MadSci Network: Physics
Query:

Re: Strange nature of electromagnetic force carriers

Date: Tue Apr 13 19:20:20 2004
Posted By: Benjamin Monreal, Grad student, Physics, MIT
Area of science: Physics
ID: 1078268566.Ph
Message:

Hello,

Those are very good questions you ask. In fact, they are very similar to the questions that everyone asked when "quantum electrodynamics" or QED (the quantum description of light waves/photons/electromagnetism) was first invented in the 1950s and 60s. One of the developers of QED (I can't remember offhand which one - Julian Schwinger? Freeman Dyson?) was able to sit down with QED equations - which are obviously very good at describing light, atoms, radiation, etc. - and proved mathematically that, yes, QED does predict the repulsion of two charged pith balls, or the attraction and repulsion of iron magnets.

This mathematical proof alone should convince us that the theory is OK. Nevertheless, it'd be nice to have a more intuitive sense of what is going on.

First off, let's ask - why doesn't a refrigerator magnet excite atoms? Why doesn't a light beam spin a compass-needle? Why doesn't a radio wave turn photographic film dark? Why does a light bulb not interfere with an AM radio? Why doesn't a microwave oven ionize atoms? Those are all electromagnetic wave phenomena, true, but they have different frequencies, or wavelengths, or energies. An atom, whose typical energies are of order 1 electron volt (1 eV), doesn't get excited by radio waves (micro-eV, or less) or by gamma rays (millions of eV). Electrons in a radio antenna will vibrate at any wavelength from nanometers (UV) to millions of meters, but only particular frequencies will excite the electronics that control the speaker. The water molecules in your microwaveable soup will vibrate at any frequency below about 2 GHz, but at that frequency in particular, they pick up a lot of power, enabling the water to heat up. A compass-needle, being fairly heavy, won't move much in response to high-frequency fields, but can certainly move in response to very low-frequency fields. It's really amazing how diverse electromagnetic waves can be, but it's all just a matter of wavelength/frequency/energy!

Here's an experiment lying somewhere between the 'static' and the 'photon' versions of electromagnetism: Take a magnetic compass - how fast does the needle swing back and forth when shaken? About once per second, say? Take a strong refrigerator magnet and hold it near the compass. You can get the compass to line up with the magnet if you hold it close, or to ignore the magnet and line up facing north if you hold the magnet far away. (So, for this experiment, let's hold the magnet not too far away. Close enough so that it works. :) ) If you shake the magnet in your hand - up and down, or perhaps by rotating your wrist, with the SAME FREQUENCY as the compass needle's natural swing, you may be able to start the compass needle oscillating. (NOTE: I think this experiment will work, but I haven't tried it, so maybe it is more difficult than I think) You know what you just did? Moving the magnet sent out an extremely low frequency electromagnetic wave - a real radio wave - that was picked up by the compass. You were broadcasting at 1Hz. In principle, a very unusual radio could pick up your weak 'broadcast', which (to an observer far enough away) contains exactly the same sort of varying electric and magnetic fields as any electromagnetic wave!

So that's a 1Hz radio wave. Slow your it down more - one cycle every ten seconds, then one cycle every minute, one per hour, one per year - and you'll find something closer and closer to the "static" fields that attract and repel magnets and charges. Maybe static magnetic fields are just the photons taken to the extremely-low frequency limit of zero cycles per second? (You should learn about what we call 'near-field' and 'far-field' radiation before believing my answer on this.)

There's another way to think of how static fields can be made up of photons; I think this is the answer most physicists would prefer, but it can be a bit counterintuitive. We usually say that the magnet and their iron are exchanging virtual photons. Photons - you can think of them as low-frequency photons, but not zero-frequency - are leaping off of the magnet and being absorbed by the iron, and vice-versa. You might think that all of this emission and absorption should consume energy, but that's not the case for virtual photons. You might also think that absorbing a photon - like stopping a bullet - could only push the magnets apart, but that is also not true for this sort of interaction. These are the same sort of photons that we high-energy physicists put into our Feynman diagrams.

Hope this helps - thanks for asking! It's always interesting and important to think about how our theory of small things - quantum mechanics - describes big things like magnets and baseballs and radio antennas. It sometimes gets complicated and counterintuitive, but it really does work out!

-Ben Monreal


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