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Thread: Mass of light

  1. #1 Mass of light 
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    Doest light have any mass .
    If yes : what is mass of light
    IF NO : then how light can be affected by gravity as gravitation force between two bodies is directly proportional to product of their masses.


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    Anti-Crank AlexG's Avatar
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    Light has no mass. The way it's affected by gravity is indirect, in that gravity bends the geodesic, and light always follow the geodesic (the shortest line between two points)


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    if light has no mass then what are photons
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    Anti-Crank AlexG's Avatar
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    The excitations of the electromagnetic field. The quantum of light, as it were.
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  6. #5  
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    Quote Originally Posted by shashi.adsure View Post
    Doest light have any mass .
    If yes : what is mass of light
    IF NO : then how light can be affected by gravity as gravitation force between two bodies is directly proportional to product of their masses.
    It has inertial mass but no rest mass. Suppose the energy of the light you have is E. Then the amount of mass is m = E/c2.
    Last edited by pmb; July 11th, 2012 at 10:18 AM.
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    I just happened to pick up the following gradualte level textbook on Classical Dynamics by Donald T. Greenwood, Dover Pub, (1977). From page 312
    Previously we found that a photon of frequency f has an inertial mass hf/c2. It follows, then, from the principle of equivalence, that it must have an equal grvitational mass, even though the rest mass is zero. The effect of the gravitational field on the motion of photons is shown in the slight bending of light rays which pass very near the sun in traveling fom a star to the earth.
    This is precisely what I was talking about. Hardly troll material if its in a graduate level mechanics text.
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    Einstein has said that fifty years of conscious brooding brought me no nearer to the reply to the question; what is quanta. But now every Tom, Dick & Harry says that he knows quanta but they are mistaken. There has to be something wrong with the theory where light is wave as well as a particle. It has to be either of the two. If it is a particle it should have mass & if it is a wave motion; then wave motion of what?
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    Unless it's actually in 2 parts travelling together.
    Everything has its beauty, but not everyone sees it. - confucius
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    Quote Originally Posted by shahidshah View Post
    Einstein has said that fifty years of conscious brooding brought me no nearer to the reply to the question; what is quanta. But now every Tom, Dick & Harry says that he knows quanta but they are mistaken. There has to be something wrong with the theory where light is wave as well as a particle. It has to be either of the two. If it is a particle it should have mass & if it is a wave motion; then wave motion of what?
    A photon dop not behave like particles and they don't behave like waves. At times a photon can have wave-like characteristics and at times it can have partilce-like characteristics. Feynman really hits it on the head when he wrote in his lectures, V-III page 1-1
    "Quntum meechanics" is the description of the behavior of matter and light in all its details and, in particular, of the happenings on the atomic scale. Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like cloudsm or billiard balls, or weighs on springs, or anything you have ever seen.
    Newton thought that light was made up of particles, but then it was discovered that it behaves like a wave. Later, however (in the beginning of the twentieth century), it was found that light indeed sometimes behave like a particle. Historically, the electron, for example, was thought to behave like a particle, and then it was found that in many respects it behaved like a wave. So it really behaves like neither. Now we have given up. We say: "It is like neither."
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