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Magnetic field ?

  1. May 14, 2010 #1
    Can B fields diffract , If I could magnetically shield something and then cut 2 slits in it would the B field diffract through the opening . If I have two B field sources how do they interfere with each other .
  2. jcsd
  3. May 14, 2010 #2
    Not quite. Waves diffract. Waves occur in a variety of media. Water waves in water, sound waves in air. Light waves are oscillations in the electromagnetic field.

    So, the magnetic field itself can't diffract, but waves in the magnetic field do. If you had a double slit or two sources they would interfere in just the same way as any other wave. But this is a roundabout way of saying that light diffracts---its the wave in electromagnetic field you are asking about.
  4. May 15, 2010 #3
    Do gravitational waves diffract .
  5. May 16, 2010 #4
    Good question. Gravitational waves are a consequence of general relativity, and the equations are non-linear. This means that there is no superposition principle for us to use when dealing with them. In other words, the way these waves interact is much more complicated that waves in linear media (like light). Not that this is very unusual -- water waves are also non-linear.

    But in these cases when the energy involved is small, the equations are nearly linear. So let's talk about that case. Suppose the gravitational waves are small. They will interact with each other just like any other wave (constructive, destructive interference, beats, whatever).

    What about diffraction? Gravity is unique in that there is no way to shield against it. For electromagnetic waves it is simple to build a single slit. I'm not so sure about gravity. I think I will have to let someone more knowledgeable than me speak to the idea of actually constructing a single slit for gravitational waves. It might be impossible. It might be easy. I'm not sure.

    But if one could build a gravitational slit then, yes, the waves would diffract.
  6. May 16, 2010 #5
    We don’t necessarily need 2 slits do we , couldn't we just have 2 sources of gravitational
    waves separated by a distance and then the waves would interact with each other .
    Just like the interfere between 2 speakers .
  7. May 16, 2010 #6
    Yes. That's what I was driving at in the second paragraph -- interference would definitely happen the standard way.

    But then I got thinking about diffraction and actually building a slit (or any other kind of diffraction). That's where I got stuck. I was thinking that it's not possible to build a gravitational mirror, for example. But one certainly has a gravitational lens. So, maybe just the right configuration of masses could warp space-time to create a single slit for a gravitational wave?
  8. May 16, 2010 #7
    If gravity cannot not be shielded then how does it interact with matter or energy.
  9. May 16, 2010 #8
    Not sure what you mean. Gravity interacts with matter and energy precisely because its not shielded.

    You can shield an electric circuit by surrounding it by a conductor. The free electrons rearrange themselves to cancel out the electric field on the inside. But for the electric field you have both sources (positive charge) and sinks (negative charge). For gravity you only have sources.
  10. May 16, 2010 #9
    if we cannot shield gravity it seems like the field would just go through the material and not pull it towards earth . Can other gravitational fields interact with other gravitational fields.
  11. May 17, 2010 #10
    When the electric field is shielded, there is no electric field (or force) on the other side. The field is absorbed by the sinks (i.e., negative charges). So, for gravity (since there are no sinks), the field does go through the material. For example, the moon still feels the gravitational pull of the sun even if the earth is directly in front of it.

    But what the field means is that any substance that participates in the force (for gravity that is anything with mass, i.e., everything) will experience a force along the field lines. This does not affect the field at all. So, an electron experiences a push when placed in an electric field not because it is absorbing or creating field lines (even though it does do that). It feels the force simply because it has charge (in fact, that is the definition of electric charge).

    As far as gravitational fields interacting with themselves this happens in two ways. The first is the almost trivial fact of superposition. Two masses will each create a gravitational field that add together.

    The second way is probably more along the lines of what you are thinking. Remember that the gravitational field is fundamentally the curvature of the surrounding space-time. So if a gravitational wave propagating through space enters the gravitational well of another mass, it will bend just like light or any other particle. It will follow the contours of the space-time curvature. But even in this case, the gravitational wave will simply pass on through like light bent around a massive star.
  12. May 17, 2010 #11
    Thanks for your response and i am enjoying this conversation , So you are saying that a gravitational wave it bent by another G field just like light , But obviously gravitational waves can propagate out of a black hole where as light cannot. And are you saying that gravitons interact with other gravitons , And you say that gravitational waves obey the principle of super position , if we apply this to photons , photons obey the principle of super position but photons do not couple to other photons , So how does a particle obey superposition but not couple with other particles of the same type.
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