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June 2009 Archives

I won't bore you with the details of the conference I've been at in Dresden, but I will mention this. As an example of a complex network, one of the speakers showed a little animation of movement of commercial aircraft across Europe. The animation consisted of a map of europe, with little dots showing the position of every commercial aircraft over the course of one 24 hour period (speeded up of course). I assume this data came from air-traffic control.

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What would you pay to know the time? Probably nothing.

But go back a couple of hundred years, and people made a living out of selling the time. They would own a particularly high quality watch, and every morning they would go up to Greenwich and set it to exactly Greenwich Mean Time (simply the time told by the most accurate clock at the observatory). Then they would take their watch to the homes of their wealthy London clients and sell them the time - in other words for a small fee the clients could look at the watch and so set their clocks to Greenwich Mean Time.

Such behaviour seems pretty bizarre today - but it does beg the question - what kind of things that we pay big money for nowadays would be viewed as utterly trivial in the future? Answers on a postcard please (or hit the 'comments' link below).


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OK, so you are in a ship in the middle of the sea, with no GPS (it is the eighteenth century after all) and you want to know where you are. It's a tricky problem - get it wrong and you end up, like Admiral Sir Cloudesley Shovell and a fair portion of the English fleet in 1707, embarrassingly and fatally wrecked on the Isles of Scilly.

Knowing latitude is easy. If you know what day of year it is (and surely every self-respecting mariner can follow a calendar, then the elevation of the sun at midday - where it is the highest - allows you to work out your latitude. For example, if it is 21 June and the sun happens to be directly overhead at midday, you know you must lie on the tropic of cancer (23 and a half degrees north).


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If ever you find yourself in London, I would very much recommend a visit to Greenwich. It's a great day out - and includes attractions such as the Maritime museum, the old Naval College, and what remains of the tea clipper Cutty Sark after a recent devastating fire. But for a physicist like me, Greenwich really only means one thing, which is the Royal Observatory, home to the Greenwich Meridian.

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Here is a question I've been mulling over for a few days since I heard a cricket commentator raise it during the recent West Indies - South Africa Twenty20 match.  How high do you need to hit a cricket ball in order for it to reach terminal velocity on its way down? - in other words, beyond what height does the height of the ball make no difference to the speed at which it hits the hands of the unlucky fielder underneath?

The commentator's question raised a flurry of email responses, which were read out during the course of the match, some of which sounded somewhat bizarre to me.

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Here's another aeroplane blog entry. I noted at Gatwick Airport in London that a well-known budget airline was proudly saying that there was no weight limit on cabin baggage - all it had to do was be less than a certain size - "If it fits in the box - it goes on the plane", or words to that effect. The allowed size I estimate to be about 50 cm x 40 cm x 30 cm, a reasonable-sized cabin bag. So my immediate thought as a physicist is just how much weight could I stuff into it.

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So you're used to reading about fuel consumption for cars, but what about planes?  The pilot of the rather aging Boeing 747 on which I travelled from Hong Kong to Frankfurt proudly stated that he had 140 thousand litres of fuel on board. (I think that's what he said). The distance is about 9200 km in a straight line (figure courtesy of Google of course) which makes it about 15 litres per km, or, in more familiar terms, 1500 litres per 100 km, or about 0.2 miles per gallon.

Compare that to my car, which will burn about 7 km per 100 km. That is, the 747 uses about the same amount of fuel as 200 cars making the same nine thousand kilometre journey. That said, it does carry close to 400 passengers - i.e. the same fuel consumption roughly as if everyone drove the same distance two to a car. Keep those oil wells pumping... that distance is only a quarter of the return distance from Auckland to Europe.    

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Physicsstop entries may slow down for three or so weeks, as I do a bit of travelling, but please be reassured that I haven't forgotten you.

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Here's a nice experiment to carry out on a freezing cold morning. Before driving to work / school / shopping centre / Auntie Betty's, look under the bonnet of your car and make a note of the level of coolant in the expansion tank. Chances are its fairly low. After you get to work or wherever, open the bonnet again and check out the level now.  See any difference? (NB I mean check the expansion tank - DON'T open the radiator cap with a hot engine)

The coolant expands quite considerably when it gets hot - and what no longer fits in the radiator / cooling system gets pushed into the tank, only to be sucked back up when the engine cools down.

I've been tracking my coolant level closely because I reckon I've got a small leak in the system somewhere, but its hard to be sure with that level constantly going up and down as temperature changes.

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While we are talking about relativity, what about evidence for special relativity? That's the area of physics which talks about the way things move at very high speeds (close to the speed of light).   For example, we talk about things contracting as they get faster (Lorentz contraction) and time slowing down (time dilation). Neither of these things are naturally demonstratable in the lab (although time slowing can be demonstrated with atomic clocks flown on aircraft)

But one lovely experimental verification of special relativity is through the phenomenon of Compton scattering.

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It's ninety years since the 1919 total eclipse of the sun, in which Sir Arthur Eddington provided the first bit of experimental evidence for General Relativity, and shot Einstein to public prominence. What Eddington did was to measure the deflection of the light from stars as it passed close to the large mass of our sun - an eclipse was obviously needed because otherwise stars close to the sun are unobservable. The results were in agreement with what Einstein had predicted a few years earlier and this event really opened up our understanding of the universe.

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