As scientific theories go, Albert Einstein’s general relativity has had an amazing run. James Clerk Maxwell’s theory of electricity, magnetism, and light gave way to a quantum theory of light within 50 years. Quantum mechanics was superseded by quantum field theory in less than 20 years. But after 100 years, Einstein’s theory of gravity, space, and time is still state of the art.

The longevity of general relativity owes as much to its complexity as it does to the wide variety of strange new phenomena it predicted. In fact, my late colleague and Nobel laureate Subrahmanyan Chandrasekhar often said that Einstein’s understanding of general relativity was quite limited! Chandra was not trying to cut Einstein down to size; he was trying to indicate how deep and profound the theory is.

While the mathematical solution describing the simplest black hole was discovered in 1916, the solution describing the more interesting spinning black hole was not found until 1963. And it wasn’t until the late 1960s, when John Wheeler coined the term “black hole,” that the mathematics describing these mysterious entities were fully understood and their reality was no longer doubted.

Likewise, debate raged into the 1970s about whether general relativity really predicted gravity waves and, if so, whether the waves actually carried energy. Singularities, places where space and time literally end, remained a mystery until Stephen Hawking and Roger Penrose clarified the subtle mathematics involved. Their singularity theorems made black holes and the Big Bang more understandable and more beautiful.

By the end of the 20th century, advances in technology allowed most of general relativity’s predictions to be tested. It began in 1919 with Sir Arthur Eddington’s apparent confirmation that gravity bends starlight as it passes near the sun. Ten years later, astronomer Edwin Hubble verified the basic tenet of Einstein’s theory—that space is flexible—when he discovered that galaxies are racing away from each other, like raisins in a rising loaf of raisin bread. In 1960 a Harvard laboratory measured the warping of time in a gravitational field, and some 20 years later researchers found indirect evidence for the existence of gravity waves.

But even with all these successes, it is clear that Einstein won’t have the final word on gravity. Like Newton’s theory before it, which was incompatible with special relativity and was not powerful enough to describe the universe, general relativity has shortcomings that point to a grander theory. That by no means implies failure. The ultimate reward for a great theory lies in its ability to raise questions that lead to its demise and help to shape its successor.

Standing on Einstein’s shoulders, physicists can now ask a new set of simpler but more profound questions. Though they are phrased in the language of general relativity, the theory cannot answer them. These questions get at the very nature of space and time and set a high bar for relativity’s successor.

**1. HOW IS GRAVITY RECONCILED WITH QUANTUM MECHANICS?**

From problems come solutions. For example, the incompatibility of Newtonian gravity and special relativity led Einstein to general relativity. Now our challenge is to reconcile the two great achievements of 20th-century physics—Einstein’s general relativity and his nemesis, quantum mechanics, the theory he helped create but never accepted.

Superstring theory, now often called M-theory, looks like the most promising approach to marrying quantum mechanics and gravity while unifying all the forces of nature at the same time. Like general

relativity, M-theory is bold: It knits together the strands of physics by describing all

particles and forces as fantastically small strings of energy vibrating in 10 spatial dimensions and one dimension of time. Even if M-theory is not the answer, it provides a taste of the kind of radical thinking about space-time that is now required as well as the wonderful surprises that lie ahead.

**2. WHAT ARE SPACE AND TIME AND WHERE DID THEY COME FROM?**

If M-theory describes the marriage of gravity and quantum mechanics, it appears that the latter got the better prenuptial agreement. Quantum mechanics provides the framework for the theory, with the description of gravity conforming to it, not vice versa. This runs counter to Einstein’s hope that the geometry of space-time would provide the new paradigm for unifying the forces. There’s already plenty of evidence that space and time are secondary, so-called emergent phenomena. For example, there are mathematical solutions of M-theory in which the number of spatial dimensions can change. This suggests that space and time are not fundamental and that they emerge from something else. From Newton’s fixed space and time to Einstein’s flexible space-time, and now to space and time as emergent phenomena, we have come a long way. Maybe we will soon be ready to tackle the question of why time moves in one direction only.

**3. WHAT LIES AT THE HEART OF A BLACK HOLE?**

According to general relativity calculations, as one passes the event horizon (the point of no return) of a black hole, space and time switch roles. The inevitability of moving forward in time becomes instead the unavoidable plunge to the singularity at the center of a black hole. And that singularity prevents our knowing what lies beyond. It could be a wormhole, or shortcut, to another place in our universe or even to another universe entirely.

The singularities—the mathematical infinities that crop up in general relativity—are a clear sign that Einstein did not have the final word on gravity. At singularities the laws of physics appear to break down. More likely, our understanding is breaking down. M-theory has tamed some of the simpler singularities of general relativity, with the fuzziness of quantum mechanics taking the sharpness off these very pointy regions of space and eliminating the mathematical infinities. While M-theory has yet to determine precisely what lies at the heart of a black hole, it has shed light on some of their fundamental mysteries.

**4. WHAT HAPPENED BEFORE THE BIG BANG?**

More than 50 years ago Fred Hoyle coined the term “Big Bang” to draw attention to what he thought was a ludicrous feature of general relativity—a colossal creation of matter and energy from nothing. After the work of Hawking and Penrose, the beginning was clarified: The Big Bang was a space-time singularity out of which space, time, matter, and energy came to be. Thus, if general relativity is correct, there is no “before the Big Bang,” as time did not exist, nor did space, matter, and energy.

When he pondered what God was doing before he created heaven and earth, St. Augustine concluded that there was no before, “as time itself was [God’s] creation.” Relativity theory suggests that the universe began in a similarly tidy way, but is that the final answer? While Hawking and Penrose demonstrated the mathematical beauty of the Big Bang singularity, it nonetheless represents a breakdown in the laws of physics, one that blinds us from seeing anything before the Big Bang. Beyond the singularity could be an earlier, collapsing phase, or “big crunch,” of our universe or even the quantum creation of a universe from nothing. There may have been countless big bangs, as Alan Guth’s cosmic inflation theory implies, each creating its own universe within a larger multiverse.