The following text is based on the report published in the Annals of the New York Academy
of Sciences , but has been formatted for HTML
presentation. Although this is published through a non-NASA
venue, the contents of this government-sponsored work are available
without copyright restrictions in the US.
Marc G. Millis
NASA John H. Glenn Research Center at Lewis Field
21000 Brookpark Rd., MS 86-2
Cleveland, OH 44135-3191
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
. 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
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
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. [
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 is
rather than using the metrics of rocketry. Discussion on the pitfalls
of using rocketry metrics for assessing breakthrough spaceflight is
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;
= incremental mass of expelled propellant
= exhaust velocity of propellant
(opposite to the motion of the rocket, hence the negative sign)
= incremental change in velocity of the rocket
= mass of the rocket (including stored propellant)
= degree of inertia modification, where a δ of 1
represents no modification, greater than 1 is an increase, and less
than 1 is a decrease.
From this starting equation, the following equation for the rocket's
change in velocity, ∆v
can be derived [ 9
∆v = change in
velocity of the rocket
= initial mass of the rocket before the expulsion of propellant
= final mass of the rocket after the expulsion of propellant
This is identical to the celebrated Tsiolkovsky equation of 1903, [ 10
] with the
exception of the presence of the term, δ
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
Rocketry Analyses for Breakthroughs:
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.
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
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:
– Both the rocket and the space
drive are assumed to be 100% efficient with their energy conversions.
– 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
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.
m = mass of the
vehicle without the propellant
required change in velocity for the mission
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
), 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.
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. [7
The comparison is made for both a flyby mission and a rendezvous
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
= "Specific Impulse" (seconds) which is
a measure of the rocket's propellant efficiency, specifically the
amount of thrust per propellant weight flow rate.
= Earth's gravitational field = 9.8
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
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.
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:
is the potential energy change associated with the altitude change, and
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
= Newton's gravitational constant
= mass of the Earth
= mass of the spacecraft
= distance from the center of the Earth
= radius of the orbit as measured from the center of the Earth
= 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
To assess the required energy for a rocket to accomplish the same
mission, the following equation is used [ 10
Where the new terms are:
= Rocket thrusting force
= 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.
Space Shuttle Main
Quantity = 3
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:
Quantity = 2
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:
Quantity = 2
Thrust, F = 6 x 10^3 lbs (27 x 10^3 Newtons) thrust/engine
Specific Impulse, Isp = 313 s
Burn Duration, t = 200s
Comparing this rocket energy value to the hypothetical space drive
energy, where the efficiency of both systems is assumed to be 100%,
indicates that the space drive is 3.58
times more energy efficient
. When compared to the benefits
of interstellar space drives, however, this gain is small. From these
cursory analyses, space drives do not appear as attractive for
launching spacecraft into low orbit as they do for high ∆v
missions that require many maneuvers. Again, such introductory
comparisons should not be taken too literally. These assessments are
provided to demonstrate that there are a variety of ways to assess the
potential benefits of propulsion breakthroughs.
Levitation is an excellent challenge to illustrate
how contemplating breakthrough propulsion is different from
contemplating rocketry. Rockets can hover, but not for very long before
they run out of propellant. For an ideal breakthrough, some form of indefinite
levitation is desirable, but there is no clear way how to represent the
energy or power to perform this feat. Since physics defines
work (energy) as the product of force acting over distance, no work is
performed if there is no change in distance. Levitation means hovering
with no change in height. Regardless, there are a variety of ways to
toy with the notion of energy and power for indefinite levitation. A
few of these approaches are listed in the next session. For now, only
one approach is illustrated, specifically the nullification of
An object in a gravitational field has the following defined value for
its gravitational potential energy:
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
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):
a. Rocketry baselines:
i. Non-relativistic Energy (velocity less
than 10% lightspeed):
Constant exhaust velocity and short thrust durations.
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
i. Non-relativistic Energy:
Simple kinetic energy differences.
2. Kinetic energy under constant
3. Kinetic energy under constant power.
ii. Relativistic Energy (cases 1-3 above
repeated with relativistic corrections).
c. Space Drive motion using geometric
i. Creating a pseudo geodesic –
reshaping spacetime to induce the preferred freefall trajectory.
ii. Warp Drive – moving a
section of spacetime. [ 4
iii. Wormhole – moving through
a shortcut in spacetime. [ 5
iv. Krasnikov tube – creating a
faster-than-light geodesic. [ 15
2. Ascent to orbit (motion in a
gravitational field with the destination being a stable orbit):
a. Rocketry ascent baselines:
Space Shuttle System data.
ii. Generic staged rocket ascent.
b. Space Drive ascent using mechanical
Simple kinetic and potential energy differences using Space Shuttle
ii. Ascent under constant power.
3. Levitating in a gravitational field:
a. Rocketry levitation baseline:
Levitation duration at the Earth's surface.
b. Space Drive levitation using
i. Normal physics definition of work,
where zero change in height equates to zero energy expenditure.
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.
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
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.
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
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
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
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 reliable
rather than as achieving a breakthrough.
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.
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
of 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
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
" 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
APPLIED SCIENCE READINESS
– deals with general underlying physics related to the
SRL-1.0 Pre-science (Unconfirmed effect
or new information connection)
SRL-1.1 Problem formulated
SRL-1.2 Data collected
SRL-1.3 Hypothesis proposed
SRL-1.4 Hypothesis tested &
– deals with an immediate unknown, critical make-or-break
issue, or curious effect relevant to the application.
SRL-2.0 Pre-science (Unconfirmed effect
or new information connection)
SRL-2.1 Problem formulated
SRL-2.2 Data collected
SRL-2.3 Hypothesis proposed
SRL-2.4 Hypothesis tested &
– deals directly with the effect required by the application
(e.g. inducing forces or generating energy in the case of breakthrough
SRL-3.0 Pre-science (Unconfirmed effect
or new information connection)
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)
accumulate progress over the long term, it is recommended to solicit a
suite of proposals every two to three years, and to let the findings of
the prior suite influence the next round of selections. This provides
an opportunity for new approaches, sequels to the positive results, and
redirections around null results. At any point, if a research task
leads to the discovery of a new propulsion or energy effect, it can be
pulled out of this process into its own advancement plan. This
strategic approach is recommended for high-gain/high-risk research,
where cycles of peer-reviewed solicitations can examine a diverse
portfolio of options, and where the decisions build on the lessons
learned from prior cycles of research.
It is far too soon, in the course of seeking spaceflight breakthroughs,
to down-select to just one or two hot topics. Instead a variety
approaches should be investigated in each review cycle. In simple
terms, this is to diversify the research portfolio. This is different
than the more common practice with advanced propulsion research where
further advancements are primarily sought on the technical approaches
already under study. Although this more common strategy can produce
advances on the chosen topics, it faces the risk of overlooking
emerging alternatives and the risk that support will wane unless the
chosen topics produce unambiguous positive results.
When inviting research on the edge of knowledge, controversial ideas
are encountered. Considering that most historic breakthroughs
originally sounded like fringe ideas, it is not surprising that many of
the proposals for breakthrough spaceflight might sound too
first, or at least unfamiliar. It is therefore difficult to sort out
the fringe ideas that may one day evolve into tomorrow’s
breakthroughs from the more numerous, erroneous ideas. During proposal
reviews, it is common to have some reviewers reflexively assume that
unfamiliar ideas will not work. To reliably
determine technical feasibility, however, is beyond the scope of a
proposal review – constituting a full research task unto
itself. Instead of expecting proposal reviewers to judge technical
feasibility, it is recommended to have reviewers judge if the task is
leading to a result that other researchers will consider as a reliable
conclusion on which to base future investigations. This includes both
the possibility of determining which approaches are nonviable as well
as which are candidates for deeper inquiry. This posture of judging
credibility rather than pre-judging feasibility is one of the ways of
being open to visionary concepts while still sustaining credibility.
When seeking advancements that can eventually lead to new technology,
there is a decided preference toward tangible observations over purely
analytical studies - all other factors being equal, such as cost,
technical maturity, etc. Experiments, being hardware, are considered
closer than theory to becoming technology. Also, experiments are
considered a more direct indicator of how nature works. Theories are
interpretations to explain observations of nature, while the empirical
nature, partially revealed within the constraints of the given
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.
The findings of over a dozen separate research tasks related to
breakthrough propulsion physics were recently published. [ 2
findings are rearranged here according to which tasks proved
non-viable, which remain unresolved, and which are candidates for
further research. Under each of these headings, the different
approaches are only briefly described, but pertinent reference
citations are offered for follow-up inquiries.
It should be stressed that even interim positive results do not imply
that a breakthrough is inevitable. Often the opportunity for sequels is
more a reflection of the embryonic state of the research. Reciprocally,
a dead-end conclusion on a given task does not imply that broader
topics are equally defunct. Both the null and positive results should
be strictly interpreted within the context of the immediate research
task. This is consistent with the operating strategy to focus on the
immediate stage of the research, and the strategy to put a higher
priority on the reliability of the information rather than on producing
It should also be stressed that these task summaries do not reflect a
comprehensive list. It is expected that new concepts will continue to
emerge in such an embryonic field and that further, more applicable
references may already be in the open literature.
& 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
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 violating conservation 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
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
controversial claim of "gravity shielding" using rotating
superconductors and radio-frequency radiation was published based on
work done at Finland's Tampere Institute. [20
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." [21
There are many variants of the original patent where high-voltage
capacitors create thrust, [22
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]." [23
Quantum Tunneling as an
A prerequisite to faster-than-light travel is to prove
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
] This topic still
serves, however, as a tool to explore this intriguing aspect of physics.
Experiments and theories published by James Woodward claim that
oscillatory changes to inertia can be induced by electromagnetic means [28
] 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. [30
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.
More than one approach attempts to use an unresolved question of
electromagnetic momentum (Abraham-Minkowski controversy [31
]) 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 an oscillatory
into a net
force remains questionable and the issue of generating external
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 [39
], but experimental
approaches to test these assertions are only beginning to enter the
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 4x10^-4 to 23x10^-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. [41
] Like the prior gravity
shielding claims, these experiments are difficult and costly to
duplicate, and remain unsubstantiated by reliable independent sources.
For Continued Research:
"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
on itself, and secondly, to satisfy 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
The Cosmic Background
is low-energy microwave radiation whose
composite motion is coincident with the mean reference frame of the
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
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
is conjectured to be an antigravity-like effect. [45
] 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. [46
] At least one perspective
views this property as being related to the gravitational potential of
the masses across the universe. [47
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 geometric
interpretation, this interpretation has been shown to be consistent
with physical observables, and transformation rules between the optical
and geometric perspectives have also been published. [50
] 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
Coupling of Fundamental
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 of E=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
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. [43
It has been shown analytically, [52
and later experimentally, [53
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
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
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. [56
] 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. [57
] 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. [58
] A report
sponsored by the European Space Agency (ESA) includes a proposal for a
"Sputnik-5" probe to explore this anomaly. [59
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
Two prominent approaches are the warp
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
] Enormous technical
hurdles face these concepts. In particular, they require enormous
quantities of "negative energy" (equivalent mass of planets or suns), [61
] and evoke time-travel
paradoxes ("closed-time-like curves"). [62
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. [63
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
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
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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