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?


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.


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.

  • 2
    I love the last paragraph. You could use Bobbles in the construction industry to flatten a slope, or at least move it around a bit. – Jontia Jun 1 '18 at 12:01

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.


I liked the post about the quantum fields creating mass. That was a cool point that I hadn't thought of.

There are two possibilities about mass distributions and whether they are detectable outside, and either one creates problems with the laws of physics. Either the information about the mass distribution is destroyed and inaccessible from outside, such that the bobble treats all internal mass as central (like a black hole), or the information about the internal mass distribution is preserved and detectable, such that the bobble itself has non-uniform gravity and moment of inertia. The former option violates conservation of angular momentum during creation and destruction of the bobble, since the angular momentum of an asymmetric rotating mass + bobble system would be instantaneously changed.

Regarding the latter: the last post is arguing that the mass distribution is preserved, which allows you to exchange angular momentum with the bobble eg. by bouncing objects off the sphere obliquely to the center of mass. This means the mass distribution is detectable, either by measurement of rotation change or elsewise by gravity.

Here's how detectability of mass distributions in the bobble breaks the laws of physics. If a bobble is created around objects with initial angular momentum non-symmetric to the bobble, this implies that upon creation the entire bobble must rotate (wobble) about a non-central mass distribution. This means the contents inside are now effectively along for the ride with a wobbling rotation. The book also says you can boost the bobble as a whole to change momentum. So presumably angular momentum can be changed too. Once the bobble ends, that change in (angular) momentum (and energy) must either be distributed among the contents of the bobble or to the region surrounding it. Otherwise it is destroyed, violating conservation of momentum.

It's transferring that momentum upon bobble collapse that creates problems. There is no way to distribute that linear or angular momentum symmetrically in a way that makes physical sense, since the bobble is not supposed to communicate with its contents. If the bobble did transfer momentum to its contents upon destruction you have FTL "timelike" communication within the sphere. Of course such issues are unavoidable during creation. So upon the start and end of the bobble there is a causality violation. Such issues are avoidable at the end of the bobble if that momentum is somehow "stored" on the surface and transferred to the immediate surroundings at the finite speed of light.

If upon collapse the bobble, the contents no longer have the momentum that the bobble contained, but rather that momentum is transferred to the surroundings in some kind of way from the surface, then bobbles would presumably cause an "explosion" once the bobble ends. But what if the net interaction/communication with the bobble is to lose momentum? It's perfectly possible to reduce the momentum of the bobble. That momentum would then have to be transferred into the spherical region when the bobble ends, eg it would have a local "implosion". This would have to imply that information about momentum change is somehow stored on the surface, which can be either positive or negative and must have vector information. There is no way for that information to be stored on the boundary because you can't store information about momentum without it being associated with mass or energy in motion, so this implies that the bobble surface itself has surface-variable energy/mass. Kind of like a freaky scifi holographic principle. OF course we still have the causality violation upon bobble creation.

Violation of causality is just as grievous a physics violation as a conservation law. For example, you can use a bobble to communicate faster than light. Let's say you create one on a light-year scale. It can either have a planet inside the bubble or outside, let's say it's on the boundary and we can control the size just right to either exclude or include it. Since the bobble then has the momentum, angular momentum, and mass of the planet (or not), that information is communicated instantaneously across the bobble surface. This is required since time is stopped inside, the creation of the bobble is automatically "timelike". So you can transfer instantly the planet's momentum to an object a light-year away by bouncing it off the bobble surface. You can also transfer momentum away from a gravitational potential, and mine this to violate the conservation of energy.

What this whole exercise tells us is that there's a deep connection between momentum, angular momentum, and causality. Ordinarily you picture causality as being associated with time, but this perhaps tells us that the real important quantity is spacetime, as we all knew. The universe is held together with such self-consistency that it's amazing how hard it is to poke holes in the laws.

All this says is that these bobble are physically impossible if you start delving into the physics, precisely because this mass distribution issue allows information and momentum to be exchanged with the bobble. If the mass distribution information is not accessible then this violates conservation of angular momentum and other things. If the mass distribution information is accessible this still violates causality and energy conservation.

  • How does this answer the question? – amflare May 31 '18 at 16:29
  • it answers the detectability of mass distributions question using the laws of physics. The question is a good one because it leads down the path of the bobbles being impossible. – sean l May 31 '18 at 16:38

Sticking to the earth does not necessarily imply mass (at least, not rest mass); photons don't have mass, yet are attracted to massive bodies. In GR, gravity is a manifestation of curved spacetime, and all object see the same spacetime, regardless of mass.

However, there are other arguments for them having mass, some of them articulated in other answers. Here's another one:

Suppose bobbles have no mass. Then you could put a massive object in a bobble, raise the bobble in a gravitational field, wait for the bobble to "pop", drop the contents, and get work out of the cycle. This violates conservation of energy. We can similarly argue that other conservation laws demand that the mass distribution remain the same.

A principle that I think is reasonable to adopt is that in science fiction, we should assume that the physics are the same as much as possible; if there is an interpretation consistent with our physics, and another inconsistent, then given a choice between the two we should consider the first to be canon, absent some compelling reason. Vinge does mention in his Witling novel, in which characters are able to teleport, that teleportation observes conservation of energy and linear momentum, but effectively admits to being unable to find a way to harmonize teleportation with conservation with angular momentum. This suggests that we should assume an intent that as many laws as possible be followed.

sean l says that "the bobble is not supposed to communicate with its contents", and this contradicts mass distribution being preserved. First, I don't think this lack of communication is of equal priority with conservation laws. Second, I don't think that it is reasonable to consider the stories involving bobbles to have a truly omniscient narrator.

They do have an "omniscient narrator" in the literary sense of descriptions of events being described without being ascribed to any particular character, and no one character being able to witness all of the events described. But I think that it is clear that the characters are initially extremely ignorant of the workings of bobbles (my understanding is that originally, it was generally assumed that bobbles were permanent), and slowly become less ignorant, but even after they have advanced bobble technology, there is still an element of their understanding only extends to the practical level of how to do things with bobbles, not what is happening or why.

Any description of the outside world not being able to interact with the contents of a bobble should be taken as merely that, in-universe, the characters' understanding is such. They bobble an object, and when the bobble ends, the object is in the state, apart from alterations such as possibly the bobble as a whole being moved to another location. Maybe the contents are frozen in time. Maybe a black hole is created, and when the bobble ends, it evaporates into Hawking radiation that exactly replicates the original object. Maybe leprechauns appear and put the object in their pocket. The characters don't know what exactly happens, and Vinge doesn't tell us (and likely doesn't know himself). All we know is what goes in and what comes out. I submit that we should take the action of a bobble is that what comes out of a bobble is whatever is most similar to the original object that is consistent with physical laws. Apart from what is demanded by physical laws, the contents are unchanged, and apart from what is demanded by physical laws, the contents do not affect the outside universe for the duration of the bobble.

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