BY ESTHER INGLIS-ARKELL
Everyone, even those contemptuous of ice skating, has seen a figure skater’s spin speed up as she draws her extended arms and legs towards her body. This is an example of the conservation of angular momentum. When the figure skater enters the spin with extended arms, the overall energy of her body has to move her hands and arms in large circles. As she pulls her arms in, there’s just as much energy as there was before, but the hands only move in small circles around the body. To conserve energy, the whole body moves faster.
Conservation of angular momentum has always been considered universal. Now, a new experiment positively demonstrates that it applies both to the way energy moves in chemicals and the way our spinning skater moves on ice. A group of chemists, lead by James McCusker, found a way to show that a luminescent reaction was also ruled by the conservation of angular momentum.
Skaters aren’t the only things that spin. Electrons do as well, and if they do it in pairs, they generally cancel out. Unpaired electrons, then, determine the ‘value’ of angular momentum for any given molecule. The molecule for this experiment involves two extensions of atoms from a core chromium atom. When energy is pushed into the system, in the form of photons, it is converted into a sort of ‘electronic’ energy – an elevated state of the electrons in the extensions of the molecule. The energy is then transferred from the extensions to the chromium core, which sends the energy back out in the form of light. This molecule, in essence, gives off light when light is shone on it. Or, at least it should do so when angular momentum is conserved in the transfer of the energy from the extension to the core.
The scientists then came up with another molecule. It is exactly the same as the first, except the chromium core is ripped out and a cobalt core is put in its place. If energy were transferred from the extensions of the molecule to its cobalt core, the core would light up, just as before. However, a different number of unpaired electrons would be spinning, and angular momentum would not be conserved.
If conservation of angular momentum wasn’t a key part of the shift of energy within that molecule, either both molecules would light up, or neither would. If conservation of angular momentum was key, then the first molecule would light up, and the second would not. To everyone’s satisfaction, the chromium molecule lit up and the cobalt molecule did not. Angular momentum is just as important to tiny systems as it is to large ones.
McCusker told “Basically, we designed a molecule that, if angular momentum was conserved, would have this reaction. The energy only transfers if the total angular momentum is conserved.”
Published in Io9 Jan2, 2012