Ideas from quantum physics have seeped into popular culture over recent decades in a way they failed to for the first half century after its development. Nevertheless, while memes about Schrödinger’s Cat or Heisenberg’s Uncertainty Principle abound on social media, the physics on which they are built is less widely known. The wave-particle duality is one of those essential pillars. Physics courses devote weeks to just introducing it – but there are still important aspects that can be covered in a short article.
A Little Light History
In the everyday world or our experience, waves and particles are very different things. An important debate of 17th century physics was whether light was a wave or composed of tiny particles. Although an ocean swell is made up of vast numbers of atoms, assumed to be particles, the breaker at the beach is definitely a wave. It was assumed things we didn’t know about, like light, had to be one or the other – even if giants such as Newton and Huygens disagreed on which it was.
Thomas Young, among others, demonstrated wave-like behavior in light the early 19th century. In Young’s classic example, light shone upon a narrow gap passes through and spreads on the other side, just as an ocean wave will when meeting narrow heads guarding a wider bay. Particles encountering such an obstruction would either bounce off or pass through unaffected. Even those that clipped an edge, like a football grazing a goalpost, would respond differently to the light Young observed.
It seemed the question was settled, a rare case of Newton having lost a scientific debate, even posthumously. The way light bends as it passes from one medium to another is also consistent with waves, and the opposite of what particles would do.
However, at the start of the 20th century, Max Planck and Albert Einstein showed that some otherwise inexplicable observations made sense if light behaved like a particle in certain circumstances. Troubling as this was, it was the main reasons cited for Einstein’s Nobel Prize in 1921, since it was then less controversial than Relativity.
In 1923, Arthur Compton proved that X-rays colliding with electrons transferred momentum like particles, but X-rays are just a higher energy (ie shorter wavelength) form of electromagnetic radiation than visible light. Light doesn’t simply shift from being a wave to a particle at some energy threshold. Instead, all light, irrespective of energy, shows both wave-like and particle-like behavior.
Wave or Particle – Why Not Both?
We now know the photons that make up light are not waves like those on the ocean when behaving in a wave-like manner. Instead, what we are seeing is now known as a wavefunction. The photon behaves as if it could be anywhere over a range of locations, with varying probabilities. Instead of being a single point, it acts – and interacts with other things – as if it was stretched and diffused like a wave of smaller components.
However, circumstances can collapse this wavefunction so that it becomes particle-like, confirmed at almost single point.
If this is hard to get your head around, don’t worry, the greatest minds of the first half of the 20th century struggled with it, and arguably even today people just pretend to really grasp it.
It was bad enough for something we were newly exploring to behave in such logic-defying ways – but physicists soon learned that electrons, previously considered to be particles, could also behave as waves when the conditions were right. Just as Young had apparently proven light a wave by shining it on slits and observing the behavior on screens behind, firing electrons at two slits was found to produce wave-like distributions on the other side. These distributions would simply not be possible if electrons had a purely particle nature.
In other words, something that is literally part of our essence, and everything around us, couldn’t be relied upon to be solid, becoming wavelike when the circumstances suited it.
From there it was a small step to discover that the other components of matter, like protons and neutrons, showed the same duality, albeit harder to demonstrate.
Successively more complex experiments have been performed, all confirming that very small entities can take on wave or particle characteristics depending on circumstances. It’s not just single particles either: Atoms made up of several, sometimes hundreds, of protons and neutrons also display wave behavior like diffraction and interference under the right circumstances, as can molecules (collections of atoms). By extension, everything is potentially a wave.
Implications
We don’t experience this wave-particle duality directly because more massive objects have smaller wavefunctions. Heisenberg’s Uncertainty Principle identifies an inverse relationship between an object’s momentum and the extent of its wavefunction. As mass rises, with even a tiny velocity the momentum increases as well, shrinking the wavefunction, and with it uncertainty about an object’s location. Almost anything large enough to see has a wavefunction so staggeringly tiny we can’t measure it, let alone see it with our own eyes.
There are exceptions. Bose-Einstein Condensates are collections of atoms, sometimes in very large numbers, that behave like a sub-atomic particle, allowing them to interact like waves. This occurs because the atoms in the condensate have quantum coherence, which is not true of familiar objects. However, since making Bose Einstein Condensates requires cooling the atoms to almost absolute zero (-273° C or -460° F) we don’t meet it outside the lab (no not even on really cold winter mornings).
That doesn’t mean wave-particle duality cannot affect us. Schrödinger’s Cat comes from a thought experiment intended to show how such duality in something microscopic could spill over into the macroscopic world. Although first proposed to demonstrate we must be misunderstanding subatomic quantum behavior, we are now confident we have this right at the scale of the very small. How what we experience as the world comes to be built up out of such quantum behaviors is something physicists are still debating.
Nevertheless, since many technologies depend on subatomic particles or photons showing wave-particle duality, we don’t need our cats, clothes, or colleagues to show wavelike behavior for the duality to shape our lives.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.
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