‘Quantum gravity’ could help unite quantum mechanics with general relativity at last
Scientists have determined a way to measure gravity on microscopic levels, perhaps bringing them closer to forming a theory of ‘quantum gravity’ and solving some major cosmic mysteries. Quantum physics offers scientists the best description of the universe on subatomic scales, while Einstein’s theory of general relativity brings about the best description of physics on huge cosmic scales. As robust and accurate as these two theories have become, they’ve refused to unite.
One of the primary reasons for this dilemma is that while three of the universe’s four fundamental forces – electromagnetism, the strong nuclear force and the weak nuclear force – have quantum descriptions, there’s no quantum theory of the fourth: gravity. But now an international team has made headway in addressing this imbalance by successfully detecting a weak gravitational pull on a tiny particle using a new technique. The researchers believe this could be the first tentative step on a path that leads to a theory of ‘quantum gravity’.
“For a century, scientists have tried and failed to understand how gravity and quantum mechanics work together,” Tim Fuchs, research team member and a scientist at the University of Southampton, said. “By understanding quantum gravity, we could solve some of the mysteries of our universe, like how it began, what happens inside black holes or uniting all forces into one big theory.”
It’s perhaps fitting that general relativity and quantum physics don’t get along; after all, Einstein was never comfortable with quantum physics. This is because while quantum physics has many counterintuitive aspects, he found one in particular very troubling. It was the notion of entanglement. Entanglement has to do with coordinating particles in such a way that changing the properties of one particle instantly alters the properties of an entangled partner particle, even if the partner is located on the opposite side of the universe. Einstein called this ‘spooky action at a distance’ as it challenged the concept of local realism.
Local realism is the idea that objects always have defined properties and that interactions between those objects are limited by distance and the speed of light, a universal speed limit introduced by Einstein as the foundation of special relativity. Special relativity is the theory that led to the formulation of general relativity in the first place. Yet despite Einstein’s protestations, scientists have indeed proven that entanglement and other counterintuitive aspects of quantum physics are truly factors of reality at subatomic scales.
Such proof has been achieved with a multitude of pioneering experiments. Fuchs and his colleagues, for instance, are following in the footsteps of physicists such as Alain Aspect, John Clauser and Anton Zeilinger, who won the 2022 Nobel Prize in Physics for experimentally verifying the non-local nature of entanglement. In their new quantum experiment, the researchers – including scientists from Southampton University, Leiden University and the Institute for Photonics and Nanotechnologies – used superconducting magnetic ‘traps’ to measure the weak gravitational pull on the smallest mass anyone has ever attempted to investigate in this way. The tiny particle was levitated in the superconducting trap at temperatures of around -273 degrees Celsius (-459.4 degrees Fahrenheit), which is just a few hundredths of a degree above absolute zero, the hypothetical temperature at which all atomic movement would cease. This frigid temperature was needed to limit the vibrations of the particles to their very minimum. With this method, the team ultimately measured a gravitational pull of 30 attonewtons on the particle.
Attonewtons represent a measure of force. To give you an idea of how tiny the gravitational force on the studied particles was, one newton is defined as the force needed to provide a mass of one kilogram with an acceleration of one metre per second – 30 attonewtons is equivalent to 0.00000000000000003 newtons. “Now we have successfully measured gravitational signals at the smallest mass ever recorded, it means we are one step closer to finally realising how it works in tandem,” Fuchs said. “From here we will start scaling the source down using this technique until we reach the quantum world on both sides.”