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A PHYSICIST WRITES . . .

(March 2005)

Some scientists must live and work in a perpetual dilemma. If they are to be respected and understood by the non-scientific public, then they must attempt to explain to the public what science is all about, in simple terms. But a few scientific words at least will have to be included. If one of these words or phrases is fancied by non-scientists and starts to be used widely, its meaning will very likely be altered ó making science even harder to explain the next time!

Probably the most glaring recent example of this (to a scientist) is quantum jump. In physics, a quantum is the smallest possible unit that something (such as energy) can exist in, or else the smallest amount that it can change by. Usually a quantum is very tiny indeed. But the word escaped into the outside world and now a quantum jump means just about the largest change you can imagine.

Feedback is another word that has become public property ó with the curious result that the two different sorts of feedback have swapped over their meanings, more or less. Positive feedback (on your driving performance, for instance) says good, negative says bad, right? But not to scientists and engineers. For us, positive feedback means that if something shifts its position, then it will be pushed even further out of balance. What we generally like is negative feedback, which pushes in the opposite direction back to stability. Negative feedback is good for you!

Especially when you are motoring. Every vehicle engine, I guess, contains a thermostat to control its temperature. When this rises, the thermostat opens further to let more coolant through and send the temperature in the opposite direction. Then if it falls too far, the thermostat closes a bit to increase it. This Ďloopí of actions, working to push the temperature in the opposite direction to the last change, produces the stability that the engine needs in order to run as efficiently as possible.

There are other negative-feedback loops that involve you, the driver. Perhaps the simplest is when you are aiming for a steady speed: the road turns slightly uphill, you notice the fall in speed and so you lean a bit on the accelerator. Further on, if the road descends a little, you donít even have to check the speedometer to know that you need to lift your foot.

I would call these actions semi-automatic feedback, because you are thinking about the foot adjustment some of the time at least. An example of fully automatic feedback (except maybe while someone is still learning to drive) is steering a planned course along the road.

It astonishes me that I can drive along a straight dual carriageway at 70 mph holding the steering wheel almost motionless, perhaps nudging it just a millimetre or two every second or so ó though I really canít consciously see why, each time. Whatís happening is that my eyes, brain, arm muscles and steering gear are joining together in a feedback loop which is largely unconscious and which generates high stability. How on earth did this ability evolve in us?

Itís as if the car is locked into the middle of the lane, or is even steering itself! The temptation to relax and look at the scenery is almost irresistible. But that would immediately break the loop and soon the car would drift. If I want to adjust the radio, I find I have to concentrate twice as hard on the road ahead while doing it. And looking at my hand while making the adjustment is a recipe for disaster.

This all goes to show how fragile the steering feedback loop is ó yet we rely on it for our lives totally, in high-speed driving. Amazingly it seems to operate almost as well after dark when the lane markings are harder to see. Negotiating a bend accurately must require even greater amounts of subconscious skill and brain-work, with all the lines in view (not to mention your own path) being curved instead of straight.

But even a negative-feedback loop can become unstable. If this happens to steering, the result is bigger movements of the wheel. I donít mean like when you are steering around obstacles, but when the wheel is simply moving to and fro more than usual, under your hands.

I try to watch for this extra movement, as a warning that something I havenít consciously noticed is trying to disrupt the steering feedback loop. It might be cross-wind, or slightly reduced visibility ahead, or haze on the windscreen, or tired eyes, muscles or brain, or maybe some combination of these ó good reason anyway to slow down a bit. In fact, what Iím looking out for is negative feedback on my driving performance!

Peter Soul

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