I’ve just had a look here and I think we can calculate the force of a normal breath. The normal transpiration volume is 0.5L which has a mass of approx 4.15 *10^-4 kg, and happens at a rate of 1-2 m/s. This gives us an acceleration of approx 0.5 - 1 m/s. Apply F=ma so that’s a force of approx 0.2 - 0.4 millinewtons (or 200-400 micronewtons).
So, if my maths is correct, the force of a normal breath is much less than that of an ion thruster, but on the same order of magnitude of the Emdrive.
Note, however, that the results for EM drive are magnitudes above the expected result for photon momentum alone. The imaginary ‘torch drive’ which uses photons to boost a rocket up to near the speed of light would also be a fantastically low thrust system.
The error, assuming there is one, probably has something to do with the fact that one end of this apparatus gets hot, and the other doesn’t.
Well, in fairness, that depends on just how many photons you’re using. Science fiction authors often postulate (as is their prerogative) photon drives capable of a sustained acceleration of 1 g or more.
I was working under the thought that a dollar bill has a mass of about a gram. Gravity pulls down at about 10N/kg, so is pulling down on the dollar bill at about 10mN, and if I’m lying on my back with a dollar bill over my mouth my normal breathing blows it off, so the force of my breath must be at least that much.
One of the assumptions of general relativity is global conservation of momentum. Note that this is just an assumption, and one we cannot (likely will never be able to) verify. It would be possible, by some extensions of a rather vaguely defined interpretation of Mach’s principle, to transfer momentum from the local frame to the rest of the mass of the universe in aggregate creating an apparently reactionless thrust. However, this has just as large of a problem as a more direct violation of conservation of local momentum, to wit, how can you causally connect a local frame to the rest of the universe? Any way you look at this, if the phenomenon were real it would be some kind of fundamental violation of the underlying assumptions of general relativity.
Cubesats (which are a standard “U” envelope and deployment specification primarily developed by Cal Poly San Luis Obispo) are generally classified as nanosatellites (1 to 10 kg) and because they are generally deployed as secondary payloads at some point during the main mission into low orbit, generally have lifetimes of less than a year, although some of the larger sats and the Planet Labs “flock of doves” deployed from the ISS will have mission durations on the order of a few years before the reenter and burn up in the atmosphere. Larger small satellites (10 kg to 100 kg, including 6U and 12U Cubesats as well as other smallsat forms) may be residual for tens of years. There is a “gentleman’s agreement” that all satellites in LEO have to be retired (e.g. deorbited) by 25 years after their end of operational life, but very few spacecraft currently have provisions for this, and some don’t even both to perform cursory analysis to demonstrate this. As there are proposals to put some larger smallsats into higher orbits and even for use in lunar and interplanetary missions, the issue becomes more significant. As noted, the persistance of such a low mass spacecraft is heavily dependant upon behavior of the thermosphere which is largely governed by the solar cycle.
Putting this device into orbit to attempt to measure thrust would significantly complicate the interpretation insofar as there would now be no fixed reference from which to measure thrust. There are many possible perturbances in space; not only the aforementioned variations in drag from the thermosphere but also magnetic perturbances, variations in the local gravitational field, outgassing and CTE effects from the extremes of temperature, et cetera, all of which could contribute to real or apparent thrust on a similar order to or gerater than the device itself, notwithstanding the necessity for providing sufficient power to operate the system. In spaceflight anomaly investigations involving potential undefined sources of thrust it is generally a lot of guesswork and statistical (‘Monte Carlo’) simulation to try to fit a hypothetical source of thrust to sensed acceleration or measured deviation, and often with frustratingly inconclusive results. Given how completely a workable device on this principle would require a retooling of physics as we know it, a falsifiable hypothesis with a sufficiently detailed model to predict behavior is absolutely necessary to verify this as being genuine rather than the result of some more prosaic but undetermined source of thrust. Students of the history of supposed discoveries such as cold fusion or polywater would correctly caution taking experimental results as conclusive evidence of a new principle of physics without a corresponding model.
Stranger
One wonders how many photons would have to spew out the back of something like that.
Metric buttload comes to mind first.
According to Noether’s theorem conservation laws are associated with symmetries and the symmetry related to conservation of momentum is (spatial) translational symmetry. Therefore global conservation of momentum implies global spatial translational symmetry (GSTS). However general relativity allows spacetimes without GSTS.
Now an actual example of a spacetime without GSTS would be Schwarzschild spacetime which would describe the vacuum outside a star for example, so in fact movement without external forces is possible if GR is correct and this has been demonstrated theoretically in Schwarzschild spacetime using “swimmers” (bodies which deform themselves cyclically). Of course the em drive is almost certainly not an example of a “spacetime swimmer”.
Couldn’t one be empirical about it? Build one in LEO, point it at the moon, switch it on and wait? A negative result might be controversial - not measured correctly or whatever - but if it plows into the moon in a week without any form of fuel we’d at least know there’s something going on that we don’t understand.
Because putting one in LEO (along with a reactor powerful enough to power it) is not cheap, and no one wants to do it with such a low probability of success.
Just because people are saying how it would break physics if it works, doesn’t mean that they aren’t being empirical right now. What do you think they’re doing, powering it up, attached to instruments?
(And I think enough thrust to get to the moon in a week would be easily noticeable in the lab. The Apollo missions took days to get there, and “I don’t know if the Saturn V is producing thrust, maybe someone is breathing on it instead” said no one ever.)
Is this to say that such a drive might (conceivably) work in an asymmetric environment (like nearish a star) but not in interstellar space? Might it require curved spacetime? If so, isn’t any local curvature at the earth’s surface due more to the earth than the sun?
Our 1U lasted 9 months, although since the solar panels did not deploy properly, it had a higher ballistic coefficient than it otherwise might have. It was deployed from the ISS. It might have lasted a full year if we had ballasted it to hit the mass limit.
Our second one was part of a 6U and projected to last ~2 years, although it was lost in the Super Strypi launch failure.
I would add another necessary condition: that the effect is small enough to be in the same range as other sources of error. If the EmDrive produced a newton per watt of thrust and the effect scaled up to kilowatt levels, there would be no need to have a falsifiable hypothesis in order to trust the results. We could build and use them before scientists had come up with anything remotely plausible. The problem of course is that the thrust levels aren’t high at all, and the effect is dominated by all kinds of local influences.
Likewise, if the cold fusion experiments had generated kilowatts of thermal energy for weeks on end, there could be no question if the effect was from something else. The range of possible errors couldn’t add up to the observed effect. Of course it might not have been “cold fusion” at all, instead being some entirely unrelated phenomenon, but nevertheless it would demonstrate that something is going on.
One thing that all pseudoscience seems to have in common is that the effects are very small and the researchers never seem to make any true progress in distinguishing the signal from the noise. Lots of real science starts with an observation that’s difficult to distinguish from randomness, but eventually the scientists manage to tease apart the signal into something they have high confidence in.
The SMART-1 spacecraft used an ion thruster to transfer to lunar orbit, and took well over a year to get there. Its ion thruster was 68 mN, which is a couple orders of magnitude above that of the EmDrive.
Stick in space and be completely unable to control for solar wind, thermal thrust across multiple degrees of freedom (which is what they’re fighting on earth), sputtering due to ionizing particles, hit by stray fleck of paint etc. etc. etc.
Figure it out down here and then test it in space if there’s a pressing need. If you need to put a 1MW reactor in space to get the same thrust a typical ion drive achieves you’re not really making a great improvement for the current time.
For comparison, the Pioneer spacecraft had an unintended thrust of about 0.15 micronewtons. It took a long time to explain it, but it turned out that it was due to non-uniform heat dissipation in the spacecraft. That’s within an order of magnitude of the effects we’re looking for here.
And that was a spacecraft with the lowest possible power it could operate at and still transmit back to Earth. The power supply and control system for this thruster would generate much more heat, and in fact, thermal expansion effects may be one of the significant factors which could generate a bias that could look like thrust.
Stranger
Here’s another interesting update. A paper is speculating that the EmDrive is all fine and dandy with newtown’s third law, because the propellant is out of phase photons that cancel each other out (so can’t be detected by normal methods).
interesting?
This shit was debunked years ago as nonsense woo. Why are we still talking about it?
Yup, has anyone shown an effect in a convincing experiment that has been replicated independently? All the experiments that I read about involved the detection of incredibly tiny forces with several possible confounding effects. As with homeopathy, it’s utterly pointless speculating about a mechanism until someone can show adequate proof that there’s a real effect that needs to be explained.
If the “propellant” is photons, why not just shove regular photons out the back?
That was my initial thought.
The forces which are supposed to be developed are orders of magnitude above the calculated thrust of a photon drive operating at the same energy. So something else is at play, either poorly understood science, magic, unidentified systematic error, or fraud.