Units, Units, Units October 23, 2009Posted by gordonwatts in physics, physics life.
Undergraduates know that Physics Professors get all wound up about units. We can’t help ourselves.
But in reading a nytimes article this morning I couldn’t help myself:
In addition, Mr. Holder said, the authorities have seized more than $32 million in American currency, 2,700 pounds of methamphetamine, 4,400 pounds of cocaine, 16,000 pounds of marijuana and 29 pounds of heroin. More arrests are expected.
Well… this is what happens when you wait until the evening to write a blog post you spotted in the morning – they change the article. That 2700 pounds? It was 2700 kilograms (which is significantly more). In short – they had mixed kilograms and pounds. I was going to get on my high horse and… well, seems someone at the times is as sensitive about this as us physicists are.
But it also occured to me that the notion of units is rather flexible. For example, when we do particle physics calculations we often set the speed of light to 1. Normally it is 300000000 meters/second (really fast!). Seriously. We just set it to 1. We are so annoyed by having to carry around that number in our calculations that we just up and set it to one. We do that with an other constant as well (called h-bar). Your unit system ends up being very weird when you do that:
Normal Every Day Units Units in h-bar = c = 1 Energy Energy Time 1/Energy Mass Energy Length 1/Energy
I know this seems weird – but you see it all the time. This is just like making the following unit conversion in the list of drugs: instead of telling us the number of pounds or kilograms, tell us how much pot they got in terms of its street value. And to tell the truth, that would have been a very useful number to have in that article.
Heck, in the old days, the unit of measure in the market was the length of the king’s forearm. When the king changed, the whole country would change its unit system…
Un physics professors getting wound up with units is ironic – we don’t really use them that heavily when we get to more advanced calculations. On the other hand, we can only drop them because we have already learned how to use them. At least, that is what we tell ourselves and everyone else!