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Comment by gaur | Hacker News Reader

Yeah, we're going to learn that some experimenters are sloppy.

Its amazing to me how many people who purportedly believe in science act more like priests than scientists. Science is about drawing conclusions from empirical data, not ignoring empirical data that doesn't jive with your predetermined conclusions.

The science is pretty settled here. No such thing can exist by laws of physics. You might as well try making a perpetual motion machine or homeopathic medicine.

>The science is pretty settled here. No such thing can exist...

Thanks, this more neatly sums up the attitude I was attempting to describe then anything I could come up with. The hubris that is endemic in much of the "scientific" community is simply amazing. Another glaring example of the gap between education and intelligence.

Extraordinary claims need extraordinary proof and until now there is no single proof or theory.

so yes, it is not hubris, it is science.

> Another glaring example of the gap between education and intelligence.

Pot kettle?

Scientific method say that if observation contradicts your theory, then your theory is wrong.

One of the more important things that you learn in a laboratory is that experimental science is hard. There are many source of experimental errors.

So it's not enough to have an experimental result, it's necessary to check very carefully the details to be convinced that it breaks the theory.

In this case, the problem is that they are measuring a very small force, and many things can cause small forces. So you must be sure that all the other possible small forces sources are under check and that the force you are measuring is created by the method you are trying to measure.

In this case, the most obvious candidates for errors are:

* Thermal: The device gets very hot, so there may be some thermal expansions and some thermal air currents.

* Electromagnetic: They use a lot of current, so the wires that connect the device may be acting as an electromagnet or something.

They try to bound this error, but it's very hard to estimated correctly how big each one of them are, and prove that they are smaller than the force they see in the experiment.

Actually, IF this were true and they really were breaking the conservation of momentum law, then by special relativity in most of the reference frames they must be breaking the conservation of energy law.

So if this were true then they could build a perpetual motion machine too. IIRC one possible design was to put two em-drives with opposite directions at the opposite sides of a carousel.

[Disclaimer: I think that this is an experimental error and that the conservation of momentum and energy laws are safe.]

It's amazing to me how many people go nuts over marginal results that lack even a thin veneer of peer review.

There are plenty of bizarre conclusions (dark matter and dark energy, for example) that have been drawn from stark empirical data and have withstood peer review. The EMdrive is not one of them.

Yes, you must draw conclusions from empirical data. However, there is a likelihood, and there is a prior. The emdrive has no theoretical basis, so the prior is very low. The likelihood is small but not very small, because the results are just about at the detection limit. They claim that thrust is linear in the 40-80 watt range at 1.2mN/kW. Rather than go to the press, why don't they just turn up the power until the effect can't be denied? The fact that they did not publish that is also information.

How many times do these results need to be replicated before you'll believe them?

So far the thrust results have been replicated more than the majority of economics and social science research (i.e. more than zero times).

The paper itself details a range of mundane error sources (including thermal expansion) that still haven't been ruled out as the cause of the observed thrust.

It violates the conservation of momentum; so it needs to replicated many many times.

What tablet did god write this law on?

Well, you're right that it's not a mathematical certainty that momentum is always conserved.

But there's such a mountain of evidence for it that we should have a very high bar for claims that it's violated. Thermal effects and measurement error are both likely sources of the discrepancy.

But another reason to be skeptical is that Noether's Theorem tells us that a violation of conservation of momentum is only possible if the laws of physics are not translation invariant, and we should be able to use the violation of conservation of momentum to illustrate a specific instance where physical laws are not translation invariant. That would be Earth-shattering. (But not, strictly speaking, impossible)

... or the law was bypassed, somehow. Also huge news if confirmed

It seems to be that extraordinary claims require extraordinary evidence.

In a tablet way more powerful than the weak evidence that this drive shows

How do you know that? Consider a pretty simple and obviously correct device: an electric engine with an internal rotating shaft + a solar panel to power it, floating in space. The faster it rotates, the higher its acceleration in the gravitational field.

The engine would spin, but it would have 0 velocity unless it sent out something as exhaust.

That's not how general relativity works. Spinning would increase its energy, hence increasing its gravitational pull, hence accelerating faster.

But momentum conservation holds in GR.

I don't know GR, but approximately the Earth would also accelerate faster.

Conservation of momentum only works because the gravitational field itself is supposed to have momentum. Similar to how momentum is conserved for an accelerating paddling boat only if you include the sea. Sea analogy is a good one I think, especially because the gravitational field interacts with itself, like water, contrary to electromagnetic fields.

Conservation laws in general are a quirky thing in GR, see: http://people.bu.edu/gorelik/ES_GG_Conservation_Laws.pdf

or http://www.preposterousuniverse.com/blog/2010/02/22/energy-i... for a shorter read

>but approximately the Earth would also accelerate faster.

Well yes, Earth has a stronger gravitational pull because of rotation. On smaller distances rotation effects become even more interesting:

https://en.wikipedia.org/wiki/Frame-dragging

While I agree with your statement I derive the opposite conclusion from it. (I.e. these results are certainly false and you can see what that implies about economics and social science.)

> So far the thrust results have been replicated more than the majority of economics and social science research (i.e. more than zero times).

No, they haven't

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