Particle Physics (IYQ 2025)

Despite 2025 being designated the International Year of Quantum Science and Technology (IYQ) we still know surprisingly little about how quantum effects “work”.

According to this year’s anniversary, the quantum story started in 1925 when Werner Heisenberg formulated his matrix mechanics while spending time on the German island of Helgoland. But Heisenberg was not the first person to consider quantum effects: a full twenty years earlier, Albert Einstein explained the photoelectric effect by treating light as discrete packets of energy, called photons. And even this mechanism built on earlier work by Max Planck, who first conceived the idea of energy being emitted from, or absorbed by, matter in discrete quanta.

Heisenberg’s contribution was to rationalise the earlier theories and observations using an underlying principle – albeit one that we still don’t really understand 100 years later.

To help celebrate IYQ, and also highlighting the gaps in our knowledge, Sean Carroll (author of The Biggest Ideas in the Universe books) has written an excellent summary that Nature has made open access. The article can be read online at https://www.nature.com/articles/d41586-025-00296-9 and there is also a link to download the document as a PDF for offline reading.

The crux of the matter, according to Carroll, is this: “Whereas in classical physics, a particle such as an electron has a real, objective position and momentum at any given moment, in quantum mechanics, those quantities don’t, in general, ‘exist’ in any objective way before that measurement. Position and momentum are things that can be observed but they are not pre-existing facts.”

This in turn links to the idea of a wavefunction that gives a value for each and every possible position of a specific particle. Unfortunately, multiple particles are not defined by multiple wavefunctions but rather by a single wavefunction in which each particle depends on every other particle, resulting in the concept of entanglement that Einstein referred to as “spooky action at a distance”. Carroll goes on to discuss what is really happening – or whether the concept of “really happening” is itself fundamentally flawed. It’s an excellent and very readable account that I highly recommend.

For those who would rather explore particle physics in a more hands-on manner there is a great sixteen-part introduction to this topic available at https://ppc.web.cern.ch/. Developed by CERN physicists Jeff Wiener and Julia Woithe, each chapter is presented as a short video (typically 10-20 minutes long) with an accompanying quiz that has carefully-worded questions, offering multiple answers, to truly test your understanding. There is a retry option for incorrectly answered questions and if you make it through all 16 chapters you will qualify for a certificate of participation.

You need to register as a guest on the CERN platform first but everything is available free of charge and I can vouch for the course being both interesting and at times challenging.