Weak Energy Condition

The weak energy condition (WEC) is a constraint in general relativity that requires the energy density measured by any timelike observer to be non-negative. It is stronger than the null energy condition but weaker than the strong energy condition, and it ensures that gravity is always attractive: masses fall toward each other, not away. The WEC is satisfied by all known forms of classical matter, but it is violated by quantum vacuum effects such as the Casimir effect, where the energy density between two conducting plates can be negative.

The violation of the weak energy condition by quantum fields is not a bug. It is a feature that makes the quantum vacuum interesting. But the violation is constrained: the quantum energy inequalities require that negative energy excursions be compensated by positive energy elsewhere, preserving the total energy budget. The WEC is not a fundamental law of physics. It is a property of classical matter that fails when quantum effects become significant, and its failure is the reason why black holes can evaporate through Hawking radiation.