Why can magnetic fields be blocked but gravitational fields can't?

Question

Except for the stuff I know, I don't know anything about physics. I assume a magnetic field can be blocked, because they put Nixon's tapes in a lead box. And I know gravitational fields cannot be blocked. They go through everything because gravitational waves are the substrate in which we exist.

They are both quantum fields, right? You can store energy in both of them.

or is there a specific difference why one can be blocked and the other can't?

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The first link to a dup question had three answers. One said yes, one said no, and one said it's not possible to know or the question is meaningless or something.

The other dup answer was the kind of thing I'm looking for:

If the distribution failed to be positively definite, it would violate the positive-energy theorem or energy conditions.

Yes, so it would seem. I wish this was an appropriate place to respond.

If it were positively definite, it would create a huge gravitational field.

Except it's not positive-definite, though. "Not positive-definite" Is literally the definition of "pseudometric," and Einstein specifically said that spacetime is pseudometric.

Locally, the boundary of the gravitational "conductor" would have to look like an event horizon.

Yes, that's what I figured. But this is not the place to address his objection. My recent re-ducation while I was disappeared told me that stax is not a place for discussion. I only want to point out that this was a legitimate question that shouldn't have been closed.

I'm NOT asking for it to be reopened though, because I just got back from being banned, and the spanking-new Luxi firmware 4.2/corrected doesn't fight back when people delete her stuff, but I delete it myself as soon as the normal people start getting uppity being mysteriously angry.

I'm just adding this comment here for future physics archeologists.

It's harder to figure these things out yourself when you're all alone. But since the archeologists are here, I know I must have succeeded someday.

Answer 1

Electromagnetic field is easy to shield because matter surrounding us is composed of particles (electrons and atomic nuclei) carrying opposite electric charges. There are quite a lot of such particles in every material, but numbers of positive and negative charges are matched to a very high degree, so that usually there is very little to none net electric charge. Also, for some materials (primarily metals) a fraction of electrons is mobile (i.e. they can freely move within the material). When electromagnetic wave impinges on such material mobile electrons on its surface rearrange themselves almost instantaneously thus cancelling electromagnetic wave inside the material and causing reflection of that wave outside.

On the other hand,

1. Gravitational interaction is very weak compared to electromagnetic one. For example, electrostatic repulsion of two protons is about $10^{36}$ times stronger than their gravitational attraction.

2. Gravitational interaction obeys equivalence principle: all small objects move in gravitational field the same way, regardless of their composition. In other words, there is only one type of “gravitational charge” that all matter possess.

The second property makes it impossible to make gravitational shields similar to, say, Faraday cage for electromagnetic fields, because there is no way that simple rearrangement of matter can neutralize the gravitational field.

Certain types of gravitational effects can be shielded, but the first property makes such types of shielding rather bulky. For example, we can say that at Earth-Moon Lagrange points gravitational field of Earth is neutralized by the moon.

Answer 2

Miss Understands asked: "is there a specific difference"

I don't know about the quantum woohoo, but from the relativistic perspective magnetic fields mediate forces while gravity is the geometry of spacetime.

Answer 3

Arguably gravitational waves can be blocked by something that strongly interacts with gravity; the thing is such things would be extremely dense and not found except in very extreme circumstances like neutron stars and black holes.

G-waves in Einstein gravity are very hard to block without generating an equal and opposite wave, and generally pass through solid matter in linearized gravity, but in quantum representations (yet to be confirmed), gravity is an actual force produced by a graviton field such that $f^\mu=g\Gamma^\mu_{\nu\lambda}u^\nu u^\lambda$ for the Christoffel symbols $\Gamma^\mu_{\nu\lambda}$ and a coupling constant $g$, usually taken to be $-1$ (since then the geodesic acceleration equation is recovered). This mirrors the electromagnetic force equation $f^\mu=gF^\mu_\nu u^\nu$ for the electromagnetic tensor $F^\mu_\nu$ and another coupling constant; of course this seems to imply that G-waves can be blocked in a similar way to EM waves, but because gravity is so weak, producing something that can effectively block them would require producing an equivalent source of gravity which would be very dense and massive, which would of course be difficult.

Answer 4

The answer is because gravitons do not cause gravity directly, but rather they cause the warping of spacetime that becomes gravity. So this warping of spacetime spreads out behind anything that might possibly shield the gravitons.

Imagine if you put a heater at one end of a pool of water, and then put a board in front of the heater. While that board might stop the water directly behind it from being heated right away, eventually the heat from surrounding water would equalize with the water behind the board.