Question
What is gravitational shield? Could we make something that can shield an object from gravity?
Answer
Chris Smith put this question to astronomer Matt Bothwell...
Matt - I really like this question. Gravitational shielding is this science fiction idea that you could create some kind of exotic material and it would shield the force of gravity so you could effectively use that to reduce the weight of objects which will have all kinds of amazing applications, as you can imagine, if you want to make really tall buildings. For example, you can make the construction materials weightless and just levitate them up.
Chris - That would be lovely wouldn’t it? Climbing the stairs would be a gift - it would be easy.
Matt - We already have experience with shielding forces. Gravity is one of the four fundamental forces of the universe and we already know how to shield one of the other fundamental forces of the universe which is electromagnetism. Electromagnetic shielding is pretty common in our phones and in our computers, and our microwave doors are electromagnetically shielded. It’s good that they are because then we don’t cook ourselves every time we turn our microwaves on.
Unfortunately, gravity and electromagnetism are pretty different and, as far as we know, we can’t build gravitational shieldings. There have been lots of experiments through the 20th century to investigate whether this is possible and they’ve all come up negative so, as far as we know, it’s not actually possible.
I think one easy thought experiment that you can prove to yourself that it would be impossible is that it would violate something called “conservation of energy,” which is one of the fundamental rules in science. If you can imagine having some sheets of gravitationally shielding material, you could put this sheet under a very heavy object. Then this object would become weightless and you could lift it up in the air with ease, and then you could just quickly whip the sheet away and the heavy object would come crashing down to earth and release loads of energy, and all that energy has just come from absolutely nowhere. If anything gives you something for nothing in this way we have reasons to be suspicious.
Comments
Can we make a gravity shield?
Well, it is difficult to answer yes/no, but different aspects can be considered -
I would prefer to call it inversion of gravity rather than shielding. There is no such thing as anti gravity as that would even in theory, require a negative mass.
1) Can there be a object/sheet kind of shield - No.
2) Can there be a mechanism - Probably yes. I do not mean fake impressions like em levitation, gyroscopic motion, centrifugal force and so on.
3) It appears to violate law of conservation of energy - There can be ways around it - energy remains conserved in a sling shot maneuver, and in a similar manner, it could be in case of a gravity modification mechanism too. It may take energy from kinetic energy of earth. We can also say we would not know where the energy came from rather than saying conservation is violated.
4) Would it violate equivalence principle - No, Because it is altering the gravitational field, and not the gravitational. mass.
5) Does General Relativity prohibit gravity shielding - Not really. GR mainly gives us is how the space curvature evolves. Natural outcome of set of tensors present in space/energy is one way (inward or say downward) curvature of space and that is what GR deals with. GR (per my knowledge) does not talk about two way curvature, or in other words, it does not talk about possibility or impossibility of inversion of that one way curvature.
I personally think inversion drive is possible.
"conservation of energy" is not necessarily violated
Chris, I do not agree that the law of conservation of energy would be necessarily violated by gravity shield. I think energy will be conserved in the same manner as it remains conserved in a slingshot maneuver. The difference however would be - In case of sling shot, the distance changes and gravity is reduced/increased (shielded) with distance. I am not suggesting that a shielding sheet/object exists, actually, they do not. But I would not consider it impossible to have some mechanism that can shield gravity that would keep energy conserved. Actually, conservation law applies to a closed system. If we are shielding gravity, the setting may not be considered closed.
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