Last week the final version of the NASA Eagleworks EmDrive paper, titled “Measurement of Impulsive Thrust from a Closed Radio-Frequency Cavity in Vacuum,” published in the American Institute of Aeronautics and Astronautics (AIAA)’s prestigious Journal of Propulsion and Power, described promising experimental results and hinted at possible theoretical models.
The experimental results:
“Thrust data from forward, reverse, and null suggested that the system was consistently performing at 1.2 ± 0.1 mN∕kW, which was very close to the average impulsive performance measured in air. A number of error sources were considered and discussed.”
It’s worth noting that the EmDrive performs much better than the other “zero propellant” propulsion systems studied to date. A modest thrust without having to carry fuel can be better, especially for long-distance space missions, than a higher thrust at the cost of having to carry bulky and heavy propellant reserves.
“NASA is looking forward to the scientific discussions with the broader technical community that will occur based on the publication of the Eagleworks team’s experimental findings,” said Jay Bolden, an Engineering PUblic Affairs Officer with NASA’s Johnson Space Center, as reported by Discover Magazine. “This is part of what NASA does in exploring the unknown, and the agency is committed to and focused on the priorities and investments identified by the NASA Strategic Space Technology Investment Plan.”
“Through these investments, NASA will develop the capabilities necessary to send humans farther into space than ever before.”
The NASA Eagleworks paper is focused on the experimental test results and doesn’t propose a detailed theoretical explanation – which is not surprising since there is no accepted theoretical explanation of the “anomalous” thrust observed in EmDrive tests. However, the Eagleworks scientists hint at a possible theoretical model, still qualitative and under development, which could lead to a theoretical explanation of why and how the EmDrive works.
This is a short and hopefully readable outline of the developing theoretical model proposed by the NASA scientists. I think reversing the order of the considerations in the paper can make the outline easier to follow.
The researchers propose that the EmDrive “pushes off of quantum vacuum fluctuations… and moves in one direction while a wake is established in the quantum vacuum that moves in the other direction.”
The quantum vacuum is not empty space where nothing happens, but a dynamic medium filled with virtual particles – “virtual” indicating particles that pop in and out of existence fast enough for the universe not to notice that conservation of energy has been violated. In fact, the energy to create virtual particles can be considered as “borrowed” and “paid back” in a very short time.
This quantum vacuum concept is part of mainstream consensus physics: according to Frank Wilczek, Nobel Laureate in Physics, “the quantum vacuum is a dynamic medium, whose properties and responses largely determine the behavior of matter.”
What is more controversial is the idea of treating the quantum vacuum as a medium capable of supporting acoustic oscillations that carry momentum in one direction, pushing the EmDrive in the other. But, according to the Eagleworks researchers, this ideas is suggested by the results of their 2015 paper titled “Dynamics of the Vacuum and Casimir Analogs to the Hydrogen Atom,” showing that “the first 7 energy levels of the hydrogen atom could be viewed as longitudinal resonant acoustic wave modes in the quantum vacuum.”
The more general idea that the quantum vacuum “could potentially be modeled at the microscopic scale as an electron-positron plasma” is explored in another 2015 paper, authored by Eagleworks team leader Harold “Sonny” White, elaborating on the ideas that White proposed in a 2009 article titled “Can the Quantum Vacuum be used as a propellant source?,” published in Space Times, the magazine of the American Astronautical Society.
According to White, the quantum vacuum can be treated as a virtual plasma made up of electron- positron (e-p) pairs, which permits engineering an apparatus to act on the virtual plasma by “squeezing the quantum vacuum,” and use it as a propellant. White argues that quantum vacuum oscillations can transfer momentum across the enclosure of the apparatus, pushing it forward. Besides explaining why the EmDrive works, White’s quantum vacuum model could have far reaching implications for fundamental physics. For example, “the gravitational constant can be shown to be a long wavelength consequence of the quantum vacuum rather than a fundamental constant,” which suggests that gravitation itself could emerge from quantum vacuum physics.
Pilot-Waves in the Quantum Vacuum
A related idea proposed by the Eagleworks scientists in the Journal of Propulsion and Power paper, which could also have far reaching implications for fundamental physics, is that quantum vacuum fluctuations (virtual particles) could be the dynamic medium that guides real particles in pilot-wave quantum physics theories. In fact, the researchers start the theory section of the paper by describing their theoretical model as “a nonlocal hidden-variable theory, or pilot-wave theory for short.”
Quantum physics is considered mysterious, and deeply counter-intuitive. Evidence seems to suggest that quantum particles can not be thought of as having definite positions when they are not observed. In other words, according to currently leading interpretations of quantum physics, speaking of a particle’s trajectory between observations is meaningless. The famous double slit experiment, which according to Richard Feynman encompasses all that is mysterious in quantum physics, is often taken as a demonstration that quantum particles are not “real particles” with clearly defined trajectories.
In pilot-wave theories, quantum particles are treated as real particles – little things that always have well-defined positions and trajectories – and their motion is driven by a real physical field (the pilot-wave). The first pilot-wave theory was proposed in the 1920s by Louis de Broglie, one of the founding fathers of quantum mechanics, and another pilot-wave theory was proposed by David Bohm in the 1950s. In the double slit experiment, a particle going through one slit knows if the other slit is open or closed, since the information is in the pilot-wave, and moves accordingly.
The pilot-wave idea “seems to me so natural and simple, to resolve the wave-particle dilemma in such a clear and ordinary way, that it is a great mystery to me that it was so generally ignored,” wrote John Stewart Bell in his seminal work “Speakable and Unspeakable in Quantum Mechanics.”
So where is the catch? The catch, as demonstrated by Bell himself, is that viable pilot-wave theories must be non-local – in other words, they must exhibit faster than light correlations between different locations.
“The universe seems to like talking to itself faster than the speed of light,” said Aephraim Steinberg, a physicist at the University of Toronto who participated in recent experiments that seem to support the pilot-wave idea. “I could understand a universe where nothing can go faster than light, but a universe where the internal workings operate faster than light, and yet we’re forbidden from ever making use of that at the macroscopic level – it’s very hard to understand.” In fact, it appears that faster than light correlations can’t be used to send signals (Bell again).
Stochastic mechanics, a related approach notably pioneered by Edward Nelson, treats quantum particle as immersed in and driven by a bath of quantum fluctuations, which Nelson models statistically like Brownian motion. Here again, non-locality is a necessary feature of a viable theory. Stochastic mechanics could be “an approximation to a correct theory of quantum mechanics as emergent,” noted Nelson in a 2012 review paper.
“But what is the correct theory?”
Perhaps the correct theory could be, as suggested by the NASA Eagleworks scientists, a detailed and quantitative theory of quantum vacuum fluctuations of virtual particles that drive the motion of real particles like a pilot-wave.
It’s worth noting that, contrary to conventional quantum mechanics, pilot-wave quantum-like behavior can be reproduced in classical fluids and explained by classical (non-quantum) fluid dynamics. In fact, fluid droplets bouncing on a vibrating fluid bath generate a pilot-wave field that, in turn, guides the motion of the droplets themselves with a striking similarity to quantum behavior. In particular, in the fluid version of the double slit experiment, a droplet goes through one slit, but the pilot wave goes trough both slits and recombines, interfering with itself, in such a way as to guide the droplet on a complex trajectory that reproduces the results of the quantum double slit experiment.
To explore the fascinating quantum-like behavior of fluid droplets, watch the video below and see two review articles by John Bush, a professor of applied mathematics at MIT: “The New Wave of Pilot-Wave Theory” and “Pilot-Wave Hydrodynamics.”
Whether this is just an interesting similarity, or has a deeper meaning, remains to be seen. It’s interesting to speculate on the possibility that quantum vacuum fluctuations could play the role of the vibrating fluid bath in the droplet experiments.
Images from John Bush/MIT and Wikimedia Commons, video from Veritasium.