, not string theory. But so far, no such experimental insight exists, and no such mathematical flaws have surfaced.

On the contrary, string theory continues to captivate seasoned researchers and aspiring students alike because of the remarkable progress that has been made in developing its mathematical framework. This progress has yielded provocative insights into long-standing mysteries and introduced radically new ways of describing physical reality.

For instance, string theory has provided unmatched insights into the surface of black holes, unraveling puzzles that have consumed some of the greatest minds, including Stephen Hawking. It has offered a novel, though controversial, explanation for the observed speedup of the universe’s expansion, proposing that our universe might be just one of many within a larger reality than conventional science ever imagined. And string theory has pioneered new approaches to thorny problems in pure mathematics, demonstrating how physical reasoning can lead toward solutions that might otherwise remain undiscovered.

Perhaps most important, string theory has realized the concept of duality: the idea that a single physical situation can be described by two distinct mathematical formulations, each offering insights that the other cannot provide. Much like the classic optical illusion that flits between a young socialite and an elderly woman, it is only by embracing both descriptions that we achieve a complete understanding.

The most far-reaching version of duality, culminating in physicist Juan Maldacena’s work at the Institute for Advanced Study, suggests that a three-dimensional realm of space can be well described by processes occurring on a thin, two-dimensional shell surrounding it, much like a hologram — a flat, two-dimensional piece of plastic that generates a three-dimensional image.

This “holographic duality” is powerful because, as we flit between the two descriptions, quantum processes in one transform into gravitational processes in the other. This remarkable and wholly unexpected connection between gravity and the quantum has provided a new approach to a vast range of problems, explaining why the paper announcing this result has become the most highly cited in the history of theoretical physics.

A groundbreaking application of holographic duality, developed by Maldacena and Stanford physicist Leonard Susskind, illustrates this power.

The application concerns two papers that Einstein wrote in the spring of 1935. One analyzed “entanglement,” an iconic quality of quantum physics in which the behavior of two distant particles can be so tightly choreographed, it’s as if they have a secret connection bridging the space between them. Einstein famously called this connection “spooky.” The other paper described “wormholes,” a curious quality of general relativity in which two distant black holes can be connected by a tunnel through the fabric of space.

Although Einstein published these papers in the span of two months, he considered them completely unrelated. Now, by leveraging holographic duality, researchers have amassed evidence that the two ways of establishing a long-distance connection — quantum entanglement and relativistic wormholes — might describe the same phenomenon, expressed in two very different languages. Roughly, it’s as if particles are tiny black holes, and the entanglement between two of them is nothing but a connecting wormhole.

If this realization holds up, we will need to shift our thinking about the unification of physics. We have long sought to bring general relativity and quantum mechanics together through a shotgun wedding, fusing the mathematics of the large and the small to yield a formalism that embraces both. But the duality between Einstein’s two 1935 papers would suggest that quantum mechanics and general relativity are already deeply connected — no need for them to marry — so our challenge will be to fully grasp their intrinsic relationship.

Which would mean that Einstein, without realizing it, may have had the key to unification nearly a century ago. That string theory can reveal such hidden connections elicits a “wow” from practitioners and propels a new generation — even in the absence of experimental results — to push the frontiers of string theory further still.

The result also reflects on how mathematics has an uncanny, almost unreasonable, capacity to illuminate reality. Einstein’s math suggested the big bang, black holes, dark energy and gravitational waves — all consequences that Einstein, who was a cautious revolutionary (human nature, again) considered too exotic to be true. But over the past 100 years, each has been observationally confirmed. In a similar vein, his 1935 papers were motivated by mathematical critiques of quantum entanglement and black holes, but rather than expose theoretical flaws, the papers are now seen to refer to bona fide features of physical reality.

So, while mathematics is no substitute for experimental adjudication, mathematical progress has proved to be a crucial indicator of a theory’s potential to unlock secrets of the universe.

Will string theory follow this well-trodden path from mathematics to reality? No one can say. But its mathematical advances so far are extraordinary. And the only way to find out is to press onward. For string theorists, a community of rigorous, skeptical and ambitious scientists, the prize — revealing nature’s deepest secrets — is well worth the risk.