"Swimming" through spacetime and the EM drive.

[cristian]

I also wanted to draw your attention to an interesting problem related to the controversial EM drive,  and NASA experimental results. Actually, as you know, there are experiments performed all over the world in this direction,  including in UK, and in China (the Chinese recently reported positive results in space,  the existence of thrust ).

In Scientific American,  August 1, 2009, there is a nice article written by Eduardo Gueron ,

"Surprises from General Relativity : 'Swimming' in Spacetime ".

https://www.scientificamerican.com/article/surprises-from-general-relativity/#

Basically,  in curved spacetime,  the procedure used to determine the center of mass of a system of particles  (or a system characterized by a certain mass distribution ) can be ambiguous,  leading to different results for different procedures   (and this does not happen in flat spacetime ). That means that a physical system with a variable mass distribution of its components,  can actually travel through a curved spacetime. There is no violation of the conservation of momentum. Even if there is a displacement of the  center of mass of the system, due to a sequence of maneuvers, there is no continuing momentum associated to the system. At the end of the sequence, the system is stationary, as before the sequence started. It is essential that the inhomogeneities of the gravitational field and the size of the system should have the same order of magnitude.

What does this have to do with the EM drive?

We consider the equivalence between mass and energy. Gravity is not only sensitive to variable mass distributions,  but also to variable energy density regions. This is also related to the problem of the indefiniteness in the location of the electromagnetic field energy (this is also a problem emphasized by Feynman). Basically, we make an analogy here. The variable distribution of the regions of high energy density of the electromagnetic field in a resonant cavity is equivalent to a variable mass distribution of the system. In the tronconic EM resonator,  the variable distribution of electromagnetic energy density regions inside the resonant cavity is essential. For each sequence of distributions, the displacement of the center of mass of the system will be very small, but a rapid succession of such sequences will lead to a measurable effect.

For a cylindrical resonator,  a nice numerical simulation can be found at the following link:

http://demonstrations.wolfram.com/CylindricalCavityResonator/

The calculations have to be done for the EM drive tronconic resonator though . So, instead of a variable mass distribution of its components, the system will have a variable distribution of the electromagnetic energy density regions inside the tronconic resonant cavity. In curved spacetime,  if properly designed, such a system will be able to travel through the curved spacetime. A starting reference would be:

http://dspace.mit.edu/bitstream/handle/1721.1/6706/AIM-2002-017.pdf?sequence=2

I emphasize that nothing in this model assumes any new physics,  but the original references must be studied for a solid mathematical model,  not the popularization references mentioned above. Once the intuition is clear,  there is an open road for improving the design, using this model.

We reach the following conclusions. Shawyer (the original inventor of the system) has suggested that thrust is caused by radiation pressure imbalance between the two faces of the cavity. This explanations was assessed as incorrect by a number of well established physicists. Other explanations require new physics, and imply phenomena that have not been observed experimentally. The system will work only in curved spacetime. Gravity Probe B showed that the curvature of spacetime in Earth's vicinity is small but measurable. I suspect that there will be a measurable effect in orbit, close to Earth. This system cannot be used for interstellar travel. There is no constant force acting on the system (and no acceleration), just a measurable displacement of the center of mass. Also, the curvature of spacetime in interstellar space within our galaxy is probably too small, in order to make the system feasible.

This would be an interesting experiment though, related to the location of the energy of the electromagnetic field, very close the Feynman's suggestion.

[tchernik]

This is a hunch some people have. That it may work but only near a strong gravity field (curved space time).

All tests so far have been done on Earth or at its immediate vicinity, if we count the Chinese tests we have been hearing about of late.

If it shows to work in orbit, even if the forces are not that good, the next obvious step is to test it far away from Earth and see how it behaves. If there is such a dependency of space time curvature as you say, we should be able to notice a difference on the thrust in function of the distance from Earth or other massive bodies.

That would be an extremely interesting result as well, of course, because if it works and can be scaled, it could be used to radically change interplanetary travel.

Of course, that means no interstellar travel with it, but no relativistic kill vehicles either.

[sanman]

This is a hunch some people have. That it may work but only near a strong gravity field (curved space time).

All tests so far have been done on Earth or at its immediate vicinity, if we count the Chinese tests we have been hearing about of late.

If it shows to work in orbit, even if the forces are not that good, the next obvious step is to test it far away from Earth and see how it behaves. If there is such a dependency of space time curvature as you say, we should be able to notice a difference on the thrust in function of the distance from Earth or other massive bodies.

That would be an extremely interesting result as well, of course, because if it works and can be scaled, it could be used to radically change interplanetary travel.

Of course, that means no interstellar travel with it, but no relativistic kill vehicles either.

And yet even if that's the case - nobody knows until it can be tested - then could it be used as a new kind of gravitational slingshot and even a gravitational version of "aerocapture"?

You use it to help build up escape velocity to leave your planet or star system, to head for a neighboring one, and once you're in that new gravity well you can use it to further slow down and avoid overshoot.

When they talk about trips to Proxima Centauri, etc, they also talk about using lots of fuel/propellant to decelerate on approach, besides what's needed to accelerate to relativistic velocity to get there.

Even if this thing only has "traction" in a gravity well / curved space, that's still something to exploit for interstellar and trips, isn't it?

It would also imply that this thing is most effective in a steeper curvature, like a massive star or even outside a black hole's event horizon. Is that how spacecraft could be sent to study massive stars and even black holes some day in the far future?

Or in the nearer future, within this century, could a future iteration of the Galileo spacecraft be sent to study Jupiter more closely within its strong gravity well, with the EMdrive obtaining better "traction" in its vicinity?

[Stormbringer]

Not all deacceleration schemes require massive amounts of fuel. One scheme for example relies on an gigantic magnetoplasma bubble and the destination star system's stellar wind. I wonder if anyone has ever produced back of the envelope figures on how fast a notional plasma bubble could slow down such a probe using reasonable numbers for the various bits of the math? Say a probe weighing 1/4 of a ton traveling at X velocity slowing down with a plasma bubble 500 KM in diameter arriving at a typical M  dwarf, a k type and a G type star. could it do it in a single swing? If not how many passes would it take? Are such additional passes possible using standard orbital mechanics?

[sanman]

Also, are we saying that if EMdrive performance is sensitive to curvature of spacetime, that it ignores the Equivalency Principle? Perhaps that makes sense, since after all, its operation does imply a preferred frame (medium) to push off.

[cristian]

Thank you rgallinat for your comments (and everybody else who commented ).  I see that you understood most of the core of the idea.  Keep in mind that for each sequence  (cycle ) of variable distributions of high energy density regions inside the resonant cavity,  the displacement of the center of mass will be very small indeed.  For each unit of time interval (second )  though, there will be a very large number of cycles completed, leading to a total displacement of the center of mass that will be measurable.

As far as I can see,  the motion of the system will still follow a geodesic of the curved space,  connecting any two desired points (this has to be checked mathematically, I could be wrong  ). 

The correct design of the system will probably require solving systems of complex PDE's, and is suitable for computer simulation and analysis . I doubt that analytical solutions,  that you can post here  (as a back of the envelope presentation ) exist.

The purpose of my post is to draw the attention of experts to a possible model that does not require any new physics. I try to build the intuition,  related to this phenomenon.  I leave the hard work of checking the mathematical details for people with a stronger background than mine, and access to significant computational power.

If the mathematical model confirms my intuition,  I have a further highly speculative question to ask. Can such a system "swim" in its own gravitational field, assuming that it has a sufficiently large mass?  This last question has nothing to do with the model presented in my post, which is based on solid mathematics.  This last question is just for fun..

The whole idea is rooted in an experiment first proposed by Feynman,  as I explained in my post.

Yours truly,
Cristian. 

[SergioZ82]

Without diminish the impact of an invention like this, the EmDrive can be seen as a rearranged microwave oven. Hence I believe it can be explained without bringing relativity into it.
I think Shawyer's hypothesis (electrodynamics) is the base where to start. First because it's what made EmDrive possible and second because it'd better to start with what scientists already know without venturing into unexplored hypothesis that makes this device even more puzzling.

My 2 cents, of course

For Emidio Laureti the key for PNN is purely electrodynamics (non existence of displacement current) and he believes it can explain EmDrive too. Actually he's going to put out into public domain his theory (after his patent will be granted and his "PNN Ltd" will be operative)

[cristian]

After a ballpark estimation, I realized that this approach cannot possibly explain the observed thrust in EM drive experiments. For a different approach, that might be able to explain it, please see my reply to EM drive developments, thread 9. It seems to me that the dynamical Casimir effect and the Schwinger effect might be the key.

New Physics for Space Technology / Re: EM Drive Developments - related to space flight applications - Thread 9

I appreciate all the useful comments.

[Vultur]

Of course, that means no interstellar travel with it, but no relativistic kill vehicles either.

Isn't the thrust way too low for anything of that sort? (at least unless you had launch platforms beyond Pluto, which certainly isn't a near-term problem)

[LowerAtmosphere]

After a ballpark estimation, I realized that this approach cannot possibly explain the observed thrust in EM drive experiments. For a different approach, that might be able to explain it, please see my reply to EM drive developments, thread 9. It seems to me that the dynamical Casimir effect and the Schwinger effect might be the key.

New Physics for Space Technology / Re: EM Drive Developments - related to space flight applications - Thread 9

I appreciate all the useful comments.

Sure. You will find a large amount of debate on this topic throughout this forum and elsewhere. EM Drives following this theory have been dubbed the Q Thruster, where Q stands for Quantum Vacuum. Hence the unresolved question whether the vacuum is mutable and degradable, in other words whether we can borrow energy from it at all. This theory of operation is a favourite of many institutional actors, but it is very difficult to prove.

Sorry, but positrons would likely not be detected since they have a tendency to annihilate with electrons rather rapidly. Even if you did detect them, then it would definitely not be conclusive proof of this theory as they could just be part of randomly occurring virtual pairs.

Edit: However, as I have argued before, an InGaAs array attached to the end should be sensitive enough - down to the individual photon - to measure additional radiation which tunnels out or penetrates the walls, but again doesn't prove much in particular. There can be an argument made that detection of anomalous gravitational fields/mass, using an interferometer (or super sensitive scale), beyond the predicted classical mass which the system should have if it only contained the input EM waves, may be a better test. The issue remains that the quantum vacuum is not an easy buffet for energy and mass. Essentially proving it is mutable and degradable AND can be used for a Q thruster, would be a revolutionary paradigm-shifting experiment to successfully perform, and it would probably be easier to do at CERN than at home.   

[Stormbringer]

If they did eliminate; wouldn't the signature to look for be either a gamma ray or an X ray? That would seem easier to detect than a single particle or anti particle...

Positron electron annihilation is one of the schemes predicted /advocated for for early Antimatter powered  propulsion drives. These schemes most often involve converting the energy of the resulting high energy photons into thermal energy.