There are a few physics terms floating around in the world that are deceptive little buggers. These jargon phrases seem to succinctly describe a topic, encapsulating a complex process or interaction into a tidy, easily digestible nugget of information. But they're liars. The concepts they're intended to communicate are actually radically different from what the jargon would suggest.
Take, for example, "virtual particles." The term is supposed to answer a very old question: How, exactly, do particles interact? Let's say we have two charged particles, and let's call them Charles and Charlene. Let's continue to say that both Charles and Charlene are negatively charged. Maybe they're electrons; maybe they're muons. Doesn't matter. What matters is that if Charlene comes racing toward Charles, they bounce off each other and end up going their separate ways. [5 Mysterious Particles Lurking Underground]
How did that bounce happen? What made it possible for Charles and Charlene to communicate with each other so that they knew to head in a new direction when the collision was all said and done?
This is a fantastically basic question, so it seems that if we could satisfactorily answer it, we could unlock Deep and Important Mysteries of the Universe.
The modern perspective of quantum field theory recognizes photons — bits of light — as the carriers of the electromagnetic force. Charles and Charlene are charged particles, so they interact with light. But obviously, Charles and Charlene aren't shooting lasers at each other, so the trite explanation for their brief dalliance is that "they exchange virtual photons."
What in the name of Feynman's ghost does that mean?
Enter the field
Let's take a step back. Back in the olden-days (i.e., the 19th century) view of physics, each charged particle generates an electric field, which is basically an instruction sheet for how other particles can interact with it. In the case of a particle, this field is strong nearby the particle and weaker farther out. That field also points out in every direction away from the particle. [The 9 Biggest Unsolved Mysteries in Physics]
So our Charles particle produces a field that permeates all of space. Other particles, like Charlene, can read this field and move accordingly. If Charlene is super-duper far away from Charles, the field she reads has very, very small numbers, so she barely notices any effect from Charles. But when she gets close, her field reader goes off the charts. Charles' electric field is very clearly saying "GO AWAY," and she obliges.
In this view, the field is just as real and important as the particle. The universe is full of stuff, and the fields tell that stuff how to interact with other stuff.
Revenge of the field
In the early to mid-20th century, physicists realized that the universe is a much, much stranger place than we had imagined. Marrying special relativity with quantum mechanics, they developed quantum field theory, and let's just say the results weren't what anybody expected.
As the name suggests, the field got a promotion. Instead of just being the bookkeeping device that showed how one particle should interact with another, it became — and here come some italics for emphasis — the primary physical object. In this modern, sophisticated view of the universe, the electron isn't just a lonely particle. Oh no. Instead, there's an electron field, permeating all of space and time like milk in French toast.
This field is it — it's the thing. Particles? They're just pinched-off bits of that field. Or, more accurately, they're excitations (like, wiggles) of the field that can travel freely. That's important, and I'll get back to it soon.
A particle party
Here's where things start to get fuzzy. A particle traveling from one spot to another doesn't exactly stay a particle, or at least not the same kind of particle.
Let's go back to Charles, the charged particle. Since he's charged, by definition he interacts with light, which is the electromagnetic field. So wiggles in the electron field (a field made up of electrons) can affect wiggles in the electromagnetic field. So, literally, as Charles zips around, he spends some of his time as an electron-field wiggle and some of his time as an electromagnetic-field wiggle. Sometimes he's an electron, and sometimes he's a photon — a bit of the electromagnetic (EM) field!
It gets worse. Way worse. Charles-turned-EM-wiggle can become other wiggles, like muon wiggles. For every fundamental particle in the universe, there's a corresponding field, and they all talk to one another and wiggle back and forth constantly.
The summation of all the wiggles and sub-wiggles and sub-sub-wiggles add up to what we call "an electron traveling from one spot to another." [Video: "What does it mean for particles to exist?"]
It all becomes really nasty mathematically very quickly, but folks like physicist Richard Feynman came up with handy tricks to get some science work done.
Now, after tons of backstory, we can get to the main question. The fields wiggle to and fro (and sometimes fro and to). If the wiggles persist and travel, we call them "particles." If they die off quickly, we call them "virtual particles." But fundamentally, they're both wiggles of fields.
When Charles encounters Charlene, they're not like two little bullets ready to slam into each other. Instead, they're complicated sets of wiggles in all sorts of fields, phasing in and out from one type of field to another.
When they do get close enough to interact, it's … messy. Very messy. Wiggles and counter-wiggles, a frenzied mishmash of intermingling. The machinery of quantum field theory — after many tedious calculations — does indeed provide the correct answer (Charles and Charlene bounce off each other), but the details are headache-inducing. [I break it down in more detail in this video.]
So, the shorthand — "they exchange virtual particles" — rolls off the tongue quite easily, a little slip of jargon to package up a very complicated process.
But, unfortunately, it's not very accurate.
Learn more by listening to the episode "What are virtual particles?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to @TanyaDavis, @AstroMatt99, Tomas A., Rae N., and many others for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutterand facebook.com/PaulMattSutter.