摘要：So-called virtual particles aren't particles at all. Some argue that they are merely mathematical figments, and that we need to find a better way to understand particle interactions
THE first thing you need to know is that virtual particles, which are impossible to avoid if you want to understand how the fundamental forces of nature animate matter, aren’t really particles at all. “The language makes people misunderstand,” says Matt Strassler, a theoretical physicist at Harvard University. The second is that they might not be proper physical constituents of the universe.
Let’s start with some basics. According to quantum theory, our best description of the subatomic realm, particles aren’t the infinitesimal snooker balls we tend to imagine, but excitations in underlying quantum fields. The Higgs boson is a spike in the underlying Higgs field, for instance, and electrons are spikes in the electromagnetic field.
These fields permeate the entire universe, but we can’t observe them directly. What we see are particles – clear disturbances in a field that persist over time and interact with other, similar disturbances to produce more particles.
Virtual particles are more subtle. So subtle, in fact, that although they can be thought of as disturbances in underlying fields, they don’t persist for long – and can’t be directly detected.
This is where things get confusing, because virtual particles do appear to affect the properties and behaviour of other particles in measurable ways. They seem to be emitted and absorbed by real particles when those particles interact, which is why we rely on virtual particles for our understanding of how three of the known fundamental forces – electromagnetism and the strong and weak nuclear forces – work.
Get to grips with the deepest layer of reality we know of with our inventory of the subatomic realm, from known particles like quarks and the Higgs boson to hypotheticals including the fifth force and strings in 11 dimensions
Another common misconception is that virtual particles appear out of nothing. “The usual story we tell is that a virtual particle is something that emerges from the vacuum, enjoying a brief, ephemeral existence before disappearing again,” says David Tong at the University of Cambridge. “While this is sometimes a useful analogy, I don’t think it captures what’s really going on.”
That is because nothing is created from nothing. “There is always energy involved in one way or another,” says Christine Aidala at the University of Michigan. A better way to think about it, she says, is to return to the idea that everything is made of fields – and recall that those fields interact with each other in complicated ways, creating ripples that constantly change.
The question of whether virtual particles are real or are merely a mathematical tool arises because they were invented – or discovered, depending on your point of view – when physicist Richard Feynman created squiggly line diagrams to render tractable the equations physicists use to describe particle interactions.
Do we observe virtual particles?
Today, some argue that virtual particles are real because we can observe the effects of them. But “what that really means is that I can detect a physical effect which can be calculated using the methods of virtual particles”, says Strassler. “You are never actually seeing the virtual particles.”
Nima Arkani-Hamed at the Institute for Advanced Study in New Jersey is similarly unconvinced. Feynman diagrams, as they are known, remain the best method we have to account for particle interactions, he says, but we shouldn’t confuse the maths we use to describe reality with reality itself.
In fact, Arkani-Hamed is among those trying to do away with virtual particles. He has been working on a radical new way to calculate particle interactions – by invoking an abstract mathematical object called an amplituhedron. Maybe one day, then, these ephemeral, endlessly confusing “particles” will disappear once and for all.