Physicists Just Caught Virtual Particles Becoming

Here's a question that's been annoying physicists for decades: Are virtual particles actually there, or are they just mathematical scaffolding we use to make the equations work?

It's not a small question. If you ask a quantum physicist what's happening in the space around you right now, they might tell you it's swarming with particles that pop into existence and immediately vanish. Virtual particles, they're called. Allegedly, they affect how electric fields behave around atoms—tiny, measurable effects that our equations predict correctly.

But measurable effects don't necessarily mean the thing causing them is real in the way you or I would use that word. You could argue—and many physicists do—that "virtual particles" is just convenient shorthand for complicated math. Shut up and calculate, as the saying goes.

Now researchers at Brookhaven National Laboratory claim they've done something different. They say they've watched virtual particles become real ones. And if they're right, it changes what "empty space" actually means.

The Experiment That Allegedly Settled It

The STAR collaboration at Brookhaven's Relativistic Heavy Ion Collider (RHIC) published their results in Nature. What they did was slam protons together at about 200 gigaelectronvolts—roughly one-tenth the energy of the Large Hadron Collider, so we're in medium-to-high territory for particle physics.

The strong nuclear force—the thing that holds atomic nuclei together—behaves strangely. Unlike gravity or electromagnetism, which weaken with distance, the strong force gets stronger the farther apart you pull particles. Physicist Sabine Hossenfelder, discussing the experiment, uses the metaphor of a stretchy string: "The more you stretch it, the more energy you need and the harder it gets to stretch it further."

That string, in this model, is full of virtual quark-antiquark pairs. When you slam protons together hard enough, you're essentially yanking quarks apart. And here's where it gets interesting: you can't isolate a single quark. Pump enough energy into separating them, and the string doesn't just stretch—it breaks. When it breaks, it creates a new particle-antiparticle pair. A virtual pair becomes real.

The researchers looked specifically at lambda particles created in these collisions. When lambdas decay, you can reconstruct their spin from the decay products. And the spin of the lambda pairs was correlated—meaning they shared a common origin.

"So this proves that the virtual particles were there and then became real," Hossenfelder explains. The correlation suggests these particles didn't just appear from nowhere. They transitioned from a virtual state to a real one.

The Interpretation Problem

Of course, proving something in physics depends heavily on what you mean by "prove" and "real."

You could still argue—and some will—that the correlation came from the quantum state itself, not from actual particles-in-waiting. That "virtual particles" is still just useful language for describing what the quantum field is doing. That nothing was actually there before the collision.

Hossenfelder is unimpressed by this objection: "I think this is nonsense because you could use the same logic to argue that real particles aren't really there. They're just convenient maths that we use to describe observations."

She has a point. If your standard for "real" is so strict that virtual particles don't qualify, you might have trouble defending the reality of the particles we already accept. The electron you're taught about in school is also, technically, an excitation in a quantum field described by math that matches observations. Where exactly do you draw the line?

"For I'm concerned, if the math describes observations correctly, that is what it means for something to be real," she says.

What Empty Space Actually Is

The philosophical debate matters because it connects to a bigger question: What is space?

If virtual particles are real, then the vacuum—what we think of as empty space—is not empty. It's seething with activity at scales we can't directly see. This has been theoretical orthodoxy for a while, but there's a difference between "our equations suggest this" and "we watched it happen."

But there's an even weirder possibility. Maybe space itself is the relationship between virtual particles. Maybe there's nothing more fundamental than the quantum fields and their excitations—virtual and real—and the structure we call "space" emerges from how they interact.

"I think physicists don't spend enough time thinking about nothing," Hossenfelder observes, and she's probably right. We've spent centuries studying what exists. The nature of what doesn't—or what exists in a probabilistic, fleeting, virtual sense—is harder terrain.

The RHIC experiment doesn't answer these deeper questions. It can't tell you whether space is made of virtual particles or whether virtual particles move through space. But it does push the "virtual particles are just math" position into an uncomfortable corner. The math now has a clearer physical interpretation.

The Hawking Connection

If this all sounds familiar, you might be thinking of Hawking radiation—the theoretical process by which black holes slowly evaporate. Hawking proposed that near a black hole's event horizon, virtual particle pairs get pulled apart by gravity. One falls in, one escapes, and the escaping particle becomes real.

No one has directly observed Hawking radiation. Black holes are far away and the effect is incredibly weak. But the RHIC experiment is mechanically similar: virtual pairs pulled apart by extreme forces (in this case, the strong nuclear force instead of gravity) until they become real.

It's not proof that Hawking was right about black holes. But it's evidence that the general mechanism—virtual particles becoming real under the right conditions—can actually happen.

The Relativistic Heavy Ion Collider, by the way, ceased operations in February. Brookhaven is building a replacement called the Electron-Ion Collider. You probably haven't heard much about it, which tells you something about how particle physics news gets covered. A collider shuts down, a new one gets built, and unless you're watching closely, it happens in near-silence.

But the questions these machines ask—What is real? What is space? What is nothing?—those don't go quiet just because the funding cycle does. The RHIC results suggest that the boundary between virtual and real is more permeable than we thought. Which means the next time someone tells you empty space is empty, you have experimental grounds to disagree.

—Nadia Marchetti, Unexplained Phenomena Correspondent