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Light some interesting questions and an observation

  1. Dec 18, 2009 #1
    I have some very vexing questions regarding light. We seem to know so little about it.

    1. What happens to light when it is not travelling ?

    2. Does it accelerate in reaching C ? If not does it "jump" to C ?

    3. Can it be destroyed ? If not it seems to me that the amount of light in the universe is constantly increasing. Is this a valid statement ?

    4. Why does it travel in a particular direction and what determines this ?

    5. Two light beams travelling in opposite directions will meet at a speed of 2C (this has to be true if C is a constant). Doesn't this contradict Einsteins theory of relativity regarding C.

    6. How is light absorbed into a particle ? ie what is the mechanism.

    7. Where does it end up ?

    8. On a slightly different topic, I have a question regarding time and the speed of an object. If time slows down for an object as it speeds up, is this not explained by the fact that it also becomes more massive as it speeds up ? As a result of this, interactions with photons will take longer thus causing a subjective slowing down of time as far as the object is concerned. In other words time is about rate of change. Are speed and mass equivalent ?

    Last edited: Dec 19, 2009
  2. jcsd
  3. Dec 19, 2009 #2


    Staff: Mentor

    Hi trogan, welcome to PF!
    On the contrary, we know more about electromagnetism than any of the other four fundamental forces.
    It always travels at c. If something is not traveling then it is not light.
    It always travels at c, its acceleration can only change its direction or its frequency, not its speed.
    Certainly it can be destroyed. To prove this simply go into a darkroom with a flashlight, turn it off, and note that it becomes dark.
    Maxwell's equations determine such behavior, although in appropriate circumstances you can use "ray optics" as a simplification.
    Light consists of electric and magnetic fields which can interact with electrically charged particles, typically the electrons.
    As mentioned above, it is destroyed. However, its energy is transferred to the absorbing particle which has increased energy after the absorption.
    I hadn't heard that interactions with photons take longer for particles with larger mass. If that is correct it is news to me.
  4. Dec 19, 2009 #3
    Thanks for your reply.

    I must admit I am totally confused by electromagnetic field theory. Fields I thought had been discredited by quantum theory. So Maxwell's equations , while being useful, have little realtion to reality. Instead an electromagnetic wave according to quantum theory is supposed to consist of photons isn't it ? i.e. light. It seems no-one has much of a clue and this has now been going on for over a century ! Problem is few seem willing to admit this.
  5. Dec 19, 2009 #4


    Staff: Mentor

    Please don't over generalize your own sense of confusion into an erroneous conclusion that no-one understands these concepts.

    Why would you think that? The modern quantum theory that governs light is called quantum electrodynamics (QED). It is a quantum field theory, so fields are a core concept in QED.

    On the contrary, Maxwell's equations have an overwhelming amount of experimental evidence supporting them. The only reason that QED and later theories are even considered is that they reduce to Maxwell's equations in the appropriate limits.

    I don't know how much scientific background you have, but it is absolutely essential that any new theory reduce to earlier experimentally validated theories in the domain of those experiments. Otherwise the new theory would immediately be falsified by existing experimental evidence. That is why we continue to teach Newtonian mechanics in college.
  6. Dec 19, 2009 #5
    I am a software developer with a chemistry background. I am researching State Machines and am looking to apply the results to generating models of reality. I have read extensively on quantum theory. I am not saying that there is anything wrong with current theory, including Newtonian, just that the theory does not come close to giving me a clear picture of reality. The questions I posed re light I think are questions that need answering before a clearer picture of reality arises. None of your answers give me the clarity I require. For example it is my understanding that a photon is emitted by an electron when it jumps to a lower orbit. I wonder what the mechanism for this is and how the hell the photion is emitted at a speed of c without any accelaration involved. The fact that nonone really knows what a electron is (or for that matter a photon) doesn't help matters !
  7. Dec 20, 2009 #6
    In science, things are defined by a list of things that they do. If you can make a list of some true statements about the behavior of an electron, that's what an electron is. Science has no other "is."

    There's no need to refer to a mechanism for an electron to emit a photon, because we don't have knowledge of some smaller steps that such a mechanism would have to consist of. If we had evidence for the existence of several more fundamental processes of which this process were known to be a combination, then we would have the challenge to determine which among the building blocks are put together to comprise this composite process.

    To form a "picture of reality", perhaps the most basic fact would be that conscious beings who receive all their information through senses must have as their limit of knowledge the sum of all information that has ever been accumulated through everyone's senses and using all existing instruments. To ask what is more basis than the most basic thing that has ever been discovered, the words can be formed into the question, but it's not the task of science to work on it.
  8. Dec 20, 2009 #7
    It seems to me that if you dig deep enough into reality and all you come up with are concepts then that is possibly what reality consists of. In which case it is likely we are a computer simulation of some kind (a la The Matrix). It is certainlty true that at some time in the future we will create computer-based entities that a. are more intelligent than we are b. consider themselves to be "real".
  9. Dec 20, 2009 #8


    Staff: Mentor

    Then I would recommend taking some courses on the subject. A few paragraphs on an internet forum are never going to be a proper substitute for organized and complete lectures and doing lots of homework. If you want a clear understanding then you will need to make the same effort that others with a clear understanding have made.
  10. Dec 20, 2009 #9
    Thanks for your advice.

    Yep, I understand what you are saying and agree that an in-depth knowledge of a subject is a great thing to have. I am essentially involved in a creative endeavour and am using my intuition as to what I think I need to know. So am pursuing fairly narrow lines of thought that may make a course a bit of an overkill. My very broad (although somewhat undisciplined) knowledge of physics is my basis for this. I tend to be more interested in concepts than maths although I will delve into the mathematics of something if I find I need to clarify it.
  11. Dec 20, 2009 #10


    Staff: Mentor

    I understand, everyone's time is limited. But it is rather presumptuous to make general statements that nobody knows something about a specific subject when you are consciously avoiding learning what many people do know about that subject.
  12. Dec 20, 2009 #11
    Let me preface this by saying that I am an engineer, not a physicist.

    It doesn't always travel at c, as c is the speed of light in vacuum. Refraction relies on the fact that light passing through different mediums will travel at different speeds. Light has also been slowed down http://en.wikipedia.org/wiki/Slow_light" [Broken]. For the original question, light is massless, so there is no acceleration period like a particle with mass would require, which is what I think you were asking.
    Last edited by a moderator: May 4, 2017
  13. Dec 20, 2009 #12


    Staff: Mentor

    True, I was being sloppy. I meant in vacuum.
  14. Dec 20, 2009 #13
    Have you read The Dreaming Universe by Fred Alan Wolf? This book implies that dreams share the same properties as quantum events. I think consciousness is built upon a reflecting of thought waves in a structure, like a laser generating coherent light.

    mirror ( < light > ) mirror

    The light reflects multiple times between two spherical mirrors and becomes coherent. The body may generate a form of mutual reflection in the neural pathways to generate coherent consciousness or self. The neurons responsible for ape-like imitation are called "mirror neurons." This is slight off topic re. properties of light but relevant to models of reality and electromagnetic waves involved in signal processing.
  15. Dec 21, 2009 #14
    Ok, so I guess a photon is destroyed when it is absorbed by an electron. And the electron has the capability to create it anew when it emits a photon. So it would seem light is not really slowed down by a transparant medium. It is constantly being destroyed and recreated by electrons and in between always travels at c.

    I wonder about the supposed impossibility of stopping light when I read articles like the followinghttp://www.wired.com/science/discoveries/magazine/15-11/st_alphageek" [Broken]

    Also, from my search of the internet there seems to be increasing evidence for super-luminary travel by photons.
    Last edited by a moderator: May 4, 2017
  16. Dec 21, 2009 #15


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    Not really so much by the electrons in the atomic orbitals. An atom only has discrete energy levels that the electrons can transition to, this greatly restricts the frequencies of light that can be emitted. So if the only mechanisms were the electrons and their orbitals, we would not have things like mirrors and such. Instead, the energy of the photon is absorbed by the bulk material, by the bonds between the atoms/molecules. This causes vibrations called phonons. Phonons have a very dense distribution of energy states. In most situations we consider it to be continuous since the separation between the energy states is less than the thermal energy of our environment. This allows for a wide range of frequencies of photons to be emitted.

    A full understanding of light would require rigorous understanding of quantum field theory which is far beyond the average person's exposure to physics. A good compromise is to explain the classical theory of electromagnetics and throw in a few quantum ideas about light, like photons and energy levels. This effectively describes light for just about any situation, in fact, classical electrodynamics is still a huge area of active research since it is still very applicable.
    Last edited by a moderator: May 4, 2017
  17. Dec 21, 2009 #16

    Thanks for that Born2bwire. It is exciting to see ever more complex states of matter being discovered. I suspect what we have discovered to date is just a small percentage of the totality of possible states. Can you tell me is a phonon a particle in its own right or a photon in a particular state ?
  18. Dec 21, 2009 #17


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    We are not using the word states as to mean states of matter. We use the word state to mean a unique, in this case, energy configuration. The physical properties that differ between states could be the spin of the electrons, the electron orbitals, the type of bonds between atoms, etc.

    My understanding of field theory is that all matter are fields. The particles, like electrons or photons, are simply the interactions of the fields. When the field interacts, it does so via a quanta of energy/momentum of the field. In the case of light, this quanta is the photon. Since we can only observe and measure via interactions, we can only see light directly as a photon, but we can still see the consequences of the field that gives rise to the photon. The photon is thus the excitation of the field (for light, the field is the scalar and vector potentials, the electric and magnetic fields are observables of the potentials).

    So if I have a field of frequency f that is excited to n levels above the ground state, then this field has n photons. Whenever the field interacts, it does so through one of these photons. If the photon is absorbed, the field drops down to the n-1 energy state. That is, the field now has one photon less of energy.
  19. Dec 21, 2009 #18
    Okayyy, I like it ! Can you recommend a book on quantum field theory that is up to date and high in concepts and low in maths :smile:
  20. Dec 21, 2009 #19


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    Not really. Quantum field theory at my university was a graduate level course that had two graduate level quantum mechanics courses as prerequisites (so two graduate and two undergraduate courses on quantum mechanics were the prerequisites for the field theory course). You will be very hard pressed to find a book that was conceptual in nature. The closest I can think of is Feynman's QED which was written for laymen but it does not delve very deeply at all into QED or field theory. The next best thing I can think of is Zee's "Quantum Field Theory in a Nutshell." The first chapter or two may be comprehensible, but he gains simplicity at the cost of deep understanding. He introduces the equations and chapters almost haphazardly because it would be too time consuming and difficult (his aim was to make just a general QFT textbook to act as a general introduction prior to a true QFT course) to explain where they came from exactly or the mathematical rigor behind them. Feynman's Path Integral text is good but it was a single run print in the 60's and can be hard to find. In addition, as the title portion suggests, it is about his path formulation more so than quantum field theory.

    EDIT: Art Hobson had a few papers discussing using a few quantum field theory concepts to help explain the wave nature of matter. The same goes for light and photons so it may help as further clarifications of the brief ramblings I introduced earlier.

    Last edited by a moderator: Apr 24, 2017
  21. Dec 21, 2009 #20
    k, thanks. I have a book called "The Light Fantastic A Modern Introduction to Classical and Quantum Optics" but not a mention in it of Quantum Field Theory.
    Last edited by a moderator: Apr 24, 2017
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