What the Dickens?
Like many people with a small amount of scientific knowledge, I love to read about things just on the edge of – or just beyond – my understanding, including such topics as black holes, elementary particles, and the nature of light. However, my all-time favourite has to be quantum mechanics. and especially the strange world of experiments with photons. I’ve mentioned this before, and I know it’s an obsession of mine. Please read on if you are interested, but, be warned, it is my take on mind bending stuff, and almost certainly poorly explained! The article this is based on is well worth reading if you would like more scientific details. [i]
Earlier this year, there was an eclipse of the sun in the US, and in Winston Salem, though partial, it was almost complete. Like many people, I wanted to take some photographs, and remembered from many years ago that you often see multiple images of the eclipse on the ground under a tree’s foliage. The gaps in the canopy of leaves create a multitude of tiny holes to allow light through, and these pinholes create images on the ground, the result looking like a series of strange, overlapping crescents, a multitude of inverted images of the eclipse above.
The properties of pinholes have been known for a long time. When I lived in Edinburgh there was a famous ‘camera obscura’ in the Outlook Tower on Castlehill. It was first set up in Short’s Popular Observatory back in 1835, in a building next to the National Monument on Calton Hill, and then in 1853 it was moved to its current location (now called the ‘Camera Obscura and World of Illusions’). Inside you are able to look at a panorama of Edinburgh in a darkened room with just one small circular opening to allow light in (by means of a mirror arrangement, the inverted image is turned the right way up). Not only is the image rather fascinating, but, unlike looking through lens of a reflex camera or binoculars, the image is sharp at all distances.
An interesting fact, but, naturally, physicists can’t leave well enough alone, and by the early 19th Century, light experiments were getting more involved. The reason for this was a long-standing debate between two famous researchers. On one side, Isaac Newton stated that light was corpuscular (in other words, emitted in particles, but don’t you like the word corpuscular?!). On the other side, the equally famous physicist, Huygens, argued light was a wave phenomenon, the waves travelling through the invisible but pervasive aether. At the end of the 19th Century, some two hundred years later, experimentation proved the mysterious aether didn’t exist, but the issue about waves and particles had continued, with the particle view slowly losing favour.
In fact, early in the 1800’s an experimenter decided to shine sunlight through two narrow slits, and saw that as the light fell on a screen, a pattern of alternating bright and dark fringes emerged, characteristic of two sets of waves interacting with each other, much as you see when you throw two pebbles in a still pond, and watch the resulting pattern. That experiment, together with the development of electro-magnetic theory, seemed to clinch things. Light was a wave phenomenon (it was just somewhat annoying that the aether wasn’t there!).
Naturally, if anyone was destined to throw a spanner in the works it had to be Einstein. In 1900, Max Planck had published a paper on ‘black body’ radiation, which solved a puzzle about the observed wavelengths of the radiation. His analysis demonstrated that radiation had to be in little parcels, quanta of energy. However, while his approach solved a mathematical quandary, it was Einstein in 1905 (the year of his series of groundbreaking papers) who successfully argued that light is made up of tiny quanta, photons, and later went on to demonstrate that quantum theory had to apply to the behaviour of atoms, too.
What a mess! Light comprises both discrete particles and waves. It was time to go back to the double slit experiment. With 20th Century technology, it was possible to do a far more ingenious experiment. How about shooting light through the two slits, one photon at a time? Let’s call this experiment #1. The first photon goes through one of the slits and appears as a spot on a photographic plate on the other side. The next goes through and hits another spot. But keep on going, then things get weird. You would expect that, rather like the pinhole camera, the spots of light would eventually ‘re-create’ the two slits on the screen. Instead, over time an interference pattern appears, with the photons mostly avoiding some areas on the screen, and building up on others – in other words, cumulatively demonstrating a wave interference effect:
“But our source is emitting light one photon at a time. The photographic plate is recording its arrival as an individual particle. And – this is crucial – the photons are going through the apparatus one at a time. There’s no interaction between one photon and the next, or the first photon and the 10th, and so on. So, what’s interfering with what?” [ii]
When in doubt, physicists often rely on an arena they like best of all: explain it by mathematics! Enter Schrödinger. Yes, he’s the one who developed a thought experiment about a cat in a box, but we’ll come back to that shortly. For now, we need to take account of his famous equations. “The Schrodinger equation is used to find the allowed energy levels of quantum mechanical systems (such as atoms, or photons). The associated wavefunction gives the probability of finding the particle at a certain position. … The solution to this equation is a wave that describes the quantum aspects of a system” [iii] As I understand it, the key word in here is ‘probability’. Applied to experiment #2, it turns out that a solution of Schrödinger’s equations predicts high probabilities for places where the photons land on the screen, and low probabilities for others.
Does this help? What physicists suggested was that there was a ‘wavefunction’, a mathematical description of the location of photon, in terms of the probabilities of finding the photon in a particular location. Good use of the Schrodinger equations, but does it help us? As Anil Ananthaswamy points out: “What’s a wavefunction and what does it mean for a wavefunction to go through two slits? Is the wavefunction something real? And how does one figure out where the wavefunction will peak when it encounters the photographic plate? Why does it peak there and not elsewhere?” [iv] After all, this is all just mathematics.
Questions led to even more elegant experiments. One used sophisticated monitoring to determine which slit each photon went through, supposedly doing so while ensuring that there was no interaction with the photon itself. Let’s call this experiment #2. When you do that, the photons hit any area of the plate, and the interference effect noted before disappears. Now that is tricky. Obviously, a photon has to go through one slit or the other. Obviously? Well, photons are discrete quanta of energy, and can’t be split up – as far as we know. But in the earlier experiment, #1, the ‘waves’ were split up (presumably the wave function went through both slits?). What changed? In experiment #2 we measured where the photon was.
And this is where we return to Schrödinger’s cat. He proposed what he considered a quite ridiculous thought experiment. A cat is put in a steel box, along with a Geiger counter, which can’t be interfered with by the cat; inside the counter there is a tiny bit of a radioactive substance, such that in the course of an hour one of the atoms decays, or, with equal probability, does not; if it happens, the counter tube discharges and through a relay releases a hammer that shatters a small flask of hydrofluoric acid, killing the cat. Leaving the cat in this box for an hour, the cat will be alive if no atom has decayed, but the first atomic decay will poison it. Clearly, one would think (as Schrödinger did), the cat is either alive or dead at any time during the hour. However, many physicists trying to make sense of the behaviour of photons argued that this was an allegory of a ‘superposition’ of states. In the unopened box, the cat is both alive and dead, just as a photon is both a particle and a wave, both somewhere and a wave function. You would only know if the cat was alive or dead by opening the box: by analogy, they argued, you would only know the state of a photon by measuring it, at which point superposition collapses.
There is more to be said as a function of the mathematical description of standard quantum mechanics. The equations do not have a variable that describes the position of a particle, a photon, as it moves. They only deal with the starting point and an end point, which can be determined by observation. And so, we can say the photon does not have a trajectory in the same way a bullet or a discuss does. In fact, in one way of interpreting the mathematics of quantum mechanics – named the Copenhagen interpretation after where this interpretation was developed – is that the photon has no objective reality until it lands on the screen!
Schrödinger appeared to have a happy belief in reality – just look in the box! Sadly, theoretical mathematical physicists are less inclined to accept that kind of empiricism. Since 1935, discussions of his thought experiment have raged on, and 80 years later have led to a number of bizarre alternative explanations of photons and cats in a box. My favourite is the ‘many worlds’ interpretation: when the box is opened, the observer and the possibly-dead cat split into an observer looking into a box with a dead cat, and an observer looking into a box with a live cat. Really? Instead of making clear how ridiculous current theory was, all Schrödinger achieved was to encourage more weird ideas. And these are scientists at work?
We haven’t given up on experiments. If you’re still reading, let’s go to experiment #3. [v] In #2, measuring where the photon was changed its observable properties to a particle. #3 tries to get around that by erasing the ‘which-way’ information (slit number one or number two). This uses a strange property called ‘entanglement’: two particles are entangled if they have the same waveform, even if they are separated by distance. It is a bit like being an identical twin, where some report they know what is happening to their twin even if he or she is hundreds of miles away. In this experiment, one photon goes through a slit. The entangled partner is used to extract which slit was used, and the information erased: as a result, the photon now behaved as a wave! Don’t ask me how they did this stuff, but, given the result, what is going on now?
Researchers never stop. Ever ingenious, another physicist came up with the idea of delaying what kind of experiment is being performed after the photon has entered the double slit device. Normally, if nothing else is done, it seems the photon, or the photon’s wavefunction, will take two paths (through both slits) and interference fringes be shown on the screen, as a result of combining the two parts of the ‘wave function’. However, in experiment #4 just before the photon arrived at the screen, the apparatus was reconfigured, so that each pathway (from each slit) is kept separate, on to two photographic plates as it were. Guess what? The photon acts like a particle, and wave properties have disappeared. What the dickens is going on now?
The technology is quite beyond me. We are talking about making changes in the testing apparatus in unbelievably short periods of time – after all, photons travel at the speed of light!! Let’s forget the technology for a moment, and instead we can concentrate on where this leaves the physicists. The conclusion seems to be that a photon has no ‘intrinsic’ nature; it is both a particle and a wave, yet neither a particle nor a wave until it is ‘detected’ on the screen. It is as if its particle like properties are only realised when the wave function collapses, and vice versa. It goes without saying that, as mysterious as the results of the experiments are, there are further experiments being explored, and many different theories to make sense of what is observed.
Some of the time, I just think this is fun stuff to read about. However, these experiments are actually probing a very important borderline. One set of theories, ‘classical physics’, is based on observations and theories about the world greater than at the atomic level, where classical mechanics make sense. The other side is the atomic-subatomic world, where quantum mechanics rule. Somehow, there has to be a bridge between the two. It is yet to be found.
To my simple mind, there is another issue, too. It takes us back to Schrödinger and the cat. Classical physics is about a world we can ‘see’, even if we have to use quite sophisticated instruments. In simple language, it is a world of real things. Quantum mechanics is about mathematics to explain phenomena that are beyond our sensory abilities, and quickly wander into many dimensional models.
As I think about it, photons are not objects like the balls on a tennis court. Indeed, experiments suggest that if we could stop them zipping along they would be found to have no mass (but, of course, we can’t). Nor are they waves like those we see on the sea, as there is no aether within which waves could exist. Rather they are some kind of smudgy package of energy, with properties that allow us to have some estimate of where they are, while at the same time they demonstrate vibrations that seem like waves. They are like nothing we can apprehend.
One explanation for the term ‘What the Dickens’ is that it is an oath, a euphemism for the Devil or Old Nick. This is claimed to fit with its use by Shakespeare: ‘I cannot tell what the dickens his name is’.[vi] One thing is certain: whatever the dickens is going on with quantum mechanics, it is devilishly mysterious!
[i] This blog is based on a wonderful article by Anil Ananthaswamy: < https://aeon.co/essays/the-elegant-physics-experiment-to-decode-the-nature-of-reality>. This is my simplification of his review.
[ii] Op cit
[iii] From < http://www.physlink.com/Education/AskExperts/ae329.cfm>
[iv] Anil Ananthaswamy, op cit
[v] There have been many more experiments, and I am leaving out some in the sequence – these are just my numbers.
[vi] If you are interested, the quote is from Merry Wives of Windsor, Act III, scene ii.
