Downloading 3,500 books to a kindle adds about one billionth of a gram in weight. In real books that’s about 2 tons.

By Using Einstein’s E=mc² formula, which states that energy and mass are directly related, Prof Kubiatowicz calculated that filling a 4GB Kindle to computer storage would increase its weight by a billionth of a billionth of a gram, or 0.000000000000000001g. This is roughly equivalent to the weight of a small virus, while the equivalent number of books – about 3,500 – would weigh approximately two tons.

E-readers could also become slightly heavier in the summer, because they would take on more energy from their exposure to sunlight, scientists explained.

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