I used to think QED was short for Quod Erat Demonstrandum, which is put at the end of a mathematical or philosophical deduction to say argument complete.
Last year I took a beginners course I quantum physics. It was as weird and wonderful as I had hoped for. One topic which was briefly touched upon was the alternative QED interpretation, namely quantum electrodynamics. Richard Feynman’s book QED – The Strange Theory of Light and Matter was recommended as further reading. This week I finally got around to read it. My vague recollections of QED from the course included matrix operations, imaginary numbers and a general sense of puzzlement. When opening Feynman’s book, I was not too optimistic about gaining a proper understanding.
Feynman’s ability to communicate this complex topic to laymen is impressive though. The book is based on four lectures given to the general public. Without using mathematics or complicated terminology, Feynman manages to explain what really goes on in QED. Or rather, what goes on in the calculations behind QED, because who knows what really goes on in quantum physics.
The Feynman Lectures
In the first lecture, Feynman explains that the quantum world is not deterministic, which is illustrated by the example of monochromatic light being reflected by glass. We cannot know for certain, if a photon will be reflected by a glass surface, but we can calculate the probability, which is 4%. This probability can be represented by an arrow which squared length equals 4% and the direction represents the timing.
If we want to calculate the probability for an event, which can occur in two different ways (the photon can be reflected by surface 1 or surface 2 to end up in point C) we will have to ‘add’ the arrows for these two events. Obviously the probability of a photon being reflected by one of the two glass surfaces is not necessarily 2 x 4% = 8%. That would be way too simple. Depending of the thickness of the class, which affects the timing and thereby the direction of the two arrows relative to each other, the probability fluctuates between 0% and 16%.
In lecture two, Feynman explains that we actually have to take into account every possible path a photon can take to point C, when calculating the probability of the photon ending up at C. This gives us an unlimited number of small arrows. As it turns out the extreme paths cancel out and only the paths relatively close to the quickest path end up contributing to the probability. The exception is when we try to control light by using a pair of blocks to limit where it can go. In a case of pure defiance the light will then spread out and seemingly unlikely paths can get a relatively high probability. We also see how the probability of a path containing several steps can be calculated by multiplying the arrows for each individual step.
The third lecture is more complex and I do feel a bit bad for the audience attending unless they had the option to stop and rewind Feynman with regular intervals. There is a lot to take in. Feynman reveals that the photon being reflected by the glass surface is a simplification. In reality the photon is being scattered by the electrons inside of the glass and a new photon is emitted by the electrons which may end up in point C.
All phenomena can be described by combining three basic actions:
- Photon goes from place to place
- Electron goes from place to place
- Electron emits or absorbs photon
Again, whenever calculating probabilities for a specific outcome all possible paths leading to that outcome must be included. Even the simple outcome ‘we start out with an electron and a photon and end up with an electron and a photon’ can happen via unfathomable paths such as ‘the electron emits a photon, then travel backwards in time to absorb a photon and then proceeds forwards in time again’. When I read this a huge smile was spreading on my face. Seriously, you got to love quantum physics!
Lecture three also describes the double slit experiment, which readers of popular science will be familiar with. It is a classic illustration of the weird behaviour of the quantum world. My favourite part is that when we try to narrow down this weirdness and measure what really happens, the weirdness disappears and the result we would rationally expect is observed. It is like nature tongue-in-cheek tells us ‘surely, you did not expect to catch me out that easily’.
The final lecture concludes with some of the challenges of QED, how it relates to other branches of physics and the standard model as it looked back then.
But what does it all mean?
So in short, light does not always travel in straight lines, nor does it always travel with the speed of light. Electrons are way ahead of us when it comes to time travel and the world around us can be explained by manipulating tiny arrows. The more you read about nature, the weirder it gets. Does that apply to any other topic than quantum physics?
Feynman’s knowledge, enthusiasm for his subject and his wicked sense of humour bounces off the pages. It is a huge privilege to have such a brilliant mind, who can communicate such complex concepts to us mere mortals.
Throughout the book, I could not help asking myself: but what does it all mean? Feynman is the first to admit that nature is absurd and we cannot understand it. Personally, I find it a separate mystery, that Feynman could come up with this extensive mathematical framework to describe nature accurately, without understanding nature itself.
It does feel a bit like reading an unfinished Agatha Christie mystery though. All the actors have been lined up, with the electron and the photon taking leading roles. We have learned about their paths, their behaviour and their interactions. Now we are all waiting for Poirot to explain what really lies behind, which puts the actions observed throughout the book into perspective and clarifies the true nature of each characters role in relation to the mystery. Except, this point never comes. Being left there yearning to get to the core of it all is utterly delightful and utterly frustrating at the same time.
But as Feynman says: ‘you can have all the philosophical worries you want as to what amplitudes mean (if, indeed they mean anything at all), but because physics is an experimental science and the framework agrees with experiment, it’s good enough for us so far’. Q.E.D.
Title: QED - The Strange Theory of Light and Matter
Author: Richard P. Feynman
Format: Book
Genre: Popular Science, Physics