Quantum physics paints a strange picture of a world filled with eerie connections, disturbing uncertainties, and—perhaps weirdest of all—particles spontaneously emerging from the void. These so-called virtual particles have indirect effects that scientists have measured before. But now, for the first time, researchers have directly tracked the evolution of these particles that came out of nowhere.
In a study published today nature, Physicists at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Long Island describe how they discovered pairs of subatomic particles with an uncanny correlation in their spin directions. Particle spin is a quantum property that can be up or down. Most groups of particles will have a random combination of up and down spins, but the researchers found that certain types of particles produced in colliders often come in pairs with matching spin directions.
Scientists believe that these particle pairs must be direct descendants of virtual groups of particles that spontaneously arise from the quantum vacuum.
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“The vacuum in quantum theory is not a vacuum,” says physicist Dmitry Khasev of Stony Brook University. “It’s a field full of virtual particles.” These particles are the result of Heisenberg’s uncertainty principle, which states that certain relevant properties, such as the energy and lifetime of a quantum state, cannot be known precisely at the same time. If a quantum state is very, very brief, its energy can be highly uncertain. This means that particle pairs—a particle and its antimatter partner—can briefly form by borrowing energy from nothingness.
Usually these particles disappear again almost immediately by annihilating each other, but this time it was different.
In RHIC, scientists smash protons together at nearly the speed of light, creating an astonishing explosion of energy. When a virtual particle pair happens to appear in the vacuum, it can commandeer the freely available collision energy, making it real. “When two particles collide at high energy, it gives the vacuum an energy boost,” said Brookhaven physicist Dunming (Kong) Du Zhou, one of the authors of the new study. “Now the virtual particles get a push without having to annihilate back into the vacuum.”
Physicists were able to track this process using a solenoid tracker on the RHIC (STAR) detector. However, the details of how they do it might make your head spin.
Because these new real particles originate as a pair, they are entangled and remain connected no matter how far apart they are. Therefore, when they fly apart after a collision, they have the same direction of rotation.
The experiment tracked pairs of “strange” quarks – cousins of the “up” and “down” quarks that make up protons and neutrons. Quarks themselves are not stable, so when new quarks appear, they quickly combine with other quarks to form aggregate particles called lambda hyperons. Instead of the proton’s two up quarks and one down quark, these exotic versions of the proton contain an up quark, a down quark, and a strange quark.
In turn, Lambda itself is unstable. They only last about 10–10 Second, they travel a few centimeters inside the collider and decay into more ordinary particles that STAR can see.
The direction of the momentum of these decaying particles reveals the spin of the lambda hyperon that produced them. A lambda’s spin is thought to be determined solely by the spin of its strange quark (because the spins of its up and down quarks cancel each other out).
When the researchers looked at their measurements, they were surprised by how correlated the particles were. “Their spins appear to be parallel,” said study co-author Jan Vanek, a physicist at the University of New Hampshire. “This suggests that we actually find these vacuum strange quark pairs in these lambda hyperons.”
The discovery confirms a prediction made 30 years ago by Kharzeev and colleagues that pairs of strange quark virtual particles must have parallel spins. “It’s exciting because you can come up with theoretical ideas in your head that seem reasonable, but you never know whether nature follows this,” he said. “So it’s very gratifying to see this finally being measured in a real experiment.”
This new window on virtual particles should help answer one of the great mysteries in nuclear physics: Where does the proton get its mass? The three quarks that make up the proton contribute only a tiny amount of mass—the remaining 99 percent is thought to result from interactions between these real quarks and virtual quark populations in the vacuum. “If we can trace a pair of quarks from a virtual particle to a real particle, maybe we can gain insight into how this mass is created through interactions with the vacuum,” Tu said.
The discovery also marks another achievement for RHIC as the collider prepares to shut down. Friday will be the last day of a 25-year streak of record-breaking collisions. Components of the machine will be reused in Brookhaven’s upcoming electron-ion collider, which is set to launch at the laboratory in the mid-2030s.