“It’s a bit like a romantic relationship,” says Christopher Isham, describing his collaboration with Andreas Döring. Certainly the two physicists can claim to share their own unique understanding of the world, as many in love do. Together they are proposing a radical new way to view reality—one that takes you into a new "mathematical universe" where notions of "truth" and "falsehood" no longer apply, but where the paradoxes of quantum mechanics suddenly make sense.

Their journey together began five years ago. In September 2004, Isham was already a prominent name in theoretical physics, researching deep conceptual questions at Imperial College in London. He was wrestling with some of the most profound mysteries of modern physics, including the quest for a theory that unifies general relativity with quantum mechanics. His work caught the attention of Döring, then still a PhD student at the Johann Wolfgang Goethe-University in Frankfurt, Germany.

Specifically, Isham and a few like-minded colleagues were attempting to use topos theory, a well-developed mathematical apparatus that had rarely been used in physics, to build a quantum theory of the universe. (See "Topos or Not Topos.")

Döring’s first e-mail was typical of a tentative young suitor—he expressed his admiration and asked for a "date." Or rather, he asked Isham if there was a chance that he could come to Imperial and work with him.

Isham remembers liking Döring right from the start. "I was very impressed with him, both as a person and a researcher," he says. "He’s obviously very bright, but he’s also a nice person. I put a lot of stock in a person’s personality."

The young scientist came to London and started working at Imperial a year later, in December 2005. The two began meeting weekly, and a collaboration into topos theory was underway.

Dynamic Duo

Isham and Döring’s working relationship was not without obstacles, however. Isham suffers from a neurological disease and needs a cane to walk, so he prefers to work from his home in London when possible. So Döring would swing by Isham’s apartment for intense brainstorming sessions that begin with tea and chatter before turning to physics.

Döring and Isham’s partnership has even inspired a movie. In 2007, filmmaker Ilian Metev shot a documentary of the partners at work, called The Physicist. "What attracted me to their work was a certain creative, almost electrifying energy with which their dialogue of ideas would relentlessly grow," Metev says.

In contrast to Isham and Döring’s dynamic partnership, the duo are working to bring together two of the biggest theories in physics that have been notoriously hard to unite. General relativity, which explains the interactions of cosmic bodies, and quantum mechanics, which defines interactions at the subatomic level, have both proven successful at their separate ends of the universe, the very large and the very small. But the two theories have so far refused to mesh with each other. A unified theory, the so-called Holy Grail of modern physics, would either meld the two together or replace them with something that works at both extremes.

For more than a dozen years now, Isham has been convinced that topos theory might provide the answer. Don’t confuse one type of theory with another, though. Unlike general relativity or quantum mechanics, topos theory isn’t a theory about how the universe works. Instead, it helps Isham and his allies think about the world using a completely different form of logic—and even different conceptions of truth.

"What Andreas and I have been arguing is that we need to change mathematics itself to solve this problem," Isham says. "I don’t mean to find a different branch of mathematics. I mean actually change mathematics, the whole whack, completely."

True or False?

Reconstructing the rules of logic sounds mind-bending—and it is. But quantum theory wreaks havoc with logic at the best of times. For example, it suggests that the universe isn’t the solid construct it seems but actually an array of possibilities collapsed into a single reality by the act of observation. So maybe mind-bending concepts are exactly what’s called for.

According to quantum mechanics, you cannot ask what the properties of a quantum particle are before it is measured. Prior to observation, the particle exists in a superposition of multiple contradictory states. There’s no simple yes/no or true/false answer to questions about the state of the particle.

Topos theory may be able to incorporate this murky quantum logic. It underlies the laws of mathematics and logic that we use, and is more fundamental than either, the physicists argue. Each topos describes a different "mathematical universe," with its own mathematical constructions and logic. And each topos can be used as a different lens through which to view the world, depending on requirements.

Isham and colleagues have identified a topos in which quantum theory appears to make logical sense—as long as you embrace a new type of logic, in which "true" and "false" are no longer your only options. There are now multiple shades in between. A different topos recreates classical reality with its firm "yes" and "no" answers. Isham and Döring believe that every physical system, from atomic particles to the universe as a whole, can be viewed through different topoi.

"In a sense, using a topos is changing the whole of mathematics," says Döring. "One chooses a ’mathematical universe’ in which to argue."

The work has won fans. "It’s really quite wonderful," says John Baez, a physicist at the University of California, Riverside.

So how do you keep grounded, when you spend your days rewriting reality? It helps that Isham’s background was originally in electrical engineering. "It’s very easy to get seduced," Isham says. "The mathematics is very intriguing, but we have to keep in mind that we’re working on theoretical physics."

Staying focused on the ultimate purpose of your intellectual endeavor is one of the many lessons that mathematician Döring has learned from working with Isham, who often reminds him to "think like a physicist."

Döring now lectures at Oxford University, UK, and shares these lessons with his students. His move makes those weekly meetings with Isham in London harder. "I strongly hope to keep up the collaboration with Chris in some form, even though the style will surely change," Döring says.

In other words, it’s not a breakup. It’s the beginning of a long-distance relationship.