Is Gravity Control Propulsion viable? Part 2

So far Gravity Control Propulsion doesn't look promising, we know spacetime tells mass how to move and mass tells spacetime how to curve (gravity) however with our current understanding of GR and QFT we don't understand the underlying quantum processes that causes this, GR however models the above very well. In QFT, a hypothetical particle called the graviton is thought to mediate gravity. Problem is there is no means to detect a single graviton by any experiment today or the foreseeable future, they are predicted to have no mass and no electric charge hence do not interact with photons and the absorption of a single graviton by a particle would only change its spin or position however the change would be indistinguishable from a quantum fluctuation. So this will remain a hypothetical particle, although detecting gravitational waves would be possible, this wouldn't  confirm the graviton hypothesis since other models can explain gravitational waves.


The Gravity Probe B experiment confirmed spacetime effects predicted by GR. Image: NASA

Is it possible for mass  not to tell spacetime how to curve? From the previous post, mass doesn't appear to influence \(h\), the gravitational shielding constant, in any way which makes sense since mass and energy \((E=mc^{2})\) are the cause of the spacetime curvature in the first place. What if we modify the vacuum energy density? In 2013 this can be done in the lab (with great difficulty) by Casimir plates.

 

Let's have a look at a third interesting paper that touches on these topics called Gedanken experiments with Casimir forces, vacuum energy, and gravity by Gordon Maclay. In the abstract it is mentioned "we demonstrate that a change \(\Delta E\) in vacuum energy, whether positive or negative with respect to the free field, corresponds to an equivalent inertial mass and equivalent gravitational mass \(\Delta M=\Delta E/c^{2}\)." He also looks at the energy considerations of a hypothetical gravitational shield. Further on "We are interested in considering several aspects of vacuum energy and Casimir forces, including the inertial mass associated with vacuum energy, the interaction of vacuum energy and gravity, and the possibilities of utilizing vacuum energy for propulsion or other purposes.". Three concepts of mass are outlined:

• gravi-inertial mass: inertial mass that resists acceleration.
• active gravitational mass: mass that generates a gravity field around it.
• passive gravitational mass: mass that reacts to a gravitational field.

In GR, all three are equal and equivalent, for the purposes of this discussion however it is useful to look at the three concepts separately. It is mentioned that the vacuum field seems to contribute to inertial mass and "the general consensus is that only changes in vacuum energy act as a source of a gravitational field".  Several interesting gedanken experiments are outlined to answer the following questions:

 

1. Is a change in inertia of a system associated with a change in the vacuum energy of the system?
2. Is a gravitational field generated by the change in vacuum energy (equivalent active gravitational mass)?
3. If an external gravitational field is present, is there a change in the gravitational energy of the system that is associated with the change of vacuum energy (equivalent passive gravitational mass)?

The interesting (yet unconfirmed by experiment) Scharnhorst effect is also mentioned. Why resort to gendanken experiments (thought experiments)? Because the quantum vacuum effects looked at are so small, it makes it a real challenge with 2013 lab technology to measure these with enough precision. High precision experiments in Physics unfortunetly means high costs. There's an interesting quote from the late Arthur C. Clarke:

 

"If vacuum fluctuations can be harnessed for propulsion by anyone besides science-fiction writers, the purely engineering problems of interstellar flight would be solved."

In Gedanken Experiment two, the author refers to another paper: A Gedanken spacecraft that operates using the quantum vacuum (Dynamic Casimir effect). Unfortunetly the thrust generated if confirmed is tiny although interesting as the Dynamic Casimir Effect was only verified recently in the lab in 2011, where mirrors (SQUIDs) are vibrated very fast at \(\frac{1}{4}c\) which created real photons out of the quantum vacuum. It's very unlikely that down the track this Gedanken spacecraft will replace rockets anytime soon.

Casimir plates partly suppressing vacuum fluctuations. Image: Wiki

In Gedanken Experiment Three: Vacuum Energy Contributes to Inertial Mass, the paper considers an isolated sphere with a battery operated motor which can move Casimir plates inside the sphere. As the plates are moved closer, what happens to the total energy of the system? Total energy of the sphere is conserved however the distribution of the energy within the system has changed from the vacuum energy between the plates and the battery. The paper "suggests that it might be possible to make components that have negative inertial mass. Such objects would tend to rise in a uniform gravitational field. Indeed negative vacuum energy in the stack of parallel plate capacitors considered theoretically by Calloni et al resulted in a force in a gravitational field that was in the opposite direction from that experienced by normal positive matter, but the positive force due to the mass of the silicon wafers, was much larger. Could one make an object that floated in a gravitational field?" 

 

One needs to be careful here with the terms "negative vacuum energy" and "negative inertial mass."  Lower vacuum energy between the Casimir plates, by restricting the wavelengths of photons and reducing the vacuum energy does not necessarily imply a negative vacuum energy compared to the vacuum energy outside the plates. It is a lower energy density state compared to the outside environment but does not imply negative energy and mass, the distinction is important and here the answer to the author's last question would be no. Negative mass also has not been observed in Nature so this is a hypothetical concept. Nevertheless the paper proposes an experiment to measure the ratio of the gravitational force to the Casimir force. Note that we are not talking about anti-matter here (which is also predicted to fall with gravity just like normal matter). Here's what Schiller also has to say on negative mass in Motion Mountain Vol 1, p98:

 

"Indeed, a negative (inertial) mass would mean that such a body would move in the opposite direction of any applied force or acceleration. Such a body could not be kept in a box; it would break through any wall trying to stop it. Strangely enough, negative mass bodies would still fall downwards in the field of a large positive mass (though more slowly than an equivalent positive mass). Are you able to confirm this? However, a small positive mass object would float away from a large negative-mass body, as you can easily deduce by comparing the various accelerations involved. A positive and a negative mass of the same value would stay at constant distance and spontaneously accelerate away along the line connecting the two masses. Note that both energy and momentum are conserved in all these situations. Negative-mass bodies have never been observed. Antimatter, which will be discussed later, also has positive mass."

Gedanken Experiment three doesn't look promising. GE 4 shows that "Vacuum energy couples to gravity the same way any other form of energy is expected to couple to gravity.", all forms of energy (mass) couple to gravity as shown further in GE 5. GE 7 concludes that "our assumption that vacuum energy does not contribute to active gravitational mass is not true."

In the last GE 8, the paper looks at energy considerations of a hypothetical gravity shield and asks "Would a box that shields against vacuum fluctuations be fundamentally impossible?"

The paper has been useful in outlining various concepts related to the quantum vacuum, gravity and mass. It is clear that more experiments are needed to answer some of the questions. It would be useful for eg to be able to modify the vacuum energy without having to resort to Casimir plates to carry out experiments in this difficult field and to be able to carry out measurements with other geometries other then parallel plates and spheres to verify the various models of Casimir forces in these conditions. Is Quantum Vacuum Engineering a viable field in the future for eg? In the next part we'll have a deeper look at how the quantum vacuum might contribute to gravity.

CI.

Update: Read this post from Sean Caroll, it has an interesting discussion in the comments section on the Higgs particle, inertia and mass.