# Could general relativity be right simply because it explains gravity?

1. Jan 26, 2008

### mdeng

We all know magnetic fields can be blocked by metal, light can be blocked by
non-transparent materials, forces exerted from object of momentum can be
blocked by placing another object in between, except gravitational pull. This
is very much counter-intuitive.

GR provides a good explanation why gravity can't be blocked. According to GR,
the only way to "block" gravity is to have a counter force (e.g., a flying
rocket, but not a stationary object) to cause us to de-accelerate to an inertia
state, or to have a counterforce to prevent the initial acceleration to start
with (e.g., place another "Earth" above our head to cancel Earth's gravity.

Can we say this gives an intuitive reason why GR should be right

2. Jan 27, 2008

### castlegates

Let me know what metal blocks magnetic fields so I can build me a motor that runs without input energy. :-)

3. Jan 27, 2008

### mdeng

Sorry, I meant dynamically changing electromagnetic fields. But you are right, I absent-mindedly overlooked the static one. This may be why Einstein tried to unify EM field with GR somehow.

Static magnetic fields can be a bit tough to use the same trick as GR, as it can either attract or repel, while gravity is only one direction.

4. Jan 27, 2008

### Fredrik

Staff Emeritus
It's no easier to "block" gravity in Newton's theory, so why should this explanation favor GR?

By the way, no theories are "right". Some just agree with experiments to a higher degree than others. I don't believe it's possible to explain why GR (or any theory) should agree with experiments.

5. Jan 27, 2008

### Staff: Mentor

Iron. A 7T whole-body MRI magnet can be shielded with ~400 tons of iron.

I don't know how you would use the fact that iron blocks magnetic fields to build a motor that runs without input energy.

6. Jan 27, 2008

### peter0302

None of the forces are really "blocked." EM can be interfered with by the intermediate presence of other force-producing matter because of the variable (+/-) nature of the charge. Thus, the EM force between two sources can be cancelled out by a third placed inbetween them.

The real issue is not "can gravirty be blocked" but rather is there anti-gravity? The answer so far is not that anyone's seen. And, by the way, the strong force can't be "blocked" either. It just doesn't have any significant magnitude until the distances between the particles is incredibly small.

7. Jan 27, 2008

### yogi

Its always been a question for me whether gravity is blockable - or if it is neutralizable - take the case of a mass within a uniform spherical shell of matter - the net force is zero for all locations therein- but does the gravitational field still exist (call it curvature if you like) - but what happens if you remove the north hemisphere of the matter shell - bingo, the mass is now attracted to the remaining structure (the South hemisphere) - so what was the state of the internal space before the North hemisphere was removed? How would it be possible to determine if the interior of the shell were in any way different from free space where there is little or no gravitational field? I am curious if the inertia of the mass within a shell of material would be any differert than what would be measured for the same object in free space. In his short book "Sidelines On Relativity" Einstein states: "The General Theory of Relativity teaches that the inertia of a given body is greater as there are more ponderable masses in promimity to it..."

Any Ideas?

8. Jan 27, 2008

### rbj

just want to second the point. a Faraday cage only works because there are both postive and negative electric charges (and the physical chemsitry of metals allow free electrons to move around freely inside the lattice of the metal atoms). if there were positive and negative masses, we might be able to construct something that would block pertubations in the gravitational field.

except i think that there would be all sorts of contradictory phenomena if there were negative masses. in free space, place a body of postive mass in the vicinity of a body of a negative mass (of the same mass in magnitude) and, keeping the equivalence principle in mind, ask yourself what would happen. would energy be conserved?

9. Jan 30, 2008

### mdeng

Thanks for the rephrasing. After reading the responses I got, I think my original statements were erroneous. So called "blocking" effects, aside from being imprecise, typically only reduce the effect to "unnoticeable". Momentums can't really be "blocked", EM can't be either unless the metal in between is grounded, and so forth. Having said that, however, I would still note that there are no known way to "reduce" the effect of gravity why we seem to be able to do that for many (if not all) "ordinary" forces/fields (including static magnetic field, which can be reduced by a large iron mass).

To cancel the effects of a force/field, "anti matter/force" would be a better way to look at it.

GR simply removes the gravity force from the picture and thus avoids the issue of whether anti-gravity exists or not, and neutralized the question of how to "reduce" it.

10. Jan 30, 2008

### mdeng

I am not clear what your question really is. However, could you think of a hallow ball of great great mass, what would it feel like in the center of this uniform ball? The moment you move away from the center, you suddenly would feel the gravity. I am not sure whether this relevant to your question though.

11. Jan 30, 2008

### mdeng

What about grounding? I think it's an essential part of the cage. For gravity, what would "ground" mean? I am not even quite sure what "grounding" means mathematically (in terms of all the great equations we have) in the case of Faraday's cage.

I can't even comprehend what it means to have a body placed between +/- masses. That would be my issue. I guess there will be other issues.

12. Jan 30, 2008

### rbj

oh? what difference would it make?

so what does it mean for the E&M case?

i don't either. i know what ground is and, in the case of E&M, grounding is not necessary for a Faraday cage. what is necessary is that the door is closed (that it's "sealed" so that any holes are much, much smaller than the wavelength of any radiation that it is meant to keep in or keep out).

13. Jan 30, 2008

### mdeng

I thought we always need it for shielding against a dynamic EM. Hmm, thinking about it again, it seems grounding is only needed to prevent the cage from becoming an EM source again.

For a static electric field, grounding would mean to bring the electrical potential of the cage wall to be at the same level as those outside of the cage. thus it would be needed. That's my understanding. I have no idea what it means for a magnetic field.

I was thinking about a static gravity field as an analogy to a static electric field.

14. Jan 30, 2008

### rbj

it has nothing todo with that.

on what basis of fact or theory do you base that statement?

the metal being "in between" is not sufficient. a Faraday cage surrounds something (either a transmitting or receiving antenna of sorts). you put something in a completely enclosed metal shell, and no appreciable external EM field will make it inside. likewize for an internal field getting out.

but Coulomb's law has the same form as Newton's law of gravitation. there is no cancelling of forces in either.

no, the "better way" to look at it is that we have both positive and negative electric charges that can be arranged to arithmetically cancel the EM field of some source. if we had both positive and negative masses (and and i don't think that what particle physicists mean by "anti-matter"), then we could arrange them similarly to arithmetically cancel gravitational fields emitted by some source. but we don't have negative masses and if we did, we could construct a perpetual motion machine with such. the energy crisis would be gone. that's what i was alluding to before.

whether it's a force or whether it's a curvature of spacetime that causes matter to fall toward other matter is not the issue.

i don't think you understand any of this.

15. Jan 30, 2008

### mdeng

I was wrong. see my reply above.

I mean between the inside and the outside.

Your point is that Coulomb's law does not tell how to "cancel" (or shield from) a electric field, and yet it can be done by a Faraday cage? If so, what conclusion did you want to draw?

However, GR says that there actually are no force to block in the first place. All we have is space curvature. Can this effect (curvature) be canceled or how would we cancel it? I have no idea, but whatever it is would seem to be different from the conventional cancellation mechanism we have seen so far.

16. Jan 31, 2008

### rbj

i'll throw the question back to you. do you understand how a Faraday cage works? how does it "block" EM fields in the inside from getting out (or fields on the outside from getting in)? Coulomb's law applied to some charge on the outside still has effect on charges on the inside. but maybe we can find another charge (of opposite sign) that will arithmetically cancel that effect.

well, if positive masses curve it one way, maybe negative masses (if they existed) would curve it the other way. but the Newtonian model ain't half bad, if you're not a black hole or a universe. it's what they still use to describe the motion of the planets and nearly everything else in our solar system (sure, it misses very slightly regarding the precession of the perihelion of Mercury's or other planets' orbits, or similar effects for GP-B, but these are tiny). so just as opposite-signed charges move around in the metallic shell of a Faraday cage to cancel (not block) EM fields (and you can use Gauss's Law to show how that works), if there were negative masses (and Newton's laws of motion and gravitation worked the same way on them, except for the sign change, just like Coulomb's law), it is conceivable that something could be constructed that would behave like a Faraday cage to cancel (not block) gravitational fields. as a field, gravity is also inverse-square so Gauss's Law works on gravitational fields, too.

sure it would be cool to somehow build one of these gravitational Faraday cages and use them to make a room where you could step in and be weightless. but, besides the fact that we don't see any of those negative masses hanging around anywhere, there are theoretical problems regarding their existence. just as positive masses attact everything (positive masses will even attract a negative mass, if you count the number of minus signs carefully), it turns out that negative masses repel everything. even other negative masses. you could also make yourself a perpetual motion machine with a mass of +1 kg and another one of -1 kg. put them both in free space a meter apart and watch what happens. the +1 kg mass will attract the -1 kg mass which will move toward it, but the -1 kg mass repels the +1 kg mass and the latter will move away from it. it's like this loser guy in high school that had a crush on this hot chick: he moves closer, she moves away, he moves closer still, she move away more. doesn't ever stop. put them on some kinda radial arms and hook 'em up to a generator and no more energy problems.

so, mdeng, if you find one of them negative masses, let's get together and make a lot of money with it. i'll bring the positive mass. to hell with the Patent Office who say explicitly that they will not patent perpetual motion machines.

17. Jan 31, 2008

### mdeng

A Faraday cage cancels the effect of an external electric field on the objects inside the cage by creating an opposite field on the cage. Thus, yes, if there are negative mass, we can build a Farday cage to block gravity using materials which "conduct" both +/- masses.

Also did some more reading about this grounding mechanism that has fused in my mind for many years (thanks for raising that question to me :). It actually has its (good/necessary) purpose, though irrelevant to our discussion. For a Farady cage which has a change inside, the grounding of the cage will remove the charge from the cage and thus block the effect of the charge inside on those external objects that are also grounded. However, grounding would be unnecessary to shield the objects inside the cage from external electric fields. For (dymanic) electromagnetic fields, grounding is not necessary for the shielding but is helpful to reduce the interference (due to capacitive effect) between the cage and surrounding EM devices.

I'll sure share with you if I ever stumble upon even only a few ounces of negative masses, for your help that rendered my original question pointless. :) So, according to GR, perhaps there is no "gravity force" about which we can talk about "blocking", there is space curvature that seems isomorphic to electric fields which are known to be blockable. So GR can't be said to be valid because "it explains away the question of blockability of gravity". Thanks.

Last edited: Feb 1, 2008