quantum vacuum

"No point is more central than this, that empty space is not empty. It is the seat of the most violent physics."
- John A. Wheeler


'Empty space' when seen at the subatomic level is far from empty but instead seethes with energy, the rapid appearance and disappearance of all sorts of elementary particles, and fluctuations in the fabric of spacetime itself. It is the source of zero point energy which is seen as the potential basis for radically new forms of propulsion and "free energy" in the far future.