Back in the 1920s, when the field of quantum physics was still in its infancy, a French scientist named Louis de Broglie had an intriguing idea.
In response to confusion over whether light and matter were fundamentally particles or waves, he suggested an alternative: what if both were true? What if the paths taken by quantum objects were guided by something that rose and fell like an ocean swell?
His hypothesis was the foundation of what would later become pilot wave theory, but it wasn't without its problems. So, like any beautiful idea that falters in the face of experiment, it swiftly became a relic of scientific history.
Today, the majority of physicists subscribe to what's referred to as the 'Copenhagen interpretation of quantum mechanics', which, generally speaking, doesn't give precise locations and momentums to particles until they're measured, and therefore observed.
Pilot wave theory, on the other hand, suggests that particles do have precise positions at all times, but in order for this to be the case, the world must also be strange in other ways which led to many physicists dismissing the idea.
Yet something about De Broglie's surfing particles makes it impossible to leave alone, and over the past century, the idea continues to increasingly pop up in modern physics.
For some, it's a concept that could finally help the Universe make sense from the tiniest quantum particles to the largest galaxies.
To better understand what a pilot wave is, it helps to first understand what it is not.
By the 1920s, physicists were baffled by highly accurate experiments on light and subatomic particles, and why their behavior seemed more like that of a wave than a particle.
The results were best explained by a new field of mathematics, one that incorporated probability theory with the mechanics of wave behavior.
To theoretical physicists like Danish theorist Niels Bohr and his German colleague Werner Heisenberg, who set the foundations of the Copenhagen interpretation, the most economical explanation was to treat probability as a fundamental part of nature. What behaved like a wave was an inherent uncertainty at work.
This isn't merely the kind of uncertainty a lack of knowledge brings. According to Bohr, it was as if the Universe was yet to make up its mind on where to put a particle, what direction it should be twisting, and what kind of momentum it might have. These properties, he maintained, can only be said to exist once an observation has been made.
Just what any of this means on an intuitive level is hard to say. Prior to quantum physics, the mathematics of probability were tools for predicting the roll of a dice, or the turning of a wheel. We can picture a stack of playing cards sitting upside down on a table, its hidden sequence locked in place. Mathematics merely puts our ignorance in order while reality exists with 100 percent certainty in the background.
Now, physicists were proposing a flavor of probability that wasn't about our naivety. And that isn't as easy to imagine.
De Broglie's idea of a hypothetical wave was meant to return some kind of physicality to the notion of probability. The scattered patterns of lines and dots observed in experiments are just as they seem consequences of waves rising and falling through a medium, little different to a ripple on a pond.
And somewhere on that wave is an actual particle. It has an actual position, but its destiny is in the hands of changes in the flow of the fluid that guides it.
On one level, this idea feels right. It's a metaphor we can relate to far more easily than one of a dithering Universe.
But experimentally, the time wasn't right for de Broglie's simple idea.
"Although de Broglie's view seems more reasonable, some of its initial problems led the scientific community to adopt Bohr's ideas," Paulo Castro, a science philosopher at the University of Lisbon in Portugal, told Science Alert.
Eminent Austrian physicist Wolfgang Pauli, one of the pioneers of quantum physics, pointed out at the time that de Broglie's model didn't explain observations being made on particle scattering, for example.
It also didn't adequately explain why particles that have interacted with one another in the past will have correlating characteristics when observed later, a phenomenon referred to as entanglement.
For around a quarter of a century, de Broglie's notion of particles riding waves of possibilities remained in the shadows of Bohr's and Heisenberg's fundamental uncertainty. Then in 1952, the American theoretical physicist David Bohm returned to the concept with his version, which he called a pilot wave.
Similar to de Broglie's suggestion, Bohm's pilot wave hypothesis combined particles and waves as a partnership that existed regardless of who was watching. Interfere with the wave, though, and its characteristics shift.
Unlike de Broglie's idea, this new proposal could account for the entangled fates of multiple particles separated by time and distance by invoking the presence of a quantum 'potential', which acted as a channel for information to be swapped between particles.
Now commonly referred to as the de Broglie-Bohm theory, pilot waves have come a long way in the decades since.
"The new main hypothesis is that the quantum wave encodes physical information, acting as a natural computation device involving possible states," says Castro.
"So, one can have whatever superposition of states encoded as physical information in the tridimensional wave. The particle changes its state to another by reading the proper information from the wave."
Philosophically speaking, a theory is only as good as the experimental results it can explain and the observations it can predict. No matter how appealing an idea feels, if it can't tell a more accurate story than its competitors, it's unlikely to win over many fans.
Pilot waves fall frustratingly short of contributing to a robust model of nature, explaining just enough about quantum physics in an intuitive way to continue to attract attention, but not quite enough to flip the script.
For example, in 2005 French researchers noticed oil droplets hopped in an odd fashion across a vibrating oil bath, interacting with the medium in a feedback loop that was rather reminiscent of de Broglie's wave-surfing particles. Critical to their observations was a certain quantization of the particle's movements, not unlike the strict measurements limiting the movements of electrons around an atom's nucleus.
The similarities between these macro scale waves and quantum ones were intriguing enough to hint at some kind of unifying mechanics that demanded further investigation.
Physicists at the Niels Bohr Institute in the University of Copenhagen later tested one of the quantum-like findings made on the oil drop analogy based on their interference patterns through a classic double slit experiment, and failed to replicate their results. However, they did detect an 'interesting' interference effect in the altered movements of the waves that could tell us more about waves of a quantum variety.
In a remarkable act of serendipity, Bohr's own grandson a fluid physicist named Tomas Bohr also weighed in on the debate, proposing a thought experiment that effectively rules out pilot waves.
While null results and thought experiments hardly disprove the basic tenets of today's version of de Broglie-Bohm's pilot waves, they reinforce the challenges advocates face in elevating their models to a true theory status.
"The wave quantum memory is a powerful concept, but of course, there is still a lot of work to be done," says Castro.
It's clear there's an aching void at the heart of physics, a gap begging for an intuitive explanation for why reality rides wave-like patterns of randomness.
It's possible the duality of waves and particles has no analogy in our daily experience. But the idea of a wave-like medium that acts as some kind of computational device for physics is just too tempting to leave alone.
For pilot wave theory to triumph, though, physicists will need to find a way to pluck a surfer from its quantum wave and show the two can exist independently. Experimentally, this could be achieved by emitting two particles and separating one from its ride by measuring it.
"Then we make this empty quantum wave interfere with the wave of the other particle, altering the second particle's behavior," says Castro. "We have presented this at the first International Conference on Advances in Pilot Wave Theory."
Practically speaking, the devices required to detect such an event would need to be extremely sensitive. This isn't outside of the bounds of feasibility, but it is a task patiently waiting for an opportunity. Empty pilot waves might even hold the key for solving practical problems in quantum computation by making the waves less prone to surrounding noise.
Future physicists could eventually land on observations that open us to a Universe that makes sense right down to its roots. Should experiments detect something, it'll be a solid indication that far from empty, the heart of physics beats with a pulse. Even when nobody's watching.
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