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[Phys-l] Reminiscences on systems of units



On 2 Feb 2010 Mike Edmiston wrote:

I whole-heartedly agree with this. We should explicitly say what we mean. But this creates the usual problem with dimensionless units... when students are making sure "the units come right" the units in fact won't "come out right" because there is nothing to cancel the dimensionless unit that we explicitly wrote into the paperwork. Students have difficulty dealing with this, and I typically don't have a good solution other than telling them just to learn to live with it.

As an undergrad I used mostly cgs, always noting fps equivalents at first. We also used rationalized mks units, later called SI. We became familiar with converting back and forth. Occasionally a professor would dabble in electromagnetic units.

When I was a grad student I took an E&M course from Robert Karplus. He thought it would be good exercise for us to use rationalized cgs units - and so we did! When I converted from solid state to astronomy and astrophysics in 1980 I learned that I would be using cgs again. That was OK, because my thesis professor, Mike Tinkham, always referred to cgs as "God's own units".

Very soon after I started teaching in 1966 the textbook fad for SI came in. I must agree that it helped students, though I used the cgs version of Purcell's E&M book sometime in the eventies. I had been using rationalized mks since junior high school because I was a ham, so it suited me well, but it rendered obsolete all the cgs-oriented constants I had internalized during my own education. The reluctance of astrophysicists to give up cgs was part of the field's attraction.

After I retired I took GR courses* and rose to a new level of appreciation for dimensionless units. Then one day a visitor came and gave a seminar. He used a new set of units based upon the "small circle approximation": h-bar = c = pi = 1. That would probably befuddle Mike Edmiston's students! I think it must be the ultimate in dimensional obscurantism.

I will turn 75 this year and my memory is deteriorating a bit. I can still remember 299 792 458 m/s and 511 keV and a few more, but it is frustrating to realize that I have forgotten something that I remember that I once knew. (At least I can still construct and parse that last phrase.)

Happy Imbolc everyone!

Leigh

*Not covered in undergrad at Berkeley in the fifties