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#1
03-29-2010, 10:08 AM
 BrainGlutton Guest Join Date: Mar 2003
Would an antigravity device also resist rotational "gravity"?

In Consider Phlebas (1987), the first of Iain Banks' Culture novels, a crew of space mercenaries are about to try some scavenging on a huge Orbital (similar to Larry Niven's Ringworld) scheduled for destruction in the Culture-Iridan War. Some of them have personal antigravity devices built into their suits; the captain warns them that AG will not work against the artificial "gravity" produced by the Orbital's spin.

SPOILER:
One of them is late to the briefing and later jumps off a precipice and falls to his death.

Is this true? In terms of physics, what is there, really, that makes a difference between a mass-induced gravity field and a rotating-frame-of-reference gravity field (or an accelerating-frame-of-reference gravity field)? My very vague understanding of relativity is that frame of reference is everything. Is it that the one form of gravity involves graviton interactions and the other does not? Or what?

Putting this in CS rather than GQ because, after all, there is no such thing as antigravity IRL, yet if ever, so it's really a question about a more plausible/defensible way to do SF blackboxing.
#2
03-29-2010, 10:29 AM
 tim314 Charter Member Join Date: Mar 2004 Posts: 3,602
Einstein tells us that there is an equivalence between gravity and acceleration (in this case the acceleration towards the center that keeps you spinning in a circle instead of flying off in one direction). There are ways you can tell them apart... e.g. an acceleration won't in general fall off as 1/r2 as you move further from some source... but I'd be a little surprised if such details affected these hypothetical anti-gravity devices.

I'm honestly not sure how one reconciles the equivalence principle with the notion that gravity is caused by the exchange of force carrying particles . . . . Maybe another poster better versed in GR will know, although it might be one of those things that we won't totally understand without a theory of quantum gravity.
#3
03-29-2010, 10:30 AM
 pancakes3 Guest Join Date: Nov 2008
the "easiest" way to construct an anti-gravity machine would probably be with magnetic repulsion since as of yet, there is no way to stop, block, or impede in any way the force of gravity. so, as long as the anti-gravity machine isn't using scientific theory unbeknownst to humanity then sure - the machine can counter centripetal acceleration too.
#4
03-29-2010, 10:47 AM
 BrainGlutton Guest Join Date: Mar 2003
Quote:
 Originally Posted by pancakes3 the "easiest" way to construct an anti-gravity machine would probably be with magnetic repulsion since as of yet, there is no way to stop, block, or impede in any way the force of gravity. so, as long as the anti-gravity machine isn't using scientific theory unbeknownst to humanity then sure - the machine can counter centripetal acceleration too.
But, is it possible such a device using currently unknown science could block the one but not the other? Or is there no difference that makes a difference? I.e., would an anti-gravity device necessarily be also an anti-force-of-acceleration device?

Last edited by BrainGlutton; 03-29-2010 at 10:48 AM.
#5
03-29-2010, 10:53 AM
 Mogle Guest Join Date: Jun 2002
It makes sense to me, normal gravity is caused by attraction between masses so even if you do not touch the other mass it will still affect you.
But in a rotating or accelerating frame of reference 'gravity' will not really affect you unless actually touch the ground. You will just keep on moving with no acceleration in the same absolute direction as you were when you stepped off the edge, unfortunately the ground also keep moving in its absolute direction and unlike you, it still has acceleration.
Of course this is what it looks like to the outside observer, inside the rotating or accelerating frame for reference it will very hard to tell the difference between this and normal gravity.

In the end it depends on how the anti gravity is generated, does it merely generate thrust to counteract the effects of gravity or does it actually prevent the attraction between masses?
If it's the former it will work anywhere, if the latter it will only work with normal gravity.
#6
03-29-2010, 11:03 AM
 tim314 Charter Member Join Date: Mar 2004 Posts: 3,602
Quote:
 Originally Posted by BrainGlutton But, is it possible such a device using currently unknown science could block the one but not the other? Or is there no difference that makes a difference? I.e., would an anti-gravity device necessarily be also an anti-force-of-acceleration device?
There isn't any known science that can "block" gravity . . . using known science, pancakes3 is right, you'd have to make an equal and opposite force to counter it out. In which case, why not just do it for everything?

As I said, it is possible to distinguish gravity from acceleration, at least for macroscopic objects (as opposed to pointmasses) . . . e.g., if you're floating over the Earth, you feel a slightly greater gravitational pull at your feet than your head, because your feet are closer to the Earth. But if you're in, say, an accelerating rising elevator, you still feel a downward force, but it's the same for your whole body.

Now if you're feeling artificial gravity due to rotation, the force will be different at your head (closer to the center) than at your feet, but it varies like 1/r instead of 1/r2. So in principle you could tell the difference.

In otherwords, if they really wanted their anti-grav device to test if it was consistent with real gravity and only apply the counteracting force in that case, they probably could . . . but they could probably just as easily make it work for rotational pseudo-gravity too if they preferred.
#7
03-29-2010, 11:15 AM
 pancakes3 Guest Join Date: Nov 2008
Quote:
 Originally Posted by BrainGlutton But, is it possible such a device using currently unknown science could block the one but not the other? Or is there no difference that makes a difference? I.e., would an anti-gravity device necessarily be also an anti-force-of-acceleration device?
there is a difference. REAL gravity is the fact that two things with mass are naturally attracted to each other. you can put a wall up between the two and it'll still be there. there's nothing you can do to block this attraction.

the simulated gravity is essentially no different than what you feel when you slam on the brakes of a car, except in the car the force you feel is lateral. when you rotate it, it's akin to swinging a bucket of water in a circle. the water sticks to the bottom of the pail no matter what direction it's facing.

if you were the water, in the pail, there's no way of you knowing you're spinning. all you know is that the bucket bottom is the "ground" and you're forced down on it. to counter this, you would put on a mini rocket pack, or charge yourself and the bucket bottom the same electric charge, or pole vault, or use some other physical force that would cancel out the "downward" force. OR you can just stop the spinning.

either way, we can at least imagine solutions to counteract the bucket gravity. there is nothing we can do to turn off the real gravity.
#8
03-29-2010, 03:11 PM
 Chronos Charter Member Join Date: Jan 2000 Location: The Land of Cleves Posts: 47,965
"Real" gravity works by making it so that what you think of as a "stationary" reference frame is actually accelerating. The effect of people falling off of buildings works just the same way in that accelerating reference frame as it does in the accelerating reference frame of a rotating space station. There's no scientific justification at all for a device (however constructed) to enable flight in "real" gravity but not in centrifugal gravity.

Quote:
 I'm honestly not sure how one reconciles the equivalence principle with the notion that gravity is caused by the exchange of force carrying particles . . . . Maybe another poster better versed in GR will know, although it might be one of those things that we won't totally understand without a theory of quantum gravity.
It's definitely in the murky, uncharted waters of quantum gravity, but compared to what we do know and understand, it's not all that weird. Even without quantum gravity, it's possible for someone in an accelerating reference frame to observe the existence of particles that are not present to someone in a non-accelerating frame.
#9
03-29-2010, 04:53 PM
 Marley23 Administerminator Administrator Join Date: Apr 2002 Location: New York Posts: 68,751
Moved to General Questions from Cafe Society to see if there is a factual answer.
#10
03-29-2010, 05:53 PM
 matt Member Join Date: Aug 1999 Location: A Brit in 'Stralia Posts: 1,964
Quote:
 Originally Posted by Chronos There's no scientific justification at all for a device (however constructed) to enable flight in "real" gravity but not in centrifugal gravity.
Sure there is. Have it work like literal "antigravity" - a force that repels matter, proportional to the mass and inverse with the distance. So on Earth, your antigravity repulsor bike pushes the earth away from you with say, up to 5000 N force at full power. On Mars it would only push with 1600 N force due to the lower mass of Mars. And on the rotating Ringworld it wouldn't get off the ground because the thin Scrith floor beneath it doesn't have enough mass for the repulsor to work against.
#11
03-29-2010, 07:48 PM
 unclelem Guest Join Date: Oct 2008
Quote:
 Originally Posted by Chronos The effect of people falling off of buildings works just the same way in that accelerating reference frame as it does in the accelerating reference frame of a rotating space station. There's no scientific justification at all for a device (however constructed) to enable flight in "real" gravity but not in centrifugal gravity.
This does not seem right to me.

First off, the force caused by the spinning, only exists if you're coupled to the rotating frame (the station).

Take your giant space ring and rotate it as much as you want to create any amount of pseudo gravity you want on the ring surface. Fly a spaceship through the ring at any point, including an inch from the ring itself, and (neglecting the boundary effects of a non-infinite cylinder) that spaceship feels no force from the ring at all.

Take a person in a space suit and have them run in the opposite direction of rotation. Assuming they can reach and maintain (essentially cancel) the speed of rotation, the will find that the "gravity" disappears.

Drop a rock while standing on the inside of the ring. It doesn't fall in a straight line to the ground. The path appears to curve in the rotating frame - because it's really following an inertial straight line along the tangent from where it was dropped.

Or for a more real-world example, swing your bucket of water around on a rope. The water stays in the bucket - but note that loose change does not fall out of your pocket and get sucked in, nor does debris get picked up off the ground and sucked in.

The problem is that as soon as (or as long as) you decouple from the rotating frame, inertia takes over and you immediately realize it was/is not real gravity.

The answer to the OP, however depends on how the anitgravity device "works".

Gravity is a force of attraction at a distance between two objects with mass.

The pseudo-gravity from the rotating frame is due to a property of matter called inertia - the object will tend to go flying off in a straight line unless a force is applied to it to change that path. That force is a result of a physical force (the force that keeps the book from falling through the table) rather than gravitational attraction.

If the supposed antigravity device can somehow eliminate the attractive force of gravity, but not eliminate the actual "massiness" of the object such that inertia remains, then yes, the device would not work to cancel pseudo-gravity.
#12
03-29-2010, 07:55 PM
 billfish678 Guest Join Date: Jun 2006
I am going with Unclelem on this one (good post BTW).

Mass creates a gravitational field which causes an acceleration. Acceleration creates the appearance of a gravitation field.

Its not obvious to my chimplike understanding of the two that those are EXACTLY the same thing. If they are not exactly the same thing, it seems reasonable to me that some voodoo science that may cancel one wouldnt neccessarily cancel the other.

Last edited by billfish678; 03-29-2010 at 07:57 PM.
#13
03-29-2010, 11:06 PM
 Chronos Charter Member Join Date: Jan 2000 Location: The Land of Cleves Posts: 47,965
Quote:
 This does not seem right to me. First off, the force caused by the spinning, only exists if you're coupled to the rotating frame (the station).
Likewise, the force caused by the presence of a mass also only exists if you're coupled to an accelerating frame. Astronauts on the Space Station, for instance, do not experience the gravity of the Earth.

Quote:
 Its not obvious to my chimplike understanding of the two that those are EXACTLY the same thing. If they are not exactly the same thing, it seems reasonable to me that some voodoo science that may cancel one wouldnt neccessarily cancel the other.
All that means is that you're not Einstein, which is nothing to be ashamed of. It wasn't obvious to plenty of people. And it probably wouldn't be obvious to me, either, if I weren't so familiar with the topic.
#14
03-30-2010, 02:25 AM
 si_blakely Guest Join Date: Jul 2002
It would be more logical for the anti-gravity device to work, but for the astronaut to hang fixed in space and then get clobbered sideways by the rotation of the station (a plot point used by Asimov in "The Billiard Ball" - if your antigravity field locally flattens spacetime, objects within the field are decoupled from all frames of reference, including earths rotation, orbital motion, and the suns motion in the galaxy, and thus instantly acquire a rather large velocity vector).

Si
#15
03-30-2010, 09:51 AM
 Pleonast   Charter Member Join Date: Aug 1999 Location: Los Obamangeles Posts: 4,954
If general relativity is correct, it is not possible to distinguish among accelerating frames of reference in general. (Just as, if special relativity is correct, it is not possible to distinguish among inertial frames of reference.) The existence of a device that counters gravitational frames of reference but not other accelerating frames would falsify general relativity.
Quote:
 Originally Posted by unclelem Take your giant space ring and rotate it as much as you want to create any amount of pseudo gravity you want on the ring surface. Fly a spaceship through the ring at any point, including an inch from the ring itself, and (neglecting the boundary effects of a non-infinite cylinder) that spaceship feels no force from the ring at all. Take a person in a space suit and have them run in the opposite direction of rotation. Assuming they can reach and maintain (essentially cancel) the speed of rotation, the will find that the "gravity" disappears.
As Chronos implies, this is equivalent to orbiting a massive body.
Quote:
 Drop a rock while standing on the inside of the ring. It doesn't fall in a straight line to the ground. The path appears to curve in the rotating frame - because it's really following an inertial straight line along the tangent from where it was dropped.
This is exactly what happens when you drop a rock on Earth--it's following an inertial path from your hand to the ground.
Quote:
 Or for a more real-world example, swing your bucket of water around on a rope. The water stays in the bucket - but note that loose change does not fall out of your pocket and get sucked in, nor does debris get picked up off the ground and sucked in.
Nor does gravity suck things in. Instead it distorts what "straight" is.
Quote:
 The problem is that as soon as (or as long as) you decouple from the rotating frame, inertia takes over and you immediately realize it was/is not real gravity.
You can decouple from gravitation as well. It's what we call escape velocity.
Quote:
 The pseudo-gravity from the rotating frame is due to a property of matter called inertia - the object will tend to go flying off in a straight line unless a force is applied to it to change that path. That force is a result of a physical force (the force that keeps the book from falling through the table) rather than gravitational attraction. If the supposed antigravity device can somehow eliminate the attractive force of gravity, but not eliminate the actual "massiness" of the object such that inertia remains, then yes, the device would not work to cancel pseudo-gravity.
It's funny that you bring up inertia, because general relativity states that inertial mass is indistinguishable from gravitational mass. Any effects you see due to inertia is exactly equivalent to effects due to gravitation.
#16
03-30-2010, 10:41 AM
 Frylock Guest Join Date: Jun 2001
Quote:
 Originally Posted by Chronos Likewise, the force caused by the presence of a mass also only exists if you're coupled to an accelerating frame. Astronauts on the Space Station, for instance, do not experience the gravity of the Earth.
What's "experience" mean here? I'd have said they do experience the gravity of Earth. If an astronaut in orbit around Earth accelerates, he will find his direction of movement after the acceleration can be determined in part by the mass and position of the Earth. In other words, the astronaut finds that what "straight" means for him has a lot to do with how big the earth is and where it is in relation to him. That's what I'd call "experiencing earth's gravity."

Meanwhile, someone floating in the middle of an rotating artificial gravity machine will not find his direction of motion affected in the same way by that machine. His direction of motion isn't affected unless he actually comes into physical contact with the machine's moving parts. "Straight" for him doesn't seem to be determined by anything about the machine.

They really look like distinct effects to me still.

When one billiard ball collides with another, imparting momentum to it, the first one hasn't changed what "straight" means for the second, has it? (Genuine question, though I do think I know the answer, but I've been surprised before.)

Last edited by Frylock; 03-30-2010 at 10:41 AM.
#17
03-30-2010, 05:35 PM
 BrainGlutton Guest Join Date: Mar 2003
OK, what if you're wearing your AG suit, and you're on the inner surface of the Orbital with local acceleration-gravity at 1G, but, you're facing the direction opposite the direction of spin, and running at the same speed as the spin, on a treadmill?
#18
03-30-2010, 06:29 PM
 Snirk Guest Join Date: Mar 2010
If there's a plane on the inside of the ring, and its wheels are moving at the same speed as the ring's rotation, would it be able to pull a boat on giant rollerskates off a treadmill into a fan?
#19
03-30-2010, 07:20 PM
 ZenBeam Charter Member Join Date: Oct 1999 Location: I'm right here! Posts: 6,891
Einstein believed that the equality of gravitational mass and inertial mass (as far as measurements could tell) was indicative that they were the same thing, and hence exactly equal. This led him to the Equivalence Principle. This eventually led him to general relativity, and those predictions have so far always been found to be correct. If the equivalence principle is correct, then you can't distinguish between acceleration and gravity the way the OP's book does.

It's barely possible that the equivalence principle isn't truly correct, and gravitation just happens to mimic its effects correctly so far as we can tell, but fails in the case of antigravity devices.

More likely, it's a software glitch feature in the way the antigravity devices are designed.
#20
03-30-2010, 08:38 PM
 Stranger On A Train Guest Join Date: May 2003
Quote:
 Originally Posted by Frylock What's "experience" mean here? I'd have said they do experience the gravity of Earth. If an astronaut in orbit around Earth accelerates, he will find his direction of movement after the acceleration can be determined in part by the mass and position of the Earth. In other words, the astronaut finds that what "straight" means for him has a lot to do with how big the earth is and where it is in relation to him. That's what I'd call "experiencing earth's gravity." Meanwhile, someone floating in the middle of an rotating artificial gravity machine will not find his direction of motion affected in the same way by that machine. His direction of motion isn't affected unless he actually comes into physical contact with the machine's moving parts. "Straight" for him doesn't seem to be determined by anything about the machine. They really look like distinct effects to me still.
Nope, Chronos is correct; for the astronauts in a freefall orbit, there are no discernible forces acting upon them (at least, as long as they aren't spread out enough in radial distance that tidal gradients come into play) and as far as the astronauts know, space is just curved. And indeed, if they are in a circular orbit and their frame of reference is rotating at the same rate that they are orbiting, they will perceive themselves to be motionless in both lateral and radial dimensions.

Similarly, for the explorer on the surface of the Ringworld or another rotating construct of sufficient size that the normal components are negligible, there is no difference in the interaction as far as he can measure from a Lagrangian perspective. In this case, instead of a mass curving space and causing an orbiting object to be accelerated, "space" itself is (artificially) curved by virtue of being constructed of a rigid plenum and the applied force is a reaction to change in momentum, which is identical to being in a powered orbit. Imagine a spacecraft orbiting a fixed point by constantly thrusting toward it at just the correct velocity to make a circle, and you have precisely the same effect, which is, of course, equivalent to acceleration due to gravity.

Realize that in order for a force to be felt, it must be counteracted by another force. For instance, in order for you to "feel" the pull of gravity, you have to be pushed against some solid frame that prevents you from moving, i.e. the electrostatic repulsion between the ground and your feet. You don't "experience" any single influence in isolation; every perceived force is a couple (hence, Newton's third law of motion).

It is possible, of course, to determine that you are in a rotating coordinate frame by measuring the Coriolis and (if you are moving radially with respect to the rotating coordinate frame) Euler components, but these drop off rapidly as the radius increases (though you could detect the tendency to rotate with a Foucault pendulum, albeit for a somewhat different reason than Foucault's experiment). So you should be able to discern between acceleration due to a fixed mass and that instigated by being in a forced rotation frame.

Above someone mentioned gravitons (which are hypothetical exchange carriers of the gravitational force) and quantum gravity. While it might seem that the fundamental nature of acceleration induced by a concentrated locus of mass-energy and that resulting from a change of inertia in a rotating frame are quite different, in fact both occur due to interactions between bodies with inertia which warps the path of the affected object. One might equally conceive of the ring exchanging "force carriers" via momentum transfer (though from the external observer such interactions degenerate into inertial reaction from being forced into a circular path). In any case, no one has ever "seen" a graviton, and it is likely that no one will ever measure the influence of a single interaction with any imaginable instrumentation.

Stranger
#21
03-31-2010, 07:09 AM
 Frylock Guest Join Date: Jun 2001
Quote:
 Originally Posted by Stranger On A Train So you should be able to discern between acceleration due to a fixed mass and that instigated by being in a forced rotation frame.
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.
#22
03-31-2010, 07:53 AM
 Anachronism Guest Join Date: May 2000
Quote:
 Originally Posted by Stranger On A Train Nope, Chronos is correct; for the astronauts in a freefall orbit, there are no discernible forces acting upon them (at least, as long as they aren't spread out enough in radial distance that tidal gradients come into play) and as far as the astronauts know, space is just curved. And indeed, if they are in a circular orbit and their frame of reference is rotating at the same rate that they are orbiting, they will perceive themselves to be motionless in both lateral and radial dimensions. Similarly, for the explorer on the surface of the Ringworld or another rotating construct of sufficient size that the normal components are negligible, there is no difference in the interaction as far as he can measure from a Lagrangian perspective. In this case, instead of a mass curving space and causing an orbiting object to be accelerated, "space" itself is (artificially) curved by virtue of being constructed of a rigid plenum and the applied force is a reaction to change in momentum, which is identical to being in a powered orbit. Imagine a spacecraft orbiting a fixed point by constantly thrusting toward it at just the correct velocity to make a circle, and you have precisely the same effect, which is, of course, equivalent to acceleration due to gravity. Realize that in order for a force to be felt, it must be counteracted by another force. For instance, in order for you to "feel" the pull of gravity, you have to be pushed against some solid frame that prevents you from moving, i.e. the electrostatic repulsion between the ground and your feet. You don't "experience" any single influence in isolation; every perceived force is a couple (hence, Newton's third law of motion). It is possible, of course, to determine that you are in a rotating coordinate frame by measuring the Coriolis and (if you are moving radially with respect to the rotating coordinate frame) Euler components, but these drop off rapidly as the radius increases (though you could detect the tendency to rotate with a Foucault pendulum, albeit for a somewhat different reason than Foucault's experiment). So you should be able to discern between acceleration due to a fixed mass and that instigated by being in a forced rotation frame. Above someone mentioned gravitons (which are hypothetical exchange carriers of the gravitational force) and quantum gravity. While it might seem that the fundamental nature of acceleration induced by a concentrated locus of mass-energy and that resulting from a change of inertia in a rotating frame are quite different, in fact both occur due to interactions between bodies with inertia which warps the path of the affected object. One might equally conceive of the ring exchanging "force carriers" via momentum transfer (though from the external observer such interactions degenerate into inertial reaction from being forced into a circular path). In any case, no one has ever "seen" a graviton, and it is likely that no one will ever measure the influence of a single interaction with any imaginable instrumentation. Stranger
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.
#23
03-31-2010, 05:42 PM
 BrainGlutton Guest Join Date: Mar 2003
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?
#24
03-31-2010, 05:54 PM
 ZenBeam Charter Member Join Date: Oct 1999 Location: I'm right here! Posts: 6,891
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.
#25
03-31-2010, 06:10 PM
 Frylock Guest Join Date: Jun 2001
Quote:
 Originally Posted by BrainGlutton 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?
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...)
#26
03-31-2010, 06:14 PM
 Frylock Guest Join Date: Jun 2001
Quote:
 Originally Posted by ZenBeam 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.
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?

Last edited by Frylock; 03-31-2010 at 06:15 PM.
#27
03-31-2010, 09:05 PM
 ZenBeam Charter Member Join Date: Oct 1999 Location: I'm right here! Posts: 6,891
Quote:
 Originally Posted by Frylock 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.
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.
#28
03-31-2010, 09:30 PM
 Frylock Guest Join Date: Jun 2001
Quote:
 Originally Posted by ZenBeam 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!

Last edited by Frylock; 03-31-2010 at 09:34 PM.
#29
03-31-2010, 09:38 PM
 ZenBeam Charter Member Join Date: Oct 1999 Location: I'm right here! Posts: 6,891
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.
#30
03-31-2010, 11:06 PM
 Frylock Guest Join Date: Jun 2001
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.
#31
04-01-2010, 12:03 AM
 Bryan Ekers Guest Join Date: Nov 2000
The best anti-gravity tech we have thus far? Parachute.
#32
04-01-2010, 01:45 AM
 Stranger On A Train Guest Join Date: May 2003
Quote:
 Originally Posted by Frylock 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?
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
#33
09-23-2010, 03:38 AM
 ivn1188 Guest Join Date: Nov 2008
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.

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