Theories of everything: Has cosmic science written its last word?
Independent, 10 September 2010
God is dead," declared Friedrich Nietzsche, but few listened or cared. "It is not necessary to invoke God to light the blue touch paper and set the universe going," announced Stephen Hawking last week, and it was picked up by the world's media. For over 20 years earlier, the world's most famous scientist had ended his phenomenal bestseller A Brief History of Time with the arresting conclusion that "If we discover a complete theory, it would be the ultimate triumph of human reason - for then we should know the mind of God."
Why is there something rather than nothing? Why do we exist? Why this particular set of laws and not some other? It is these "ultimate questions of life" that Hawking's now sets out to answer, with the help of the American physicist and science writer Leonard Mlodinow, in his fascinating new book The Grand Design (Bantam, £18.99). Philosophers have traditionally tackled such questions, while most physicists have stayed well clear from addressing the "why" of things and concentrated instead on the "how".
Not any more. "To understand the universe at the deepest level," says Hawking, "we need to know not only how the universe behaves, but why." He believes that "philosophy is dead" because it failed to keep up with the latest developments, especially in physics.
For it is possible to answer these questions purely within the realm of science and without resorting to God. And the answers hinge on a candidate for a theory of everything called M-theory, "if it indeed exists", the authors admit. Unfortunately, no one seems to know what "M" stands for; it could be "master", "miracle" or "mystery".
The story of M-theory could be said to begin with the desire of physicists to unify and simplify. Just as ice, water and steam are different manifestations of water, in 1864 James Clerk Maxwell showed that electricity and magnetism were likewise different manifestations of the same underlying phenomenon - electromagnetism. He managed to encapsulate the disparate the behaviour of electricity and magnetism into a set of four elegant mathematical equations. Using these equations, Maxwell was able to make the startling prediction that electromagnetic waves travelled at the speed of light, approximately 670 million miles per hour. Light was a form of electromagnetic radiation. Maxwell's unification of electricity, magnetism and light was the crowning achievement of 19th-century physics.
In the 20th century, to go with gravity and electromagnetism, physicists discovered two new forces - the weak, which is responsible for radioactivity, and the strong that binds together, for example, the nucleus of an atom. They believed that these four forces, which appeared so different, would be reunited a single all-encompassing theory of everything.With exception of general relativity, Einstein's theory of gravity, it's possible to "quantize" the other three forces, since quantum mechanics deals with the atomic and sub-atomic domain. In effect, we have three trains running on the same-sized track.
Unfortunately, Einstein's gravity train was running on a completely incompatible track. Yet the impulse for unity and simplicity is so strong that theorists have pursued a quantum theory of gravity, without success, for decades. Then in the 1980s there appeared a new theory that looked promising - superstrings.
The theory assumes that all observed particles are different manifestations of the same fundamental entity. According to the superstring idea, all particles previously thought off as little points are in fact not points at all but basically little oscillating bits of "string" which move through space. The different levels of "vibration" of these strings correspond to the different particles.
Superstrings vibrate in 10 dimensions. But we don't notice these extra dimensions because they are curled up into a space that's infinitesimally small. Alas, it was discovered that there were at least five different string theories and millions of ways the extra dimensions could be curled up - an embarrassment of riches for those who hoped that string theory was the longed for theory of everything.
As others despaired, the American physicist Ed Witten led the way, beginning in the mid-1990s, in showing that the different string theories and a theory called "supergravity" were all just different approximations to a more fundamental theory: M-theory.
"M-theory is not a theory in the usual sense," admits Hawking. "It is a whole family of different theories, each of which is a good description of observations only in some range of physical situations. It is a bit like a map." Faithfully to map the entire earth, one has to use a collection of maps, each of which covers a limited region. The maps overlap each other, and where they do, they show the same landscape.
M-theory needs 11 space-time dimensions and contains not just vibrating strings but other objects that are impossible to visualize. The extra space dimensions can be curled up in a mind-blowing 10 to the 500th different ways, each leading to a universe with its own laws. To get an idea how many that is, Hawking and Mlodinow ask the reader to imagine a being capable of scanning each of those universes in just one millisecond and who started working on it at the Big Bang. Today that being would have only have scanned just 10 to the 20th of them.
This plethora of universes, the multiverse, explains what appears to be the mystery behind the remarkable coincidences that have fine-tuned natural laws to make our universe habitable for us. With so many universes, it's a lot less remarkable that there is at least one in which conditions are Goldilocks-like: just right to have given rise to us, since we exist it has to be this one. This is the anthrophic principle that effectively says that things are the way they are because they were the way they were. From here, Hawking goes on to argue "Because there is a law like gravity, the universe can and will create itself from nothing."
"'Think of an expanding universe as a surface of a bubble," writes Hawking. "Our picture of the spontaneous quantum creation of the universe is then a bit like the formation of bubbles of steam in boiling water. Many tiny bubbles appear, and then disappear again. These represent mini-universes that expand but collapse again while still of microscopic size. They represent possible alternative universes, but they are not of much interest since they do not last long enough to develop galaxies and stars, let alone intelligent life. A few of the little bubbles will grow long enough so that they will be safe from recollapse. They will continue to expand at an ever-increasing rate and will form the bubbles of steam we are able to see. These correspond to universes that start off expanding at an ever-increasing rate."
Spontaneous creation is the reason there is something rather than nothing; why the universe exists, why we exist. God is surplus to Hawking's requirements.
Why are the fundamental laws as they are? The ultimate theory must be consistent and must predict finite results for quantities that we can measure. There must be a law like gravity and, for a theory of gravity to predict finite quantities, the theory must have what is called "supersymmetry" between the forces of nature and the matter on which they act. "If the theory is confirmed by observation," says Hawking, "it will be the successful conclusion of a search going back more than 3000 years."
"Yet in the history of science," he admits, "we have discovered a sequence of better and better theories or models, from Plato to the classical theory of Newton to modern quantum theories. It is natural to ask: Will this sequence eventually reach an end point, an ultimate theory of the universe, that will include all forces and predict every observation we can make, or will we continue forever finding better theories, but never one that cannot be improved upon?"
Though Hawking is probably being rhetorical, Russell Stannard, a former professor of physics at the Open University, looks at the unanswered questions of modern physics in his book The End of Discovery (Oxford, £14.99). Stannard believes that eventually, but he doesn't know when, fundamental science will reach the limit of what it can explain. On that day, the Scientific Age, like the Stone Age and the Iron Age before it, will come to an end. He believes that not only technological limits, but maybe humanity will have reached the limits if its mental capacities to unravel the nature and workings of reality.
Stannard takes readers on a tour of some of the deepest questions facing science: questions to do with consciousness, free will, the nature of space, time, and matter. He covers much of the same territory as Hawking and Mlodinow, and points out that to understand the subatomic world, scientists depend of particle accelerators; but to understand the very smallest units of nature, it has been calculated that we would need an accelerator the size of a galaxy.
In A Brief History of Time, Hawking said that a scientific theory "may originally be put forward for aesthetic or metaphysical reasons, but the real test is whether it makes predictions that agree with observations". As they have waited for the next generation of particle accelerators and experiments, the research of physicists from superstrings to quantum cosmology has had a tendency to take on a metaphysical character in recent decades.
So maybe philosophy isn't as dead as Stephen Hawking thinks. For those having a difficult time wrapping their head around "spontaneous creation", he has this tip: "If you like, you can call the laws of science 'God'."