Q:

Are the mass properties of bobbles well defined?

In Peace War and Marooned in Realtime, bobbles are spheres that are completely reflective of electromagnetic energy.  Within them, time stops.  From my recollection, it seems like the bobble still has as much mass as it's contents (meaning it stays stuck to the Earth).
Is the distribution of mass within a bobble detectable from the outside? If there was mostly air but there was a ton of bricks on one side, could you detect that distribution from the outside?  Does the fact that time is stopped within cause any issues with the idea that it still has mass?  Is any of this spelled out or hinted at in the books?

A:

The bobble technology is ill defined but its properties were were able to be interpreted by what couldn't be done to them. It can be assumed the mass of the object bubbled would remain constant. So a city put into a bobble and connected to a thrusting platform would need sufficient propulsion to move the expected mass.
Bobbles were a multi-use technology and were defined as:

a device which can create a "bobble", a spherical stasis field in which time stands still, allowing one-way time travel into the future. These frictionless, perfectly reflective spheres are also used as weapons, as shields against other weapons, for storage, for space travel (combined with nuclear pulse propulsion), and other purposes.

From within the bobble, nothing has changed. The objects have the exact same mass, density and overall existence they have always had. In fact, they are frozen in space-time, linked to the position on the planet they have when the bobble is activated. All appreciable activity is ceased. Activity at the quantum level is questionable, since it barely constitutes activity at all.
Nor can any aspect of the bobbled subjects be changed as long as the bobble is active. No entropy, no time, no molecular activity takes place. This technology literally stops all interaction with the universe at large. This makes bobbles invulnerable to all forms of harm, from the cellular to the nuclear, it is impossible to harm something that has been placed in a bobble field. 
From the outside of a bobble, it is impossible to determine anything about what is inside the bobble, except for its mass, since no electromagnetic energy can penetrate the sphere of frozen reality inside. From the outside of a bobble, it might as well be nothing since no interaction with any form of the unified field, except for the bobble tech itself is active.

A:

This is never directly addressed in the books, but I think we know the answer.
Bobbles retain their mass/density properties, as shown in the key plot point in Peace War when a bobble floats when the air temperature changes. And the use of flicker bobble techniques implies that the mass distribution is unchanged while bobbled.  Otherwise mass would effectively shake around during flickers.  Thus, while Vandenberg was bobbled, the lower half of the bobble had the density of earth, and the upper half of the bobble had the density of air.  
Because there's zero friction between the skin of the bobble and the outside world, this implies that if you bobbled something that was lopsided in mass distribution, the bobble would rotate so that the center of mass was below the center of the sphere.  

A:

It is certainly made clear in The Peace War that bobbles retain their mass-density properties.  However, it's not clear how this can be made consistent with physics.
In addition (obviously) to not knowing anything about bobbles beyond what's described in the Realtime series, we also don't really have a final theory of gravitation or mass.  In particular, the best theory we have of gravitation is known to be wrong.  However, it seems certain that the gravitational interaction propagates at c.  As far as we know, 5% of the mass of ordinary matter comes from interactions with the Higgs Field, and the other 95% comes from the force that binds protons and neutrons together -- the strong nuclear force.  But of course it takes time for the strong force to work (the mass comes from virtual gluons constantly popping into and out of existence), and this can't happen if time is stopped.  Since time doesn't pass inside a bobble, the mass inside the bobble must be zero.
However, the bobble itself must have mass, from E = mc^2 if nothing else.  There isn't much to the bobble, so the mass must be proportional to the volume enclosed, radius, or surface area.  The last seems most likely, since the bobble is hollow and this gives unit mass per surface area.  If that's the case, the density of the bobble is inversely proportional to its radius, so in an atmosphere, bobbles will float at an altitude given by their radius: small bobbles will be near the ground or on it, larger ones floating at increasing heights.  
This is also one of the many properties that make bobbles the dream vehicles for space travel...more on that later.

