Is it a particle or is it a wave?

Sometimes I think I’d gladly be locked up in a dungeon ten fathoms below ground, if in return I could find out one thing: What is light?
Galileo, from the play Life of Galileo, by Bertolt Brecht

 The single greatest source of debate among physicists in the early decades of the last century was to do with the nature of light. Come to think of it, this concept has probably caused more angst than any other to scientists and philosophers right back to the ancient Greeks.
To take just one aspect; we can prove that light is a particle (via the photoelectric effect) and we can prove that light is a wave (via interference, or the famous ‘double slit’ experiment) yet particles and waves are two completely different phenomena. Particles are ‘things’ and are therefore supposed to be localised in space and have mass. And while there are  different varieties of waves, they are not supposed to be ‘in one place’ or have mass.
So what gives?


Answer: nobody knows. To this day there are different interpretations, but none that is accepted by all. The YouTube clip below shows some of the world’s greatest physicists coming together for one of a series of conferences to try to make sense of it all back in the 1920’s. Needless to say they did not reach a consensus. There is wonderful book called QUANTUM which describes in great detail the history of this debate at the beginning of the last century. See here  for a previous post on the book itself.

Now in leaving cert physics we need to know the evidence for light being both a particle and a wave. But there is room in the syllabus or any of the textbooks that I have come across to highlight the bizarre nature of this. It lies at the heart of one of the greatest problems scientists have ever faced, and our response is to simply pretend that there is nothing of note here.

It’s simply not good enough.


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