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The magnetic field as dark matter

  1. Jan 28, 2008 #1
    The magnetic field as "dark matter"

    Is it possible that the magnetic field has energy?

    If so, how much would that energy be?

    Would the energy have an impression on the space-time continuum?

    Since they extend infinitely, could the magnetic field of stars have more energy than the stars themselves?

    Could magnetic fields be the "dark matter" we are looking for?
  2. jcsd
  3. Jan 28, 2008 #2
    Magnetic fields, by themselves, cannot do work. So they can't have energy, like say an object orbiting the Earth would have gravitation potential energy.

    Do you mean gravity? Magnetism does not produce gravity.
  4. Jan 28, 2008 #3

    Jonathan Scott

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    Gold Member

    Magnetic fields do contain energy, which is assumed to have a density proportional to the square of the field strength. They don't do overall work on electric charges, but can change the momentum of a moving charge and can change the energy of a magnetic dipole.

    The gravitational effect of the energy of a static magnetic field would be incredibly tiny; I think it would generally be completely negligible compared with the gravitational effect of the masses of the charged particles necessary to give rise to the field.
  5. Jan 28, 2008 #4
    I stand corrected.
  6. Jan 28, 2008 #5
    Magnetic feilds do have energy given by [tex]\mu_{B}[/tex] (the energy density)=[tex]\frac{1}{2\mu_{0}}[/tex]B[tex]^{2}[/tex].

    I would say 'mass' by itself cannot do work but itself has a substantial amount of energy.

    Space-time continnum has energy which is refered to as http://en.wikipedia.org/wiki/Stress-energy_tensor" [Broken]
    Last edited by a moderator: May 3, 2017
  7. Jan 29, 2008 #6
    Energy of a 1 nanotesla magnetic field within 1 cubic light year:


    1/2 (0.000000001 tesla)^2 / magnetic constant * 1 cubic light year

    3.3690412*10^35 joules !!!

    Through E=mc^2, this translates into:

    3.74856388*10^18 kilograms

    And that's just for one cubic light year and for assuming the whole light year is 1 nanotesla, which it probably is not. Does someone know how to model this in a more complex system?
    Last edited by a moderator: May 3, 2017
  8. Jan 29, 2008 #7
    The relevant comparison would be energy density, not total energy ; )
  9. Jan 29, 2008 #8
    Energy density of 1 joule per cubic nanometer would be a more signficant extent in a volume of say, 1 billion cubic light years, instead of 1 cubic nanometer.
  10. Jan 29, 2008 #9
    this is true, however magnetic fields are created by currents and so need to be in the vicinity of matter. I think you are suggesting a comparison of the gravitational effect of the matter to the gravitational effect of the mass equivalent of the energy in the magnetic field created by the matter. This will be very small.

    If you are considering the effect the magnetic field would have on nearby charged particles then that is a different story, but not what you were asking I think.
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