Planet Earth is about three trillion tons lighter than the sum of its parts. But never fear, the ground beneath your feet is not draining away into a cosmic sinkhole. No, what sounds at first like an accounting error of global proportions is just the gravitational binding energy of Earth— the effect of Earth's gravity acting on Earth's six billion trillion tons. Like all binding energies, this small self-energy is a negative quantity. And, as Albert Einstein told us, energy is mass, in this case, three trillion tons' worth subtracted away by the force of gravity.
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