How Much Heavier Is Your E-Reader after You Download a Book?

Granted, it's not a lot. But adding a book to your Kindle does make it heavier, as computer scientist John Kubiatowicz discovered:

Although the electrons were already present, keeping them still rather than allowing them to float around takes up extra energy – about a billionth of a microjoule per bit of data.

Using Einstein's E=mc² formula, which states that energy and mass are directly related, Prof Kubiatowicz calculated that filling a 4GB Kindle to its storage limit 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.

Link -via Marginal Revolution | Photo: Flickr user jimmiehomeschoolmom

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@lewis. Do you mean to say you don't consider a billionth of a billionth of a gram to be negligible weight? You are more likely to need to be concerned about the weight added by the oils from your fingers and dust that collects on the case.
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Flash memory is based on NAND gates, so there are electrons present whether the value is 0 or 1. No weight change. Well, there might be one, due to stray capacitance or something like that, but we can assume it to be negligible.
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