
EmDrive: China claims success with this 'reactionless' engine for space travel - Osiris30
http://www.popsci.com/emdrive-engine-space-travel-china-success
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madengr
So they used twisted pair DC cabling to minimize torque errors due to DC
magnetic field (and the torque follows direction of cavity orientation),
but.... I'm wondering if there is DC current circulating in the cavity due to
rectification from oxides or impurities on the copper surface; i.e. Passive
Intermodulation Distortion (disclaimer I'm an RF EE). They said the torque was
strongly non-linear with applied power; which is a hint.

DC cavity currents were not listed as a potential error source, but these
phenomenon could show up with high power and high Q.

Anyway, I just emailed a couple of the NASA guys; see what they say.

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londons_explore
I don't understand why they don't just use a battery to power the
experiments... Now all power supply related concerns are gone.

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walrus01
on the scale of micronewtons being measured, batteries change significantly
thermally as they're discharged. You would have to account for the chemical
and thermal activity of the battery which is not a 100% inert thing.

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londons_explore
Is there a paper or technical source? All the articles I find seem to be copy
pasta of one another, none of which have sources...

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vanattab
Here is a link to the most recent nasa eagle works paper.

[http://arc.aiaa.org/doi/10.2514/1.B36120](http://arc.aiaa.org/doi/10.2514/1.B36120)

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kobeya
He's asking about the Chinese test. That one appears to be hearsay.

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madengr
Why after several positive tests, has the physics community not gone full
throttle to try and disprove it, like they did with cold fusion?

So far the tests have been static torque? Could there be an interaction with
the earths magnetic field?

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the8472
China is shipping one up to their space station to test it there. That is
already a pretty big allocation for research resources considering that there
is a large prior against this working: conservation of momentum.

So every time a positive result turns up incrementally more effort will be
applied to test it, assuming the new result improves on previous ones.

Seems like a reasonable approach to me.

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colordrops
I still haven't heard a good explanation why this must mean that conservation
of momentum is being violated. Couldn't there be a new phenomenon at which
forces act at a larger or more distant scale on other masses, similar to
gravity? That seems more plausible than beating conservation of momentum.

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gus_massa
> _Couldn 't there be a new phenomenon at which forces act at a larger or more
> distant scale on other masses, similar to gravity?_

Gravity doesn't act at large distance. If you have a star like the Sun, it
modifies the gravitational field locally. The modifications travel for a while
and when the modification hits a planet like the Earth it applies a force to
the Earth and modifies it's direction. The modifications are like the
gravitational waves that have been recently measured. They travel at the speed
of light.

If you have a force that can truly act at distance, you can use it to transmit
information faster than light and it will break special relativity.

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colordrops
Well all experiments have been near large large objects so far, so if such a
force exists nothing is being said about it being faster than light.

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gus_massa
Near is very near here.

Let's analyze a similar case. The weak nuclear force is transmitted by the W+,
Z0 and W- particles. They live approximately 1E-25 seconds. Assuming they
travel near the speed of light, they can travel 3E-17 meters. That is a very
small distance

You can think that an electron and a neutrino at 3E-17 meters can somehow
interact at distance, because it's very small. But this is not true. The
electron has to create one of the W+, Z0 and W- particles, send it to the
neutrino, and the neutrino has to absorb it. Even 3E-17 meters is a long
distance here, and it's possible to measure indirectly the properties of the
intermediate particles.

If the Em-Drive device somehow interacts with the floor, walls or the Earth
that are a few inches away, this is a extremely very big distance for the
current theories. The Em-drive must interact with some kind of field and
create some intermediate particles, like photons, gravitons or some new
particle.

The problem is that if the Em-drive using some field and particles to conserve
the energy and momentum locally, then the maximal theoretical output of a
device that don't break the currently accepted physics laws is 1/c = 0.0033
mN/KW. I couldn't find in this article the experimental results in China, but
it the recent "successful" experiment in NASA measured 4000x this value. [In
my opinion, it was an experimental error.]

Getting 0.0033 mN/KW is "easy" with a perfect light source, like a perfect
laser. Less is easier with a real light source, like a real laser. More is
theoretical impossible with the current physics laws.

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trelliscoded
I just wanted to point out that thrust without expending mass is already
possible without an emdrive.

Anything that asymmetrically radiates photons will generate a net
acceleration. This very mechanism has been experimentally verified as the
pioneer acceleration anomaly.

If the emdrive is some kind of special case of this mechanism, it's entirely
compatible with existing physics. The proposals so far are based on mechanisms
which are different, and incompatible with the existing understanding of the
universe.

It's also worth noting that in 2016 the Chinese research team was unable to
replicate their findings after accounting for a power cable that was
magnetically inducing a force on the experimental apparatus.

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beevai142
Photon rockets are not actually reactionless, because the emitted photons
carry momentum and energy out of the system. This makes conservation laws to
hold for such systems, cf.
[https://arxiv.org/abs/1506.00494](https://arxiv.org/abs/1506.00494)

Now, the emdrive people are apparently claiming that it is truly reactionless.
If so, you can build a perpetual motion machine of the first kind (produces
energy from nothing) out of it --- which should demonstrate the degree of
plausibility of such claims.

For photon rockets, there's apparently a hard upper bound on the efficiency,
which limits the practical usability.

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trelliscoded
You're right, what I really meant was radiation pressure doesn't require
reaction mass.

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gabrielblack
It smells like bullshit: no source, no paper, no verifiable test available. It
seems the classic scientific fraud that , resurrecting like a cockroach again
and again in the socials, has only the purpose to feed the conspiracy
theories.

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sfifs
I kind of expected China would fund a let's try it where measurements are
easier amd obvious and see approach.

Some experiments just cannot be done properly in of our gravity well. EmDrive
claims are tailored made for testing in space.

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IgorPartola
I thought it was shown that the thrust was due to the power cable's
electromagnetic field.

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madengr
No; See page 9 section 3 of paper.

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choxi
I'm surprised at how quiet SpaceX has been on this, the tech seems right up
their alley.

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akvadrako
You shouldn't be - all signs point to this being crackpot science. A couple of
the biggest reasons are:

1\. The magnitudes recorded in the several badly done experiments are very
small and there are plenty of possible experimental errors.

2\. The original inventor's own theory of how it works plainly violates very
fundamental physics, like how all frames of reference are equivalent.

There is really no reason to think this will work, except hope. Still, it's a
good idea to do some more research and put the idea to rest.

