one of spacetimes most requested topics is the controversial emdrive they flooded in since eager works labs published a paper detailing their tests of an e/m drive in a hard vacuum chamber those tests were consistent with the positive result a real thrust today we inaugurate the space-time journal club by taking a very careful look at this result we've avoided the emdrive in the past because the results were inconclusive and the proposed physics was either very wrong or way too speculative we walk a fine line of science communicators talking about high peace stuff is fun and popular but excuse the public impression of what is credible and what is not sometimes bizarre new results do turn out to be real most often they turn out to be in error like operas faster than light neutrinos and the bicep2 primordial gravitational waves my guess is the emdrive results we'll end up with a similarly mundane explanation but you guys been asking for it that's why I want to cover this as a discussion of the paper what we call a journal Club in academic circles so for edition one of the spacetime journal Club we're jumping into Harold sunny white at our 2016 measurement of impulsive thrusts from a closed radiofrequency cavity in vacuum first a bit of background the structure of the emdrive is a tapered copper cylinder with two flat ends a resonant radiation field is induced inside so microwaves standing waves reflecting between the ends the device was invented by Roger Shire in 1999 and a couple of similar proposals followed most notably gwido fetters can a drive these fall under the family of microwave resonant cavity thrusters they propose that thrust a net force in one direction the fat end for the e/m Drive can be achieved by extracting momentum from the internal radiation field but with no propellants this would be pretty revolutionary because conventional rocket engines are fundamentally limited by the fact that they have to carry not just an energy source but propellant a bunch of mass to shoot out the back end to push them forward the law of conservation of momentum demands this Rajesh Iyer explains the e/m drives a legend thrust as resulting from differential radiation pressure between the two ends the best quip I've heard against this explanation is by Caltex Jim Woodward we pointed out that it's like trying to accelerate a car by getting in the driver's seat and pushing on the windshield which is to say this explanation breaks the law of conservation of momentum unless those photons actually escape the cavity then any momentum exchange between the cavity and radiation field gets redistributed again because the system is closed and if photons do escape then you've just built a photon thruster the thrust they produce is tiny and much smaller than the thrust that sharor reported when he tested his own emdrive the Eagle works result is the latest in several attempts to reproduce Troyer's measurements none of which are close to being conclusive Eagle works itself had previously tested the device in air at atmospheric pressure and found a positive thrust the main criticism of all non vacuum tests is that thermal convection in the surrounding air may have produced the observed force you're basically running a microwave oven and so things heat up but the latest test was in a vacuum chamber at one ten-billionth of sea level atmospheric pressure so no convection is possible thrust was tested with a torsion balance basically a stiff metal wire on which the device is hung with a counterbalance twisting of the WHI gives an extremely precise measurement of any force it's the same tool that henry cavendish used to first measure the teensy-tiny gravitational constant over 200 years ago the Eagle works test was performed at 3 power levels 40 60 and 80 what's the author's claim a positive thrust in the expected direction at all power levels and that direction reversed when device orientation was switched the thrusts were reported to be the same as those observed in a non vacuum the average thrust to input power observed was around 1.2 million Newton's per kilowatt that's vastly smaller than the thrusts observed by Soyuz experiments but still much much larger than for a photon thruster by the way at 1 million Newton per kilowatt it would take around a gigawatt of power to levitate a good sized human so the power output of a typical commercial nuclear power plants or the power consumption of a time-traveling DeLorean but eliminating the need for propellants may mean that measly thruster power ratio is useful for very long range space flight definitely not powerful enough for hoverboards though sorry so I have a gripe about the reporting of these thrusts the author suggests a consistent thruster power response and fit a straight line to the thrust versus power graph however it's really clear that the 60 and 80 watt powers are statistically equivalent and there's a huge scatter that straight line is misleading that said the positive displacement still looks statistically significant so does the a.m. Drive generate thrust well not until all other possible causes are ruled out the big one seems to be that thermal effects could still be a factor in particular deformation of the device or the scale due to thermal expansion a simple test would be to heat the device without a radiation field to see if that heating produces a similar false positive signal but this hasn't been done yet the authors identify several other potential sources of false positive signal and either make reasonable arguments against them or propose tests to rule them out before we can believe that thrust is real those tests need to be done any result that requires new exotic physics demands that all mundane causes be exhaustively eliminated beyond reasonable doubt so the last part of the paper talks about a connection between the am drive and pilot-wave theory this isn't something we can get into properly without first doing some quantum field theory so I'll keep it brief the paper invokes pilot wave theory as a way to justify treating the quantum vacuum as a sort of plasma with which it can exchange momentum however it's highly speculative and isn't necessarily even an obvious outcome of pilot wave theory our understanding of the quantum vacuum and standard quantum field theory doesn't allow you to push off it like you might row a boat on a lake to exchange momentum with virtual particles over a distance longer than a Planck length those particles need to become real photons will need to give up their energy producing particle-antiparticle pairs that is an understood process and it does happen but it's not what's happening here if it were those particles would also be trapped in the cavity or if they escaped they'd be a propellant and momentum would be exchanged with no more efficiency than a photon thruster the authors don't claim this by the way instead they invoke pilot wave theory to justify treating the quantum vacuum as a deformable medium in two separate papers Harold white performs computer simulations which he argues demonstrates that such a medium can reproduce CERN quantum observables like the energy levels of the hydrogen atom if the quantum vacuum is some sort of almost classical medium then they argue that the emdrive could push off it leaving a wake of degraded vacuum behind it however none of this is relevant unless the founding assumption is right that the vacuum is something very different than described by the otherwise amazingly successful quantum field theory all of that said if the observed effect really is a thrust then something is causing it the most likely explanation is that more careful experimentation will eliminate the apparent thrust the second most likely explanation is that it'll be explainable with old physics a distant third is that it's something brand-new like this quantum vacuum stuff so I look forward to works and other teams getting to the bottom of it okay that's my take on this paper this is space time journal club which means it's now open for discussion feel free to pick apart my analysis in the comments back to our regularly scheduled slightly less speculative science next week on space time everyone