How Physics Falls Apart If the EMdrive Works

http://www.forbes.com/sites/startswithabang/2016/11/23/how-physics-falls-apart-if-the-emdrive-works/#1673149e4b0c

by allenleein • 10 years ago

29 33 10 years ago

api3• 10 years ago

Wow... what a particularly bad article.

The Emdrive is an experiment that has so far provided an anomalous result. If that result holds up to replication and deeper investigation, only then will we proceed to the theory stage.

There is presently no theory to explain this result, but there are many speculations and hypotheses. Only the most extreme of these invoke a true violation of conservation laws, and these as a result would be considered last after others had been ruled out.

Less extreme hypotheses include interaction with dark matter or some other normally invisible or weakly interacting medium, unknown physics surrounding gravity or magnetic fields, etc.

Personally my favorite is dark matter. Could this be a weakly interacting particle thruster? Perhaps it is somehow accelerating ubiquitous but weakly interacting particles, essentially using dark matter as a propulsive medium.

That could have massive practical implications and would also lead to new physics, but would not violate conservation laws since momentum is being transferred.

A craft powered by such a thruster would have a "jet wash," just one that is harmless and largely undetectable through normal means. It might also only work in the vicinity of massive objects or in other places where a sufficiently rich concentration of propellant is found. Planets and stars have been hypothesized as being surrounded by dark matter clouds, but e.g. it may not work well in interstellar space.

Of course it would only be useful for propulsion if it could be made efficient.

madaxe_again4• 10 years ago

See the NASA paper - they have a hypothesis, which is that they're essentially pushing against the quantum background with pilot waves - which doesn't really violate anything but does mean we took a wrong turn on QM 80 years back.

kordless• 10 years ago

Here's a detailed hypothesis by a team out of Finland: http://scitation.aip.org/content/aip/journal/adva/6/6/10.106...

api3• 10 years ago

Where does the momentum go when the virtual particles re-annihilate?

Even if it is pushing against QM background I suspect that could only be part of the story. There has to be an "and then" unless we want to toss a conservation law.

Many physicists think there is likely dark matter and that it is actual matter with mass, so I've repeatedly seen that thrown out as a potential answer for where momentum might be going eventually. Maybe messing with the QM background is how that happens... somehow. Maybe weakly interacting dark matter particles interact with the microwave background or something.

mrob• 10 years ago

The dark matter hypothesis has already been discussed here, and raattgift posted a good explanation for why it is very unlikely:

https://news.ycombinator.com/item?id=13003811

obi1kenobi2• 10 years ago

This is such a bad article. The Emdrive working as advertised does not immediately imply that conservation of momentum does not hold.

There are a variety of more plausible alternatives, precisely because conservation of momentum is so well-tested. Perhaps the most famous one is that the Emdrive is pushing against virtual particles that arise from a quantum vacuum.

madaxe_again4• 10 years ago

Indeed - it appears the author didn't read or didn't understand the paper.

If the EMDrive works as postulated, then physics doesn't fall apart, and conservation of momentum isn't violated - however it does mean that our statistical view of quantum mechanics is nothing but elegant mathematical shorthand ( which remains perfectly workable for much of QM), and reality is far more de Broglie-Bohmian than commonly believed.

So. The worst case (if it works as advertised) is that physics doesn't fall apart and rather a whole new field of physics opens up. Although if that field opens up it was always going to open up, so does the progression of time and therefore progress mean anything in a deterministic universe?

New metaphysics too, I guess.

krastanov3• 10 years ago

Guys, to both the parent and grandparent, please do not throw out word like that haphazardly. Even if you know what Bohmian mechanics or vacuum virtual particles actually means, you are just spreading rumors at this point, unless you try to define those terms.

About virtual particles: First of all, the virtual particle picture is a useful picture to build intuition, the math behind it is not as simple as the picture implies. Second, no, just pushing on the virtual particles is not a magical way to save conservation of momentum - where does that momentum go after those virtual particles re-annihilate for instance?

About Bohmian mechanics: How do you even reach that conclusion? This is a really wild claim, that can not be taken seriously without any argument. Bohmian mechanics already has philosophical problems (stemming from computation theory, c.f. some of Aaronson's writing) even if mathematically it is equivalent to QM in the case of a single particle. But look at how ugly its generalization to multiple particles looks like (I do concede, ugliness is not the best guide, but it is something to consider when searching for a good description of nature).

jdeisenberg• 10 years ago

Towards the end of the article, the author says this: "The point isn’t that physics is wrong, nor is the point that the Eagleworks team is wrong. The point is that this is the beginning stages of actual science being done to examine an effect. The most likely outcome is that momentum really is conserved and there’s something funny going on here."

api3• 10 years ago

Yes, but "funny" may still involve new physics unless it turns out to be something mundane but very sneaky.

akuma73• 10 years ago

I'm with Sean Carroll here. Extraordinary claims require extraordinary evidence (Bayesian reasoning).

http://www.preposterousuniverse.com/blog/2015/05/26/warp-dri...

beevai1422• 10 years ago

Emdrive does not warrant any hype:

- There is no reason to expect that it "works". Shawyer's original "theory" on which the design is based is wrong, the EW do not even mention it in the paper.

- Two other groups attempted to measure a similar device and published results. TU Dresden [1] people failed to get a signal, and the Chinese NWPU people reported failure (in the 2016 update [2], after debugging the first measurement). Both are consistent with null hypothesis that there's nothing to see here.

- Bringing up "quantum vacuum plasma" and Bohmian theory, but failing to give a working argument why they would explain anything in the experiment, or even be relevant for it. The likely reason for the failure to produce something intelligible likely is that quantum vacuum does not help here --- it's a well-understood concept (perhaps not to EW), as is momentum conservation.

[1] http://dx.doi.org/10.2514/6.2015-4083 [2] http://dx.doi.org/10.13675/j.cnki.tjjs.2016.02.022

swasheck• 10 years ago

Hm. Seems to be about what I expected from a business/investment-oriented magazine publishing a scientific article.

cjensen• 10 years ago

The was the best explainer I've seen for what a lot of skeptics have pointed out: the science here is extremely dubious and by far the most likely explanation is experimental error.

The concepts in the article are pretty much consensus science.

tzs• 10 years ago

If you violate momentum conservation by different amounts, you violate energy conservation, too; energy is not only not conserved, it’s not conserved by different amounts in different reference frames. The most sacred law of particle physics — one that has been observed to apply to every system and every interaction set in history — would be busted.

But we've known since the 1920s, when we started figuring out that spacetime changes, that energy is not conserved.

Sean Carroll has a good article explaining this [1]. Some quotes:

It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.)
The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved.

(skipping some elaboration and examples he gives)

It is possible to interpret GR in a way that lets you still say that energy is conserved but it is kind of a hack:

Having said all that, it would be irresponsible of me not to mention that plenty of experts in cosmology or GR would not put it in these terms. We all agree on the science; there are just divergent views on what words to attach to the science. In particular, a lot of folks would want to say “energy is conserved in general relativity, it’s just that you have to include the energy of the gravitational field along with the energy of matter and radiation and so on.” Which seems pretty sensible at face value.
There’s nothing incorrect about that way of thinking about it; it’s a choice that one can make or not, as long as you’re clear on what your definitions are. I personally think it’s better to forget about the so-called “energy of the gravitational field” and just admit that energy is not conserved, for two reasons.
First, unlike with ordinary matter fields, there is no such thing as the density of gravitational energy. The thing you would like to define as the energy associated with the curvature of spacetime is not uniquely defined at every point in space. So the best you can rigorously do is define the energy of the whole universe all at once, rather than talking about the energy of each separate piece. (You can sometimes talk approximately about the energy of different pieces, by imagining that they are isolated from the rest of the universe.) Even if you can define such a quantity, it’s much less useful than the notion of energy we have for matter fields.
The second reason is that the entire point of this exercise is to explain what’s going on in GR to people who aren’t familiar with the mathematical details of the theory. All of the experts agree on what’s happening; this is an issue of translation, not of physics. And in my experience, saying “there’s energy in the gravitational field, but it’s negative, so it exactly cancels the energy you think is being gained in the matter fields” does not actually increase anyone’s understanding — it just quiets them down. Whereas if you say “in general relativity spacetime can give energy to matter, or absorb it from matter, so that the total energy simply isn’t conserved,” they might be surprised but I think most people do actually gain some understanding thereby.
Energy isn’t conserved; it changes because spacetime does. See, that wasn’t so hard, was it?

So, getting back to the Emdrive, if it works it won't be it not obeying conservation laws that surprises me. It will be how simple it is.

From Noether's theorem, we know that conservation laws are equivalent to various symmetries in the laws of physics. Translation invariance gives conservation of linear momentum. Rotational invariance gives conservation of angular momentum. Time invariance gives conservation of energy.

The universe looks pretty much the same in every direction from our point of view, and when we look into the past (by looking at things farther away), it looks like the laws of physics are the same. If we do NOT have translation, rotation, or time invariance, the variation appears to be very small. Thus, I'd expect actually exploiting any such variation to require something very very big, probably galaxy cluster size at least.

[1] http://www.preposterousuniverse.com/blog/2010/02/22/energy-i...

krastanov3• 10 years ago

I am not certain your argument holds for the system being considered here: it is important to mention that local energy conservation in a lab in flat non-expanding spacetime is very much a thing. And flat non-expanding spacetime is where this experiment is being done. Sure, if you are talking about cosmology and GR then some new effects appear, but is it reasonable to talk about those here - I doubt that the sensitivity of the equipment warrants talking about those.

gohrt• 10 years ago

Energy is always conserved, because that's the definition of energy. When an experiment discovers that energy is not conserved, that means we've discovered a new kind of energy (kinetic energy, gravitational potential energy, electrical energy, etc). This is semantics, not physics.

The only way to disprove conservation of energy would be to show that measurable energy -- in _all_ forms combined, varies in quantity in unpredictable ways. And it would be hard to differentiate "non-conservation of energy" from "dark energy" we can't measure with current tools.

yk• 10 years ago

Could we get a nice red background for links to forbes? That is a site which distributed malware at least twice [1][2][3] and tries to force users to enable JS.

[1] November 2014 http://www.businessinsider.com/forbescom-was-infected-by-chi...

[2] Sept. 8 to Sept. 15 https://www.fireeye.com/blog/threat-research/2015/09/malvert...

[3] Possible distinct incident before January 2016 http://www.businessinsider.com/forbescom-was-infected-by-chi...

DanielBMarkham• 10 years ago

Hear hear.

And they refuse to allow any access through their paywall for folks with AdBlockers, even for Google referrals.

I don't think we should be sending them any traffic at all.

kristianp• 10 years ago

Interesting, no problems here with uBlock Origin.

This whole thing about distributing malware, isn't that a widespread problem with any website that uses ad networks? Is it fair to single out Forbes in this regard?

hrxn• 10 years ago

Can't wait to see Forbes ending up in Facebook's new fake news filter...

PaulHoule• 10 years ago

That Emdrive paper has all the smells of bad science around it.

Like the original cold fusion papers, the experiment is way too complex for what it is.

flukus2• 10 years ago

That Emdrive paper has all the smells of bad science around it.

I've seen this thrown around a lot but no one has bothered to mention what the papers shortcomings were.

trhway• 10 years ago

the cavity is asymmetric, has EM waves bouncing inside and isn't a Faraday cage - thus generates radiation outside (by Maxwell's classic theory, not virtual particles QM), asymmetric at this. EM radiation, ie. photons, have impulse, so according to Newton's and impulse preservation laws the EM drive should have an impulse in the direction opposite of the total vector sum impulse of that outside radiation.

The impulse is larger than the same energy would generate using visible light frequency photons in photonic drive or solar sail - that is obvious too because they use radio frequencies in EM drive, ie. much lower frequency photons. The same basic math works here like the one for specific impulse, thrust and energy relations in the classic situation of high by-pass jet engine vs. low by-pass vs. rocket engine.

Btw, for current working example of IR frequency photonic drive look up the slowdown of Voyager by IR radiation from its RTG bouncing off the back of its antenna (which is pointed to Earth/Sun and the RTG is behind it thus that IR photons bounce slows the Voyager)

How Physics Falls Apart If the EMdrive Works

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