![]() On the other hand, you raise a very good point. assuming frictionless motion of heavy macro objects because you approximate the space to be empty). But the density is so low that it can be assumed to vanish for considering the kind of physics we know from every day life (e.g. trying to figure out how this dust behaves). But since there is also a lot of stuff in the intergalactic medium (ultra low density dust?), for some questions this can not be assumed to vanish (e.g. Usually space between galaxies (or even stars) is assumed to be empty when considering questions in cosmology. I think that depends on how you define a "gravity field".īut it also depends on how crude you approximate or what situation you are interested in. But out there, where it's lightyears and lightyears to the nearest object, you could be careening through the universe at thousands of miles per hour, but how would you know? We can tell that we are experiencing gravity in free fall because we can see the ground approaching. We can tell that we are experiencing gravity here on Earth because we are constantly pushing down on the Earth. ![]() Between galaxies, for example, gravitational acceleration would seem to be non-existent because of the vast amount of space. The good news is that for practical purposes, space is so big that there are plenty of places you could go that would seem like there is no gravity. Of course, in any practical sense, there would be no difference between the stationary object and the one moving in a straight line, and in a universe with completely flat spacetime, neither space nor time would seem very familiar to us, and we wouldn't have a reference for either. If we gave it a push, it would continue in a straight line forever. If we could somehow add a non-object with no mass, it would simply stay in one place forever. Supposing we did have a universe with no matter, what we would find is that spacetime would be completely flat. ![]() This might seem like a meaningless nitpick since the object won't affect its own trajectory, but spacetime itself would be affected (this is what gravity is, afterall). If you added an object to check its trajectory, there would no longer be no gravity. To my knowledge, the only way to have a region of space greater than a single point with no gravity whatsoever is to have no matter in the universe. On the second part of your question, about the trajectory of an object in space free of gravity, no such situation could exist. It may be possible that there are points somewhere in the universe where, at least momentarily, net gravitational acceleration is zero because all the accelerations from every object in the universe sum to zero, but again I don't know how we would recognize this situation if it ever occurred. It would be assumed negligibly small but technically speaking would still exist. But even so, external bodies not counted in the model would still exert gravitational force on anything placed in those points. There are five such points, L1 through L5. This is not exactly what you're looking for, but demonstrates how gravitational acceleration via multiple bodies can net out to a value less in magnitude than either individual acceleration. The precedent is laid down in the concept of Lagrangian points, points in a two-body system where net gravitational acceleration is matched perfectly by the centripetal acceleration required for the point to retain its relationship to the bodies. Given the above principle, it would probably be impossible to say for certain whether any point is exactly zero in a universe with more matter than we could possibly take into account, but in theory, such points could exist. Or three or more might combine to form a net of zero. Acceleration is a vector, and as such acceleration equal in magnitude but opposite in direction would net out to zero. However, there does exist the potential for net gravitational acceleration to be equal to zero. Gravitational acceleration, then, exists at every point in the universe if there is any matter at all. Without finding one, we don't know for sure, and if we did, I don't know how we'd recognize it, but one of the basic principles of gravity in theory is that gravitational forces exist between every pair of objects in the universe.
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