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How will a stream of photon (being light) looks like?

  1. May 14, 2004 #1


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    I know this sounds stupid but I still find it hard to digress QM. Could anyone give me a picture of how a photon is like? And how will a stream of photon (being light) looks like? At least an approximation, if possible.
  2. jcsd
  3. May 14, 2004 #2
    A photon is typically a fairly localized excitation of the electromagnetic field. It looks like a sine wave modulated by an exoponentially decreasing function.

    This actually represents its wavefunction, and its square is proportianal to the probability that it will be detected at a particular point in space if its position is measured.

    Please bear in mind that this is only a "typical" picture of a photon. It's wavefunction might be more spread out in general and it might exhibit entanglement with other particles.
  4. May 14, 2004 #3
    Actually I have been wondering if there is such a thing as "photons" or is it just an assumption? I am also confused about the properties of the photon. It is a wavepacket, right? Would it expand in space and time? If I recall correctly, the wavepackets of photons have continuous distribution of momentum. Why?
  5. May 17, 2004 #4
    You may think about the photon as a point-particle, but the uncertainty principle makes its position to spread out. The wave packet shows you the probability distribution. But you can have a photon being emmited from the nucleus of an atom, which is a very small volume, or produced in elementary-particle collisions. I am not familiar with quantum loop gravity or superstring theory, but probably in these theories the photon occupies a very small volume instead of a point in space.
  6. May 17, 2004 #5
    Photons have continous distributions of momentum because they are not "bound" particles like the electron in the atom. A free electron or any other free particle also has a continous distribution of momentum.
  7. May 17, 2004 #6
    If you take the classical equations of electromagnetism and quantize them, using any of the standard methods of quantization, then you end up with a bunch of creation and annihilation operators for quanta of the electormagnetic field. These are analogous to the quanta of a simple harmonic oscillator and they are what we call the photon modes of the field.

    Their mathematical existence, as solutions to quantum field theory, is unambiguous. Whether they represent things that really exist out there and what happens to them when you make measurements on the field is an unresolved question in the foundations of the theory. However, they exist for all practical purposes, so you can generally believe a physicist when he tells you about photons.

    If you take a nonrelativistic limit of quantum field theory, you get solutions that look like wave-packets. Under time evolution, they spread out over all space and it is not possible to find solutions that don't do this. This was worrying for Schroedinger, who wanted to describe all of non-relativistic physics in terms of waves via his equation and their are obviously things in the world that don't diffuse over all space. To resolve this, Born postulated a rule connecting the waves to the probability of detecting a particle at a particular location. Hence, the measurement problem was born (no pun intended).
  8. May 18, 2004 #7


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    I'm confused. I have been under the impression that a free particle has a well-defined momentum, i.e. [tex]e^i^k^x[/tex].
  9. May 19, 2004 #8
    That's a solution to the free particle Schroedinger equation, but so is any superposition of such states. In the nonrelativistic theory, this is an energy eigenstate for a massive free particle, so we might expect such systems to be in one of these states to a close approximation.

    To treat photons properly, we really have to go to the relativistic theory - quantum field theory. There, causality means that photons are typically created in states which are much more localized.
  10. Jun 13, 2004 #9
    Light and electrons

    Light is pure energy. Light opperates at a temporal state where there is no dilation in time. That is to say light has no entropy because it has no time.
    Light is an EM field. EM fields display the properties of both particles and waves. This is a temporal question, asking a EM field what it is implies that light occupies the same temprol reality that we do.

    More to the point when an electron recieves the energy of light it transfers the energy from one temporal state to another. From a nutral state to that of that of a entropic state. Thier is no such thing as a photon in real terms. This is becuase photons have no mass, and opperate in a seperate temporal state. The photon is a means by understanding the relationship between matter and energy. Matter ubsorbs unitsof energy known as Quanta, the exact quantity used to exite electrons stays the same. It is that amount that is said to be the photon.

    The electron is particle that owes its exsistance to the magnetic field of an atom. The electron has mass however it also displays the same temporal charactaristics of light. The relationship between the photon and electron is the relationship between the EM field and the Magnetic field. The magnetic field is created by the movement of charged particles. The field itself guides the movement of charged particles with in timespace. This is why Light carries its own magnetic field as it is the movement of charged particles with in timespace. Magnetic fields differ as they have a polarized field, that is the movement of particles acording to thier charge either to the nourth or south.
    Now light differs in that it does not actualy have a charge until it enters our temporal state, that is to say light must be slowed from the speed of light by the magnetic field of the atom for the photon to release a charge. I understand that electrons can create photons and photons can create electrons.

    Ergo the photon is a unit of exchangeing energy between the two temporal states manifest as the EM field and the Magnetic Field
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