Nuclear and Emerging Technologies for Space (2012) - Advanced Propulsion Physics: Harnessing the Quantum Vacuum.
Can the properties of the quantum vacuum be used to propel a spacecraft? The idea of pushing off the vacuum is not new, in fact the idea of a “quantum ramjet drive” was proposed by Arthur C. Clark (proposer of geosynchronous communications satellites in 1945) in the book Songs of Distant Earth in 1985: “If vacuum fluctuations can be harnessed for propulsion by anyone besides science fiction writers, the purely engineering problems of interstellar flight would be solved.”. When this question is viewed strictly classically, the answer is clearly no, as there is no reaction mass to be used to conserve momentum. However, Quantum Electrodynamics (QED), which has made predictions verified to 1 part in 10 billion, also predicts that the quantum vacuum (lowest state of the electrodynamic field) is not empty, but rather a sea of virtual particles and photons that pop into and out of existence stemming from the Heisenberg uncertainty principle. The Dirac vacuum, an early vacuum model, predicted the existence of the electron’s antiparticle, the positron in 1928, which was later confirmed in the lab by Carl Anderson in 1932. Confirmation that the Quantum Vacuum (QV) would directly impact lab observations came inadvertently in 1948 while Willis Lamb was measuring the 2s and 2p energy levels in the hydrogen atom. Willis discovered that the energy levels were slightly different, contrary to prediction, but detailed analysis performed within weeks of the discovery by Bethe at Cornell predicted the observed difference only when factoring in contributions from the QV field The Casimir force, derived in 1948 by Casimir in response to disagreements between experiment and model for precipitation of phosphors used with fluorescent light bulbs, predicts that there will be a force between two nearby surfaces due to fluctuations of the QV. This force has been measured and found to agree with predictions numerous times in multiple laboratories since its derivation.
What is the Casimir force? The Casimir force is a QV phenomenon such that two flat plates placed in close proximity in the vacuum preclude the appearance of particles, whose wavelength is larger than the separation gap, and the resultant negative pressure between the two surfaces is more negative than the pressure outside the two surfaces, hence they experience an attractive force.
Using the Plank frequency as upper cutoff yields a prediction of ~10^114 J/m3. Current astronomical observations
put the critical density at 1*10^-26 kg/m3. The vast difference between QED prediction and observation
is not currently understood.
What is the dynamic Casimir force? The dynamic Casimir force arises as a result of Unruh radiation where an accelerated observer sees the vacuum as a higher temperature photon bath, and is the mechanism that facilitates Hawking radiation around a black hole where relativistic acceleration separates a virtual pair such that one particle goes in the horizon, while the other escapes. Recent findings reported earlier in 2011 show that the dynamic Casimir effect may have been detected in the lab. The simplest mechanical construct to help visualize using the dynamic Casimir force to generate thrust is through the use of vibrating mirrors where the mirror trajectory is designed to generate radiation in a preferred direction. The magnitude of thrust arising from using the dynamic Casimir force derived numerous times in the literature has been shown to be very small in comparison with conventional propulsion systems, but has been clearly shown to be theoretically possible.
Recent models developed by White suggests that there are ways to increase the net force, and these models have been validated against data at both the cosmological scale, the quantum level, and test devices have been fabricated/ tested in the lab and found to agree with model predictions.
The near term focus of the laboratory work is on gathering performance data to support development of a Q-thruster engineering prototype targeting Reaction Control System (RCS) applications with force range of 0.1-1 N with corresponding input power range of 0.3-3 kW. Up first will be testing of a refurbished test article to duplicate historical performance on the high fidelity torsion pendulum (1-4 mN at 10 to 40 Watts). The team is maintaining a dialogue with the ISS national labs office for an on orbit DTO.
How would Q-thrusters revolutionize human exploration of the outer planets? Making minimal extrapolation of performance, assessments show that delivery of a 50 mT payload to Jovian orbit can be accomplished in 35 days with a 2 MW power source [specific force of thruster (N/kW) is based on potential measured thrust performance in lab, propulsion mass (Q-thrusters) would be additional 20 mT (10 kg/kW), and associate power system would be 20 mT (10 kg/kW)]. Q-thruster performance allows the use of nuclear reactor technology that would not require MHD conversion or other more complicated schemes to accomplish single digit specific mass performance usually required for standard electric propulsion systems to the outer solar system. In 70 days, the same system could reach the orbit of Saturn.
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