Throwing Light On the Matter
My father was a physics teacher, and I suspect that it was in one of his collection of books that I first read about the ‘double slit’ experiment. He had a bookcase full of tantalising volumes, some of which were technical, even esoteric, and others which were fascinating, well, at least as far as the titles suggested. He was interested in light and how light was understood by astronomers, and part of this interest was that he would carry out experiments at home. They were part of his plan to complete a PhD, a plan that was never fully realised. I have no idea what he was trying to study, and he certainly made no attempt to explain it to me. Nor did he ever complete his research.
All I remember is that at some point in his studies his research involved taking photographs of light sources through long tubes, made up, as I remember it, from the carboard cores of toilet rolls! There were strange moments in our London house when all the lights were out, a toilet core tube extended through the hall, and, on the floor, my dad would take photographs of the light source at the other end of the tube. And you thought your childhood was strange?
From time to time I would take one of the books from his bookcase and read it. I’d try to understand what I was reading. The ones that boggled me were those concerned with relativity, the expanding universe, and similarly wonderful issues. I seem to remember reading various books on the nature of light, and some, written around the early 1960s, offered an overview of various recent experiments, which were turning out to take physicists a further few key steps forward in the understanding the nature of light. More to the point, I hadn’t realised that these were merely the most recent stages in a series of experiments that had begun back in the early 1800s.
Perhaps I should have gone back a little future, as in many accounts of the study of light the real starting point was Newton. In 1666, Newton had shone light through a prism, and observed the spectrum of colours that exited, ranging from red at one extreme, through orange, yellow, green, blue and indigo, to violet at the other (to be remembered by schoolchildren when I was young by ‘Richard Of York Gave Battle In Vain!). This led to a series of experiments, including one clever demonstration that the colour spectrum could be brought back together (using a lens), to recreate white light.
However, other work demonstrated a beam of a single colour, when shone on various objects, always remained the same colour. The light was the colour. All of which was a little perplexing, as I knew from messing around in art classes that if you mixed all those colour paints together you ended up with a very dark mess, and a ‘colour’ that was certainly not white.
Newton was interested in the properties of light, and concluded it was composed of particles or corpuscles, which were refracted by accelerating into a denser medium (from travelling through the air to travelling through glass), the different colours being diffracted to differing degrees. In 1675, in his Hypothesis of Light, of 1675, he proposed the existence of the ether, through which these particles travelled, (this ether being the medium containing air). In 1704, Newton published Optiks, in which he expounded his corpuscular theory of light. He considered light to be made up of extremely subtle corpuscles, that ordinary matter was made of grosser corpuscles and speculated that through a kind of alchemical transmutation “Are not gross Bodies and Light convertible into one another, … and may not Bodies receive much of their Activity from the Particles of Light which enter their Composition?” Little did he know where this would end up!
Newton was a dominant figure, but within 100 years, his views were challenged. Thomas Young came up with an ingenious experiment in 1801. On the presumption that Newton was correct in his model of particles of light, then when these particles were fired in a straight line through a slit and allowed to strike a screen on the other side, the result should be a pattern corresponding to the size and shape of the slit. However, when this ‘single-slit experiment’ is actually carried out, a diffraction pattern appears on the screen, with bands of light and dark. This pattern is the result of the interference of light waves from the slit. At that time it was thought that light consisted of either waves or particles. Young’s experiment showed light was a wave form!
With the beginning of modern physics, about a hundred years later, it was realized that light could, in fact, show bothwave and particle characteristics. All this became evident once physicists embarked on a series of increasingly complex ‘double path’ experiments, in which a wave is split into two separate waves (the wave is typically made of many photons and better referred to as a wave front, not to be confused with the wave properties of the individual photon) that later combine into a single wave.
In the basic version of this experiment, a carefully structured and controlled light source, such as a laser beam is directed towards a non-transparent plate pierced by two parallel slits. The light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to ‘interfere’, the result being the production of bright and dark bands on the screen – this was a result that would not have been expected if light consisted of classical particles. If only it were that simple! Of course, there was yet more experimentation, and this further research discovered the light is always found to be absorbed at the screen at discrete points, as evidence that it is individual particles (not waves) which are striking. What we see as the interference pattern appears as a result of the varying density of these particle hits on the screen.
By now, there was more and more experimentation and further versions of the experiment that included adding ‘detectors’ at the slits. As a result, experimenters discovered that each detected photon passes through one slit only (as would a classical particle), and not through both slits (as would a wave). That seems clear enough (!), which has to be a comment suggesting there had to be a further complication. And there was! These same experiments demonstrated something quite weird: it seems that the particles do not form the interference pattern if one detects which slit they pass through. The particles knew what the experimental scientist was up to? I guess there is only one word for that: spooky! In fact, the ‘spooky’ behaviour of things is a very popular word among some theoretical physicists.
This last finding was one guaranteed to ensure more and more experimentation. An important version of these experiment involved single particle detection. Sending coherent particles through a double-slit apparatus one at a time results in single particles being detected as white dots on the screen, as expected. Remarkably, however, an interference pattern emerges when these particles are allowed to build up one by one. Now the particles know how to create an interference pattern! Even more spooky.
For scientists, these results demonstrated the principle of wave-particle duality, which isn’t as sophisticated as you might imagine. It is, quite simply that all matter exhibits both wave and particle properties. Without getting too complicated, the particle is measured as a single pulse at a single position, while the ‘modulus squared of the particle’s wave’ describes the probability of detecting the particle at a specific place on the screen, in other words a function that results in the observed statistical interference pattern. This wave-particle phenomenon has been shown to occur with more than just photons, as it is also true for electrons, atoms, and even some molecules. Continuing experiments revealed, for example, that electrons are found to exhibit the same behaviour as photons when fired towards a double slit.
Did you think that was the end of the puzzle. By no means. Soon it was found that, in addition to the interference pattern, the the detection of individual separate impacts was observed to be inherently probabilistic, indeterminate for any individual photon or electron, a finding which was – and is – completely inexplicable using classical (Newtonian) mechanics.
The researchers have continued, and it was found the original photon experiments could be carried out with entities much larger than electrons and photons, although, much to the physicists’ interest, the outcome is found to become more difficult to observe as the size of the entity increases. Despite challenges, the largest entities for which the double-slit experiment has been performed have been molecules made up of some 2,000 atoms.
The double-slit experiment (and its variations) has become a classic, mainly for its clarity in expressing the central puzzles of quantum mechanics. For the very reason it demonstrates the fundamental limitation of the ability of the observer to predict experimental results, one physicist, Richard Feynman, has called it “a phenomenon which is impossible […] to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery (of quantum mechanics).” This comes from The Feynman Lectures on Physics, Vol 3, Chapter 1, Quantum Behaviour, and, no, I haven’t attempted to read and understand them: way beyond my ability.
However, I am aware that if you can’t explain something, make the inability to explain an axiom or a theory. This leads us to Werner Heisenberg and his ‘uncertainty principle’. Although this is a horrible simplification, the uncertainty principle is that if you make measurements on an object at the atomic level, and you determine its momentum, you cannot, at the same time, know its position accurately.
How does this help us with the double slit experiments? The implication of Heisenberg’s principle is that one cannot design equipment in any way to determine which of two alternatives is taken by an electron, say, without, at the same time, destroying the pattern of interference. Ah, but that is true only if you are observing the electron. Of course, a clever physicist came up with an approach to get round that problem.
We imagine a modification of the experiment in which a screen with two holes consists of a plate mounted on rollers so that it can move freely up and down. “By watching the motion of the plate carefully we can try to tell which hole an electron goes through. … We would expect that an electron which passes through hole ‘1’ must be deflected downward by the plate to reach the detector. Since the vertical component of the electron momentum is changed, the plate must recoil with an equal momentum in the opposite direction. The plate will get an upward kick. If the electron goes through the lower hole, the plate should feel a downward kick. It is clear that for every position of the detector, the momentum received by the plate will have a different value for a traversal via hole ‘1’ than for a traversal via hole ‘2’. So! Without disturbing the electrons at all, but just by watching the plate, we can tell which path the electron used.”
Like many of the weird issues that pop up in sub-atomic physics, the double-slit experiment is often used to highlight the differences and similarities between the various interpretations of quantum mechanics. Do we want to go any further? Well we might just note there are several different interpretations, and, so far, no easy way to determine why one is ‘correct’ and the others are not.
Briefly, the first approach is called the ‘Copenhagen interpretation’. This is based on the view the idea that quantum mechanics is intrinsically indeterministic, where objects have certain pairs of complementary properties that cannot all be observed or measured simultaneously. Moreover, the act of “observing” or “measuring” an object is irreversible, and no truth can be attributed to an object, except according to the results of its measurement: on this basis a particular experiment can demonstrate particle behavior (passing through a definite slit) or wave behavior (interference), but not both at the same time. Copenhagen-type interpretations hold that quantum descriptions are objective, in that they are independent of physicists’ personal beliefs and other arbitrary mental factors.
This is quite different from the ‘relational interpretation’ of quantum mechanics, which argues that observations such as those in the double-slit experiment result specifically from the interaction between the ‘observer’ ( the measuring device) and the object being observed (physically interacted with), but not any absolute property possessed by the object. In the case of an electron, if it is initially “observed” at a particular slit, then the observer–particle (photon–electron) interaction includes information about the electron’s position. This partially constrains the particle’s eventual location at the screen. If it is “observed” (measured with a photon) not at a particular slit but rather at the screen, then there is no “which path” information as part of the interaction, so the electron’s “observed” position on the screen is determined strictly by its probability function. This makes the resulting pattern on the screen the same as if each individual electron had passed through both slits.
If you think that one is bad enough, there is, of course, worse! This is the ‘many worlds interpretation’. Here theunifying theme is that physical reality is identified with a wavefunction, and this wavefunction always evolves unitarily. This means, Consequently, there are many parallel universes, which only interact with each other only through interference. At least one physicist has argued that the way to understand the double-slit experiment is that in each universe the particle travels through a specific slit, but its motion is affected by the interference with particles in other universes. This creates the observable fringes.
Confused? So am I. Have I thrown any light on the nature of light? Unlikely. This is one of those fascinating areas of science that are delightfully unclear. There is considerable evidence to justify the statement that light comprises discreet particles, usually referred to as photons. Shining a light on an object it to direct a stream of photons, that then interact with and bounce back from the object being illuminated. However, the challenge sits within what we call a photon. It shows the behaviour of both an object and a wave. We know what waves of water are, but they are waves in one medium, pushing aside another. Photons are just wave packets, and waves are just waves. It is confusing, and it is one of the glorious puzzles of our world.
