
How Physics Falls Apart If the EMdrive Works - allenleein
http://www.forbes.com/sites/startswithabang/2016/11/23/how-physics-falls-apart-if-the-emdrive-works/#1673149e4b0c
======
obi1kenobi
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.

~~~
jdeisenberg
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."

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

~~~
greglindahl
The most likely "cause" is (still) experimental error.

------
api
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_again
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.

~~~
api
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.

~~~
madaxe_again
Maybe the quantum background ends up with the momentum. Honestly, don't have a
better hypothesis, but if we're to accept pilot waves we end up back at
luminiferous aether, so it's anyone's guess.

I still reckon there's an experimental error somewhere - but if there isn't,
it's undeniably a big deal.

------
akuma73
I'm with Sean Carroll here. Extraordinary claims require extraordinary
evidence (Bayesian reasoning).

[http://www.preposterousuniverse.com/blog/2015/05/26/warp-
dri...](http://www.preposterousuniverse.com/blog/2015/05/26/warp-drives-and-
scientific-reasoning/)

~~~
beevai142
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](http://dx.doi.org/10.2514/6.2015-4083)
[2]
[http://dx.doi.org/10.13675/j.cnki.tjjs.2016.02.022](http://dx.doi.org/10.13675/j.cnki.tjjs.2016.02.022)

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

~~~
cjensen
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.

------
PaulHoule
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.

~~~
flukus
> 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
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)

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

------
tzs
> 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...](http://www.preposterousuniverse.com/blog/2010/02/22/energy-
is-not-conserved/)

~~~
krastanov
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.

------
yk
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...](http://www.businessinsider.com/forbescom-was-infected-by-chinese-
malware-2015-2?IR=T)

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

[3] Possible distinct incident before January 2016
[http://www.businessinsider.com/forbescom-was-infected-by-
chi...](http://www.businessinsider.com/forbescom-was-infected-by-chinese-
malware-2015-2?IR=T)

~~~
DanielBMarkham
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
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?

~~~
flukus
Do the run their own ads like a lot of the bigger news sites do?

