The transactional interpretation of quantum mechanics (TIQM) takes the wave function of the standard quantum formalism,
and its complex conjugate, to be retarded (forward in time) and
advanced (backward in time) waves that form a quantum interaction as a Wheeler–Feynman handshake or transaction. It was first proposed in 1986 by John G. Cramer,
who argues that it helps in developing intuition for quantum processes.
He also suggests that it avoids the philosophical problems with the Copenhagen interpretation and the role of the observer, and also resolves various quantum paradoxes. TIQM formed a minor plot point in his science fiction novel Einstein's Bridge.
More recently, he has also argued TIQM to be consistent with the Afshar experiment, while claiming that the Copenhagen interpretation and the many-worlds interpretation are not.
The existence of both advanced and retarded waves as admissible solutions to Maxwell's equations
was explored in the Wheeler–Feynman absorber theory. Cramer revived
their idea of two waves for his transactional interpretation of quantum
theory. While the ordinary Schrödinger equation does not admit advanced solutions, its relativistic version does, and these advanced solutions are the ones used by TIQM.
In TIQM, the source emits a usual (retarded) wave forward in time, but it also emits an advanced wave
backward in time; furthermore, the receiver, who is later in time, also
emits an advanced wave backward in time and a retarded wave forward in
time. A quantum event occurs when a "handshake" exchange of advanced and
retarded waves triggers the formation of a transaction in which energy,
momentum, angular momentum, etc. are transferred. The quantum
mechanism behind transaction formation has been demonstrated explicitly
for the case of a photon transfer between atoms in Sect. 5.4 of Carver Mead's book Collective Electrodynamics. In this interpretation, the collapse of the wavefunction
does not happen at any specific point in time, but is "atemporal" and
occurs along the whole transaction, and the emission/absorption process
is time-symmetric. The waves are seen as physically real, rather than a
mere mathematical device to record the observer's knowledge as in some
other interpretations of quantum mechanics. Philosopher and writer Ruth Kastner
argues that the waves exist as possibilities outside of physical
spacetime and that therefore it is necessary to accept such
possibilities as part of reality.
TIQM is explicitly non-local and, as a consequence, logically consistent with counterfactual definiteness (CFD), the minimum realist assumption. As such it incorporates the non-locality demonstrated by the Bell test experiments and eliminates the observer-dependent reality that has been criticized as part of the Copenhagen interpretation. Cramer states that the key advances over Everett's Relative State Interpretation are that the transactional interpretation has a physical collapse and is time-symmetric. Cramer also states that the TI is consistent with but not dependent upon the notion of an Einsteinian block universe. Kastner claims that by considering the product of the advanced and retarded wavefunctions, the Born rule can be explained ontologically.
The transactional interpretation is superficially similar to the two-state vector formalism (TSVF) which has its origin in work by Yakir Aharonov, Peter Bergmann and Joel Lebowitz of 1964. However, it has important differences—the TSVF is lacking the
confirmation and therefore cannot provide a physical referent for the
Born Rule (as TI does). Kastner has criticized some other time-symmetric
interpretations, including TSVF, as making ontologically inconsistent
claims.
Kastner has developed a new Relativistic Transactional Interpretation (RTI) also called Possibilist Transactional Interpretation
(PTI) in which space-time itself emerges by a way of transactions. It
has been argued that this relativistic transactional interpretation can
provide the quantum dynamics for the causal sets program.
In his book, The Quantum Handshake, Cramer has added a
hierarchy to the description of pseudo-time to deal with Maudlin's
objection and has pointed out that some of Maudlin's arguments are based
on the inappropriate application of Heisenberg's knowledge
interpretation to the transactional description.
Transactional Interpretation faces criticisms. The following is partial list and some replies:
"TI does not generate new predictions / is not testable / has not been tested."
TI is an exact interpretation of QM and so its predictions must be the same as QM. Like the many-worlds interpretation
(MWI), TI is a "pure" interpretation in that it does not add anything
ad hoc but provides a physical referent for a part of the formalism that
has lacked one (the advanced states implicitly appearing in the Born rule).
Thus the demand often placed on TI for new predictions or testability
is a mistaken one that misconstrues the project of interpretation as one
of theory modification.
"It is not made clear where in spacetime a transaction occurs."
One clear account is given in Cramer (1986), which pictures a
transaction as a four-vector standing wave whose endpoints are the
emission and absorption events.
"Maudlin (1996, 2002) has demonstrated that TI is inconsistent."
Maudlin's probability criticism confused the transactional
interpretation with Heisenberg's knowledge interpretation. However, he
raised a valid point concerning causally connected possible outcomes,
which led Cramer to add hierarchy to the pseudo-time description of
transaction formation.Kastner has extended TI to the relativistic domain, and in light of this
expansion of the interpretation, it can be shown that the Maudlin
Challenge cannot even be mounted, and is therefore nullified; there is
no need for the 'hierarchy' proposal of Cramer.
Maudlin has also claimed that all the dynamics of TI is deterministic
and therefore there can be no 'collapse'. But this appears to disregard
the response of absorbers, which is the whole innovation of the model.
Specifically, the linearity of the Schrödinger evolution is broken by
the response of absorbers; this directly sets up the non-unitary
measurement transition, without any need for ad hoc modifications to the
theory. The non-unitarity is discussed, for example in Chapter 3 of
Kastner's book The Transactional Interpretation of Quantum Mechanics: The Reality of Possibility (CUP, 2012).
"It is not clear how the transactional interpretation handles the quantum mechanics of more than one particle."
This issue is addressed in Cramer's 1986 paper, in which he gives many
examples of the application of TIQM to multi-particle quantum systems.
However, if the question is about the existence of multi-particle wave
functions in normal 3D space, Cramer's 2015 book goes into some detail
in justifying multi-particle wave functions in 3D space. A criticism of Cramer's 2015 account of dealing with multi-particle
quantum systems is found in Kastner 2016, "An Overview of the
Transactional Interpretation and its Evolution into the 21st Century,
Philosophy Compass (2016). It observes in particular that the account in Cramer 2015 is
necessarily anti-realist about the multi-particle states: if they are
only part of a 'map', then they are not real, and in this form TI
becomes an instrumentalist interpretation, contrary to its original
spirit. Thus the so-called "retreat" to Hilbert space (criticized also
below in the lengthy discussion of note)
can instead be seen as a needed expansion of the ontology, rather than a
retreat to anti-realism/instrumentalism about the multi-particle
states. The vague statement (under)
that "Offer waves are somewhat ephemeral three-dimensional space
objects" indicates the lack of clear definition of the ontology when one
attempts to keep everything in 3+1 spacetime.
Reactionless drives have been compared to perpetual motion machines. The October 1920 issue of Popular Science covered the topic.
Reactionless drive refers to hypothetical and unproven forms of spacecraft propulsion that would generate thrust without expelling propellant or other reaction mass. A propellantless drive is not necessarily reactionless if it functions as an open system interacting with external fields, but a reactionless drive is generally conceived as a self-contained or closed-system device, which is why such claims are commonly treated as conflicting with Newton's third law and the conservation of momentum. Reactionless-drive claims are therefore often compared to perpetual motion machines or other impossible-energy schemes if their reported performance is taken at face value.
The expression has been in English-language newspaper use since
at least 1963 and later became a recurring public label for alleged
no-propellant engines. Concepts described this way have included the Dean drive, electrogravitics and lifters, gyroscopic and oscillation devices, microwave-cavity thrusters such as the EmDrive and Cannae drive, the helical engine, Mach-effect or MEGA drives, quantum drive claims, and the quantum vacuum thruster.
No reactionless drive has become an accepted propulsion
technology, and reported effects from devices such as a thrusting
antenna and the EmDrive
have later been described as null results, below photon-thrust limits,
or experimental artifacts rather than evidence of new physics. Speculative discussion continues in propulsion and space drive literature, including proposals involving Mach-principle coupling, inertial frame manipulation, distant-matter interaction, Heim-theory propulsion, and zero-point field vacuum-inertia models.
History and terminology
In English-language popular writing, a reactionless drive is
generally described as a hypothetical propulsion concept that would
generate thrust without expelling propellant or reaction mass.These explanations usually define the term by contrast with ordinary
rockets, which move by throwing exhaust out the back, whereas a
reactionless drive would supposedly produce forward motion without that
reaction mass exchange. The term has often been used for electrically powered or internally
driven concepts said to convert stored energy directly into thrust,
including microwave-cavity devices, inertial schemes, and other
self-contained engine proposals. In this usage, the core claim is not merely low-propellant propulsion
but propulsion without expelled mass, which is why these articles
repeatedly tie the idea to Newton's third law and the conservation of momentum.
Several of these sources also describe reactionless-drive claims
as bordering on perpetual-motion or free-energy arguments if the
reported thrust and power figures were accepted at face value. Some writers have treated reactionless drives as a long-running science fiction trope as much as a technical claim, tying their appeal to the rocket equation and to the desire for a space engine that avoids the mass penalties of conventional rocketry. Winchell Chung, creator of the website Atomic Rockets that records the history of rocketry in science fiction, has described reactionless drives as hypothetical spacecraft drives
that would move without propellant and, in a closed system, are
impossible because they would violate conservation of momentum, and
would radically alter the strategic logic of spaceflight by allowing
even small spacecraft to function as relativistic weapons. Taken together, these sources use reactionless drive
as a broad public-facing label for hypothetical self-contained
propulsion claims, especially those said to generate thrust without
propellant, rather than as the name of one single device or one settled
academic field.
The expression family was in newspaper use by at least 1963, when
a California newspaper quoted inventor Allen Fisher referring to "My
reactionless space drive" in describing a proposed propulsion device. A newspaper profile of Harry E. Holden in 1967 stated that he "holds a
U.S. patent for his invention of a reactionless drive mechanism used in
space vehicles." In 1980, a United Press International
report on inventor Richard Foster described his proposed "space drive"
as a "reactionless drive" in explaining his self-contained propulsion
concept. By 1981, the term had also appeared in newspaper book coverage. An Iowa newspaper roundup described Joel Dickinson's The Death of Rocketry
as presenting "a scientific explanation of the reactionless drive," and
quoted the book's claim that "The reactionless drive would ... enable
us to attain almost unlimited speeds in outer space." Newspaper feature coverage through 1990 was using reactionless drive in straightforward descriptive prose: a Santa Maria Times
article on inventor Bob Cook stated that "The reactionless drive
utilizes circular motion to create a centrifugal force and, most
importantly, focus it in one direction."
Various other related technologies were proposed through the 20th
century. In 1992, Rex L. Schlicher patented a "nonlinear electromagnetic
propulsion system" described as creating a unidirectional propulsive
force without particulate reaction mass. NASA Glenn later tested Schlicher's "thrusting antenna", which had been
claimed to produce thrust exceeding photon radiation pressure, and
reported no correlated motion, concluding that any thrust was far below
practically useful levels.
In the 21st century, the term by 2008 was already appearing in coverage of Roger Shawyer's EmDrive, which Universe Today
described as a "reactionless propulsion system" and placed in a lineage
of earlier reactionless-drive ideas dating back to the 1950s. The same wave of coverage also pulled in related microwave-cavity claims, including the Cannae drive, as part of a broader family of alleged no-propellant thrusters. During the mid-2010s, skeptical reporting on EmDrive tests helped make reactionless drive
a recurring public label for these claims, even when the thrust signals
were being interpreted as experimental error or artifact rather than
evidence of new physics. The same term was being applied to David Burns's proposed helical engine, with explanatory pieces defining reactionless drives in general before using Burns's concept as the latest example. In the 2020s, popular coverage continued to use the label for other
speculative concepts, including Jim Woodward's Mach-effect or MEGA drive
and IVO's quantum drive. The term has also appeared by analogy in discussion of warp drive
research, where weak-field or limited warp effects were compared to
reactionless-drive proposals even while remaining conceptually distinct
from them.
NASA engineers Marc G. Millis and Nicholas E. Thomas published Responding to Mechanical Antigravity, a paper based on the agency's Breakthrough Propulsion Physics project and its experience with large numbers of unsolicited breakthrough-propulsion proposals. The report summarized that these devices can give "the appearance that a
net thrust is being produced without expelling a reaction mass or
having a direct driving connection", which is why they can appear to be
breakthrough propulsion concepts. They wrote that such submissions arrived at a rate of about one per workday.
Types of related systems
A variety of drives directly characterized as, or functionally presented as reactionless drives, are listed here.
The Dean drive
was a claimed reactionless device built by Norman L. Dean, who said
that his working models functioned as a "reactionless thruster". The Dean drive received extensive promotion from John W. Campbell in Astounding Science Fiction beginning in 1960.
Dean held several private demonstrations but never revealed the exact
design of the models nor allowed independent analysis of them. Campbell published photographs of the device operating on a bathroom scale, and the June 1960 cover of Astounding featured a painting of a United States submarine near Mars supposedly propelled by a Dean drive.
In 1984, physicist Amit Goswami wrote that the Dean drive had become so
embedded in genre consciousness that "it is now customary in SF (science fiction) circles to refer to a reactionless drive as a Dean drive".The Visual Encyclopedia of Science Fiction catalogued the Dean drive as a distinct propulsion concept for space travel in the genre. Dean's claims of reactionless thrust generation were later argued to be mistaken, with the apparent "thrust" likely caused by friction between the device and the surface on which it rested rather than any effect that would operate in free space.
Electrogravitics began with Thomas Townsend Brown's 1920s experiments on a Coolidge tube, which Army Research Laboratory authors Thomas B. Bahder and Christian Fazi treated as the origin of the later Biefeld-Brown effect.
In their account, Brown believed the energized tube produced thrust and
sought British patent protection, receiving Patent 300,311 in 1928 for
producing force or motion. Bahder and Fazi then traced a longer patent arc through Brown's U.S. Electrokinetic Apparatus patent in 1960, his Electrokinetic Transducer patent in 1962, and a further Electrokinetic Apparatus patent in 1965.
Their appendix also noted parallel and later claimants, including A. H.
Bahnson's electrical-thrust patents in 1960 and 1966 and two NASA
asymmetric-capacitor patents in 2002, showing that the concept persisted
as a continuing propulsion claim rather than a single episode in
Brown's career.
Reporting on ARL's own work, they said they had verified a net force on
asymmetric capacitors of several shapes while also stressing that the
physical basis of the effect remained unresolved.
Various historians placed Brown's devices in the realm of Asymmetrical Capacitor Thrusters, distinguishing rotating ACTs from vertical "lifters", comparing them with Alexander de Seversky's 1960s Ionocraft and Robert Talley's later vacuum tests. By 2003 The Guardian described skeletal high-voltage lifters as the closest thing to Brown's original vision, while Wired portrayed them as the work of grassroots antigravity subcultures trading designs online. Canning's report, published the next year, treated these machines not
as forgotten curiosities but as active propulsion problem with a long
history of interest and no proven mechanism.
Millis's 2005 NASA review grouped "Biefeld-Brown effect," "lifters,"
"electrostatic antigravity," "electrogravitics," and "asymmetrical
capacitors" as labels for the same family of high-voltage thrust claims.
Gyroscopic Inertial Thruster (GIT)
Gyroscopic thrust claims have typically centered on forcing a spinning gyroscope to precess in a way supposed to convert internal torques into a net linear force. In simpler terms, the claim was that the motion of a precessing gyroscope could be turned into one-way thrust. A 2006 NASA review of "mechanical antigravity" proposals treated Eric Laithwaite's gyroscope demonstrations as a famous example of this class of claim. Official material from the Royal Institution
describes Laithwaite's 1974 lectures as controversial because he argued
that the behavior of gyroscopes violated the law of conservation of
energy.
Laithwaite patented, with William Dawson, a propulsion system in
which gyroscopes mounted for remote-axis precession were claimed to move
a vehicle through alternating precession-dominated and
translation-dominated portions of motion. Later gyroscopic variants continued to make similar claims. Sandy
Kidd's 1991 U.S. patent for a "gyroscopic apparatus" described a pair of
opposed rotatable discs driven in different directions and periodically
forced toward one another to generate a pulsatile force, with the
claimed effect of producing upward thrust. In NASA's analysis, however, the apparent lift in such devices is not a
true upward thrust but a torque acting through the pivots and stops of
the mechanism.
Millis and Thomas concluded that gyroscopic devices of this kind
misinterpret torques as linear thrust, and distinguished them from
reaction wheels, which can change a spacecraft's attitude but cannot
change the position of the system's center of mass.
In 2019, NASA Marshall Space Flight Center engineer David M. Burns proposed the helical engine,
a closed-loop ion-beam propulsion concept intended for long-term
satellite station-keeping without refueling and for interstellar travel. Burns described it as an engine in which ions confined in a helical beam guide are accelerated to relativistic speeds and then decelerated to create unequal momentum exchange at the top and bottom of the engine.
The proposed architecture used two concentric helical beam-guide cores,
with ions traveling upward in an outer accelerating core and returning
downward through an inner decelerating core.
Popular coverage described the idea as a
particle-accelerator-based space drive that sought to avoid conventional
propellant expenditure by exploiting relativistic changes in momentum
inside a helical path. Burns stressed his concept required testing to prove it could produce
thrust within real engineering constraints, and wrote that his
Relativistic Momentum Transfer Model needed validation through tests of
helical beam-guide shapes in a synchrotron. Secondary coverage treated the proposal cautiously. Newsweek, citing Burns and comments reported to New Scientist, described the basic concept as unproven. Universe Today
described the paper as an outline rather than peer-reviewed work and
treated the helical engine as a reactionless drive proposal akin to the EmDrive.
James F. Woodward's
Mach-effect propulsion program grew out of his long interest in using
Mach's principle to pursue propellantless propulsion. By 1995, according
to a later Wired
profile, Woodward's ideas about Mach effects had coalesced into a full
theory, and he turned to building a thruster based on stacks of
piezoelectric disks that he believed could exploit tiny transient mass
fluctuations. Earlier conference literature framed the work in
propellantless-propulsion terms: Thomas L. Mahood's 1999 AIP conference
paper was titled Propellantless propulsion: Recent experimental results exploiting transient mass modification, while a 2006 AIP paper by Paul March and Andrew Palfreyman described
the Woodward effect as a transient mass fluctuation in energy-storing
ions and reported experimental verification efforts at 2 to 4 MHz.
By 2020, the device was being referred to publicly as the Mach Effect Gravitational Assist, or MEGA, drive. Wired
reported that Woodward and Hal Fearn secured NIAC funding in 2017 and
used it to develop improved thrusters and the conceptual SSI Lambda
interstellar probe. The same report described the device as an
electricity-powered propulsion system designed to operate without
propellant, while also noting mixed test results, the small scale of the
reported forces, and the need for independent replication before the
effect could be accepted.
Microwave cavity thrusters comprise a class of reactionless drive claims. In 2008, Universe Today described Roger Shawyer's EmDrive as a "reactionless propulsion system" that supposedly generated thrust by converting electrical energy via microwaves. Later coverage treated Guido Fetta's Cannae drive as a related device, describing both as closed microwave systems with no exhaust that purported to generate thrust. The subject drew wider attention after NASA Eagleworks tested
radio-frequency cavity thrusters and later published a paper reporting
small thrust measurements under vacuum conditions.
Coverage presented those results as extraordinary because, if
valid, they would imply propulsion without expelled reaction mass and
conflict with conservation of momentum. By 2018, however, some independent testing reported as pointing to
ordinary experimental artifacts rather than new propulsion physics. National Geographic
reported that tests by Martin Tajmar's group suggested the apparent
thrust was due to electromagnetic interaction rather than the drive
itself. Tajmar and colleagues later published an open-access study reporting no
thrust across a wide frequency band and concluding that any anomalous
thrust was below the photon-thrust limit, ruling out earlier reported
values by at least two orders of magnitude.
Oscillation thrusters
Oscillation thrusters are mechanical devices claimed to create net
thrust through cyclic motion of internal masses. NASA engineer Marc G.
Millis and University of Miami
researcher Nicholas E. Thomas described this family as "oscillation
thrusters", also referred to as sticktion drives, internal drives, or
slip-stick drives, and identified the 1959 Dean drive as one of its best-known examples. They wrote that, despite many variations, such devices generally rely
on an asymmetric cycle in which internal masses move more quickly in one
direction than the other, causing the whole apparatus to surge across
the ground and give the appearance of thrust without expelled reaction
mass.
The idea persisted in patent literature for decades, from Dean's 1959
"System for Converting Rotary Motion into Unidirectional Motion" to
Brandson R. Thornson's 1986 "Apparatus for Developing a Propulsion
Force" and Richard E. Foster Sr.'s 1997 "Inertial Propulsion Plus/Device
and Engine".Campbell's September 1960 "Report on the Dean Drive" presented
Wellesley Engineering as having built a duplicate Dean model and several
modified set-ups, all reported to have produced thrust.
The concept also acquired a broader magazine afterlife in Astounding and Analog: Campbell's June 1960 "The Space Drive Problem" treated the Dean drive as part of the search for non-rocket space propulsion, Analog
put William O. Davis's "The Fourth Law of Motion" on its May 1962 cover
as a science-fact breakthrough, and G. Harry Stine was still revisiting
the "controversial Dean Drive" in a June 1976 retrospective.
Foster's patent was especially explicit that later variants could
require "external force assist" from friction wheels, air blast, jets,
rockets, or force derived from the pathway to prevent the craft from
returning to its prior position during the return stroke.
Millis and Thomas concluded that oscillation thrusters are not
self-contained propulsion devices but misinterpretations of differential
friction, with the ground serving as the reaction mass, and later
analytical work on Dean-drive mechanics likewise treated the system as
open once ground forces are induced and any unidirectional motion as
limited and friction-dependent rather than sustained self-contained
propulsion.
Quantum drives
A number of named no-propellant drive claims, including devices
presented as quantum drives or related reactionless engines, have been
proposed. Notable "quantum drive" claims have centered on IVO Ltd.'s
Quantum Drive, which the company introduced in 2022 as a claimed "pure
electric thruster" using zero fuel and which the University of Plymouth linked to Mike McCulloch's theory of quantized inertia. Independent coverage in 2023 described the device as a controversial or
"impossible" propulsion claim that purported to generate thrust without
propellant and framed the planned orbital test as a decisive moment for
the concept. The proposed flight test was tied to IVO's Barry-1 CubeSat
satellite mission, which multiple outlets described as an attempt to
determine whether the Quantum Drive could produce measurable thrust in
orbit under operating conditions.
In early 2024, however, Futurism
reported that contact had been lost with the spacecraft before the
drive could be tested, leaving the concept without a completed in-space
demonstration. Coverage of the Quantum Drive also consistently presented it as a
disputed reactionless or no-propellant drive claim rather than an
accepted propulsion technology. Universe Today noted that many physicists regarded the underlying theory as fringe, while Popular Mechanics, Forbes, and Futurism
all stressed that the device was being promoted as something that would
defy ordinary expectations about propellant-based spaceflight or
Newtonian mechanics.
Quantum vacuum thruster
The Q-thruster, or Quantum Vacuum Plasma Thruster, was a proposed propulsion concept associated with Harold G. White's Eagleworks Laboratories program (also called the Advanced Propulsion Physics Laboratory) at NASA's Johnson Space Center.
In a 2011 Eagleworks presentation, White and his coauthors said the
laboratory would commission its torsion pendulum with an existing
Quantum Vacuum Plasma Thruster and described earlier QVPT work as
suggesting very high specific impulse and specific force, with
speculative applications ranging from human Mars missions to one-year
Neptune transits at higher power levels.
In a 2013 NASA brief, White described Q-thrusters as a low-TRL form of
electric propulsion that would "push off of the quantum vacuum" using
magnetohydrodynamics, treating the vacuum as a sea of virtual particles,
and argued that recent model development and test data suggested
performance competitive for in-space propulsion.
NASA project summaries in 2014 for the Q-Thruster Breadboard
Campaign said three FY13 test campaigns had produced measurable thrust,
raising the concept from TRL 2 to early TRL 3, and described the
technology as a mission-enabling form of electric propulsion with about
seven times the thrust-to-power ratio of Hall thrusters and a target of
0.4 N/kW at maturity.
B. Kent Joosten and White in a 2015 IEEE mission-analysis paper
described the Q-thruster as a system that uses quantum vacuum
fluctuations as its "propellant" source, eliminating the need for
conventional on-board propellant, and modeled round-trip Mars missions,
rapid Jupiter and Saturn transfers, and interstellar performance under
those assumptions.
In 2016, H. Fearn and James F. Woodward treated White's
quantum-vacuum-plasma interpretation as part of the broader
breakthrough-propulsion literature, but argued that the proposal led to
incorrect results and noted that they had not found the underlying
Q-thruster physics written up in any detailed peer-reviewed paper.
In propulsion literature, "space drive" has been used as a broad term
for hypothetical self-contained propulsion in which a vehicle would
move by interacting with its surrounding space rather than by expelling
propellant. Marc G. Millis described it as a generic term encompassing
attempts to induce motion through vehicle-space interactions, with the
motivating goal of eliminating the need for propellant.
Earlier work by Millis treated the idea as a way to frame the
unresolved momentum-conservation and inertial-reference problems that
any propellantless drive would have to solve.
Mach-principle coupling
One speculative route within space-drive theory has been to revisit
Mach's principle, the idea that inertial frames arise from the
distribution of surrounding matter. In 1996, Millis wrote that a
propulsion-useful formalism of Mach's principle would need to show how
reaction forces could be transmitted to surrounding matter, so that
"pushing against that frame with a space drive is actually pushing
against the distant surrounding matter". In a 2005 review he again described reexamining Mach's principle as one
theoretical approach to momentum conservation for space drives, noting
that such Machian perspectives treated inertial frames as connected to
the surrounding mass in the universe.
Inertial frame manipulation
A related but somewhat narrower line of thought has been to ask
whether propulsion might be obtained by altering the properties of
inertial frames themselves. In the same 1996 paper, Millis suggested
that if asymmetries could somehow be created in the spacetime properties
that give rise to inertial frames, they might produce net inertial
forces.
He returned to that possibility in 2017 in a paper devoted to inertial
frames, treating variable inertial-frame strength as a speculative
"what-if" framework for space-drive thought experiments rather than as
an established theory.
Distant-matter interaction
These Machian and inertial-frame ideas overlapped in proposals that a
hypothetical drive might somehow exchange momentum with distant matter
rather than with onboard reaction mass. Millis's 1996 discussion
explicitly framed the problem in those terms, while his 2017 paper
recast older direct-interaction versions of Mach's principle as part of a
broader attempt to understand whether surrounding matter might endow
space with inertial-frame properties that a drive could interact with.
Heim-theory propulsion
Another speculative route in breakthrough-propulsion literature drew on Burkhard Heim's Heim theory,
a higher-dimensional field theory that Walter Dröscher and Jochem
Häuser said implied two additional interactions beyond the known
fundamental forces. In their account, one of these, the "graviphoton
force", would permit a distinct "gravitophoton field propulsion" scheme
that could accelerate a material body without propellant.
Later propulsion overviews likewise listed Heim's quantized space-time
ideas among speculative alternatives being considered alongside
vacuum-energy and Machian approaches.
Vacuum-inertia models
Another speculative route in space-drive theory linked inertia, and
in some versions gravitation, to interactions with the electromagnetic
zero-point field. In The Challenge to Create the Space Drive,
Millis treated the electromagnetic fluctuations of the vacuum, or ZPF,
as a promising candidate medium for hypothetical space-drive research,
noting that discovering any way to react asymmetrically with the ZPF
would likely create a space drive.Bernard Haisch
and Alfonso Rueda's NASA workshop paper likewise described the
zero-point field as the basis of inertia and gravitation and treated the
concept as relevant to radically new propulsion schemes.
Millis cautioned, however, that such theories had not been developed in
the context of propulsion and did not directly show how inertia or
gravity might be manipulated for thrust.
Selected claims, tests, and proposals
The following table summarizes selected reactionless-drive systems
and related proposals discussed in this article, including claimed
demonstrations, laboratory tests, and later assessments.
Bahder & Fazi (ARL, 2003) verified net force on asymmetric capacitors but physical basis unresolved.Millis (2005) grouped Biefeld-Brown, lifters, and electrogravitics as one family of high-voltage thrust claims.
Private demonstrations on bathroom scale; no independent analysis permitted
1959
Norman L. Dean (US Patent 2886976)
Unverified
Promoted by John W. Campbell in Astounding beginning 1960.Goswami (1985): "it is now customary in SF circles to refer to a reactionless drive as a Dean drive." Apparent thrust later attributed to friction.
Royal Institution describes Laithwaite's lectures as controversial. Millis & Thomas (2006): apparent lift is torque through pivots and stops, not true thrust.
NASA Eagleworks reported small thrust under vacuum. Tajmar et al. (2022): no thrust across wide frequency band; any
anomalous signal below photon-thrust limit by at least two orders of
magnitude.
White (2013): proposed to "push off of the quantum vacuum" via MHD. Fearn & Woodward (2016) critiqued Q-thruster physics; noted no detailed peer-reviewed paper found.
University of Plymouth linked device to McCulloch's quantized inertia theory. Contact lost with spacecraft before drive could be tested.
Mach-principle coupling
Theoretical
Speculative framework: reaction forces transmitted to distant matter
–
–
Theoretical
Millis (1996): propulsion-useful Mach formalism would need to show how reaction forces transmit to surrounding matter.
Inertial frame manipulation
Theoretical
Speculative: asymmetries in spacetime inertial-frame properties might produce net forces
–
–
Theoretical
Millis (2017): variable inertial-frame strength treated as "what-if" framework for thought experiments, not established theory.
Heim-theory propulsion
Theoretical
Higher-dimensional field theory implying "graviphoton force" for propellantless acceleration
–
–
Theoretical
Dröscher & Häuser (2005): two additional interactions beyond known forces, one permitting propellantless acceleration. Listed among speculative alternatives in later propulsion overviews.
Vacuum-inertia models (ZPF)
Theoretical
Speculative: inertia linked to electromagnetic zero-point field; asymmetric ZPF reaction as space drive
–
–
Theoretical
Millis (1996): ZPF as promising candidate medium; not yet developed in propulsion context. Haisch & Rueda (1999): ZPF as basis of inertia and gravitation.
Normally, genetic chimerism is not visible on casual inspection;
however, it has been detected in the course of proving parentage. More practically, in agronomy,
"chimera" indicates a plant or portion of a plant whose tissues are
made up of two or more types of cells with different genetic makeup; it
can derive from a bud mutation or, more rarely, at the grafting
point, from the concrescence of cells of the two bionts; in this case
it is commonly referred to as a "graft hybrid", although it is not a
hybrid in the genetic sense of "hybrid".
In contrast, an individual where each cell contains genetic
material from two organisms of different breeds, varieties, species or
genera is called a hybrid.
Another way that chimerism can occur in animals is by organ transplantation, giving one individual tissues that developed from a different genome. For example, transplantation of bone marrow often determines the recipient's ensuing blood type.
Classifications
Natural chimerism
Some level of chimerism occurs naturally in the wild in many animal
species, and in some cases may be a required (obligate) part of their
life cycle.
Sponges
Chimerism has been found in some species of marine sponges. Four distinct genotypes have been found in a single individual, and
there is potential for even greater genetic heterogeneity. Each genotype
functions independently in terms of reproduction, but the different
intra-organism genotypes behave as a single large individual in terms of
ecological responses like growth.
It has been shown that male yellow crazy ants
are obligate chimeras, the first known such case. In this species, the
queens have arisen from fertilized eggs with a genotype of RR
(Reproductive × Reproductive), the sterile female workers show a RW
arrangement (Reproductive × Worker), and the males instead of being haploid,
as is usually the case for ants, also display a RW genotype, but for
them the egg R and the sperm W do not fuse so they develop as a chimera
with some cells carrying an R and others carrying a W genome.
Artificial chimerism
Chimeric trait distribution by generation
Artificial chimerism refers to examples of chimerism that are produced by humans, either for research or commercial purposes.
Tetragametic chimerism
African violets exhibiting chimerism
Tetragametic chimerism is a form of congenital chimerism. This
condition occurs through fertilizing two separate ova by two sperm,
followed by aggregation of the two at the blastocyst
or zygote stages. This results in the development of an organism with
intermingled cell lines. Put another way, the chimera is formed from the
merging of two nonidentical twins. As such, they can be male, female, or intersex.
The tetragametic state has important implications for organ or stem cell transplantation. Chimeras typically have immunologic tolerance to both cell lines.
Microchimerism is the presence of a small number of cells that are
genetically distinct from those of the host individual. Most people are
born with a few cells genetically identical to their mothers' and the
proportion of these cells goes down in healthy individuals as they get
older. People who retain higher numbers of cells genetically identical
to their mother's have been observed to have higher rates of some
autoimmune diseases, presumably because the immune system is responsible
for destroying these cells and a common immune defect prevents it from
doing so and also causes autoimmune problems.
The higher rates of autoimmune diseases due to the presence of
maternally derived cells is why in a 2010 study of a 40-year-old man
with scleroderma-like disease (an autoimmune rheumatic disease), the
female cells detected in his blood stream via FISH (fluorescence in situ
hybridization) were thought to be maternally derived. However, his form
of microchimerism was found to be due to a vanished twin, and it is
unknown whether microchimerism from a vanished twin might predispose
individuals to autoimmune diseases as well. Mothers often also have a few cells genetically identical to those of
their children, and some people also have some cells genetically
identical to those of their siblings (maternal siblings only, since
these cells are passed to them because their mother retained them).[citation needed]
Germline chimerism
Germline chimerism occurs when the germ cells (for example, sperm and egg cells) of an organism are not genetically identical to its own. It has been recently discovered that marmosets
can carry the reproductive cells of their (fraternal) twin siblings due
to placental fusion during development. (Marmosets almost always give
birth to fraternal twins.)
Types
Animals
As the organism develops, it can come to possess organs that have different sets of chromosomes. For example, the chimera may have a liver composed of cells with one set of chromosomes and have a kidney
composed of cells with a second set of chromosomes. This has occurred
in humans, and at one time was thought to be extremely rare although
more recent evidence suggests that this is not the case.
This is particularly true for the marmoset. Recent research shows most marmosets are chimeras, sharing DNA with their fraternal twins. 95% of marmoset fraternal twins trade blood through chorionic fusions, making them hematopoietic chimeras.
In the budgerigar, due to the many existing plumage colour variations,
tetragametic chimeras can be very conspicuous, as the resulting bird
will have an obvious split between two colour types – often divided
bilaterally down the centre. These individuals are known as half-sider budgerigars.
An animal chimera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic.
Innate chimeras are formed from at least four parent cells (two
fertilised eggs or early embryos fused together). Each population of
cells keeps its own character and the resulting organism is a mixture of
tissues. Cases of human chimeras have been documented.
Some consider mosaicism to be a form of chimerism, while others consider them to be distinct. Mosaicism involves a mutation
of the genetic material in a cell, giving rise to a subset of cells
that are different from the rest. Natural chimerism is the fusion of
more than one fertilized zygote in the early stages of prenatal development. It is much rarer than mosaicism.
In artificial chimerism, an individual has one cell lineage
that was inherited genetically at the time of the formation of the
human embryo and the other that was introduced through a procedure,
including organ transplantation or blood transfusion. Specific types of transplants that could induce this condition include
bone marrow transplants and organ transplants, as the recipient's body
essentially works to permanently incorporate the new blood stem cells
into it.
Boklage argues that many human 'mosaic' cell lines will be "found to be chimeric if properly tested".
In contrast, a human where each cell contains human genetic material as well as that from another species would be a human–animal hybrid.
While German dermatologistAlfred Blaschko described Blaschko's lines in 1901, the genetic science took until the 1930s to approach a vocabulary for the phenomenon. The term genetic chimera has been used at least since the 1944 article of Belgovskii.
In nonidentical twins, innate chimerism occurs by means of blood vessel anastomoses. The likelihood of offspring being a chimera is increased if it is created via in vitro fertilisation. Chimeras can often breed, but the fertility and type of offspring
depend on which cell line gave rise to the ovaries or testes; varying
degrees of intersex differences may result if one set of cells is genetically female and another genetically male.
On 22 January 2019, the National Society of Genetic Counselors released an article Chimerism Explained: How One Person Can Unknowingly Have Two Sets of DNA,
where they state, "where a twin pregnancy evolves into one child, is
currently believed to be one of the rarer forms. However, we know that
20 to 30% of singleton pregnancies were originally a twin or a multiple
pregnancy".
Most human chimeras will go through life without realizing they
are chimeras. The difference in phenotypes may be subtle (e.g., having a
hitchhiker's thumb
and a straight thumb, eyes of slightly different colors, differential
hair growth on opposite sides of the body, etc.) or completely
undetectable. Chimeras may also show, under a certain spectrum of UV
light, distinctive marks on the back resembling that of arrow points
pointing downward from the shoulders down to the lower back; this is one
expression of pigment unevenness called Blaschko's lines.
Another case was that of Karen Keegan,
who was also suspected (initially) of not being her children's
biological mother, after DNA tests on her adult sons for a kidney
transplant she needed seemed to show that she was not their mother.
Plants
The green cells in the centres of the leaves of this Pelargonium
plant have formed from the epithelium cell layer, which has normal
chlorophyll. That cell layer does not extend all the way to the edges of
the leaves, which therefore show the chlorophyll-deficient cells of
other developmental layers. This is a periclinal chimera.
Structure
The distinction between sectorial, mericlinal and periclinal plant chimeras is widely used.Periclinal chimeras involve a genetic difference that persists in the descendant cells of a particular meristem layer. This type of chimera is more stable than mericlinal or sectoral mutations that affect only later generations of cells.
These are produced by grafting genetically different parents, different cultivars or different species (which may belong to different genera). The tissues may be partially fused together following grafting to form a single growing organism that preserves both types of tissue in a single shoot. Just as the constituent species are likely to differ in a wide range of
features, so the behavior of their periclinal chimeras is like to be
highly variable. The first such known chimera was probably the Bizzarria, which is a fusion of the Florentine citron and the sour orange. Well-known examples of a graft-chimera are Laburnocytisus 'Adamii', caused by a fusion of a Laburnum and a broom,
and "Family" trees, where multiple varieties of apple or pear are
grafted onto the same tree. Many fruit trees are cultivated by grafting
the body of a sapling onto a rootstock.
Chromosomal chimeras
These are chimeras in which the layers differ in their chromosome constitution. Occasionally, chimeras arise from loss or gain of individual chromosomes or chromosome fragments owing to misdivision. More commonly cytochimeras have simple multiple of the normal
chromosome complement in the changed layer. There are various effects on
cell size and growth characteristics.
Nuclear gene-differential chimeras
These chimeras arise by spontaneous or induced mutation of a nuclear
gene to a dominant or recessive allele. As a rule, one character is
affected at a time in the leaf, flower, fruit, or other parts.
Plastid gene-differential chimeras
These chimeras arise by spontaneous or induced mutation of a plastid
gene, followed by the sorting-out of two kinds of plastid during
vegetative growth. Alternatively, after selfing or nucleic acid thermodynamics,
plastids may sort-out from a mixed egg or mixed zygote respectively.
This type of chimera is recognized at the time of origin by the
sorting-out pattern in the leaves. After sorting-out is complete,
periclinal chimeras are distinguished from similar looking nuclear
gene-differential chimeras by their non-mendelian inheritance. The majority of variegated-leaf chimeras are of this kind.
All plastid gene- and some nuclear gene-differential chimeras
affect the color of the plasmids within the leaves, and these are
grouped together as chlorophyll chimeras, or preferably as variegated leaf chimeras. For most variegation, the mutation involved is the loss of the chloroplasts in the mutated tissue, so that part of the plant tissue has no green pigment and no photosynthetic
ability. This mutated tissue is unable to survive on its own, but it is
kept alive by its partnership with normal photosynthetic tissue.
Sometimes chimeras are also found with layers differing in respect of
both their nuclear and their plastid genes.
Origins
There are multiple reasons to explain the occurrence of plant chimera during the plant recovery stage:
The process of shoot organogenesis starts from the multicellular origin.
The endogenous tolerance leads to the ineffectiveness of the weak selective agents.
A self-protection mechanism (cross protection). Transformed cells serve as guards to protect the untransformed ones.
The observable characteristic of transgenic cells may be a transient
expression of the marker gene. Or it may due to the presence of
agrobacterium cells.
Detection
Untransformed cells should be easy to detect and remove to avoid
chimeras. This is because it is important to maintain the stable ability
of the transgenic plants across different generations. Reporter genes
such as GUS and Green Fluorescent Protein (GFP) are used in combination with plant selective markers (herbicide,
antibody etc.). However, GUS expression depends on the plant development
stage and GFP may be influenced by the green tissue autofluorescence. Quantitative PCR could be an alternative method for chimera detection.
Boiling Springs Lake, California, is where the first natural chimeric virus was found in 2012.
In 2012, the first example of a naturally occurring RNA-DNA hybrid virus was unexpectedly discovered during a metagenomic study of the acidic extreme environment of Boiling Springs Lake in Lassen Volcanic National Park, California. The virus was named BSL-RDHV (Boiling Springs Lake RNA DNA Hybrid Virus). Its genome is related to a DNA circovirus, which usually infects birds and pigs, and a RNA tombusvirus,
which infect plants. The study surprised scientists, because DNA and
RNA viruses vary and the way the chimera came together was not
understood.
Other viral chimeras have also been found, and the group is known as the CHIV viruses ("chimeric viruses").
Research
The first known primate chimeras are the rhesus monkey twins, Roku
and Hex, each having six genomes. They were created by mixing cells from
totipotent
four-cell morulas; although the cells never fused, they worked together
to form organs. It was discovered that one of these primates, Roku, was
a sexual chimera; as four percent of Roku's blood cells contained two x
chromosomes.
A major milestone in chimera experimentation occurred in 1984 when a chimeric sheep–goat was produced by combining embryos from a goat and a sheep, and survived to adulthood.
To research the developmental biology of the bird embryo,
researchers produced artificial quail-chick chimeras in 1987. By using
transplantation and ablation
in the chick embryo stage, the neural tube and the neural crest cells
of the chick were ablated, and replaced with the same parts from a
quail. Once hatched, the quail feathers were visibly apparent around the wing
area, whereas the rest of the chick's body was made of its own chicken
cells.
In August 2003, researchers at the Shanghai Second Medical University in China reported that they had successfully fused human skin cells and rabbit
ova to create the first human chimeric embryos. The embryos were
allowed to develop for several days in a laboratory setting, and then
destroyed to harvest the resulting stem cells. In 2007, scientists at the University of Nevada School of Medicine created a sheep whose blood contained 15% human cells and 85% sheep cells.
In 2023 a study reported the first chimeric monkey using
embryonic stem cell lines, it was the only live birth from 12
pregnancies resulting from 40 implanted embryos of the crab-eating macaque,
an average of 67% and a highest of 92% of the cells across the 26
tested tissues were descendants of the donor stem cells against 0.1–4.5%
from previous experiments on chimeric monkeys.
Chimeric mice are important animals in biological research, as they
allow for the investigation of a variety of biological questions in an
animal that has two distinct genetic pools within it. These include
insights into problems such as the tissue specific requirements of a
gene, cell lineage, and cell potential.
The general methods for creating chimeric mice can be summarized
either by injection or aggregation of embryonic cells from different
origins. The first chimeric mouse was made by Beatrice Mintz in the 1960s through the aggregation of eight-cell-stage embryos. Injection on the other hand was pioneered by Richard Gardner and Ralph
Brinster who injected cells into blastocysts to create chimeric mice
with germ lines fully derived from injected embryonic stem cells (ES cells). Chimeras can be derived from mouse embryos that have not yet implanted
in the uterus as well as from implanted embryos. ES cells from the inner
cell mass of an implanted blastocyst can contribute to all cell
lineages of a mouse including the germ line. ES cells are a useful tool
in chimeras because genes can be mutated in them through the use of homologous recombination, thus allowing gene targeting. Since this discovery occurred in 1988, ES cells have become a key tool in the generation of specific chimeric mice.
Underlying biology
The ability to make mouse chimeras comes from an understanding of
early mouse development. Between the stages of fertilization of the egg
and the implantation of a blastocyst into the uterus, different parts of
the mouse embryo retain the ability to give rise to a variety of cell
lineages. Once the embryo has reached the blastocyst stage, it is
composed of several parts, mainly the trophectoderm, the inner cell mass, and the primitive endoderm.
Each of these parts of the blastocyst gives rise to different parts of
the embryo; the inner cell mass gives rise to the embryo proper, while
the trophectoderm and primitive endoderm give rise to extra embryonic
structures that support growth of the embryo. Two- to eight-cell-stage embryos are competent for making chimeras,
since at these stages of development, the cells in the embryos are not
yet committed to give rise to any particular cell lineage, and could
give rise to the inner cell mass or the trophectoderm. In the case where
two diploid eight-cell-stage embryos are used to make a chimera,
chimerism can be later found in the epiblast, primitive endoderm, and trophectoderm of the mouse blastocyst.
It is possible to dissect the embryo at other stages so as to
accordingly give rise to one lineage of cells from an embryo selectively
and not the other. For example, subsets of blastomeres can be used to
give rise to chimera with specified cell lineage from one embryo. The
Inner Cell Mass of a diploid blastocyst, for example, can be used to
make a chimera with another blastocyst of eight-cell diploid embryo; the
cells taken from the inner cell mass will give rise to the primitive
endoderm and to the epiblast in the chimera mouse.
From this knowledge, ES cell
contributions to chimeras have been developed. ES cells can be used in
combination with eight-cell-and two-cell-stage embryos to make chimeras
and exclusively give rise to the embryo proper. Embryos that are to be
used in chimeras can be further genetically altered to specifically
contribute to only one part of chimera. An example is the chimera built
off of ES cells and tetraploid embryos, which are artificially made by
electrofusion of two two-cell diploid embryos. The tetraploid embryo
will exclusively give rise to the trophectoderm and primitive endoderm
in the chimera.
Methods of production
There are a variety of combinations that can give rise to a
successful chimera mouse and – according to the goal of the
experiment – an appropriate cell and embryo combination can be picked;
they are generally but not limited to diploid embryo and ES cells,
diploid embryo and diploid embryo, ES cell and tetraploid embryo,
diploid embryo and tetraploid embryo, ES cells and ES cells. The
combination of embryonic stem cell and diploid embryo is a common
technique used for the making of chimeric mice, since gene targeting can
be done in the embryonic stem cell. These kinds of chimeras can be made
through either aggregation of stem cells and the diploid embryo or
injection of the stem cells into the diploid embryo. If embryonic stem
cells are to be used for gene targeting to make a chimera, the following
procedure is common: a construct for homologous recombination for the
gene targeted will be introduced into cultured mouse embryonic stem
cells from the donor mouse, by way of electroporation; cells positive
for the recombination event will have antibiotic resistance, provided by
the insertion cassette used in the gene targeting; and be able to be
positively selected for. ES cells with the correct targeted gene are then injected into a
diploid host mouse blastocyst. Then, these injected blastocysts are
implanted into a pseudo pregnant female surrogate mouse, which will
bring the embryos to term and give birth to a mouse whose germline is
derived from the donor mouse's ES cells. This same procedure can be achieved through aggregation of ES cells and
diploid embryos, diploid embryos are cultured in aggregation plates in
wells where single embryos can fit, to these wells ES cells are added
the aggregates are cultured until a single embryo is formed and has
progressed to the blastocyst stage, and can then be transferred to the
surrogate mouse.
The US and Western Europe have strict codes of ethics and regulations
in place that expressly forbid certain subsets of experimentation using
human cells, though there is a vast difference in the regulatory
framework. Through the creation of human chimeras comes the question: where does
society now draw the line of humanity? This question poses serious legal
and moral issues, along with creating controversy. Chimpanzees, for
example, are not offered any legal standing, and are euthanised
if they pose a threat to humans. If a chimpanzee is genetically altered
to be more similar to a human, it may blur the ethical line between
animal and human. Legal debate would be the next step in the process to
determine whether certain chimeras should be granted legal rights. Along with issues regarding the rights of chimeras, individuals have
expressed concern about whether or not creating human chimeras
diminishes the "dignity" of being human.
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