# photons mass and energy

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how is the assumption zero rest mass accepted for photon?and not for an e-!! both are particles they have relative masses during motion and also both have energy during motion. while e- rest mass is not zero!photon's mass is zero. we also know set of photons can be converted into particles having rest mass,can we convert a single photon(energy) to mass by bringing into rest? if mass is found then it contradicts the assumption!! it will be a paradox!

also the mass of an electron is approximately 9.109×10−31(rest mass) kilogram or 5.489×10−4 atomic mass unit. On the basis of Einstein's principle of mass–energy equivalence, this mass corresponds to a rest energy of 0.511 MeV.

now why not photon(energy) in motion is not having rest energy?

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There's no paradox. A photon has zero rest mass but will have non-zero relativistic mass (more commonly referred to as energy). An electron has non-zero rest mass and non-zero relativistic mass. You will find that there is a minimum mass-energy that an electron may possess (when you slow it down as close to zero as possible). A photon on the other hand, can't be slowed down but can have as little energy as you can measure. There is no minimum energy a photon may have.

Think of it this way: how much kinetic energy does a 100,000 ton train at rest have?

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E^2 = p^2c^2 + m^2c^4

For an object at rest, this reduces to E = mc^2

But only for an object at rest. The mass is the rest mass.

For a photon, E = pc

i.e. it has no contribution from a rest mass term, and there is no frame where it will be at rest.

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There's no paradox. It's just the energy levels of photons and electrons are different. Obviously, electrons don't move at the speed of photons.

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Obviously, electrons don't move at the speed of photons.

In a vacuum.

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The reason that photon does have a zero rest mass is that its gamma is infinity. Therefore, mo=m/infinity=zero.

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The reason that photon does have a zero rest mass is that its gamma is infinity. Therefore, mo=m/infinity=zero.

I'm not happy with dividing by infinity. Also, v=c is not allowed as the Lorentz group is noncompact.

You cannot really define a "rest mass" of a photon as there is no rest frame of photon. The "invariant mass" or just "mass" is a better term, this is zero.

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As we move from the sub-atomic structure of matter, into the sizes of the macro-world, matter moves toward the pure particle state, away from the particle-wave duality. On the other hand, energy retains the particle-wave duality, regardless of scale-up or scale-down.

For example, a ball sitting on the table is not a wave, nor is it under the rules of uncertainty. It is more or less a particle at position (x,y,y,t). However, the color of the ball, or its energy refection will still exhibit particle-wave properties.

The main difference between matter and energy has to due with the loss of wave characteristics, and the movement away from uncertainty. With energy, whether we go small to large or large to small, it will stay the same.

The difference is connected to time potential. The motion blur effect of photography is an analogy. If the shutter frequency is too slow, such that time potential is left in the photo. Since time has stopped in a still photo, the excess time potential becomes uncertainty in distance; blur.

If you look at the photo above, it has particle-wave duality. The part of the photo with certainty in distance is analogous to the particle aspect. This is where there is little excess time potential. The wave aspect has more uncertainty in distance due to more time potential. As we scale up matter into macro-form, the excess time potential goes into mass instead of into distance creating certainty in space-time.

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Discussion of time potentials has its own thread in speculations, and should not be discussed here.

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O.K. ajb. Do you know any other deduction for the zero "mass" of the photon?

I thought the previous was the reason.

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O.K. ajb. Do you know any other deduction for the zero "mass" of the photon?

I thought the previous was the reason.

You want theoretical reasons why the photon is believed to be massless?

You have hinted at one already, the fact it travels at c. What you have hinted at is that there is no Lorentz transformation from an arbitrary frame to the "photon rest frame".

Other maybe deeper reasons is the fact that the electromagnetic interaction has a gauge symmetry.

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Isn't 'zero' mass here being referred because the mass is negligible? Or is it that the mass is theoretically zero. With zero mass, I doubt energy can transport itself through vacuum. That is, energy is transferred only when an entity with lower energy state is found. Only a particle can transport energy, right? Even when we talk about waves transporting energy, its actually particles in motion.

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No, it's referred to as zero because it's zero.

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The simple way out of this is to remember E=mc² and think of "rest mass" as "rest energy". How much of the photon energy is at rest? None of it. And you can't make a photon go slower or faster like a canonball, so mass doesn't apply.

Note though that this rest energy doesn't have to be actually at rest, just not moving in aggregate with respect to you. For example it can be a photon bouncing back and forth inside a mirrored box. The photon is still moving at c, and whilst it isn't at rest, in aggregate it isn't going anywhere. Hence it increases the mass of the box+photon system.

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If you entered the energy (I suppose kinetic in this case) of a photon and applied the standard equation (E=MC^2) you could calculate the mass due to velocity. I don't believe a photon at rest would exist so there is no "at rest mass". The only mass possible is due to the photon's velocity.

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The concept of photons not existing at rest isn't very clear. Because if they did, their acceleration at one instant (from 0 to c) would approach infinity and from the next instant it would approach zero.

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Coulomb's law: m < 10^-14 eV

with upper bound of: m < 3 x 10^-27 eV

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