The episode on In Our Time about photons was summed up near the end by all the experts agreeing with an Einstein quote that if you think you understand what a photon is then you’re deluding yourself! So that makes it a trifle daunting to write up the episode but is reassuring in that the reason the subject feels slippery & hard to grasp is because it is 🙂 The three experts who joined Melvyn Bragg in discussing photons were Frank Close (University of Oxford), Wendy Flavell (University of Manchester) and Susan Cartwright (University of Sheffield).
Close opened the discussion by giving a summary of the 19th Century view of light. The key idea at this time was that light was a part of the electromagnetic spectrum. The electromagnetic spectrum is the name given to waves formed electromagnetically – an electrical field builds up, which generates a magnetic field, the electrical field fades away as the magnetic field builds up, and a new electrical field builds up as the magnetic field fades away. These waves can have any frequency, and scientists showed the light was a part of this spectrum (i.e. that this is what light is). The existence of non-visible frequencies was predicted after this.
This didn’t, however, explain all the known observations of light. Cartwright discussed the “black body problem”: as you heat something up it starts to emit light, first red, then yellow and so on up to the bluer wavelengths. Planck figured out that this sequence can be explained if you assume that the light comes in little packets of energy (quanta), and that the amount of energy in each packet is determined by the frequency of the electromagnetic light wave. I don’t think I’d heard of the “black body problem” before, but I was aware of the existence of Planck’s constant – which is part of this theory.
At the time Planck was thinking about this problem it was assumed that the quanta were a property of the heated object and not of light itself – after all it was “known” that light was a wave and waves don’t come in discrete particles. Flavell explained that Einstein suggested that light might need to be thought of as a particle as well, but most people thought that was ludicrous. It wasn’t until after experiments done by Compton on interference patterns, which produced results that could only be caused by light being made up of particles, that it became accepted that photons are both waves and particles.
Having brought us up to speed on the history behind the theory of light’s paradoxical existence as both a wave & a particle the experts moved on to discuss more of the properties of photons. Photons are massless, consisting only of energy. This is why they travel at “the speed of light” – that’s the speed of a massless particle, anything with mass must travel slower. Photons are bosons one of the two broad classes of particles – the other being fermions. The classes are distinguished by how many can exist in the same quantum state at the same time. There can only be one fermion in each quantum state (and this is why we don’t fall through matter), but there can be more than one boson in each quantum state. Photons are also the mechanism by which the electromagnetic force is transferred around between objects.
The wave/particle duality of photons is one of the pairs of things that can’t be measured at the same time. This is the Heisenberg Uncertainty Principle, which I had heard of before but hadn’t realised applied to more than position/speed of particles. The experiments that demonstrate this practically are some of the weirder experimental data I’ve heard of, a proper demonstration of the counter-intuitive nature of quantum physics. If you look at light passing through a diffraction grate, then you see interference patterns – this is light acting as a wave. However, if you measure it at the level of single photons passing through, then you have “forced” the light to act like a particle by counting them and there are no interference patterns. And bizarrely if you measure like this and then delete the data the patterns reappear!?
My write-up of this has definitely not done the subject justice – physics is my weakest subject by far, especially quantum physics. Still interesting to learn a bit about, tho 🙂