Here’s what you’ll learn when you read this story:

• Seemingly empty vacuums are actually teeming with perturbations known as virtual particles that are nearly impossible to detect.
• Virtual particles vanish quickly, but by smashing protons at ridiculously high speeds, researchers were able to give them an energy boost to last longer.
• After they conglomerated into larger particles, the spin of the virtual particles made it possible for the researchers to trace them back to the vacuum.

If there weren’t any planets, stars, comets, asteroids, or other cosmic debris flying around, there would be merely the vacuum of space, an infinite expanse of nothing. Or at least, it would seem logical to think that space is totally empty in the absence of matter—but what if even a void isn’t as empty as it seems?

Welcome to the quantum vacuum. What seems to be total emptiness is actually full of bizarre subatomic particles that elude observation. These are virtual particles, not actual particles of matter, but disturbances in the vacuum caused by the presence of other particles. They materialize out of nowhere and are so ephemeral that they vanish in tiny fractions of a second. Because virtual particles can’t be observed directly, the only way to detect them is through their interactions with other particles, which affect measurable properties such as particle mass and the forces that get exchanged between two particles. Virtual particles are important to scientists because they provide a window on fundamental forces of the universe: the strong and weak nuclear forces and electromagnetism.

These strange particles aren’t exactly particles, and they can’t be directly observed, a paradox rooted in quantum mechanics. The energy-time uncertainty relation, a consequence of Werner Heisenberg’s famous Uncertainty Principle, allows fleeting fluctuations in energy, meaning particle-antiparticle pairs can briefly pop into existence from the vacuum before vanishing again. Using the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory on Long Island, a research team led by physicist Zhoudunming Tu found evidence of such virtual quarks and antiquarks—the building blocks of protons and neutrons—by analyzing the particles’ spin orientations, which revealed signatures consistent with their origin in vacuum fluctuations.

“The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields and a condensate of virtual quark-antiquark pairs,” Tu said in a study recently published in Nature. “High-energy proton–proton collisions could liberate virtual quark–antiquark pairs from the vacuum that subsequently…form hadrons.”

Using the RHIC, Tu’s team smashed protons together at close to the speed of light, releasing immense amounts of energy in the process. That energy was absorbed by virtual quark-antiquark pairs— fleeting fluctuations that normally appear and vanish undetected in the vacuum—transforming them into real, detectable particles. Specifically, the collisions produced pairs of so-called strange quarks and strange antiquarks, which share the same mass but carry opposite charges. Because each pair originated from a single vacuum fluctuation, the quark and antiquark emerged quantum-entangled, meaning their properties remained correlated regardless of how far apart they traveled. The team confirmed this entanglement using RHIC's Solenoidal Tracker (STAR) detector, which showed that the quarks and antiquarks in each pair consistently spun in the same direction—a telltale signature that their shared origin in the vacuum had linked them at the quantum level.

Quarks are notoriously unstable. They can’t survive for long on their own, so they stick together with other particles to form lambda hyperons, electrically neutral subatomic particles composed of three quarks, one of which needs to be a strange quark. The spin of these hyperons is determined by the spin of the strange quark. Lambda hyperons are also unstable and start to decay after one ten billionth of a second, but the upside is that they decay into particles visible to STAR. How these particles spin is a direct reflection of the spin of the lambda hyperon they came from (and therefore the spin of the hyperon’s strange quark). In Tu’s experiment, quarks and their corresponding antiquarks from the vacuum kept spinning parallel to each other, as they had been before forming a hyperon.

By tracing pairs of quarks and antiquarks from their beginnings as virtual particles to their transition into real particles, it may be possible to finally figure out where protons get most of their mass, since quarks are so light that they only count for a small percentage. It’s thought that the bulk of proton mass is created by processes occurring inside the proton. Future research that includes virtual particles may finally be able to pull back the curtain on this mysterious mass generation.

“[We found a link between] the virtual spin-correlated quark pairs from the [vacuum] to their final-state hadron counterparts,” said Tu. “Our findings provide a new experimental model for exploring the dynamics and interplay of quark confinement and entanglement.”

The findings suggest that the universe’s most fundamental building blocks may owe their heft not to the particles themselves, but to the seething vacuum from which they spring. It turns out that “nothing” may be the most important something there is.