Gravitee, Gravitoe; What It Might Be We Just Don't Know

Ever since experiments revealed the quantum nature of much of the universe, most physicists have assumed that all of the four forces -- electromagnetism, the strong nuclear force, the weak nuclear force and gravity -- could eventually be "quantized." In simplest terms, they believed that all four forces would one day be explained by quantum theory.

After many decades of theories and experiments, three of the forces are indeed explained by quantum theory -- the idea that at its most basic level, everything is made up of infinitesimal particles that work together to create the effect of a field. The only force not playing along is gravity. There have been some supposin's about why this is the case.

One is that gravity, even though it plays a very important role in everyday life, is the weakest of all the forces. Waves in all the three forces had been discovered early in the history of this research. But although gravitational waves were first proposed in 1905, they are so weak they were not actually detected until 2015. As it is so weak, the idea goes, our ability to examine it to find out if it is quantized and how so is also very weak.

There are other reasons suggested for why we have not been able to develop what is usually called a "theory of quantum gravity." But some physicists, like Jonathan Oppenheim of University College London, have offered a much simpler reason that theorists can't quantize gravity: Gravity can't be quantized.

This possibility has been around as long as quantum theory has, but it's been exiled to the fringes because it requires reality to be explained by a mix of quantum theory and classical theory. But none of the thinking going on in quantum land allows that to be true, because quantum theory's new discoveries and experiments all hinged on the idea that quantum would replace classical Newtonian physics. It explained so many things so much better that it just had to explain gravity too.

But Oppenheim offers several conundrums about what things might look like if quantum gravity is ever discovered, none of which I understand well enough to relate with any confidence. We can just sum it up by saying he believes the universe has three quantum forces and one non-quantum force.

His suggestion is, of course, about as popular as a co-ed dorm would be to a dad with a daughter in 1962: Classical and quantum don't mix, and neither do my precious princess and those slavering Lotharios in button-downs and penny loafers.

It could be interesting to see how things happen. Gravity may not be a force like the other three. Some as-yet-unknown experiment could reveal the path to quantum gravity. Or something else entirely. The goal for a scientist -- despite the language they may use to title a paper -- is simplicity. A hybrid quantum-classical universe is not as simple as a fully quantized one, but it may be as simple as it gets.