Casimir effect
Since there are fewer virtual particles on the inside of the plates rather than the outside, there is a greater force on the outside of the plates. This causes the plates to be pushed inwards. Now, the energy density of the universe is zero. This is because every particle has positive energy, but it also has gravity, which balances it out with negative gravitational energy. Since there are fewer virtual particles within the plates, that means that the energy density of the spacetime between the plates must be less than the energy density of spacetime outside the plates. However, since the energy density of the spacetime outside the plates is zero, that means that the energy density of the spacetime between the plates is less than zero, or negative. Negative spacetime has odd properties, such as being saddle-shaped and allowing travel into the past.