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Characteristics of a photon

  1. Feb 14, 2014 #1
    How do photons carry information? For instance, when a photon is emitted by the sun, it seems to carry information about the sun- color, intensity, etc... If that photon bounces off a blade of grass then it carries new information- color, shape, etc... What is it it about the blade of grass that allows its information to "stick" to and be carried by the photon? Is any energy lost by the blade of grass in the transmission of information from itself to the photon?

    Also, if a photon is emitted by the sun, does it expand like ripples in a pond, or is it like a point-particle emitted in a certain direction? Or if a single elementary particle in empty space emitted a photon, would it be seen in every direction from the point of emission?

  2. jcsd
  3. Feb 14, 2014 #2


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    Intensity is just the number of photons per second. So that information isn't carried by a single photon.

    The color depends on the frequency of the photon. Objects have color because certain frequencies of light reflect much better off of them than others. So a blade of green grass reflects a lot of green light and not so much of the other colors.

    No information is transferred from the grass to the photon. It's simply that a percentage of the light that hits the grass will reflect off of it instead of being absorbed.

    Don't imagine light as a particle. Instead, imagine it as a wave first. The wavefront expands in all directions and has a chance of interacting and transferring energy to anything it interacts with. The wave can only transfer energy in "packets", not in a smooth, continuous manner. These packets are what we call "photons".

    If a particle emits an EM wave with only enough energy for one photon, the wavefront still expands like normal, but the wave will only have enough energy to interact with an object once. So once that photon is detected the wave has transferred all of its energy and exists no longer.
  4. Feb 14, 2014 #3
    Thanks Drakkith,

    That clears things up quite a bit. What determines the photon's frequency when it is created? Also, how is a photon 'reflected' off a blade of grass without losing all of its energy?

    Also, now I am imagining a photon as an expanding wavelike sphere of light in space. How is it that the whole wave collapses when it interacts with an object at a single point in space? Does the energy from the other part of the wave get instantly 'sucked in' to the object that it comes in contact with? Is that like the collapse of the waveform in QM?
  5. Feb 14, 2014 #4


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    Again, think of EM waves instead of photons. The EM wave has a frequency which is determined by whatever means is used to create it. For example, an antenna with voltage and current oscillating back and forth at 100 MHz gives off EM waves with a frequency of 100 MHz. A wave with that frequency will interact via photons with a specific energy.

    Reflection of an EM wave off an object is a little too complicated for me to explain.

    Don't. The photon is just the energy packet. The EM wave is what is expanding. A photon has no size, it is merely the way the EM waves interacts with matter.

    If you can answer those questions you'd probably win a nobel prize.

    Kind of. The wavefunction in QM determines the probability of finding the photon at a specific position, time, and with a specific momentum (energy).
  6. Feb 14, 2014 #5


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    First you need to understand the classical model. Then you will be in a position to take on board the QM model. All these simple examples can be described / explained in terms of waves. It is only much later that you need to consider photons. Photons are much much harder to deal with than waves (if you want to have a worthwhile understanding). You have to ask yourself what you actually mean when you say
    That is a million steps past the original question. 'Gut feelings' have no place in QM.
  7. Feb 14, 2014 #6


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    The only 'information' a photon can carry is
    1. Its energy
    2. Whether its there or not
  8. Feb 14, 2014 #7
    But a whole lot of photons can carry "information". For example the temperature of an object is done by comparing the spectrum of light to the black body radiation of objects at a known temperature. The spectrum of light is the relative intensity of different wavelengths. One photon does not a spectrum make. It takes many photons, or a population of photons to know the temperature of the object emitting them.
  9. Feb 14, 2014 #8
    Can a particle emit an EM wave that is too weak to create a photon?
  10. Feb 14, 2014 #9


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    No, no emission of photons = no EM wave emission

  11. Feb 15, 2014 #10


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    You should re-examine your personal model of a photon if you ask a question like that. Even on this thread, there are statements which would be of help for you. You should read them, and much more.
  12. Feb 15, 2014 #11


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    Information is conserved in several contexts of physics:

    • Liouville's theorem in classical mechanics.
    • Unitary operations in quantum mechanics.
    • The so-called information paradox related to black holes, as debated by Hawking and Susskind.
    • The so-called holographic principle in cosmology.

    In the black hole case, the experts agree that energy/mass falling into the black hole contain information. More mass/energy more information. This applies to photons too. Thus photons must carry information, and more energetic photons carry more information. But the kind of information in question seems to be an elusive concept.

    But a photon is always described by quantum physics. Therefore, if I understand correctly, the information contained in a photon is proportional to the number of orthonormal basis vectors required to completely describe its state. But that definition does not seem to allow for photon information proportional to its energy. It thus seems to contradict the black hole case.

    A robust definition of what information is seems nearly as elusive as the definition of entropy. We must deal with a handful of hard-to-relate definitions of entropy. It seems the same with definitions of information.

    My follow-up request on the OP is, please define the word information as applied to a photon.
  13. Feb 15, 2014 #12

    My use of the word 'information' was based on my misunderstanding of what a photon was; which Drakkith cleared up. I had thought that all matter- being energy- was somehow tied to the same spectrum as light, and that the EM wave of a photon became entangled with the energy of an object that it reflected off of- retaining information from the object encountered. I now understand that is not the case.

    Thanks for your reply!
  14. Feb 15, 2014 #13
    Quote by johnny_cloud: Can a particle emit an EM wave that is too weak to create a photon?

    No. EM waves are photonic by nature. A weak wave carries weak photonic energy. The photon is not a standard unit of energy.
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