Quantum gravity
Is a single theory of reality in sight?
Watching a rocket as it slowly starts to heave itself out of Earth’s deep gravity well and then streaks up into the blue, you suddenly grasp on a visceral level the energies involved in space exploration. One minute that huge cylinder is sitting quietly on its launching pad; the next, its engines fire up with a brilliant burst of light. Clouds of exhaust fill the sky, and the waves of body-shaking thunder never seem to end.
To get anywhere in space, you have to travel astounding distances. Even the Moon is about 400,000 km away. And yet the hardest part – energy-wise, anyway – is just getting off the ground. Clear that hurdle, slip the bonds of Earth, and you’re off. Gravity’s influence falls away and suddenly, travel becomes a lot cheaper.
So it might be surprising to hear that the most exciting new frontier in space exploration starts a mere 2,000 km above the terrestrial surface. We aren’t talking about manned missions, automatic rovers or even probes. We’re talking about satellites. Even more prosaically, we’re talking about communications satellites, in low Earth orbit. Yes, they’ll be fitted with precision laser equipment that sends and receives particles of light – photons – in their fundamental quantum states. But the missions will be an essentially commercial proposition, paid for, in all probability, by banks eager to protect themselves against fraud.
Perhaps that doesn’t sound very romantic. So consider this: those satellites could change the way we see our Universe as much as any space mission to date. For the first time, we will be able to test quantum physics in space. We’ll get our best chance yet to see how it meshes with that other great physical theory, relativity. And at this point, we have very little idea what happens next.
Let’s back up. Since its discovery in 1900 and its formalisation in the 1920s, quantum mechanics has remained unchallenged as our basic theory of the submicroscopic world. Everything we know about energy and matter can (in principle) be derived from its equations. In an extended form, known as quantum field theory, it underlies the ‘Standard Model’ – which is to say, all that we know about the elementary particles.
It’s difficult to overstate the explanatory power of the Standard Model. Physics has identified four fundamental forces at work in the Universe. The Standard Model accounts for three of them. It explains the electromagnetic force that holds atoms and molecules together; the strong force that binds quarks into protons and neutrons and clamps them together in atomic nuclei; and the weak force that releases electrons or positrons from a nucleus in the form of beta decay. The only thing the model leaves out is gravity, the weakest of the four. Gravity has a theory of its own – general relativity, which Albert Einstein published in 1916.
Many physicists believe we should be able to capture all our fundamental forces with a single theory. It’s fair to say that this has yet to be achieved. The problem is, quantum theory and relativity are based on utterly different premises. In the Standard Model, forces arise from the interchange of elementary particles. Electromagnetism is caused by the emission and absorption of photons. Other particles cause the strong and weak forces. In a way, the micro-scale world functions like a crowd of kids pelting each other with snowballs.
Gravity is different. In fact, according to general relativity, it’s not really a force at all....MUCH MORE
