4 ms·

The Emdrive takes energy, though, doesn't it? It's not like you can plug it in and have it run, unless I misunderstand a lot of claims.

sethrin 10y ago
Put two emdrives on the ends of a rotating beam. Use the violation of conservation of energy/momentum to drive a generator. Infinite speed and energy are yours to command!
- MPSimmons 10y ago
  Driving the generator is going to rob the system of energy. That should be obvious, right? Nothing in the matter/energy equation is changing. This drive would just take electrical energy and convert it into kinetic energy.
Tuna-Fish 10y ago
Yes. However, any true reactionless thrust is equivalent to a perpetual motion machine, because reactionless thrust drives produce the same acceleration regardless of speed, but the kinetic energy produced by the same acceleration goes up as the speed goes up. If you accelerate one kg from 0m/s to 1m/s, you impart 0.5joules of energy on it. If you accelerate it from 1m/s to 2m/s, you impart 1.5joules of energy on it. If you accelerate it from 2m/s to 3m/s, you impart 2.5 joules of energy on it, and so on and so on. Each added m/s costs more in energy. A reactionless drive working in a system with no preferred frame would add the same amount of acceleration for the same amount of cost, regardless of how much energy there already is. This would mean that eventually it would be going fast enough that the added kinetic energy would be more than however much energy it draws in. Then you can build a gigantic carousel that is spinned on the rim and takes energy from the middle and feeds some back in to move it, and now you have perpetual motion. This is why no real physicist actually thinks that this will be reactionless thrust. However, that does not necessarily mean it's useless. If it, for example, allows you to push against the earth's magnetic field more weight-efficiently than current magnetic propulsion systems, it would be a major win for satellite stationkeeping.
- MPSimmons 10y ago
  > reactionless thrust drives produce the same acceleration regardless of speed A) Where is that in the paper? Or am I missing something elementary to this? B) We don't even know if it produces the same level of thrust as speed increases. If it produces the same level of thrust across all speeds, then we have a problem, because as velocity increases, so does mass, and the same level of thrust would produce less acceleration over time, because of the increase in mass. Of course, that may only effect things at relativistic speeds.
  - Tuna-Fish 10y ago
    Speed relative to what, exactly? There are no preferred reference frames. A true reactionless thruster could always treat itself as if it's initial speed was 0, and so it took 0.5J to accelerate by 1m/s. Rockets do that too, but they get away with it because they stored the necessary extra energy into the kinetic energy of the fuel. If there was a reactionless thruster that did, in fact, get less efficient at higher speeds, then that would also be world-shattering physics news, because then you could measure it's performance after accelerating in different directions and eventually get a true rest frame that is privileged over the rest of the reference frames out of it.

sethrin 10y ago

Put two emdrives on the ends of a rotating beam. Use the violation of conservation of energy/momentum to drive a generator. Infinite speed and energy are yours to command!

MPSimmons 10y ago
Driving the generator is going to rob the system of energy. That should be obvious, right? Nothing in the matter/energy equation is changing. This drive would just take electrical energy and convert it into kinetic energy.

Tuna-Fish 10y ago

Yes. However, any true reactionless thrust is equivalent to a perpetual motion machine, because reactionless thrust drives produce the same acceleration regardless of speed, but the kinetic energy produced by the same acceleration goes up as the speed goes up. If you accelerate one kg from 0m/s to 1m/s, you impart 0.5joules of energy on it. If you accelerate it from 1m/s to 2m/s, you impart 1.5joules of energy on it. If you accelerate it from 2m/s to 3m/s, you impart 2.5 joules of energy on it, and so on and so on. Each added m/s costs more in energy. A reactionless drive working in a system with no preferred frame would add the same amount of acceleration for the same amount of cost, regardless of how much energy there already is. This would mean that eventually it would be going fast enough that the added kinetic energy would be more than however much energy it draws in. Then you can build a gigantic carousel that is spinned on the rim and takes energy from the middle and feeds some back in to move it, and now you have perpetual motion. This is why no real physicist actually thinks that this will be reactionless thrust. However, that does not necessarily mean it's useless. If it, for example, allows you to push against the earth's magnetic field more weight-efficiently than current magnetic propulsion systems, it would be a major win for satellite stationkeeping.

MPSimmons 10y ago
> reactionless thrust drives produce the same acceleration regardless of speed A) Where is that in the paper? Or am I missing something elementary to this? B) We don't even know if it produces the same level of thrust as speed increases. If it produces the same level of thrust across all speeds, then we have a problem, because as velocity increases, so does mass, and the same level of thrust would produce less acceleration over time, because of the increase in mass. Of course, that may only effect things at relativistic speeds.
- Tuna-Fish 10y ago
  Speed relative to what, exactly? There are no preferred reference frames. A true reactionless thruster could always treat itself as if it's initial speed was 0, and so it took 0.5J to accelerate by 1m/s. Rockets do that too, but they get away with it because they stored the necessary extra energy into the kinetic energy of the fuel. If there was a reactionless thruster that did, in fact, get less efficient at higher speeds, then that would also be world-shattering physics news, because then you could measure it's performance after accelerating in different directions and eventually get a true rest frame that is privileged over the rest of the reference frames out of it.