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Question about light waves (and speed of)

  1. Jan 28, 2004 #1

    I have recently read a book called E=MC^2 (David Bodanis). It is an interesting book that goes over the history of the famous equation, but it left me with a few questions.

    The story goes into the issue of light being a combination of electric and magnetic fields, but doesn’t go much further.

    So far, I understand that light is an electromagnetic wave (or a photon, but ill stick with the wave idea), how exactly do these waves work? Could someone please explain this, or link me to a good online explanation? For instance, with a light bulb where electricity is passed through a thin (wire?) coil which then glows. I get that when electricity passes through a wire, a magnetic field is created looping around it. But how does this create light waves?

    I also am having trouble understanding why it is not (yet) possible to travel faster than the speed of light. In the book, ‘c’ is compared to –273 degrees C, or absolute zero, and that just as nothing can be colder than this, nothing can be faster than c. I understand that nothing can be colder as temperature is a measure of the vibration of particles, and at –273 these are no longer moving, but what is the reasoning behind the speed of light?

    All replies (or links) are greatly appreciated.

    Thank you in advance


    P.S. – I apologise if this question has already been posted elsewhere on the forum. I searched but found no obviously appropriate posts, if there is one please link me to it.
  2. jcsd
  3. Jan 28, 2004 #2


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    We know that there is an essential connection between magnetic field and electric fields - a changing electric field produces a changing magnetic field. (As in an electromagnet.) A changing magnetic field also produces a changing electric field. (As in a dynamo.)

    The essence of the EM wave is exactly this. A magentic field, which produces an electric field, which produces a magnetic field, which produces an electric field... Thus propagating itself through space.

    The simplest explanation I can think of (and hope I remember correctly!) is to see spacetime as a 4d volume, and space-like travel/time-like travel as components, or directions in this vacuum. In this, we can say that the magnitude of velocity of all bodies is always c, but that c is spread out over the four directions. In a 2D analogy, imagine yourself to be travelling on a map, with the restriction that you aways travel at the same speed, c. You can change directions, but clearly there is no way that you can travel north faster than your preset speed.
  4. Jan 28, 2004 #3


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    Homework Helper

    A photon is not light in the same way that an atom is not a solid object. Photons are discrete amounts of energy that you can extract from the light (or that something can add to it). Light is, as you say, electromagnetic radiation (or so is the consensus).

    That is a very deep question. I can't answer "exactly," but basically, there is a disturbance of the electromagnetic field, and this disturbance propogates at the speed c.

    I don't think this magnetic field has anything to do with the light emitted from the filament in a light bulb. The filament heats up, and emits black body radiation. This radiation has a particular peak colour, depending on the temperature. It is more orange if it is cooler; it is more blue if it is hotter. I'm pretty sure this is the light producing mechanism in a light bulb. There is, of course, a magnetic wave involved, but it is not the steady magnetic field that encircles the filament due to the DC passing through it. The main reasoning that I am using is that, if this were considered the light, the frequency would be zero (or 60 Hz if this is a household light bulb). At any rate, this is well below the optical range of frequencies, so you would not see the light.

    The most popular explanation is that it would require more than an infinite amount of energy to exceed this speed barrier. On a little bit more technical not, any massive body must follow a timelike geodesic to maintain causality. This is a postulate proposed by someone, I'm not sure whom, but I read it in Rindler's "Relativity: Special, General and Cosmological."

    I have heard this kind of comparison suggested before. I have no idea about it.
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