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Thread: in between light rays

  1. #1 in between light rays 
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    is it possible to be so far away from a light source (such as a star, galaxy, etc) that you can actually occupy the space in between the light that it emits, effectively making it invisible to any form of detection.

    I'm not really confident in the way I understand light so maybe I'm missing something.


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  3. #2  
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    I'm not sure I understand the question really...

    But the further away you get from a light source the more distorted the light becomes, I think...


    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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  4. #3 Re: in between light rays 
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    Quote Originally Posted by Dwayne Dibley
    is it possible to be so far away from a light source (such as a star, galaxy, etc) that you can actually occupy the space in between the light that it emits, effectively making it invisible to any form of detection.
    I think you mean "making you invisible" - i.e. the same "you" that is "far away and between the light rays".

    In other words, you are asking if Adam can be invisible to Bob if Adam stands between Bob and a distant star - the star being so distant that Adam fits between its rays, so Bob cannot see Adam as a dark outline against the light because Adam doesn't occlude any of the radiation.

    (I replaced "you" and "me" with names so we don't get confused who is "you" and who is "me").

    Did I understand the question correctly? If so, I don't think I know the answer, but others around here surely will.
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  5. #4 Re: in between light rays 
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    Quote Originally Posted by Dwayne Dibley
    is it possible to be so far away from a light source (such as a star, galaxy, etc) that you can actually occupy the space in between the light that it emits, effectively making it invisible to any form of detection.

    I'm not really confident in the way I understand light so maybe I'm missing something.
    I guess the picture you have in mind is that light is emitted in linear rays. This picture is incorrect. Light comes in waves. Light rays are only used in certain visualisations, where the propagation can be assumed to be in a linear way. In essence, there is no space "between" the rays, because the electromagnetic wave, that is the light, occupies the entire space. However, you can use the light of two sources to produce interference. Then, you might have valleys and peaks of the superimposed resulting wave. The electromagnetic field in these valleys is then zero, there is no light there.
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    Forum Ph.D. Leszek Luchowski's Avatar
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    This is where I find the corpuscular-wave dualism hardest to handle. An electromagnetic disturbance, say the waves from an electric coil that was powered for a short time, propagates outwards from its center. Maxwell's equations allow us to predict when, and at what amplitude (and power density), the waves will reach any point in space. If it's very far away, the amplitude will be tiny, and might be unmeasurable, but it will have a nonzero value.

    At the same time, it comes in the form of photons, and if the energy passing, say, through one square meter (of a plane perpendicular to the radiation), throughout the duration of the pulse, is less than 1 photon at that particular frequency, what happens?
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    so in other words in nature light is more of a wave than a particle in most cases that we interact with it. Also light waves can interfere and cancel out much in the way sound can if you layer the original source with one out of phase to it. right?

    can a light source, say a sphere like a star, interfere with its own light frequency causing gaps in its wave projection? Would you count a star as one single light source or would you fraction it off as many sources?
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  8. #7  
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    Quote Originally Posted by Dwayne Dibley
    so in other words in nature light is more of a wave than a particle in most cases that we interact with it. Also light waves can interfere and cancel out much in the way sound can if you layer the original source with one out of phase to it. right?

    can a light source, say a sphere like a star, interfere with its own light frequency causing gaps in its wave projection? Would you count a star as one single light source or would you fraction it off as many sources?
    That's the strange thing about subatomic particles. They are both particles and waves. Only depending on what aspect you investigate, you tend to find one of these characteristics.

    Interference needs coherent waves, i.e. phase and wavelength have to agree for more than at least one wavelength. It is easiest for single mode monochromatic light (e.g. a laser). In stars, every ion or atom that emits a photon/wave is an individual light source. Light that contains lots of different wavelengths coming from lots of different sources - e.g. a star - practically only shows interference, if you set up a lab experiment that is especially designed to do this. This means that you split up the light and recombine it in a suitable way so that the two signals of the same photon meet again and interfere. It does not happen in nature, as far as I know.

    Optical interferometers like the VLTI can use starlight. Since it contains more than just one wavelength, a lot of technical effort has to be put into the instrument in order produce interference. The most challenging part is to maintain the same path length of the light that is emitted from one atomic process and then detected by the different telescopes until it reaches the instrument, where the interference is produced.
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  9. #8  
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    Quote Originally Posted by Leszek Luchowski
    This is where I find the corpuscular-wave dualism hardest to handle. An electromagnetic disturbance, say the waves from an electric coil that was powered for a short time, propagates outwards from its center. Maxwell's equations allow us to predict when, and at what amplitude (and power density), the waves will reach any point in space. If it's very far away, the amplitude will be tiny, and might be unmeasurable, but it will have a nonzero value.
    The wave-like property is in the fact there is always a probability of a photon striking. That probability is the part of the wave that can spread infinitely thin, but you're right that this creates a problem for short pulses.

    In the course of a short pulse, it may randomly turn out that not even one photon strikes at a given location, but that is just because you were unlucky. There is always a chance of it happening, regardless of how short the pulse is. If it's a very very short pulse, or the detector is very far away, then the odds would be very very bad, but never zero.


    At the same time, it comes in the form of photons, and if the energy passing, say, through one square meter (of a plane perpendicular to the radiation), throughout the duration of the pulse, is less than 1 photon at that particular frequency, what happens?
    Randomly, either a photon appears somewhere in that square meter, or it doesn't. If the energy-per-meter at that distance is 1/4 of a photon, then the odds are about 25% of a photon appearing there vs. a photon not appearing there.

    If you repeated the experiment a number of times, and then took the average, then 1/4 photons would be the average number.
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  10. #9 Re: in between light rays 
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    Quote Originally Posted by Dwayne Dibley
    is it possible to be so far away from a light source (such as a star, galaxy, etc) that you can actually occupy the space in between the light that it emits, effectively making it invisible to any form of detection.

    I'm not really confident in the way I understand light so maybe I'm missing something.
    Light comes to us in photons (they are not particles but for this, think of them as such). For the very distant objects large telescopes collect these to build up a picture after some time. In theory something could be far enough away that only one photon would hit you in the space of a minute so for the other 59 seconds-odd of the minute, no photons from that object would hit you, so you would not see it, assuming you could see single photons.
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