Anyone with a high-school diploma knows about Newton's law of universal gravitation: it's that the gravitation force behaves like 1 over the distance squared as you separate from a gravitating mass. And these days most everybody has also heard of quantum mechanics, thanks to the advent of quantum computing, which even Canada's prime minister is able to explain.
The fascinating behaviour of quantum systems is that they can basically allow something to be in two (or more) states at the same time. A massive particle, for instance, can be put in two places simultaneously. This is not science fiction: atom interferometers routinely put single atoms of caesium or rubidium into configurations where the single atom quantum state is split between two places separated by as much as several centimetres.
Such states are very sensitive to gravitation, giving rise to the most precise measurement scientists have been able to make of the earth's gravitational field, one part in 10^15, for instance. But what gravitational field does a massive particle create in such a quantum state?
To find out, Université de Montréal particle physicists Richard MacKenzie and Manu Paranjape have been working since 2012 with their colleague Urjit Yajnik at the Indian Institute of Technology Bombay, in Mumbai, India, supported by a Cooperation Québec-Maharashtra grant of Quebec's Ministère des relations internationales et de la Francophonie.
Along with many students and other collaborators, they have produced a large body of research in theoretical particle physics, and their most recent study, just published in Physical Review Letters , the journal of the American Physical Society, asks the question: "What is the gravitational field of a mass in a spatially non-local quantum superposition?" Their surprising finding: seeming not to know that a massive particle is split between two places at the same time, the gravitational field appears to be emanating from only one place, given the average position of the massive particle.
To reach that conclusion, the scientists thought about scattering experiments by other particle physicists, for example in the Large Hadron Collider, that over the last century have probed the interior of atoms, nuclei and other sub-atomic particles. The deep inelastic scattering experiments done in the late 1970s also demonstrated the existence of quarks inside the nucleus and confirmed the theory of strong interactions, called quantum chromodynamics.
