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How do EM and Gravitaional Fields Propagate?

  1. Aug 28, 2005 #1
    I have always wondered about this question. How do 2 paritcles sitting at distance x come to know about each other? I know, you can say that they feel field from each other. But again, how does this field propagate? Remember electromagnetic or gravitational fields don't travel through a medium as pressure waves do. These fields don't require no medium at all. Then, what is it that they ride on?

    I read in one book - "The charges exert a force on one another by means of disturbances that they generate in the space surrounding them. These disturbances are called electric fields."

    Disturbances in space?? Sounds intriguing? Doesn't it mean that space is much more than what we see. This is just amazing.

    Any thoughts???

    Manu Garg
    "Truth will set you free!"
  2. jcsd
  3. Aug 29, 2005 #2


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    Electromagnetic disturbances can travel through a vacuum. Light, like any other electromagnetic wave, needs no medium for propagation. (Sunlight reaches earth, see?).
    The problem of what it is that makes light propagate has been debated alot in the end of the 19th century. It was supposed light traveled through some invisible jelly that permeated all of space (it was there for the sole purpose of mediating electromagentic waves and was called the 'ether'). Einstein's theory of relativity solved this problem in 1905. The ether doesn't exist.
    We know now that electromagnetic forces are mediated through particles we call photons.

    By the way, is you get the chance to read something about the general theory of relativity you'll see that space is indeed much more than what we see it to be.
  4. Aug 29, 2005 #3
    Well, I have read general theory of relativity. But I have always seen it in theoretical terms. As a means to explain a force. But, never really thought that this space concept can be so creepy :))

    In physics, you never really come to know when you cross the boundary between theoretical and practical concepts. You said photons carry electromagnetic waves which is a theoretical concept in itself to explain quantum theory. What about electrical field. No waves. It's carried by just disturbances in space.
  5. Aug 29, 2005 #4
    Any other views. To make it more clear let me put it this way.

    Say there is a charged particle somewhere in space. It's producing some electric field in it's vicinity (are you sure?). We can observe this electric field only by putting in another charged particle. (right? that's the only way). But before putting in another charged particle, is there anything already in that space which makes it different for a new charged particle.

    Manu Garg
    "Truth will set you free!"
  6. Aug 30, 2005 #5


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    manugarg --You want practical? Read some electrical engineering books, particularly, say, ones dealing with antennas. Radar works; TV works, radio works -- exactly as the physics theory says they will. The issue you are worrying about was pretty much resolved more than a century ago. So, best to do some serious reading and thinking.
    Reilly Atkinson
  7. Aug 30, 2005 #6
    guys, Let me introduce myself a bit so that you don't assume I am just a sorta stupid trying to make sense of physics just for timepass. i am electrical engineer and i have read the books that you are talking about. But, I haven't seen many books trying to explain the real reason behind the existense of electrical field.

    reilly, i know all these things work. But, I want to know how. Books start with already assuming that there is something called electrical field because of charged particles. I just wanted to have a discussion on what this electrical field is exactly.

    I tend to think that it's space-time distortions as einstein said for gravitional fields. But I never saw anybody talking about it that way. May be it's just too naive talk about.

    I just wanted to know the opinion of the people here.

    Manu Garg
    "Truth will set you free!"
  8. Aug 30, 2005 #7
    I have been doing serious thinking that's why I am getting these questions. I am sure you're not even thinking so deep as I am thinking. It's very basic. I know all the equations. I just want to know the answer of a more basic question.
  9. Aug 30, 2005 #8
    And did you read the complete thing or just replied to one of my posts?
  10. Aug 30, 2005 #9


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    What is a field? I have no idea nor do I think anyone else does. Before I retired, I worked with many physicists, none were able to give a satisfactory (to me) response to “what is a field?”. I just lump it in with a virtual particle as a mathematical abstraction.

    I'm hoping some of the "big brains" post a good reply.

  11. Sep 5, 2005 #10
    it's a place where you can go to feel something that you can't feel in unlike places.
  12. Sep 5, 2005 #11
    Well, continue to hope, because I am not one of them. What I do know about the issue is this:

    1. Einstein attempted to link his "warping of space time theory" to the atom to create a universal theory bridging the gap between the gravitation and electrical attractions. He never completed it. As to why I cannot tell you, because I don't really understand the original theory all that well either.

    2. We don't currently know exactly what is going on with gravity much less with the electrical field. There are a bunch of basic questions you can torture yourself with. For example, if electrons are attracted to protons via the inverse square law, what is the mechanism that keeps them from falling into the nucleus? We know they aren't falling into the nucleus, we just don't know why. Even if we did come up with a theory for the electrical field around a proton it would have to account for things like this that seemingly contradict the theory of the electrical field to begin with.

    3. If you can resolve this issue by finding out how the electrical field works, and find some way to verify your explanation through experiment, you will be famous! We can name the new phenomenon the "manugarg effect" and it will send off a vicious cycle of infighting in the science field as everyone claims they thought of it first! It's a very attractive situation to be in (the science chicks will totally dig you), so better get cracking :wink:
  13. Sep 6, 2005 #12


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    You will find the answers to your questions (the "EM part") in the theory of quantum electrodynamics (QED), a quantum field theory of electromagnetism. I am afraid I am not enough competent to explain this to you in a comprehensible manner, but I can give you some hints. In quantum field theory the notion of field is introduced to explain phenomena with a variable number of particles which can create or annihilate. The field itself is a physical entity whose localized excitations in spacetime are interpreted as particles. Basically QED describes the interaction between light and charged matter as the coupling between two fields, the bosonic electromagnetic field mediated by photons and the fermionic Dirac field (electrons and positrons). When interactions occur, virtual particles are created and annihilated. These are a tool (as they are not directly observable) to explain forces and other non-classical phenomena like the lamb-shift. May be these references will help:
  14. Sep 6, 2005 #13
    a field is actually just a force that a particle feels.
    so asking how fields propagate is the same as asking how forces propagate
    and i think the answer is no one really knows they just do the only thing we know is that forces propagate at a constant speed of c (the speed of light)
  15. Sep 6, 2005 #14


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    Back when I was in highschool, a physics teacher did this:

    He gave one of us a stick with a little rubber ball on the end (an "electron on a stick" he called it). He gave another a little rubber ball on a string( an "electron on a string").

    Now, imagine that those really are electrons. While you can hold the "electron on a stick" rigidly, the "electron on a string" will, because of the force between like particles, be push at a slight angle to the vertical. Move your "electron on a stick" slightly toward the "electron on a string" and, of course, it will be pushed farther. Move your "electron on a stick" back and the the "electron on a string" will move forward. Wave the "electron on a stick" back and forth and the "electron on a string" will move back and forth. That's how you send electro-magnetic (light) waves.
  16. Sep 6, 2005 #15


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    If a tree falls in the woods and no one hears it, does it make a sound? If you're looking to ask "why?" or "what is behind it?" questions, then you're getting into the philosophical. The "only" thing a theory can do is provide a model and make predictions. These "more basic" quesitons are not answerable by physics.
  17. Sep 6, 2005 #16
    Do we know they aren't falling into the nucleus? I asked this question once but received no reply. Even if you just take classical electrostatic forces (i.e. ignore quantum theory), as the electron nears the proton, its potential energy decreases and its kinetic energy increases. If it were to 'fall' towards the nucleus, by the time it gets there it would have a huge kinetic energy, enough to carry on through the nucleus to the other side. In fact, no matter where it is, a bound electron ALWAYS has enough kinetic energy to reach some maximum distance away from the nucleus where its kinetic energy would be zero. I don't get why people thought that if it 'fell' to the nucleus it would just... stick there.

    In QM, the ground state of the electron has no angular momentum (L = 0). n > 0, so it must have kinetic energy. Does this not mean that whatever kinetic energy it has must be directed along a line straight through the nucleus and it? Any side momentum would give it angular momentum, I'd have thought.

    Also, in QM, isn't the most probably place to find an 1s electron in the nucleus anyway? I've only done 1D quantum wells, but in the ground state the wavefunction is at its greatest magnitude slap bang in the middle. I'd have thought, translating this to 3 dimensions, this would suggest you are much more likely to find the electron at 0 distance from the nucleus than any other specific distance, though maybe that translation is invalid. Any thoughts?

    As for fields (lest I get chastised for being too off-topic), my tuppenceworth: they're a theoretical construct to explain action at a distance. Gravitational fields were replaced in GR by spacetime curvature, which lends some physicality to gravitation. EM fields are replaced in QFT by excitations in the vacuum manifesting as virtual particles, which again lends some physicality to EM. EM waves are replaced in QED by photons, a satisfactory physical explanation. Classical fields for both forces are, I guess, unlikely to be explained as physically describable phenomena since physics has dumped them for a younger model, so I'm not sure wondering exactly what they are, physically, is a particularly worthy past-time.
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