## Saturday, 12 May 2012

### 27. Actions, Reactions, Interactions, and Causality

The question of free will, which I mentioned briefly in Part 26, is linked to that of causality. The causality principle says that every phenomenon or event is an 'effect' for which there must be a 'cause' that precedes it. I shall argue here that this principle is, at best, only an effective theory, often useful in day-to-day situations, particularly when it comes to understanding simple (or simplifiable) macroscopic systems. But an effective theory may not always have a sound logical basis, and is often just a label or nomenclature for something we do not understand very well.

Habitually we tend to subscribe to the idea that any action is followed by a reaction. In fact, we have Newton's 3rd law of motion, which says that to every action there is an equal and opposite reaction. But can we really talk in terms of actions and reactions, or causes and effects, always? No.

Consider two protons, moving towards each other. The force of repulsion between them is not much when they are far apart, but increases as they approach each other, resulting in a bending of their trajectories. They cannot get too close to each other because of the repulsion, and go their separate ways after coming as close as they can.

Both proton trajectories have been affected. Can you tell which is the cause and which the effect? No. Instead of cause and effect, or action and reaction, it makes better sense here to talk only of an interaction. Bring in a third proton and it would become even easier for you to agree with me.

Cut to our solar system. Does the Sun go around the Earth, or does the Earth go around the Sun?

There are complications because of the Moon and other planets etc. (and other celestial bodies), so imagine a simpler situation in which we have only the Sun and the Earth. Most people will still say that it is the Earth that goes around the Sun. The psychology of such an attitude is that the Sun is much heavier than the Earth. But the reality is that the two go around each other. There is a 'centre of mass' for the Sun-plus-Earth system taken as whole, and they both go around that point.

The human tendency is that the larger object is taken as causing the effect on the smaller object(s). The real thing is that there are only interactions, rather than actions and reactions. Philosophers have been tying themselves into knots by carrying the action-reaction or causality idea too far. The absurdity of it all becomes palpable when they even talk of 'downward causality'. Why not just talk of interactions, rather than actions (causes) and reactions (effects)?

As I said, the cause-and-effect approach is at best an effective theory, albeit convenient to use in a large number of practical situations, with the proviso that the effect never precedes the cause. And the last part of this statement is subject to what the special theory of relativity theory demands (cf. Part 10), namely that a signal from the cause cannot travel faster than the speed of light. What this implies is that the meaning of the word 'simultaneous' is observer-dependent. The cause precedes the effect for all inertial observers. The cause and the effect are separated by a 'timelike' interval, and the effect is in the future of the cause.

And according to the general theory of relativity, the effect must belong to the future 'light cone' of its cause, even when the spacetime is curved.

When we come to quantum field theory, the causality idea gets linked to the much-debated 'principle of locality'. But let us not get into those things here.

Consider a beehive. It is a complex system. It has 'swarm intelligence'. No one is in command, not even the queen bee. Each bee follows some very simple 'local rules', and interacts with other bees in the hive. The effect here is the 'emergent' property of swarm intelligence. What is the 'ultimate' cause of this intelligence? Not the action of any one bee. The beehive is the archetypal example of a system in which it is meaningless to talk about causes and effects, or actions and reactions. It is interactions, through and through. This is not an isolated example. Complex systems are generally like that.

The causality idea is well-entrenched in the human psyche, in spite of the above-mentioned limitations. There is no need to abandon it, of course. In fact, much of our conventional science is based on it (conventional or 'traditional' science follows reductionism and constructionism). Logical reasoning in conventional science is one big chain of cause-effect-cause-effect-cause- .... interpretations: An effect becomes the cause for the next event or process, and so on. But conventional science is often quite inadequate for tackling complexity-related, highly highly nonlinear, problems. Radically new thinking is needed when any simplifying assumption can destroy the very essence of the complex system being investigated, or when it is impossible to model a system in terms of an adequate number of differential equations, or difference equations. But some of the unconventional approaches formulated have met with strong resistance from many practitioners of conventional analytical science. Mindsets do not change easily.

Be prepared to think in terms of interactions and correlations when necessary, rather than actions and reactions all the time. Such an approach will help you better understand the properties of complex systems, and keep you away from absurdities like 'downward causality'.