Title-> Can’t get there from here; quantum physics puts a new
twist on Zeno’s paradox.
Authors-> Powell, Corey S.
Can’t Get There from Here
Two thousand years ago the Greek philosopher Zeno noted that an object
moving from one place to another must first reach a halfway point, and
before that a point half of the way to the halfway point, and so on.
Any movement involves an infinite number of intermediate points, and
so any motion must require an infinite amount of time. Motion, Zeno
concluded, is logically impossible.
In fact, things do move. Zeno did not consider that an endless series
could have a finite sum. But in the counter-intuitive realm of
quantum physics, something akin to Zeno’s paradox can occur: atoms can
be paralyzed if they are closely scrutinized. The act of observing
prevents the atom from passing a halfway point between two energy
In 1977 E. C. George Sudarshan and Baidyanath Misra of the University
of Texas at Austin realized that an unstable object, such as a
radioactive atom, would never decay if it were observed continuously.
They called this surprising phenomenon the quantum Zeno effect. Now
Wayne M. Itano and his colleagues at the National Institute of
Standards and Technology (NIST) have observed a variant of this effect
in the real world. Their work will appear in Physical Review A.
The reason for the Zeno effect lies at the heart of quantum physics,
which states that the energy of an atom moving between two energy
states is somewhat uncertain and that (for short intervals) the
uncertainty grows over time. For an atom to shift from one state to
the other, the uncertainty must be large enough to bridge the two. A
measurement that determines the atom’s energy “collapses” the atom to
the measured state. Afterward the uncertainty grows again, but it
should be possible to “freeze” an atom in one energy state by taking
measurements so frequently that its energy never becomes uncertain
enough to let it jump to another state.
To observe the Zeno effect, the NIST team confined 5,000 beryllium
ions in an electromagnetic trap and exposed them for 256 milliseconds
to a radio frequency that bumps beryllium ions to a higher, excited
energy state. During the test they fired short, 2.4-millisecond laser
pulses at the ions to determine their energy state. Ions in the
bottom state scattered the light pulse back; those in the excited
state did not. Each measurement pulse returned a scatter proportional
to the number of ions still in the bottom energy state.
When a single measurement pulse was sent at the end of the test,
nearly all the ions were found to be in the higher state, as one might
expect. More frequent laser pulses caused the number of ions in the
higher energy state to decrease. When 64 pulses–the largest number
used–were sent, essentially none of the atoms was able to jump to the
higher level. The measurement pulses occurred so often that there was
no time for each ion’s uncertainty to become large enough to permit it
to reach the upper level.
The NIST experiment sheds some interesting light on the question of
the role of the observer in a system like this. The scattered laser
light, used to determine the energy states of the atoms, was observed
after the end of the 256-millisecond test period. The energy states
of the ions, however, collapsed when hit by the pulses during the test
period, before the return scatters were actually observed.
Despite the apparent link between the viewer and the behavior of the
ions, it was the act of measurement–not the act of observing the
measurement–that immobilized the ions. Even so, the experiment may
strengthen the conviction of those who believe the old adage: “A
watched pot never boils.”