In the great classroom of physics — otherwise known as the universe — gravity doesn’t work or play well with the three other fundamental forces of nature: electromagnetism, the weak force and the strong force.
These three forces are enormously powerful but operate mainly over short distances. Most important to physicists, all three can be neatly explained by the theory of quantum mechanics.
Gravity is different. Physicists can’t quite wrap their heads around the graviton, the quantum particle of gravity that’s almost impossible to detect.
Needless to say, all of this makes gravity — and the search for the graviton — a core challenge of contemporary physics.
The Odd Force Out
On the one hand, compared to the other fundamental forces of nature, gravity is incredibly weak. The weak nuclear force makes nuclear bombs explode, while magnetism (which is much weaker) allows a toy magnet to pick up a steel needle against the whole gravitational force of the Earth.
But weak as it is, gravity dominates the universe because all mass, including mysterious dark matter, produces gravity. The force of gravity makes the Earth and other planets round. In fact, without gravity, there would be no objects in the universe, only an ever-thinner gas of individual particles.
All of which makes the search for a theory of quantum gravity a very big deal.
Black Holes and Unsolvable Messes
The core challenge for physicists is that gravitons are exceedingly difficult to detect because gravity is so weak. In fact, per Scientific American, detecting the graviton the old fashioned way, by observation of particle collisions in a classical atom smasher, is outright impossible. The device would need to be so big and massive that it would collapse into a black hole.
Even if it still worked, we’d never get the results.
There are other complications as well, as Space.com pointed out. Physicists working in quantum mechanics have come up with a mathematical tool called renormalization that can be used to gauge the interaction of quantum particles on the subatomic scale. But because gravity involves changing the shape of space, introducing gravitons to the renormalization equations makes them go haywire.
Physicist Netta Engelhardt said this “effectively means that you need an infinite number of experiments to determine anything,” which, she noted, is “not a realistic theory.”
The Cheshire Cat’s Grin
The Stanford Encyclopedia of Philosophy labeled the development of the physical theory of quantum gravity as “under construction.”
But physicists are still looking for clever ways that may allow us to confirm the existence of the graviton without ever actually detecting one.
As Quanta magazine described, this hat trick would work by showing that two microdiamonds have become entangled — linked together in their properties by quantum effects — as a result of the tiny gravitational force exerted between them. Physicist Miles Blencowe described it as detecting “the grin of the Cheshire cat,” proving that quantum gravity exists.
At the same time, reported Physics World, a research team is using the motions of the planets to pin down a maximum limit to the mass of the graviton. Observations of the solar system over the last 100 plus years show that gravitons must have a mass at most about a trillion trillion times less than the mass of a neutrino, the least massive particle we know about for sure.
Progress in science is hardly ever straightforward or easy. It is full of workarounds, hat tricks, and making the most of circumstantial evidence, which is what makes it so interesting.