It Might Be Possible to Detect Gravitons After All

Einstein proposed a solution in 1905: A wave of light is made of many discrete units called “quanta,” each with energy related to the wave’s frequency. The higher the frequency of the wave, the more energetic its quanta. And the brighter the wave, the more quanta there are. If you try to start an electric current in a metal plate with low-frequency red light, you’ll be no more successful than if you tried to knock over a refrigerator with ping-pong balls; no number would suffice. But using higher-frequency blue light is like switching to boulders. Each of those units has enough oomph to excite an electron, even in dim light with very few of them.

Einstein’s theory was met with skepticism. Physicists felt fiercely protective of James Clerk Maxwell’s then-40-year-old theory of light as an electromagnetic wave. They had seen light refracting, diffracting, and doing all the things waves do. How could it be made up of particles?

Even after Einstein won the 1921 Nobel Prize in Physics for his theory of the photoelectric effect, debate continued among physicists. The effect suggested that something is quantized; otherwise there wouldn’t be a minimum threshold required to get electrons flowing. But some physicists, including Niels Bohr, who is considered one of the founders of quantum theory, continued to explore the possibility that only the matter was quantized, not the light. Today, this type of theory is called “semiclassical” because it describes a classical field interacting with quantized matter.

To see how a semiclassical theory can explain the photoelectric effect, imagine a kid on a swing. They’re kind of like an electron in a metal. They have a ground state (not swinging) and an excited state (swinging). A classical wave is like giving the kid a series of pushes. If the pushes come at some random frequency, nothing happens. The kid might bounce around a little, but they will basically stay in their ground state. It’s only when you push with just the right frequency — the swing’s “resonant” frequency — that the kid accumulates energy and starts swinging. (Electrons in a metal are a little different; they resonate with a whole continuous “band” of frequencies instead of just the one. But the upshot is the same: Any wave below that frequency band does nothing, whereas any wave in that frequency band excites electrons and makes a current flow.)

Einstein was eventually vindicated, but not on the strength of the photoelectric effect alone. Later experiments that collided electrons and photons like projectiles found that momentum, too, came in chunks. This research eventually ruled out the main alternative — a semiclassical theory of light and matter from Bohr and his collaborators. In 1925, seeing the data, Bohr agreed to “give our revolutionary efforts as honorable a funeral as possible” and welcomed light into the quantum fold. Light quanta became known as photons.

Few doubted the photon after 1925, but physicists are nothing if not thorough. Just because no one could think of a viable semiclassical theory didn’t mean one didn’t exist. The final proof that photons are real came in the late 1970s, when quantum optics researchers showed that light arrived at a detector in a pattern no semiclassical theory could mimic. The experiments were akin to firing a photon gun once per second and confirming that the detector clicked once per second in response. The photon wars ended with a whimper.

“There were just mountains of evidence that this photon concept was useful and vital,” Wilczek said.

The Graviton Wars Begin

In August of 2023, Daniel Carney and his collaborators fired the first shot in a new war.

It started when Carney’s colleague Nicholas Rodd had an insight similar to Pikovski’s, about a possible way to detect a graviton. “We got super pumped,” said Carney, a physicist at Lawrence Berkeley National Laboratory.

But when he and his collaborators dug into the literature, they uncovered the messy history of the photon, and the lengths to which quantum optics researchers had gone in the 1970s to close the final loopholes. They translated those more stringent tests into the gravitational context and found that Dyson had been right. Really proving quantumness by detecting lone gravitons one after another — as opposed to plucking one out of a tsunami, in the style of Pikovski’s proposal — would indeed take planetary-scale machinery.