When will we have anti gravity?

[Event_Horizon]

It would probably be the biggest discovery known to mankind if we found out how to counteract the "force" of gravity. It kind of seems impossible; given that GR is correct, gravity is geometric, and is a result of curved spacetime.

Perhaps if we found out about the "dark energy" that's expanding the universe and acting against gravity, will we be able to create antigravity devices?

Imagine the manipulation of gravity! No more oil, no more pollution, etc. Space travel far and beyond, maybe even time travel.

 

[Jorrie]

Imagine the manipulation of gravity! No more oil, no more pollution, etc. Space travel far and beyond, maybe even time travel.

Don't you think it will take energy to "manipulate gravity"?

 

[RandallB]

given that GR is correct, gravity is geometric, and is a result of curved spacetime.

Well I doubt we will be able to consider it till we have a real TOE that resolves the assumptions like the one you've made here.
Regardless of what high level physics put out in books or TV shows where they are trying to combine GR and QM, the key in all that is that they are TRYING. They have not done so, and as GR and QM stand now and always have they are not compatible and contradict each other in their basic understanding of reality. That means one or both must be wrong until something new is added or changed to give a complete description of physics. IMO only one of the two theories can survive not both, maybe nether one will.

Once we do have a GUT and TOE, I suspect we have a good chance of learning that gravity cannot be counteracted or manipulated.

 

[cesiumfrog]

as GR and QM stand now and always have they are not compatible and contradict each other in their basic understanding of reality. That means one or both must be wrong until something new is added or changed to give a complete description of physics. IMO only one of the two theories can survive not both, maybe nether one will.

Clearly both theories are correct in appropriate limits, as they have both individually made quantitative (and surprising) predictions that have since been confirmed experimentally. Both theories must survive in the appropriate limits of any "unified" theory.

 

[RandallB]

Clearly both theories are correct in appropriate limits, as they have both individually made quantitative (and surprising) predictions that have since been confirmed experimentally. Both theories must survive in the appropriate limits of any "unified" theory.

I disagree, a theory only good "in appropriate limits" is an incomplete theory. As both, GR & QM show the explanation offered by Newtonian Classical theory to be incomplete, it is never the less still quite useful.
Einstein worked to complete GR by building a GUT to explain Electricity, Magnetism, and Gravity in one unified theory. (IMO doing so would also explain weak and strong as well for a full TOE, even though Einstein never worked on that).

Niels Bohr claimed QM is complete in that nature will never let us see beyond the HUP, to find an unknown hidden variable. To which Einstein never agreed, and to this day GR vs. QM and the Standard Model are incompatible.
Although I agree both are quite useful, neither have been shown to be complete, both are still at odds with each other.
If you have trouble accepting that, pick from a few of the many credible Physicists you can find on a Brain Greene DVD to confirm it.
The same Elegant Universe DVD will also talk about the DREAM of STRINGS where it requires a simple combining of GR & QM, but that “simple” result of Strings does not seem to come with a simple explanation of how or why they are compatible at all. We are to just trust and wait for the theory to produce a TOE.
For me I do not buy that dream, and expect the string folks to come up with a proof before I will.

IMO a simple understanding of the difference between GR demands of gravity based on four-dimensional curves and warps vs. QM expectation of particle exchanges, clearly shows they cannot both survive.

 

[pmb_phy]

It would probably be the biggest discovery known to mankind if we found out how to counteract the "force" of gravity. It kind of seems impossible; given that GR is correct, gravity is geometric, and is a result of curved spacetime.

EM just as much about as geometry is as gravity.


Pete

 

[Office_Shredder]

How to counteract gravity?

Have you tried a pulley?



Imagine you have some sort of anti-gravity shield. Take a big wheel, and put the shield under half of it. The wheel starts to spin. Free energy! Yay

 

[pmb_phy]

Actually anti-gravity is observed in nature. It is now apparent that the universe is expanding at an accelerated rate. This can be accomplished by introducing a non-zero cosmological constant into Einstein's equations of gravity. This non-zero cosmological constant acts in nature as anti-gravity. Einstein introduced this constant so as to allow for what he assumed as a static universe. Such a universe was held apart by anti-gravitational forces. Objects in GR such as a vacuum domain wall act as objects which forces/gravitationally accelerates particles away from the wall - i.e. antigravity.

Pete

 

[Aether]

Imagine you have some sort of anti-gravity shield. Take a big wheel, and put the shield under half of it. The wheel starts to spin. Free energy! Yay

How about that column of air above the anti-gravity shield. If it no longer has weight, then the atmospheric pressure due to the gravity acting on the rest of the atmosphere will cause the entire atmosphere to be purged into space.

So, if your invention ever does turn this big wheel for free, then it will also immediately wipe out all life (aerobic life anyway) on earth...seems like a serious glitch.

 

[RandallB]

Not really, the air above say a “shield” with a 5 ft radius is not held in place by gravity acting directly on it mass. And the weight of the air close to the shield would have an insignificant effect in the local air pressure. Whoever the loss of weight for the air just a few feet above would be minor and even smaller the high you go in the air column as most of the effect of gravity on the individual parts would be small as most of the Earth is still visible as a source of gravity for miles around this tiny little shield.

Also I except conservation laws to hold, and we shouldn’t expect the gain of “Free Energy” created by the shield local would result in even more energy being created in the form of gases jetting out away from earth.

Not included with the idea of a shield is an accounting for just where or how the total energy balance will be maintained. It should explain what reaction is expected where to balance the apparent gain in local energy in the turning “anti-gravity” wheel. Such a shield is not going to provide a free lunch by moving the wheel and creating some weird local air currents. The real glitch is the idea holds out no option to explain where the energy would be lower as a result of the local gain.

 

[selfAdjoint]

Not really, the air above say a “shield” with a 5 ft radius is not held in place by gravity acting directly on it mass. And the weight of the air close to the shield would have an insignificant effect in the local air pressure. Whoever the loss of weight for the air just a few feet above would be minor and even smaller the high you go in the air column as most of the effect of gravity on the individual parts would be small as most of the Earth is still visible as a source of gravity for miles around this tiny little shield.

I don't believe this analysis. Consider gravitational potential energy. The region immediately outside the circle is at zero gravitational potential relative to itself. Immediately inside the circle the potential would be "infinite"; at least a very huge number, relative to the Earth's surface. The physics at this discontituity cannot just be handwaved away. I remember an old discussion of this issue in Astounding Science Fiction magazine (as it still was at that date in the fifties of tha last century). They concluded that the physics discontinuity was a deal buster.

 

[Aether]

Not really, the air above say a “shield” with a 5 ft radius is not held in place by gravity acting directly on it mass.

It's not?

Also I except conservation laws to hold, and we shouldn’t expect the gain of “Free Energy” created by the shield local would result in even more energy being created in the form of gases jetting out away from earth.

My point is to illustrate why we wouldn't be well served by having such things around even if it were possible. Some people seem to think that it would be great if we had access to such things.

 

[pervect]

There would be a lot of unpleasant consequences to having some "lump form" of exotic matter (i.e. matter with a negative mass) as well.

The key point is that if you push on it, the exotic matter will move towards the push, because it has a negative inertial mass.

This leads to a rather large variety of unpleasant behaviors.

 

[DaveC426913]

How about that column of air above the anti-gravity shield. If it no longer has weight, then the atmospheric pressure due to the gravity acting on the rest of the atmosphere will cause the entire atmosphere to be purged into space.

So, if your invention ever does turn this big wheel for free, then it will also immediately wipe out all life (aerobic life anyway) on earth...seems like a serious glitch.

You assume way too many absolutes. Absolute range (as opposed to diminishing), absolute speed (as if it would pour out of the Earth all at once) and all sorts of others.

That's like saying 'if we Earthlings ever invented the magnet, we would immediately be crushed by the Moon!'

 

[Aether]

You assume way too many absolutes. Absolute range (as opposed to diminishing), absolute speed (as if it would pour out of the Earth all at once) and all sorts of others.

I did assume that the gravitational mass of the Earth was concentrated at a point which is a standard assumption for modeling orbits, but it doesn't work out here as RandallB said. I didn't assume absolute speed: the air would pour out at the speed of sound; that and the radius of the evacuating cylinder would determine the rate at which the air poured out...then the seas would vaporize, I forgot to mention that.

 

[Mickey]

Maybe when we have anti-mass?

Or something that curves spacetime in the other direction?

 

[RandallB]

It's not?

Of course not - how much does one cubic foot of air just above the "shield" weigh do you think? How is that tiny weight going to compare with the 14lb/sq in pushing from all sides going to compare?

 

[RandallB]

I don't believe this analysis. Consider gravitational potential energy. The region immediately outside the circle is at zero gravitational potential relative to itself. Immediately inside the circle the potential would be "infinite"; at least a very huge number, relative to the Earth's surface. The physics at this discontituity cannot just be handwaved away. I remember an old discussion of this issue in Astounding Science Fiction magazine (as it still was at that date in the fifties of tha last century). They concluded that the physics discontinuity was a deal buster.

I have no clue what you are talking about.
SHREDDER was talking about a “Shield” not some negative mass or some "infinite" potential “inside a circle”. (I understand gravitational potential inside a sphere but not inside a flat circle.)

If you are trying to construct some mathematical model for such an impossible Shield, worrying about the interior of the shield is like claiming to know what a singularity looks like.
Showing that such a shield will not blow off our atmosphere is easy.
But building a rational explanation of how the law of energy conservation could be maintained is so unlikely that I suspect someone, much better at math than I, could show the law would have to be violated for such a shield to exist.

 

[DaveC426913]

Again, said as if a fridge magnet would suck the Moon down from the sky.

Why do you assume the effect would extend to the edge of the atmosphere instead of falling off logarythmically and having a barely useful range?

Why would it not be dependent on the power of the device? Say, 10,000 watts gets you antigravity in a one cubic inch volume?

Now you've got a one cubic inch volume of air that weighs nothing. Or almost nothing (nobody said the effect was a simple on/off).

You could still scale it up, but - like almost every other engineering effort we're familiar with, it would be difficult (though not impossible) to scale it up to commmercially-viable levels.

And nobody said it would be energy-efficient. An anti-gravity engine might replace the jets in a 747, but might be as heavy and use as much fuel.

 

[Aether]

Of course not - how much does one cubic foot of air just above the "shield" weigh do you think? How is that tiny weight going to compare with the 14lb/sq in pushing from all sides going to compare?

For simplicity, let's consider just one nitrogen molecule at mean sea level (MSL) and standard temperature and pressure (STP) instead of one cubic foot of air, and then see if we can predict what would happen to it if gravity were not acting directly on its mass, ok? My initial guess is that it would be out in space within a matter of minutes.

 

[DaveC426913]

BTW, are we all assuming that this shield is a simple material, i.e. does not need power to keep it running?

 

[DaveC426913]

For simplicity, let's consider just one nitrogen molecule at mean sea level (MSL) and standard temperature and pressure (STP) instead of one cubic foot of air, and then see if we can predict what would happen to it if gravity were not acting directly on its mass, ok? My initial guess is that it would be out in space within a matter of minutes.

Why???

 

[Aether]

My initial guess is that it would be out in space within a matter of minutes.

Why???

The vertical pressure gradient in the atmosphere applies a force against every particle in the atmosphere, a "pressure gradient force". I suppose that vertical equilibrium exists within the atmosphere only when this force has an average magnitude of 1g acceleration, is directed vertically outward toward space, and is exactly balanced by the force of gravity thus allowing air molecules to remain suspended. If the force of gravity is removed from any particle thus suspended, then the pressure gradient force is unopposed and would act to eject the particle from the atmosphere.

 

[DaveC426913]

The vertical pressure gradient in the atmosphere applies a force against every particle in the atmosphere, a "pressure gradient force". I suppose that vertical equilibrium exists within the atmosphere only when this force has an average magnitude of 1g acceleration, is directed vertically outward toward space, and is exactly balanced by the force of gravity thus allowing air molecules to remain suspended. If the force of gravity is removed from any particle thus suspended, then the pressure gradient force is unopposed and would act to eject the particle from the atmosphere.

This is oversimplifying.
What is 6" below the particle? What is 6" above the particle? In a vacuum, the particle will simply move freely - there is no force applied to it except its own kinetic energy. And after 6", it will enocounter some stopping barrier, likely a whole bunch of air.

 

[Aether]

This is oversimplifying.
What is 6" below the particle? What is 6" above the particle? In a vacuum, the particle will simply move freely - there is no force applied to it except its own kinetic energy. And after 6", it will enocounter some stopping barrier, likely a whole bunch of air.

Here's an article on hydrostatic equilibrium wherein they split a gas into a large number of cuboid volume elements, and then show that $$0=P_{top} \cdot A - P_{bottom} \cdot A + \rho \cdot g \cdot A \cdot h$$ -- http://en.wikipedia.org/wiki/Hydrostatic_equilibrium. Clearly, the pressure gradient force on a parcel of still air imparts an acceleration of -g to exactly balance the gravitational acceleration g. If gravity were removed from the equation would this volume of rising air have to overcome some drag on its way to space? Yes.

 

[DaveC426913]

Yes, locally. But it isn't a clear vacuum all the way to space, there's 100mi -6" of air. The particles would bounce around until they encountered the air and/or until it left the influencing anti-grav field.


Let's look at this again. You've got a single nitrogen particle in a steel box filled with hard vacuum (this keeps the air at bay). You zero the gravitational field inside the box (which by the way, we could do by putting it in orbit, but never mind).

What does the particle do? It does nothing more interesting than what it was doing. It bobs around with its kinetic energy. There is no net upward force.

The only net upward force that's going to affect that particle is if you let air into the box (or remove the box) The weight of air pushing down will float the bouyant particle. That's the upward force.

But the particle isn't going to stay within the anti-grav field forever. It will rise out of the field (if allowed to) and be influenced by gravity agaijn.

I could see you point if we could remove the influence of gravity from a particular particle or particles regardless of where it is or where it goes, but no one is proposing that, they are proposing a shield that removes the influence of gravity on anything within a certain defined boundary. Particles leave that volume and they act normally again.

 

[RandallB]

For simplicity, let's consider just one nitrogen molecule at mean sea level (MSL) and standard temperature and pressure (STP) instead of one cubic foot of air, and then see if we can predict what would happen to it if gravity were not acting directly on its mass, ok? My initial guess is that it would be out in space within a matter of minutes.

Sure - I like simple.
The tiny wieght of the one molucle will not hold it against the top surface of the 5 foot flat shield even if gravity is working. Remember it has tempature and will hit and rebound off the surface at a speed of over 300m/sec. Granted not escape speed but it should be on its way into some kind of orbit with or without gravity – why doesn’t it?
Other molecules of air keep kgetting in the way!
= AIR PRESSURE with or without the shield.
AH but wait, maybe the shield may change the net pressure holding it in place – let us take a look.
Say just 100 feet up – all the moulucles here for 400 yards around and more will have their wieght reduced due to shield blocking gravity but by how much? They may not “see” Earth gravity through the shield but much more than 90% of the Earth is in view around the shield so their wieght reduction is small.
Likewise for the thousands of feet of air above that.
So no; Even with a gravity "shield" the only way that nitrogen molecule is going to be out in space within a matter of minutes is if it can find its way onto some space launch before they close the door.

 

[Aether]

I could see you point if we could remove the influence of gravity from a particular particle or particles regardless of where it is or where it goes, but no one is proposing that, they are proposing a shield that removes the influence of gravity on anything within a certain defined boundary. Particles leave that volume and they act normally again.

Ok, we're not in disagreement then.

So no; Even with a gravity "shield" the only way that nitrogen molecule is going to be out in space within a matter of minutes is if it can find its way onto some space launch before they close the door.

I am not disagreeing with you about what we might expect from a hypothetical gravity shield. As I already said, my initial comment was based on this bad assumption:

I did assume that the gravitational mass of the Earth was concentrated at a point which is a standard assumption for modeling orbits, but it doesn't work out here as RandallB said.

which would mean that a gravity shield located on the surface of the Earth would remove the gravitational influence of the Earth's mass everywhere above the shield within a cone having the center of the Earth as its vertex and the gravity shield's perimeter as its directrix.

As long as we all agree that the nitrogen molecule is going to be out in space within a matter of minutes if we could remove the influence of gravity from it regardless of where it is or where it goes, then we're not in disagreement about anything.

 

[RandallB]

As long as we all agree that the nitrogen molecule is going to be out in space within a matter of minutes if we could remove the influence of gravity from it regardless of where it is or where it goes, then we're not in disagreement about anything.

But we don't agree, as I said even with gravity acting on it and nothing else it would be heding for space or at least near space, its individual mass and wieght has litle to do withit staying in there area it is in.
With gravity in a vacuum chamber with only that one molecule in it and that tube extended all the way to the moon it could get there - no need for gravity shield.

 

[Aether]

But we don't agree, as I said even with gravity acting on it and nothing else it would be heding for space or at least near space, its individual mass and wieght has litle to do withit staying in there area it is in.

Wrong.

With gravity in a vacuum chamber with only that one molecule in it and that tube extended all the way to the moon it could get there - no need for gravity shield.

Wrong.

The tiny wieght of the one molucle will not hold it against the top surface of the 5 foot flat shield even if gravity is working. Remember it has tempature and will hit and rebound off the surface at a speed of over 300m/sec. Granted not escape speed but it should be on its way into some kind of orbit with or without gravity – why doesn’t it?

It doesn't because Earth's gravity (with an acceleration of about $$9.8m/s^2$$) will stop the ascent of any ballistic projectile having an initial vertical velocity of 300m/s in about 31 seconds or 4600 meters.

 

[DaveC426913]

As long as we all agree that the nitrogen molecule is going to be out in space within a matter of minutes if we could remove the influence of gravity from it regardless of where it is or where it goes, then we're not in disagreement about anything.

Anti-gravity device premises:
1] a device or material where a volume near it has gravity reduced or negated. Anything in that volume does not experience gravity, anything outside that volume does.

2] a device or process that targets mass, instilling that mass with the ability to ignore gravity.

If you used #2, pointed it at your nitrogen atom and fired, your N atom would now have zero mass, and would retain zero mass for an undetermined time, regardless of what else it did. In this case, the N atom would rise up to the near the edge of the atmosphere. It is infinitely bouyant. Any pressure from air would force it up. But it would do so by bouncing off all the other atoms on its way up. It would not happen rapidly.

But that is only one idea for anti-gravity, and it's not the one I usually think of. I doubt we can imbue matter with the property of weightlessness. I expect that it will be more like a field or volume, inside of which, mass does not experience the effect of gravity. This does not affect the matter, only the gravity reaching the matter.

 

[pervect]

Anyone up for moving this to the PF lounge? Doc Al, ZapperZ?

 

[RandallB]

Anyone up for moving this to the PF lounge? Doc Al, ZapperZ?

Or just close it.

 

[Aether]

Anyone up for moving this to the PF lounge? Doc Al, ZapperZ?

Or just close it.

I would like to quantify the time required for a weightless nitrogen molecule to work its way out into space before this thread is closed, but that's not a subject for either relativity or PF lounge.

 

[SpaceTiger]

I would like to quantify the time required for a weightless nitrogen molecule to work its way out into space before this thread is closed, but that's not a subject for either relativity or PF lounge.

How are we defining "space" in this discussion? If gravity were suddenly turned off for the atmosphere, the pressure gradient would act (at the speed of sound) to force many of the molecules away from the Earth's surface.