If light is energy, then according to $E =mc^2$, it should have mass. My reasoning is that if light has speed, and it is a form of energy, it must have mass, otherwise how can it satisfy the equation?
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4Possible duplicate of Einstein equation $E=mc^2$: Does it mean an object without mass does not have energy? – StephenG - Help Ukraine Mar 06 '17 at 14:20
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Related/possible duplicate: http://physics.stackexchange.com/q/143652/50583 and its linked questions. – ACuriousMind Mar 06 '17 at 14:20
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Wrong starting point - confusion in terms and words. Light is not energy, if you mean a photon. Photon is a photon. You can speak about his energy $E$ as an attribute of existence, but he himself is not $E$. And thus photon $\neq mc^2$. Although $E=mc^2$, it is not much useful to speak about photon mass. Energy is ok, you can speak about frequency as $E=h\nu$ – jaromrax Mar 06 '17 at 14:54
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Have you looked at the formula for the famous m in
So in the Lorenz transformation system of special relativity , when the velocity of a particle approaches the speed of light, m becomes undefined, i.e. 0/0 for a zero mass particle.
m as dependent on velocity cannot characterize a particle in a one to one correspondence. What characterizes a particle is m_0, the invariant mass, which is the "length" of the Lorenz energy momentum four vector
So your reasoning is not considering all the mathematical consequences of Lorenz transformations.
The zero invariant mass of the photon is consistent with observations and the Maxwell equations for light, which consists of innumerable photons.
anna v
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The term "mass" has two meanings here. Each particle has a rest mass $m_0$ such that $E^2=m_0^2c^4+c^2p^2$ with $p=\frac{Ev}{c^2}$ the momentum. For light, $v=c$ so $cp=E$ so $m_0=0$. Rest mass is also called invariant mass because it's unchanged by a Lorentz transformation such as a frame shift, whereas $E,\,p$ are not. Indeed, the rearrangement $m_0^2c^2=p^\mu p_\mu$ with $p^0:=\frac{E}{c}$ makes this manifest.
J.G.
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