Wave Particle Duality Debunked Part 2
Particles are still never waves
In my last article on wave particle duality, We debunked the common myth that particles can sometimes turn into waves. We also talked about what the double slit experiment really means, and how each particle is always detected at one spot. In today’s article, we will be taking this idea even further to demonstrate how we can determine the actual size of particles. Let’s begin.
What is a Particle?
We’ve used the word particle a lot, so it would be nice to define it: A particle is an object that is localized in space (in a specific spot). In other words, particles are just really small objects. Waves on the other hand are the oscillations in a medium. It’s clear from these definitions that particles and waves are very different entities. While particles are very small objects, waves are spread out and are “ripples” of some underlying medium. For example, a water wave is a ripple in the medium of water. Some people may say that waves don’t always need mediums, like electromagnetic waves. It is true that EM waves don’t need mediums, but that is because they are not matter waves (they’re mathematical waves). So a physical wave, by definition, would need to be a medium of some larger space. If this is confusing, don’t worry; we’ll talk more about the distinction between particles and waves in the last article of this series.
How do we detect particles?
A standard way of detecting particles is to observe their footprints in interactions. An example of this is a bubble chamber. In a bubble chamber, alcohol vapor is filled. When a particle like an electron enters the bubble chamber, it ionizes (interacts with) the gas and leaves tracks behind. We can visually see these tracks
In the image above, we can see an electron-positron pair scattering off each other in the bubble chamber. The paths taken are clearly particle-like, not wave-like. This demonstrates that in real experiments we detect particles — not waves.
The particle nature of particles isn’t just seen in bubble chambers. The above image shows what is called a dijet event. A dijet event is the production of 2 jets (streams) of particles which are caused by the color of quarks. In the visual image of a dijet event, we clearly see particles travelling in well-defined streams (the pink and blue lines). If these particles were waves, they’d be spread into space, and you would not see well-defined streams.
What is the Size of a Particle?
In theory, all elementary particles are pointlike. But wait a minute — aren’t particles “vibrations” of fields? That’s another can of worms which will be addressed in the next article, but the reason why physicists say particles are pointlike is due to the fact that pointlike particles interact the same regardless of energy scale. When we scatter particles off eachother (like the dijets seen from the CERN image), the resulting particles are at an angle relative to each other. This angle does not depend at the energy in which you probe the interaction. In physics, larger energy corresponds to “zooming into” our system, and since our system “looks” the same when we zoom in, it implies it has pointlike constituents, since points look the same regardless of how you zoom in.
In physics, point particles are treated very differently from extended objects. In scattering experiments, extended objects are described by “Form factors” which encode the size of a particle. If the form factor of an object changes with energy scale, it implies that it is “smeared out”. But if the form factor of a particle does not depend energy scale, it implies that the particle is pointlike. This property is known as Bjorken Scaling, and it has been observed for quarks at high energy collisions. More information on form factors can be found here.
Since the standard model agrees well with particle experiments, it is believed that these elementary particles are pointlike at the energy scales in which we probe them. This doesn’t mean that particles are literally points though. It means that at the scales in which we observe particles they look like points. Many physicists speculate that at even higher energies (=smaller distances) particles will stop behaving as pointlike entities, and instead like strings. This is the basis for string theory, which I might make an article about sometime.
Why do Physicists talk about Wave Particle Duality?
A common question I got from my last article is why physicists still talk about wave particle duality if it has been invalidated by experiments. The truth is most professional physicists don’t talk about wave particle duality, and it is a dying concept. You may often hear physics popularizers talk about wave particle duality in order to “mystify” their audience with a heavily simplified concept. This gives viewers the false impression that wave particle duality is still a legitimate concept, when in reality it is not. Historically though, wave particle duality was a thing in the early days of QM, but as Feynman says in one of his lectures,
“The wave function ψ(r) for an electron in an atom does not, then, describe a smeared-out electron with a smooth charge density. The electron is either here, or there, or somewhere else, but wherever it is, it is a point charge”
In modern textbooks of quantum mechanics and quantum field theory, talk of wave-particle duality is absent. Examples include Weinberg and Peskin and Schroeder.
In conclusion, wave-particle duality is not a legitimate concept since it mistakes the wave-function for a real physical wave, and ignores how (fundamental) particles act as pointlike in scattering theory. Perhaps the real wave particle duality were the friends we made along the way. It’s time to wave goodbye, till next time :)
But wait! There’s still more! We haven’t gone over why quantum field theory does not prove wave-particle duality, but don’t worry — that’ll be the next article in this series (and hopefully the last). In the meantime, you can check out my channel Fermion Physics where I’ve made some videos on quantum field theory and other topics. I’ll be going over second quantization in a week or so, so if you’re into that kind of stuff, then maybe consider subscribing and or sharing.
Thanks again, till next time (for real this time!) :)