(Image: Claudia Marcelloni/CERN)
A RATHER glib distinction is often made between the two pillars of modern physics. Quantum mechanics is the physics of the very small, while general relativity is the physics of the very large. That’s not quite accurate – for example, quantum-mechanical effects have been observed spanning hundreds of kilometres. And at some scale, surely these two supremely accurate theories must come together.
Yet wherever they do cross paths, the two theories fail to play nicely together – such as around black holes (see “General relativity at 100: The paradox of black holes“). Efforts to establish a quantum theory of gravity have stumped many physicists over the past century. Einstein himself became extremely unproductive in his later years as he sought such a “theory of everything“.
To understand why, we must start with a fundamental tenet of quantum physics. Heisenberg’s uncertainty principle embodies the fuzziness of the quantum world. It allows particles, such as electrons or photons of light, the equivalent of an interest-free loan: they may borrow energy from empty space and use it to make mass, according to Einstein’s famous equation E = mc2. This mass takes the form of short-lived “virtual” particles. The only caveat is they must pay this energy back – the particles must disappear once again – before anyone asks any questions. The more energy they borrow, the quicker this must happen.
Given such freedom, one can imagine an electron, photon or any other particle going to town, taking out many zero-interest loans in succession. As a result, calculating even a prosaic quantum process – an electron travelling from left to …
