4 ms·

While it's true that most reactionless drives break some of the most basic physical laws, that's not the case for all of them. An Alcubierre drive is a good ex

by chton 12y ago

While it's true that most reactionless drives break some of the most basic physical laws, that's not the case for all of them. An Alcubierre drive is a good example of a propulsion method that works within the current laws, yet does not emit anything. The idea that the designs they tested (and EmDrive) would be reactionless is exactly why everybody is so baffled and excited by this. If they are, they must have some way to maintain those basic laws, through a process we don't know about yet. Aside from the practical uses, it could lead to a whole new physics to study.

gus_massa 12y ago
Alcubierre drive is not a thruster, you can't attach it to a spaceship to move it. It's something like: "If you put a spaceship inside this, and it stabilizes the gravity field using that, then it will travel faster than light in an external reference frame." It travels at a constant speed, so it doesn't need to emit particles to change the momentum. (On the other side, it uses exotic matter that no one has ever seen, so it may be impossible to construct.) When you say "reactionless" you mean that the device doesn't emit anything and it breaks the momentum conservation law? The previous experiments tried to give some theoretical explanations, but usually they were equivalent to emitting photons or gravitons. The conservation of momentum is a very fundamental law. It's possible to change the physics laws, but you need to have very good experimental evidence. (Hey! I believe in the Higgs boson!) This is like the faster than light neutrinos. It's very exiting if it were true, but the easier explanation is an experimental error.
- jliechti1 12y ago
  I asked a particle physicist to compare the validity of this to the faster than light neutrino claim that came up and his response was: "Although several people are drawing that comparison, they're really not in the same class. OPERA did an extremely thorough analysis of the faster than light neutrino result before they published. In the end it turned out to be a very subtle experimental error that cause the problems. In contrast, this is just junk science from start to finish. The initial claim was likely an outright lie to lure investors."
  - gus_massa 12y ago
    Just for the record: I agree. I was just comparing that there was also a lot of excitement.
  - smellf 12y ago
    But there's three independent labs working on this concept, plus NASA. _Who_ is lying, the EmDrive folks? And if they are, then how are NASA, the Chinese and the other lab all replicating their results?
    - XorNot 12y ago
      By not testing it in a vacuum, which would eliminate most of the immediate problems with thrust being produced by outgassing from heated wires (10kW is more power then your stove top) or coronal discharge. I don't know what's going on with whoever is doing this at NASA, but the joke of testing IN A VACUUM CHAMBER while not actually pumping it down is ridiculous. These people are scientists who have access to the device - how did they not turn it on? For lack of a suitable capacitor? That's a nothing part for such a massive potential breakthrough.
      - chton 12y ago
        This was not a huge experiment for NASA, most likely the budget wasn't there for vacuum-capable capacitors and the time to convert the equipment. It was either go with what they had, or don't go at all. I do hope that, with a tentatively positive result, they can now get a bit more budget to perform the test again with an improved setup.
      - deleted 12y ago
        [deleted]
      - pavlov 12y ago
        The OP (Wired article) says that the vacuum chamber was indeed pumped down: While the original abstract says that tests were run "within a stainless steel vacuum chamber with the door closed but at ambient atmospheric pressure", the full report describes tests in which turbo vacuum pumps were used to evacuate the test chamber to a pressure of five millionths of a Torr, or about a hundred-millionth of normal atmospheric pressure.
- chton 12y ago
  In an Alcubierre drive the total system does accelerate forward. The spaceship itself won't, but the total system ('bubble', contents and ship together) will increase in momentum to an outside observer. I agree that it may be impossible to construct, but it's theoretically possible. According to the experiments, the drive seems reactionless so far. It's possible that it does emit something that we can not measure, it's also possible that it transfers momentum to something else, or that it works on something else entirely. We simply don't know yet. There is potential here, if there does turn out to be experimental evidence for it being truly reactionless, for that to require an addition to physics. Even if the evidence turns up something else, it'll at least be interesting. I agree that there are a lot of ifs, ors and buts. That's why everybody is calling for independent confirmation. Just like the neutrino case, that's how science is supposed to work. But any potential for expanding our understanding should be given proper attention and enthusiasm.
ethbro 12y ago
I think it's fair to place Alcubierre drive and something that's been physically built and tested in two very different camps. I understand this thread was about the theory, but from an engineering perspective that's a substantial distinction.

by chton 12y ago

gus_massa 12y ago

Alcubierre drive is not a thruster, you can't attach it to a spaceship to move it. It's something like: "If you put a spaceship inside this, and it stabilizes the gravity field using that, then it will travel faster than light in an external reference frame." It travels at a constant speed, so it doesn't need to emit particles to change the momentum. (On the other side, it uses exotic matter that no one has ever seen, so it may be impossible to construct.) When you say "reactionless" you mean that the device doesn't emit anything and it breaks the momentum conservation law? The previous experiments tried to give some theoretical explanations, but usually they were equivalent to emitting photons or gravitons. The conservation of momentum is a very fundamental law. It's possible to change the physics laws, but you need to have very good experimental evidence. (Hey! I believe in the Higgs boson!) This is like the faster than light neutrinos. It's very exiting if it were true, but the easier explanation is an experimental error.

jliechti1 12y ago
I asked a particle physicist to compare the validity of this to the faster than light neutrino claim that came up and his response was: "Although several people are drawing that comparison, they're really not in the same class. OPERA did an extremely thorough analysis of the faster than light neutrino result before they published. In the end it turned out to be a very subtle experimental error that cause the problems. In contrast, this is just junk science from start to finish. The initial claim was likely an outright lie to lure investors."
- gus_massa 12y ago
  Just for the record: I agree. I was just comparing that there was also a lot of excitement.
- smellf 12y ago
  But there's three independent labs working on this concept, plus NASA. _Who_ is lying, the EmDrive folks? And if they are, then how are NASA, the Chinese and the other lab all replicating their results?
  - XorNot 12y ago
    By not testing it in a vacuum, which would eliminate most of the immediate problems with thrust being produced by outgassing from heated wires (10kW is more power then your stove top) or coronal discharge. I don't know what's going on with whoever is doing this at NASA, but the joke of testing IN A VACUUM CHAMBER while not actually pumping it down is ridiculous. These people are scientists who have access to the device - how did they not turn it on? For lack of a suitable capacitor? That's a nothing part for such a massive potential breakthrough.
    - chton 12y ago
      This was not a huge experiment for NASA, most likely the budget wasn't there for vacuum-capable capacitors and the time to convert the equipment. It was either go with what they had, or don't go at all. I do hope that, with a tentatively positive result, they can now get a bit more budget to perform the test again with an improved setup.
    - deleted 12y ago
      [deleted]
    - pavlov 12y ago
      The OP (Wired article) says that the vacuum chamber was indeed pumped down: While the original abstract says that tests were run "within a stainless steel vacuum chamber with the door closed but at ambient atmospheric pressure", the full report describes tests in which turbo vacuum pumps were used to evacuate the test chamber to a pressure of five millionths of a Torr, or about a hundred-millionth of normal atmospheric pressure.
chton 12y ago
In an Alcubierre drive the total system does accelerate forward. The spaceship itself won't, but the total system ('bubble', contents and ship together) will increase in momentum to an outside observer. I agree that it may be impossible to construct, but it's theoretically possible. According to the experiments, the drive seems reactionless so far. It's possible that it does emit something that we can not measure, it's also possible that it transfers momentum to something else, or that it works on something else entirely. We simply don't know yet. There is potential here, if there does turn out to be experimental evidence for it being truly reactionless, for that to require an addition to physics. Even if the evidence turns up something else, it'll at least be interesting. I agree that there are a lot of ifs, ors and buts. That's why everybody is calling for independent confirmation. Just like the neutrino case, that's how science is supposed to work. But any potential for expanding our understanding should be given proper attention and enthusiasm.

ethbro 12y ago

I think it's fair to place Alcubierre drive and something that's been physically built and tested in two very different camps. I understand this thread was about the theory, but from an engineering perspective that's a substantial distinction.