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Question about photons

  1. Dec 19, 2013 #1
    I am a physics student at the university of Athens (1st year) .
    Recently i have been studying special relativity by myself and i have question that i cant understand by myself
    When we speak about photons and their energy we use this formula : E=hf
    where h : planks constant and f:frequency
    my question : How can we speak about the energy of photons and use frequenscy .. how can a photon have its own frequency ?

    and something else . Can an electron ( and in general every particle) be considered as a wave also ? (like the photons)
  2. jcsd
  3. Dec 19, 2013 #2


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    Staff: Mentor

    If you're studying special relativity, then you don't need the quantum mechanical concept of photon at all - it will do more harm than good. Instead, every time you find yourself saying "photon", say"light signal" or "flash of light" instead. Light is an electromagnetic wave, so there's no problem assigning it a frequency and a wavelength.

    Yes, although again this is quantum mechanics not special relativity. You can learn and understand SR without going anywhere near quantum mechanics, using just the classical concepts of light as an electromagnetic wave - and I strongly recommend doing exactly that.
  4. Dec 19, 2013 #3
    Understood and thank you ...
    Actually i am taking physics courses via the internet and one of the lessons i am attending is called " introduction to relativity , nuclear physics and cosmology" so i believe i will have no problems ..
  5. Dec 19, 2013 #4
    If your search for anything like 'photon energy' in these foums you'll find loads of prior discussions.

    Briefly, in the standard model of particle physics relativistic quantum theories describe
    'particles' as probabilistic wave functions, not like 'little balls'. When we detect such phenomena we observe them as point particles and physical action at small scales takes place in discrete steps as in multiples of 'h'. What they 'really are' between observations is less clear and you can find discussions in these forums...

    The particular quantum theory for light and matter is QED. Roughly, you can think of particles as detectable excitations or a quanta of the underlying quantum fields.... Planck's constant doesn't appear in Einstein equations so you won't find anything quantum-mechanical from them.

    here are a few views I like to get you started:

    tomstoer[I think]

    One recent discussion including energy is here :

    There is a long and valuable discussion here...at least I learned a lot from it:

    What is a particle

    As you might infer from the above, this is really the best way to view an electron. The old view of an electron like a planet 'orbiting' a sun [nucleus] doesn't work so well. Viewing an electron as a resonant, standing, wave analogous to a vibrating string works well. In such a view, the electron is not in a specific location, it is to be found at various places in the electron 'cloud' with different probabilities. The modern term is electron 'orbital'.

    Youll find some great illustrations of these in Wikipedia under ATOMIC ORBITAL. http://en.wikipedia.org/wiki/Atomic_orbital be sure to scroll down the page as there as a number of interesting illustrations.
  6. Dec 20, 2013 #5

    Claude Bile

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    Science Advisor

    On this point; the frequency uncertainty of a photon is intrinsically linked with its position uncertainty. In other words, the better known the position of the photon is known, the larger the spread in frequencies and vice versa.

    A photon with a perfectly known position will have an arbitrary frequency. Conversely, a photon with a perfectly known frequency will have an arbitrary position.

    This property is linked to the linearity of waves and the Fourier transform relationships between quantities like time and frequency.

  7. Dec 20, 2013 #6
    Isn't that Heisenberg uncertainty principal ? oh and
    thank you .!!!
    Last edited by a moderator: May 6, 2017
  8. Dec 20, 2013 #7
    That's a really nice synopsis from Claude if you have been exposed to Fourier analysis...

    If you are interested in additional insights, see this discussion

    "How big is a photon."

    It's long and involved, but if you plow through it you'll really find out the subtelies
    of 'particles'....read different/conflicting views

    Here is a quote I like from a famous scientist [maybe in the second link of the prior post above]:

    Carlo Rovelli:

    It is from that last sentence that the ambiguity arises.
  9. Dec 20, 2013 #8
    yes...the issue is what HUP 'means'.....

    HUP is another quantum phenomena whose 'meaning' [interpretation] is still debated.....
    Just search the that name in these forums and you will find dozens of discussions/arguments/viewpoints.
  10. Dec 20, 2013 #9


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    It's worth noting that basic QM and QFT treat this a little different. In QM, photon is a wave, which has uncertainty in momentum and position. Momentum, of course, being directly linked to its frequency.

    Quantum Field Theory would say that no, a photon has an exact frequency and an exact position. But the uncertainty is in which photon you are observing. (A photon with which frequency, and at what location.) It's just semantics when you deal with a single particle, but becomes important when you consider many-particle systems.

    Just thought you should know that's out there, because you might run into statements about photons with exact frequency and location on this forum or in textbooks, and it might be confusing.
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