
The 'impossible' EmDrive could reach Pluto in 18 months? - jonbaer
http://www.wired.co.uk/news/archive/2015-07/24/emdrive-space-drive-pluto-mission
======
searine
I've been following this story pretty closing for several months now. So
here's the opinion of an actual research scientist.

Q. Does the Em drive produce thrust?

A. Undetermined. Several tests from multiple independent labs have reported
thrust, including in a vacuum. However several of the labs that reported
positive thrust come from a sketchy background.

Q. Does it violate conservation of momentum?

A. Undetermined. There are several competing theories trying to explain the
phenomenon, but we lack raw data. I think the last thing anyone wants to see
here is "new physics".

Q. Roger Shawyer seems like a scummy kook?

A. Yes. This is why more testing is really important.

Q. What about more tests?

A. A few dozen people have taken it upon themselves to hack together emDrives
and test them. The first couple so far were plagued by poor construction, RF
interference, and poor experimental design. However, the hacking community is
slowly getting its shit together.

Q. Pluto in 18 months?

A. If it works. My intuition screams "its cold fusion all over again", but
there is still a chance.

~~~
javert
Thanks, nice summary.

> I think the last thing anyone wants to see here is "new physics".

I really want to see new physics, because I want to see us get off this rock
and far away. I think most people want to see new physics. I think that's why
this topic is so popular.

That said, I can understand _physicists_ not wanting to see new physics.

Wait. In a way, that sentence is _crazy_. In a way, physicists should want to
see new physics more than anybody.

Where does the "not wanting to see new physics" sentiment come from? Just not
wanting to be wrong?

~~~
cabinpark
I'd love to see new physics! Every physicist I know loves new physics.

But if you claim to violate the conversation of momentum, everyone is going to
be skeptical and not believe it. We have hundreds of years of experiments and
observations that support the law and have looked in many places from small to
large and not seen violations. This is what makes people not believe it and
thus physicists are going to assume you are wrong and not physics. It's not so
much a negative concern, but more of a practical one. But should it be tested
in a rigorous manner and reproduced, then physicists will believe it. It's
just it hasn't and thus we are going to stick with the established very well
understood and tested theory. Like are you sure you have properly counted all
the initial momentum? Are you 100% you are not introducing energy to the
system? The possible explanations is long and a good experiment will eliminate
as many of these as possible.

As an example, look at the OPERA experiment which found to have superluminal
signals. New physics? Nope, just a faulty piece of equipment. BICEP showing
gravitational waves? Nope, just poor data interpretation and neglecting
interstellar dust. So if you want to claim new physics, be damned sure you
have ironclad proof.

~~~
ttctciyf
> Every physicist I know loves new physics.

If physicists don't want new physics, they should stop doing research!

~~~
javert
Only as long as the new physics is in your little sphere of influence so you
can keep the grant money gravy train flowing. Presumably.

------
Niten
It's good to see that someone finally got around to testing this in a vacuum.
Here's the paper mentioned in this story:

[http://arc.aiaa.org/doi/abs/10.2514/6.2015-4083](http://arc.aiaa.org/doi/abs/10.2514/6.2015-4083)

High levels of skepticism are still warranted, as much as I would love for
this to pan out. Every time this gets tested—presumably more and more
carefully—the measured force gets smaller. The original test at Chinese
Northwestern Polytechnical University measured 720 mN; this latest test in a
vacuum chamber, only 20 μN. This is so tiny that a test anomaly would seem a
much more plausible explanation than a violation of the conservation of
momentum.

Of course the ultimate test would be to put one of these in space and try
accelerating something with it, in the same way that the ultimate test of a
perpetual motion machine would be to make one perform macro-scale net work.

~~~
Natsu
What would it take to get one of these into space?

~~~
lawlessone
from the looks of how testing is going, a good old fashioned rocket.

~~~
Natsu
Well yes, I don't expect a tiny fraction of a Newton of force to get them into
orbit, but I meant more as a way to test whether the anomalous force is usable
for spaceflight.

After all, it will either sit there and do nothing or it will move.

------
DenisM
Let's cut to the chase:

 _5\. Q. Why does the EmDrive not contravene the conservation of momentum when
it operates in free space?

A. The EmDrive cannot violate the conservation of momentum. The
electromagnetic wave momentum is built up in the resonating cavity, and is
transferred to the end walls upon reflection. The momentum gained by the
EmDrive plus the momentum lost by the electromagnetic wave equals zero. The
direction and acceleration that is measured, when the EmDrive is tested on a
dynamic test rig, comply with Newtons laws and confirm that the law of
conservation of momentum is satisfied._

[[http://emdrive.com/faq.html](http://emdrive.com/faq.html)]

So, the momentum simply "builds up". That's an exceptionally weak explanation.

~~~
yincrash
Can anyone explain electromagnetic momentum? I thought that em waves (photons)
are massless. How can they have momentum (mass * velocity)?

~~~
gizmo686
Because photons travel at the speed of light, we need to consider relativistic
effects. Under special relativity, classical momentum (mass * velocity) is not
actually conserved. To achieve conservation of mass, we consider the "inertial
mass" of an object instead of the classical mass, where the intertial mass is
given by:

    
    
        m'=ym
        y=1/sqrt(1-(v/c)^2)
    

As you can see, y>1 and y approaches infinity as you speed approaches the
speed of light. This is why we say that the mass of an object increases with
its velocity, and that any object with mass traveling at the speed of light
will have infinite mass.

Now consider the momentum, p, of a photon. We have:

    
    
         p=m*y*v
         m=0
         y=infinity
         v=c
    

this gives us p=0*infinity, which is indeterminate, so we cannot use this
equation to determine the momentum of a photon.

Instead, we can use the engery-momentum relationship, which states:

    
    
        E^2 = (mc^2)^2 + (pc)^2
    

(This is a generalization of the famous E=mc^2 equation to also consider the
momentum). In this equation m refers to the rest mass, not inertial mass.
Because we are dealing with a photon, we have m=0, which gives us:

    
    
        E^2=(pc)^2
        p=E/c
    

Indicating that the momentum of a massless object is proportional to its
energy.

~~~
nitrogen

        E^2 = (mc^2)^2 + (pc)^2
    

Is it just coincidence that this looks like the Pythagorean theorem?

~~~
jimmahoney
It's not entirely coincidence.

Four dimensional space-time has a metric analogous to but not quite the same
as euclidean space: the time part has the opposite sign from the space parts.

    
    
      ds^2 = (c dt)^2 - (dx^2 + dy^2 + dz^2)
    

ds is an "invariant proper time" which has the same value in all frames of
reference. Check any special relativity textbook for the details.

Just as you can start with distance and then build up to momentum and energy
in classical physics, in relativistic mechanics you can start with this metric
and build up vectors in 4-space for velocity and energy/momentum. (Turns out
that the time part is an energy while the space parts are momentum.) The
upshot is that

    
    
      (m_0 c^2)^2 = E^2 - (p c)^2
    

which is the frame-invariant length of the energy-momentum 4-vector. (I think
that m is better written as m_0, the rest mass, since the "m" notation
sometimes means relativistic mass, which is different.)

------
duskwuff
Making projections like "this drive could get a probe to Pluto in 18 months"
seems kind of premature, given that scientists still aren't certain that the
drive works _at all_ , let alone how efficient it actually is.

~~~
Semiapies
I'd say science reporting is a sad ghetto of journalism, but that's giving too
much credit to the profession as a whole.

------
dicroce
"WIRED understands that there are multiple labs around the world working on
their own EmDrives, although Tajmar's is the only new one willing to go public
so far."

I'm very curious about this bit. I wonder who is working on it?

~~~
Thorondor
NASA has a group working on it:
[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2014000...](http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140006052.pdf)

The Northwestern Polytechnical University in China is testing a version of the
EmDrive:
[https://dx.doi.org/10.1088%2F1674-1056%2F22%2F5%2F050301](https://dx.doi.org/10.1088%2F1674-1056%2F22%2F5%2F050301)

Finally, a number of amateurs are interested as well:
[http://emdrive.wiki/Building](http://emdrive.wiki/Building)

------
idlewords
Has any independent laboratory confirmed this effect and published on it in a
peer-reviewed journal?

~~~
gilgoomesh
Not exactly a journal but NASA reproduced the Cannae and EmDrive style devices
and documented it:

[http://ntrs.nasa.gov/search.jsp?R=20140006052](http://ntrs.nasa.gov/search.jsp?R=20140006052)

Turning 17 watts of microwave power into 40-91 micronewtons of thrust isn't
exactly stunning, though.

~~~
baq
from the wikipedia ion thruster article:

> Ion thrusters have an input power spanning 1–7 kilowatts, exhaust velocity
> 20–50 kilometers per second, thrust 20–250 millinewtons and efficiency
> 60–80%.[1][2]

3 orders of magnitude less thrust than for not needing to accelerate any fuel
all the way into orbit? i'll tell you space companies will send trucks full of
money to anyone who can make a reliable one.

------
foobarian
I wonder how they rule out the effects of the Earth's magnetic field. I can
see there being some small effect due to some kind of eddy currents pushing
ions into one direction. Maybe they turn the apparatus at multiple angles and
check the bias to north-south?

~~~
jboggan
I seem to recall that they could not find any variance in the force by
orienting the apparatus in different directions. Some good work has been put
into finding the cause of this, and most of the easy and obvious errors have
been tested and found to be unrelated to the effect.

------
leni536
Conversation of momentum is strongly tied to the conservation of energy
through relativity theory. If it actually works (I doubt it) it has _really_
strong implications.

~~~
repsilat
Conservation of momentum is a much bigger deal than conservation of energy. We
already know of situations in which energy isn't conserved (like the energy of
a photon decreasing when there is metric expansion where it is.)

Conservation of momentum is a consequence of the translational symmetry of
physics via Noether's theorem, so that's what we'd need to give up to hope to
see it broken.

~~~
leni536
Much like conservation of energy is a consequence of time translation
symmetry. If you mix spatial and time coordinates through relativity theory
you can see that these are strongly coupled.

Your photon example works because there is no time translational symmetry in
the case of metric expansion and it's a general relativity model. In general
relativity there is no conservation of momentum either. It would be hard to
even define since coordinate systems change from point to point. We can only
say that the energy-momentum vector is going through parallel transport, which
is like conservation in the local coordinate system.

------
stillsut
Question: why did someone build this drive if theory predicts it won't work?
Were they just tinkering or did they have an alternative theory from which
they could deduce it might work?

------
aaron695
Reddit has a sub forum that seems pretty active -

[https://www.reddit.com/r/EmDrive/](https://www.reddit.com/r/EmDrive/)

------
seesomesense
"They laughed at Columbus, they laughed at Fulton, they laughed at the Wright
brothers.

But they also laughed at Bozo the Clown."

------
ChuckMcM
Seriously, it isn't _that_ expensive to get a shot on a possibly blowing up
test rocket that is going into orbit, launch something and fly it out to mars
and back. We'll believe you then.

------
n0us
The worlds biggest unsolved mystery about microwaves has still not yet been
answered; how exactly does one microwave on HIGH?

~~~
jdmichal
While your comment doesn't add much to this discussion, I won't turn down a
chance to educate.

Conventional microwave ovens only have two states: on and off. (I won't rule
out the possibility of some exotic build having multiple powers, but the ones
you interact with on a normal basis don't.) So, power levels cannot change the
output level of the magnetron. Instead, the power levels control what
percentage of time the magnetron is on. A power level of 50% means the
magnetron is on for roughly 50% of the indicated time. These off periods will
typically result in more even heating at the cost of time. The off periods
give the heat a chance to conduct through the food and balance, reducing the
lava-and-ice problems seen when heating frozen things in microwaves.

HIGH just means 100% power level.

~~~
eropple
_> I won't rule out the possibility of some exotic build having multiple
powers, but the ones you interact with on a normal basis don't._

Mine does, and it cost $120.

[http://www.amazon.com/Panasonic-Countertop-Microwave-
Technol...](http://www.amazon.com/Panasonic-Countertop-Microwave-Technology-
NN-SD681S/dp/B005BFZ5N6)

(I don't find that it actually makes a difference when using it, though.)

~~~
PhantomGremlin
I have a Panasonic and it makes a difference. (or maybe it's just confirmation
bias?)

Where the Panasonic shines is at lower power settings. E.g. if you defrost at
30% power for 3 minutes you get much more pleasant results than using the
typical microwave. In a typical microwave the thinner areas actually start
cooking rather than simply defrosting when operating at 30% power.

~~~
jdmichal
I would expect it to make a bigger difference at low power settings, so I
don't think it's confirmation bias at all. 10 seconds of almost-high output
compared to 9 seconds of high output won't see much difference. However, 10
seconds of low output vs 2 seconds of high will see a big difference. Same
amount of energy, but delivered over a 5x longer time period.

