# Em drives

#### kamaz

##### Unicorn hunter
The argument made by the model is that the Poynting vector decreases over distance in such a way that when it hits the other wall, there's less energy flow and less momentum flow. <--- Herein lies the problem. If the energy flow and momentum flow weakens, where does it go? If we don't want to throw the conservation of momentum out of the window, it has to be reabsorbed by the device or dumped overboard. Dumping overboard would just create a typical photon rocket.
Precisely.

This demonstrates that there is a net energy flow from the small end towards the big end. Now, if the energy is flowing into the big end, it must be dissipated there somehow, either by localized heating or production of thrust by whatever mechanism.

Next, observe that radiation pressure of photons hitting a surface (think solar sail) is <S>/c, where <S> is average of Poynting vector over time, and c is the speed of light -- thus the force exerted on the sail with surface A is <S>*A/c. The simulation shows that <S> is zero on the wide end, while it is non-zero on the narrow end. This means that an (inward!) force is exerted on the narrow end, but not on the wide end. Thrust!

But there is still an (apparent?) violation of conservation of momentum. The force comes from the momentum of photons hitting the wall being transferred to the frustum: a photon flying towards the wall carrying the momentum of +p hits the frustum and transfers its momentum to it. So far so good; at this point conservation of momentum holds. But, conservation of momentum should also hold at the antenna, so the act of emission of a photon with momentum +p should have exerted momentum -p on the antenna. So the frustum should have received momentum -p at photon's emission and +p at absorption, resulting in the net momentum of zero.

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#### Urwumpe

##### Not funny anymore
Donator
But there is still an (apparent?) violation of conservation of momentum.
Apparent very likely. Its a very tiny factor that is not accounted in the energy balance.

#### RisingFury

##### OBSP developer
Precisely.

This demonstrates that there is a net energy flow from the small end towards the big end. Now, if the energy is flowing into the big end, it must be dissipated there somehow, either by localized heating or production of thrust by whatever mechanism.
Energy is NOT momentum!!! They're NOT the same, even if there's a link between the two!

When you emit a photon, the photon pushes you away. When that same photon hits the other wall, it stops you.

Next, observe that radiation pressure of photons hitting a surface (think solar sail) is <S>/c, where <S> is average of Poynting vector over time, and c is the speed of light -- thus the force exerted on the sail with surface A is <S>*A/c. The simulation shows that <S> is zero on the wide end, while it is non-zero on the narrow end. This means that an (inward!) force is exerted on the narrow end, but not on the wide end. Thrust!
Except that it ISN'T 0! That's the mistake in the model! That would mean that the energy just poofs and disappears! And with that, momentum would as well.

But there is still an (apparent?) violation of conservation of momentum. The force comes from the momentum of photons hitting the wall being transferred to the frustum: a photon flying towards the wall carrying the momentum of +p hits the frustum and transfers its momentum to it. So far so good; at this point conservation of momentum holds. But, conservation of momentum should also hold at the antenna, so the act of emission of a photon with momentum +p should have exerted momentum -p on the antenna. So the frustum should have received momentum -p at photon's emission and +p at absorption, resulting in the net momentum of zero.
This part is true, which is why the device doesn't produce any thrust and all observations so far have effects that are unaccounted for.

#### kamaz

##### Unicorn hunter
Except that it ISN'T 0! That's the mistake in the model! That would mean that the energy just poofs and disappears! And with that, momentum would as well.
Yes, but the program does not know that.

All this program does is solve Maxwell equations inside the em-drive cavity. And the solution is that at one end of the cavity, <S> != 0, while at the other end, <S>=0. We don't want to throw out Maxwell simply because we don't like the result, do we?

If Maxwell holds, then there is a good chance that the Poynting theorem will also hold:

This is where it gets interesting. If <S> is non-zero, then the first term of the equation is also non-zero. But since the equation must balance out to zero, then the energy pumped in via the first term must go out either via the middle terms or via the last term. It cannot be the last term, because this is simply a Lorentz force moving carriers in the direction of the Poynting vector (see textbook solution for the coax cable), but you don't have a conductor in the direction of the Poynting vector, because the cavity wall is perpenticular the Poynting vector!

That leaves the two middle terms which simply describe the energy stored locally in the E and B fields. So what this program tells you is that once you power the device on, it starts building up potentially very large E and B fields near the wide end.

And there is really nothing questionable up to this point. Assuming that the guy has simulated this correctly of course.

So then...

Assume hypothetically that there exists a yet undiscovered electrogravitic effect which manifests at very high densities of e-m fields. Or, to put it differently, assume that E/B and G fields are indeed coupled, but the coupling coefficient is very low -- so low, that the arising force is not observable most of the time.

But, since the device can build up a very strong E/B field at one end, at one point the field becomes strong enough that the electrogravitic effect starts to manifest itself. This explains the behavior seen in this video, where the force appears only several seconds after the device is powered up: it takes time to build up the field to values at which the EB/G coupling kicks in. Once that happens, the field energy starts being converted into kinetic energy (force).

---------- Post added at 08:17 AM ---------- Previous post was at 08:03 AM ----------

When you emit a photon, the photon pushes you away. When that same photon hits the other wall, it stops you.
Yes! This is why I'm saying that we still have an apparent CoM violation, even if we don't have CoE violation.

That said, observe that the wavelength of the photon is the same as the length of the cavity, so the photon cannot be treated as a particle and it is subject to Heinseberg's uncertainity. Does CoM really hold in such regime?

See also Casimir effect, which also appears to violate CoM: the plates are being pushed together, but where is the reaction mass?

#### RisingFury

##### OBSP developer
The field can't just build up. Why? Because if it gets high enough - in air to about 10 000 volts per cm, you'd get discharge. If the field did build up, this would essentially become a capacitor.

So you're arguing that a very dense EM field would couple with gravity. But we can make extremely dense EM fields and we haven't found anything yet. We can make fields dense enough to cause discharge and so can nature - you've seen lightning.

---------- Post added at 07:41 ---------- Previous post was at 07:28 ----------

That said, observe that the wavelength of the photon is the same as the length of the cavity, so the photon cannot be treated as a particle and it is subject to Heinseberg's uncertainity. Does CoM really hold in such regime?
That is completely wrong.

You can always treat a photon as a particle, no matter what its wavelength is. Keep in mind that wavelength gives it energy, however, where Heisenberg's uncertainty principle comes in is that you can't accurately determine the energy.

Heisenberg uncertainty stages that
$\Delta x \cdot \Delta p \geq \frac{\hbar}{2}$

And a photon has energy equal to
$E = p \cdot c$

So if you bring those together, you get
$\Delta x \cdot \Delta E \geq c \cdot \frac{\hbar}{2}$

The only thing this tells you is that you can't accurately know the energy of a photon. You can only know it to
$E\pm\Delta E$

This can be seen most clearly with the thickness of spectral emission and absorption lines. There are several effects that contribute to their thickness, but one of them is the uncertainty.

You can't just go throwing out quantum physics because you want to support a result you like.

---------- Post added at 07:44 ---------- Previous post was at 07:41 ----------

See also Casimir effect, which also appears to violate CoM: the plates are being pushed together, but where is the reaction mass?
There is no reaction mass - the photons deliver momentum!

#### kamaz

##### Unicorn hunter
The field can't just build up. Why? Because if it gets high enough - in air to about 10 000 volts per cm, you'd get discharge.
But of course!

So you're arguing that a very dense EM field would couple with gravity. But we can make extremely dense EM fields and we haven't found anything yet.
Here's something to think about: let's say that a 1kV/cm field over 1 cm^2 surface generates 1 mN of force via electrogravitic coupling. Do you really think that the effect could be noticed without specifically looking for it? Yes, such fields are common in electric machinery, but the alleged effect is so small that it is drowned by Lorentz forces, vibration, etc.

Or, to put it differently: the Romanian guy's em-drive genererated 5mN of force after pumping 1kW or so of power into it. A 1kW electric motor at 2000rpm has a torque of 5 N*m. It will be difficult for you to notice the anomalous 5mN force, particularly given the fact that the motor is axially symmetrical, so the e-m drive forces, even if present, will cancel themselves out.

The only thing this tells you is that you can't accurately know the energy of a photon.
But we are in a resonance cavity, so the energy is constrained by the cavity geometry!

---------- Post added at 09:43 AM ---------- Previous post was at 09:34 AM ----------

There is no reaction mass - the photons deliver momentum!
Yes, virtual photons deliver real momentum. Do you say that this is a CoM violation or not?

Now: if quantum vacuum is a sea of charged virtual particles, then if you apply E/B field you will cause these particles to move -- and there is your reaction mass.

#### kamaz

##### Unicorn hunter
Tajmar's paper is out:

https://hacked.com/scientists-confirm-impossible-em-drive-propulsion/
http://arc.aiaa.org/doi/abs/10.2514/6.2015-4083

We continued with testing on a torsion balance inside a vacuum chamber. Here we also found thrusts but quickly realized that there was a strong interaction with our magnetic damping system. Still we used this setup to test an EMDrive for the first time in high vacuum down to 4´10-6 mbar observing similar thrusts (although at somewhat lower power levels) ruling out any air influence in this configuration. After changing the position of the magnetron (outer position) and replacing the magnetic damping with oil fluid damping, surprisingly we could still observe thrusts that are indeed reversing with thruster orientation but with control runs in vertical direction producing similar thrusts compared to the positive direction. However, negative thrusts were only observed with firing the thruster indeed in a negative direction. Running the magnetron also in this direction at lower voltages produced similar positive values as the vertical control experiment. The thrusts observed with the oil-damped torsion balance were close to the original prediction taking our small Q factor into account (around +/- 20 μN for 700 W of microwavepower – still an order of magnitude more effective than pure radiation thrust). We also observed that the thrust appeared not to go down to zero immediately after power is switched-off but rather noted a gradual decrease as if the EMDrive was charged up and slowly reduced its thrust effect.

The nature of the thrusts observed is still unclear. Additional tests need to be carried out to study the magnetic interaction of the power feeding lines used for the liquid metal contacts. Our test campaign can not confirm or refute the claims of the EMDrive but intends to independently assess possible side-effects in the measurements methods used so far. Nevertheless, we do observe thrusts close to the magnitude of the actual predictions after eliminating many possible error sources that should warrant further investigation into the phenomena. Next steps include better magnetic shielding, further vacuum tests and improved EMDrive models with higher Q factors and electronics that allow tuning for optimal operation. As a worst case we may find how to effectively shield thrust balances from magnetic fields.

#### jedidia

##### shoemaker without legs
I would rather they find out if the minus first generation works before writing papers on the second generation... :shifty:

#### Urwumpe

##### Not funny anymore
Donator
I would rather they find out if the minus first generation works before writing papers on the second generation... :shifty:
I would do some many more things different there.

if you read past the clickbaiting, its really just a big Socrates saying "We know, that we don't know. But our not-knowing has become much better by now."

Not that you misunderstand me there: I also think that knowing that and what you don't know is a virtue. I just have the impression, that a lot of the unknowns are presented as "already solved" or "almost already solved" in the presentations, instead of sticking to a more sober, honest "We answered one question and got five far more exciting new questions to solve."

#### kamaz

##### Unicorn hunter
Tajmar's paper is pretty solid experimental work: we have accounted for all things we could think of, we still have force, we don't know why, don't ask us. Fair enough.

Shawyer's paper though...

Fig. 3 shows a diagram of a superconducting cavity mounted in a Dewar containing a liquefied gas coolant.
The internal cavity walls are coated with a High Temperature Superconducting material such as Yttrium Barium Copper Oxide (YBCO) which is superconducting at liquid Nitrogen temperature and below.

Microwave power, typically 20kW for a lift engine, is fed to the cavity via an input waveguide. To compensate for the decrease in frequency due to the Doppler shift under acceleration, the axial length of the cavity is increased by piezoelectric elements, mounted between the sidewall and the small end plate. A change in axial length of 22e-6m was calculated for a 915MHz cavity accelerating at 0.3m/s/s. The voltage controlling the length of the piezoelectric element is determined by a processor, fed by the output of an accelerometer.
What.

He's saying that if I am sitting in an accelerating car and I'm shining a laser pointer at the dashboard then I should see a change in light frequency. Hmmm.... Let me consult some SR textbook...

#### Urwumpe

##### Not funny anymore
Donator
He's saying that if I am sitting in an accelerating car and I'm shining a laser pointer at the dashboard then I should see a change in light frequency. Hmmm.... Let me consult some SR textbook...
Special Relativistics are wrong. General Relativistics as well. All old backwards traditionalist science meant to keep creative minds down.

#### mmendelbaum

##### New member
Good afternoon!
My name is Mikhael Mendelbaum. I'm from Russia.
I wrote an article about EmDrive. The paper describes version of the physical model and numerical calculations of mechanical force EmDrive.
Numerical calculations are in good agreement with experiments.
The model is based on two fundamental physical processes. It is
1) electronic polarization of the copper atoms of the surface layers by radiation of the magnetron and the reflected radiation and
2) the electric interaction of dipoles with this radiation.
Article in Russian. Sorry. But the formulas and numbers are international.
The article can be viewed at
http://new-idea.kulichki.net/pubfiles/150824164502.pdf
or
http://x-faq.ru/index.php?topic=3748.msg82636 .
I want to be useful to you.
Regards, Mikhail

Donator

#### RisingFury

##### OBSP developer
Both of these are the same as all others that came before. They find something shaking and attribute it to EMDriveForce. In Commic sans MS...

Nobody actually managed to eliminate any variables.

#### MaverickSawyer

##### Acolyte of the Probe
Okay, then. Time to build a flight article and test it in space. Can we fit one and its power supply in a cubesat?

#### Andy44

##### owner: Oil Creek Astronautix
Okay, then. Time to build a flight article and test it in space. Can we fit one and its power supply in a cubesat?
Sure. You want to pony up the cash for that? Maybe you can talk Elon Musk into it.