Physicists Catch a Paradox in a Jar : Science: Reaffirming a theory named for the philosopher Zeno, researchers demonstrate, in effect, that a watched pot doesn’t boil.


Government researchers have for the first time proved a scientific principle that, in the arcane and often bizarre world of quantum physics, is equivalent to demonstrating that a watched pot never boils.

In proving what is known as the quantum Zeno effect, named after the Greek philosopher, the physicists at the National Institute of Standards and Technology in Boulder, Colo., reaffirmed the now well-known paradigm that attempting to measure the characteristics of atoms at the submicroscopic level can alter the characteristics that are being measured.

One particularly intriguing aspect of that principle--and one that still remains to be proved--is that continuous observation of unstable radioactive isotopes can prevent them from disintegrating radioactively. Theoretically, by the tenets of the Zeno effect, if a nuclear bomb were watched intently enough, it could not explode.


The new research, which is to be published in the journal Physical Review A, has no immediate practical applications, said physicist Wayne M. Itano of NIST, but it does indicate that there are fundamental limits to the accuracy that can be achieved with atomic clocks.

It is “a fantastic result,” said physicist Peter Coveney of the University of Wales in Bangor, United Kingdom. “It seems that the act of looking at an atom prevents it from changing. It is completely against what common sense would tell us, and implies that quantum mechanics does not . . . operate at the scale of real kettles.”

Zeno of Elea, who lived from 490 to 430 BC, was one of the first proponents of the rhetorical technique of reductio ad absurdum, in which he would derive impossible conclusions from the opinions of his philosophical opponents. He is best known for four apparent paradoxes that have been adequately explained only in this century through sophisticated applications of the concepts of space and time.

One of his most famous paradoxes involves a flying arrow. Because an object cannot occupy two places at the same time, he argued, the arrow is at only one place at any given moment during its flight. But to be in one place is to be at rest. Therefore, the arrow is at rest during every moment of its flight, he concluded, and motion is impossible.

In the 1970s, theoretical physicist Ennackel Sudershan and his colleagues at the University of Texas in Austin showed in theory that “a continuously observed quantum state cannot decay”--that is, an atom in one energy state cannot change its energy as long as it is being observed. Sudershan dubbed this phenomenon the quantum Zeno effect, but until the recent NIST experiments, no one had demonstrated it directly.

Itano and his colleagues undertook the experiment, he said, “because we had some apparatus we were using to make a better atomic clock that could easily be modified” to do the research.


In their experiment, the “kettle” was a magnetic field that was used to hold the “water,” a cluster of about 5,000 positively charged beryllium ions, in a fixed position. The “heat” they applied to the kettle was a radiofrequency field that would drive the beryllium ions from a low initial energy state to a higher energy state.

By shining a brief pulse of light from a laser on the beryllium atoms, they could readily determine which state the atoms were in.

The transition from the low- to the high-energy state takes about 250 milliseconds, or one-quarter of a second. If the researchers turn on the radiofrequency field and then check the atoms with the laser about 250 milliseconds later, virtually all will be in the high-energy state.

But if the researchers check the atoms only 4 milliseconds after the radiofrequency field is turned on, they find that virtually all are still in the low-energy state. But more important, the process of measuring their energy state gives “a little nudge” to those atoms that had started a trek to the higher energy state, forcing them back down to the lower state--in effect, resetting the system to zero.

As long as the researchers keep checking the atoms every 4 milliseconds, the atoms never make it to the higher energy state, despite the outside force driving them toward it. As long as the observers are watching them, in other words, they never boil.

The same principle should apply to the radioactive disintegration of atoms, Itano said, but he added that it does not “look feasible” to test it now. The disintegration of an atom occurs in a time interval roughly a million billion times shorter than the energy change in beryllium atoms, he said, and physicists simply cannot measure it fast enough to prevent it from occurring.