Jim Baggott [Baggott - Quantum Space: Loop Quantum Gravity and the Search for the Structure of Space, Time, and the Universe

Lets get one thing straight.

This is a book about loop quantum gravity, one of several contemporary approaches to the development of a quantum theory of gravity, perched right on the very edge of our current understanding of space, time, and the physical universe. One hopes that science at the frontiers will always make for entertaining reading but, make no mistake, like all such theories, as of today there is not one single piece of observational or experimental evidence to support it.

You might then wonder why I think you ought to be interested in this.

Heres why. Theres little doubting that in these first few decades of the twenty-first century we face some tremendous economic, political, and environmental challenges, some much more stubborn and intractable than others. But when it comes to our ability to comprehend the nature of space and time, to understand the very fabric of physical reality, the quantum theory of gravity is simply the greatest scientific problem of our age. It addresses the ultimate big question of existence. Resolving this problem demands a real depth of scientific expertise; it demands unique moments of insight and inspiration; and it demands intellectual creativity likely to be unsurpassed in the entire history of physics.

The reason is simple. Today we are blessed with two extraordinarily successful theories. The first is Albert Einsteins general theory of relativity, which describes the large-scale behaviour of matter in a curved spacetime. It tells us how gravity works: matter tells spacetime how to curve, and curved spacetime tells matter how to move. This theory is the basis for the so-called standard model of Big Bang cosmology. We use it to describe the evolution of our universe from almost the very beginning, which on current evidence happened about 13.8 billion years ago. The discovery of gravitational waves at the LIGO observatory in the USA (and now Virgo, in Italy) is only the most recent of this theorys many triumphs.

The second is quantum mechanics. This theory describes the properties and behaviour of matter and radiation at its smallest scales; at the level of molecules, atoms, sub-atomic, and sub-nuclear particles. In the guise of quantum field theory it is the basis for the so-called standard model of particle physics, which builds up all the visible constituents of the universe (including stars, planets, and us) out of collections of quarks, electrons, and force-carrying particles such as photons. It tells us how the other three forces of nature work: electromagnetism, the strong force, and the weak interaction. The discovery of the Higgs boson at CERN in Geneva is only the most recent of this theorys many triumphs.

But, while they are both highly successful, grand intellectual achievements, these two standard models are also riddled with holes. Theres an awful lot they cant explain, and they leave a lot of important questions unanswered. If anything, their successes have only served to make the universe appear more elusive and mysterious, if not downright bizarre. The more we have learned, the less we seem to understand.

The two theories are also fundamentally incompatible. In the classical mechanics of Isaac Newton, objects exist and things happen within a container of absolute space and time which somehow sits in the background. If we could take everything out of Newtons universe we must suppose that the empty container would remain. General relativity gets rid of this container. In Einsteins universe space and time become relative, not absolute, and the theory is said to be background independent. Spacetime is dynamic; it emerges as a result of physical interactions involving matter and energy.

Quantum mechanics, though exasperatingly bizarre yet unfailingly accurate in its predictions, is formulated in a different way. Interactions involving the elementary particles of matter and radiation are assumed to take place in precisely the kind of absolute spacetime container that general relativity eliminates. Quantum mechanics is background-dependent.

And there you have it. We have a classical (non-quantum) theory of spacetime which is background-independent. And we have a quantum theory of matter and radiation which is background-dependent. Our two most successful theories of physics are built on incompatible interpretations of space and time. They are woven on different kinds of fabric, one co-generated by the physics and the other pre-supposed and absolute.

We have two incompatible descriptions but, as far as we know (and certainly as far as we can prove), weve only ever had one universe. This is a problem because we also know that in the first few moments following its birth in the Big Bang, the universe would have existed at the quantum scale, at the mercy of a quantum mechanics. Now, the fact that we cant explain the origin and earliest moments of the universe might not trouble you overmuch, but the track-record of physics in the past hundred years or so has encouraged us to have greater expectations. What we need is a quantum theory of gravity.

So, do I have your attention yet?

The Chinese philosopher Laozi once said that a journey of a thousand miles begins with a single step. The first thing we can do is recognize that the only way to bring together quantum mechanics and general relativity is to find a new fabric, a new way of conceiving of space and time, one that is compatible with physics on any scale.