Quantum Physics / Thought Experiments

13
Jan

Much Ado About Nothing

Nothing is not as simple as it seems.

The concept of nothing has fascinated philosophers and scientists throughout history. The search for an ever-deeper understanding of nothing has driven scientific discovery since the age of ancient Greece, and today the pursuit of nothing defines the frontier of modern particle physics. But before we talk about nothing, let’s talk about something: air.

For millennia, philosophers thought that “empty” air was nothing. Aristotle and the ancient Greeks, though, recognized air as a “thing” in its own right. Wind, after all, is nothing but air, yet it can be felt powerfully. Indeed, the Greeks considered air to be one of the basic elements, along with earth, water, and fire. These elements, in turn, were believed made of some basic something which they called “ur-matter.” A familiar modern example, sucking on a drinking straw, seems to illustrate the impossibility of creating a vacuum: The straw doesn’t fill up with vacuum but instead “implodes,” apparently confirming the Greek belief that “Nature abhors a vacuum.”

About two millennia would pass before Galileo and others realized that the implosion is due to the external pressure of the air, and not a cosmic law against nothingness. This soon led to the invention of the barometer and a remarkable discovery: Air pressure decreases with altitude. The reason is that the atmosphere has a finite height and the nearer you get to the surface, the less air there is pressing down on you. This inspired the thought that above the atmosphere is nothing—or, at least, no air.

By the end of the 17th century, then, when people talked about “nothing,” they were no longer talking about air: They were talking about the void of space. Today, we know that though space is empty of air, it is filled with gravitational forces which guide the planets and order the galaxies. It is also full of electric and magnetic fields that give us sunlight and starlight in the form of electromagnetic waves.

This created great problems for 19th century scientists: Since the electromagnetic waves from the sun and stars were making it all the way to Earth, they must be traveling through something. After all, they knew that sound waves need a medium through which to travel. I speak and air molecules bump into one another until some hit your eardrums, making them vibrate, generating signals that your brain interprets as sound. The absence of air in space leaves the sun silent, yet we can see it.

To resolve this paradox, scientists argued that there must be some medium through which the electromagnetic waves traveled. “Waves in what?” was answered with: “The ether.” And so began one of the greatest wild goose chases in the history of science, as many of the leading lights in the field went in search of this weird ether that was capable of transmitting light at about 300,000 km every second while still allowing the planets to pass through as if there were nothing there at all. The search did not end until Einstein finally introduced his theory of relativity in 1905, which eliminated the need for the ether. (But that’s a story for another day.) The tables had turned on nothing: Aristotle was wrong. Nothing could exist—or so we thought. And then came quantum mechanics.

In the quantum realm of tiny subatomic particles, the more closely you look at nothing, the more things you discover. What looks empty to our gross senses turns out to be effervescing with particles of matter and anti-matter. The apparent void is a medium filled with stuff, a froth of will-o’-the-wisp particles of matter and antimatter.

This new quantum mechanical view of nothing began to emerge in 1947, when Willis Lamb measured spectrum of hydrogen. The electron in a hydrogen atom cannot move wherever it pleases but instead is restricted to specific paths. This is analogous to climbing a ladder: You cannot end up at arbitrary heights above ground, only those where there are rungs to stand on. Quantum mechanics explains the spacing of the rungs on the atomic ladder and predicts the frequencies of radiation that are emitted or absorbed when an electron switches from one to another. According to the state of the art in 1947, which assumed the hydrogen atom to consist of just an electron, a proton, and an electric field, two of these rungs have identical energy. However, Lamb’s measurements showed that these two rungs differ in energy by about one part in a million. What could be causing this tiny but significant difference?

When physicists drew up their simple picture of the atom, they had forgotten something: Nothing. Lamb had become the first person to observe experimentally that the vacuum is not empty, but is instead seething with ephemeral electrons and their anti-matter analogues, positrons. These electrons and positrons disappear almost instantaneously, but in their brief mayfly moment of existence they alter the shape of the atom’s electromagnetic field slightly. This momentary interaction with the electron inside the hydrogen atom kicks one of the rungs of the ladder just a bit higher than it would be otherwise.

This is all possible because, in quantum mechanics, energy is not conserved on very short timescales, or for very short distances. Stranger still, the more precisely you attempt to look at something—or at nothing—the more dramatic these energy fluctuations become. Combine that with Einstein’s E=mc2, which implies that energy can congeal in material form, and you have a recipe for particles that bubble in and out of existence even in the void. This effect allowed Lamb to literally measure something from nothing.

This suggests that the contents of the vacuum—the “stuff” of nothing—could be organized in different ways at different times in the history of the universe. Think of water molecules: They can roam freely in the liquid or lock tightly to one another in ice crystals. This analogy hints at an intriguing possibility: Could the contents of the quantum vacuum be in a different configuration in today’s cool universe than they were in the first moments after the hot Big Bang?

At creation, the thinking goes, particles had no mass and moved through the vacuum at the speed of light. Around a trillionth of a second after the Big Bang, the universe was cool enough that a mass-giving field called the “Higgs field” condensed in the vacuum, as water condenses from steam.

The Higgs field is believed to disturb the motion of fundamental particles like electrons as they move through it, producing the effect that we call mass. If this is correct, there should be particle manifestations of the Higgs field, known as Higgs bosons, just waiting to be discovered. The Large Hadron Collider (LHC) at CERN is hot on the trail of these particles, but decisive evidence of the Higgs boson—which is very massive and can only be produced in an enormous blast of energy—is still elusive. Scientists working on the LHC expect that they may see the first glimpse of the Higgs by the end of 2012. Whether this is the real deal or whether we are being fooled by some cruel, random throw of Nature’s dice, time will tell.

Aristotle was right: There is no thing that is nothing. Is the Higgs field part of the something? Within a few months we may know the answer.

Go Deeper
Editor’s picks for further reading

FQXi: Much Ado About Nothing
Ted Jacobson investigates the nature of the cosmic vacuum.

The New York Times: There’s More to Nothing Than We Knew
In this article, Dennis Overbye reviews why physicists believe that something—like our universe—can come from nothing.

World Science Festival: Nothing: The Subtle Science of Emptiness
Journalist John Hockenberry leads Nobel laureate Frank Wilczek, cosmologist John Barrow, and physicists Paul Davies and George Ellis in a discussion of the physics and philosophy of nothing.

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Frank Close

    Frank Close is professor of theoretical physics at the University of Oxford, and Fellow of Exeter College, Oxford. He is a high energy particle physicist and has published some 200 research papers, specializing in the quark structure of nuclear particles, with a particular interest in glueballs. He is the author of several popular books on physics, including Antimatter, Neutrino, and Nothing - A Very Short Introduction (all published by Oxford University Press). His latest book is The Infinity Puzzle (Basic Books), the story of half a century of discoveries that have led to the Large Hadron Collider.