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It appears that you're using the equation for the energy of a photon in terms of its wavelength. In that case the correct relationship isE = hfIf you want the relativistic mass of the photon the start with the relationship between the photons energy and its momentum which is E = pc. The relation between its momentum and speed is p = mc. Solve for m and you get E = mc^{2}.

Yes I know you think E=hf but in reality PE=hf the ''energy'' is not ''created'' until the interaction with ''matter''. It remains potential ''energy'' until interaction.

Quote from: Thebox on 24/06/2016 11:29:07Today for some reason I understood E=mc², however after considering this I do not ''see'' why we square c. Should this equation not be E=*c?It appears that you're using the equation for the energy of a photon in terms of its wavelength. In that case the correct relationship isE = hfIf you want the relativistic mass of the photon the start with the relationship between the photons energy and its momentum which is E = pc. The relation between its momentum and speed is p = mc. Solve for m and you get E = mc^{2}

Today for some reason I understood E=mc², however after considering this I do not ''see'' why we square c. Should this equation not be E=*c?

If a Photon was travelling at 1mph

Quote from: TheBoxIf a Photon was travelling at 1mph But a photon cannot travel at 1 mph in a vacuum - it must always travel at 300,000 km/s when you measure it in a vacuum in your lab.

Thebox

The definition of momentum is mass times velocity (mv). If we replace v with c (mc) then we have defined an upper bound on momentum.

Acceleration can replace c to ma which signifies force.

It is a simple step to multiply by a distance over which the force operates to get the energy (mc^2). I could have broken it down into simpler steps but didn't.

The definition of momentum is mass times velocity (mv). If we replace v with c (mc) then we have defined an upper bound on momentum. Acceleration can replace c to ma which signifies force. It is a simple step to multiply by a distance over which the force operates to get the energy (mc^2). I could have broken it down into simpler steps but didn't.

I am explaining something to Thebox remember.

OK forget the above. You are over-thinking this. Instead of mass lets call it physical 'stuff'. If you want to move that stuff in a particular direction at a particular speed then you have to apply a force. This is what you understand as F = ma. The acceleration of stuff is moving it from being stationary to some non stationary speed. If the force is maintained then that speed continues to increase so we have a continued acceleration. If the force is constant then the acceleration of the stuff is constant. It gets faster and faster. So in order to apply this force we need to add energy. Just as we expend energy when push starting a car. Now acceleration is stuff times distance divided by time squared. If we have a constant speed then there can be no acceleration of our stuff since the speed isn't changing. Therefore a non accelerating amount of stuff moves via stuff times velocity. Velocity is simply speed in a particular direction. So coming back to mass instead of stuff we have p = mv where p is the momentum. Momentum can be imagined as constant speed with no force applied. We stop pushing the car and it keeps moving on its own.

a ''stationary'' object can gain ''energy'' and only involves the entropy of the object absorbing ''light'' without any complication of momentum which is seemingly not needed?

The relativistic concept of energy is discussed under two important aspects: the behaviour of the energy under coordinate transformations and its general definition in the presence of external gravitational and electromagnetic fields. On the basis of energy conservation and a nonrelativistic approximation it is argued that only the zeroth component of the covariant 'generalized' momentum should be called 'energy'.

When an object absorbs light and heats up,

this energy goes into increasing the velocity of the constituent atoms and molecules. This increases both kinetic energy and momentum of the particles.

Because these velocities are in random directions, the object as a whole does not move (it just expands a bit with temperature). This increase in temperature has an impact on Entropy.

When General Relativity talks about momentum and kinetic energy, it is usually talking about motion of the object as a whole.