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Comment by cjensen | Hacker News Reader

For those new to this, the emdrive is very likely bogus.

1. It violates Conservation of Momentum. If the phenomena is real, a large number of basic Physics principles would need to be discarded.

2. It was designed based on certain principles. Those principles have been shown to be wrong. But it works anyway because reasons?

3. The results of each experiment are suspiciously close to the limits of measurement.

There are two choices here: (a) experimental error or (b) profound alteration to fundamental physics. Occam's Razor applies. Also, "extraordinary claims require extraordinary evidence" and this evidence is far from extraordinary.

That this is being published in a journal which does not cover basic physics is not helping.

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Agree with everything you write except,

> That this is being published in a journal which does not cover basic physics is not helping

AIAA is an extremely reputable journal and society. All the members are extremely familiar with conservation of momentum.

The observation of superluminal photons (later disproved) was published in reputable journals as well.

No scientist took either result very seriously. However, the observation of anomalous results is a result worth of publication and communication to other scientists and engineers.

As a practicing scientist, I can assure you that peer-review does not mean something is 'correct', but that is it not provably false, and that the results may be important to the community.

The hope is that this gives the idea some legitimacy, which will encourage others to test the idea too. It's like a win-win situation, if you test it and you discredit the effect, that's a free paper. If you verify it or add your own little spin, you'll go down in the history books along with this team.

"Basic Physics" means the underlying principles of physics. AIAA is reputable, but "underlying principles" is simply not a thing they do.

The tone you set by using the term "bogus", is IMHO, inappropriate at this point. There's been at least 4 separate groups who've all reported similar results. This latest one has been peer reviewed, presuming it's not a crapshoot journal there aren't any glaring issues (granted that's not saying much given the state of peer review). Nonetheless it's more likely that an unknown systemic experimental error is causing groups to report these kind of results, but not bogus as in the case of a truly bogus "discovery" cold fushion.

> 1. It violates Conservation of Momentum. If the phenomena is real, a large number of basic Physics principles would need to be discarded.

According to our current understanding of the system yes, however, it's also plausible that this works via in a non-trivial interaction of general relativity and quantum mechanics. Which is still a very open question, and may not actually be violating conservation of momentum. To state it more correctly, it appears the em drive violates conservation of momentum by all means we currently understand. Given the minuscule amount of thrust it's plausible there could be a unknown quantum effect. One group tested the drive in vacuum which makes it certainly more interesting if there is an actual novel physics interaction.

> 2. It was designed based on certain principles. Those principles have been shown to be wrong. But it works anyway because reasons?

That's a non-sequitur. If there weren't several repeated experiments showing similar results it might be a reasonable caution. However, many of the best scientific discoveries arose in similar mis-comprehensions.

> 3. The results of each experiment are suspiciously close to the limits of measurement.

This is the most problematic portion of the experiments. It's only an order of magnitude smaller thrust per kW than more conventional drives. However, this article at a cursory reading was designed to try various configurations of thrust to check for potential systematic experimental errors in measurement. At this point asking more serious questions would be required rather than a "this doesn't fit into my preconceptions". E.g. explain how the particular experimental design in question could be flawed, in particular, what conditions could cause the null thrust test to not remove systemic measurement.

In short, scientific skepticism doesn't mean incredulity.

> That's a non-sequitur. If there weren't several repeated experiments showing similar results it might be a reasonable caution. However, many of the best scientific discoveries arose in similar mis-comprehensions.

It's not a non-sequitur! It's firm evidence they don't understand what is going on and therefore aren't in a position to explain it. The fact that they have found a consistent effect does not mean they found anything interesting, a tiny wiggle very close to the detection range when you put a lot of power into it isn't by itself interesting unless you have a good basis to know it is. Which they DON'T.

I would add that by Noether's theorem, a violation of the conservation of momentum implies that the laws of physics are different from one place to another. So these results are even more suspicious than they might sound to a non-physicist.

Agreed, Occam's Razor would select that this is a case of experimental error. However...

The "Laws of Physics" are our current best way to describe the phenomenon that we see; but they may simply be the best approximations we have given the limited set of our observations. It does not follow that these laws are perfectly correct.

I think of it as a best-fit curve, which does a great job of describing the real curve so long as we have sufficiently accurate samples.

Sometimes I worry that we do not provide enough funding to experiments which fly in the face of our current understanding of Physics, and so skew our sample data towards fitting the curve we are presently comfortable with.

This is not a good description of how physics works. You can't just say we don't know everything, so maybe anything goes.

That's not what I claimed. We have a very good curve to fit our sample data; this doesn't mean that "anything goes" because we don't know everything, it only means that we should be careful not to assume that we know everything because our curve fits so well to the data we do have.

Part of that is confirming that things we expect not to work do not work. We shouldn't fear or avoid testing things simply because they disagree with our understanding, because testing them may confirm our understanding.

I think I get what you were going for, but I think David Deutsch's idea[1] of "explanations that are hard to vary" may be a better metaphor. The basic idea is that we can come up with all sorts of ad-hoc explanations for any particular phenomenon, but if you can just arbitrarily change your explanation (the means) in response to events (data) then it's manifestly not very good at prediction (the goal).

Just to get back on track: The point is that we have a huge number of explanations, currently, which are "hard to vary". You need a commensurate amount of "hard to vary" explanations to counter that. That's why it's very hard to overturn e.g. the 2nd Law of Thermoses, erm, Theormodynamics.

[1] I think, from "The Beginning of Infinity"... Actually, I can't remember if he cited anybody else for this idea, but I can say that it's the first place I came across it.

Hmm, sounds like I have something interesting to read, now. :)

I encourage anyone to read it, even laypeople. It's a great read if you want to think about science itself as opposed to just thinking about the "latest development in $SCIENCE". (Where $SCIENCE could be Physics, Biology, Medicine, ...)

There are a few bits in there that may be hard for the layperson to understand, but I think the overall point is amazingly well made.

I don't agree at all: being so quick to dismiss energy-momentum conservation is pretty much the same as anything goes. Noether's theorem runs really deep. For an example of an existing (non-crankpot) effort, have a look at the attempts to find Lorentz violations: https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_vi... That's more like what it looks like when people try to do it seriously.

I don't think this will result in dismissing of conservation of momentum.

But because many thought it did, it's been an incredible uphill battle to get this thing tested; yet throughout tests it shows strange outcomes. Something odd is going on, and perhaps the simple outcome is that we find a better way to perform the tests.

To me, that's a fantastic outcome and I'm dismayed it's taking such a _political_ fight to arrive at it.

You're wrong if you think this is "political", as you put it.

Conservation of momentum is something 'tested' throughout physics. It's not like we're short samples and it will turn out it's 99.9907132% conserved.

And that will likely remain true. Thanks to testing this, we'll end up with better testing processes.

Why is it such an uphill battle to engage in actions that have positive outcomes like these, simply because they challenge our core understanding of the world? At the least, we end up better equipped to measure and test future experiments.

> 1. It violates Conservation of Momentum. If the phenomena is real, a large number of basic Physics principles would need to be discarded.

Relativity didn't make us discard Newtonian physics though. It just turned out to be a good approximation of something more complex in certain conditions. Similarly, we don't discard the descriptions of macroscopic events when they turn out to be just emergent phenonema, good approximations of complex quantum events at certain scales. Why would this be any different?

> 2. It was designed based on certain principles. Those principles have been shown to be wrong. But it works anyway because reasons?

Those 'reasons' are imo more interesting than whether it really works or not. Sure, the measurements may be off, or the setup of the experiments could be flawed. But until someone can point out exactly why we're measuring what we're measuring, I think it's worth pursuing.

> 3. The results of each experiment are suspiciously close to the limits of measurement.

If nothing else, maybe it will motivate scientists to come up with more exact ways of measuring, or better experimental designs that shield from unintended effects.

I guess my point is, regardless of whether it works as a drive or not, we shouldn't discard it as a hoax or consider its study a waste of time as some scientists seem to do.

The trick is that this is not a closed discussion. The thrust was suspiciously repeated in many experiments, and question is whether it's some fundamental flaw with experiments, or it is real.

I guess, the more info is available to the public, the better. Hopefully someone will test it properly. But no way it can be considered as "provably works"

"The thrust was suspiciously repeated in many experiments" <--- How sure are you of that?

http://emdrive.wiki/Experimental_Results claims that there are six orders of magnitude difference between the lowest detected thrust and the highest detected thrust.

Going from the assumption that some effect actually exists and all the physical principles we have painstakingly tested ten million times over are wrong... Most of these studies are still complete BS, because they don't agree closely enough to be a replication.

What do you mean by "closed discussion", specifically?

We've been hearing about this for ~1 year at this point, so I'm not sure what you mean.

> I guess, the more info is available to the public, the better.

This is up for debate -- esp. given the whole "fake news" thing and the observation that people basically love echo chambers (as observed in the field of Behavioural Economics).

If, by "info" you mean "actually provable[1] info", then I probably agree.

[1] To some reasonable standard of "proof". Certainly we don't need "mathematical" proof, but we need something better than "a study says". (See: reproducibility crisis[2], the fact that a "study" doesn't require any particular standard of evidence, etc.)

[2] Meta: I wonder if the BE field is itself suffering from a reproducibility crisis? Anyone know?

Oh yeah, I wasn't clear, yes.

Not "everything is clear, the case is closed" - that's what I meant

For those who can't imagine how easy is to produce such an experimental error, the measurements are comparable to what's appeared "unexplained" in the case of the "Pioneer anomaly" until a very careful analysis was made:

https://en.wikipedia.org/wiki/Pioneer_anomaly#Explanation:_t...

It's much easier to produce "some results" then to prove that the researcher didn't fool himself.

Re Pioneer: "once the thermal recoil force is properly accounted for, no anomalous acceleration remains."

Now from the EmDrive paper's conclusion actually confirms that not everything was proved to be "sure" even from the point of view of the authors:

"Although thermal shift was addressed to a degree with this test campaign, future testing efforts should seek to develop testing approaches that are immune to CG shifts from thermal expansion."

Why too much public attention before the whole thing is actually proved can be dangerous:

http://blogs.discovermagazine.com/outthere/2014/08/06/nasa-v...

"When the results of the Cannae Drive prove impossible to validate (as will almost surely happen), it may produce an unjustified cynicism about how NASA has failed us once again."

It's just a very small team "devoted to exploring unorthodox propulsion ideas." "The reality is that space exploration is expensive, difficult, and time-consuming. Therein lies both its challenge and its glory."

If the phenomena is real then it means that some theory is wrong or incomplete, but not necessarily conservation of momentum. In the paper they give some ideas how conservation of momentum can be preserved. More research needs to be done for sure, but the phenomena being real does not necessarily mean a rewrite of physics. We have to wait and see if it is real and what the underlying physics is. Only then we will know how wrong/right our theories are.

See, i don't get the whole "we need to trash Physics!". Surely principles can still hold true at some level. I mean, physicist are perfectly happy dealing with "singularities" that do not fit their nice little models.

or c) an alternative explanation that still fits with most of what we know.

Anyway I agree with you, although it has already surpassed initial expectations stronger evidence is required.

Most alternate explanations make it not very exciting, though.

Assuming it is not actually violating conservation of momentum, it's pushing off something. Either that something is some part of the apparatus (in which case it's a boring old reaction drive) or else it's something environmental (in which case it's probably not going to work in the very different environment of deep space).

For this thing to be exciting as a drive per se, one of the following must apply:

1. Violates conservation of momentum, and like literally everything we know about physics is wrong.

2. Pushes against some kind of omnipresent ether-like medium, and darn near everything we know about physics is wrong.

3. Pushes against something in the device in some kind of very efficient way (ie accelerates reaction mass to a much higher exit velocity than conventional drives do), in which case why haven't we detected these high energy emissions?

The Lambda-CDM model ("standard cosmological model") states that the majority of matter in the universe is non-baryonic dark matter. It's generally found around the same locations as ordinary matter, and has only very weak interaction with ordinary matter (like neutrinoes do). It sounds a lot like ether except without the relativity problems. Maybe the EmDrive can somehow interact with non-baryonic dark matter and accelerate it.

I don't actually think this is true, because measurement error sounds more likely, but unlike conservation of momentum violation it would require only a minor tweak to known physics.

Agreed. When I make hard-ish science fiction settings that feature better drive technology than the real world, I assume that either:

1. A process can convert normal matter to dark matter and at the same time accelerate that dark matter to some absurd speed -- so you can take a small reaction mass with you and use it very efficiently, but you don't have an ultra-efficient weapon system.

2. A process can push against some background medium of dark matter.

But. If there is dark matter in the vicinity of Earth (and while I agree it's "in the same places as ordinary matter," that's at a macroscopic, galactic level), it's unlikely to be at some convenient rest-y-from-human-perspective frame of reference. Which would suggest that the EM drive would work better facing one direction than others. Which is, I think, not congruent with the experimental evidence in front of us.

Local dark matter energy-density is very small, around 0.01 times the mass of the sun per cubic parsec or in SI units about 10^-16 kg / m^3. In concordance cosmology CDM neither dissipates nor collides, so it cannot easily clump into warmer (and brighter) dense structures, so it is always diffuse outside of black hole event horizons.

If we accept for the moment the existence of a pseudo-CDM identical to the concordance CDM except that it can interact with EmDrive microwaves, then the first question must be: why doesn't it interact with astrophysical masers in GMCs, affecting the structure evolution therein? (e.g., if astrophysical microwave hotspots move pseudo-CDM then there will be a bias in the location and age of young stellar objects and H II regions due to the change in energy-density). Some astrophysical megamasers have luminosities on the order of a thousand suns -- that's a lot of watts to shuffle dark matter around in your idea about the EmDrive, and that shuffling will drag dust and gas along with it.

Tweaking CDM into pseudo-CDM without it collapsing gravitationally by radiating photons directly or by colliding with charged particles is probably impossible for any but a tiny fraction of the overall mass-density of the dark matter sector. Collision or direct dissipation removes angular momentum from the particles such that they can fall a little closer to the centre of the galaxy. On a galactic scale, that leads to a much higher total energy density at galactic core at even fairly small fractions of the local energy density.

Avoiding that would require extremely demanding work and likely would have you adding a lot more new particles to the standard model to capture all the additional (broken) symmetries -- and it gets worse the greater the local energy density or equivalently the greater the interaction cross-section you require in order to interact with a physically small device like an EmDrive.

The mere existence of some kind of ether wouldn't disprove much, as far as I know. It wouldn't impact relativity any more than a swimming pool.

An ether that can be pushed against is an ether with mass. There's a sibling comment in this thread talking about dark matter as a candidate for an ether and the problems with such a concept.

An ether that can be pushed against is also an ether that has a frame of reference, and that frame of reference is unlikely to be one that is extremely favorable to use of the EMDrive as a drive.

An ether that is in some way massless or frame-of-reference-less but still can be pushed against is a fundamental rethinking of physics.

Radio have mass, right!? It would be nice if we could suck in all kind of radiation as a turbo.

Exactly, it's fake. It won't work.

Rather than 'fake' it seems more genuine to describe it as a mystery that conflicts with existing scientific understanding. It would seem that it's a problem worth solving, if for no other reason than determining what's wrong with our measurement instruments or practices.

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