1

If we can shield a charge from electrical forces by putting it inside a hollow conductor. Can we shield a body from gravitational influence of nearby matter by putting it inside a hollow sphere or by some other means ?

Sabbir Vadharia
  • 67
  • 2
    Possible duplicates: http://physics.stackexchange.com/q/2767/2451 , http://physics.stackexchange.com/q/19896/2451 and links therein. – Qmechanic Apr 28 '14 at 10:25
  • The substance you want is called cavorite, and it is unfortunately fictional. http://en.wikipedia.org/wiki/The_First_Men_in_the_Moon – Eric Lippert Apr 28 '14 at 13:31
  • 1
    Experimental evidence to date indicates that no such effect exist http://en.wikipedia.org/wiki/Gravitational_shielding. Gravitational shielding is considered to be a violation of the equivalence principle and therefore inconsistent with both Newtonian theory and general relativity. – Tea is life Apr 28 '14 at 13:43

3 Answers3

10

No. Shielding works in electrostatics because there are both positive and negative electric charges. In gravitation, there is only one kind of charge, i.e. mass, which is positive. As a consequence, gravity can only attract, and there is no way to counteract this by a repulsive force also originating from gravitational effects.

Frederic Brünner
  • 15,722
  • Just as an aside. Why is gravity "positive"? Because it attracts? But "positive" charge attracts "negative" one just as much as the negative attracts positive. And two positive charges repel each other just as much as negative ones do. Isn't positive-negative just a convention? – bright magus Apr 28 '14 at 13:10
  • @brightmagus: Mass is positive and the comparison to charge is just an analogy. "Positive repels positive" is only true for the electrostatic force. – Frederic Brünner Apr 28 '14 at 13:13
  • That's why I asked: why gravity is positive. Is it because attraction is in general considered a "positive" force? Like people who attract others are "positive" and those who repel are "negative"? (Any link, as I don't want to make a mess here :-) ). – bright magus Apr 28 '14 at 13:27
  • 1
    @brightmagus: I am not sure what you are asking. Is it about semantics? Are you asking me why gravity is attractive? Or why mass is positive? – Frederic Brünner Apr 28 '14 at 13:29
  • I'm asking what is "positive" about gravity? The direction of the force? Is attraction considered positive (and repulsion negative)? But if my question is not self-evident, then lets skip it. – bright magus Apr 28 '14 at 13:35
  • @brightmagus: What I refer to as "positive" is mass, it is a positive number. Gravity, as a force, is negative. This is to be understood as the sign in Newton's law of gravity. – Frederic Brünner Apr 28 '14 at 13:38
  • @brightmagus: The gravitational force on a mass $m_1$ due to a mass $m_2$ is $F=-Gm_1m_2/r^2$. The sign of $m_1$ and $m_2$ is not important since $m_1m_2>0$ as long as they both have the same sign. There isn't anything inherently positive about mass, since the physics here would be unaffected by a change in sign. If you chose mass to be negative though, Newton's second law would be $F=-ma$, which doesn't look as clean as $F=ma$. – George G Apr 28 '14 at 14:02
  • Thanks George, that's what I wanted to know (and suspected): a convention that simply tells the direction of force. – bright magus Apr 28 '14 at 14:08
  • So will we be able to do sheilding using antimatter? (If we can make it?) – Kartik Apr 28 '14 at 14:39
  • @Kartik: Antimatter also has positive mass/energy, so the answer is no. – Frederic Brünner Apr 28 '14 at 14:42
  • @FredericBrünner OK. I did not know about that. I always thought that Antimatter has negative mass (because of 'anti' in its name.). – Kartik Apr 28 '14 at 14:47
  • @Kartik It refers to other properties, e.g. opposite charge. Mass, however, is the same. – Frederic Brünner Apr 28 '14 at 14:48
2

Shielding works because the electrons (or other free charge particles) migrate in such a way that the electric field the conductor generates inside itself is equal and opposite of the external field applied to the outside.

This only works when there is a repulsion force.

ratchet freak
  • 299
1

Your question brings up an interesting sidelight. Schiff and Barnhill[1] showed that an electron inside a conducting cylinder will be subjected to an induced electric field that will counteract the pull of gravity. Their reasoning can be summarized as follows.

  1. The atoms in the crystalline metal that the cylinder is made up of do not fall through each other because of the Coulomb forces between the electrons of the individual atoms.

  2. When the conducting cylinder is placed with it's axis of rotation perpendicular to the ground, the force of gravity places an additional force on each atom that makes up the cylinder.

  3. The atoms still do not fall through one another and Barnhill and Schiff surmise this is due to an increase in the electric field between the electrons surrounding each atom. The increase in field is just enough to support each neighboring electron against the downward force of gravity.

  4. Near the surface of the cylinder, this electric field that separates the electrons in neighboring atoms can be observed outside the cylinder wall.

  5. If the cylinder is long enough and narrow enough,the electric field inside the cylinder will be uniform. An electron introduced inside the cylinder will simply float in this induced field which is strong enough to support the weight of an electron against gravity.

One of the reasons physicists care about all of this is that we're interested in finding out if the anti-particle of the electron, (the positron) falls up or down in gravity[3]. To test this, we need to understand what the positron will do if we contain it in a metal vessel, a cylinder for example, to perform the experiments. If positrons fall down under gravity and Schiff and Barnhills' theory was correct, then a positron should fall down through the cylinder at twice it's normal rate of descent. If they fall up under a gravitational pull, then the positron should also be suspended within the cylinder.

This might all sound a bit outlandish, but Witteborn, and Fairbank performed an experiment that confirmed Schiff and Barnhill's theory. There were some issues with the experiment. The experimenters surmised that the walls between crystaline domains within the material making up the cylinder should have influenced the results. It appears that they didn't. This behavior is not yet theoretically understood and consequently the results of the experiment are not widely agreed upon.

References

  1. Schiff and Barnhill https://journals.aps.org/pr/abstract/10.1103/PhysRev.151.1067

  2. Witteborn and Fairbank https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.19.1049

  3. More on falling positrons https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.64.237

dolphus333
  • 667