Sometimes I encounter people who proclaim to be of a “pragmatic nature”, and who somewhat oppose theories – perhaps because they think that for instance quantum theory is to remote from the reality they inhabit. However, such a “theory” about theories seems to be ignorant about the stunning precision of these theories. As we wil see in this short text, quantum theory (i.e., Quantum electrodynamics), describes the “real world" with a precision “quivalent to measuring the distance from Los Angeles to New York to the thickness of a human hair” (Feynman, 2006).
“There is no God, and Dirac is his prophet.” ― Wolfgang Pauli (in: Heisenberg, 2013)
Quantum electrodynamics (QED) explains how a spring stretches, and how dynamite explodes; the working of your eye, and how grass is green. More technically, it is a theory that describes all interactions involving light (photons) and charged particles, and, in particular, all interactions involving photons and electrons. Because the interactions between atoms depend on the arrangement of electrons in the clouds around the nuclei, that means that, among other things, QED underpins all of chemistry. In fact, as far as the everyday world is concerned, QED explains everything that isn't explained by gravity. Importantly, there are two other forces of nature, which only operate on a very small scale – within the nucleus of an atom – and are responsible for holding those nuclei together and for radioactivity. But outside the nucleus, on the scale of atoms and above, all that matters is QED and gravity. Both QED and gravity (in the form of Einstein's General Theory of Relativity) are extremely accurate and well-understood theories. In terms of experiments actually carried out in laboratories (on Earth), QED is the outstanding example of a successful theory – that is, one which predicts with unbelievable precision the outcome of experiments. One example is the so-called magnetic moment of the electron. Using Dirac's theory of the electron, theory and experiment agree to an accuracy of two parts in ten decimal places, or 0.00000002 per cent. In his book QED: The Strange Theory of Light and Matter, Richard Feynman points out that this is equivalent to measuring the distance from Los Angeles to New York to the thickness of a human hair – and this is just one example of the many precise agreements between QED and experiment. Very recently, the General Theory of Relativity has been checked to a similar accuracy by studying the behaviour of an astronomical object known as the binary pulsar; but somehow, that isn't quite the same as doing the experiments here on Earth. In that sense, QED is the most successful and accurate of all scientific theories, although both kinds of observation are really equally valid (Gribbin & Gribbin, 2018).
However, even though extremely precise, it seems that also this theory can be further tailored to fit with reality. Put in the words of Paul Dirac himself: “It seems clear that the present quantum mechanics is not in its final form. Some further changes will be needed, just about as drastic as the changes made in passing from Bohr's orbit theory to quantum mechanics. Some day a new quantum mechanics, a relativistic one, will be discovered, in which we will not have these infinities occurring at all. It might very well be that the new quantum mechanics will have determinism in the way that Einstein wanted.”
We began by observing the quote by Pauli that “There is no God, and Dirac is his prophet”. Why? Well, apart from Einstein, Paul Dirac was probably the greatest theoretical physicist of the 20th century. For those of you who are not familiar with Dirac's theories, we are fortunate to have this TED talk from Professor of Physics at Northeastern University in Boston, Graham Farmelo, on Paul Dirac and the Religion of Mathematical Beauty. Dirac, co-inventor of quantum mechanics, is now best known for conceiving of anti-matter and also for his deeply eccentric behavior. For Dirac, the most important attribute of a fundamental theory was its mathematical beauty, an idea that he said was “almost a religion” to him – perhaps the beauty of the dress attests to its degree of being tailored to reality?
References
Feynman, R. P. (2006). QED: The strange theory of light and matter. Princeton University Press.
Gribbin, J., & Gribbin, M. (2018). Richard Feynman: A life in science. Icon Books.
Heisenberg, W. (2013). Der Teil und das Ganze: Gespräche im Umkreis der Atomphysik. Piper Verlag.
