Are virtual particles for real?

The universe is apparently filled with particles and anti-particles dancing into and out of existence. Are they for real? In this week's "Ask a Physicist," we'll find out.

Several readers took my previous Crackpot column as an invitation. To you, I say, "bravo!" If you want to send me a question about the universe in the hopes of internet fame or just to see if I'll read your anti-Einstein manifesto, please email me.

This week's much more coherent question comes from Alex Andreachi, who asks:

Virtual particles coming "in and out of" existence sounds contradictory to the basic law that energy can not be created or destroyed. I am hoping you can do your thing and explain how virtual particles work, what they really are, and how it is similar to quantum fluctuations.

I've said it before and I'll say it again. One of the problems with living in a quantum universe is that everything's just so uncertain. Those of you familiar with The Heisenberg Uncertainty Principle probably recall that it has something to do with not being able to measure the position and momentum of a particle to better than a particular combined uncertainty. Measure the position well, and you'll be very uncertain about the momentum, and vice-versa.

But there's another part to the Uncertainty Principle, and one that makes a mockery of the conservation of energy: You can create particles for a short while, but the more massive they are, the shorter they'll last. To a physicist, mass and energy are interchangeable, so this is basically the same as saying that you can have an uncertainty in energy, and the greater the uncertainty, the shorter the time that you're uncertain about whether the particle even exists.

Think of it like the fractions of pennies in Superman III or Office Space. The roundoff error is so small that nobody would notice, and it's all supposed to cancel out in the end. So on the whole, energy conservation will work out eventually, after microscopic fractions of a second. However, it turns out that black holes and other systems can be the Richard Pryors of virtual particles - perhaps less coke-fueled, but equally manic.

Are virtual particles for real?

What does it mean to be virtual, anyway?

Go back and read Alasdair's excellent Field Guide to Subatomic Particles. Among other things, you'll learn that the photon (aka light) is the mediator particle for the electromagnetic force. The idea is that two charged particles, electrons for example, constantly exchange "virtual photons" between them, telling the electrons to repel one another.

There's a sense in which this is just a math trick. You don't actually get to see the photons, so who's to say they're real? (A: I do. But give me a few minutes) To figure out the overall interaction between the electrons, we sum up the effects of an infinite number of possible photon configurations. Not only are there infinitely many possibilities, some of them don't even seem to make sense.

Photons are supposed to be massless, and so they travel at the speed of light (of course). But in the calculations, there are small contributions from photons that have mass, and others that effectively have a negative mass. Electrons, gluons, W bosons, you name it, show up in these calculations, and whenever it comes time to include their mass component, the "normal" mass turns out to be just a good suggestion, not a hard and fast rule. These gross violations can be forgiven because they're only virtual. At the end of the calculations, all of the particles are their regular masses once again and all the virtual particles drop back into the vacuum where they belong.

And it's not just in force calculations, either. Even in the so-called vacuum, virtual particles pop into and out of existence all the time, but they do so in pairs. At this very moment and all around you, electrons and their anti-particles, positrons are being created and, about a billion-trillionth of a second annihilated again. The same is true (with even shorter lifetimes) for virtually every other possible set of particle/anti-particle pairs.

That's what the equations say, but if virtual particles are only in the equations to account for the difference between the "before" and "after," how can we say they're even real at all?

Here's how.

Throughout it all, you might be tempted to think that they really are just a math trick. They don't last very long, and presumably since they always come in pairs, the positive charges will always cancel the negative ones, so it's hard to imagine how we'd detect them. But virtual particles, or at least the net effect of them, really can be detected, at least indirectly.

One of the most famous measurements supporting the idea that virtual particles really are real is the so-called Casimir Effect. Suppose you have a pair of conducting plates. Because they're conducting, you can't produce an electric field inside them. As a result, only certain wavelengths of "virtual photons" are allowed to exist. Net-net, this means that there is a lower energy density between the plates than outside, and the two plates are pushed together, because the universe really likes to make energy as low as possible. Hendrik Casimir actually measured this in 1948, but I warn you not to try it at home, since the effect is very small and requires a serious vacuum.

Are virtual particles for real?

What's more, if these virtual particles are real, then gravity should "see" them. After all, gravity is supposed to respond to all of the energy in the universe. This is one possible explanation for "Dark Energy." However, as I explained in a previous article doing the most simple-minded version of this calculation yields a result that's about 10100 times larger than the dark energy that cosmologists actually measure. This is not a small problem.

And they don't have to stay virtual forever.

Let me wax philosophical for a moment, if you don't mind. Somewhere in the heart of the Andromeda galaxy, about 2.5 million light-years away, there's an electron. There are a huge number, of course, but bear with me. The electron creates an electric field, which means that it exchanges virtual photons, including some that travel through space for 2 1/2 million years, eventually to be absorbed by an electron here on earth. Was the photon really virtual the whole time? Well yeah, I suppose it is. The only reason that virtual particles are considered virtual is because they're created and destroyed before we can measure them.

Are virtual particles for real?

But even if they start virtual, they don't have to end up that way. Let's deal with a more concrete example: black holes.

Suppose you're hanging out near the edge of a black hole for some reason. All of a sudden - poof! - an electron and a positron come into existence out of nowhere. "No big deal," you think to yourself, "They'll be gone soon enough." But as luck would have it, the electron falls beneath the event horizon of the black hole and is gone forever. The positron, (like every malfunction on the holodeck ever), goes from being virtual to real, and flies off into the cosmos. This, as all good io9 readers probably know, is the origin of Hawking radiation. I should point out that Hawking radiation has not been detected yet, but we're pretty confident about it.

A variant of this may also have happened in the very early universe. In the tiny fraction of a second after the big bang, particles pairs were created constantly. But during the period of "Inflation" (which I'd love to talk about in a future column, if only somebody would ask), the universe exploded in size, and particles which were initially near one another quickly became separated by such huge distances that they couldn't possibly recombine. These particles, too, went from virtual to real, and are, incidentally, the origin of all structure today.

In a sense, you yourself are virtual particles that have become real. Good for you!

Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (follow us on twitter, facebook, twitter or our blog.) He is an Associate Professor of Physics at Drexel University. Feel free to send email to askaphysicist@io9.com with any questions about the universe.