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London - When the Kindle e-reader was launched, we all thought it meant the end to lugging heavy tomes around.
But perhaps you shouldn’t put away your book bag quite yet.
For a full 4GB Kindle - stocked with 3,500 books - is ever so slightly heavier than an empty one. The reason is rather complicated, and owes a debt to Einstein.
Storing new data on a Kindle involves holding electrons - tiny particles that help make up an atom - in a fixed place in the device’s memory. Although the electrons are already present in an empty Kindle, keeping them still uses more energy than allowing them to flow around.
As Einstein pointed out in his equation E=MC2, energy and mass are directly linked - so filling up a 4GB Kindle makes it weigh 0.000000000000000001g more than an empty one.
It’s still better than the real thing though - 3,500 ordinary books would weigh around two tons. - Daily Mail