1.2   The Evidence, m’Lud

 

The first thing we should consider is whether the whole thing might be a load of cobblers. Maybe your teacher was simply ill-informed and anything can actually exist in any energy state, in which case there would be nothing to worry about.  So let's ask the question: are electrons in atoms ​really​ restricted to specific energy levels? 

 

Think about flame tests: you probably have done these - different colour flames for different elements and all that. If you haven’t, then you should: it’s part of growing up. If all else fails, try setting fire to some newspaper (using appropriate safety precautions and being sure to select a publication with politically obnoxious views) - and ask yourself why the flames are tinged with green. 

Even better, have a look at the spectrum of the emitted light (you probably have a diffraction grating from a Christmas cracker lying around somewhere). This reveals that the yellow light of a sodium flame corresponds just to one narrow line in the spectrum (actually it’s a closely spaced pair but if you can see that with your home built spectrograph, you’re an engineering genius).

 

So what does this mean? Well in the heat of the flame, two things happen. First, a little bit of the sodium chloride decomposes to form some sodium atoms. Then it's the old story: electron, bored of its unchanging existence is excited by the flame but can’t take the high energy lifestyle for long and pretty soon comes crashing back down to where it came from. The energy the electron couldn’t handle is emitted as a photon, a brief testament to its adventure. The crucial thing is that every time this happens, the photon is the same colour - hence the narrow yellow line in the spectrum - which means the amount of energy released by the electron is always the same:  as if every cormorant that ever swooped out of the sky to scoop an unsuspecting fish from the waves started from the same altitude. Only if we embrace the idea of discrete energy levels does it seem to make sense: the electrons are jumping from a specific excited state with an energy that is always the same, back to their ground state which, also, is always the same. 

 

So electrons are strange. But that’s just the beginning. Quantisation - that’s the proper word for specific energy levels - turns out to be much more general than just a quirk of electrons. Its recognition and understanding was one of the great turning points in the evolution of science. A gaggle of ghostly physicists gathers, eager to explain (and to establish their individual claims to immortality). Max Planck is first: he was fascinated by the radiation emitted by hot bodies. “It’s just human nature” he insists, “and it led me to the conclusion that matter must have discrete energy levels.” We’re not going to get into that but it's worth knowing that classical physicists at the time called their inability to explain this distribution “the ultraviolet catastrophe”, just so you got the point that it really upset them.

 

There is, then, plenty of evidence that electron energies – and other energies – are quantised but this just returns us to our original question: why? If an electron is just a lump of stuff, how can there be such restrictions on the energy it can possess? The key to answering this question is for us – as it was for the folk who originally worked it out at the beginning of the 20th century – to forget to about matter for the moment and turn our faces to the light.