Photon?

Discussion in 'Physics & Math' started by Fredrik, Jan 2, 2015.

  1. Fredrik Registered Member

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    I have a question about particle physics, especially about photons.
    If a photons energy is calculated by taking Plancks konstant and multiplying it with it's velocity, then doesn't than mean that if a photon were to go through Earth's atmosphere, the energy would decrease since the it's slowing down which shouldn't be possible right?

    Also if elektromagnetism is essentially photons then why don't electromagnetic materials emit light?
     
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  3. Russ_Watters Not a Trump supporter... Valued Senior Member

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  5. exchemist Valued Senior Member

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    Since Russ has answered the first part, I'll have a pop and the second.

    Photons are electromagnetic (EM) radiation. Any electric charge that oscillates will radiate EM waves, that is, it will emit photons. A good example is a radio antenna, in which electrons oscillate at the frequency of the radio emission and thereby induce radio-frequency photons to be emitted. You can't see these photons of course, because they have a wavelength that is far too long to be what we call "light". Another example is the electrons in an atom. When one of these drops from a higher energy orbital to a lower energy one, it creates an oscillating dipole that emits a photon, which may well be visible as light, since the frequency of this process is far higher than that of the electrons moving in a radio antenna. Sodium street lamps use this principle (the yellow is due to the famous "sodium D line", caused by excited sodium atoms emitting photons with the frequency of yellow light.)

    So in a way electromagnetic materials do emit light, but you have to be careful what you mean by an "electromagnetic material". It needs to be something with moving electric charges.
     
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  7. zxen Registered Member

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    Human Retinas perceive a certain part of the electromagnetic spectrum. We call this light. Animals such as sharks can perceive things like metal, and bats can see sound. Apparently dogs can smell in 'colour'. Charles Darwin proposed that over hundreds of years, certain traits were kept if successful individuals of a species didn't die in bad conditions such as droughts, famines and ice ages etc, and were able to find a mate to transfer their unique DNA into new offspring. Charles Darwin proposed that these changes occurred randomly or through selective breeding. But quantum physics seems to indicate that there is a fifth force of nature that still seems magical to us at this point in history: Mind. We can change matter at a subatomic level just by concentrating on it, or in the case of plants, perhaps another mind makes a plant figure out how to suck the blood out of flies or make prettier flowers. The electromagnetic force, to which human-perceived light belongs, travels at various speeds through various materials, especially the main material of the universe, primitively known at this point in history simply as 'the fabric of spacetime'. If the gravity is low, the electromagnetic force travels slower. And vice versa. Light travels fastest in a black hole. Also look at red shifting effects of galaxies. Materials such as liquid slow it down, and solids stop a lot of it from penetrating at all, which is why you can't see through walls. But rest assured, the photons (electromagnetic radiation) will still eventually tear a hole through the American flag on the moon. Radio waves and x rays can penetrate a certain amount of solid material. I'm basically giving you layperson responses here. Hopefully someone will correct me. I think that electromagnetism and all forces might work by rippling along the fabric of spacetime from one location in the universe to another. This might mean that the fabric has a density that we cannot compare to the outside of the universe. The fabric is not empty and uniform either. Its foaming matter/energy out of itself all the time.
     
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  8. Farsight Valued Senior Member

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    The photon energy is E=hf, where f is frequency. The photon doesn't slow down as it goes through the atmosphere. However the "coordinate" speed of light is lower at a lower elevation, so in a way the photon goes slower as it gets lower. See this Baez article for more on that. However it doesn't lose any energy. Instead it appears to gain it, wherein we talk of "blueshifted" photons. But: it doesn't actually gain any energy. There is no magical mechanism by which a photon gains energy from something else. Gravity is not a force in the Newtonian sense. Conservation of energy applies. What happens is that you and your clocks go slower when you're lower, so you measure the self-same photon frequency to be higher. It's a bit like what happens if you accelerate towards a photon in free space. You measure it to be blue-shifted, but the photon didn't change a bit. Instead, you did.

    They do. But you have to heat up some material to make it emit light you can see. Strike a match, and you can see an electromagnetic material emitting light.
     
  9. Fredrik Registered Member

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    Thanks for all the answers!
     
  10. krash661 [MK6] transitioning scifi to reality Valued Senior Member

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    it's wise to ignore farsight.
     
  11. James R Just this guy, you know? Staff Member

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    Fredrik:

    Probably you have seen the equation \(E=h\nu\) for the energy of a photon. The second symbol on the right-hand side is not a \(v\) for velocity. It's a Greek letter: 'nu', which in this context is used to represent the photon frequency.

    The speed of light in air is almost the same as in vacuum, so photons don't slow down very much at all when they enter Earth's atmosphere.

    Also, when a photon goes from one medium to another - say from air into water, it's frequency doesn't change, even though the speed of light and the wavelength both change. So, the photon energy remains constant.

    They do - under the right conditions.

    But I'm not quite sure what you mean by an "electromagnetic material". What would be an example of such a material?

    It is important to appreciate that the emission and absorption of light (photons) is only one kind of electromagnetic phenomenon. There are many kinds of electrical, magnetic and electromagnetic effects that don't involve photons*.

    *Or at least not real photons, but let's not get into that yet.
     
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  12. James R Just this guy, you know? Staff Member

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    Farsight,

    Actually, there's no way to tell the difference, since there are no preferred reference frames. A technical point, which I'm sure you understand.
     
  13. Farsight Valued Senior Member

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    Sorry, I don't know what you mean James. Maybe it would be clearer if I replaced the photon with a star: when you accelerate towards it, the starlight appears blueshifted. But it hasn't changed, instead you have.
     
  14. QuarkHead Remedial Math Student Valued Senior Member

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    Here again, Farsight, you pose yourself as a "relativity denier". ALL relativistic theories - yes, even Galileo's - make no distinction between a hypothetical "you" at rest and the star moving toward you on the one hand, and the star at rest and the hypothetical "you" moving toward it on the other.

    In either case - which is a silly way to frame it, as the 2 situations are relativistically equivalent - the star's light is blue shifted. And if there is no way to to sure who exactly is doing the moving, the statement that "it was you that changed, not the star" lacks content in any relativistic theory.

    I wonder what Einstein would have said - do you have a (partial) quote, Farsight?
     
  15. paddoboy Valued Senior Member

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    Mis-placed confidence James old matey!
     
  16. James R Just this guy, you know? Staff Member

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    Ah. I overestimated you, it seems.

    There's no way to tell whether you're moving towards a star or whether it is moving towards you. Only relative motion matters when it comes to blue shift. So once the period of acceleration is over and we've settled down to a constant relative velocity, there's no way to know which party did the accelerating (i.e. who changed and who didn't).
     
  17. Farsight Valued Senior Member

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    I'm not a relativity "denier". There's a clear distinction in that you accelerated, and you could feel it. You know you changed. And you know the star didn't change just because you put the pedal to the metal. You know that your observation of it changed.

    Garbage! It's crystal clear who's doing the moving! And we can observe that the star didn't change one iota just because you decided to accelerate towards it.

    No. This is simple stuff.


    I doubt it.

    You felt the acceleration, you can see the star getting bigger in your viewscreen while Sol is getting smaller in your rear-view mirror. You know full well you're moving towards it. Along with everybody else who's been watching you.
     
  18. Dr_Toad It's green! Valued Senior Member

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    Why is Farsight allowed to post in any sections here except alt theories? For that matter, why is he allowed here at all?
     
  19. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    Because Farsight is a real man!
     
  20. Farsight Valued Senior Member

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    Because of all the people here, my physics knowledge is the best.

    Why are you allowed here at all? You're just a troll.
     
  21. OnlyMe Valued Senior Member

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    When did you FEEL or measure your acceleration relative to any star? As far as any velocity we have relative to any star, we are part of an inertial system dominated by the earth's graviationnal field. Any acceleration you feel is realive to the earth or at best if you ever made it into space, perhaps our sun.

    Come on Farsight, you are confusing what you imagine as a hypothetical situation and reality. I mean Einstein, your hero produced a convincing argument against you current position in 1905, when he introduced special relativity.., and he was really consolidating evidence and theory that predated his insights.
     
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  22. krash661 [MK6] transitioning scifi to reality Valued Senior Member

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    says the one who never stepped into an actual scientist lab(s). says the one who never ever touched science through experiments or models. says the one who never touched higher levels of physics. says the one who continues to incorrectly interpret everything. says the one who continues to deceive when confronted . it's referred to as experience as an actual scientist, not some typical inexperienced piss-ant.
     
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  23. PhysBang Valued Senior Member

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    Then why have you never, in about a decade of posting, produced an equation to support your claims? Why have you insulted the efforts of scientists by claiming that they calculate things correctly, but you never point out the error in calculation?

    I submit that you are a liar.
     
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