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A PHYSICIST WRITES . . .
(April 2007)
I do try, in these columns, to disguise the fact that physics is firmly based on mathematics. On the other hand, motoring matters often involve numbers too, so perhaps I ought not to protect you from them so much! Let’s see what you make of this news item I heard on Radio 4 last month: “Ministers are urging drivers to ease off the accelerator, cut down on revving the engine, clear the clutter to reduce weight and keep tyres at the correct pressure. It’s claimed that the measures could cut CO2 emissions from cars by 8% a year.”
I’ve missed some of it out, but the last sentence is exactly what was broadcast. Does it make sense to you? Could these changes really reduce CO2 emission more and more every year? Of course not — it would decrease just once, and then stay at the new level (other things being equal). The last two words in the news item should not have been there. Later I tracked down the original report. In it the claimed reduction was “5.5 million tonnes of CO2 a year, or 8% of what cars emit annually now.” Can you see how this step-change turned into an annual rate of decrease when the news item was being written?
The media continually bombard us with statistics, percentages, rates of change and other numbers. They are sometimes misleading, sometimes incorrect (like my example above) and sometimes merely puzzling. Just occasionally they’re expressed accurately and clearly. But even when they’re not, we ought to try to understand what’s being said. Don’t let the blighters get away with it!
Take those full-page adverts from a certain airline proclaiming: “Aviation accounts for just 2% of CO2 emissions — so Hands off our Holidays!”. They are saying this because all flights (globally) contribute about 2% to all emissions (globally). But the total carbon output of the UK as a country, from all types of source, is also 2% of the global figure I believe. Does the smallness of this percentage mean that the UK shouldn’t be bothering to try to cut back at all?
In any case, if you look at just us British travellers, I’ve read that when we’re in the air our share of the UK carbon output is more like 10%, or nearly as much as when we are driving our cars. And whilst there is every hope of installing cleaner technology in cars in the future, there’s very little prospect of making aircraft less polluting [in October 2007 I backtracked on this statement!]. So, as with most statistics, the figures are being bent to suit the interests of the messenger ... but I must get off this hobby-horse of mine.
Last month, after describing the paths of rays of sunlight across the sky, I said that light didn’t always travel in straight lines. One of the effects I was thinking of was scattering. This occurs everywhere you look: clouds are white because all the colours in sunlight are scattered equally by the little water droplets. The sky is blue because the much smaller air molecules scatter this colour more than they do the others — it was Einstein who worked out how this happens.
Later in the day as the sun sets and has to shine a much greater distance through the atmosphere, the other colours are scattered too (especially when there is dust about) except for red. Thus the sun illuminates any clouds directly with reddish light and we see the shepherd’s delight, or a red sky at night.
And what’s the opposite of this? It’s the shepherd’s warning (a red sky in the morning) of rain possibly to come. Suppose you’re at a T-junction, peering right and left through your side windows which been made almost opaque by the rain-drops on them — another example of random scattering of light. Here’s a trick that sometimes helps you to see out: move your head rapidly forward and back (in line with the car, I mean) a few inches. This will average out the effects of the scattering to some extent and you should be able to observe the approaching traffic. If not, then you will just have to wind down the window!
Sometimes light finds itself travelling in a very gentle curve. This happens, for example, when it is passing almost horizontally through layers of air that have different temperatures. You must have noticed the shimmer on a hot road in the distance, as if it had a reflecting liquid surface. What you are seeing is light that came down at a glancing angle to the road, met the hot air next to it after passing through the slightly cooler air above, and curved away from this hotter layer (sound waves can behave likewise — see my October 2002 column).
A similar thing is taking place when the sun appears to be flattened a bit at sunset. And in very rare circumstances, on a desert trek perhaps, you might see a classic mirage, in which light is refracted many miles around the curve of the earth’s surface through the atmosphere, showing you distant wonders (or rather, wonderful optical distortions of distant things that are probably quite ordinary).
Finally, I congratulate John Connor [a fellow columnist in our newsletter] on referring to Einstein in only his second column, last month — when I haven’t found an excuse to do so in nearly five years, until this month! What Einstein said and what John misquoted (deliberately) was: “When a man sits with a pretty girl for an hour, it seems like a minute. Let him sit on a hot stove for a minute and it's longer than any hour. That's relativity.” But I also like another law attributed to him: “Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.”
Peter Soul
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