Last week Daedalus invented a novel aero-engine. Its air duct was threaded by a.c. electric and magnetic fields at right angles. As the electric field came on, it polarized the air, driving a transient displacement current upon which the magnetic field exerted its motor action. As the electric field turned off again, a reversed displacement current flowed through the air; but since the magnetic field had also reversed, the air felt an impulse in the same direction as before. Thus the motor exerted a unidirectional pump action on its gaseous dielectric.

Daedalus now wonders what would happen if he ran this motor in a vacuum. Vacuum is a perfectly good dielectric: it can sustain a transient displacement current like any other insulator. His electromagnetic pump should exert a force on it. So how can you pump a vacuum?

The modern quantum vacuum, he points out, is a very crowded place. It is dense with transient virtual particles appearing and vanishing again all the time, lasting just long enough for their energy not to infringe the uncertainty principle. Clearly it is these particles that carry a displacement current through a vacuum, and on which the electromagnetic vacuum pump exerts its thrust.

Yet any pump or motor must create a reaction equal to the action of its thrust. If the thrusted particles promptly vanish, their reaction vanishes with them, which seems impossible. But Daedalus notes the theory of particle collisions: a high-energy collision dumps so much energy in a small volume that the virtual particles in the vicinity can seize it to pay off their debt to Heisenberg, and thus become real. His pump must do the same thing. It dumps energy into the vacuum, allowing real particles to appear. And these newly created particles carry away its reaction.

The fields in the electromagnetic vacuum pump have an extremely low energy density; but their volume as a particle source is enormous. In creating its thrust, the pump would pour out vast numbers of particles of amazingly low energy: languid pions, lethargic neutrinos, feeble photons, all travelling in exactly the direction and with exactly the energy needed to balance its thrust. A splendid new field of physics awaits development. So does a wonderful propellant-free space thruster, which could reach the stars on energy alone.

The Further Inventions of Daedalus is published by Oxford University Press.