Jan 23, 2011
Everyday Relativity
Many people think that relativity is some obscure science concept that is only important to academics and nerds like myself, when, in fact, relativity is important on a daily basis. GPS? Without taking relativity into account, they’d be wildly inaccurate on the order of kilometers per day. And recently there was a new bit of science done that shows it’s important in another place: your car battery!
First, however, some background. I’m a chemist (or used to be one), so I’m a super nerd when it comes to relativistic chemistry. It turns out that as elements get heavier, relativity becomes more and more important. As a theoretical chemist, one thing you learn quickly is that if you don’t take relativity into account, your calculation will be way off.
Without relativity, for example, my thesis work on IBr- had to take into account relativity. Iodine is a heavy element so relativity is important, so, I had to consider it. I did this the easy way: I let others take care of it! I used what’s called an ECP that sort of built in the relativity without my needing to worry.
I can hear you saying: so what, IBr isn’t that important and I’ve never heard of it. But, let’s say you want to calculate the what color gold metal should be or what mercury should be like at room temperature. Without relativity, gold looks like silver and mercury is a solid. With relativity, gold is gold and mercury is a liquid.
So now to the main event: why your car battery only works because of relativity. Couple weeks ago, a paper came out in Physical Review Letters (PRL, 106, 018301 (2011) if you have access to it) by Pekka Pyykkö and collaborators called “Relativity and the Lead-Acid Battery”. This paper uses relativistic quantum chemistry to model the reactions in a lead-acid battery (like the 12V battery in your car) and it found something rather amazing. Of the voltage generated by a lead-acid cell, which is ~2.1 V (there are 6 cells in a 12 V car battery), fully 1.7-1.8 V is due to relativistic effects. 80-85% of the voltage in a car battery is there because of relativity!
As the authors note in the paper, while the lead-acid cell has been known for 150 years, this paper is the first doing ab initio, or first principles, calculations, compared to the more modern lithium-ion cells of which ab initio calculations abound. The major reason for this is that lead is heavy and a heavy atom is a hard atom to calculate.
(They also show that without relativity, it’d make for a crap battery. They do calculations with lead’s lighter brother tin and find that a tin-acid battery wouldn’t be worth it’s weight in, well, tin.)
I just love it when something I studied, I’ve used, I know about has such a cool result like this.
Yours in NERD!,
Matt
