IT HAS to be one of the world鈥檚 most sluggish spacecraft, taking 10 years to reach a speed of just 10 centimetres per second. But don鈥檛 knock it. The craft, as envisioned by two American physicists, would reach the stars by extraordinary means: pushing against empty space itself.
Quantum theory says that the apparently empty space of a vacuum is not empty at all. Instead, it is seething with electromagnetic waves. 鈥淎nd it鈥檚 these that could be exploited to propel a spacecraft,鈥� says Jordan Maclay, chief scientist of Quantum Fields, a company based in Wisconsin that studies the use of quantum effects in engineering.
The key to Maclay鈥檚 bizarre plan is what quantum physicists call the 鈥渄ynamic Casimir effect鈥�, which manifests itself whenever a metal plate is rapidly accelerated through a vacuum. Electric charge inside a piece of metal always rearranges to cancel out any localised electric field on its surface. But for an accelerating plate that is continuously encountering the fluctuating electric fields of a vacuum, this must happen quickly. At large accelerations, however, the electric charge rearrangements cannot keep up. The surface electric field can only be reduced to zero if it is dissipated as electromagnetic radiation. 鈥淐onsequently, the plate sprays out photons,鈥� says Maclay.
Advertisement
Photons are like tiny bullets of light and their recoil slows the plate. 鈥淭his is the opposite of what you need for a spacecraft,鈥漢e points out. 鈥淗owever, it is possible to achieve a net push in one direction if the plate is vibrated.鈥�
The form of the vibration is key. If the plate is oscillated sinusoidally it would accelerate one way through the vacuum half the time and the opposite way the other half. The net effect would be zero. 鈥淭he two halves of the cycle must be different 鈥� you鈥檝e got to wave it just right,鈥� says Maclay.
Maclay calculates that a metal plate vibrated 10 billion times a second with an amplitude of 1 nanometre could accelerate a spacecraft by just 3 脳 10鈭�20 metres per second per second. 鈥淵ou鈥檇 have to be patient,鈥� admits Maclay.
But he points out that if a second, stationary plate were held parallel to the vibrating one, forming a 鈥渞esonant cavity鈥�, this could boost the emission of photons and increase the acceleration 10 billion times. In fact, if the plate were made of something much stronger 鈥� perhaps a single crystal 鈥� that could tolerate a vibration amplitude of 1 millimetre, it would be possible to accelerate faster than a chemical rocket. 鈥淎nd I鈥檓 sure there are more ingenious ways of boosting the acceleration,鈥� says Maclay.
Using the vacuum this way is 鈥渟exy鈥� says John Cramer of the University of Washington in Seattle. 鈥淏ut it鈥檚 very inefficient.鈥� Maclay agrees, but adds: 鈥淣ow we鈥檝e opened the door to the possibility of driving a spacecraft with the vacuum, we hope others will find ways of boosting the efficiency.鈥�
But there鈥檚 no free lunch 鈥� energy is needed to vibrate the plate, so there is no net energy gain from the vacuum. The plate is simply pushing against the medium of the vacuum like a swimmer pushing against water.
However, Maclay says there are still huge advantages. 鈥淎 spacecraft need not carry large amounts of mass to throw out the back as exhaust,鈥� says Maclay. 鈥淚n chemical rockets this can be 99 per cent of the mass.鈥�
A paper describing the plan, by Maclay and his late colleague Robert Forward, will appear in a future edition of the American Journal of Physics.