Mach Effect Thrusters

The Mach Effect Thruster, or Woodward effect, proposes that when an object's internal energy is changing and it is simultaneously being accelerated, its rest mass fluctuates transiently. Drive a piezoelectric stack so that the mass fluctuation and the acceleration are correctly phased, and the claim is a net time-averaged force. Momentum is not violated on this account — it is exchanged with the distant matter of the universe through a Machian gravitational interaction, formalised through Wheeler–Feynman-style advanced waves and Hoyle–Narlikar theory. The proponents state it explicitly: “the advanced wave concept is a means to conserve momentum of the system with the universe. There is no momentum violation in this theory.”

The single most important thing to say about this device is that it is not the EmDrive. The EmDrive asserted thrust from a closed cavity with no reservoir named. The Mach effect names a reservoir, derives the effect from an existing if minority formulation of Mach's principle inside general relativity, and predicts a specific frequency and phase dependence. That makes it falsifiable in a way the EmDrive never was, and it is why it has been tested rather than dismissed. Reactionless Propulsion owns the momentum-conservation frame and the EmDrive as the worked example; this brief owns the thruster.

The framing under test — the Mach effect is a real and testable propulsion mechanism — splits into two halves with opposite answers, and the brief's job is to keep them apart. Testable: yes, unambiguously. It has been tested repeatedly, by proponents and by an independent group, on purpose-built instruments, and the tests were meaningful. Real: no evidence survives the cleanest tests, and what remains is a methodological dispute about how those tests were run rather than a settled physical question. That live dispute is this brief, not its disclaimer.

Speculative The original claim, 1996, in a peer-reviewed journal and on the originator's own apparatus. Woodward's A laboratory test of Mach's principle and strong-field relativistic gravity reports a transient mass fluctuation of “a few tens of milligrams”, a stationary weight shift of “several milligrams”, an apparatus sensitive to about a tenth of a milligram, and results described as “signals some 10 to 15 times the standard error”. Established A milligram of weight is about 10 micronewtons, so this is a claim of roughly 50 micronewtons measured at about 1 micronewton resolution.

Frontier 2012, and this is where the proponents earn something the EmDrive community never did: they built their own null test. Fearn and Woodward's Experimental Null Test of a Mach Effect Thruster reports approximately 1.5 micronewtons with unequal reaction masses — thick brass at one end, a thin aluminium cap at the other — at 32.35 kHz; and with identical brass masses on both ends, at about 42 kHz, “we have lost any linear thrust signal, and we are seeing noise”. Established A symmetry control, run by the claimants, reported against their own signal. That is real methodology and this brief credits it explicitly, because it is the single strongest thing in the proponents' favour.

Speculative 2015, the theory paper, with operating parameters and a venue problem. Fearn, Zachar, Wanser and Woodward's Theory of a Mach Effect Thruster I gives 8 lead-zirconate-titanate discs of 2 mm by 19 mm, resonance at 39.3 kHz, about 234 V and about 140 W, with claimed forces “in the single digit microNewton range up to approximately 10–20 microNewtons maximum”. Established At 10 to 20 micronewtons on 140 W, that is roughly 70 to 140 micronewtons per kilowatt. The journal is published by Scientific Research Publishing, which is not a recognised venue in mainstream physics — a fact about the citation rather than about the physics, and it is recorded in the reading list rather than used as an argument.

Handwave 2018, and the claim jumps by three orders of magnitude. Heidi Fearn of the Space Studies Institute is reported as claiming that separating the brass from the reaction mass “improved the thrust by about 65 times up to 60 millinewtons per kilowatt”, which would be competitive with the best Hall thrusters. Frontier Do the arithmetic across the claims, because nobody does. 60 millinewtons per kilowatt is about 500 times the same group's own 70 to 140 micronewtons per kilowatt from three years earlier, while the stated improvement mechanism accounts for 65. The community's own quoted specific thrusts are internally inconsistent by roughly an order of magnitude, and rose by two to three orders across three years with no independent replication in between. Whether that is an artefact of different devices or of the reporting is something this brief cannot distinguish from the sources available, and it says so.

Established Put every claim against the two reference numbers, because that is what makes the scale legible. The photon limit — the thrust a device gets for free by radiating its power away as light — is 3.34 micronewtons per kilowatt. So 70 to 140 micronewtons per kilowatt is 20 to 40 times it, and 60 millinewtons per kilowatt is about 18,000 times it. Established Meanwhile NASA's qualified 12-kilowatt Hall thruster delivers 48.8 millinewtons per kilowatt with propellant, a 4,500-hour wear test and a delivery schedule (see Electrodynamic Propulsion Concepts). The 2018 claim is precisely a claim to have matched a flight-qualified Hall thruster with no propellant.

Established The funding belongs in the brief rather than in a footnote, because it is one of the two typed requirements. Woodward held a NASA NIAC Phase II award in 2018, Mach Effect for In Space Propulsion: Interstellar Mission, with the Space Studies Institute as performing organisation; NASA's own page describes the concept as TRL 1, states the Phase II objective as testing new designs at higher frequency to increase output thrust, and describes a conceptual probe to Proxima b. Established The Space Studies Institute's Exotic Propulsion Initiative, announced 2013 and led by Woodward and Fearn, is privately funded — the institute states it raised thousands of dollars, and that Woodward donated the royalties from his own book to it. Frontier Its public page lists the drive work, the workshops and the NIAC selection, and makes no mention of replication attempts or null results. The silence is itself reportable.

Established The independent tests were run by the group with the best instruments in this field, and the sequence matters. In 2016, at the proponents' own Estes Park workshop, Tajmar reported “first tests show thrust values in the sub-micronewton range” with balance calibration, thermal drifts and power-feeding-line interactions still under investigation, on a balance reaching thrust noise down to the sub-nano-newton regime with attocube laser interferometry and 25 kg of instrument capacity — and said of his own data, “I cannot then say for sure that what I have seen is real or some noise.” Established In 2018 the first results, reported by Marc Millis, gave 0.6 micronewtons measured against 0.02 expected for the Mach-effect device, alongside about 4 micronewtons at just 2 W for the EmDrive against 2.6 expected. Millis assessed the Mach-effect results as “less pessimistic” than the EmDrive's because they showed directionality, and wrote that “it's too early to firmly dismiss the possibilities”.

Established In 2021 came the result, and its provenance has to be stated carefully. Monette, Kößling, Neunzig and Tajmar presented The SpaceDrive Project — Mach-Effect Thruster Experiments on High-Precision Balances in Vacuum at Space Propulsion 2020+1 in March 2021; TU Dresden's own research-information record notes that the conference contribution was not peer-reviewed, with the journal version appearing in Acta Astronautica 182:531 (2021). The conference abstract's conclusion, as quoted in contemporaneous coverage: “our tests reveal the presence of mechanical artifacts but no thrust”, with the apparent effect identified as a vibration artefact. Frontier This brief could not obtain the peer-reviewed paper's full text and does not cite it as read — the quoted conclusion comes from a news item quoting the abstract, and the reading list names it as such.

Established The artefact claim is stronger than a bare null because there is a simulated mechanism for it. At the 2018 Estes Park workshop, software engineer Jamie Ciomperlik presented numerical simulations showing that with the right parameters a false positive thrust could result from vibrational effects. Frontier A null says the signal was not there; a demonstrated route by which the apparatus could manufacture the signal says why it appeared, and that is a materially different evidential position.

Established And the weight of the Dresden nulls comes from who produced them. The group's entire research line is breakthrough propulsion; the balances were built specifically to find effects like this; Tajmar published a Mach-effect thruster model paper in 2017, meaning he modelled Woodward's theory seriously before testing it. Established Interest runs against the finding at every step, which is the same structural argument that makes the electrogravitics and EmDrive nulls heavy and is the strongest form evidence takes in this cluster.

Speculative Hypothesis one, and it is the brief's centre of gravity: the Mach effect is real, engineerable, and the independent nulls tested the wrong configuration. Woodward's objections to the 2018 Dresden tests were made publicly in a 71-slide presentation and are specific. Wrong frequency — the device was operated at a frequency “that could never trigger any thrust signature”. Missing transformer — the team “didn't use the mandatory stepup/isolation transformer”. Wrong voltage, and thermal damage — they ran at 75 V where Woodward asks what would have happened at “a voltage amplitude two to two and a half times the 75 volts they used”, and says that running voltage high for too long “managed to toast the initially good-working device”. Wrong settling protocol — correct operation requires frequency sweeps with specific timing, or constant-frequency operation long enough for the balance to settle, about five seconds.

Frontier Assess that dispute honestly, because it is the whole value of this brief. These are not unfalsifiable objections. Contrast the quantised-inertia defence covered in Reactionless Propulsion — that a symmetric metal box cancels the thrust — which explains why the null happened in a way that cannot be checked. Woodward's objections name a frequency, a voltage multiplier, a specific transformer and a settling time. They constitute a runnable experiment. Frontier They were also made after the null, with the drive configuration apparently not agreed in advance, which is a protocol failure with two signatures on it and is why they carry less weight than a pre-registered specification would.

Established And one boundary of the dispute is genuinely open in the record available here. The objections apply to the 2018 interim results. Whether they apply equally to the 2021 vacuum-balance campaign could not be established, because the peer-reviewed paper's full text was not obtainable. Frontier This brief therefore asserts neither that the 2021 tests answered Woodward's objections nor that they failed to. Stating the gap is the honest position and it is a small one, but it is exactly the kind of gap that gets quietly closed in the wrong direction by whichever side is writing.

Speculative Hypothesis two is nobody's stated position and is what the proponents' own numbers imply: the effect is real but the claimed thrusts are wrong by orders of magnitude. 70 to 140 micronewtons per kilowatt in 2015 and 60 millinewtons per kilowatt in 2018, from the same laboratory line, with an improvement mechanism that accounts for 65 of a factor of about 500. Frontier When a claimed effect grows between independent tests rather than shrinking, that is the opposite of what happens to a real effect being characterised. Compare the EmDrive, whose claimed figure fell by two orders of magnitude between a 2011 laboratory briefing and a 2016 paper as the measurement improved — that is at least the right direction.

Frontier Hypothesis three is the best-supported position: the apparent thrust is a vibration artefact. It rests on a demonstrated mechanism rather than on absence of signal, which is what raises it above a bare null. Frontier Hypothesis four is the mainstream default: the theory is wrong, and Woodward's mass-fluctuation term does not exist at usable magnitude even if Mach's principle holds. There is no direct test of it, and the term's magnitude has never been independently derived — which is a real gap in the sceptical case and should be recorded as one.

Speculative Hypothesis five is largely disjoint from the propulsion claim and deserves to be separated from it. Hoyle–Narlikar theory, described by the proponents as a fully Machian version of Einstein's relativity that reduces to the field equations in the smooth-fluid limit, is a respectable minority formulation. Mach's principle is not part of standard general relativity — general relativity is not fully Machian, and how far it should be has been a live question since Einstein. Frontier The theoretical base is a minority formulation, not a fringe invention, which is exactly why this device has been tested rather than ignored. Whether inertia is Machian at all belongs to Inertial Manipulation; whether a piezoelectric stack can exploit it is this brief's.

Speculative Hypothesis six is the charge this brief takes seriously and declines to endorse: that the device is unfalsifiable in practice because the operating configuration is retuned after each null. Nobody states it in the fetched sources, and the record supports it less strongly here than it supports the equivalent charge against quantised inertia, because Woodward's objections are checkable. Frontier But the 2015-to-2018 claim jump is evidence for it, and the brief states the tension rather than resolving it.

Frontier Hypotheses seven and eight are institutional, both live, and this brief declines to choose between them. One: NIAC-class funding for TRL-1 concepts is working as designed, and a decade of nulls on a cheap award is a good outcome rather than a bad one — NIAC's remit is exactly this population. Frontier Two: repeated null results should terminate funding, and the fact that they did not is a discrimination failure. Speculative Hypothesis nine, and the most likely to be true of any on this list: the Mach effect's real contribution is the artefact catalogue — vibration false positives, thermal drift, feedthrough surface tension — all found because somebody kept trying to measure micronewton forces.

Established The instrument is not the obstacle, and this is the sharpest single statement available about this subject. Dresden's balances resolve below a nano-newton. The smallest claim under dispute is 10 to 20 micronewtons; the largest implies about 8 millinewtons at the stated 140 W. The measurement capability is four orders of magnitude better than the smallest claim it is asked to test.

Frontier The obstacle is an agreed protocol and an agreed number, and both are free. Nothing has to be invented, purchased or discovered for the outstanding experiment to be run. What has to happen is that both parties fix the drive configuration in advance, and that the proponents name one specific thrust figure the test is meant to confirm or exclude. Frontier Until they do, the instrument requirement cannot even be written down, because at 60 millinewtons per kilowatt the predicted force is enormous by this field's standards and at 70 micronewtons per kilowatt it is modest.

Frontier A third bottleneck is thermal and it is the claimants' own. Woodward's account of the 2018 tests includes the device being destroyed by running voltage high for too long. A drive whose claimed operating point is near its own thermal destruction threshold is hard to test at that operating point, and any protocol has to state a duty cycle rather than a voltage alone.

Speculative And a fourth, which is about the theory rather than the hardware. The magnitude of Woodward's mass-fluctuation term has never been independently derived. Without that, a measured null cannot distinguish “the theory predicts nothing measurable” from “the device failed to implement the theory”, which is the ambiguity every result in this brief sits inside.

Established This brief records two requirements and neither of them is a brief, which is unusual and is the point. One is an instrument running a specific configuration; the other is the funding structure that keeps such a programme alive through a decade of nulls. Unlike everything else in this cluster, what is missing here is not a physical result but a specific experiment that both parties agree on the shape of.

Established The physics dependency is handed to another brief rather than restated. Whether inertia has a Machian origin at all is Inertial Manipulation's question; this brief takes the minority formulation as given and asks only whether a driven piezoelectric stack can exploit it. Frontier That division is doing real work, because the two questions have different answers: the first is respectable open physics and the second has been measured.

Established One number is imported and not argued here. The 48.8 millinewtons per kilowatt benchmark from a qualified 12-kilowatt Hall thruster belongs to Electrodynamic Propulsion Concepts; this brief quotes it because the 2018 claim is a claim to have matched it without propellant, and then moves on.

Established The outstanding experiment can be specified completely, from the dispute itself, which is why this brief is prescriptive rather than plaintive. Seven items. A pre-registered drive configuration agreed by both parties before the run: frequency, voltage amplitude, transformer, piezoelectric stack construction, reaction-mass geometry and settling protocol. Proponent-supplied, proponent-tuned hardware, delivered working — Fearn was reported in 2018 as intending to bring a working device to Dresden.

Frontier Third, voltage at the claimed operating point — about two to two and a half times the 75 V used — with a stated thermal duty cycle that does not destroy the stack. Speculative The reason voltage matters is a scaling inference and it is the weakest link in this brief's reasoning, so it is flagged as one: if the predicted effect scales steeply with drive amplitude, a test at 75 V against a claimed operating point two to two and a half times higher is a test at a small fraction of the claimed signal. The proponents' claimed 65-fold improvement from a purely mechanical change suggests strong parameter sensitivity generally, but that is suggestive rather than derived.

Established Fourth, vibration isolation with an independent accelerometer channel, because the identified artefact is vibrational and there is a simulation showing it can mimic thrust at plausible parameters. Fifth, a symmetry null in the same session — identical reaction masses, per the proponents' own 2012 protocol, which is the control they invented and should therefore be uncontroversial. Sixth, orientation reversal and a power-attenuator control, the two cheap tests that killed the EmDrive.

Frontier And seventh, resolution sufficient for the claim under test — which cannot be specified until somebody names the claim. At 60 millinewtons per kilowatt and about 140 W the predicted force is roughly 8 millinewtons, which is enormous by this field's standards and would appear six orders of magnitude above a sub-nano-newton noise floor. At 70 to 140 micronewtons per kilowatt it is 10 to 20 micronewtons. Frontier The instrument requirement therefore depends entirely on which claim is being tested, and the proponents need to name one figure before the experiment can be written down. That is the sharpest single criticism available here and it is arithmetic rather than opinion.

Handwave No engineering requirements for a vehicle can be stated, because no effect has survived independent measurement — and this brief will not write a specification for a drive whose central figure of merit varies by 500 times across three years of its own literature.

Established What can be specified is the device under test, and the proponents have specified it. Eight lead-zirconate-titanate discs of 2 mm by 19 mm, resonance at 39.3 kHz, about 234 V, about 140 W, with a reaction mass whose geometry is itself a disputed variable — unequal masses giving a signal, identical masses giving noise, and separating the brass from the reaction mass claimed to improve thrust by 65 times. Frontier A device whose performance depends this strongly on mechanical arrangement is either exquisitely sensitive to the phasing the theory requires, or exquisitely sensitive to how it vibrates. That ambiguity is the engineering statement of the whole dispute.

Established The instrumentation requirements are real, met, and worth stating. A balance with thrust noise in the sub-nano-newton regime, attocube laser interferometry, 25 kg of instrument capacity, vacuum operation, and a documented systematic budget covering thermal drift, magnetic coupling and power-feeding-line interaction. Established All of that exists and has been run against this device.

Frontier And one engineering requirement falls on the claimants rather than the testers. The device must survive its own claimed operating point long enough to be measured at it. An account in which the correct voltage destroyed the article is a statement about the hardware's readiness, not only about the test's competence.

Established The most important boundary is with Reactionless Propulsion, and it is a division of labour rather than a difference of opinion. That brief owns the momentum-conservation frame and the EmDrive as the worked example of a complete claim-to-resolution cycle. This brief needs the EmDrive only as a contrast — the Mach effect names a momentum reservoir and the EmDrive did not — and borrows its two controls, orientation reversal and a 10,000-fold power attenuator, as the model for the protocol in section 6. The EmDrive numbers are not re-run here.

Established The second boundary is with Inertial Manipulation and it is a clean one-line hand-off. Whether inertia is Machian is that brief's question, as physics. Whether a driven piezoelectric stack can exploit it is this brief's, as a device. Frontier Keeping those apart matters because the first is a respectable open question and the second has an experimental record, and collapsing them makes the device look better or the physics look worse than either deserves.

Established The third is a supplier relationship rather than a boundary. Electrodynamic Propulsion Concepts supplies the benchmark — 48.8 millinewtons per kilowatt from a qualified Hall thruster with a 4,500-hour wear test — which is what the 2018 claim would have to match, with no propellant, to matter commercially.

Established And two briefs that look adjacent are not, which is worth saying explicitly because the association is common and wrong. History of Electrogravitics covers a 1950s aviation-industry episode; the Woodward lineage is 1990s-onward academic physics, and conflating them is a category error in both directions. Frontier Electrogravitics is an electrostatic claim while this is a gravitational-inertial one — different mechanism, different literature, and a different verdict structure, since that brief's question is closed and this one's is not. If the two are mentioned together anywhere it should be to say they are unrelated.

Frontier The institutional question here is genuinely two-sided and this brief poses it rather than answering it. NIAC exists to fund concepts too early for anyone else. A TRL-1 concept whose central effect has returned null in independent testing is precisely the population NIAC was built to fund — and also precisely the population that consumes speculative funding indefinitely. Both readings are defensible on the same facts, and picking one would be a preference dressed as a finding.

Established The cleanest available discriminator belongs to a sympathiser rather than a critic. Marc Millis's Breakthrough Propulsion Physics selection rule: “reliability of assertions is more important than the implications”, with criteria scores multiplied rather than added, so low credibility cannot be offset by a large payoff. That programme's own outturn on 16 tasks was about a third not viable, a quarter with clear sequels, and the rest unresolved. Frontier The Mach effect sits in the unresolved bucket, and it has sat there for thirty years.

Frontier Millis's criteria for whom to believe are worth applying directly, because they are a practitioner's. Check whether experimenters systematically test for false positives; whether theoreticians engage conflicting viewpoints; whether they focus on make-or-break issues; and ignore both believers and pedantic pundits. Frontier By those criteria the Mach-effect community scores better than the EmDrive community — they built their own symmetry null in 2012 and published it — and worse than the independent testers, because their claimed performance rose two orders of magnitude between independent tests. That scoring is this brief's, against Millis's stated criteria.

Established The funding organisation's own public record has a gap that is itself a datum. The Space Studies Institute's initiative page lists the programmes, the workshops and the NIAC selection, and makes no mention of replication attempts or null results. Frontier A funder's page is not a literature review and nobody should expect one to be. But a decade of independent testing that does not appear at all on the page of the organisation funding the concept is a description of how a claim survives its own evidence.

Frontier And the institutional failure that both sides share is cheap to fix and nobody has fixed it. The drive configuration was not agreed before the run, so the objections arrived after the null and were unanswerable in principle. Established Pre-registration is standard in fields with far larger stakes and far less contested measurements, and its absence here is the single most consequential institutional fact in this brief.

Established The primary ethical question here is about how a null gets reported, and it cuts against the sceptical side as often as the credulous one. This brief could not obtain the peer-reviewed 2021 paper, and the widely quoted conclusion — mechanical artefacts but no thrust — reaches it through a news item quoting a conference abstract. Established Citing a journal paper for a figure that came from a conference abstract is a naming defect even when the conclusion is right, and this page names the source it actually has.

Frontier Second, the fair treatment of a claimant who behaved well. Fearn and Woodward built and published their own symmetry null in 2012, and their objections to the independent test are procedural and checkable rather than unfalsifiable. Frontier Flattening that into “proponents dispute the result” loses the distinction that is the entire content of this page, and it is the distinction that separates a testable claim from an untestable one.

Established Third, the corresponding obligation in the other direction. A claimed specific thrust that rises 500-fold in three years with no independent replication in between is the strongest available evidence against the claim, and it comes from the claimants' own numbers. Frontier A brief that credited the good methodology without stating the arithmetic would be doing a different kind of unfairness.

Speculative And fourth, a modest point about public funding. The sums here are small — a NIAC seed award and thousands of dollars of private donation — which makes the institutional question interesting rather than urgent. The cost of a decade of nulls on a cheap award is low; the cost of the discrimination question going unasked across a whole portfolio is not.

Handwave If the effect were real at the 2018 claimed figure, the consequence would be immediate and enormous: a propellantless drive matching a flight-qualified Hall thruster's specific thrust, with no tank, no throughput limit and no erosion life. Interstellar probe concepts follow from that, which is why NASA's own award description mentions Proxima b.

Established At the 2015 figure the consequence is far smaller and almost nobody says so. 70 to 140 micronewtons per kilowatt is two to three orders of magnitude below what a qualified Hall thruster delivers today. Frontier A real Mach effect at the earlier claimed magnitude would be a profound physics result and a marginal propulsion technology, and the fact that the claim's civilizational significance depends entirely on which of the proponents' own numbers is used is itself an argument for making them name one.

Frontier The durable contribution so far is the artefact catalogue and the instrument, exactly as in the neighbouring briefs. Vibration false positives with a simulated mechanism, thermal drift, power-feeding-line interaction, and current-dependent surface tension in a liquid-metal feedthrough — all found because somebody kept trying to measure micronewton forces in vacuum. Established That capability qualifies real thrusters and drag-free spacecraft, and it exists because of thirty years of chasing this.

Frontier And the subject's most transferable value is as a worked example of a dispute that could be settled and is not being. Both parties agree what the disputed configuration is. The instrument that would test it exists and is four orders of magnitude better than the smallest claim. Speculative That a question this tractable has stayed open for a decade is a fact about protocols and incentives rather than about physics, and it generalises far beyond propulsion.

These horizons track a protocol rather than a technology, because the instrument that would settle the question already exists:

• 10 yr: Frontier The decisive question is whether a pre-registered joint test is ever run — agreed frequency, voltage, transformer, stack, reaction-mass geometry and settling protocol, with proponent-supplied hardware on an independent balance. Nothing has to be invented for it to happen. Speculative If it is run and returns null at the disputed configuration, the subject closes in the way electrogravitics closed. If it is not run, expect the current state — a null, a procedural objection, and no resolution — to persist unchanged. Established Expect no independent confirmation of 60 millinewtons per kilowatt; none exists and no group is pursuing one.
• 25 yr: Frontier The institutional question is likely to be answered before the physical one: whether speculative-grant programmes adopt an explicit discrimination rule of the kind Breakthrough Propulsion Physics wrote down, and whether pre-registration reaches this corner of the field. Speculative A plausible trajectory is that the theory work continues as minority physics — Hoyle–Narlikar and fully Machian formulations are respectable and largely disjoint from the device — while the thruster stops being funded as propulsion. Handwave A working drive at this horizon requires a mass-fluctuation term at usable magnitude that nobody has independently derived.
• 50 yr: Speculative Contingent on whether Mach's principle turns out to have any operational content, which is Inertial Manipulation's question rather than this brief's, and which no experiment currently addresses. Frontier The device question, by contrast, is not a 50-year question at all. It is a one-campaign question that has been available for a decade, and stating it that way is the most useful thing this brief can do.
• 100 / 250+ yr: Handwave Beyond useful forecasting. The defensible statement is about the shape of the record: the best independent measurement found mechanical artefacts and no thrust, with a simulated mechanism for the artefact; the proponents' own claimed figure rose 500-fold in three years; and the disputed configuration has never been tested as specified. Frontier That combination makes “unresolved” an accurate terminal position rather than a hedge, and it is the only topic in this cluster where that is true.

• Depends on No depends_on edge is recorded, and the reason is the finding rather than an absence of one. Unlike everything else in this cluster, what is missing here is not a physical result but a specific instrument running a specific configuration, and both parties agree on which one. The physics question upstream — whether inertia has a Machian origin at all — belongs to Inertial Manipulation, and this brief takes the minority formulation as given in order to ask only whether a driven piezoelectric stack can exploit it. The theoretical base is Hoyle–Narlikar theory, a fully Machian formulation that reduces to Einstein's field equations in the smooth-fluid limit; it is a minority position rather than a fringe invention, which is why this device gets tested rather than dismissed. The load-bearing step, and the one flagged handwave in the chain, is the move from “inertia has a Machian origin” to “a driven transducer produces useful thrust”.
• Requires (not on this map) The instrument is not the obstacle, and saying so precisely is what this requirement is for. Dresden's balances already resolve below a nano-newton, which is four orders of magnitude better than the smallest claim under dispute. What is missing is a thrust balance running the disputed configuration, and the dispute is specific and checkable: Woodward objects that the independent tests used the wrong frequency, omitted the mandatory stepup and isolation transformer, ran at 75 V against a claimed operating point two to two and a half times higher, and used the wrong settling protocol. Those are procedural objections that constitute a runnable experiment — unlike the post-hoc shielding defence offered for quantised inertia in Reactionless Propulsion, which explains a null in a way nobody can check. They were made after the null, with the configuration apparently not agreed in advance, which is a protocol failure with two signatures on it. And the instrument requirement cannot even be written down until the proponents name one number: at 60 millinewtons per kilowatt on about 140 W the predicted force is roughly 8 millinewtons, and at 70 to 140 micronewtons per kilowatt it is 10 to 20 micronewtons — figures from the same laboratory line three years apart, differing by about 500 times with no independent replication in between. The second requirement is the funding structure that keeps such a programme alive: a concept NASA's own page rates at TRL 1, held on a NIAC Phase II award with a privately funded institute behind it, sustained through a decade in which independent testing returned null. Whether that is the system working as designed or failing to discriminate is a live question this brief poses and declines to settle.
• Enables Nothing measurable, and no typed enabling edge is claimed. If the effect were real at the 2018 claimed figure of 60 millinewtons per kilowatt it would match a flight-qualified Hall thruster with no propellant, which is what NASA's own award description has in mind when it describes a conceptual probe to Proxima b. At the proponents' 2015 figure of 70 to 140 micronewtons per kilowatt it would be a profound physics result and a marginal propulsion technology — two to three orders of magnitude below what Electrodynamic Propulsion Concepts already delivers. That the enabling case depends on which of the claimants' own numbers is used is itself an argument for the requirement below.
• Adjacent Precision force metrology, vibration isolation and vacuum engineering supply every measurement in this brief; piezoelectric transducer design supplies the device. Within this map: Reactionless Propulsion owns the momentum frame and the EmDrive contrast and lends this brief its two controls; Inertial Manipulation owns Mach's principle as physics; Electrodynamic Propulsion Concepts supplies the commercial benchmark. Two briefs that are commonly associated with this one are not adjacent to it: Electrogravitics is an electrostatic claim with a closed verdict, and History of Electrogravitics is a 1950s industry episode; conflating either with a 1990s-onward academic physics line is a category error in both directions.

Established “The Mach effect thruster is the EmDrive.” It is not, and this is the misconception that most distorts the subject. The EmDrive asserted thrust from a closed cavity with no reservoir named. The Mach effect names a reservoir — the distant matter of the universe, through a Machian gravitational interaction — derives from a minority formulation of general relativity, and predicts a specific frequency and phase dependence. Frontier That is why it has been tested rather than dismissed, and why its verdict structure is open where the EmDrive's is closed.

Established “The Mach effect thruster has been demonstrated.” The best independent measurement found “mechanical artifacts but no thrust”, with the apparent effect identified as a vibration artefact. Frontier And the artefact claim is stronger than a bare null, because a simulation shows that with the right parameters a false positive thrust can result from vibrational effects — a mechanism, not merely an absence.

Established “The Mach effect thruster has been definitively refuted.” It has not, and overclaiming here would cost this page its credibility on everything above. The proponents' objections to the 2018 tests are specific and procedural — frequency, transformer, voltage, settling time — and were not answered in any source this brief could obtain. Frontier Whether they apply equally to the 2021 vacuum-balance campaign could not be established, because the peer-reviewed paper's full text was not obtainable, and this brief asserts neither direction.

Frontier “Proponents always object after a null, so their objections are worthless.” Not equivalent, and the difference is the content of this page. McCulloch's defence of quantised inertia — that a symmetric metal box cancels the thrust — explains the null in a way that cannot be checked. Woodward's objections name a frequency, a transformer, a voltage multiplier and a settling time, and constitute a runnable experiment. Frontier They still carry less weight than a pre-registered specification would, because they arrived after the result; but treating the two as the same move is wrong.

Established “The claimed performance figures are consistent.” They differ by roughly 500 times across three years from the same laboratory line: 70 to 140 micronewtons per kilowatt in the 2015 theory paper's own numbers, 60 millinewtons per kilowatt attributed to Fearn in 2018 — with the stated improvement mechanism accounting for 65 of that factor. Frontier When a claimed effect grows between independent tests rather than shrinking, that is the opposite of what happens to a real effect being characterised, and the EmDrive, whose figure fell two orders of magnitude as measurement improved, at least moved the right way.

Established “60 millinewtons per kilowatt has been measured.” No independent measurement of it exists in any source this brief could obtain. Established For scale: that figure is about 18,000 times the photon limit of 3.34 micronewtons per kilowatt, and it is a claim to have matched NASA's qualified 12-kilowatt Hall thruster at 48.8 millinewtons per kilowatt — with no propellant, and without a wear test.

Established “NIAC funding is validation.” NASA's own page rates the concept TRL 1 and describes the Phase II objective as testing new designs at higher frequency to increase output thrust. Frontier NIAC exists to fund concepts too early for anyone else, so an award is evidence that a concept is early rather than evidence that it works — and whether a decade of nulls should have ended it is a live institutional question with two defensible answers.

Frontier “The outstanding experiment is blocked on instrumentation.” It is not. Dresden's balances resolve below a nano-newton, four orders of magnitude better than the smallest disputed claim. Frontier It is blocked on an agreed protocol and an agreed target number, both of which are free, and neither of which anyone has produced in a decade.

Established “This is the same subject as electrogravitics or the 1950s antigravity story.” It is not, in either direction. Electrogravitics is an electrostatic claim whose verdict is closed; the 1950s episode is aviation-industry history owned by History of Electrogravitics; and the Woodward lineage is 1990s-onward academic physics with a peer-reviewed origin paper and a minority-but-respectable theoretical base. Frontier Conflating them is a category error in both directions, and it flatters one subject while unfairly damaging the other.

Established And the framing itself, which comes apart into two halves with opposite answers. Testable is comprehensively vindicated: this is the most thoroughly tested speculative propulsion claim in the cluster, examined by an independent group with better instruments than the proponents, with the proponents' own symmetry null adopted as a control. Frontier Real has no surviving support — and yet the disputed configuration has never been run as specified, on an instrument that already exists. That is why this is the one topic in the cluster where “unresolved” is an accurate terminal position rather than a hedge, and why the requirement recorded in section 13 is a real gap rather than a polite fiction.