The term breakthrough propulsion refers to concepts like space drives and faster-than-light travel, the kind of breakthroughs that would make interstellar travel practical.
This research falls within the realm of physics instead of technology, with the distinction being that physics is about uncovering the laws of nature while technology is about applying that physics to build useful devices. Since existing technology is inadequate for traversing astronomical distances between neighboring stars (even if advanced to the limit of its underlying physics), the only way to circumvent these limits is to discover new propulsion physics. The discovery of new force-production and energy-exchange principles would lead to a whole new class of technologies. This is the motivation of breakthrough propulsion physics research.
Objectively, the desired breakthroughs might turn out to be impossible, but progress is not made by conceding defeat. Reciprocally, breakthroughs have a habit of taking pessimists by surprise, but can equally remain elusive. By proceeding in small, incremental steps that focus on the immediate questions and by emphasizing the reliability of the findings rather than their long-range implications, relevant and dependable knowledge will result. Regardless of whether the breakthroughs are found, this inquiry provides an additional perspective with which to seek answers to the lingering unknowns of our universe.
Status of the NASA Breakthrough Propulsion Physics (BPP) Project
All NASA support to sustain cognizance on these possibilities has been withdrawn as of October 1, 2008. The final NASA contribution was to assist in the compilation of a graduate-level technical book, Frontiers of Propulsion Science, which is due out in early 2009. This book (750 pages, hardback) will be volume 227 of the series, Progress in Astronautics and Aeronautics Series, which will be published by American Institute for Aeronautics and Astronautics (AIAA).
Prior to this point, the project’s leader, Marc G. Millis, continued to monitor and assess a variety of ongoing research with the assistance of an informal network of volunteers scattered across academia, industry, various NASA Centers, and other Federal labs. During that time, several publications were completed to document the progress made. When funding for active research was available, which ran from 1996 to 2002, the project oversaw research into 8 different approaches, produced 16 peer-reviewed journal articles, and an award-winning website (Warp-When), all for a total investment of less than $1.6M. Also during that funded time, the BPP Project coordinated with related research funded at the NASA Marshall Space Flight Center. With the implementation of the 2003 Federal Budget (p.325), all advanced propulsion research was deferred, including these research efforts.
Status of Research
No breakthroughs appear imminent. This is a nascent field where a variety of concepts and issues are being explored in the scientific literature, beginning since about the early 1990s. The collective status is still at step 1 and 2 of the scientific method, “defining the problem” and “collecting data,” but a small number of approaches are already at step 4, “testing hypotheses;” with experiments underway.
Cautionary note: On a topic this visionary and whose implications are profound, there is a risk of encountering, premature conclusions in the literature, driven by overzealous enthusiasts as well as pedantic pessimists. The most productive path is to seek out and build upon publications that focus on the critical make-break issues and lingering unknowns, both from the innovators’ perspective and their skeptical challengers. Avoid works with broad-sweeping and unsubstantiated claims, either supportive or dismissive.
Breakthrough Propulsion — Interim Report
The term, propulsion breakthrough, refers to concepts like propellantless space drives and faster-than-light travel, the kind of breakthroughs that would make interstellar exploration practical. Although no such breakthroughs appear imminent, a variety of investigations have begun. From 1996-2002, NASA supported the Breakthrough Propulsion Physics Project to examine physics in the context of breakthrough spaceflight. Three facets of these assessments are now reported: (1) predicting benefits, (2) selecting research, and (3) recent technical progress. Predicting benefits is challenging since the breakthroughs are still only notional concepts, but energy can serve as a basis for comparison. A hypothetical space drive would require many orders of magnitude less energy than a rocket for journeys to our nearest neighboring star. Assessing research options is challenging when the goals are beyond known physics and when the implications of success are profound. To mitigate the challenges, a selection process is described where: (a) research tasks are constrained to only address the immediate unknowns, curious effects or critical issues, (b) reliability of assertions is more important than their implications , and (c) reviewers judge credibility rather than feasibility . The recent findings of a number of tasks, some selected using this process, are discussed. Of the 14 tasks included, 6 reached null conclusions, 4 remain unresolved, and 4 have opportunities for sequels. A dominant theme with the sequels is research about the properties of space, inertial frames, and the quantum vacuum.
Confronted by the physical limits of rocketry and space sails, NASA supported the Breakthrough Propulsion Physics Project from 1996 to 2002. [1-3 ] As its name suggests, the project specifically looked for propulsion breakthroughs from physics rather than refinements of technology. By breakthroughs, it is meant new propulsion methods to make human voyages to other star systems possible. Theories and phenomena in recent scientific literature provide new approaches to seek such breakthroughs, including warp drives, [4 ] wormholes, [5 ] vacuum fluctuation energy, [ 6 ] and emerging physics in general.
This report focuses on the following 3 challenges of this pursuit: (1) predicting benefits, (2) selecting the best research approaches, and (3) summarizing recent technical progress. To predict benefits, a number of different assessments are offered. Since little has been published toward quantifying benefits, a variety of analyses are offered to set the groundwork for future assessments. The second challenge, that of selecting the best research approaches, is addressed by summarizing the key management strategies from the NASA Breakthrough Propulsion Physics Project. [ 3 ] And finally, extracts from recent research findings [ 2 ] are compiled with attention drawn to the most immediate research questions.
Gauging the potential benefits of undiscovered propulsion breakthroughs is challenging, but addressable. The major difficulty is that such breakthroughs are still only notional concepts rather than being a specific method from which performance can be unambiguously calculated. One prior assessment considered a Voyager-sized spacecraft using a hypothetical space drive to show that the trip time to reach our nearest neighboring star could be decreased by a factor of 6.5 just by using the leftover power of Voyager’s generators. [7 ] Another recent assessment considered a rocket with hypothetical modifications of inertia and gravity and showed that the benefits would be trivial. [ 8 ] Performance estimates vary considerably depending on the methods and assumptions. To pave the way for a more complete suite of assessments, a variety of methods are introduced here along with a few examples that are worked out. A key feature is that the basis of comparison isenergy , rather than using the metrics of rocketry. Discussion on the pitfalls of using rocketry metrics for assessing breakthrough spaceflight is also provided.
Assessing Hypothetical Inertial Modifications: A recent publication took a first step toward assessing the potential benefits of hypothetical inertial and gravitational control, but did so in terms of rocketry. [8 ] A modified rocket equation was used to demonstrate that naive modifications of gravity or inertia do not produce much benefit. Although an important first step to help correct misconceptions, this assessment did not include many other relevant comparisons. As an example of a limitation, the analysis applied its hypothetical inertial change equally to both the propellant and the vehicle. There is no benefit in this case. One could equally assume that only the inertia of the expelled propellant were increased while the inertia of the vehicle remained the same, in which case there would be more benefit.
To illustrate this alternative, the rocket equation can be derived for the hypothetical case where the expelled propellant’s inertia is increased as it is accelerated out of the rocket. The inertial modification is not applied to the rest of the rocket or the stored propellant. It is important to stress that this is only a hypothetical example to illustrate the sensitivity of the findings to the methods, rather than to suggest that this is a realistic potential breakthrough. Numerous variations on this analysis are possible. Starting with conservation of momentum, where the momentum of the rocket in one direction must equal the momentum of expelled propellant in the other, a coefficient, δ , has been inserted to represent this inertial modification:
The left side of the equation represents the momentum of the expelled propellant and the right side represents the corresponding momentum of the accelerated rocket, and where;
From this starting equation, the following equation for the rocket’s change in velocity, ∆v, can be derived [ 9]:
This is identical to the celebrated Tsiolkovsky equation of 1903, [ 10] with the exception of the presence of the term, δ, for the inertial modification. This means that a delta of 1.10, representing a 10% increase in the expelled propellant’s inertia, would yield a 10% increase in delta-v. While this appears encouraging, it should be remarked that there are at present no known techniques to affect such a change in propellant inertia and that this result only illustrates the potential advantage of hypothetical inertial modifications.
An additional issue to pursue would be to calculate the energy required to support this hypothetical change in propellant inertia. Again, the main point of the exercise is to reveal that different approaches will yield significantly different results. The implications of Equation 2 are considerably different than the null finding that occurs when one applies the inertial modification to both sides of the equation.
Limits of Rocketry Analyses for Breakthroughs: When using the metrics of an incumbent technology to assess the potential of a new technology, results can be misleading. The example above is just one illustration of how two different assumptions of hypothetical inertial control can lead to very different predictions.
Another misleading use of the rocket equation is the common practice of assigning an infinite specific impulse to describe a propellantless space drive. Although based on a reasonable extrapolation where higher specific impulse leads to less propellant, this also leads to the conclusion that a propellantless space drive would require infinite energy. As shown later, this is not necessarily the case. Furthermore, since specific impulse is a measure of the thrust per propellant weight flow rate, it has no real meaning if there is no propellant flow.
Using the rocket equation to describe something that is not likely to involve a rocket is about as misleading as using the metrics of sailing ships to assess steamships. [ 11] Although reduced sail area is indeed a consequence of steamships, the true benefit is that shipping can continue regardless of the wind conditions and with far more maneuvering control. Similarly, the benefits of breakthrough inertial or gravity control would likely surpass the operational conventions of rocketry. Although comparisons built on the incumbent methods might be useful for introductory purposes, a deeper understanding of the benefits and research approaches are better illustrated by using a common and more basic metric. For spaceflight, whether via rockets or space drives, energy is a better basis for comparison.
Deep Space Propulsion Energy: This next assessment deals with deep space travel. Both a rocket and a hypothetical space drive will be compared in terms of energy. A space drive is defined as: “an idealized form of propulsion where the fundamental properties of matter and spacetime are used to create propulsive forces anywhere in space without having to carry and expel a reaction mass.” [ 12] For this exercise it can be thought simply as a device that converts potential energy directly into kinetic energy. Since issues such as momentum conservation are addressed in the cited reference, they will not be repeated here.
For this introductory exercise, the following assumptions are used. To more fully understand the challenges, it would be fruitful to repeat the analysis using different assumptions:
– The thrusting duration is assumed to be much shorter than the trip duration, which for interstellar travel is reasonable.
– For the rocket, constant exhaust velocity is assumed.
– Non-relativistic trip velocity and exhaust velocity are assumed.
– The energy requirements for a rendezvous mission are based on equal ∆v’s for acceleration and deceleration.
Since a space drive has been defined as a device that converts potential energy into kinetic energy, the basic equation of kinetic energy is used to represent the space drive energy, where the values of vehicle mass and mission delta-v will be the same as with the rocket.
To compare the energy of a rocket to a space drive that does not use propellant, we need an equation for rocket energy where the propellant mass is represented in terms of the vehicle’s mass and the∆v of the mission. Combining the Tsiolkovsky rocket equation with the equation representing the energy imparted to the propellant from the rocket’s frame of reference, the following approximation for rocket energy can be derived. [ 9] This is consistent with the previously stated assumptions:
Two things are important to note regarding the energy differences between a rocket and a hypothetical space drive. First, the energy for a given ∆v scales as an exponent for a rocket and scales as a square for a space drive. This by itself is significant, but it is important to point out that a rocket and a space drive treat additional maneuvers differently. For a rocket it is conventional to talk in terms of increases to ∆v for additional maneuvers. For space drives, however, the additional maneuvers are in terms of additional kinetic energy. To illustrate this difference, consider a mission consisting of multiple maneuvers ( n) each having the same incremental change in velocity ( ∆v i). Notice the location of the term representing the number ( n) of repeated maneuvers (∆v i), in the following two equations:
In the case of the space drive, additional maneuvers scale linearly, while for rockets they scale exponentially. This is another example to highlight why rocket conventions can be misleading when contemplating space drives.
Numerical Example: To put this into perspective, consider a representative mission of sending a 5000 kg probe over a distance of 5 light-years in a 50-year timeframe. This range is representative of the distance to our nearest neighboring star (4.3 light-years) and the 50-yr time frame is chosen as one short enough to be within the threshold of a human career span, yet long enough to be treated with non-relativistic equations. This equates to a required trip velocity of 10% lightspeed. The probe size of 5000 kg is roughly that of the Voyager probe plus the dry mass of the Centaur Upper Stage (4075 kg) that propelled it out of Earth’s orbit.  The comparison is made for both a flyby mission and a rendezvous mission.
Before proceeding, a limit should be brought to attention. For these introductory exercises, the comparisons are non-relativistic. For rockets, this implies limiting the exhaust velocity to ≤ 10% lightspeed. This corresponds to a specific impulse limit of 3 x 10^6 s, which is found by setting the exhaust velocity to 10% light speed in the following equation relating specific impulse to exhaust velocity [ 13]:
g = Earth’s gravitational field = 9.8 m/s^2.
The results of the comparisons are listed in Table I. The rocket case is calculated for two different specific impulses, one set at the upper non-relativistic limit previously described, and another set at an actual high value achieved during electric propulsion lab tests. [ 14] The Space Drive Improvement column is the ratio of the rocket energy to the space drive energy.
TABLE 1 – COMPARISON OF DEEP SPACE MISSION ENERGY REQUIREMENTS
Even in the case of the non-relativistic upper limit to specific impulse – an incredibly high-performance hypothetical rocket – the space drive uses a factor of 2 to 3 less energy. When compared to attainable values of specific impulse – values that are still considerably higher than that currently used in practice – the benefits of a space drive are enormous. Even for just a flyby mission, the gain is 72 orders of magnitude. When considering a rendezvous mission, the gain is almost 150 orders of magnitude. Again, though these results are intriguing, they should only be interpreted as the magnitude of gains sought by breakthrough propulsion research. Other assessments and results are possible.
Earth to Orbit Energy: Consider next the case of lifting an object off the surface of the Earth and placing it into orbit. This requires energy expenditures both for the altitude change and for the speed difference between the Earth’s surface and the orbital velocity. For the hypothetical space drive, this energy expenditure can be represented as:
Where ∆U is the potential energy change associated with the altitude change, and ∆K is the kinetic energy change associated with different speeds at the Earth’s surface and at orbit. The change in potential energy, which requires expending work to raise a mass in a gravitational field, is represented by:
M Earth = mass of the Earth
m = mass of the spacecraft
r = distance from the center of the Earth
r Orbit = radius of the orbit as measured from the center of the Earth
r Surface = radius of the Earth’s surface
The change in kinetic energy requires solving for the orbital velocity and the velocity of the Earth’s surface and can be shown to take this form [ 9]:
For the case of placing the shuttle orbiter ( m = 9.76 x 10^4 kg ) into a typical low Earth orbit (altitude = 400 km; r orbit = 6.67 x 10^6 m), the energy required is found to be 3.18 x 10^12 Joules.
To assess the required energy for a rocket to accomplish the same mission, the following equation is used [ 10]:
Where the new terms are:
t = Thrusting duration
The parenthetical term is the rocket power, which is mentioned for two reasons: to show this additional form of the rocket equation and to introduce the idea of contemplating power in addition to just energy. While power implications are not explored here in detail, they constitute a fertile area for further study.
Entering the following values for the Space Shuttle System (extracted from “STS-3 Thirds Space Shuttle Mission Press Kit, March 82,” Release #82-29), the total energy for delivering the Shuttle orbiter via rockets is found to be 1.14 x 10^13 Joules.
Thrust, F = 470 x 10^3 lbs (2.1 x 10^6 Newtons) thrust/engine
Specific Impulse, Isp = 453 s
Burn Duration, t = 514 s
Solid Rocket Boosters:
Thrust, F = 2.9 x 10^6 lbs (12.9 x 10^6 Newtons) thrust/booster
Specific Impulse, Isp = 266 s
Burn Duration, t = 126 s
Orbital Maneuvering System Engines:
Thrust, F = 6 x 10^3 lbs (27 x 10^3 Newtons) thrust/engine
Specific Impulse, Isp = 313 s
Burn Duration, t = 200s
Usually this definition is used to compare energy differences between two relatively short differences in height ( r) but in our situation we are considering this potential energy in the more absolute sense. This same equation for potential energy can also be derived by calculating how much energy it would take to completely remove the object from the gravitational field, as if moving it to infinity. This is more in line with the analogy to nullify the effect of gravitational energy. This is also the same amount of energy that is required to stop an object at the levitation height ( r) if it were falling in from infinity with an initial velocity of zero.
Using this equation, it could conceivably require 62 mega-Joules to levitate 1-kg near the Earth’s surface. This is roughly twice as much as putting 1-kg into low Earth orbit. Again, these assessments are strictly for illustrative purposes rather than suggesting that such breakthroughs are achievable or if they would even take this form if achievable. Some starting point for comparisons is needed, and this is just one version.
List of Possible Assessments: As illustrated with these introductory examples, there are a number of different ways to assess the potential benefits of breakthrough physics propulsion. To continue with deeper inquiry, a variety of missions and assumptions can be addressed. The following list is just a starting point for further analyses. Those items marked in bold font are the ones already introduced in this paper.
1. Deep space travel (motion from point A to B without external forces):
2. Constant thrust.
3. Constant acceleration.
4. Optimized for minimum trip times.
ii. Relativistic Energy (cases 1-4 above repeated with relativistic corrections).
b. Space Drive motion using mechanical analogies:
2. Kinetic energy under constant acceleration.
3. Kinetic energy under constant power.
ii. Relativistic Energy (cases 1-3 above repeated with relativistic corrections).
c. Space Drive motion using geometric spacetime analyses:
2. Ascent to orbit (motion in a gravitational field with the destination being a stable orbit):
ii. Generic staged rocket ascent.
b. Space Drive ascent using mechanical analogies:
ii. Ascent under constant power.
3. Levitating in a gravitational field:
b. Space Drive levitation using mechanical analogies;
ii. Comparison to continual down thrust of a reaction mass (rocket and helicopter analogy).
iii. Comparison to normal accelerated motion in free space, where distance is traversed.
iv. By negating gravitational potential, as if moving a mass to infinity.
v. Comparing to kinetic energy associated with escape velocity.
vi. Thermodynamic approach: Seeking equations for the energy and power to keep a system in a stationary state away from its equilibrium condition, where the equilibrium condition is defined as free-fall motion in a gravitational field and the stable non-equilibrium condition is defined as levitation at a given height.
vii. Assuming a “gravity shield,” but for illustrative purposes consider it located under half of a vertical wheel to calculate the energy associated with the increasing rotation rate of the wheel.
viii. Calculating the energy of oscillation about an median hovering height, but where an energy cost is incurred for both the upward and downward excursions, and where damping losses are included.
ix. Analyze using the “impulse” treatment (force x duration, rather than force x distance).
c. Space Drive levitation in terms of geometric general relativity – inducing a null geodesic where the local freefall path is a stationary trajectory.
Selecting the Best Approaches
A normal challenge of any research project is directing limited resources to the best prospects. The hunt for incredible breakthroughs faces the additional challenge of making credible progress. Because the desired propulsion breakthroughs are presumably far from fruition and provocative, specific strategies were devised in the course of the NASA Breakthrough Propulsion Physics Project to mitigate the risks and maximize progress. [ 3] This Project employed the operating strategies described below. Other details, such as the specific selection criteria, evaluations equations, review process, and lessons learned, are presented in the cited reference.
Reliability: Although it is a common practice when advocating research to emphasize the ultimate technical benefits, this practice is not constructive on topics as visionary and provocative as breakthrough spaceflight. Instead, it is more constructive to emphasize the reliability of the information being offered. Compared to other propulsion research, new propulsion physics is at its infancy. It is expected, therefore, that any practical embodiment is years, perhaps decades away, if not impossible. Although breakthroughs, by their very definition, happen sooner than expected, no breakthrough is genuine until it has been proven to be genuine. Hence, the reliability of the information is a paramount prerequisite to the validity of any conclusions. To place the emphasis where it is needed, no research approach should be considered unless credibility is satisfactorily addressed, regardless of the magnitude of claimed benefit. Success is defined as acquiring reliableknowledge, rather than as achieving a breakthrough.
Immediacy: Another technique to shift the emphasis away from provocative situations and toward constructive practices is to focus the research on the immediate questions at hand. These immediate unknowns, issues, and curious effects can be identified by comparing established and emerging physics to the ultimate propulsion goals. The scope of any research task should ideally be set to the minimum level of effort needed to resolve an immediate “go/no-go” decision on a particular approach. This near-term focus for long-range research also makes the tasks more manageable and more affordable. Specifically, it is recommended that any proposed research be configured to reach a reliable conclusion in one to three years. Should the results be promising, a sequel can be proposed in the next solicitation cycle.
Measured: To help identify a suitable research increment and to provide managers a means to measure progress, the Scientific Method can be adapted as a readiness scale in a manner similar to how the Technology Readiness Levels are used to measure technological progress. [ 16] The readiness scale developed for the BPP Project consists of three stages that gauge the applicabilityof the work (reflecting how research can evolve from the more general, to the more specific application), and within each of these 3 stages, the 5 steps of the scientific method are repeated (from recognizing the problem, through testing the hypothesis). This equates to 15 levels of relative maturity, with the most advanced level being equivalent to Technology Readiness Level 1 (Basic principles observed and reported). An abbreviated version of these ” Applied Science Readiness Levels”at topic to the next readiness level. This is consistent with the incremental research strategy. Applied Science Readiness Levels” is presented in Table 2. After a given a research objective has been ranked relative to this scale, the next logical increment of research would be to advance that topic to the next readiness level. This is consistent with the incremental research strategy.
APPLIED SCIENCE READINESS LEVELS
General Physics – deals with general underlying physics related to the application.
SRL-1.1 Problem formulated
SRL-1.2 Data collected
SRL-1.3 Hypothesis proposed
SRL-1.4 Hypothesis tested & results reported
Critical Issues – deals with an immediate unknown, critical make-or-break issue, or curious effect relevant to the application.
SRL-2.1 Problem formulated
SRL-2.2 Data collected
SRL-2.3 Hypothesis proposed
SRL-2.4 Hypothesis tested & results reported
Desired Effect – deals directly with the effect required by the application (e.g. inducing forces or generating energy in the case of breakthrough propulsion applications)
SRL-3.1 Problem formulated
SRL-3.2 Data collected
SRL-3.3 Hypothesis proposed
SRL-3.4 Hypothesis empirically tested & results reported
(Equivalent to TRL 1: Basic principles observed and reported)
Published: The final recommendation to mitigate the risks of pursuing visionary, high-gain research is to ensure that the research findings are published, regardless of outcome. Results, pro or con, set the foundations for guiding the next research directions. Although there can be a reluctance to publish null results – where a given approach is found not to work – such dissemination will prevent other researchers from repeatedly following dead-ends.
Recent Technological Progress
Oscillation Thrusters & Gyroscopic Antigravity:
Mechanical devices are often claimed to produce net external thrust using just the motion of internal components. These devices fall into two categories, oscillation thrusters and gyroscopic devices. Their appearance of creating net thrust is attributable to misinterpretations of normal mechanical effects. The following short explanations were excerpted and edited from a NASA website about commonly submitted erroneous breakthroughs. [ 17]
Oscillation thrusters move a system of internal masses through a cycle where the motion in one direction is quicker than in the return direction. When the masses are accelerated quickly, the device has enough reaction force to overcome the friction of the floor and the device slides. When the internal masses return slowly in the other direction, the reaction forces are not sufficient to overcome the friction and the device does not move. The net effect is that the device moves in one direction across a frictional surface. In a frictionless environment the system’s components would simply oscillate around their center of mass.
A gyroscopic thruster consists of a system of gyroscopes connected to a central body. When the central body is torqued, the gyros move in a way that appears to defy gravity. Actually the motion is due to gyroscopic precession and the forces are torques around the axes of the gyros’ mounts. There is no net thrust created by the system.
To keep an open, yet rigorous, mind to the possibility that there has been some overlooked physical phenomena with such devices, it would be necessary to explicitly address all the conventional objections and pass at least a pendulum test. Any test results would have to be impartial and rigorously address all possible false-positive conclusions. There has not yet been any viable theory or experiment that reliably demonstrates that a genuine, external, net thrust can be obtained with one of these devices. If such tests are ever produced, and if a genuine new effect is found, then science will have to be revised, because it would then appear that such devices are violatingconservation of momentum.
Hooper Antigravity Coils:
Experiments were conducted to test assertions from US Patent 3,610,971, by W. J. Hooper that self-canceling electromagnetic coils can reduce the weight of objects placed underneath. No weight changes were observed within the detectability of the instrumentation. More careful examination led to the conclusion that Hooper may have misinterpreted thermal effects as his “Motional Field” effects. [ 18]
Schlicher Thrusting Antenna:
Tests of a specially terminated coax, that was claimed to create more thrust than attributable to photon radiation pressure, revealed that no such thrust was present. [ 19]
Podkletnov Gravity Shield:
A controversial claim of “gravity shielding” using rotating superconductors and radio-frequency radiation was published based on work done at Finland’s Tampere Institute.  A privately funded replication of the Podkletnov configuration “found no evidence of a gravity-like force to the limits of the apparatus sensitivity,” where the sensitivity was “50 times better than that available to Podkletnov.” 
There are many variants of the original patent where high-voltage capacitors create thrust,  many of which claim that the thrust is a new affect akin to antigravity. These go by such terms as: “Biefeld-Brown effect,” “lifters,” “electrostatic antigravity,” “electrogravitics,” and “asymmetrical capacitors.” To date, all rigorous experimental tests indicate that the observed thrust is attributable to coronal wind. [23-25] Quoting from one such finding: “… their operation is fully explained by a very simple theory that uses only electrostatic forces and the transfer of momentum by multiple collisions [with air molecules].” 
Quantum Tunneling as an FTL venue:
A prerequisite to faster-than-light travel is to prove faster-than-light information transfer. The phenomenon of quantum tunneling, where signals appear to pass through barriers at superluminal speed, is often cited as such empirical evidence. Experimental and theoretical work indicates that the information transfer rate is only apparently superluminal, with no causality violations. Although the leading edge of the signal does appear to make it through the barrier faster, the entire signal is still light-speed limited. [26, 27] This topic still serves, however, as a tool to explore this intriguing aspect of physics.
Woodward’s Transient Inertial Oscillations:
Experiments and theories published by James Woodward claim that oscillatory changes to inertia can be induced by electromagnetic means  and a patent exists on how this can be used for propulsion.  Conservation of momentum is satisfied by evoking interpretations of Mach’s principle. Independent verification experiments, using techniques less prone to spurious effects, were unable to reliably confirm or dismiss the claims.  Woodward and others continue with experiments and publications to make the effect more pronounced and to more clearly separate the claimed effects from experimental artifacts. This oscillatory inertia approach is considered unresolved.
Abraham-Minkowski Electromagnetic Momentum:
More than one approach attempts to use an unresolved question of electromagnetic momentum (Abraham-Minkowski controversy ) to suggest a new space propulsion method. [32-34]. The equations that describe electromagnetic momentum in vacuum are well established (photon radiation pressure), but there is still debate concerning momentum within dielectric media. In all of the proposed propulsion methods, the anticipated forces are relatively small (comparable to experimental noise) and critical issues remain unresolved. In particular, the conversion of anoscillatory force into a net force remains questionable and the issue of generating external forces from different internal momenta remains unproven. Even if unsuitable for propulsion, these approaches provide empirical tools for further exploring the Abraham-Minkowski controversy of electromagnetic momentum.
Inertia and Gravity Interpreted as Quantum Vacuum Effects:
Theories are entering the peer-reviewed literature that assert that gravity and inertia are side effects of the quantum vacuum. The theories are controversial and face many unresolved issues. In essence this approach asserts that inertia is related to an electromagnetic drag force against the vacuum when matter is accelerated, and that gravity is the result of asymmetric distributions of vacuum energy caused by the presence of matter. [35-38] The space propulsion implications of these theories have been raised , but experimental approaches to test these assertions are only beginning to enter the literature. 
Podkletnov Force Beam:
On an Internet physics archive it is claimed that forces can be imparted to distant objects using high-voltage electrical discharges near superconductors. Between 4×10^-4 to 23×10^-4 Joules of mechanical energy are claimed to have been imparted to an 18.5-gram pendulum located 150 meters away and behind brick walls of a separate building.  Like the prior gravity shielding claims, these experiments are difficult and costly to duplicate, and remain unsubstantiated by reliable independent sources.
“Space drive” is a general term to encompass the ambition of propulsion without propellant. To identify the unresolved issues and research paths toward creating a space drive, seven hypothetical space drives were conceived and cursorily addressed. [ 12] The two largest issues facing this ambition are to find a way for a vehicle to induce external net forces on itself, and secondly, tosatisfy conservation of momentum in the process. As discussed below, several avenues for research remain, including: (1) investigate space from the perspective of new sources of reaction mass, (2) revisit Mach’s Principle to consider coupling to surrounding mass via inertial frames, and (3) investigate the coupling between gravity, inertia, and controllable electromagnetic phenomena. These are very broad and open areas where a variety of research sequels could emerge.
Reaction mass in space: A key aspect of conservation of momentum is the reaction mass. When an automobile accelerates, its wheels push against the road using the Earth as the reaction mass. Helicopters and aircraft use the air as their reaction mass. In space, where there are no roads or air, a rocket must bring along propellant to thrust against. To contemplate space travel that circumvents the propellant limits of rockets, some other indigenous reaction mass must be found along with the means to induce net forces on the reaction mass.
Recent observations reveal a number of interesting phenomena of space. Although none are directly suitable as reactive media, they are at least indicative that space has substantive properties whose further study pertains to breakthrough spaceflight. Cosmological observations have revealed the Cosmic Microwave Background Radiation, dark matter, and dark energy,  and quantum physics has revealed zero point energy.  The Cosmic Background Radiation is low-energy microwave radiation whose composite motion is coincident with the mean reference frame of the universe.  Although too weak to be used as a reactive media, its existence and directional dependence is thought provoking in the context of space travel. Dark matter is the term used to encompass observations that there is more gravitating matter at galactic scales than can be observed. Some estimates are that more than 90% of the matter in galaxies is not directly visible. One of the key supporting empirical observations are the anomalous rotation rates of galaxies, where the galaxies appear to hold together more strongly than can be accounted for by the visible matter. From the propulsion point of view, the suitability of dark matter as a reaction mass has not yet been rigorously studied. On even larger scales, anomalous red-shifts from the most distant matter of the universe suggest that the universe is expanding at an accelerating rate. The working hypothesis for this anomaly is dubbed dark energy and it is conjectured to be an antigravity-like effect.  Again, the propulsion implications of such phenomena have not been explored. And lastly, the quantum phenomenon of zero point energy suggests that even the most empty of spaces still contain some non-zero amount of energy. This last item is discussed separately later.
Revisit Mach’s Principle: One of the theoretical approaches in dealing with momentum conservation for space drives is to reexamine Mach’s Principle. Mach’s Principle asserts that an inertial frame, specifically the property of a space to be a reference frame for acceleration, is actually created by and connected to the surrounding mass in the universe.  At least one perspective views this property as being related to the gravitational potential of the masses across the universe.  A related issue is that a literal interpretation of Mach’s Principle implies an absolute reference frame, coincident with the mean rest frame for all the matter in the universe.  From the space-propulsion point of view, this is a convenient perspective. Curiously, a known phenomenon that is coincident with this reference frame is the Cosmic Microwave Background Radiation.
These Machian perspectives imply a Euclidean view of space-time. Within general relativity, there do exist such Euclidean interpretations, which are often referred to as ” optical analogies.” In this interpretation, space is represented as an optical medium with an effective index of refraction that is a function gravitational potential. [49, 50] Although different from the more common geometricinterpretation, this interpretation has been shown to be consistent with physical observables, and transformation rules between the optical and geometric perspectives have also been published.  Conveniently, it also casts the coupling between gravity and electromagnetism in more simple terms. Little attention is typically focused on this optical analogy because it does not predict any new effects that aren’t already covered by the more common geometric perspective and because it raises unanswered issues with coordinate systems choices. Another consequence is that wormholes are indescribable in this perspective. From the propulsion point of view, however, issues of coordinate frames are of keen interest.
Coupling of Fundamental Forces: Electromagnetism, gravity and spacetime are coupled phenomena. Given our technical proficiency at manipulating electromagnetism, this coupling hints that we might be able to use electromagnetism to affect gravity. In principle this is true. In practice, at least from the perspective of general relativity, it would take an enormous amount of electromagnetic energy to produce a perceptible gravitational effect – energy levels in the regime ofE=mc^2, where m represents the induced gravitational mass effect. While general relativity pertains to large-scale couplings, quantum and particle physics pertains to the couplings on the atomic scale and smaller. One example of an unresolved small-scale question is the unknown inertial and gravitational properties of antimatter. Although presumed to be equal to their normal-matter counterparts, long-duration low-gravity experiments could resolve minor differences that have not been testable in terrestrial labs.  Such experiments might also help resolve the lingering incompatibility between general relativity and quantum mechanics. As much as these pertain to general physics, they may also have implications for propulsion physics.
Quantum Vacuum Energy Experiments:
The uncertainty principle from quantum mechanics indicates that it is impossible to achieve an absolute zero energy state. This includes the energy state of empty space.  It has been shown analytically,  and later experimentally, , that this vacuum energy can squeeze parallel plates together. This “Casimir effect” is only appreciable for very small cavity dimensions (microns). Nonetheless, it is evidence that empty space can present situations where forces exist when none were naively expected. Theoretically it might be possible to induce net forces relative to this background energy, but the forces are extremely small. [ 6]. More recent experiments have explored the physics of the quantum vacuum using MEMS technology – micro-electro-mechanical structures of machined silicon. [54, 55] Continued research on this phenomenon and through these techniques is expected.
In addition to the unanswered questions of reaction mass in space or the viability of vacuum energy for practical purposes, there are a variety of other provocative effects and theoretical questions that pertain to the search for new propulsion physics. One example from general relativity is that a propulsive effect could be induced by frame dragging from a twisting toroid of ultra dense matter, where an acceleration field is induced inside the toroid.  Although the magnitude of the induced effect is trivial compared to the energy expenditure, this serves as an analytical approach to investigate the implications of such notions. Another curiosity is the anomalous trajectories of the Pioneer 10/11, Galileo, and Ulysses spacecraft.  Once these spacecraft were farther than about 20 astronomical units from the Sun, their actual trajectories show an unexpected deceleration on the order of 10^-10 g’s.  A report sponsored by the European Space Agency (ESA) includes a proposal for a “Sputnik-5” probe to explore this anomaly.  This same ESA study further suggests checking for evidence of violations of the equivalence principle in long duration free-fall trajectories (i.e. orbits).
Faster than Light:
As a consequence of Einstein’s general relativity, the notion of warping space to circumvent the light-speed limit is an open topic in scientific literature. This approach involves altering spacetime itself rather than trying to break the light-speed limit through spacetime. Two prominent approaches are the warp drive and the wormhole. The warp drive concept involves moving a bubble of spacetime that carries a vehicle within. [ 4] A wormhole, on the other hand, is a shortcut through spacetime created by extreme spacetime warping. [5, 60] Enormous technical hurdles face these concepts. In particular, they require enormous quantities of “negative energy” (equivalent mass of planets or suns),  and evoke time-travel paradoxes (“closed-time-like curves”).  Given the magnitude of energy requirements to create perceptible effects, it is unlikely that experimental work will be forthcoming in the near future. Even though these theoretical concepts are unlikely to be engineered, they are at least useful for teaching the intricacies of general relativity. While laboratory experiments are still prohibitive, astronomical searches for related phenomena could be undertaken, such as looking for the characteristic signatures of a wormhole. 
Summary of Research Findings:
The majority of open research paths involve further study of the fundamental properties of spacetime and inertial frames, looking for candidate sources of reaction mass and the means to interact with it. As much as these are basic areas of investigation for general physics, their investigation in the context of breakthrough spaceflight introduces additional perspectives from which to contemplate these lingering unknowns. This alternative perspective might just provide the insight that would otherwise be overlooked.
The potential benefits of breakthrough propulsion cannot be calculated yet with certainty, but crude introductory assessments show that the performance gains could span from a factor of 2 to a factor of 10^150 in the amount of energy required to move an object from one point to another. The more demanding the journey, the higher the gain. For a hypothetical non-relativistic space drive, the energy scales as the square of the ∆v, while rocket energy scales exponentially for ∆v. This is a considerable difference, particularly for high ∆v missions.
Because of the profound implications of success and the fledgling nature of the research, special management methods are recommended to ensure credible progress. Lessons from the NASA Breakthroughs Propulsion Physics Project include: (1) constraining the research tasks to only address immediate unknowns, curious effects or critical issues, (2) putting more emphasis on the reliability of assertions than their implications, and (3) having reviewers judge credibility rather than feasibility.
The search for breakthrough propulsion methods is an embryonic field encompassing many differing approaches and challenges. The majority of open research paths involve further study of possible reaction masses in space, the physics of inertial frames, the properties of the quantum vacuum, and the coupling of electromagnetism, spacetime and gravity. As much as these are basic areas of investigation for general physics, their investigation in the context of breakthrough spaceflight introduces another perspective from which to contemplate these lingering unknowns. This alternative perspective might just provide an insight that would otherwise be overlooked.
Much of the research is conducted as individual discretionary efforts, scattered across various government, academic, and private organizations. In addition to the research already described, there are many more approaches emerging in the literature and at aerospace conferences. At this stage it is still too early to predict which, if any, of the approaches might lead to a breakthrough. Taken objectively, the desired breakthroughs might also remain impossible. Reciprocally, however, history has shown that breakthroughs tend to take the pessimists by surprise.
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Marc Millis is NASA’s leading expert on Breakthrough Propulsion Physics – covering such visionary goals as gravity control, space drives, and faster-than-light travel. This topic requires a challenging blend of vision and rigor to identify and chip away at the relevant unknowns. When funded (1996-2002), the Breakthrough Propulsion Physics Project assessed 8 different research approaches and documented its findings in 16 peer-reviewed journal reports. With NASA’s emphasis on returning to the Moon, this work is no longer supported.
Millis joined Cleveland’s Glenn Research Center in 1982 after earning a degree in Physics fromGeorgia Tech. His assignments evolved from engineering support into research and eventually into project management. The work spanned designing ion thrusters, electronics for rocket monitoring, rocket fuel equipment, and even a cockpit display that guides aircraft flights to create weightlessness. All the while Millis spent his discretionary time pondering how to make rocketry obsolete, which eventually led to the creation of the Breakthrough Propulsion Physics Project. This work gained wide public attention, being cited in Newsweek, Wired, Popular Science (May 2001 cover), New York Times and most recently in the books Centauri Dreams (Gilster 2004) and in I’m Working On That (Shatner & Walter 2002). This work also earned Millis a nomination for a 2004 World Technology Award. Millis recently completed a Masters of Science degree inPhysics Entrepreneurship from Case Western Reserve University (2006) and is an alumnus of the International Space University Summer Session (1998).
Millis pursues futuristic visions outside of NASA too. Leveraging the allure of science fiction beyond what can be done in government and academia, Millis founded the nonprofit Tau Zero Foundation in 2006, to accelerate progress and education toward practical interstellar flight. In 2005, Millis authored: “Making the jump to light-speed” a chapter in the National Geographic book: Star Wars – Where Science Meets Imagination. For hobbies, Millis enjoys craftsmanship; building award-winning scale models, Halloween costumes, and other mischief. With specialties in science fiction models built from scrap plastic and 1960’s slot cars, he occasionally publishes “how-to” articles and photographs. Amidst all of this, Millis enjoys time as a husband and father.