ow do you imagine Albert Einstein? Mention of his name brings to mind the amiable face of a wise old man with unruly white hair. Yet at the time he made the greatest strides in science,Einstein was a vigorous young man with thick, dark hair and penetrating eyes. He was brilliant, imaginative and a lively correspondent.

All the letters written to or by Albert Einstein are stored in an archive in the Hebrew University in Jerusalem. Letters from fellow physicists and from admiring members of the public are filed alongside those containing racist and anti-Semitic abuse. One section is devoted to the letters of children - to whom Einstein always made a point of replying. This principle led him into a touching correspondence with Tyfanwy Williams, a pupil at St Cyprian’s Boarding school in Cape Town, South Africa, in 1946.

I was not aware that you were still alive. Tyfanwy’s first letter, on July 10, asked him, as “the greatest scientist that ever lived”, for his autograph. Not that she collected autographs, she explained; but she would like his. The truth was that she had thought him dead. Most great scientists “had lived in the 18th century, or somewhere around that time”. Maybe she had “mixed him up with Sir Isaac Newton or someone”. Anyway, she had got really excited when the maths mistress “who we can always side-track” began talking about brilliant scientists and mentioned his escape from the Nazis. When Tyfanwy asked if Einstein was buried in America, the teacher had said that he was not dead yet. “I got so excited when I heard that, that I all but got a maths detention!” She added, however, “I am sorry that you have become an American citizen; I would much prefer you in England.”

I am awfully interested in Science. Tyfanwy, with her dormitory friends the Woodrow twins, stole out of bed after lights out to study the stars. “For the first part of the year we had the Pleiades, and the constellation of Orion, then Castor and Pollux, and what we thought to be Mars and Saturn”. But their observations came under threat: “We have been caught a few times now, so it’s rather difficult.” What bothered Tyfanwy was “How can Space go on for ever?” She had difficulties with Einstein’s theories - “If you do not mind me saying so, I do not really see how Space can be spiral”. She concluded, “I trust you are well, and will continue to make many more great scientific discoveries.”

I have to apologise to you that I am still among the living. Einstein replied on August 25. “I have to apologise to you that I am still among the living. There will be a remedy for this, however”. He reassured her about “curved space”. “Used in the right sense, the word “curved” has not exactly the same meaning as in everyday language”. He hoped that her astronomical studies would “not be discovered any more by the eyes and ears of your school government. That is the attitude taken by most good citizens toward their government and rightly so.” ‘The letter was signed “Albert Einstein”. Tyfanwy had got her autograph.

I cannot tell you how thrilled I was. “I cannot tell you how thrilled I was to receive your letter yesterday,” Tyfanwy wrote on September 19. She wrote during a mathematics prep period. “Outside the birds are singing and all that sort of thing, and here we sit and learn that x and y is equal to something divided by something else!” But her letter had brought her fame - “the news that I had your signature went round the school in no time”.

Tyfanwy assured him that she was not dis-appointed to find that he was still alive. “It’s much nicer for one’s favourite scientist in history to be alive, than to know he died something like a century ago”. She was glad to be reassured about curved space. She told him about the Southern Cross, which she could see from her window: “It’s an awfully fine constellation and when I am feeling fed up after a day at school, I look at it and it cheers me up no end”.

I hope you will not think any the less of me for being a girl. She added: “I forgot to tell you, in my last letter, that I was a girl. I mean I am a girl.” Although Tyfanwy had always regretted this, she was “by now more or less resigned to the fact”.

Einstein’s reply was simple. “I do not mind that you are a girl. But the main thing is that you yourself do not mind. There is no reason for it.”

A single bomb of this typeI might very well destroy the whole port In August, 1939, Einstein and a number of other scientists, concerned at the progress the Nazis in Germany were making in nuclear science, wrote to President Roosevelt. Einstein alone signed the letter. While he predicted nuclear bombs - “the element uranium may be turned into a new and important source of energy in the immediate future” - he could not conceive of delivery by any means but ship - “such bombs might prove to be too heavy for transportation by air”. However, he suspected that a ship sailing into harbour with “a single bomb of this type... might very well destroy the whole port”.

He and his colleagues were aware that Nazi Germany had control of uranium from Czechoslovakia, while the United States’ own reserves were poor.

Franklin Roosevelt replied in October, assuring Einstein that he was taking action. The Manhattan Project was launched, which led to the atomic bomb and the destruction of Hiroshima and Nagasaki. Einstein never forgave himself for this outcome. He spoke out passionately against nuclear weapons. In 1955, a week before he died, he wrote an open letter urging scientists to unite to prevent nuclear war.

LIGHT YEARS AHEAD - WHAT DO EINSTEIN’S THEORIES MEAN?

Light and time are inextricably interwoven. Suppose you were in a racing car that could travel at the speed of light. As it blasts off at 186,000 miles per second you look back at a clock on the launch pad. It is noon, as it is every time you look back. You are in a box of your own time and space, and there is no universal time.

Relativity The racing car example is at the centre of the Principle of Relativity. You in your racing car - and a friend back home - are both subject to the same laws about time, distance, mass and force; but the values you get for them will be different.

At the speed of light As you approach the speed of light, relativity makes things change for you. Three minutes for you at half the speed of light is three-and-a-half minutes for your friend back home. Horizontal distances collapse.

Things you pass seem more crowded together. People look thin and tall. Tops of buildings seem to bend inwards, colours change ...

As your racing car whizzes towards your friend, it seems blue. As it shoots away it seems red. These changes are due to both the behaviour of light and to relativity.

Bending light Einstein saw all nature as a unity; and so he predicted, from his General Theory of Relativity, that the gravity of the Sun would distort space itself, causing a ray of light to bend towards it. This was proved during a solar eclipse.

E=mc2 In his famous equation E=mc2, Einstein predicted that, if light and gravity could be linked, so could energy and matter. Loss of matter would produce enormous energy. The atomic bomb, years later, devastatingly proved his predictions correct.

TEN FACTS ABOUT ALBERT EINSTEIN

1 Einstein did not talk until he was three years old; but at 12 he taught himself Euclidean geometry.

2 At the age of 14, he asked:“What would the world look like if I rode on a beam of light?” This was a question of genius. It is harder to frame the question than to postulate the answer - which is that time would stand still.

3 He did not enjoy school, using a family move as an excuse to avoid it for a year at age 15. He finally graduated in 1900 by studying the lesson notes of a classmate.

4 He worked as a supply teacher for two years and in the patent office in Bern, of which he later said: “The inventions now look idiotic”.

5 When American physicist Robert Andrews Millikan confirmed Einstein’s theory on the behaviour of light a decade after it was proposed, he was surprised and unsettled by the result.

6 Einstein’s special theory of relativity was based on the realisation that our measurements of time and space depend upon human judgments on whether two distant events occur simultaneously. “A clock fixed at the Earth’s equator will run slower by a very small amount than (one) fixed at one of the Earth’s poles.”

7 He predicted that light would bend in the vicinity of a massive body such as a star. This was confirmed during an eclipse of the Sun in 1919, making Einstein internationally famous.

8 He declined an offer made by the leaders of the State of Israel to become its first President.

9 Einstein declared that, given his time again, he would “choose to be a plumber or a pedlar”, because of their relative independence. He had a letter from R. Stanley Murray of The Stanley Plumbing and Heating Co. in New York, who suggested: “as a team we would be tremendously successful, being possessed of knowledge and independence” and offered to change the firm’s name to The Einstein and Stanley Plumbing Co.

10The face of the Jedi Master Yoda, in the Star Wars films, is based on Einstein, because of his apparent wisdom (see below).