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Propellantless propulsion flies in the face of the conservation of momentum. This is a law which is baked in the current Physics theories, including the standard model and general relativity.
From a theoretical perspective, it follows from the Lagrange function being independent under certain coordinate transformations with Noether's theorem.
The steelman version of this propellantless propulsion would be the claim that of course momentum is conserved, there are just previously undetected particles or fields which carry momentum. Just like a plane can accelerate while staying at the same height without violating the conservation of momentum by transferring some momentum to the air with a propeller, a spacecraft might do the same. Of course, the particles could not be reacting with anything else (like satellites or these fancy detectors we use for dark matter search), otherwise they would have been found long ago. A fundamental part of the universe being discovered by chance through an commercially interesting engineering application seems unlikely -- it would be like if Edison had created the light bulb and physicists had only discovered electricity afterwards to figure out how it works. (By contrast, my priors for observing complex systems exhibiting unexpected behaviors which will surprise physicists are much more relaxed, high temperature (that is, liquid nitrogen) superconductors were a total surprise, and the early experiments with heavier-than-air flight probably took place before we had any idea how a plane is generating lift.)
The priors for that would at least be slightly higher than "Archangel Uriel personally pushes the spacecraft forward", but still lower than for room temperature superconductors or even room temperature fusion.
The best way to convince the world that the "emdrive" works would be to put one in LEO in a cubesat. Even if you can only generate a very moderate thrust from solar power, the ability to create that thrust continuously will integrate to a tremendous delta v. A year at a thousandths of Earth surface acceleration would work out to 309km/s delta-v. Within three years, your spacecraft would pass Voyager 1 in distance. Humans have some capabilities to track satellites, so we could check easily enough.
Of course; anything that works (beyond VSIMR or solar sails, as another commenter helpfully pointed out) requires some law-of-physics updates. But I will point out that this is exactly what is claimed.
I’m not strongly arguing for this being The Real Deal; as another commenter pointed out, put it on a satellite and prove it. Rather, my interest in this is as a thought exercise: consistent force production from electricity allows us to do all kinds of wacky stuff, up to and including interstellar travel on reasonable timeframes, pursuant to your definition of reasonable. 1G acceleration, as is claimed in this particular instance, would get us to Alpha Centauri in a little over six years; 12 years if we are slowing down at the halfway point. This is well shy of “generation ship” type speculation, and would turn intersolar travel into something feasible in a lifetime.
Now, hefty grain of salt and all that. I’m skeptical myself, and recognize this is extremely speculative. Not only are there large engineering challenges in building such a spacecraft (or proving that one of these propellantless engines can produce thrust), there are also a whole slew of known unknowns (interstellar hydrogen or small molecule impacts at an appreciable percentage of C?) and unknown unknowns.
At the same time, it also solves some problems. Consistent acceleration, likely even under 1g, removes a lot of the problems of extended stays in microgravity, and if we’re hypothesizing advanced extrasolar civilizations anyways… Then it stands to reason that we would not be the only ones who would discover such things. It would “raise the ceiling” on intrastellar travel, so to speak.
I’m happy to be able to discuss it. While my own priors are low for any individual ‘game changing technology’ coming to fruition, we do know there are yet-unexplained physics; we live in exciting days, in both the positive and negative sense, to be able to more seriously start investigating these fringes.
Even if we grant them that they have discovered a new thing which can carry momentum, I am kind of puzzled about the implications for conservation of energy.
Friction and air resistance aside, the most effective method to convert energy into momentum of your vehicle is a railway (or car). The other mass involved in the conservation of momentum is Earth, which is much heavier than your train, so almost all of the energy you invest ends up as kinetic energy in your train. We know from high school physics that the energy you have to invest to reach velocity v is E=m/2vv.
Rockets are a lot less energy efficient than that. Because their momentum-balancing mass is much smaller, they end up with most of the energy being carried by the exhaust. Tyranny of the rocket equation and all that.
Photon drives powered by onboard reactors may or may not fall under some weird relativistic version of the rocket equation (after all, your reactor will become slightly lighter as it provides energy), but are in any case laughably inefficient.
A drive which provides a useful constant, rate of acceleration while using a constant amount of power would be better than the train, eventually, thereby violating conservation of energy.
Another way to think about it: If you are using undiscovered massive particles (perhaps dark matter) to dump your momentum into, the rest system of these particles will define an unique frame of reference. If you are in the rest system, you can accelerate very efficiently with your magical drive: just suck in particles and expel them with a tiny velocity (say 1m/s) to carry your momentum. If you do that for a while and now move through the particles with 10 km/s, you will notice that your job becomes much harder: to carry the same momentum, you will have to accelerate the incoming particles, which you see at 10km/s, to 10.001 km/s. This costs a lot more energy than accelerating them from 0m/s to 1m/s. (You will also see more particles per time, but this will not save you, fundamentally, the amount of energy you require to dump a marginal amount of momentum (dE/dp) will become very unfavorable.)
This of course suggests another test for the emDrive: Michelson-Morley experiment, dark matter edition. Measure the thrust per energy (probably in z direction, so don't pick the poles?) at different times of the day and the year, so that the relative velocity of the particles in the direction of the thrust is different. If you get fluctuations consistent with Earth moving through some particle field, this should be enough for at least one Nobel.
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Whatever happened to Heim theory? Did it get disproven?
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To be fair it's not like that kind of stuff never happens. A lot of the history of semi-conductors is engineers trying various random things intuitively to get a particular defined effect and only explaining why that worked after the fact.
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Photons have momentum. If you're collecting light that's hitting your craft always at the same angle this momentum has to be transferred somewhere, and your craft is this somewhere as there is nowhere else for it to go.
Yes they do. Wikipedia points out that the force is (1/c) times the power, and helpfully converts 1/c to 3.34 Newtons per Gigawatt. The article also helpfully does the calculation for the solar radiation near 1 AU (i.e. Earth) and comes to a value of ten Micronewtons per square meter.
If one wants to use this force, the best thing one can do is have a very large and very light mirror, which is better than first taking the momentum of the suns photons on your solar collectors and then sending a small fraction of that momentum out in the direction you actually want to accelerate in. This is not completely hopeless: metallized Mylar foil might weight some 50 micrograms per square meter, so a space craft where most of the mass is in the foil might accelerate at 40 centimeters per second squared (though there are some constraints on the direction, similarly to sailing). Of course, having a spacecraft with two hectares of foil per kilogram of payload might be difficult from an engineering point, and micrometeorites might become a problem. I would probably play a Kerbal mod which adds Kerbol radiation pressure and giant sails, though.
Or you could actively shine an Earth- or Moonbound laser on your spacecraft.
In general, there is a tradeoff between getting the most momentum out of your propellant mass, which benefits from higher exhaust velocities on the one side (with photons being the optimal choice, and ultrarelativistic ions only slightly worse) and getting the most momentum per energy invested, which favors throwing out a huge mass at minimal velocity. For propulsions where the energy source is decoupled from the reaction mass, such as ion drives, the sweet spot seems to be at a mere 20-50km/s -- which is far away from the 300,000km/s you would get with photons.
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