Can't Get There from Here Quantum physics puts a new twist on Zeno's paradox
Scientific American May 90

A 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 ffifinite 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 counterintuitive 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 levels. 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. After ward 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-milli -second 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.' -Corey S. Powell

The Quantum Physics of Time Travel

David Deutsch and Michael Lockwood Scientific American Mar 94

Deutch and Lockwood devote their article to demonstrating that neither relativity, nor quantum theory forbid paradoxical time loops. They then use the many universes interpretation which says that whenever a quantum probability occurs, all outcomes occur with the given probability instead of one only as is hypothesized in 'reduction of the wave packet'. They use a special case of many universe to suitably partition the possibilities from paradox.

The authors firstly note that there is nothing in the equations of relativity which forbids time loops (see figure at left). They then go on to explain that if we take a certain view of the many universes interpretation we can remove any resulting paradoxes.

They apply this to the paradox of a grand-daughter entering a time machine and dissuading her grand-father marrying her gran-mother leading to herself not being born - paradox. Below is illustrated the similar dilamma of Sonia resolving to enter a time machine tomorrow and travelling back to today only if she doesn't emerge from the time machine today - again paradox.

In the diagram below two probability universes A and B. In the A universe, Sonia doesn't enter the machine tomorrow because she arrived out of it today. In the B universe, the opposite happens, she doesn't come out of it and thus does enter the time machine. However she exits into universe A. A thus ends with two Sonias and B none at all. This is somewhat of concern to her boyfriend, or that component of him in universe B. The one in A would however have a ball or two much of a good thing.


Non-paradoxical time loop with paired probability universes A and B.

This article says some important things about the idea of connectivity between contingency branches, which qualifies the many universes. This qualifies the many-universes by effectively editing out the paradoxical ones. This changes the many universes interpretation so that a connectivity limits the possibilities. This is very akin to the notion of transactional supercausality.

Quantum Biology Scientific American May 1989

Hydrogen tunneling contributes to an enzyme reaction

It has been known for some years that electron tunneling-a quantum-mechanical effect that enables an electron to circumvent an energy barrier-has a crucial role in many biological reactions, such as photosynthesis. Now researchers at the University of California at Berkeley report that tunneling by hydrogen contributes to an enzyme-reaction mechanism under biologically relevant conditions. The enzyme, a yeast alcohol dehydrogenase, speeds up the conversion of benzyl alcohol into benzaldehyde, a transformation that involves cleaving a hydrate (a hydrogen atom vath an electron) from an alcohol molecule. The enzyme boosts the rate of this transformation enormously by lowering the energy barrier that must be surmounted in order for the reaction to take place. A semi-classical model compares the binding of the hydrogen to the alcohol molecule with the behavior of a mass on a spring. The model predicts that an ordinary hydrogen nucleus-a proton-can hurdle the energy barrier more easily than the heavier isotopes deuterium and tritium. This means that the reaction rate for ordinary hydrogen should be faster than that for deuterium and tritium. How much faster can be calculated precisely. If however, quantum tunneling contributes to the reaction, the reaction, rate for ordinary hydrogen should be greater than the rate predicted by the semiclassical model. According to quantum mechanics, a particle's position is uncertain; the probability of finding it at a given point is smeared out in space. Tunneling can occur if the region of uncertainty extends to the other side of an energy barrier. Because particles with smaller mass have a greater uncertainty in their position, they have a higher probability of tunneling. In biological molecules, electrons tunnel readily across distances of tens of angstroms, whereas a proton should tunnel less than one angstron- The heavier hydrogen isotopes, deuterium and tritium, are even less likely to tunnel. To see if tunneling is a factor in the reaction rate. Yuan Cha, Christopher J. Murray and Judith P. Klinman prepared two versions of the alcohol, with specific sites on the molecule occupied by an ordinary hydrogen and a tritium in one, and a deuterium and a tritium in the other. During the reaction a benzyl alcohol molecule loses a hydrogen atom to a molecule of nicotinamide adenine dinucleotide (NAD). The workers determined how much of each isotope became bound to NAD. As they report in a recent issue of Science, the rate at which ordinary hydrogen was transferred was greater than the rate predicted semi-classically, which would indicate that the reaction is assisted by tunneling. What is more, the enzyme may facilitate tunneling not only by lowering the energy barrier but also by narrowing it. This could occur if the enzyme brings the active sites on the NAD and the alcohol very close together. "the next step is to see what happens if the molecules are kept farther apart," Cha notes. The observation of hydrogen tunneling could have wide implications.


Caught on camera the first image of a Bose-Einstein condensate, shown above (right) captured by shining a laser through the material. Previous plots (left) were calculated after watching the condensates fly apart
(Scientific American Mar 98).

New State of Matter Pictured at Last New Scientist 1 Jun 96

Last July, when scientists in Colorado announced the creation of a new state of matter caIled a Bose-Einstein condensate, there was only one problem: no one could see it. Now a team at the Massachusetts Institute of Technology has managed to do just that. In the early 1920s, Albert Einstein and the Indian physicist Satyendra Nath Bose realised that certain types of gas would stop behaving as a collection of individual particles if they were cooled to within a hair's breadth of absolute zero. The particles would instead become a single, large, diffuse entity. In quantum mechanical terms, the particles, wave functionsmathematical representations of their positions and other attributes-would expand until they completely overlapped. For 70 years it was impossible to test this prediction directly, because nobody knew how to chill a gas to less than one-millionth of a kelvin. But last year a team led by Eric Cornell of the National Institute of Standards and Technology in Boulder and Carl Wieman at the University of Colorado used a combination of lasers and magnetic fields to cool a sample of rubidium until some two thousand atoms coalesced into a Bose-Einstein condensate (New Scientist, Science, 22 July 1995, p 16). Or so Cornell and Wieman calculated. They could not observe the condensate t directly because it was a mere 10 E micrometres in diameter. Instead, they I tumed off the magnetic trap to let the cloud v of rubidium atoms expand. By measuring r the size of the cloud a short time later, they k were able to determine the velocities of " the atoms before the trap had been opened and show that the condensate had existed. Wolfgang Ketterle and his colleagues at MIT began creating Bose-Einstein con densates soon after the Colorado team's announcement, and have now seen what other researchers have only been able to infer. On 18 May, at a meeting of the American Physical Society's Division of Atomic, Molecular and Optical Physics in Ann Arbor, Michigan, Ketterle described the creation of pencil-shaped Bose-Einstein condensates that are much larger than those produced in previous experiments. The condensates, which survive for more than 20 seconds, contain about 5 million sodium atoms and are about 8 micrometres wide and 150 micrometres long. This is about 1000 times larger than any other group has achieved. Ketterle and his team observed their condensate by shining a laser on it and measuring the light's deflection. Like a glass lens, the condensate bends light that passes through it. Before these experiments, physicists did not know whether Bose-Einstein condensates would be black, highly reflective to light, or transparent which is how they turn out to be. Now that this most basic of questions has been answered, Ketterle aims to create more large Bose-Einstein condensates and clear up some of the other mysteries surrounding this strange quantum state of matter. How will sound propagate through a condensate? And how will excited states of a condensate behave? The excitation of individual atoms is well understood-their orbiting electrons move to higher energy levels. But nobody knows what to expect from an excited "superatom" consisting of 5 million atoms acting as a single entity.