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I Can the presence of a field be detected w/o any charge?

  1. Apr 20, 2017 #1
    Is there anyway to detect the earth's magnetic field without a compass? or putting it differently,
    Is there anyway to detect an electric field without bringing a charged particle in proximity of the field?
     
  2. jcsd
  3. Apr 20, 2017 #2

    BvU

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    Hi sasan, :welcome:

    To detect the presence of an electromagnetic field, you need something that interacts with that field. So: no. Only particles with charge can show some interaction.
     
  4. Apr 20, 2017 #3

    mfb

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    Very intense fields can lead to pair production, that would be detectable - but the produced particles are charged, of course. It just means you don't need your own particles.
     
  5. Apr 20, 2017 #4

    BvU

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    Note that your second question is not equivalent ('putting it differently' is an unfortunate wording) to the first: A compass is a permanent magnet, you could also use the Hall effect or a rotating coil to detect the earth magnetic field.
     
  6. Apr 20, 2017 #5

    vanhees71

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    Well, if only we had these very intense fields at hand. There's yet no observation of this socalled Schwinger mechanism :-(.
     
  7. Apr 20, 2017 #6

    mfb

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    Not yet, but the lasers are making progress towards it.
    Pair production at ultraperipheral lead-lead collision is a related effect.
     
  8. Apr 20, 2017 #7

    BvU

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    Hey guys, we had a simple question in this thread o_O
     
  9. Apr 21, 2017 #8
    Wow, this was my first post and I did get my question answered. I am so thankful. I truly am deeply thankful.
     
  10. Apr 21, 2017 #9

    vanhees71

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    Yes! This has a long history in heavy-ion collisions. As far as I know, so far there's also no (clear) evidence for the Schwinger effect there either. It's already tough to get the "normal dileptons" (which are very interesting in themselves, because they provide the most direct probe for in-medium effects of hadrons in the fireball created in heavy-ion collisions).

    To get the Schwinger mechanism you need indeed ultra-peripheral collisions to avoid the creation of all this (from the point of view of the Schwinger pairs "background"). On the other hand you need the very large fields, which you may reach formally in such "grazing collisions", however for a very short time only.

    The same holds for the large magnetic fields from the spectators in non-peripheral collisions and their impact on the heavy-ion observables in connection with the socalled "chiral magnetic effect".
     
  11. Apr 21, 2017 #10

    sophiecentaur

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    You can always observe a charge that's in a position when you expected to be in another position. Without seeing any actual change taking place, you can 'infer' that there must be a field there.(Same as a stretched spring could imply that it has a mass hanging on it or that a gravitational field is present.) But of course, that implies there was a change at some time in the past, when the present situation was set up.
     
  12. Apr 22, 2017 #11

    tech99

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    But magnetic detectors such as these contain charges.
     
  13. Apr 22, 2017 #12

    hilbert2

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    In principle, the energy density of an EM field gravitates just like any form of mass/energy, I guess, but the effect is way too small to be detectable.
     
  14. Apr 22, 2017 #13

    mfb

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    Depends on what we count as detection. The mass of a singly ionized helium atom, the mass of an electron, and the mass of doubly ionized helium can all be measured to better than 10 eV. The first one is the sum of the other two minus 54 eV (the second ionization energy of helium). A +54 eV contribution from kinetic energy and a -109 eV contribution from the electric field. That contribution is measurable.
     
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