Physicists at the University of British Columbia have conducted experiments which successfully modelled an as yet untestable quantum phenomenon.
Quantum mechanics, often described as the ‘spooky’ behaviour of the smallest states of matter, does not align with general relativity, Einstein’s theory which describes the way everything else in the universe interacts.
However, a different theory, quantum electrodynamics, successfully describes how light and matter interact at the quantum level. It's considered the most precise and well-tested theory in all of physics, and it was pioneered by physicists like Julian Schwinger.
New research, led by Dr. Philip Stamp, concerns the Schwinger effect - a quantum electrodynamic phenomenon that has long been considered impossible to observe directly.
The Schwinger effect predicts the spontaneous creation of matter from a vacuum. In a strong enough electric field, it is theorised that a virtual pair of particles can gain enough energy to become real particles.
This counterintuitive idea suggests a vacuum isn't truly empty; it's filled with fluctuating virtual particle-antiparticle pairs - which is to say that the possibility of the particle pairs exists.
"In the popular imagination, I guess empty space is empty space. There's nothing there," Dr. Stamp said. "The two great theories of physics, upon which all of physics is founded, quantum mechanics and general relativity, actually have a different point of view... in quantum mechanics, not only is the physical state of a system not definite, but the vacuum itself is this roiling sea of energy and fluctuations."
Crucially, the Schwinger effect relies on an electrical field far exceeding what any modern lab can produce. These fields would provide the energy to create the particles and then immediately pull them apart, preventing them from annihilating each other.
To overcome this challenge in a practical setting, the researchers worked with an analog physical system: superfluid helium. What’s special about this substance is that it exhibits zero-resistance flow at extremely low temperatures.
Essentially, within a thin film of this fluid, they theorised that the flow itself could act as a substitute for the electric field, leading to an equivalent phenomenon.
"The superfluid behaves like a vacuum," Dr. Stamp explained. "You can imagine that there are these incipient vortex anti-vortex pairs, and you can pull them apart. Now, with the Schwinger effect, you have to apply these monstrous electric fields, but in the superfluid, all you have to do is apply an external flow."
This external flow, when strong enough, would cause the spontaneous creation of vortex-antivortex pairs - microscopic whirlpools spinning in opposite directions - from the "nothingness" of the superfluid vacuum.
The team's research was not initially motivated by the Schwinger effect, but rather by the desire to study superfluid helium itself. However, in pursuing the mathematical framework for the superfluid model, they made a shocking discovery about the Schwinger effect itself.
"We realized that actually the Schwinger calculation is not correct," Dr. Stamp stated. "We realized that just as the vortices change their mass as they pull apart, so would electrons and positrons. And that changes the calculation of the Schwinger effect."
This finding challenges a 70-year-old pillar of quantum electrodynamics. Dr. Stamp and his collaborators showed that the effective mass of the vortices (and analogously the particles) is not constant as they are pulled apart, and that the creation of particles is inhibited by interactions with surrounding "quasi-particles," effects that Schwinger’s original calculation did not account for.
The research is a testament to the power of using physical analogs to explore concepts that are otherwise inaccessible. While acknowledging that there are "many, many differences" between a superfluid and the vacuum of deep space, Dr. Stamp is optimistic about this parallel between superluids and quantum mechanics.
Looking ahead, the team is focused on further investigating the variability of effective mass in both vortices and electron-positron pairs, and on the broader implications for the instability of the vacuum. The work could even provide insights into the behavior of spacetime near black holes.
