Comment by gus_massa | Hacker News Reader

Comment by gus_massa | Hacker News Reader

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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.

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."

Just for the record: I agree.

I was just comparing that there was also a lot of excitement.

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?

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.

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.

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.

Yeah just saw that part. The abstract doesn't mention it, which is super-weird, and neither did the original Wired article.

But if they found the effect persisted when pumped down then that puts this way further into "interesting" territory.

EDIT: Ok I've just been through the paper, but it's buried at the end - "Vacuum compatible RF amplifiers with power ranges of up to 125 watts will allow testing at vacuum conditions which was not possible using our current RF amplifiers due to the presence of electrolytic capacitors"

There is a big description of the test rig, and how it can be pumped down to vacuum during tests, but they do not explicitly say that they pumped down to vacuum before running tests on the thruster. And the vacuum is never mentioned again until the very end, where we get this one ambiguous sentence. They might be referring to a new optimized thruster they couldn't test, or they might be saying they couldn't test in vacuum at all - it's never made clear and it's pretty damn important.

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.

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