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  1. Does light have mass?

  2. If yes, will it exert force?

  3. If no, then how are light particles are travelling at light speed?

  4. If light doesn't have mass how is it attracted by gravitational force (black holes)?

  5. And what actually is light?

Qmechanic
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Ashif
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  • Light has zero rest mass. – Anubhav Goel Feb 18 '16 at 09:03
  • Light has momentum but the classical formula for momentum $p=mv$ does not apply. Rather $p=\frac h \lambda$, where $h$ is Planck's constant and $\lambda$ is the wavelength of the light, should be used. – Farcher Feb 18 '16 at 09:03
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    Duplicate of http://physics.stackexchange.com/q/3541/ – Anubhav Goel Feb 18 '16 at 09:08
  • Introduction to Light: The Physics of Light, Vision, and Color by Gary Waldman , a book without the maths , for any scholar from 14 to 140 years old –  Feb 18 '16 at 14:58
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    In addition to the duplication issue, we don't do lists of questions. It's just not part of the stack exchange model. You should have search carefully for the answer to each of these (which would have found answers to #1 and #3 (Anubhav's link), #2 (http://physics.stackexchange.com/q/112866/), #4 (http://physics.stackexchange.com/q/107930/) and #5 (http://physics.stackexchange.com/q/146253/); which is to say all your questions here. – dmckee --- ex-moderator kitten Feb 18 '16 at 15:08
  • Please note that the question is asking also about "light". i.e. an ensemble of photons, not only the particles from which it emerges. The answers is : unless the photons from which light emerges are collinear, the emergent light has an invariant mass. The best example is the pi0 which decays into two photons. – anna v Sep 20 '18 at 07:08

2 Answers2

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Photons having zero rest mass is precisely the reason why they travel at light speed. A particle can have momentum without having mass, the only condition is $$(mc^2)^2=E^2-(pc)^2$$ in the case of light, left side is zero and $E=pc$ where $E$ is energy and $p$ is linear momentum.

Furthermore, the gravitational attraction isn't really attraction in the traditional sense. Gravitation curves the space-time so that the geodesic (the locally straightest and shortest path, which the light takes) is curved. So the light just goes along its way, as straight as it can get, and the curvature of the space makes this geodesic path deflect in angle when the ray passes a massive object.

Light is an electromagnetic wave - change in electric field induces magnetic field, and change in magnetic field induces electric field again... this cycle propagates in space with the speed of light.

orion
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  • Orion, your last sentence implies a shift of B and E fields by 90° like in this picture? – HolgerFiedler Feb 18 '16 at 20:18
  • Yes, just like that (the orientation of the "cross" is polarization of light). Maxwell equations are all about perpendicular things (which is why electromagnetism is a bit difficult at the high-school level, as you can't "dumb it down" to 2D). Just recall how changing magnetic field perpendicular to a circuit loop/coil induces voltage in the loop, and how a current induces circular magnetic field, perpendicular to the direction of the wirte. – orion Feb 19 '16 at 07:55
  • Please note that the question is asking also about "light". i.e. an ensemble of photons, not only the particles from which it emerges. The answers is: unless the photons are collinear, the emergent light has an invariant mass. The best example is the pi0 which decays into two photons. – anna v Sep 20 '18 at 07:06
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As orion stated, photons in vacuum have zero rest mass and travel at light speed. There are, however, some cases where you can have massive photons, such as in the Abelian Higgs model which describes $U(1)$ symmetry breaking. This model applies to superconductivity : in a superconductor you have the Meissner effect, which is the fact that magnetic field penetrates over a very small depth inside of the superconductor. Its is a signature that the electromagnetic forces become short-ranged, or, in a field theory language, that the photon becomes massive.

Dimitri
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  • Please note that the question is asking also about "light". i.e. an ensemble of photons, not only the particles from which it emerges. The answers is: unless the photons are collinear, the emergent light has an invariant mass. The best example is the pi0 which decays into two photons. – anna v Sep 20 '18 at 07:07