Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnetic fields in vacuum

  1. Oct 26, 2010 #1
    So this question might be a little simple, or not. But I was wondering about the exact nature of magnetic fields. Now obviously magnetic fields work in a vacuum(due to the fact that it's not a perfect vacuum), but would a magnetic field exist in a theoretical perfect vacuum? Now I think this would be a rather difficult experiment to perform, in order to see if the field is there, we would have to insert particles into the space were we suspect the field might be. Is there a mathematical model or theory to prove of disprove this without altering the experiment itself? Or could this be a question along the line of "a tree falling in the woods without anyone around.. blahblah" Am I missing something fundamental? Any thoughts or insight would be peachy, thanks.
  2. jcsd
  3. Oct 26, 2010 #2
    why do you think magnetic fields don't exist in a vacuum?
  4. Oct 26, 2010 #3
    Well, there is no real way to see if these fields exist in a perfect vacuum without altering or disturbing the outcome of the experiment. You see, if want to see if there is a magnetic field, you have to insert particles to the Perfect vacuum thereby making the vacuum not so perfect anymore.
    I suppose the more appropriate question I am asking is, what is it exactly that makes a magnetic field? Now, I know in a magnetic field you have the E. field traveling through space and alternating magnitudes over time to create this 90 perpendicular magnetic field...... Perhaps this magnetic field can only be thought of as a quantitative potential force that can act, and cause a reaction to particles, if they happen to come into it's field of effect, or within it's flux lines.
  5. Oct 26, 2010 #4
    okay so now we are getting into something. You said that you know magnetic fields and electric fields are hand in hand. So you would assume that a travelling electrical wave comes along with a magnetic wave. But a vacuum is a space void of matter. Can't you have electromagnetic waves without having matter?
  6. Oct 26, 2010 #5
    Well yes, but that's kinda different... Isn't it? these EM waves we call light. A traveling packet of energy. An EM wave is just an EM field moving in space and time?... but the only way to get a magnetic component to the EM field is to have a varying E. field traveling through space and time??? argggg!! lets try this.....

    If say, we have a bar magnet, and there are particles within it's flux line, we use a special "thingy" to see the particles that will align themselves with/along the flux lines, thereby seeing the magnetic flux lines, by seeing the particles. If we clear this region of all matter- no more particles, we do not see these flux lines anymore?, but are they still there? If so what are they made up of?
  7. Oct 26, 2010 #6


    Staff: Mentor

    If I understand your point then I believe the answer is as follows: if you have no detector you cannot detect anything. That seems to be rather self-evident.
  8. Oct 26, 2010 #7
    Hmmm, perhaps that's the right idea Dale. The magnetic field is only made up of what it interacts with... There is a real possibility that the very logic is which i'm trying to envision these fields is fundamentally flawed, and that they have to be thought of differently.
  9. Oct 26, 2010 #8
    How about this. Take an empty box with a theoretical perfect vacuum inside it, and wrap a coil with area A and N turns around the box in the x-y plane. Put the box in a large electromagnet and turn on the magnetic field Bz in the z direction. Monitor the coil with a voltage integrator circuit (to integrate the induced Faraday Law voltage):

    V(t) = - NA dBz/dt

    Using a current integrator circuit*, we get the change in magnetic field inside the box to be equal to

    ΔBz = - (1/NA)∫V(t) dt = +(RC/NA) Vout

    where Vout is the current integrator output.

    *See current integrator circuit and theory about half way down in


    Bob S
    Last edited: Oct 27, 2010
  10. Oct 26, 2010 #9
    It doesn't matter that the vacuum is no longer perfect. Charged particles will behave differently on entering the formerly-perfect-vacuum depending on whether or not it contains a magnetic field. It's empty space prior to the entry of the test particle, but there's clearly something different about it.

    And besides, in the case of a single particle or small number of particles, what do you call all the empty space around the particles? Yes, magnetic fields can exist in perfect vacuum.
  11. Oct 26, 2010 #10
    I seriously hope that no-one here is suggesting that a magnetic field in a perfect vacuum does not exist, or that a magnetic field un-detected does not exist.
    Silly me, of course no-one is thinking anything of the sort. Sorry.
  12. Oct 27, 2010 #11
    tonyo, the notion of a magnetic field is simply a representation of what we see experimentally. You don't necessarily want to tie your mind up by even thinking about what a magnetic field is made up of.
  13. Oct 27, 2010 #12
    Ok so this magnetic field will exist in a theoretical perfect vacuum even without an observer, or matter of any kind for it interact with. That's great, although I'm still not overly convinced. I was really kind of hoping that someone would have posted a real good though experiment to prove it's existence, without altering the very nature of the silly question.
    Now, this question was presented to me by a very intelligent bio-chemist, who thinks in terms of particles, and mainly molecules, thinking in such a way can make it slightly difficult to think in terms of "fields of force", which I myself don't fully comprehend, upon being asked such a question.
    This force is not so much like a gravitational/mass force, where it will warp the very fabric it exists in, or does it???? I don't know if we can open up a whole new can of worms and suggest that such a field can warp the fabric of a unknown dimension which interact with our known(or observable) 4.
    What is it precisely that enforces this magnetic force upon our universe?
  14. Oct 27, 2010 #13
    "What is it precisely that enforces this magnetic force upon our universe?"

    Why would magnetic fields be so different from gravitational fields?
  15. Oct 27, 2010 #14
    I don't know how to answer your question but if I were you (which I might as well be), what about gravity do you understand that you don't about magnetic fields.
  16. Oct 27, 2010 #15
    What don't you understand about the experiment proposed in post #8 to prove the existence of a magnetic field in a theoretical perfect vacuum? The experiment relies on two simple and tested concepts; the Faraday Law of induction (based on Maxwell's equations), and a voltage integrator (common op-amp circuit). Is it one of these, or something else?

    Bob S
  17. Oct 27, 2010 #16
    Ok, so upon further research and discussions with some physicist co-workers, it appears as though the solution I'm looking for can be found in somewhat Dirac's theory, but mainly it's evolved form: Feynman's QED theory. I'm still not in full understanding of it right now, but if what precedes makes sense to anyone else please say so lol.
    There are some particles/anti-particles/virtual particles(idk!) and energies which pop in and out of existence causing a disturbance in space. Virtual particles borrow energy from the future to create anti-virtual particles in the present, thereby interacting with the virtual particles, and both annihilate each other, thereby returning the future borrowed energy very quickly in the very immediate future or somewhat present, restoring the thermodynamical equilibrium. These energies are what will cause a electro-magnetic field disturbance in space, EVEN if there is no matter what so every in the perfect vacuum......

    Does that make any sense to anyone??? if not, I give up, and will go far away and find some kind of government desk job or something...... good god.....
  18. Oct 27, 2010 #17
    If you have a perfect vacuum containing a magnetic field, and fire electrons into it, their paths will curve. If you fire electrons under the exact same conditions into a perfect vacuum that has no magnetic field, their paths will not curve. The electrons didn't change, so clearly, there's something different about that volume of space, in spite of the fact that there was no matter in it before you shot an electron into it. Real perfect vacuums are hard to come by, but there's no sign of electromagnetic fields fading as vacuums are drawn.

    What's unconvincing about this? Why do you need to complicate things by bringing in Dirac and virtual particles? I don't see what you gain by it...how can a low level theoretical model be more convincing than direct observation?

    In GR, gravitational fields don't warp space-time, gravitational fields *are* warped space-time. Electromagnetic fields are treated differently, but I don't know why you allow space-time the geometrical attributes that allow gravitation, but balk at the idea of electromagnetic fields in a vacuum. Or why you prefer a QED theoretical description that doesn't include gravitation at all as an answer.
  19. Oct 27, 2010 #18
    I think you missed some posts Cjames, or miss-read the very nature of the question.
  20. Oct 28, 2010 #19


    User Avatar
    Science Advisor
    Gold Member

    Well you should explain it again because I too fail to see what the problem is. How is this any different from say gravitational or electric fields? What is wrong with Bob S' thought experiment? It's nothing more than a cathode tube being subjected to an applied magnetic field.
  21. Oct 28, 2010 #20
    The very heart of the question is "what makes up a magnetic field"? The only reason we know they exist is due to the way they interact with normal matter. If we choose to remove all matter, how would we preform an experiment to quantify this field without subjecting the field to any kind of external variables. It's something of a thought experiment, one that cannot be done, obviously one cannot measure something without disturbing it. Placing a cathode tube in a magnetic field completely kills our perfect vacuum.
    So the question becomes, does a magnetic field exist in a perfect vacuum void?? The answer can depend on how philosophical you want to be. Pallidin's reply hit the nail right on the head. If we cannot see something or interact with this something, then CLEARLY it does not exist..........*sigh*.....
    It's my understanding now that a few theoretical physicists and mathematicians some 50 years ago were asking questions along similar lines, I think they were trying to find a unified quantum field theory, but in the end came up with QED. The QED explains this "field" and how to quantify it WITHOUT needing it to interact with any normal matter. I only have a few hours so far of researching this theory so I'm not going to pretend I have it completely understood, until I comprehend the maths involved. But it seems this is the mechanism which makes it so the field can interact with matter, the stuff a magnetic field is made up of. So even though we have a perfect vacuum, there is still "stuff" happening there. This then allows a field to exist in the absence of any kind of measuring apparatus.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook