Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics · HackerPapers discussions

Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by roywiggins

Thu, 03 Sep 2020 15:07:31 GMT

It's worth noting that if a propellentless drive works, it will probably generate free energy. This may adjust your expectations on how likely this idea is. How likely is a free energy device (ie, one that reliably gives you more energy out than you put in)? Well, this thruster can't be any more likely, because it would produce free energy. https://arxiv.org/abs/1506.00494 > the kinetic energy of a mass with a constant force applied will increase with the square of time, while the electrical energy input only increases linearly with time. A mass with such a drive attached would eventually…

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by roywiggins

Thu, 24 May 2018 15:17:52 GMT

I found this paper pretty convincing. Any "reactionless" drive more efficient than a photon drive is probably a perpetual motion machine: https://arxiv.org/abs/1506.00494 The claimed thrust of the EMDrive was incompatible with photon thrust- it was much too powerful.

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by roywiggins

Wed, 23 May 2018 21:37:06 GMT

It would have been a perpetual motion machine of the first kind (you could get an unlimited amount of energy out of it): https://arxiv.org/abs/1506.00494 The evidence for it was no better than the evidence for any other perpetual motion machine, there was nothing that made it special other than not claiming to be a perpetual motion machine (despite being one).

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by roywiggins

Wed, 23 May 2018 21:30:36 GMT

Such a drive would necessarily be a perpetual motion machine. Perpetual motion machines are a dime a dozen and we don't test all of them rigorously because approximately none of them will ever work. https://arxiv.org/abs/1506.00494

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by roywiggins

Wed, 23 May 2018 21:29:06 GMT

Any reactionless drive better than a photon drive is probably a perpetual motion machine. https://arxiv.org/abs/1506.00494

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by roywiggins

Fri, 18 May 2018 15:06:40 GMT

Does this sidestep the perpetual motion problem? https://arxiv.org/abs/1506.00494

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by roywiggins

Fri, 18 May 2018 14:22:43 GMT

Conservation of momentum is almost as important as conservation of energy, and indeed, if you have a reactionless drive you can extract unlimited energy: "applying a constant force results in a constant acceleration, the kinetic energy of a mass driven by such a device increases quadratically with time, while the energy input increases only linearly with time. Thus, at some point, the kinetic energy of the device-driven mass exceeds the energy input, and if this energy is collected via decelerating the mass (via regenerative electromagnetic braking, for example), then there would be a net…

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by roywiggins

Fri, 18 May 2018 14:20:53 GMT

And indeed, you can build a perpetual motion machine out of a reactionless drive. "any device with a thrust-to-power ratio greater than the photon rocket would be able to operate as a perpetual motion machine" https://arxiv.org/abs/1506.00494

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by akvadrako

Wed, 26 Apr 2017 10:18:27 GMT

Basically, it's because as the drive keeps accelerating without losing mass, the velocity is proportional to the energy expended. However, it's kinetic energy increases with the velocity squared. Eventually, there is a cross-over point. You can create infinite energy by placing a bunch of drives on the spokes of a huge flywheel. See this paper for a more detailed answer: https://arxiv.org/abs/1506.00494

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Reconciling a Reactionless Propulsive Drive with the First Law of Thermodynamics — mentioned by beevai142

Sat, 24 Dec 2016 23:13:40 GMT

Photon rockets are not actually reactionless, because the emitted photons carry momentum and energy out of the system. This makes conservation laws to hold for such systems, cf. https://arxiv.org/abs/1506.00494 Now, the emdrive people are apparently claiming that it is truly reactionless. If so, you can build a perpetual motion machine of the first kind (produces energy from nothing) out of it --- which should demonstrate the degree of plausibility of such claims. For photon rockets, there's apparently a hard upper bound on the efficiency, which limits the practical usability.

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