Would an antigravity device also resist rotational "gravity"?

Factual Questions
1

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.

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

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/r[sup]2[/sup] 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

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

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?

5

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

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/r[sup]2[/sup]. 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

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

“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.

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

Moved to General Questions from Cafe Society to see if there is a factual answer.

10

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

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

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.

13

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.

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

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

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.

As Chronos implies, this is equivalent to orbiting a massive body.

This is exactly what happens when you drop a rock on Earth–it’s following an inertial path from your hand to the ground.

Nor does gravity suck things in. Instead it distorts what “straight” is.

You can decouple from gravitation as well. It’s what we call escape velocity.

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

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.)

17

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? :smiley:

18

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

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 [del]glitch[/del] feature in the way the antigravity devices are designed.

20

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