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Energy Density"Vacuum energy" or "zero-point energy" is the energy density of empty space. It is the amount of energy stored in a given system or region of space per unit volume, and is most commonly denoted u. It therefore has units of energy per length cubed. In modern physics, we have two fundamental theories: quantum field theory and general relativity. Quantum field theory takes quantum mechanics and special relativity into account, and it's a great theory of all the forces and particles except gravity. General relativity is a great theory of gravity, but it ignores quantum mechanics. Nobody knows how to reconcile these theories yet. That's what people working on "quantum gravity" are trying to do. Quantum field theory and general relativity have really different attitudes towards the energy density of the vacuum. The reason is that quantum field theory only cares about energy differences. If we can only measure energy differences, we can't determine the energy density of the vacuum - it's just a matter of convention. As far as we know, we can only determine the energy density of the vacuum by experiments that involve general relativity - namely, by measuring the curvature of spacetime.
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