More on alpha, the fine structure constant

Wayne,

Here’s a section of a blog post by Prof. Chad Orzel at Forbes Magazine. The entire post is good, and its third part deals with one of your questions to me about the fine structure constant.

Bernard

Three Experiments That Show Quantum Physics Is Real

Precision Measurement

Comparison of high-precision measurements of the fine structure constants, providing an incredibly sensitive test of quantum electrodynamics. (Figure by Chad Orzel)

The last idea I want to mention is not, in fact, included in my recent list of six core principles, but is one of the weirdest quantum predictions of all. It comes from Feynman’s formulation of quantum electrodynamics, colloquially known as “QED”, the strange theory of light and matter.

The motion of a single electron interacting with an electromagnetic field seems like something that ought to be really simple to explain, and the quantum theory of the 1930′s almost gets it right. Getting the right answer, though, requires glossing over some effects that everybody in theoretical physics knew ought to be real, which only worked until new experiments enabled by WWII technology made them impossible to ignore. The famous Shelter Island Conference in 1947 threw these issues into stark relief, and kicked off a big effort to solve them. A year later, at the Pocono Conference, Julian Schwinger and Richard Feynman had solved the problem and developed working models of QED; Sin-Itiro Tomonaga in Japan had his own version at about the same time, and Freeman Dyson showed that all three versions were mathematically equivalent.

Feynman’s version of the theory is the most user-friendly and consequently the most famous– most modern descriptions of Schwinger’s theory end up falling back on Feynman’s language to talk about what’s going on. It introduces the idea of “virtual particles” popping into existence out of empty space, interacting with the real particles whose properties we measure for a very short time, then vanishing again. This provides a convenient conceptual explanation for what’s going on, and harnesses the power of narrative to make really abstract mathematics comprehensible. (And is also the inspiration for the silly blog post that launched my book-writing career…)

One of the great things about Feynman’s virtual-particle approach is the way it makes clear that the effect of these virtual particles gets smaller as the scenarios get more complicated. Each virtual particle you add reduces the contribution to measurable properties by about a factor of 137 (a number called the “fine-structure constant” for historical reasons). But if the influence of these particles is so tiny, how can we know this is real?

The answer is that atomic physicists are phenomenally good at measuring tiny shifts effects. Modern precision spectroscopy experiments can measure the interaction between an electron and a magnetic field to something like 14 decimal places. Putting that together with a QED prediction lets you find a value for the fine-structure constant and compare it to the best measurements by other techniques (shown in the image above, a slide from a talk I gave last summer; the paper is in Physical Review Letters and on the arxiv. And, of course, I have an old blog post on this). To get the near-perfect agreement they see requires the QED model to include scenarios involving up to eight interactions with virtual particles, including the baseball-shaped mess in the figure above. These experiments are the most sensitive test we have of QED, and make QED arguably the most precisely tested theory in the history of science.

So, as strange as the idea of material particles briefly appearing from empty space might seem, it’s absolutely essential to explain our best measurements of physics. Virtual particles, like wave nature and non-locality, are an essential part of quantum physics, and absolutely confirmed by experiment.

Bernard

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