The way I see it, an anti-grav device* could work one of two ways: either it would make an object transparent to gravity (I’ll call it grav-glass** for the sake of brevity), or it would reduce/negate the effects of gravity for an object (shortened to null-grav).
What would happen as a result of this? Assume that objects within these areas of effect still experience/exert gravity relative to each other (it’s only gravity from external objects that is modified)
[ol]
[li] Would null-grav blow a column of atmosphere away in the shadow of null-gravity, or would gravity flow around it like a fluid?[/li][li] For null-grav and grav glass, would the object float upwards due to buoyancy (it “weighs” less than the air it’s displacing)[/li][li] What other side effects could happen?[/li][/ol]
If such a thing were somehow possible
** Yes, I know glass affects, and is affected by light. It’s just used as a convenient label here.
This is exactly as meaningful as asking for the resting pulse rate of a unicorn. Such a device couldn’t exist, so any details someone thinks up are just illusory and impossible to prove.
I don’t like raining on parades, but sometimes its the only response.
I agree that when conceiving something not possible with known physics, there is no answer unless you specify what alternate theory you want to propose. However, in this case, there is a simple possibility, at least within classical physics. You could posit the existence of negative gravitational mass. Then the behavior in the classical limit will be like static electricity, except that like charges attract and unlike charges repel. If you want to null out gravity, simply add enough negative mass to an object so that it cancels the positive mass. Then it will be neutrally bouyant, neither attracted nor repelled by other massive objects.
All objects are already transparent to gravity, so we can ignore your proposed grav-glass.
It would neither blow the atmosphere away, nor would gravity flow around it like a fluid.
If you neutralize the mass, it will be neutrally bouyant. If you add even more negative mass, it will be repelled.
Gravitational charges would tend to separate out into clumps of positive mass repelling clumps of negative mass. It will take a large input of energy to neutralize the mass of an object.
Having negative mass would not be quite as exciting as one might think. If an object has a tonne of negative mass, it will still be attracted by the gigantic mass of the Earth, so it would just behave as if it were a little bit lighter.
What would be really exciting would be negative inertia, where the pull of the Earth would be translated into a push in the opposite direction. Now you can fly. Unfortunately it would be difficult to control this sort of flight.
Asimov wrote a short story called “The Billiard Ball” which was about an anti-gravity machine. The conceit was that the “anti-gravity field” flattened space-time to the equivalent of absolute zero.
The upshot of which was that anything that entered the field instantly stopped moving due to the gravity of earth/the sun/the milky way/the local galactic cluster - which means it was suddenly moving very fast indeed relative to everything on the planet.
Maybe I need to be a bit clearer. The anti-grav field I’m postulating creates an area where the mass of objects outside the field is neither attracted nor repelled (due to gravity-based effects) to the mass of objects inside the field, and vice versa. Chemical interactions, momentum/inertia, electrostatic interactions, and the like would not be affected by the field. The field itself would be pervious to air, light, sound, etc. though you could easily have a vessel inside the field seal against those elements.
Do we even know enough to understand the impact such a hole or blindspot would have on the rest of the universe?
Well, you seem to have a very clear idea of what you want to happen in an anti-grav field; perhaps we should be asking you questions. Perhaps it would be useful to imagine the curvature of space around such a field.
Congratulations! You have just invented a perpetual motion machine. Position a vertically mounted wheel such that half of the wheel is in the anti-grav field and half is outside. Gravity will pull down on the half outside the field, causing it to rotate, since the other half is now weightless. Attach a generator to the axle and you have solved the energy problem.
Yes. We just do the math. And as newme says, it would make perpetual motion machines possible. It would also do weird things to air. This is based on purely mental calculations, which I often make grave mistakes in, but gas molecules entering from below would, due to the higher pressure, have higher average kinetic energy, and without gravity slowing them down, would leave the top of the field with more total energy than they entered, making, in effect another sort of perpetual motion machine.
Your field probably needs to account for that energy in some way …
Wouldn’t you have to weight it unevenly and then give it a push? Just having half in/half out wouldn’t cause it to rotate on its own, and if it was evenly weighted, it wouldn’t actually gain energy from gravity either and wouldn’t be much different than an evenly weighted ring in full gravity.
More specifically, it would exhibit the (apparent) centrifugal force of Earth’s rotation, its revolution around the Sun, and its revolution around the galactic center (and etc. for larger structures) without the corresponding (actual) gravitational forces.
In the first case (Earth’s rotation), gravity far exceeds the centrifugal force (which is why we don’t fly off into space). So, the effect of no gravity would be well below 1G, which would be easily contained, though not on a billiard table. In the other cases, the forces are in balance (which is why we’re in orbit). I wonder what the magnitude of these apparent centrifugal forces would be, and which would be the most significant. If my intuition isn’t leading me astray, the greatest curvature is the biggest force, so it’d be the Sun, by many orders of magnitude.
It would be weighted unevenly by default: the half of the wheel outside the field would fall according to gravity, rotating the other (weightless) side up. It’d be like a water wheel, with the field causing the bins on one side to seem empty.
But the bottom line is that the answer is the same as “what if there was magic?” Without the equations, we can’t do the math. (I can’t do it even with the equations, unless they’re easy!)
And what happens when you add an anti-tonne of negative mass to something that originally weighed a traditional tonne of positive mass? That’s the interesting interpretation of the negative mass idea.
Your spoiler indicates you may have misunderstood the plot twist in that story.
The device in question was, in OP’s parlance, a null-grav device. In the field, it nullified gravity by an unknown mechanism. (The backstory of the story was the perpetual competition between a brilliant but methodical and completely obscure theoretical physicist, and his lifelong frenemy, an equally brilliant but careless and shamelessly self-promoting experimental physicist and engineer. The theoretical physicist claimed antigrav was impossible. The experimentalist decided to publicly prove him wrong, and came up with a mechanism that seemed to do the trick, but couldn’t be bothered to figure out HOW it worked.)
The null-grav field, when activated, is surrounded by a brilliant blue aura (in atmosphere). This is a clue to the mechanism of the device.
The great reveal, after the dramatic and shocking events of the story, is that the device “nullified gravity” by[spoiler]actually suppressing all mass within its field – matter becomes a massless particle, like a really big photon. Something with no rest mass must travel the speed of light, which it does… until it leaves the nullification field, at which it resumes its mass but retains its .99999999999c velocity. All physical Conservation laws have just taken a flying leap at themselves.
The blue aura is Cherenkov radiation, cause by atmospheric molecules passing into the field and then resuming mass and shedding some of their impossible velocity.[/spoiler]
