Light - solid, liquid or gas?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by PsychoticEpisode, Nov 4, 2005.

  1. PsychoticEpisode It is very dry in here today Valued Senior Member

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    This may sound dumb to the educated but I'm just a hayseed from the country and I know absolutely nothing about light. Photons have been described as being both a wave and a particle. If it is a particle then is it a liquid, gas or solid? They fly thru space, affected by gravity, bounce off things and mix with water, so if not one of those three then is it in a special class by itself?
     
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  3. orcot Valued Senior Member

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    try putting light in a jar. If it hovers around it would be like a gas.
    If it moves when you move the jar, it would be like a liquid, and if it keeps still it would be a solid.
     
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  5. abyssoft Registered Senior Member

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    In a perfectly mirrored jar, light would react most like a gas
     
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  7. Light Registered Senior Member

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    Don't be concerned about asking the question, it's probably one that others wonder about also.

    It's non of the three. It's a small bundle of energy that exhibits some of the properties of a particle. It also exhibits the properties of a wave. One term that been in use for a long time is better at describing it - the term is "wavicle" (also sometimes spelled "waveicle") which indicates that it behaves as both particle and wave.

    It's not really correct to say they "mix with water." They simply pass through it just as they do air or glass unless they are absorbed.
     
  8. PsychoticEpisode It is very dry in here today Valued Senior Member

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    Does that mean the light energy bundle is half particle - half wave? or are certain circumstances required for either the wave or particle property to manifest itself?
     
  9. Light Registered Senior Member

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    Another good question!

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    No, it isn't a half-and-half situation, it's simply that it acts like one or the other under certain conditions - which also answers your second question.

    For example, a photon will travel in a straight line if unimpeded and gravity will change it's path. There it is acting as a particle.

    But a beam of light can be refracted and diffracted and if passed through a narrow slit will form a spreading wavefront. There it is acting like a wave.
     
  10. PsychoticEpisode It is very dry in here today Valued Senior Member

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    The doppler effect, is that because of wavelengths or does each primary color have its own speed?
     
  11. Light Registered Senior Member

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    It streches the wavelengths because the source of light is moving away from the person seeing it. All colors are stretched out.

    All colors travel at exactly the same speed through a vacuum - the speed of light - as does all other forms of electromagnetic energy. Radio waves, microwaves, X-rays, infrared, ultraviolet and the like.
     
  12. PsychoticEpisode It is very dry in here today Valued Senior Member

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    I picked up Hawking's "A Brief History of Time" and started reading it last nite. I'm wading thru the light section and just trying to understand it, thus all the questions. I have another that popped into my head while reading and not sure if this has ever been asked.... Can more than one observer see the same photon?
     
  13. Light Registered Senior Member

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    Good - you're on a nice path. And your questions are more than welcome.

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    No, only a single observer. In order to see the photon, it has to be absorbed in the retina of the eye. Thus it ceases to exist and cannot be detected by anyone else.
     
  14. PsychoticEpisode It is very dry in here today Valued Senior Member

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    Thanks for the help. I'm having a bit of trouble trying to understand how a photon goes from zero to c instantaneously. When I try to think of why, I get an image in my head where spacetime is actually moving at c (like a river) and the photon gets swept up upon its creation. Aside from what my thoughts are, what provides the instantaneous propulsion of a photon?
     
  15. Light Registered Senior Member

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    Yet another valid question.

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    A photon is generated whenever an electron drops from a higher level of excitation (higher energy level) to a lower one. This happens instantanously and the energy released is conserved ( saved/converted) into the exact amount of energy carried by the photon. It's the fact that this happens with different kinds of atoms where the electrons exist at differing energy levels (compared to other atoms) that result in the photons created having different energy levels. And those different energy levels are what determine the wavelength of the emission. The higher the energy, the shorter the wavelength.
     
  16. PsychoticEpisode It is very dry in here today Valued Senior Member

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    Is this brightness? You say that energy is released. Is this the same as what happens when nuclear fusion takes place. Hawking says that according to Einstein's E=mc² that an equal amount of mass is also lost when enrgy is released. This leads to my next question, how goes an electron that generates a photon lose no mass but still loses energy. Is this energy released first absorbed from an external source then released? I think I'm not reading something right here.

    I would recommend this book (A Brief History of Time) to anyone who has trouble getting thru normal scientific verbiage. I still have to read things more than once in an attempt to understand but at least its possible to do so.

    Space and time appear linked and I was wondering if light and time are thought to be similarly linked?
     
  17. Light Registered Senior Member

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    No, it's not brightness (intensity), that depends on the number of photons being released at the same time. And it isn't related to fusion or any other nuclear activity. It happens at what we would consider fairly ordinary temperatures and with no real pressure applied. Such as in a light bulb or burning match or candle.

    There's no loss in mass involved. It can best be thought of as like the release of energy when someone throws a ball very hard and you catch it barehanded. The burning, stinging sensation is due to the release of energy when you stop the ball from moving. That energy of impact is converted directly into heat and pressure and it's clear that no loss of mass has occurred. The ball weighs the same before, during and after the event.

    Yes, you've figured it out.

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    The energy comes first from elsewhere. In a light bulb, it's the electricity. In a burning match or candle, it's from the chemical conversion (burning/oxidation) that's taking place. As you know, there's no such thing as a free lunch and the same applies to energy - it must come from somewhere, excites the atom which raises the energy level of the electron, and is then passed along in the form of a photon when the electron drops back to it's rest level.
     
  18. PsychoticEpisode It is very dry in here today Valued Senior Member

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    The double slit experiment shows the wave/particle duality of light. My problem with this experiment is that light is massless and travels at c but the electron has mass, cannot travel at c and should slow down after being fired at slits. Does this not mean the experiment is possibly flawed? Do we really know enough about light to say this is what's happening? Is there something else going on we don't see?
     
  19. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    To PsychoticEpisode:

    Light has been basically correct but made a two errors.

    (1) An atom in an excited state does weight more by m = E/c^2 and this is reduced when the photon is emitted. For example, some excited states are meta-stable and may persist for micro seconds, storing the mass/energy until the photon is emitted. Perhaps they were excited by a collision with an electron or are the lower state (but not the ground state) of an earlier transition from an even higher upper state. (Often an atom may be collisionally excited into a relative high excited state and then cascade down thru several levels with the emission of several photons, one after the other.) It may help you to think of florescent crystal where the light that excited it has been turned off and even 30 minutes later, some of the energy that has been stored in it (probably as “quasi free electrons” fall into a lattice defect trap) is released as a photon. Yes, this excited crystal is losing mass as it glows (as time passes).

    (2)The emission process does take time. If the transition probability is low then the photon emitted is long, perhaps several meters long. You can measure it length as follows: First you must accept the strange fact that in a two path interferometer, EACH photon goes by both paths. If you make one path 10 cm longer than the other the interference pattern is still sharp and clear. As you increase the path difference, the pattern fades and does not exist if the path length difference exceeds the photon length. For the photons associated with the “Sodium D” pair of yellow lines from a typical high pressure sodium lamp, you will find that they are about 30 cm long. I have done this experiment and the pattern was completely gone with path length difference of 50 cm. It is impossible to have a classical concept of the emission of a photon, but you will not be far wrong if you think of it as if you were holding a static stretched rope with the far end tied to a distant tree and then began to oscillate your hand to “pump out” a set of waves in the rope. If you stop hand motion when the set of waves is half way to the tree, you can see your finite length “photon” travel to the tree and know it took a finite time to make it. Low probability transitions, make long photon, all else being equal. (If you know about Fourier transforms, you will understand why these long ones correspond to very well defined frequencies and the shorter one to broader spectral lines.)
     
  20. Azeperak Registered Member

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    If I remember a course I took in modern physics - The double slit experiment shows that light exhibits a classic interference pattern - however, light also behaves as a particle in other circumstances.

    The compromise model of light is that it is made up of pieces of waves. A discrete package made of a wave section is the description of a photon that I was given - but that was 20 years ago.
     
  21. Light Registered Senior Member

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    Hello, Billy T,

    Actually, they weren't errors at all.

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    This individual is very new to the whole concept and I was purposly making things overly simple in order to get the basic concepts across. I think it's a mistake to jump into the deep water before first giving them time to get their feet wet.

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    For example, you're quite correct about the added energy increasing the mass but once the photon(s) have been released, the mass returns to it's original value. And there was never any nuclear reactions involved, that's one of the primary points I wanted to make.
     
  22. http://www.newscientist.com/article.ns?id=dn2497

    article on how they propose to develop liquid light, how could be considered a (unique) gas, literally, and some of the possible uses for this liquid light (optical computing)

    Interesting.
     
  23. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I will risk confusing some again it the interest of being as fully correct as possible: There are no "nuclear reactions" in the sense you intend (some "nuclear chemisty" taking place), but the nucleus is often influencing the exact frequency of the light emitted. First, by the isotope effect and if the nucleus has "spin" by sort of a Zeeman effect.

    In a classical model of a nucleus with one electron in orbit, it is really both orbit about the center of mass and this makes the quantized levels of the "electron's orbit" depend upon the mass of the nucleus. For a specific example, the line spectra from atomic hydorgen, atomic deterium and tritium are all slightly different. (Once the second or the meter, I forget which, was defined using the radiation of mercury, but it had to be the specified isotope of mercury.)

    To understand the "spin effect" (better called the "fine structure") note that if both the spin of the electron and the proton in the hydrogen atom are "aligned" the energy levels are slightly higher (wider spaced) than the case where one is "spin up" and the other is "spin down"

    But as you said, the beginner should not worry about these small effects. You need good spectral resolution to see them.

    However, I do think that when the truth can be told without being burried in sophisticated math, it should be. I rarely "baby down" physics even for beginners. I want them to know it is complex, strange, and exciting and that most of us will never fully understand more than a small part of it. That is I want them to know that if they go into physics, a great adventure is ahead and that it will last a lifetime and probably provide them a reasonable living too!
     
    Last edited by a moderator: Nov 6, 2005

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