Every year, as part of a final year statistical physics course, I have a lecture on correlation vs. causation. This is a very important distinction in science. For example, the average temperature over the Earth’s surface, and the number of pirates in the world’s seas, are correlated

However, this is not cause and effect, unlike a plot of current in a wire as a function of the voltage across the wire.

In the above plot (click on it to see a larger version), what is happening is that over the last 200 years we have cracked down on pirates, so reducing their number, at the same time as producing a lot of carbon dioxide, so increasing global temperatures. The fact that we have done both simultaneously has created a correlation between the number of pirates, and global temperatures.

The situation is different for the current in a wire, we know that increasing the voltage increases the current, so they are not just correlated, the current is caused by the voltage.

These are two simple examples. We have understood for a long time that voltages create currents, and few people would be fooled into thinking that increasing piracy will counteract global warming. The pirates example is a deliberately silly example, devised in the debate over teaching creationism.

In research, things are not so simple, unfortunately. One of the things I research on is a protein called Dystrophin – it is the protein that when missing causes the disease Duchenne Muscular Dystrophy. This disease causes muscle cells to die. The question is: What kills the cells that lack this protein?

The cells are known to have tears or rips in the membranes, and also to have the wrong amount of calcium in them. Changes in calcium concentrations are what triggers a muscle cell to contract, so calcium concentrations in muscle cells are crucial.

The problem is: What is cause, and what is effect? Do the tears in the membrane cause the change in amount of calcium, or does the calcium change lead to the tears? Unfortunately, muscle cells are sufficiently complex, that we do not know. We will have to get more data, test more ideas, and find out.