I have to be misunderstanding something; The abovequoted, by itself anyway, seems to me to mean that there is a distinction between the two kinds of gravity (or “gravity”) and so there could be tech that negates the one without negating the other.
The difference is if someone was motionlessly hovering motionless above a rotating ringworld (assume no other forces for this example) and the ringworld disappeared, they would remain in the same location. If you where in orbit and the earth disappeared your ‘direction’ would change and you would fly off into space.
I have been reading quite a bit on the subject, trying to figure out if gravitons and mass curving space (space/time?) are two different theories or just two different ways of looking at the same thing.
To me it seems if gravitational attraction is caused by gravitons then theoretically there could be some way to ‘block’ the gravitons. To block ‘gravity’ felt from acceleration you would have to find a way to block mass? Would they be the same thing?
I’m sure it’s painfully obvious but I have no formal education on this subject, it’s just something I do some reading on because I find it intersting. And the book ‘Three roads to Quantam Gravity’ doesn’t seem to be about quantum gravity at all unless that part was over my head.
I figured there would be a clearly correct answer to the question raised in the OP, but with the level of disagreement here I’m wondering if it is actually an unsettled question in physics.
Could anybody from the SDSAB help us out here? Or even, The Master?
The biggest problem is that there’s nothing in our understanding of gravity to suggest that anti-gravity is possible, so it’s kind of hard to answer intelligently a question that assumes that it is.
Well, but who’s disagreeing with who? Are there demonstrably knowledgable people disagreeing, or is it that the knowledgable people are saying one thing while others are basically expressing a lack of understanding? (I take myself to be in the latter category, of course…)
I’m less interested in the question of anti-gravity than I am in the question of whether artificial gravity (due to rotation) can be treated, mathematically, as being completely of a piece with natural gravity. It looks like the knowledgable people here are saying that it can–but I haven’t understood how this is possible yet.
I think it boils down ultimately to the fact that I’ve never understood how it can be that accelerational forces and gravitational forces are supposed to be indistinguishable. I imagine an inhabitant of a constantly accelerating flat surface (a three dimensional inhabitant, I clarify, just to make sure you guys don’t think I’m pulling a Flatland here) could take a look at objects floating around his world (his world being the flat surface) and notice that they do not move as though they are attracted to his world in the same way he himself seems to be attracted to his world. So he would realize, it seems to me, that the “gravitational” force he feels is actually an accelerational force and not a gravitational force.
What am I doing wrong here?
If I’m understanding your description correctly, the flat surface is accelerating in the direction perpendicular to the plane of the surface. But then there wouldn’t be objects floating around weightlessly and not being hit by the surface. An object above the surface, and stationary WRT the surface at some point in time, will collide with the surface when the accelerating surface reaches it a short time later. The only way that won’t happen is if the object is also being accelerated like the surface, but then the object isn’t just floating. To someone standing on the surface, the object will hit the surface in just the same way it would if the surface were stationary, above a large mass attracting the object.
Say the surface is a circle with radius r and I’m standing in the center of it. Then an object which, once it intersects the plane of the surface, would be at some distance from me greater than r, will not behave “gravitationally” (so to speak) with respect to the surface I’m standing on. It will not be attracted toward the surface I’m standing on, rather, it will zoom past that surface–and continue accelerating even once it’s gone past the surface (rather than, as I would expect if the force I’m feeling is gravitational, beginning to decelerate and eventually beginning to come back towards the surface I’m standing on.)
It occured to me shortly after I made my previous post that I guess this is equivalent to a situation where there’s some huge massive object “below” the surface I’m standing on, w/ the surface somehow supported against it, and with everything else in the world around me attracted to that object. I mean, I can look down below my surface and see there’s no such object there, but I suppose that’s not important? It’s just important that the force I’m experiencing is equivalent, is “as though” there were such a huge object there exerting a gravitational force on everything I can see?
ETA: I mean look, this is how bad my lack of understanding is. If accelerational and gravitational forces are of a piece, I’ve never understood why we can’t therefore say that the earth (and everything else that exerts a gravitational pull) is constantly expanding at a more and more rapid rate. If my being attracted to the center of the earth is equivalent to the surface of the earth accelerating upwards toward me, then shouldn’t we be able to treat the entire surface of the earth as constantly accelerating “upwards,” i.e., treat the entire earth as though it is constantly expanding?
I know it’s nonsense. The nonsense is in me! Get it out! Get it out!
Yes. The “gravitational field” in the case where you’re on the disk, and want to describe things in your frame of reference, would be uniform throughout the universe.
Can we say this?:
Acclerational and gravitational forces are equivalent. In both cases, what you have can be treated aptly as a gravitational field. But gravitational fields can be caused by different things. Some gravitational fields are caused by the mere presence of massive objects. Other gravitational fields are caused by the acceleration of massive objects. There may be no way to determine which kind of field you’re in with certainty–but you can usually get a pretty good idea by looking around and seeing if there’s a massive object around that would account for the accelerations you’re seeing. If there’s not one, chances are the accelerations you’re seeing are due your own acceleration rather than being due to the presence of a massive object somewhere.*
And since gravitational fields can be caused in different ways, it is at least “theoretically possible” (as they say) for there to be tech that negates some gravitational fields but not others–because it interferes with whatever the mechanism is that causes one kind of gravitational field, but does not interfere with the mechanism that causes the other kind of gravitational field.
*You’re in freefall, and you see an earth-sized object accelerating toward you at 32 feet per second per second. You’d like to know the cause of this acceleration before the earth sized object collides with you. You whip out a pencil and paper, take some measurements, do some math, and discover that the mass of this earth sized object, together with its position relative to you, would exactly account for the acceleration you are witnessing. In general, you know that massive objects cause gravitational fields to be present, the shape of the field relating to the mass of the object–and the shape of this field correlates with the mass of that earth-like object in exactly that way. So the acceleration is due to a gravitational field caused by the presence of a massive object.
Scenario two: You’re in freefall, and you see a smallish object acclerating toward you at 32 feet per second per second. You’d like to know the cause of the acceleration before the inevitable collision, so you get out your pencil and paper etc. You find that the mass of the object accelerating towards you doesn’t account for the acceleration you’re witnessing. The acceleration, then, is not due to a gravitational field produced by the presnce of that particular massive object. Meanwhile, you don’t see any other candidate massive object around. Furthermore, using your handy dandy space probes (did I forget to mention those?) you check around the back of the small object–and see that it is emitting tiny masses at a rate that would exactly account for the object’s acceleration through laws of intertia and so on. You conclude that the gravitational field here is caused by inertial acceleration, and not by the presence of a massive object.
These scenarios were meant to illustrate how it seems to me that the two fields can be equivalent, yet have demonstrably (and for the most part measurably) different causes. And if they have different causes, then technology could (in theory) treat them differently.
Is how it seems to me right now.
The best anti-gravity tech we have thus far? Parachute.
In a sense, it is. That is, the presence of the mass of the Earth curves the underlying plenum of spacetime in the same way that a cannon ball dropped onto a kitchen floor will cause the linoleum to crater. This distortion in spacetime is indistinguishable from thrusting against an object in flat space. Both act against the inertial properties of the object in question, i.e. its resistance to deviate from a geodesic (a line that is “straight” with respect to the topology of space when flattened out. When taken from this perspective, there is no different between gravitational force and “accelerational force” (by which I take it you mean the force resulting from an application of impulse over a time interval). Whether the body in question is being accelerated by rocket thrust, forced into a curved path by a rigid rotating body, or diverted by a distortion in the plenum of spacetime due to a concentration of gravitationally-bound energy (mass), the effect is the same, and indistinguishable from the reference frame of the body (save that a system under rotation will have some additional components).
The basic problem with the premise of the o.p. is that he imagines blocking the interaction that we observe as gravitational attraction with some kind of shield, as if the gravitational bond is a string that can be broken. However, it doesn’t work this way; one body can’t exert (or attempt to exert) a force upon another and not be affected in turn. In order to disable gravitational effects, you’d need to remove the property of inertia for the body…and then you’re back in the same situation that any other impulse will have the same effect. (This also causes some very serious problems with relativity in general, which doesn’t like objects that are massless but don’t move at c; currently the only massless particles we know of are the gauge bosons.)
The concept of “blocking” gravitons seems so intuitive, and yet, completely wrong. Gravitons aren’t appearent independent of mass, and indeed, it is a mistake to think of them as being little blobs of mass-affecting stuff that you can watch as it flies past. Gravitons are (per quantum field theory) quantized interactions between masses that are very tiny (but extremely high energy) distortions that are propagated in the underlying spacetime plenum. The only way to detect a graviton–which has no charge or mass–is to interact with it, and the only way we know of doing that is to put a mass in front of it–which then exerts its own field. (You can, of course, null out a gravitational field by suspending an equal mass opposite of it, but it would obviously be impractical to carry around a second Earth-sized mass in order to put one’s self in freefall.) If you could create some kind of discontinuity in spacetime (which would require quantized spacetime and unimaginable energies) you can potentially “block” gravitons, but honestly, it is just a lot easier at that point to take up hang gliding, which is almost as good and far less likely to piss off the local Physicists’ Guild.
Stranger
Yes, I know this is an old thread, but…
The book(s) in question involve technologies so ridiculously in advance of our own that it’s silly to argue against them given our current knowledge of physics, at least if you assume the technological premises of the books.
As Stranger mentions in his post above, you could do some sort of blocking of gravity with distortions of spacetime. Fortunately, the technology or Mr. Banks’ novels includes many ways to do exactly that, with all kinds of fields and 4 or more dimensional computer intelligences and hyperspace dimensions (in both directions), artificial wormholes, faster-than-light travel and communications, unlinked inertias, etc. etc.
It’s true enough that it’s impossible to tell the difference between acceleration from gravity and from other sources in a single reference frame, but in this case it’s a red herring, since we’re dealing with technologies that are non-extant. Imagine that the AG units actually do create some sort of point-sized mass that can be adjusted to counteract the gravity force you feel; a little bit of thinking about how that would work would show that it would not behave the same way in a non-gravity frame (like an orbital) as opposed to on a planet or something, even if that could be adjusted by different control software to mimic the effect in a rotating object. And that’s not even considering the other possible ways (in the book universe) that an antigravity effect could be achieved (they also have artificial gravity systems, which would make the opposite pretty simple).
So back to the OP’s question, I think, given the premises of the novels, and without an explanation of the underlying mechanism, it’s not an error in understanding the physics, it’s simply a magic technology that could plausibly act in the way he describes.