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Einstein: Letters
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The number of contemporaries with which Einstein corresponded, is hard to estimate. According to the Jerusalem Einstein Archives there are about 570 known and important names of scientists and far more than 200 non-scientific ones Einstein exchanged letters with. This category includes, among other persons, crowned heads, politicians, writers and artists. Einstein had ... uncountable writing contacts with unknown or scientifically interested persons or people looking for help and with children.
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Einstein signed a letter that informed President F. D. Roosevelt of the possibility of nuclear bombs, warning that the Germans might try to build them. The next year Einstein became an American citizen.
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Others say that Einstein came up with E=MC² because these were the only three letters of the alphabet that he knew of. The addition of the '²' and the order was pure coincidence.
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