The Perfect Theory (36 page)

There is more. Astronomers and relativists are mobilizing to build the telescope that will actually
see
the black hole. Called the
Event Horizon Telescope, it will have a resolution of a billionth of an angular degree, a fraction of the size the black hole takes up in the sky, so that it will actually be able to see Schwarzschild's shroud, the surface of the black hole that Oppenheimer and Snyder showed is a snapshot frozen in time. It will be a dark shadow surrounded by the swirling mess that Zel'dovich and Novikov conjectured would surround the black hole, the accretion disk of stars, gas, and dust being shredded by the gravitational pull of the singularity.

The accumulating evidence is very compelling. While Weinberg's reticence was understandable, it is now difficult to find anyone who would argue against there being a black hole in the center of the Milky Way. And just like the Milky Way, all the other galaxies should have black holes firmly in their centers like massive engines surrounded by gargantuan spirals of stars.

 

The media find anything to do with general relativity and Einstein's great ideas both enticing and newsworthy. Images of the center of our galaxy lead to headlines like
“Black Hole Confirmed in Milky Way” on the BBC, and “Evidence Points to Black Hole at Center of the Milky Way” in the
New York Times
. On the day I am writing this, the BBC news website features a comment from an Oxford colleague of mine on a recent observation of a quasar now shown to be a super-massive black hole with a mass of a billion suns. What stuns me is that almost fifty years after Maarten Schmidt's measurements and the first Texas Symposium, black holes can still create such a stir.

Not a month goes by without something in the news about cosmology or black holes, about the beginning of the universe or echoes of other universes, signatures of the mysterious multiverse. Words like
black holes,
Big Bang, dark energy, dark matter, multiverse, singularity,
and
wormholes
have penetrated the farthest recesses of popular culture, from Broadway plays and songs to comedy shows and Hollywood movies. And then there are the countless ways in which general relativity has been folded into science fiction, from novels to TV shows to movies. They have surpassed even Wheeler's wildest dreams in terms of imagination and creativity. Everyone seems to consider himself or herself an expert on general relativity.

This fascination is exhilarating but sometimes also ludicrous. When my son called me irresponsible for, in some indirect way, willing the Large Hadron Collider into existence, he was not alone. The media had repeatedly advertised the idea that string theory, one of the candidate theories of quantum gravity, predicted that black holes would be formed when the Large Hadron Collider switched on. When the beams of protons actually collided, among the multitude of stuff that would spew out into the detectors would be microscopic black holes, mini portals into other dimensions. My son also knew that black holes suck up everything around them. Everyone knows that. So why on earth would I, or anyone in his or her right mind, want to produce these incredibly dangerous things? It was obviously a stupid thing to do.

One physicist of sorts, of all people, actually tried to stop the
LHC from being switched on by going to court. When interviewed on the Jon Stewart show, he was asked about the probability of a catastrophe actually happening, and in a remarkable flourish of on-air reasoning he said, “Fifty percent.” He lost, the LHC was switched on, and we are still here. Unfortunately, no miniature black holes have been found.

 

Every time I give a public lecture about what I do, I am asked the same thing: “What was there before the Big Bang?” I resort to the various explanations. There is the “There was no before, no time, before the Big Bang” answer. Or there is my colleague Jocelyn Bell Burnell's more Zen-like answer: “That is like asking what is north of the North Pole.” It would be so much easier if I could resort to mathematics, but I can't because most of my audience would find that it went over their heads. And for decades, because of Stephen Hawking's and Roger Penrose's singularity theorems, we have believed that, indeed, there was nothing before the Big Bang. It is one of those truths, those
mathematical
truths, we can't get around that came out of the Golden Age of General Relativity.

Very recently, I've found my answers to the Big Bang questions becoming much more diverse and much less definitive. Over the past few years, the beginning of time has been thrown wide open by developments in quantum gravity and cosmology. When you wind back the clock and make the universe denser, hotter, and messier, that is when quantum foam, strings, branes, or even the loops have a say. It is where, it seems to some, spacetime breaks down and it no longer makes sense to talk about the initial singularity.

So what happened before the Big Bang? One possibility is that our universe popped into existence out of a vacuum, a bubble of spacetime that grew and grew to become what we are today. And like ours there are many universes that just popped up out of the vacuum. Another guess comes out of ideas in string and M-theory, which posit that the universe has many more than four dimensions and that we live on a three-dimensional “brane” in this spacetime and roll with it. Our domicile, our brane, feels just like a three-dimensional universe that every now and then collides with another brane just like ours. When they collide, they heat up, and as a result our universe feels as if it has undergone a hot Big Bang. There is no singularity, just an infinite succession of hot Big Bangs, a cyclic universe that would have made the Soviet orthodox philosophers, and possibly even Fred Hoyle and his cronies, proud. The model's creators have dubbed each new Big Bang
Ekpyrosis
, an ancient Greek term for the periodic destruction of the universe, inevitably followed by a rebirth.

But, of course, so much of quantum gravity seems to point to the fragmentation of spacetime if looked at under an all-seeing microscope. If we wind back the clock so that spacetime is concentrated at a point, surely we must run up against the bits and pieces that make up the fabric of space. Before any initial singularity is reached, when the chunkiness comes into play, known physics breaks down. Those who believe that loop quantum gravity is the answer say that there was a before, a time when the universe was collapsing until it reached the quantum wall and magically started expanding again. The universe underwent what has prosaically become known as a “bounce.”

It might not even be necessary to resort to that weird, dark era where quantum gravity comes into play, where so many differing opinions lead to so many different conjectures. A grander possibility is that spacetime is much vaster than we previously envisioned and our universe is only one of countless universes that together make up the multiverse. All over the multiverse, universes are breaking out into existence, growing to cosmic proportions, each one at its own pace and made up in its own particular way. If we follow back the existence of our own universe, we find that it is embedded like a pustule in a much wider spacetime that has existed for all eternity. The multiverse is a wild, immense realm of what is ultimately stasis: a steady state of creation and destruction.

The multiverse, along with something called the anthropic principle, has emerged as the favorite solution for the cosmological constant problem. With the great successes of observational cosmology, many believe that the cosmological constant actually exists in the real universe, even though quantum theory predicts an obscenely large value for it, much larger than what we observe. String theorists now apply the lack of predictivity in string theory to posit a landscape of different possible universes, each one with its own symmetries, energy scales, types of particles and fields, and, most crucially, its own cosmological constant. Any of these universes is possible, even ones with a very small cosmological constant. The anthropic principle, first proposed by Robert Dicke and further developed by Brandon Carter, argues that the universe is the way it is because if it were any other way, we wouldn't be around to see it. We exist and are sentient because the universe has exactly the right set of constants, particles, and energy scales—including the cosmological constant—that allow for our existence. There are countless possible universes, but only the ones with the right values for physical constants, including the cosmological constant, allow us to exist. Given that such a universe is possible, it is natural that it will be the one, of all the universes in the multiverse, that we observe.

Some argue that cosmology has become so rich and complex that we may be at the frontier of what should be called science. George Ellis is one skeptic who thinks this approach goes too far. A relativist who, with Hawking and Penrose, cemented the existence of singularities in the cosmos in the late 1960s, Ellis has been at the forefront of using the whole of the universe as an immense laboratory and testing ground for Einstein's theory.
“I do not believe the existence of those other universes has been proven—or could ever be,” he says. “The multiverse argument is a well-founded philosophical proposal but, as it cannot be tested, it does not belong fully in the scientific fold.” On this landscape of possibilities anything can be predicted somewhere. Even among the string theorists there is a sense that things have gone too far. The new approach abandons the ultimate goal of modern physics to find a unique and simple unified explanation for all the fundamental forces, including gravity. Accepting the multiverse is tantamount to giving up. Even Edward Witten, the pope of modern string theory, is unhappy with how things are turning out and says, “I hope the current discussion of the string theory isn't on the right track.”

Yet the multiverse's following is growing. It solves some of the great unsolved problems, such as why there is a cosmological constant and why the constants of nature are tuned to be exactly what we measure them to be. On a regular basis, there are press releases and media reports on parallel universes and evidence for the immensity and plurality of spacetime. It is, of course, a wonderful setting for speculation, a vast blank canvas for storytelling. But, to Ellis, it simply isn't science.

 

In 2009 I visited Príncipe, a small, lush speck of greenery in the armpit of Africa. It was from there that, ninety years before, Arthur Eddington had telegraphed a message to Frank Dyson, then the president of the Royal Astronomical Society, saying simply, “Through cloud. Hopeful.” Eddington's measurements of starlight during a solar eclipse had established Einstein's general theory of relativity as
the
modern theory. The eclipse expedition established Eddington and Einstein as international superstars.

I traveled to the small island nation of São Tomé and Príncipe with a motley collection of Brits, Portuguese, Brazilians, and Germans to lay a plaque donated by the Royal Astronomical Society and the International Astronomical Union at the site where Eddington and Cottingham had made their measurements.

São Tomé and Príncipe had emerged from centuries of colonial rule to become, for a while, yet another African socialist state. It joined the world of free markets, and its jumbled collection of shiny new houses for affluent Angolan holidaymakers contrasted with grand, decrepit colonial farmhouses.

The main house at Roça Sundy, where Eddington made his measurements, was in better shape than most of the abandoned colonial homes scattered throughout the green countryside. The regional president of Príncipe, a tiny island of not more than five thousand people, had taken it as his holiday home. This turned out to be wishful thinking—it was still ramshackle, rusty, and uninhabitable.

I found that perfect little corner of the world deeply moving. My grandmother was born in São Tomé and Príncipe in the early twentieth century, and I had heard much about the place from her. But more important, I felt that I was witnessing a turning point in history. This is where Einstein's theory was proved right, insofar as any scientific theory can be proved right. This is where general relativity became fact.

Scattered around were relics of the bygone era when Eddington passed through. There was the tennis court, cracked concrete fighting a losing battle with the inexorable vegetation seeping up from the ground. Everywhere I looked was lush, overwhelming green. It was a far cry from the bleak, manicured landscape of the fens where Eddington had spent almost all his life. Now, with our visit, there was a shiny plaque marking Eddington's achievement and, we hoped, explaining to any passerby of this remote location how stupendous the event had been.

Looking back to 1919, it is amazing how Einstein's and Eddington's ideas developed. The simple idea that light would be deflected by warped spacetime, the key to testing Einstein's theory, was now, ninety years later, one of the most powerful tools in astronomy. Over the past twenty years it has become the norm to look at how light is deflected by spacetime to learn about the universe. By looking at stars in nearby galaxies and waiting to see if their light is suddenly focused due to the passage of a dark heavy object in front of them, it has been possible to look for dark matter in our galaxy. The nuggets of dark matter, if they exist, will play the role of the sun in Eddington's experiment, bending starlight, lensing it, as the effect has become known. On a grander scale, we now use lensing to look at clusters, swarms of tens to hundreds of galaxies. These behemoths sink into spacetime, creating gigantic warps that scatter and align the light from distant galaxies. Astronomers now use the distortions and shifts in the light of these distant galaxies to weigh the clusters.

Why stop there? With typical hubris, astronomers, cosmologists, and relativists have now set their sights on mapping the distortions of spacetime all the way out, as far as can possibly be observed. By observing slices of the universe and seeing how the light of those galaxies is affected by intervening spacetime, it should be possible to build up a detailed description of what spacetime actually looks like all around us. Taking Einstein's and Eddington's ideas to a new level, we harness the universe, learning what it is made of and whether our current laws for the way spacetime behaves are correct.