Saturday, October 15, 2011

How atoms are tiny antennas

In my last post I said the universe is full of tiny radio antennas, and they are called atoms. I'm sure people must wonder what I'm exactly talking about. There are few ideas in physics so strongly entrenched as the idea of the Bohr atom with its planetary orbitals and "quantum leaps": the principle that an electron can be in this orbital or that orbital, but nowhere inbetween; and that the mysterious leap from one orbital to the next is accompanied by the emission of a particle of energy called the "photon". How is this an antenna?

In 1926 the Bohr atom was replaced by the Schroedinger atom. The Bohr atom lasted all of eleven years but it is the iconic picture of an atom that everyone remembers. It is the symbol for atomic energy. But what about the Schroedinger atom? What does it look like? If you took high school chemistry, you might have seen pictures of cloudy-looking "orbitals: the s, the p, and the five "d" orbitals including the peculiar one with the equatorial ring. But the real beauty of the Schroedinger atom is what happens when you combine two orbitals at the same time. I've found a nice applet from the University of Saskatchewan that lets you see what they look like:
If you clear the applet and then select the first s orbital and the first p orbital, you'll see the cloud of charge oscillating back and forth. That's what the charge is doing when the electron is in a superposition of two states. It looks just like an antenna, and everybody knows this. Schroedinger was ecstatic when he was able to show you didn't need the quantum leap to explain atomic transitions, and you didn't need to throw out Maxwell's Equations to explain radiation.
But incredibly, the other physicists scoffed at Schroedinger's interpretation! Heisenberg, Born, and Lorentz were firm believers in their own alternative interpretation based on probabilities and wave function collapse. This philosophy was an outgrowth of the original Bohr paradigm with its mysterious quantum leaps, concocted to accomodate the Schroedinger equation without allowing for the physical reality of the Schroedinger picture. I never understood how deeply ingrained these prejudices were until after being shot down in numerous internet forum arguments by physicists who basically agreed with me that the superposition of two atoms "technically" looked like an oscillating charge, but we were not allowed to use Maxwell's Laws to analyze it! When I pointed out that if you did treat them as little antennas, you get the correct values for things like the Einstein A and B coefficients, they said "even if that was true", it was just a coincidence. Yes, technically an atom may be in a superposition of two states, and the "probable" charge location may be oscillating, but there is no radiation associated with that oscillating "probability".
I'm going to try and say it clearly once and for all: everything that atoms do on a thermal level can be understood by following the motion of the charges and applying the classical Maxwell equations of electromagnetism. Schroedinger tried to argue this case for a couple of years but he was howled down by the "mean" physicists. There was actually an experiment done in 1929 which was considered decisive at the time, but which I am convinced has been misinterpreted. The problem with these arguments is that they are conducted on a very abstract level. Physicists, even those at the very top, are desperatley lacking in good physical pictures. That is why my explanation of the crystal radio is so very important. I am continually encountering people who should know better who make grevious mistakes in their explanation of the photoelectric effect because they simply do not have the picture in their heads, which I have in mine, of how a crystal radio absorbs power. I'm going to name one Marlon Scully in particular, a leading proponent of the Jaynes school which famously promoted the "semi-classical" interpretation of quantum optics. I think I'm going to save this discussion for my next blog post.


Anonymous said...

Taking a antenna theory class now and enjoying your posts.

Marty Green said...

Thanks. I'm awfully glad to hear that.

Anonymous said...

This insight is exactly spot on. I'm a "formally" trained physicist and the lack of physical intuition for the theories we hold up as gold standards is deeply unnerving. These mathematical tools are convenient ways to derive results however there are always other ways to arrive at a solution in physics and view a system. Viewing atoms as an antenna has no problem when viewed from a quantum perspective. It is an antenna with one electron and the oscillating fields and transient kinetic energy arising from its oscillation is one energy eigenmode or state of the system so are states of the system where the electron is chilling and a photon is propagating in one of many directions with a probability dependent on the local fields of oscillating electron as suggested by fermi's golden rule (rate of transition between modes is proportional to both coupling (fields) and density of states (which is related to the speed of light in material)). The only reason no one treats a radio antenna as a quantum problem is the sheer complexity of the quantum setup. You need a many body electron wave function with a non equilibrium population distribution oh god it gets bad quick.