Two-century-old journey of the Quantum riddle

Newton believed light was a particle, and Huygens believed light was a wave. Both were correct but this was not acceptable to the 17th-century scientists

Through two doors at Once 

The elegant experiment that captures the enigma of our quantum reality 

Anil Ananthaswamy 

Penguin Viking,

299 pages; Rs 599

Quantum theory — the mathematical physics of very small things — is among the most divisive realms of science. The mathematics works, the predictions  quantum theory makes check out experimentally. But it also seems like utter nonsense in terms of the normal-sized reality we all experience. 

Particles disappear; they reappear mysteriously and unpredictably in different places; they seem to know what the observer wants them to do; when we can tell the position of a particle, we don't know its momentum, and when we know its momentum, we don't know where it is.  For that matter, a particle is not necessarily a particle — it could be a wave instead. Indeed, it seems to be both at the same time and it may be in two different places at the same time. 

Wave-particle duality is one of the many, many puzzles that make quantum theory impossible to understand in terms of our everyday intuition. The debate about whether light was a wave or a particle actually started way back in the 17th century when Isaac Newton (who believed light was a particle, or “corpuscular” as he termed it) and Christiaan Huygens (who believed light was a wave) ended up on the opposite sides. Both these great scientists brought sound logic and strong experimental data to the table. Both were correct, which was not something that any 17th-century scientist could have swallowed. 

In the early 19th century, Thomas Young described and performed what is now known as “Young’s double-slit experiment”.  He made a small slit in a window shutter and held up a playing card sideways to split the sunbeam that came through. He placed a screen to capture the pattern that resulted from this split beam. In effect, this is equivalent to making two slits (the card acts as a barrier). 

Imagine for the moment that such an experiment is performed by shooting very small bullets through these slits. This will give two areas of bullet holes on the screen with a gap in the middle. If light is composed of particles, we should, therefore, see two bright spots on the screen with a gap in-between. 

On the other hand, think of a wave of water, hitting a sea-wall with two openings. The wave breaks up into many waves as it comes through those slits. The water sloshes through, creating a pattern where there are multiple areas of wetness, interspersed by dry patches.  The wet patches occur when two (or more) waves peak together reinforcing each other, while dry patches occur where two waves interfere with each other, cancelling out. If light is a wave, it should create a similar patchy pattern with bright areas alternating with darkness. 

Young discovered that light is indeed a wave because that alternating light-dark pattern is precisely what occurs if both slits are open. However, when one of those slits is blocked, light seems to behave like particles, creating a bright area, without interference. 

What happens if this experiment is performed, millions of times, over a period of 220 years, with increasingly sophisticated equipment and carefully calibrated variations? What happens if one single photon is let through each slit at a time? Or, since light and electromagnetic waves are equivalent, what if it's an electron? Suppose there is a tiny mirror to deflect the photon? Suppose there are two entangled particles being swung through? Suppose one of the slits is shut as the photon is pushed through? Suppose measurements are taken in staggered ways? 
 

Over the years, as physicists learnt more about the quantum realm, and as our technology has evolved in sophistication, all these variations and many more of the slit experiments have been tried. The great men who worked out quantum theory — Max Planck, Albert Einstein, Niels Bohr, Paul Dirac, Erwin Schrödinger, Werner Heisenberg, Wolfgang Pauli, Richard Feynman, Alain Aspect, Roger Penrose, and many others —  conceptualised and carried out an insane number of variations of Young’s experiment. They have learnt more from studying the data. But more puzzles have also been thrown up by the data. 

Einstein went to his grave wondering if there was something missing from quantum theory — “hidden variables” that would make it easier to explain. He and the others , like Schrödinger, were bothered by weird phenomena like entanglement, when two entangled particles change their states simultaneously even though they are separated by great distances. That debate about what, if anything, underlies the present state-of-the-art of quantum theory continues.

Through Two Doors at Once tells the story of these experiments and the history of quantum research seen from this perspective. It is a wonderful way to introduce quantum concepts and it's a well-written book, amply illustrated by a series of superb diagrams. Read it without fear or preconceptions.