# Objects that can produce a shadow of gravity

First of all, I must confess that my knowledge of Physics is rudimentary (high-school level). So, the question I am going to ask may not be make much sense, and I apologize in advance if this is the case.

Question: Are there objects that can block graviational waves just as matter blocks light and produces a shadow? That is, a body in the shadow will not be able to detect any gravitational force due to the body that is being blocked.

A related question then will be: can there be places where there can be no gravity at all?

I did some search on the Internet which came up with references to Lesage's theory, a book on the discovery of gravitational waves, and a similar question at http://van.physics.illinois.edu/qa/listing.php?id=42143 though the answer to this last question seems to talk about shadows of light rather than that of gravity.

Let's first make a difference between blocking gravitational waves and blocking a static gravitational field. The difference is similar to that between blocking light and blocking a static electric field.

Let's start with the blocking of gravitational waves/light. The reason objects cast a shadow is because the light interacts with the electrons in the material. The electrons get accelerated by the light and thereby the light loses some of its intensity. If the material is thick enough all intensity is lost and the material casts a shadow. A similar reasoning holds for gravitational waves. When waves from for instance a black hole merger reach the earth, the earth gets deformed and this costs energy. The gravitational wave intensity will thus be slightly lower behind the earth. To create a proper shadow you would have to place many many many earths together, but I guess it would be possible in theory.

The second question if it is possible to shield a static gravitational field. For electric fields we know that we can shield them by placing a grounded electrical conductor between us and the source. The electric charges in the conductor will be moved by the electric field. These charges will thereby create an electric field of their own. Inside and behind the conductor this field exactly cancels the original electric field, thereby shielding the electric field. For gravity I would not know any material/mechanism giving the same behavior. A fundamental difference between gravity and electromagnetism is that in electromagnetism there are both positive and negative charges, while in gravity there is only positive mass. It is therefore not so trivial to cancel a gravitational field.

Gravitational waves and gravity are not the same thing, as the question seems to suggest.

According to Einstein's theory, matter produces a gravitational field, by the virtue of curving up of spacetime. I assume the cosmological constant to be zero. Therefore, by its very definition, any object would have a gravitational field associated with it, and cannot "block" gravity. There can be places where the gravitational field is negligible, although they are usually very far away from sources.

The part about "blocking" gravity waves is different however. Assuming they obey the wave equation when gravity is linearized, they can be deflected (see this) given precise boundary conditions for the linearized wave equation. Deflecting gravity waves is one way of such a "blocking".

• There's also a bit of absorption -- the energy used to move the ligo arms and wobble the earth is "lost"... although, energy of a gravitational wave is a sketchy subject and also isn't exactly conserved in this case (energy is just a component of a stress-energy tensor and you can't really integrate it if the space isn't flat). – orion Aug 24 '16 at 10:17
• @orion i was thinking about including attenuation too, but didn't as i dont have a proper idea to think about it..thanks for pointing this out... – Bruce Lee Aug 24 '16 at 10:20
• Thanks for your answer. A clarification regarding your second paragraph: when I wrote "object" I did not mean to say matter, I meant a theoretical construct that does not have its own gravitational field. Otherwise, as you say there will not be a shadow. An object which is not a source of gravitational waves and, in addition blocks them. For example, for light, glass is not a source but lets light pass, whereas a stone is not a source but blocks light. Does your answer change with this clarification? – user137846 Aug 24 '16 at 10:31
• @user137846 the problem with your definition is, any theoretical construct occupying spacetime, as you suggest, contributes to stress-energy tensor and as a result has its own gravitational field. That is the nature of gravity, it simply couples to everything. – Bruce Lee Aug 24 '16 at 10:42
• A stone interacts with EM waves (well so does glass) -- has charged particles and responds to the incoming wave. For gravitational wave, the "charge" is mass which produces gravitational field. So... no interaction with gravitational field (no own field), no effect on the wave. It's the essential symmetry of physics, interaction goes both ways (generalization of 3rd Newton's law). And as Bruce said... anything with energy and/or mass curves space, so there is no room for such a construct in the theory. – orion Aug 24 '16 at 10:45