Rick Robinson

Nov 29th 2020

When Quantum Mechanics and Relativity Collide


The two greatest achievements of modern physics — quantum mechanics and Einstein’s theory of general relativity — share one serious problem: They don’t agree. As a result, physicists find themselves in the awkward position of seeking to explain the universe using two theories that are not quite able to explain or account for each other.

In fact, these two theoretical frameworks differ not just in their abstruse mathematics, but in the basic ways in which they view the world. As puts it, general relativity treats the universe as fundamentally smooth and curved. What looks like sharp edges are simply zones of more rapid change, like musical notes recorded on tape or old-fashioned vinyl.

In contrast, quantum mechanics treats the universe as essentially lumpy and sharp-edged, like a digital music recording. Even transitions that appear smooth, if closely examined, resolve into a series of tiny but abrupt changes.

The Large and Small of It

Most of the time, these very different approaches to nature do not get in each other’s way. Einstein’s relativity theory, in spite of its popular association with nuclear energy, is mainly used to explain how the universe works at large scale: the physics of baseballs, planets, galaxies. Quantum mechanics, in contrast, deals with the very small scale: atomic nuclei and smaller, down to the most fundamental building blocks of matter, quarks.

The phenomenon we call gravity exemplifies the tremendous effects of scale. Observed on a large scale, gravity is the most powerful force in the universe. In fact, it shaped the universe, causing the expanding gas left over from the Big Bang to form into galaxies, and galaxies in turn to form stars. Gravity also causes some of those stars to collapse into black holes, isolated from the rest of the universe by gravitational fields so strong that not even light can escape.

Yet gravity is also an astonishingly weak force, many orders of magnitude weaker than any of the other fundamental forces of nature, such as magnetism, that physicists have identified, according to PBS. Thus, a horseshoe magnet from a toy store can lift an iron nail, outpulling the entire gravitational force of the earth.

How can something so weak be so strong? For physicists trying to make sense of it all, this paradox is not the only troublesome thing about gravity. The other fundamental forces of nature: electromagnetism, and two nuclear-scale forces known as the weak and strong nuclear forces, all could be shown to work correctly under both relativity theory and quantum mechanics.

Misadventures in Renormalization

When the mathematical technique called renormalization is used to demonstrate how this unity is applied to gravity, it doesn’t work. In the quantum mechanics framework, gravity should be associated with theoretical particles called gravitons. But gravitons, by reshaping space, render renormalization calculations endlessly complicated, says

According to physicist Netta Engelhardt, quoted at, introducing gravitons into standard renormalization calculations “effectively means that you need an infinite number of experiments to determine anything. That’s not a realistic theory.”

Wanted: A Theory of Quantum Gravity

Physicists would like a more realistic theory — one that would allow useful experiments, so that they could test the theory to see if it actually works. They have not yet quite gotten there; the Stanford Encyclopedia of Philosophy succinctly describes the current state of quantum gravity theory as “under construction.”

But in the meanwhile, a look at the construction site suggests that quantum gravity could have some mind-bending consequences. According to Live Science, applying quantum gravity effects to the behavior of spaceships flying through powerful gravity fields could reverse cause and effect — so that an event triggers an earlier event, which can in turn trigger the original event.

If you can re-read that without your head exploding, you might have a big future in theoretical physics.

And the weirdness keeps coming. According to one cutting edge theory, reports Science Alert, space-time does not exist on its own, but is the product of quantum forces shaking themselves out, like miso soup settling after it is poured into a bowl.

At the frontier of modern physics, there are no textbook answers you can look up in the back of the book. Cutting-edge science is all about finding out what those answers might be.