The Pioneer Anomaly and Vacuum Energy

[josephwouk]

One of the proposed explanations for the Pioneer Anomaly is that the additional deceleration is caused by interaction with the vacuum energy.

http://en.wikipedia.org/wiki/Pioneer_anomoly#Definition_of_inertia_modified

This makes sense to me, given that some of the particles must actually be impacted during their extremely short lifetime during the course of the flight. Clearly it is a very small number, such that the effect would only be noticed over the course of a very long distance such as that traversed by the Pioneer spacecraft .

Do any of you know of any experiments, proposed or accomplished, that attempt to measure whether or not vacuum energy in fact creates a "drag," however small, on objects passing through it?

Thanks again for your help...

 

[JesseM]

One of the proposed explanations for the Pioneer Anomaly is that the additional deceleration is caused by interaction with the vacuum energy.

http://en.wikipedia.org/wiki/Pioneer_anomoly#Definition_of_inertia_modified

This makes sense to me, given that some of the particles must actually be impacted during their extremely short lifetime during the course of the flight. Clearly it is a very small number, such that the effect would only be noticed over the course of a very long distance such as that traversed by the Pioneer spacecraft .

Do any of you know of any experiments, proposed or accomplished, that attempt to measure whether or not vacuum energy in fact creates a "drag," however small, on objects passing through it?

Thanks again for your help...

That doesn't seem to make much sense in quantum field theory (where the notion of 'vacuum energy' comes from) because there is no notion of being at rest relative to the vacuum or in motion relative to it, vacuum energy should be measured to behave the same way in all frames. "Drag" seems to presuppose such a rest frame, the way we experience drag when in motion relative to the average rest frame of the air around us but not when we are at rest relative to that frame.

 

[josephwouk]

So you explain the casimir effect how?

 

[JesseM]

So you explain the casimir effect how?

Not sure what you're getting at, the an observer traveling at 0.99c relative to the Earth, and carrying a pair of parallel plates would observe the casimir effect to work exactly the same as an observer at rest relative to the Earth with her own identical pair of plates. The basic explanation I've seen is that only virtual photons whose wavelength fits an even number of times into the distance between the plates can arise, so there are fewer possible virtual photons between the plates than in an open region of space and thus the vacuum energy is lower between the plates than in free space and this pulls the plates towards one another.

 

[josephwouk]

Jesse...

You say it "pulls" the plates together. Why not that the excess force outside the plates "pushes" them together?

Thanks again!

 

[JesseM]

You say it "pulls" the plates together. Why not that the excess force outside the plates "pushes" them together?

Yes, verbal formulations are by nature less precise than mathematical ones so I'd say either version is fine, the point is that it's the difference in vacuum pressure between the inner side and outer side of the plates that causes them to move towards each other.

 

[josephwouk]

Jesse...

I agree that words are often pesky things when applied to physics. While I'm a writer by profession and try to use words in the clearest and most accurate fashion, it is often extremely difficult and sometimes seemingly impossible to use them to accurately describe the meaning behind the mathematics of physics.

That being said, I don't believe this is one of those cases. The Casimir effect shows that vacuum energy is capable of exerting force on macro objects. I assume that it is this fact that underlies those who theorize that the Pioneer Anomaly is caused by the vacuum energy.

Do you understand how they possibly get around the rest/motion objection that you raised earlier to that possibility?

 

[JesseM]

That being said, I don't believe this is one of those cases. The Casimir effect shows that vacuum energy is capable of exerting force on macro objects. I assume that it is this fact that underlies those who theorize that the Pioneer Anomaly is caused by the vacuum energy.

Do you understand how they possibly get around the rest/motion objection that you raised earlier to that possibility?

The speculation in the paper by Mike McCulloch in the wikipedia article you linked to seems to be that the effect has to do with acceleration of the craft, something to do with the Unruh effect seen by accelerating observers in quantum field theory. The Unruh effect doesn't require the vacuum to have any particular rest frame, as indeed it doesn't in quantum field theory. There's some discussion of McCulloch's idea in this blog entry if you're interested, but my impression is that the idea of explaining inertia (the resistance of massive objects to being accelerated) in terms of the Unruh effect is a speculative idea that most physicists don't think is likely. This page on a modification to general relativity known as MOND offers some problems with the idea:

While the Unruh-like radiation may well serve as a marker for non-inertial motions it is still difficult to implicate it directly in the generation of inertia: 1. It is not clear that it carries all the information on the motion needed to produce inertia. For example, even for hyperbolic motion, can the direction of its acceleration be told by the accelerated observer (it should be remembered that the radiation is characterized by more than just its spectrum. For example, a finite size observer can compare the radiation in its different parts.) 2. If inertia is local-as it is to a very good approximation in the non-MOND regime-it has to adjust instantaneously to the state of motion. The latter may change however on time scales that are short compared with the typical period of the Unruh-like radiation.