Avi Loeb’s new foray into the remote future had me thinking of the Soviet physicist Leonid Shkadov, whose 1987 paper “Possibility of Controlling Solar System Motion in the Galaxy” (citation below) discussed how an advanced civilization could get the Sun onto a new trajectory within the galaxy. Why would we want to do this? Shkadov could imagine reasons of planetary defense, a star being moved out of the way of a close encounter with another star, perhaps.
All of this may remind science fiction readers of Robert Metzger’s novel CUSP (Ace, 2005), which sees the Sun driven by a massive propulsive jet. A more recent referent is Gregory Benford and Larry Niven’s novels Bowl of Heaven (Tor, 2012) and ShipStar (2014), in which a star is partially enclosed by a Dyson sphere and used to explore the galaxy. In 1973, Stanley Schmidt would imagine Earth itself being moved to M31 as a way of avoiding an explosion in the core of the Milky Way that threatens all life (Sins of the Fathers, first published as a serial in Analog).
Loeb’s paper mentions technologies for moving stars because several recent proposals involve this kind of ‘cosmic engineering’ to change civilizational outcomes. The accelerated expansion of the universe will, after the universe has aged by a factor of 10, make all stars outside the Local Group of galaxies disappear as they recede. The process continues with continued acceleration. Wait long enough and we fall prey to cosmic winter, and as Loeb writes:
Following the lesson from Aesop’s fable “The Ants and the Grasshopper,” it would be prudent to collect as much fuel as possible before it is too late, for the purpose of keeping us warm in the frigid cosmic winter that awaits us. In addition, it would be beneficial for us to reside in the company of as many alien civilizations as possible with whom we could share technology, for the same reason that animals feel empowered by congregating in large herds.
There are places where we might do this, says Loeb, and I’ll talk about his ideas on them tomorrow. For today, note that in response to his previous papers on the oncoming ‘cosmic isolation,’ Freeman Dyson proposed to Loeb his own project that would move stars, bringing large-scale formations of stars down to a more manageable volume so that they will be bound by their own gravity. Closely spaced, the stellar collection avoids being dissipated in the accelerated expansion of the cosmos. And it turns out that Dan Hooper (University of Chicago) has also been pondering induced stellar motion, using the energy output of stars to cluster large numbers of them into an astronomically tight radius so that their energies can be harvested.
Image: A Shkadov thruster as conceived by the artist Steve Bowers.
Like Dyson and Loeb, Hooper has his eye on the acceleration of cosmic expansion and its consequences. So what kind of ‘stellar engines’ can we envision that could move objects as large as stars? Leonid Shkadov suggested using a star’s radiation pressure. The Shkadov thruster extracts energy from the star by using a vast mirror to take advantage of photon momentum. Let me turn back to an earlier post to reprint a diagram that Duncan Forgan uses in describing a Shkadov thruster. Forgan (University of Edinburgh) points out the difference between these thrusters and Dyson spheres, the latter being spherical and shaped so as to balance gravitational forces on the sphere by way of collecting a maximum amount of stellar energy. But here is the Shkadov thruster as diagrammed by Forgan:
Image: This is Figure 1 from Duncan Forgan’s paper “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves.” Caption: Diagram of a Class A Stellar Engine, or Shkadov thruster. The star is viewed from the pole – the thruster is a spherical arc mirror (solid line), spanning a sector of total angular extent 2?. This produces an imbalance in the radiation pressure force produced by the star, resulting in a net thrust in the direction of the arrow. Credit: Duncan Forgan.
Thus the whole idea of the Shkadov thruster is not balance but imbalance. And Forgan goes on to say this about the idea:
In reality, the re?ected radiation will alter the thermal equilibrium of the star, raising its temperature and producing the above dependence on semi-angle. Increasing ? increases the thrust, as expected, with the maximum thrust being generated at ? = ? radians. However, if the thruster is part of a multi-component megastructure that includes concentric Dyson spheres forming a thermal engine, having a large ? can result in the concentric spheres possessing poorer thermal ef?ciency.
Efficient or not, Shkadov thrusters interest Forgan as a possible SETI detection (Hooper also notes the possibility). Like Dyson spheres, their sheer scale and unusual features could make them visible in a lightcurve, perhaps with the aid of radial velocity follow-ups. Arguing against the idea, though, is the fact that the Shkadov thruster is probably not the technology our hypothetical future civilization would use to move its star (or stars).
I found this out when I wrote Avi Loeb in reaction to his new paper and mentioned the Shkadov idea. Loeb found Dan Hooper’s ideas (in “Life Versus Dark Energy: How An Advanced Civilization Could Resist the Accelerating Expansion of the Universe,” citation below) to be a better solution to the problem. Here is what Loeb told me in our email exchange yesterday:
The use of the momentum associated with the radiation emitted by the star for its propulsion, as envisioned in Shkadov’s thruster, is much less efficient than using the energy associated with same radiation. The radiation momentum equals its energy divided by the speed of light, c. However, the momentum gained by converting this energy to the kinetic energy of a massive object moving at a speed v is larger by a factor of 2(c/v). This is a huge factor of which Dan Hooper is taking advantage to argue that Sun-like stars can reach a percent of the speed light, 0.01c, in a billion years. If he would have used the momentum of the light emitted by the star as in Shkadov’s thruster, then the attainable speed would have been a hundred times smaller, only of order 30 km/s (similar to the speed of chemical rockets), and the journey being contemplated would have not been feasible. During the age of the Universe (10 billion years) one would only be able to traverse a million light years and not leave the Local Group of galaxies. This is not sufficient for gaining more fuel than available within the Local Group of galaxies.
Thus the problem of the Shkadov thruster: By using the momentum of stellar photons, Shkadov loses efficiency. Loeb adds: “One way to improve the efficiency of Shkadov’s thruster (by employing the energy and not the momentum of the star’s radiation) is to harvest the energy from the star through a Dyson sphere and then use it to ablate its surface on one side, generating a rocket effect.” This seems to be what Dan Hooper has in mind in his paper. The civilization in question would harvest the energy of stars through Dyson spheres that, quoting Hooper, “…use the collected energy to propel the captured stars, providing new and potentially distinctive signatures of an advanced civilization in this stage of expansion and stellar collection.”
Hooper has a SETI prospect in mind, while also thinking about collecting sources of energy, maximizing its amount in the form of starlight by a factor of several thousand. It is this prospect that interests Loeb. His paper makes the case that we already have massive reservoirs of fuel in the visible universe in the form of galactic clusters, each containing the equivalent of 1000 Milky Ways. Perhaps, then, we need no star-moving and collection project a la Hooper or Dyson, but rather need to think about reaching a galactic cluster for our fuel. Given the magnitude of that challenge, whether or not we take our home star along is inconsequential.
The speculative buzz I get from this is science fictional indeed. The nearest cluster is Virgo, whose center is some 50 million light years away, with the Coma cluster six times farther still. Thinking in terms of a civilization that could cross such gulfs takes us into Olaf Stapledon territory, a region of spacetime with which I share Loeb’s obvious fascination. I’m running out of time today, but want to look deeper into this with the help of Loeb’s paper tomorrow. I’ll also have more thoughts on ‘stellar engines’ and their origins.
Avi Loeb’s new paper is “Securing Fuel for Our Frigid Cosmic Future” (preprint). Leonid Shkadov’s paper on the Shkadov thruster is “Possibility of controlling solar system motion in the galaxy,” 38th Congress of IAF,” October 10-17, 1987, Brighton, UK, paper IAA-87-613. The Forgan paper is “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves,” accepted at the Journal of the British Interplanetary Society (preprint). The Hooper paper is “Life Versus Dark Energy: How An Advanced Civilization Could Resist the Accelerating Expansion of the Universe” (2018). Preprint.
I always look at the NASA Innovative Advanced Concepts (NIAC) awards with interest as well as a bit of nostalgia. When I began researching the book that would become Centauri Dreams, NIAC was an early incentive. Then known as the NASA Institute for Advanced Concepts, it was under the direction of Robert Cassanova (this was back around 2002), and its archive of funded studies was a treasure house of deep space ideas, from antimatter extraction in planetary magnetic fields to exoplanet imaging through starshades. I spent days going through any number of reports and interviewed many NIAC study authors.
You can still see the NIAC reports from that era on the older site (go to NIAC Funded Studies). The current NIAC site makes the point that the program looks for “non-traditional sources of innovation that study technically credible, advanced concepts that could one day ‘change the possible’ in aerospace.” And it’s here that we get the 2017 Phase 1 proposals, a $125,000 award for a nine month period that can result in a two-year Phase II follow-up.
Laser-Fed Ion Drive for Interstellar Precursor
2017 proposals with specifically interstellar intent include one from JPL’s John Brophy, who describes what he calls “A Breakthrough Propulsion Architecture for Interstellar Precursor Missions.” Brophy is looking for a way to deliver a high spacecraft velocity change (delta-V) without using much propellant. He envisions a lithium-fueled ion thruster with a specific impulse of 58,000 seconds (compare this with the Dawn spacecraft’s 3,000 seconds).
How to achieve this? By use of a 10-km orbital laser array that allows his craft to operate far beyond the point where sunlight loses its useful effect, the array feeding a lightweight, photovoltaic array aboard the spacecraft that can produce the electric power to drive the thrusters. Brophy believes laser power in this configuration can be converted to electric power at an efficiency of 70 percent, to produce an output voltage of 12 kV. The lithium fueled system would be capable of a 12-year flight-time to 500 AU, close to the point where gravitational lensing begins to get interesting. Time to Pluto: 3.6 years, and Jupiter in a single year.
So much depends upon that laser array, a ground-based version of which is under active study by the Breakthrough Starshot effort. The problems of implementing such an array are daunting, but its uses if ever built are so game-changing that the investigation continues. Breakthrough Starshot itself works with a 30-year horizon, so we are not talking about near-term implementation, but rather sketching out concepts a laser array could make feasible.
Image: Breakthrough Propulsion Architecture for Interstellar Precursor Missions
Credit: John Brophy.
Counting on Ernst Mach
Like Brophy’s proposal, Heidi Fearn’s “Mach Effects for In Space Propulsion: Interstellar Mission” is a Phase 1 initiative, an interesting entry because it shows NIAC beginning to open to the kind of breakthrough propulsion concepts NASA once investigated through its Breakthrough Propulsion Physics project (led by Tau Zero founder Marc Millis). In this case, the work goes toward a so-called Mach Effect Thruster (MET). Mach effects are transient variations in the rest masses of objects as predicted by standard physics where Mach’s principle applies. Proponents believe they offer the possibility of producing thrust without the ejection of propellant, as discussed in James Woodward’s Making Starships and Stargates: The Science of Interstellar Transport and Absurdly Benign Wormholes (Springer-Verlag, 2012).
What Fearn proposes is to investigate such thrusters by continuing the development of laboratory-scale devices while designing and developing power supply and electrical systems that will determine the efficiency of the Mach Effect Thruster. The analytical task is to improve theoretical thrust predictions and build a reliable model of the device. At the theoretical level, this team is definitely talking deep space, with part of the proposal being to:
Predict maximum thrust achievable by one device and how large an array of thrusters would be required to send a probe, of size 1.5m diameter by 3m, of total mass 1245 Kg including a modest 400 Kg of payload, a distance of 8 light years (ly) away.
Image: Mach Effects for In Space Propulsion: Interstellar Mission. Credit: Heidi Fearn.
Can such a device work? Remember, we are in the realm of advanced concepts, where the technology is at the TRL 1 level, but the benefits of working with a thruster that does not expel fuel mass could make high velocities possible, enough of an incentive to pursue these early steps. Mach’s principle relates the motion of the farthest objects in the universe to the local inertial frame or, as Hawking has it, “Local physical laws are determined by the large-scale structure of the universe.” The principle is vague enough that Hermann Bondi and Joseph Samuel have compiled eleven different variations on it, all of which could be called Machian.
Gravitational Lensing and Exoplanet Imaging
JPL’s Slava Turyshev is behind a Phase 1 study called “Direct Multipixel Imaging and Spectroscopy of an exoplanet with a Solar Gravity Lens Mission,” a study focusing on a mission to exploit the Sun’s gravitational lens, where (at 550 AU and beyond) the Sun’s gravity has bent the light of objects directly behind it to allow powerful lensing effects.
Long-time Centauri Dreams readers will know that the Italian physicist Claudio Maccone has championed a gravitational lens mission called FOCAL for many years, proposing it to the European Space Agency and writing numerous papers as well as two books on the topic (there is much available in the archives here about this concept). The definitive book, Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer, 2009) lays out the principles of such lensing, the history of its study, and potential solutions to the many imaging issues raised by a space mission. Maccone has discussed lensing as one solution to the communications challenges of Breakthrough Starshot.
What Turyshev is proposing is to study not the means of getting to 550 AU and beyond, but the question of spacecraft operations at such distances. From the proposal description:
Specifically, we propose to study I) how a space mission to the focal region of the SGL (Solar Gravitational Lens) may be used to obtain high-resolution direct imaging and spectroscopy of an exoplanet by detecting, tracking, and studying the Einstein’s ring around the Sun, and II) how such information could be used to unambiguously detect and study life on another planet.
Image: Direct Multipixel Imaging and Spectroscopy of an exoplanet with a Solar Gravity Lens Mission. Credit: Slava Turyshev.
Other proposals of interest are “Pluto Hop, Skip and Jump Global,” from Benjamin Goldman (Global Aerospace Corporation), an innovative study of a Pluto lander with a novel approach to landing and in situ science; “Gradient Field Imploding Liner Fusion Propulsion System” from Michael LaPointe (NASA MSFC), exploring magneto-inertial fusion in a new configuration that could lend itself to in-space propulsion; and “Fusion-Enabled Pluto Orbiter and Lander,” from Stephanie Thomas (Princeton Satellite Systems, Inc.), a Direct Fusion Drive concept based on work under development at the Princeton Plasma Physics Laboratory, and its application in a Pluto orbiter with lander (this one is a Phase II study).
The full list of 2017 Phase I and Phase II selections is here, including Nan Yu’s interesting take at JPL on possible dark energy interactions with a ‘Solar System laboratory.’ Have a look at these and ask yourself which we may still be talking about a decade hence. When I look back to the NIAC studies I examined in 2002 and later, I find many ideas that we wound up discussing at length here on the site, and I wouldn’t be surprised if several of the new selections move on to Phase II status and a great deal of future scrutiny. Which ones will they be?
The concluding part of the Tau Zero Foundation’s examination of what is being called the ‘EmDrive’ appears today. It’s a close analysis of the recent paper by Harold ‘Sonny’ White and Paul March in the Journal of Propulsion and Power. Electrical engineer George Hathaway runs Hathaway Consulting Services, which has worked with inventors and investors since 1979 via an experimental physics laboratory near Toronto, Canada. Hathaway’s concentration is on novel propulsion and energy technologies. He has authored dozens of technical papers as well as a book, is a patent-holder and has hosted and lectured at various international symposia.
Hathaway Consulting maintains close associations with advanced physics institutions and universities in the US and Europe. Those familiar with our Frontiers of Propulsion Science book will know his paper on gravitational experiments with superconductors, which closely examined past methods and cast a skeptical eye on early claims of anomalous forces (an earlier paper, “Gravity Modification Experiment using a Rotating Superconducting Disk and Radio Frequency Fields,” appeared in Physica C). Like Marc Millis, Hathaway calls for continued testing of EmDrive concepts and increased rigor in experimental procedures.
By George Hathaway
Comments on “Measurement of Impulsive Thrust from a Closed Radio Frequency Cavity in Vacuum” (White, March et al, published online by Jnl. Prop. & Power November 17, 2016).
White et al are to be congratulated for attempting to measure the small thrusts allegedly produced by a novel thruster whose operating mechanism is not only not understood but purportedly violates fundamental physical laws. They have made considerable effort to reduce the possibility of measurement artifact. However it appears that there are some fundamental problems with the interpretation of the measurement data produced by their thrust balance. This document will analyse the measurement procedure and comment on the interpretation.
The following comments roughly follow the order in the original text by White et al
Analysis and Comments
1. Null Test Orientation
Tests were performed in both the “Forward” and “Reverse” direction as well as in a “Null” direction where the alleged force vector pointed towards the rotational axis of the balance (pg 23). Apparently no Null tests were performed with the force vector pointing away from the balance axis nor were any tests performed with the “test article” force vector pointing up or down. These additional orientations would have provided much needed control data given the magnitude of the allegedly purely thermal signal seen in their “Null” test.
In addition, the Forward and Reverse tests should also have been performed by just re-orienting the test article whilst keeping all other rotating components untouched. In this type of control experiment, the spurious effect of the rest of the components is largely eliminated.
2. Axis Verticality
An optical bench was used as a platform to mount the vacuum chamber containing the balance. It is not stated whether the optical bench was itself mounted on pneumatic legs, however, this is usually the case with optical benches. The correct operation of any balance of this geometry requires that the pivots around which the balance arm rotates must be perfectly aligned vertically one above the other (for a 2-pivot system). When the pneumatic legs of the table are inflated, the axis of the balance is not typically able to be kept perfectly vertical, as required to obtain the maximum balance sensitivity and repeatability. There is no indication in the text stating how such verticality was assured throughout the test campaign, especially since the balance was housed in a large vacuum chamber.
3. Flexural Bearings
There is no information presented to indicate whether the linear flexure bearings were operating within the manufacturer’s axial loading specification, especially when additional ballast weight was required for the non-“split configuration” tests. It would also have been useful to see data on the natural frequency of the balance when loaded with the equivalent weights used in the thrust tests, given the damping method described. Also missing is an explanation of why none of the traces of the optical displacement sensor return to starting baseline after the calibration and “thrust” pulses. There seems to be an inherent bearing stiction problem preventing the balance from returning to its original baseline after a test. This is not due to general balance drift and is typical for overloaded bearings of this type. Long-term balance stability/drift plots would be useful.
4. Electrostatic Calibrator
Evidently the calibration of the electrostatic “fin” method of applying calibration pulses was performed using an electronic balance (Scientech SA-210). Unfortunately no data was provided to show exactly how this calibration was performed. In particular, no data was provided to show that there was no electrostatic interaction between the high-voltage calibration voltages and the operation of the balance. Since the Scientech balance properly reports vertical forces only, was care taken to translate these vertical forces into the horizontal calibration forces required by the thrust balance? It would have been useful for the authors to have employed a second, independent horizontal force calibration to verify the Scientech method such as a strain gauge-type force gauge with interpolation.
5. Vacuum System
The authors note that although turbomolecular pumps were used to evacuate the vacuum chamber, they caused no artificial vibrational signals. Turbo pumps require mechanical backing pumps to evacuate them to atmosphere. These mechanical pumps are connected to the turbo pumps typically via thick and stiff vacuum hoses. These hoses can transmit backing pump vibrations to the turbo pumps which are usually rigidly connected to the vacuum chamber. Was this source of vibration taken into account as well?
Additionally, no evidence is provided to show how the interior of the test article was evacuated coincidentally with the chamber evacuation. This is a different concern to that stated in the paper (pp 27, 28) regarding outgassing of the dielectric. The concern here is that if the test article cannot be fully evacuated coincidentally with the chamber evacuation, residual gas inside the test article can possibly escape during the time of a test, causing spurious force signals. Moreover, if the test article is rather well-sealed, the shell of the test article, especially the end plates, could expand upon evacuation of the chamber due to air trapped inside prior to chamber pump-down. This would alter the center of gravity (COG) of the balance causing a spurious signal, especially if the trapped air is heated upon application of RF power of tens of watts.
6. Liquid Metal Connections
“Galinstan screw and socket” rotary connections were employed to prevent any unwanted torques from upsetting the balance due to hard-wire connections between the rotating test article and the power supplies, analytical instruments etc fixed to the lab frame. There must have been quite a few of these connections for DC power, Forward and Reverse RF power, various tuning and drive signals etc. The authors failed to indicate how these connections were arranged geometrically. The ideal mounting arrangement is for such liquid metal connections to be stacked one on top of the other exactly coaxial with the main rotational axis of the balance. It seems unlikely that the design constraints of the balance within the chamber shown would accommodate this tall a stack of connections. Thus it is assumed that these connections were not arranged coaxially with the balance axis. If so, there could be spurious side thrusts generated by Ampère currents set up within the galinstan. This should have been tested and reported.
7. Thermal Expansion and Control Tests
The White et al paper contains considerable information on the effects of thermal expansion of the various test article components. It would be beneficial to see control experiments in which the test article is replaced by a suitable control article such as a purely cylindrical cavity of approximately the same dimensions, materials and construction and which supports similar RF modes as the frustrated conical test article.
According to pg 10, the heat sink unsurprisingly is the greatest source of heat during operation. It would be useful to perform control tests by separating the heat sink mechanically from the rest of the rotating components in such a way as to allow it to be oriented in any direction relative to the rest of the components to see the effect on the optical displacement signal.
Evidently, the test article assembly produces a relatively large thermal “thrust” signal as measured by the optical displacement sensor. The only explanation given is the change in center of gravity (COG) due to thermal expansion of various components causes a spurious torque on the balance. In fact the presence of a thrust signal due to thermal effects is only inferred, not proven. Not only that but it is stated (pg 10) that this thermal effect causes the balance arm to shift “with the same polarity as the impulsive signal” in Forward or Reverse tests. Here also it is implied but not proven that an “impulsive thrust” signal is even present (see below). The authors need to perform such control tests as to ascertain with certainty that there is indeed a “thermal thrust” before assuming without proof that it causes the balance arm to shift “with the same polarity”. One such test would be to construct a “control article” of the same shape, material and weight as the test article but with guaranteed no “impulsive thrust” and substitute it for the test article. Instead of powering it with an RF signal, put a resistor or light bulb inside to simulate the thermal characteristics.
This lack of proof of the presence of either a thermal thrust or an impulsive thrust thus precludes statements such as “the thermal signal in the vacuum runs is slightly larger than the magnitude of the impulsive signal [due to convective issues]”.
8. Confirmation Bias in Thrust Analysis
The entire edifice of the analysis of the signals from the optical displacement sensor rests on the assumption of the correctness and correct application of Fig. 5 to the present test situation. Fig. 5 shows an ad-hoc superposition of two assumed signals, namely a thermal signal and a pulse (impulse) signal. This is presented initially as a “conceptual simulation” and is reasonable in its own right. However, it then takes on the value of an accepted fact throughout the rest of the paper. Fig. 5 represents what the authors expect to see in the signal from the optical displacement sensor. When they see signals from this sensor which vaguely look like the expected superposition signal as represented in Fig 5, they assume that Fig 5 must actually represent what is going on in their system under test. This is a clear inductive reasoning fallacy called Confirmation Bias. This problem leads to baseless assumptions about the timing of the onset of expected effects after application of the stimulus (RF power), their proper shapes, and the joint amplitudes and thus the individual (impulse vs thermal) magnitudes.
In particular, the authors assume that the “true” impulse signal from the test article will look just like the assumed signal shown in Fig. 5, namely that it will look just like their calibration signal. This will include an initial fast-rising but well-behaved exponential slope up to a flat-topped constant thrust followed by a slower exponential falling section back to baseline. Next they assume that the thermal signal will be a well-behaved double exponential starting exactly at the same time as the impulse signal, also as shown in idealized form in Fig. 5. An additional assumption made by the authors is that there are no other spurious effects which might be represented as additional curves in Fig.5. The simple addition of the amplitudes of the thermal and impulse signals produces the resulting superposition signal. This signal is used as a template against which the actual sensor signal is compared. By stretching the imagination, the sensor signal can be force-fit onto the idealized superposition signal and, voila, the simple analysis can proceed to extract the magnitude of the true impulse signal.
This method is applied to all the sensor signals except that in Fig. 10 showing the “split configuration”.
There are additional problems with this force-fitting routine. For example, in Fig. 7, which is analysed in some detail, the initial rising slope of the displacement sensor signal should be an asymptotically flattening exponential according to Fig. 5. But it is clearly an asymptotically rising signal, perhaps exponential in shape. About half-way through the RF power application period, this rising slope suddenly breaks into a markedly linear (rising) slope. According to Fig. 5, this part of the signal should show an asymptotically decreasing (flattening) exponential slope, definitely not a linear slope. The authors even use linear curve fitting in this region, evidence that even they do not consider this part of the slope exponential. All the optical displacement signals shown in the other relevant figures (Figs. 13, 16) show this characteristic as well.
Then a sleight-of-hand is used to tease out the contributions of the assumed thermal vs the impulsive signal. According to pg. 11, “the characteristics of the curve [superposition curve in Fig. 5] after this discontinuity [the break in slope of the rising exponential due to the onset of steady thrust] are used as the baseline to be shifted down so that the line projects back to the “origin” or moment when RF power is activated.” The amount of this baseline shift is taken to represent the “true” impulse signal. Naturally, this assumes that the onset of thrust (and the thermal signal) are all coincident exactly with the application of RF power (and are all of the ideal shape according to Fig. 5). According to Fig. 7, it also assumes that a straight line can be used as this “baseline shift” rather than the more likely broken exponential shaped line depicted in Fig. 5. This has the added bonus of arbitrarily increasing the “calculated” impulsive thrust.
Pg. 13 introduces a “Slope Filtering: Alternate Approach” to the force-fitting approach discussed above whereby the time derivative of the displacement sensor signal is plotted. This procedure produces a curve of magnitudes of slopes (Fig. 9). Sadly, this method starts off with the same assumptions as in the above approach. It compounds these problems by invoking an arcane procedure whereby the parts of the original displacement sensor curve with slopes lower a particular arbitrary (and unstated) value are removed and what’s left of the curve allegedly represents the “true” impulse curve. None of this procedure is shown in detail and only the final result is shown which, conveniently for the authors, is within ~20% of the previous analysis method. Of course, this convenient coincidence is entirely dependent on the arbitrary slope magnitude removal value.
9. Split Configuration
On pg. 15 we learn that by splitting the test article from the rest of the electronics – one on each end of the balance arm, the response time is reduced as expected due to the reduction in ballast weight required, and that the “true” thrust amplitude has been reduced from 106 uN to 63 uN, all other things being equal! Additionally, the displacement sensor curve (Fig. 10) is completely different in shape from the non-split configuration tests. The only explanation proffered for this discrepancy is that “the thermal contribution…is smaller in magnitude compared to the impulsive signal.” No proof of the correctness of this statement is provided. Since the split and non-split configuration curves are so radically different, the authors chose not to apply either of the analysis methods discussed above. They arbitrarily take the amplitude of the displacement signal at the instant it starts an exponentially asymptotic downward slope as the correct point. Why not use a variant of this method and apply it to the non-split configuration? Because it would result in relatively and apparently unacceptably huge (eg ~260 uN at 60 W) thrusts!
10. Difference between Forward and Reverse Thrusts
Tables 2 and 3 allow us to compare “calculated” thrusts (using the ideal curve force-fitting method discussed above) from Forward and Reverse non-split configurations. The Reverse thrusts are consistently lower than their Forward thrust counterparts. For example for 60 W, average Forward thrusts are 108 uN vs 60 uN for Reverse thrusts. For 80 W, these numbers are 104 uN vs 71 uN. No explanation is given for these differences, nor for the fact that in the Forward configuration, the 80 W thrust is lower than the 60 W thrust.
11. Null Thrust Test
It is stated on pg. 23 that “The [COG] shift from thermal expansion causes a downward drift in the optical displacement sensor.” Why not an upward drift? There is no justification given for this statement as no control tests were performed to ascertain what the result of a purely thermal effect might be, expansion or otherwise.
Further, the authors state “The results from the null thrust testing show no impulsive element…only the thermal signal.” This is also an unproven statement since no purely impulsive or purely thermal signal has been positively identified in shape or amplitude. The authors appear to have forgotten the thermal curve they used in Fig. 5, namely a double exponential. There is no evidence for any exponential part of the supposedly “thermal only” curve of the Null Test in Fig. 18. It appears completely linear and if there is a slight hint of an exponential, it is in the wrong sense (asymptotically falling, not flattening)! Another hint as to the problem of assigning a purely thermal explanation of the curve in Fig. 18 is the fact that exactly at the time of shutting off the RF power, there is no thermal lag or overshoot: the linear slope breaks suddenly to become essentially flat.
The implication of the Null Thrust test is that the thermal signal apparently seen in the Null Test would be the same as that seen in the Forward and Reverse tests. If so, then the curve force-fitting routine discussed above is invalid as it assumes a double exponential thermal curve (Fig. 5).
The Null Thrust test depicted in Fig. 18 was run at 80 W RF power. The Reverse Thrust test in Fig. 16 run at 80 W shows an apparent thermal signal of approx. 70 uN using the force-fitting routine. For the same period, the Null Thrust test shows an apparent thermal signal of approx. 275 uN. This is a huge discrepancy begging for detailed explanation.
In addition to mechanical and related considerations, the authors’ methods of analysis of sensor data to derive thrusts rests on untenable grounds. Not only is there an assumption of the presence of only a “true” impulse signal as well as a thermal signal, there is an assumption that the observed signal can be broken down into just these 2 components and amplitudes can be calculated based on an idealized superposition assumption. Therefore, until more control tests are performed allowing a more accurate method for estimation of thrusts, no faith can be placed in the thrust magnitudes reported in the paper.
Now that the EmDrive has made its way into the peer-reviewed literature, it falls in range of Tau Zero’s network of scientist reviewers. Marc Millis, former head of NASA’s Breakthrough Propulsion Physics project and founding architect of the Tau Zero Foundation, has spent the last two months reviewing the relevant papers. Although he is the primary author of what follows, he has enlisted the help of scientists with expertise in experimental issues, all of whom also contributed to BPP, and all of whom remain active in experimental work. The revisions and insertions of George Hathaway (Hathaway Consulting), Martin Tajmar (Dresden University), Eric Davis (EarthTech) and Jordan Maclay (Quantum Fields, LLC) have been discussed through frequent email exchanges as the final text began to emerge. Next week I’ll also be presenting a supplemental report from George Hathaway. So is EmDrive new physics or the result of experimental error? The answer turns out to be surprisingly complex.
by Marc Millis, George Hathaway, Martin Tajmar, Eric Davis, & Jordan Maclay
It’s time to weigh in about the controversial EmDrive. I say, controversial, because of its profound implications if genuine, plus the lack of enough information with which to determine if it is genuine. A peer-reviewed article about experimental tests of an EmDrive was just published in the AIAA Journal of Propulsion and Power by Harold (Sonny) White and colleagues: White, H., March, P., Lawrence, J., Vera, J., Sylvester, A., Brady, D., & Bailey, P. (2016), “Measurement of Impulsive Thrust from a Closed Radio-Frequency Cavity in Vacuum,” Journal of Propulsion and Power, (print version pending, online version here.
That new article, plus related peer-reviewed articles, were reviewed by colleagues in our Tau Zero network, including two who operate similar low-thrust propulsion tests stands. From our reviews and discussions, I have reached the following professional opinions – summarized in the list below and then detailed in the body of this article. I regret that I can only offer opinions instead of definitive conclusions. That ambiguity is a significant part of this story that also merits discussion.
(1) The experimental methods and resulting data indicate a possible new force-producing effect, but not yet satisfying the threshold of “extraordinary evidence for extraordinary claims” – especially since this is a measurement of small effects.
(2) The propulsion physics explanations offered, which already assume that the measured force is real, are not sound.
(3) Experiments have been conducted on other anomalous forces, whose fidelity and implications merit comparable scrutiny, specifically Jim Woodward’s “Mach Effect Thruster.”
(1) If either the EmDrive or Mach Effect Thrusters are indeed genuine, then new physics is being discovered – the ramifications of which cannot be assessed until after those effects are sufficiently modeled. Even if it turns out that the effects are of minor utility, having new experimental approaches to explore unfinished physics would be valuable.
(2) Even if genuine, it is premature to assess the potential utility of these devices. Existing data only addresses some of the characteristics necessary to compare with other technologies. At this point, it is best to withhold judgment, either pro or con.
Pitfalls to Avoid
(1) The earlier repeated tactic, to attempt fast and cheap experimental tests, has turned out to be neither fast nor cheap. It’s been at least 14 years since the EmDrive first emerged (2002) and despite numerous tests, we still lack a definitive conclusion.
(2) In much the same way that thermal and chamber effects are obscuring the force measurements, our ability to reach accurate conclusions is impeded by our natural human behavior of jumping to conclusions, confirmation biases, sensationalism, and pedantic reflexes. This is part of the reality that also needs understanding so that we can separate those influences from the underlying physics.
(1) Continue scrutinizing the existing experimental investigations on both the EmDrive and Mach Effect Thrusters.
(2) To break the cycle of endlessly not doing the right things to get a definitive answer, begin a more in-depth experimental program using qualified and impartial labs, plus qualified and impartial analysts. The Tau Zero Foundation stands ready to make arrangements with suitable labs and analysts to produce reliable findings, pro or con.
(3) If it turns out that the effects are genuine, then continue with separate (a) engineering and (b) physics research, where the engineers focus on creating viable devices and the physicists focus on deciphering nature. In both cases:
- Characterize the parameters that affect the effects.
- Deduce mathematical models.
- Apply those models to (a) assess scalability to practical levels, and (b) understand the new phenomena and its relation to other fundamental physics.
- On all of the above, conduct and publish the research with a focus on the reliability of the findings rather than on their implications.
Pitfall 1 – The Fog of Want
Our decisions about this physics are influenced by behaviors that have nothing to do with physics. To ignore this human element would be a disservice to our readers. To get to the real story, we need to reveal that human element so that we can separate it from the rest of the data, like any good experiment. I’m starting off with this issue so that you are alert to its influences before you read the rest of this article.
As much as I strive to be impartial, I know I have an in-going negative bias on the EmDrive history. To create a review that reflects reality, rather than echoing my biases, I had to acknowledge and put aside my biases. Similarly, if you wish to extract the most from this article, you might want to check your perspectives. Ask yourself these three questions: (1) Do you already have an opinion about this effect and are now reading this article to see if we’ll confirm your expectation? (2) Do you want to know our conclusions without any regard to how we reached those conclusions? (3) Are you only interested in this EmDrive assessment, without regard to other comparable approaches?
If you answered “yes” to any of those questions, then you, like me, have natural human cognitive dysfunctions. To get past those reflexes, start by at least noticing that they exist. Then, take the time to notice both the pros and cons of the article, not just the parts you want to be true. Deciphering reality takes time instead of just listening to reflexive beliefs. It requires that one’s mind be open to the possibility you might be right and equally open to the possibility you might be wrong.
This history is a recurring theme of incredible claims with non-credible evidence for those claims. In all cases, the effect is assumed to be real before the tests – which reflects a blinding bias. This dates back to at least 2002 when Roger Shawyer claimed to invent a device that “provides direct conversion from electrical energy to thrust, without expelling propellant.” I was still at NASA and vaguely remember reviewing it then. Regardless of the claims, the fidelity of the methods were below average. Over the years I heard about several other tests, but never saw any data. Eventually there was a press story about tests in China, along with this photo. It turns out that this photo is not a Chinese rig, but one of Shawyer’s:
Shawyer’s device and supporting equipment are on a rotating frame, where that rotation is used to determine if the device is thrusting. Note, however, the radiator and coolant lines. Any variation in the coolant flow would induce a torque that would obscure any real force measurements. Knowing the claimed thrusting effect is small and having enough experience to guess the likely variations in coolant flow, I considered this test set-up flawed.
Regarding the Chinese tests, I did not previously know they are described in peer-reviewed articles. Since many of us did not know either, I’m listing them here along with cursory impressions:
Juan, Y., et al, (2012). Net thrust measurement of propellantless microwave thrusters. Acta Physica Sinica, Chinese Physical Society.
Due to all of the impressions below, I do not have any confidence in their data:
- Assumes first that the EmDrive is genuine.
- Verbally describes theory, but without predicting experimental findings.
- The experiment is not described in enough detail to assess its fidelity, but is similar to the one in the photo. Regardless, there is absolutely no discussion of possible influences on the rotation from tilting, power lead forces, vibration effects, thermal effects, or others.
- The behavior of the thrust stand was not characterized before installing the EmDrive. Testing the two together without first having characterized the thrust stand separately prevents separating their distinct characteristics from the data.
- The data plots lack error bands.
Juan, Y., et al (2013). Prediction and experimental measurement of the electromagnetic thrust generated by a microwave thruster system. Chinese Physics B, 22(5), 050301.
Due to all of the impressions below, I do not have any confidence in their data:
- The description of the experiment is improved from the 2012 paper and appears to be the same configuration. This time possible effects from tilting and the power lead forces are mentioned, but they still do not address vibration, thermal, coolant loop, or other effects.
- Again, they fail to characterize the thrust stand separately from the EmDrive.
- Unlike the 2012 paper, they attempt to make numerical predictions. Details are provided for their physics derivations (which I did not scrutinize). That theory is then applied to make predictions for their specific hardware, but only verbally described it, rather than showing an explicit derivation. They show plots of the predicted force versus power, but only up to 200W, where the experimental runs span about 100W to 2400W.
- The experimental results do not match their linear predictions for the ratio of force-to-power. These differences are then evasively dismissed.
Juan, Y., et al. (2016), “Thrust Measurement of an Independent Microwave Thruster Propulsion Device with Three-Wire Torsion Pendulum Thrust Measurement System,” Journal of Propulsion Technology, vol. 37, no. 2, pp 362-371.
The text is in Chinese, which I did not translate, but the figures and plots are captioned in English. Therefore I comment only on those diagrams. Again, what is shown is not enough to support claims of anomalous forces:
- From figures 2, 3, 6, 7, 16, and 19, it appears the prior apparatus is now hung from torsion wires instead of a rotating support from below. This time the coolant loop is explicitly shown, but in a conceptual drawing instead of showing specifics. Again, the influence of the coolant loop is ignored.
- The only “measurement results” plot is “force versus serial number” – which conveys no meaningful information (without being able to read associated text).
- I learned later from Martin Tajmar, that the observed thrust drops by more than an order of magnitude when the device is powered by batteries instead of the external cables (cables whose currents can induce forces).
I chose not to cite and comment on the many non-peer-reviewed articles on Shawyer’s website and related AIAA conference papers.
Shawyer eventually published a peer-reviewed article, specifically: Shawyer, R. (2015), “Second generation EmDrive propulsion applied to SSTO launcher and interstellar probe,” Acta Astronautica, vol. 116, pp 166-174. Shawyer states: “Theoretical and experimental work in the UK, China and the US has confirmed the basic principles of producing thrust from an asymmetric resonant microwave cavity.” That assertion has not held up to scrutiny. Therefore, all related assertions are equally unfounded. Instead of offering substantive evidence, this article instead predicts the performance for three variations of EmDrives that now claim to use superconductivity. From these, he presents conceptual diagrams for their respective spacecraft. He also mentions the “Cannae Drive,” by Guido Fetta, as another embodiment of his device.
Latest EmDrive Paper
The latest paper, in the AIAA Journal of Propulsion and Power, is an improvement in fidelity on the prior tests and may be indicative of a new propulsive effect. However, the methods and data are still not crossing the threshold of “extraordinary evidence for extraordinary claims” – especially since this is a measurement of small effects. With the improved fidelity of the reporting and the data traces themselves, I have to question my earlier bias that the prior data was entirely due to experimental artifacts and proponent biases.
The assessment offered below is a summary of discussions with the coauthors of this report plus a few other colleagues. Both Martin Tajmar and George Hathaway operate similar low-thrust propulsion test stands and thus are familiar with such details. George Hathaway’s more focused analysis will be posted in a future Centauri Dreams article.
The major problems with the paper are (1) lack of impartiality, (2) the test hardware is not sufficiently characterized to separate spurious effects from the test article’s effects, (3) the data analysis is marred by the use of subjective techniques, and (4) the data can be interpreted in more than one way – where one’s bias will affect one’s conclusions.
The first shortcoming of the paper is that it is biased. It assumes that the propulsion effect is genuine and then goes on to invent an explanation for that unverified effect. This bias skews how they collect and analyze the data. To be more useful, the paper should have reported impartially on its experimental and analytical methods to isolate a potential new force-producing effect from other contaminating influences.
The next shortcoming is insufficient testing for how spurious causes can affect the thrust stand. While this new paper is a significant improvement over the previous publications, it falls short of providing the needed information to reach a definitive conclusion. They use techniques comparable to engineering tests of conventional low-thrust electric propulsion. While such engineering techniques might be passable for checking electric propulsion design changes, it is not sufficient to demonstrate that a new physics effect exists. The specific shortcomings include:
- Thrust stand tilting: The thrust stand has a vertical axis, where even slight changes of that alignment will affect how the thrust stand behaves. There are three parts to this, none of which are quantified: the fidelity of the thrust stand flexures and pivots, the alignment fidelity of that structure to the vacuum chamber, and the sustained levelness of the “optical bench” upon which the vacuum chamber is mounted.
- Thrust stand characterization: The thrust stand does not return to its original position after tests, even for most calibration events. Additionally, the thrust stand is over-damped, meaning that it is slow to respond to changes, including the calibration events. Those characteristics (time for the thrust stand to respond to a known force and the difference between its before/after positions) are important to understand so that those artifacts can be separated from the data. These facets are largely ignored in the paper. The report does mention that the location of the masses on the thrust stand affects its response rate (“split configuration” versus “non-split”), but this difference is not quantified. The thrust stand uses magnetic dampers. Similar dampers used on one of Martin Tajmar’s thrust stands were found to cause spurious effects (subsequently replaced with oil dampers). Given the irregular behavior, it is fair to suspect that other causes are interfering with the motion of the thrust stand. The flexural bearings might be operated beyond their load capacity or might be affected by temperature.
- Forces from power cables: To reduce the influence of electromagnetic forces from the power leads, Galinstan liquid metal screw and socket connections are used. While encouraging, it is not specified if these connections (several needed) are all coaxially aligned with the stand’s rotation axis (as required to minimize spurious forces). Also, there are no tests with power into a dummy load to characterize these possible influences.
- Chamber wall interactions: Though mentioned as a possible source of error, the electromagnetic forces between the test device and the vacuum chamber walls are dismissed without quantitative estimates or tests. One way that this could have been explored is by using more variations in the position and orientation of the test device relative to the chamber. For example, in the “null thrust” configuration, only one of four possibilities is used (the device pointed toward the pivot axis). If also pointed up, down, and away from the pivot, more information would have been collected to help assess such effects.
- Thermal effects: The paper acknowledges the possible contributions from thermal effects, but does not quantify that contribution. For example, there are no measurements of temperature over time compared to the thrust stand’s deflection. Such measurements should have been made during operation of the device and when running power through a dummy load. Absent that data, the paper resorts to subjectively determining which parts of the data are thermal effects. For example, without any validation, the paper assumes that the displacement measured during the “null thrust” configuration is entirely a thermal effect. It does not consider chamber wall interactions or any other possible sources. The paper does speculate that temperature changes might shift the center of gravity of the test article in a way that affects the thrust stand, but no diagrams are offered showing how a slight change in one of those dimensions would affect the thrust stand.
The third and most egregious shortcoming in the report is that they apply a vaguely described “conceptual simulation” (which is never mathematically detailed) as their primary tool to deduce which part of the data is attributable to their device and which is due to thermal effects. They assume a priori the shapes of both the “impulsive thrust” (their device) and thermal effects and how those signals will superimpose. There is no consideration of chamber wall effects, power lead forces, tilting, etc. As a reflection of how poorly defined this assumed superposition, the ‘magnitude’ and ‘time’ axes on the chart showing this relation (Fig. 5) are labeled as “arbitrary units.” Another problem is that their assumed impulsive thrust curve does not match the shape of most of the data that they attribute to impulsive thrust. Instead of the predicted smooth curve, the data shows deviations about halfway through the thrusting time. They then apply this subjective and arbitrary tool to reach their conclusions. Because they are biased that the effect is genuine and because their methods overlook critical measurements, I cannot trust the authors’ interpretations of their results.
Absent an adequate accounting for the magnitude and characteristics of secondary causes and how to remove those possible influences from the data, the fourth major problem with the report is that its data can then be interpreted more than one way.
Rather than evoking subjective techniques here, the comments that follow are based only on examining their data plots as a whole. To illustrate how this data can then be interpreted in more than one way, both dismissive and supportive interpretations are offered. In particular, we compare the traces from the “forward,” “null,” and “reverse” thrust configurations and then the force versus power compilation of the runs.
The data for the 80W operation of the device in the “forward,” “null,” and “reverse” thrust configurations is presented in Figures, 9c, 18, and 10c, respectively. Recall from the above discussions that this data includes all the uncharacterized spurious causes (thermal, chamber wall interactions, power lead forces, tilting of the thrust stand, and seismic effects), plus any real force from the test device. The values shown in the table below were read from enlarged versions of the figures.
Table of Noteworthy Data Comparisons Between Forward, Null, and Reverse Thrust Orientations
For a genuine thrusting effect, one would expect the results to show near-matching magnitudes for forward and reverse thrust and a zero magnitude for the null-thrust orientation. If one looks only at the “Total deflection,” all the magnitudes are roughly the same, including the null-thrust. Pessimistically, one could then infer that the spurious effects are great enough to be easily misinterpreted as a genuine thrust.
Conversely, if one considers how quickly the deflections occur, then the attention would be on the “Rate of deflection.” In that case, the thrusting configurations are roughly twice as large as the null-thrust configuration. From only that, one might infer that a new force-producing effect is larger than spurious causes.
To infer conclusions based on the deflection rates, one must also examine the rate of deflection for the calibration events, which should be the same in all configurations. The calibration deflection rate appears roughly the same in the forward and reverse thrust configuration, but more than 2.5 times larger in the null thrust configuration. That there is a difference compounds the difficulty of reaching conclusions. There are also significant inconsistencies with how the thrust stand rebounds once the power is turned off between the thrusting and null-thrust configurations, again compounding the difficulty of reaching conclusions.
Because a possible positive interpretation exists within those different perspectives, I cannot rule out the possibility that the data reflects a new force-producing effect. But as stated earlier, given all the uncharacterized secondary effects and the questionable subjective techniques used in the report, this is not sufficient evidence. Given the prominent role played by the rate of deflections, the dynamic behavior of the thrust stand must be more thouroughly understood before reaching firm conclusions.
Next, let’s examine the compilation of runs, namely Fig. 19. Based on a linear fit through the origin with the data, they conclude a thrust-to-power ratio of 1.2 ± 0.1 mN/kW (=µN/W). While this is true, the data can be interpreted more than one way. Note that the averages for 60 and 80 watts operations are the same, so a linear fit is not strictly defensible. One could just as easily infer that increasing power yields decreasing thrust, a constant 50 µNewton force, or an exponential curve that flattens out to a constant (saturated) thrust of about 100 uN. Note too that the null-thrust data (which could be interpreted to be as high as 211 µN) is not shown on this chart.
Recall too that they did not quantify the potential spurious effects, so their presumed error band of only ±6 µN does not stand up to scrutiny. Note, for example, the span in the 40W data is about ± 17µN, the 60W about ± 50µN, and the 80W about ± 32µN. What is not clear is if these 40, 60, and 80 Watt runs represent different operating parameters (Q-factor?), or if instead, these are the natural variations with fixed settings.
The pessimistic interpretation is that the deviations in the data represent variations for the same operating conditions, in which case the data are too varied from which to conclude any correlations. Conversely, the optimistic interpretation is to assume the variations are due to changes in operating parameters, but then that additional information should be made available and be an explicit part of the analysis.
In summary, this most recent report is a significant improvement, but has many shortcomings. Questionable subjective techniques are used to infer the “thrust” from the data. Other likely influences are not quantified. But also, despite those inadequacies, the possibility of a new force-producing effect cannot be irrefutably ruled out. This is intriguing, but still falling short of defensible evidence.
EmDrive and Other Space Drive Theories
First, I cannot stress enough that there is no new EmDrive “effect” yet about which to theorize. The physical evidence on the EmDrive is neither defensible nor does it include enough operating parameters to characterize a new effect. The data is not even reliable enough to deduce the force-per-power relationship, let alone any other important correlations. What about the effects of changing the dimensions or geometry, changing the materials, or changing the microwave frequencies or modulation? And then there is the unanswered question, what are the propulsion forces pushing on?
Assuming for the moment that the EmDrive is a new force-producing effect, we know at least two things (1) it is not a photon rocket, because the claimed forces are 360 times greater than the photon rocket effect, and (2) a force, without an “equal and opposite force,” goes beyond Newton’s laws. Note that I did not evoke the more familiar “violating conservation of momentum” point. That is because these experiments are still trying to figure out if there is a force. We won’t get to conservation of momentum until after those forces are applied to accelerate an object. If that happens, then we must ask what reaction mass is being accelerated in the opposite direction. If the effects are indeed genuine, then new physics is being discovered or old physics is being applied in a new, unfamiliar context.
For those claiming to have a theory to predict a new propulsion effect, it is necessary that those theories make testable numeric predictions. The predictions in Juan’s 2013 paper did not match its results. The analytical discussions in White’s 2016 experimental paper do not make theoretical predictions. The same is true with his 2015 theoretical paper: White (2015), “A discussion on characteristics of the quantum vacuum,” Physics Essays, vol. 28, no. 4, 496-502.
Short of having a self-consistent theory, any speculations should at least accurately echo the physics they cite. The explanations in the White’s 2016 experimental paper, White’s 2015 theory paper, and even White’s 2013 report on the self-named “White-Juday Warp Field Interferometer” (White (2013), “Warp Field Mechanics 101,” Journal of the British Interplanetary Society, vol. 66, pp. 242-247), did not pass this threshold. I’ll leave to other authors to elaborate on the 2015 and 2016 papers, while a review of the 2013 warp drive claims is available here. It is Lee & Cleaver (2014), “The Inability of the White-Juday Warp Field Interferometer to Spectrally Resolve Spacetime Distortions,” [physics.gen-ph].
In contrast, it is also important to avoid pedantic reflexes – summarily dismissing anything that does not fit what we already know, or assuming all of our existing theories are completely correct. For example, the observations that lead to the Dark Matter and Dark Energy hypotheses do not match existing theories, but that evidence has been reliably documented. Using that data, many different theories are being hypothesized and tested. The distinction here is that both the proponents and challengers make sure they are accurately representing what is, and is not yet, known.
If a propulsion physics breakthrough is to be found, it will likely be discovered by examining relevant open questions in physics. A relevant theoretical question to non-rocket propulsion concepts (including the EmDrive) is ensuring conservation of momentum. One way to approach this is to look for phenomena is space that might serve as a reaction mass in lieu of propellant, perhaps like the quantum vacuum. Another approach is to dig deeper into the nature of inertial frames. Inertial frames are the reference frames upon which the laws of motion and the conservation laws are defined, yet it is still unknown what causes inertial frames to exist or if they have any deeper properties that might prove useful.
Woodward Tests and Theory
In addition to the overtly touted EmDrive, there are about two-dozen other space drive concepts of varying degree of substance. One of them started out as a theoretical investigation into the physics of inertial frames which then advanced to make testable numeric predictions. Specifically I’m referring to what is now called the “Mach Effect Thruster” concept of James F. Woodward, which dates back at least to this article:
Woodward, James F. (1990), “A new experimental approach to Mach’s principle and relativistic gravitation,” Foundations of Physics Letters, vol. 3, no. 5, pp. 497-506.
A more in-depth and recent publication on these concepts is available as:
Woodward, James F. (2013) Making Starships and Stargates: The Science of Interstellar Transport and Absurdly Benign Wormholes. Springer Praxis Books.
Experiments have been modestly underway for years, including three recent independent replication attempts by George Hathaway in Toronto Canada, Martin Tajmar in Dresden Germany, and Nembo Buldrini in Wiener Neustadt, Austria. A workshop was held to review these findings in September 20-23, 2016, in Estes Park, Colorado. I understand from an email conversation with Jim Woodward that these reports and workshop proceedings are now undergoing peer review for likely publication early in 2017.
The main point here, by citing just this one other example, is that there are other approaches beyond the highly publicized EmDrive claims. It would be a disservice to our readers to let a media fixation with one theme blind us to alternatives.
If either the EmDrive or Mach Effect Thruster is indeed genuine, then new physics is being discovered or old physics is being applied in a new, unfamiliar context. Either would be profound. Today it is premature to assert than any of these effects are genuine, or conversely, to flatly rule out that such propulsion ambitions are impossible. When the discussions are constrained to exclude pedantic disdain and wishful interpretations, and limited to people who have either the education or experience in related fields, one encounters multiple, even divergent, perspectives.
Next, even if new physics-to-engineering is emerging, it is premature to assess its utility. The number of factors that go into deciding if a technology has an advantage over another are way beyond what data is yet available. Recall that the performance of the first aircraft, jet engine, transistor, etc, were all tiny examples of what those breakthroughs evolved to become. Reciprocally, we tend to forget about all the failed claims who have faded into obscurity. We just do not know enough today, pro or con, to judge.
I realize the urge within human behavior for fast, definitive answers that we can act on. This lingering uncertainty is aggravating, even more so when peppered with distracting hype or dismissive disdain. To get to the underlying reality, we must continue with a focus on the fidelity of the methods to produce reliable results, rather than jumping to conclusions on the implications.
What to Do About It
If you want definitive answers, then we must improve the reliability of the methods and data, and remain patiently open for the results to be as they are, good news or bad news. I alluded earlier to the broken tactic of trying to get answers with fast and cheap experiments. How many inadequate experiments and over how many years does it take before we change our tactics? I’ve had this debate more than once with potential funding sources and I hope they are reading now to see… “I told ya so!” Sorry, I could not resist that human urge to emotionally amplify a well-reasoned point. To break the cycle of endlessly not doing the right things to get a definitive answer, we must begin a more in-depth experimental program using qualified and impartial labs, plus qualified and impartial analysts. Granted, those types of service providers are not easy to find, where impartiality is the hardest to come by. Also, it might take three years to get a reliable answer, which is at least better than 14 years. And the trustworthy experiments will not be cheap, but quite likely far less than the aggregate spent on the repeated ‘cheap’ experiments. If any of those prior funding sources (or new) are reading this and finally want trustworthy answers, contact us. Tau Zero stands ready to make arrangements with suitable labs and analysts to conduct such a program.
And what if we do discover a breakthrough? In that case, we recommend distinguishing two themes of research, one from an engineering point of view to nudge the effect into a useful embodiment, and another from an academic point of view, to fully decipher and compare the new effects to physics in general. In both those cases we need to:
1. Characterize the parameters that affect the effects. Instead of just testing one design, vary the parameters of the device and the test conditions to get enough information to work with.
2. Deduce mathematical models from that more complete set of information.
3. Apply those models to (a) assess scalability to practical levels, and (b) explore the new phenomena and its relation to other fundamental physics.
4. On all of the above, conduct and publish the research with a focus on the reliability of the findings rather than on their implications.
For those of you who are neither researchers nor funding sources, what should you do? First, before reposting an article, take the time to see if it offers new and substantive information. If it turns out to be hollow click-bait, then do not share it. If it has both new information with meaningful details, then share it. Next, as your read various articles, notice which sources provide the kind of information that helps you understand the situation. Spend more time with those sources and avoid sources who do not.
Regarding questionable press stories, I’m not sure yet what to make of this: “The China Academy of Space Technology (CAST), a subsidiary of the Chinese Aerospace Science and Technology Corporation (CASC) and the manufacturer of the Dong Fang Hong satellites, has held a press conference in Beijing explaining the importance of the EmDrive research and summarizing what China is doing to move the technology forward.” Some stories claim there is a prototype device in orbit. If true, I would expect to see at least one photo of the device being tested in space. But we’ll see…
When faced with uncertain situations and where the data is unreliable, the technique I use to minimize my biases is to simultaneously entertain conflicting hypotheses, both the pro and con. Then, as new reliable information is revealed, I see which of those hypotheses are consistent with that new data. Eventually, after enough reliable data has accrued, the reality becomes easier to see.
The cited devices have gone by multiple names (e.g. EmDrive, EM Space Drive; Mach Effect Thruster, Mach-Lorentz Thruster), and the versions used in this article are the ones with the greatest number of Google search hits.