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by Rachel Covault
You can walk through walls. You can cheat death by being alive and dead at the same time, and you may even have uncountable identical twins acting out variations on your life in zillions of parallel universes.
But before you start using brick walls as back doors, read the fine print. These are everyday happenings in the sub-nanoscopic kingdom of fleeting photons and escaping electrons, tiny crumbs of matter that behave as both particles and waves. These are the qualities that inspired the inventions of the laser and electron microscope, superconductivity and semiconductivity, the diode and transistor, and other fantasies in the making. The direction of our technological future is pointed by our understanding of these properties, dubbed quantum mechanics.
One of Albert Einstein’s chain of epiphanies from 1905 (an idea that Max Planck had also championed earlier) was the revelation that light only comes in multiples of a certain base amount. In other words, light is quantized into indivisible particles called photons. This realization explained the mysterious photoelectric effect, the observation that it is not the intensity of light (the amount of photons) but the amount of energy each photon carries that affects the voltage of a photocell when exposed to light.
Scientists thought this was very peculiar, since light does not always behave as a particle. If light shines through a small double slit in a barrier, it shows up on a wall on the other side as an interference pattern of alternating bright and dark bars, analogous to water waves passing through a narrows. The ripples that emerge from each slit interfere with each other when the crest from two ripples combine at the wall and appear as a bright spot. When a crest and a trough meet, they cancel each other out and show up as a dark spot on the wall. Light can be viewed as both a particle and a wave.
In 1924, Louis De Broglie showed that the opposite was also true: Particles could also be seen as waves. Repeating the double slit experiment with electrons, interference patterns appeared again. When one electron was allowed through the slits at a time, each appeared as a dot upon reaching the wall, but the telltale stripes still emerged when the final destination of many individuals were marked. It was as if each electron turned into a small packet of waves while passing through the slits, let the waves interfere with themselves, then returned to its particle incarnation in one of the bright stripes on the wall.
Enter Erwin Schrödinger and his feline martyr. Schrödinger was the man who applied a bit of mathematical formalism to the situation, forming an equation describing the wave function of any particle. Solving the equation for the wave function results in a picture of how pieces of matter look like waves.
So what does matter look like? What happened to those idealized models of solar-system-like atoms wrapped with dutiful electrons following perfectly circular orbits that school children are taught?
Bernd Thaller, a professor at the University of Graz, Austria, where Schrödinger once worked, is currently translating his predecessor’s opaque numbers and letters into colorful, three-dimensional windows into the atomic world.
"In the strange world of quantum mechanics the application of visual techniques is particularly rewarding, for it allows us to depict phenomenon that cannot be seen by any other means," writes Thaller in the prologue to his book, Visual Quantum Mechanics.
With the help of Mathematica software, he has depicted the hydrogen atom in various energetic states. The rubbery surfaces correspond to the electron’s wave function, and the hue to the phase of the function’s imaginary parts. The imaginary parts are as important as the real ones because when they are combined by integrating the product of both parts, they produce the particle’s "probability wave." This wave represents the places where the particle is most likely to be found. Far from the simplistic, concentric spheres of Niels Bohr’s model, this quantum mechanical view of the atom reveals a convoluted complexity to the tiny things that make up the universe.
Any wave can be seen as the combination, or superposition, of many smaller waves, and particle waves are no exception. This means that matter can be thought of as a superposition of many different states, and the resulting wave function is used to determine the probability that matter is in one of those states.
Schrödinger volunteers his cat to demonstrate this. It is shut in a windowless box equipped with a time bomb set to expel a lethal gas the instant a certain radioactive atom decays. Scientists outside the box have no way of determining the imprisoned cat’s status. Even if they knew the average decay rate of the triggering substance, it is impossible to say when this particular atom will chose to radiate. The cat could be in one of two states, life or death, but until the researchers open the box, they regard their pet as being in a superposition of both states. To them, the cat is both alive and dead at the same time.
The moment the suspense is broken, the wave function describing the opposing states "collapses" into either dead or perfectly healthy. Thus, observation is not a passive act. The universe is built on statistics calculated from wave functions (for example, the likelihood of decay), but nothing can be said of a single event. This means that if Schrödinger kept a zoo of his deadly cages, at some point half of the animals would remain brighteyed and bushy-tailed, while the other half would have embarked on their next lives. But that fact says nothing about the well being of Polly the Platypus, who remains in both states until an observation proves otherwise. Such a nondeterministic viewpoint is known as the Copenhagen Interpretation of quantum mechanics and is the standard school of thought.
The Many Worlds Interpretation is more exotic. It admits to the nondeterminism of our universe, but goes on to suppose there are infinitely many universes forming a deterministic multiverse. At every instant, every possibility hinted at in every wave function occurs somewhere in the multiverse.
Returning to the menagerie, Polly is lively in one world but lifeless in another. Since zookeepers can observe only the events of their resident universe, each universe’s keepers can make bets on their platypus’ outcome, but a theoretical inter-universal traveler would see the multiverse as a whole and could inform the keepers of the vitality of their charges.
If only the beasts knew how to walk through walls, they would be out of danger. When a particle or a platypus encounters a barrier that it does not have enough energy to overcome, it retreats in search of other escape routes. Similarly, a marble rolling up a ramp stops and reverses its direction when it runs out of momentum. Particles, however, also have the ability to take advantage of their wave duality to tunnel through the barrier to the other side. When a particle bounces off a wall, a fraction of the particle’s wave function seeps through the wall and there is a very small chance that the particle will be observed on the other side of the wall. (Unfortunately, the wave function for mammals is so complex that Polly would have to spend longer than the age of the universe in front of her cell door before she would suddenly find herself in the hedges outside.)
Radioactive decay works off this principle. Particles spit out by the atom do not have enough energy to break away from the clutches of the nucleus, but occasionally they do escape. Scanning tunneling electron microscopes take advantage of this "tunneling" effect as well, recording electrons as they bridge supposedly impassible gaps between the object material and the head of the measuring device.
Quantum mechanics is still a bustling field of study. Someday PCs may be replaced by quantum computers that use qubits (quantum bits) to calculate, and secret messages may be sent by quantum cryptography. Perhaps someday Polly will even be able to teleport from her black cell into a friendlier parallel universe, leaving us to wonder about the bizarre nature of our surroundings.
