““We know that the uncertainty relation implies not only the absence of empty space but also the nature of the minimal content of every space-fluctuating energy and virtual particle-antiparticle pairs. The fields of the general theory of relativity by themselves are permitted to have zero values, and thereby truly empty space. But a quantum theory of gravity must preclude that; its energy carriers will create fluctuations. These fluctuations may include the value zero but will never violate the uncertainty relation for energy. Since we do not yet have a theory that unifies general relativity and quantum mechanics, anything we might say about fluctuations of the fields of general relativity must remain mere speculation. We can say for sure that quantum theory, when joined to the special theory of relativity, mandates the permanent and ubiquitous existence of fluctuations that include the appearance and disappearance of virtual particle pairs. This mandate includes the emptiest of all imaginable space, a void we might surround with an impenetrable wall at temperature zero.””
