Light has momentum but no mass. Doesn't that make it pure energy? Some argue that because photons have momentum they are not pure energy. Some even argue that photons have mass. Can someone explain please.
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Qmechanic
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zane scheepers
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Photons have zero mass, which means that E=p in units where the speed of light is 1. They have both energy and momentum so that a photons invariant mass is zero. Two non colinear photons have an invariant mass, so light that is a confluence of innumerable photons may have an invariant mass. (pi0 decays to two photons). Pure energy means nothing because energy is just one of the four components of a four vector http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/vec4.html – anna v Apr 28 '17 at 10:58
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Light is classically an electromagnetic wave (a periodically changing electric and magnetic field at any one point). Therefore it is not strictly energy, but a time-varying force on any test charge. For example, it rattles electrons as it hits our eyes, which we register as "light", or it can heat up an object that contains charged particles. In quantum field theory it is an exchange of photons between two charged objects which thereby experience a sudden momentum transfer. A large numbers of such events will average to exactly the classical electromagnetic wave of force fields. – Zardos Apr 28 '17 at 16:12
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The question is interesting in its own right and shouldn't have been subsumed under 'what is pure energy' – Zardos Apr 29 '17 at 08:05