麻豆传媒

IN MARCH, a team of physicists announced that they had detected ripples in space-time produced at the universe鈥檚 birth. Their claims took the world by storm. One of my colleagues declared it the most important discovery of this century, while another told me it was the most exciting thing he had experienced in his scientific life. Someone even said that it was more important than the discovery of the Higgs boson.

I, too, was thrilled by the news. For if this discovery holds up to scrutiny 鈥� and some of my colleagues aren鈥檛 sure that it does 鈥� it is the first direct sighting of the gravitational waves that Albert Einstein predicted shortly after he proposed his general theory of relativity in 1915. It is also further evidence that the universe was very different in the distant past from what it is now, and that it has evolved over many billions of years 鈥� another prediction from Einstein鈥檚 theory.

Yet, oddly enough, I wasn鈥檛 completely surprised by the discovery. Over the last few years, I have come to believe that we are in a new golden age for the general theory of relativity and that we should expect fantastic things to happen. There are many great discoveries to look forward to, from the direct imaging of a black hole to untangling the fundamental nature of space and time. To understand my optimism, we need to look at the biography of Einstein鈥檚 theory and follow it over a century of discovery and turmoil.

鈥淲e are in a new golden age of relativity. We should expect fantastic things to happen鈥�

When Einstein started thinking about gravity in 1907, he had already figured out his special theory of relativity, which brought together Newtonian mechanics 鈥� how things move, push and pull 鈥� and Maxwell鈥檚 theory of electricity and magnetism. To achieve this, the rules of physics had to change. Space and time became intertwined and the speed of light become sacrosanct and invariant, a cosmic speed limit on any physical process. It all worked beautifully 鈥� except for one thing. Isaac Newton鈥檚 gravitational force, which explains how the planets move around the sun and why things fall to the surface of the Earth, didn鈥檛 fit in. And so Einstein set out to come up with a more general formulation that could include gravity. It took him eight years.

The new general theory of relativity needed a completely different form of mathematics and a fresh way of thinking about physics. It was as if Einstein had learned Sanskrit from scratch and then used it to write a novel. When he finally worked it all out, he had the most stunning set of equations. They were compact yet complex, a veritable treasure trove of discoveries waiting to happen. They changed our view of the nature of reality. From then on, space and time had life: they were malleable and dynamic; they responded to the presence of stuff and, in turn, made stuff move around as if it were in an invisible gravitational force field. It was, in all ways, a perfect theory.

鈥淚t was as if Einstein had learned Sanskrit from scratch and then used it to write a novel鈥�

When Einstein offered his theory up to the world, it took on a life of its own. And in the decade that followed, a series of brilliant yet idiosyncratic explorers used it to come up with the most amazing discoveries. There was the British astronomer Arthur Eddington, a Quaker and conscientious objector during the first world war. In 1919, he sailed to the small island of Pr铆ncipe to witness a total solar eclipse and showed that the light from a distant star cluster was bent around the sun, exactly as predicted by Einstein鈥檚 theory. Or there was the German astronomer Karl Schwarzschild who, amid fighting in Russia on the Eastern Front, conceived of what would later become known as a black hole. And then there was the Soviet meteorologist and mathematician Alexander Friedmann who, along with the Belgian priest Georges Lema卯tre, showed that Einstein鈥檚 theory implied that the universe was evolving and expanding. And of course Einstein himself predicted the existence of gravitational waves.

By the early 1930s, Einstein鈥檚 theory had captivated many of the leading lights of physics. Eddington, Wolfgang Pauli, Werner Heisenberg and Erwin Schr枚dinger all wrote textbooks with their own take on how the theory should be understood.

Except that, once the low-hanging fruit had been picked, general relativity slowly faded as an object of physicists鈥� interest. The discovery of quantum physics pushed Einstein鈥檚 theory into the long grass. Quantum physics was a far more practical theory that explained things that could actually be measured in the laboratory and could be used to build bombs. General relativity was lost. It had become a beautiful yet esoteric theory, with little to say about the real world. The world had moved on.

鈥淨uantum physics pushed relativity into the long grass. The world had moved on鈥�

Relativity鈥檚 revival

And then, after almost a quarter of a century, the green shoots of recovery started to break out. In the 1950s, a new generation of astronomers working at radio frequencies started to map out a universe littered with incredibly energetic objects at staggering distances. These powerful beacons seemed far too heavy to be explained using Newtonian gravity and the general theory of relativity beckoned. A new generation of physicists began turning their attention to the mysteries of Einstein鈥檚 theory and, slowly but surely, began to unpick many of the intriguing and bizarre results that had been ignored.

Mathematics revealed the inner workings of black holes in exquisite detail, while observational evidence for them started to amass. The discovery of a relic bath of light 鈥� the cosmic microwave background 鈥� showed that the universe was hot and dense early on, adding weight to the idea that an expanding universe was a plausible description of our cosmic history. Things just seemed to fall into place during this 鈥済olden age of general relativity鈥�, as of the California Institute of Technology called it.

I began working in science in the early 1990s, mesmerised by this 鈥済olden age鈥� and all the wonderful thinkers involved. General relativity still had a bit of an esoteric, almost tainted, aura about it and working on it wasn鈥檛 particularly recommended. But it was at the heart of what was really interesting in modern physics. It seemed to me that, if anything, it was driving the really new discoveries, both in theory and observations.

Now things are moving quickly again. For a start, satellites are the latest outposts of science. These unspeakably sophisticated laboratories float in space at the boundaries of our reach doing once-unimaginable experiments. A few years ago, the put out a call for the next big missions that it should support. Einstein鈥檚 theory seems to underpin the case for many of the satellites being considered.

One mission, called eLISA, proposes to pick out the waves of gravity expelled from the explosive collisions between black holes. Another mission, called Euclid, would measure how much the universe has expanded since it was half its current age to figure out the effects of the elusive dark matter and dark energy which contemporary cosmology has thrown in. Yet another mission, called ATHENA, would look at the powerful X-rays given off as matter and light are shredded by the titanic gravitational forces near a black hole鈥檚 surface. We would, for the first time, be delving into the most extreme conditions of space imaginable.

But we don鈥檛 need to wait for such giants to be launched into space to explore these extremes. Next year, the advanced Laser Interferometric Gravitational Observatory, or advanced LIGO for short, will be switched on. When it is we expect to see the echoes of embryos of black holes as they slowly orbit, collide and merge into one massive compact object. With the Event Horizon Telescope, a network of telescopes scattered across the globe, we may actually see the black hole at the centre of the Milky Way. For the first time we could image the black hole鈥檚 dark shadow surrounded by the swirling mess of stars, gas and dust being torn apart by its gravitational pull.

And then there is my favourite: a collection of tens of thousands of radio antennas scattered across many thousands of kilometres. Known as the Square Kilometre Array, or SKA, because the collecting area of all the antennas should add up to a square kilometre, it will be based in two continents: Australia and South Africa. And, while Eddington used a small telescope on Pr铆ncipe to establish the primacy of general relativity, the SKA will be the beast that can test Einstein鈥檚 theory on galactic and cosmological scales with unprecedented precision. The SKA will detect if there are any cracks in his grand idea.

While general relativity seems to be driving observations and experiments to new levels of sophistication, the realm of ideas is also undergoing dramatic changes. One notable example is when discussing the beginning of time. Every time I give a public lecture about what I do I am asked the same thing: 鈥淲hat was there before the big bang?鈥� I resort to various explanations. There is the 鈥渢here was no before, no time, before the big bang鈥� answer. Or there is my colleague 鈥榮 more Zen-like answer: 鈥淭hat is like asking what is north of the north pole.鈥� But recently, I鈥檝e found my answers becoming much more diverse and much less definitive.

Over the last few years, the beginning of time has been thrown wide open by developments in quantum physics and cosmology. When you wind back the clock the universe becomes denser, hotter and messier 鈥� the perfect conditions for quantum physics to have more sway. One possibility is that our universe popped into existence out of a vacuum, a bubble of space-time that grew and grew to become what we are today.

A grander possibility is that space-time is much vaster than we had previously envisioned and our universe is just one of countless universes that together make up the 鈥渕ultiverse鈥�. Throughout the multiverse, universes are coming into existence and growing to cosmic proportions, each one at its own pace and made in its own particular way. The multiverse is a wild, immense realm of what is ultimately stasis: a steady state of creation and destruction. In this scenario our own universe is like a pustule in a much wider space-time that has existed for all eternity. It is a hugely speculative idea that is pushing the boundaries of what we can reasonably call science, but it is still in the realm of space-time.

Before the big bang

But there is a much deeper revolution quietly taking place that is shaking the foundations of Einstein鈥檚 theory. Attempts to unite general relativity and quantum theory into a theory of everything, and all the issues that these approaches raise, are shattering our hallowed notions of space-time. And they all seem to indicate that, at a fundamental level, we should give up on space-time as smooth and malleable, evolving, twisting and warping according to Einstein鈥檚 theory. Instead we should think of it as something fragmented and atomised, where notions of locality 鈥� the here and now that we are so familiar with 鈥� are thrown away.

If we do away with space-time then we need to recast the laws of physics. The new rules may be unfamiliar but, ultimately, they may be much simpler and many of the paradoxes and conundrums should disappear. A simple example is if we wind back the clock in our universe鈥檚 history towards the moment when the whole of space-time was concentrated at a point and our current laws of physics break down. Given that we can鈥檛 say much about what happened then, it is fair to speculate that there was an era before the big bang, a time very different from our own.

All the experiments, observations and ideas that are captivating so many of us are part of the vibrant and active life of the general theory of relativity. It has had a long and convoluted history that isn鈥檛 over yet. While the discovery of gravitational waves, if it is confirmed, is one fantastic example, I expect there will be many more. And so, while the 20th century was dominated by the weird and wonderful discoveries to come out of quantum physics, from the atom and quantum electrodynamics to the standard model of particle physics, we should brace ourselves to witness the glorious consequences of Einstein鈥檚 Perfect Theory.

This article appeared in print under the headline 鈥淭he universe expects鈥�