Quantum thoughts...

Wave-Particle Duality and the Double-Slit Experiment

“Readers familiar with the philosophical debates in twentieth-century physics and the impact which these have had on the postmodern understanding of natural science will recognize the similarity between Shevek’s approach to the understanding of the nature of time in The Dispossessed and the way in which advocates of the Copenhagen Interpretation of quantum mechanics, like Niels Bohr and Werner Heisenberg, understand the nature of light and of electromagnetic radiation more generally—the so-called wave-particle duality” (Burns 199).

Wave-particle duality concerns the fundamental properties of matter and light. Matter/light was previously understood as being either/or. Either “particle” or “wave” was the property that identified a specific “particle” of matter or “wave” of light. These are fundamentally different properties. A particle is localised at a point in space, such as a grain of sand or a ball. A wave is not – like waves in the sea or ripples from a rock dropped in a lake. Waves spread out and exist in many places at once, while the particle can only exist in one. Waves and particles also interact with themselves differently. Particles bounce off one another and waves pass through one another. They have seemingly opposing or contradictory properties and behaviours. However, experiments have shown that these fundamentally different things can behave like one another: a wave can behave like a particle, and a particle can behave like a wave (Barad, Meeting the Universe Halfway 100-101; Davis “Explainer: what is wave-particle duality).

“One of the strangest experimental results ever observed has got to be that of the single particle double- slit experiment” (PBS Space Time: “The Quantum Experiment that Broke Reality” 00:07–12).

The double-slit experiment, used to show whether something is a wave or a particle, shows that both light and matter can display characters that are both wave-like and particle-like. Both matter and light display the behaviours of the opposing identifiers and their own – at the same time. The result is these seemingly opposing entities – matter/light – are “both simultaneously necessary and mutually exclusive” (Barad “Erasers and erasures...” 444).

In classical physics, a particle is understood as a piece of matter with a set of properties that make it X particle. It will behave in predictable ways and occupies a localised point in space. However, “the closer we look at things, that is, at the scale of subatomic particles, the more we discover that they are in constant flux”, things are no longer “stable entities” (West-Pavlov 48-49).

The Copenhagen Interpretation of the double-slit experiment ushers in the idea of change and possibility. Rather than particles being stable entities that have X set of properties, the double-slit experiment showed that under certain conditions a particle will behave like a wave (And conversely that a wave under certain conditions will behave like a particle (Barad, Meeting the Universe Halfway 105-16). )

The double -slit experiment is used to determine wave or particle properties because when wave or particles go through a double-slit experiment they produce different patterns on a detection screen set up on the other side (see fig.1).

The “Ball Machine” shows how a particle (say an electron) should behave and the wave machine how a wave should. The black line on the far right indicates the pattern expected to be found on the detection screen. For the particle, you would expect a pattern of two separate points, corresponding to the two slits, where the individual particles have passed through. As the wave passes through, it gets diffracted and the waves start to overlap, shown in the illustration above, and a pattern gets produced on the detection screen like the black squiggly line on the wave machine detection screen. This is called a diffraction pattern. However, a strange thing happens when electrons are released through the apparatus. If just one electron goes through everything seems as it should, the atom hits the screen and a localised point is detected, as would be expected. However, when a host of electrons are released over time while each electron leaves an individual mark as an electron should, the pattern from the collective electrons is not in two distinct marks corresponding to the two distinct slits, rather a diffraction pattern emerges; like the squiggly black line from the wave machine (Barad, Meeting the Universe Halfway 97-106; PBS Space Time: “The Quantum Experiment that Broke Reality” 00:00 – 4:50).

When the particle is released, it acts like a particle. When it hits the detection screen, it acts like a particle, but at some point, between being released and hitting the screen, the way the particles pass through the detector creates a diffraction pattern (see fig. 2).

If we compare the pattern on the detection screens of the wave and ball machines in fig. 1 to the detection screen of the ball machine (“electron source”) in fig. 2, we can see that in fig. 2 the electrons are hitting the detection screen as particles but creating a wave-like diffraction pattern Why this happens is one of the fundamental questions of quantum physics. This is known as the “duality paradox” and it has not yet been resolved. Because waves and particles are fundamentally different, they were thought to be mutually exclusive. However, with the advent of quantum physics and the double-slit experiment they can be thought of as being both a wave and a particle at the same time (Barad Meeting the Universe Halfway, 453).

The Copenhagen Interpretation

“According to the Copenhagen Interpretation, when asked to consider whether light is either a wave or a particle we should reject such a stark “either-or” choice and, despite the fact that these two ways of thinking about light are not logically consistent with one another, take seriously the possibility that light might legitimately be thought of as being both a wave and a particle” (Burns 199).

The Copenhagen Interpretation was the first attempt to understand the quantum world, devised by pioneers of quantum physics, Niels Bohr and Werner Heisenberg in the 1920s. According to the Copenhagen Interpretation, after the electron goes through the slit it is not comprised of matter but of possibility. It exists as a wave of possible locations that encompass all possible paths. It is only when it is detected (so upon measurement) that it becomes a definite thing with properties and location (PBS Space Time: “The Quantum Experiment that Broke Reality” 8:00-45).

To try and pin down which slit each particle was travelling through, Bohr imagined something known as the “which-path device” – a device that can detect exactly which slit the particle travels through. This was a Gedanken experiment that has since been tested in a lab. The device is placed before the slits, so the observer can detect exactly which slit the particle is travelling through. The strange thing about the experiment is that when it is set up, and the observer tracks which slit the particle goes through – the electrons pass through one by one and behave like particles. The particles keep functioning as particles throughout, and there is no diffraction pattern on the detector (Barad Meeting the Universe Halfway, 104-105).

Bohr concluded from this that we can either know which slit the particle goes through by using the “which path” device and have the electron behave like a particle throughout, or the electron displays wave-like behaviour, without the “which path” detector. But we cannot have both at once. They are mutually exclusive, yet necessary to one another. Together they form the whole picture (Barad Meeting the Universe Halfway 106).

“It is impossible to observe both the wave and particle aspects simultaneously. Together, however, they present a fuller description than either of the two taken alone” (“Complementarity Principle”).