by MATHIAS FRISCH
Either cause and effect are the very glue of the cosmos, or they are a naive illusion due to insufficient math. But which?
Do early childhood vaccinations cause autism, as the American model Jenny McCarthy maintains? Are human carbon emissions at the root of global warming? Come to that, if I flick this switch, will it make the light on the porch come on? Presumably I don’t need to persuade you that these would be incredibly useful things to know.
Since anthropogenic greenhouse gas emissions do cause climate change, cutting our emissions would make a difference to future warming. By contrast, autism cannot be prevented by leaving children unvaccinated. Now, there’s a subtlety here. For our judgments to be much use to us, we have to distinguish between causal relations and mere correlations. From 1999 and 2009, the number of people in the US who fell into a swimming pool and drowned varies with the number of films in which Nicholas Cage appeared – but it seems unlikely that we could reduce the number of pool drownings by keeping Cage off the screen, desirable as the remedy might be for other reasons.
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There were two reasons for this. First, causes seemed too vague for a mathematically precise science. If you can’t observe them, how can you measure them? If you can’t measure them, how can you put them in your equations? Second, causality has a definite direction in time: causes have to happen before their effects. Yet the basic laws of physics (as distinct from such higher-level statistical generalisations as the laws of thermodynamics) appear to be time-symmetric: if a certain process is allowed under the basic laws of physics, a video of the same process played backwards will also depict a process that is allowed by the laws.
The 20th-century English philosopher Bertrand Russell concluded from these considerations that, since cause and effect play no fundamental role in physics, they should be removed from the philosophical vocabulary altogether. ‘The law of causality,’ he said with a flourish, ‘like much that passes muster among philosophers, is a relic of a bygone age, surviving, like the monarchy, only because it is erroneously supposed not to do harm.’
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Astronomical observations provide a particularly stark example. How do we know that the points of light in the night sky are stars? The approach using laws and initial (or, in this case, final) conditions to calculate backward in time to the existence of the star would require data on the surface of an enormous sphere of possibly many light years in diameter. Stuck here on Earth as we are, that just isn’t going to happen. So what do we do? Well, we can make use of the fact that we observe points of light at the same celestial latitude and longitude at different moments in time, or at different spatial locations, and that these light points are highly correlated with one another. (These correlations can, for example, be exploited in stellar interferometry.) From these correlations we can infer the existence of the star as common cause of our observations. Causal inference may be superfluous in some idealised, superhuman version of physics, but if you actually want to find out how the Universe works, it is vital.
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