Quantum Physics


Quantum Foam, Virtual Particles and Other Curiosities

Quantum physicists regularly ask you with a straight face to accept what seems to be complete nonsense. Particles are also waves; cats are alive and dead at the same time. But some of the most incredible creatures of the quantum realm get far less attention than Schrödinger’s famous cat. They’re called virtual particles, and they might be the reason the universe exists in the first place. In the pencast below, I’ll explain the basics of virtual particles. Then read on to learn more.

While the Big Bang theory explains how the universe has expanded and cooled since it began, it is quite silent on what “pulled the trigger,” so to speak. We simply don’t know what started the process. How there could be nothing at one moment and an entire baby universe the next?

It turns out that getting something from nothing is just business as usual for virtual particles. The most straightforward way to explain virtual particles is by an example. Consider a particle collision in which one electron hits another and the two scatter. In the classical view, the electric field from one electron interacts with the other and the two feel a repulsive force. However, this approach neglects Einstein’s Nobel Prize realization that light—and, by extension, every electromagnetic field—is quantized. So a quantum treatment of electron scattering needs to include not only the quantum nature of the electrons, but also the quantum nature of the photon. We now treat electron scattering as the two quantized electrons exchanging a quantized photon and, in the process, changing their directions.

So how do virtual particles enter in? Well, you can calculate the properties of the photon that must be emitted to scatter the electrons. Simple energy and momentum conservation considerations tell us what the energy and momentum of the photon must be. However, when you do the calculation, you find that the photon has a mass! Since photons are massless particles, this seems to invalidate the whole idea. It sure sounds like physicists are pulling your leg, just to see how long it will be before somebody is willing to say that the subatomic Emperor has no clothes.

As crazy as this seems, it is true. To see how, we need to invoke another hard-to-swallow axiom of quantum mechanics: the Heisenberg Uncertainty Principle, named after its inventor, Werner Heisenberg. In classical physics, energy and momentum are always conserved. But Heisenberg spotted a loophole in this rule: in the quantum realm, energy and momentum don’t have to be conserved, as long as the non-conservation doesn’t persist for very long. It’s kind of like having a shady accountant. If you audit the books, the amount of money you send him has to agree exactly with the amount of money he uses to pay your bills. But, while he has your funds, he is free to temporarily lend or borrow money so that momentarily he will have the “wrong” amount of money. Further, the larger the amount of money loaned or borrowed, the shorter the period of time it will occur. Similarly, in the quantum realm, energy and momentum can briefly be “wrong,” but the larger discrepancy, the shorter the period of time for which it is allowed.

So in our example of electrons scattered by exchanging photons, the photon can briefly have the “wrong” amount of energy and momentum. Now, it is understandable if you find this a bit hard to take; perhaps an instance of physicists making stuff up to save their theories. And, truth be known, that would be my reaction if there were not an extensive list of experimental measurements that demonstrate that virtual particles exist. In fact, virtual particles play a critical role in most of the experiments performed at large particle physics laboratories like CERN, Fermilab and many other similar facilities.

While I’ve described the idea of a single virtual particle, the idea is actually much richer than that. Virtual particles also exist in association with real particles. For instance, suppose you have an ordinary, garden-variety, electron. A reasonable mental image of the electron would be a little subatomic marble, carrying electrical charge, mass and spin. Anyone with even a cursory understanding of quantum mechanics know that image is a bit dodgy, as electrons exhibit lots of crazy quantum behavior.

The life of an electron is much more complex than that, though. In addition to the usual quantum craziness, where an electron is both a particle and a wave and the position of the electron is generally indeterminate, electrons are surrounded by virtual particles. For instance, an electron can briefly emit a photon. That photon will be reabsorbed quickly in such a way that the energy and momentum conservation laws aren’t violated. But it gets crazier than that. The virtual photon can also turn into a virtual electron/positron pair. Thus, for a brief moment, what was once just an electron becomes an electron plus an additional electron and positron. As long as the virtual particles coalesce before the universe notices, it’s all within the rules. Indeed an electron never exists as a single “bare” electron. Rather, it is always enshrouded in an ephemeral cloud of virtual particles, flickering in and out of existence, and vastly complicating what an electron “really” is.

It might seem far-fetched, but experiments can actually detect the presence of this cloud. That is because every electron acts like a mini-magnet. We can calculate exactly how strong the magnet should be. But when we make very precise measurements of its strength, we find that the measured magnetic moment is about 0.1% off from the simple prediction. It turns out that when you take into account the virtual cloud around the electron, it exactly matches this small 0.1% discrepancy, showing that the cloud is definitely present. Further, the data and prediction exactly match to nine digits!

If your mind isn’t blown, wait…it gets crazier still. Empty space—that is, space that contains nothing—no energy, no charge, no matter, nothing—is filled with a writhing, active population of virtual particles that physicists call “the quantum foam,” with bubbles appearing and popping in wild abandon. At the subatomic level, space is never truly empty.

You’d think that if empty space were filled with a constant roiling boil of quantum activity, you’d see it. The fact that you don’t could give you yet more reason to disbelieve, yet the effects of the quantum foam have been directly observed.

The first observation of the quantum foam came from tiny disturbances in the energy levels of the electron in a hydrogen atom. A second effect was predicted in 1947 by Hendrik Casimir and Dirk Polder. If the quantum foam was real, they reasoned, then the particles should exist everywhere in space. Further, since particles also have a wave nature, there should be waves everywhere. So what they imagined was to have two parallel metal plates, placed near one another. The quantum foam would exist both between the plates and outside of them. But because the plates were placed near one another, only short waves could exist between the plates, while short and long wavelength waves could exist outside them. Because of this imbalance, the excess of waves outside the plates should overpower the smaller number of waves between them, pushing the two plates together. Thirty years after it was first predicted, this effect was observed qualitatively. It was measured accurately in 1997.

Quantum foam also has astrophysical implications. In 1974, Stephen Hawking was thinking about quantum mechanics and black holes. He realized that the quantum foam would exist near the event horizon of the black hole. If an electron/positron virtual pair popped into existence just outside the event horizon, one of the two particles might spiral down and get trapped in the black hole, while the other would escape. As it happens, more energy would escape than be captured, so the energy of the black hole would get slightly smaller. Over the eons, this “Hawking Radiation” would cause the black hole to evaporate until it totally disappeared.

Virtual particles and the quantum foam are one of the craziest of the quantum phenomena. They have no classical analog and they certainly seem like something that physicists dreamed up to save the counterintuitive world of quantum mechanics. Borrowing from the movie “The Maltese Falcon,” quantum mechanics is said to be the dreams that stuff is made of, but virtual particles are no dreams. They have been experimentally observed, and indeed it could be that a quantum fluctuation similar to virtual particles was the thing that pulled the trigger on the creation of the universe itself: a crazy start for a universe where, we’re learning, the bizarre is the norm and dreams are reality.

Go Deeper
Author’s picks for further reading

Alice in Quantumland: An Allegory of Quantum Physics
Join Alice on an illustrated adventure in quantum mechanics. By physicist Robert Gilmore.

Of Particular Significance: Virtual Particles: What are they?
Theoretical physicist Matt Strassler presents an alternative explanation of virtual particles in this blog post.

QED: The Strange Theory of Light and Matter
Physicist Richard Feynman’s classic introduction to quantum electrodynamics offers a more advanced description of the field.

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Don Lincoln

    Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.