What Makes Photons, Bosons, and Gluons Massless?

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    Massless Photons
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Discussion Overview

The discussion revolves around the nature of mass in relation to photons, bosons, and gluons, particularly why these entities are considered massless despite the relationship between mass and energy as described by Einstein's equation. The scope includes theoretical considerations and conceptual clarifications regarding mass and energy in the context of particle physics.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that according to Einstein's equation E=mc², mass and energy are related, questioning why massless particles like photons exist.
  • One participant points out a flaw in the initial reasoning, suggesting that not all energy must have a rest mass, and clarifies that energy can exist without being equivalent to mass.
  • Another participant mentions that high-energy photons can convert into electron-positron pairs, introducing the concept of energy conversion into rest mass under specific conditions.
  • A participant provides an analogy involving a mirrored box of photons, explaining that while photons have no rest mass, their kinetic energy contributes to the mass of the ensemble, similar to confined atoms in a gas.
  • There is a suggestion that the concept of a photon having zero rest mass implies it is never at rest, with some agreement on this point among participants.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between energy and mass, with some agreeing on the implications of special relativity while others challenge the initial assumptions. The discussion remains unresolved regarding the fundamental reasons for the masslessness of photons, bosons, and gluons.

Contextual Notes

Limitations include the dependence on interpretations of mass and energy, as well as the assumptions underlying the application of Einstein's equation to massless particles. The discussion does not resolve the complexities surrounding these concepts.

khil_phys
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According to Einstein's equation E=moc2, any object which has mass has energy, and, conversely, anything which has energy must also have mass. Then, why are photons, bosons and gluons said to be massless?
 
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khil_phys said:
According to Einstein's equation E=moc2, any object which has mass has energy, and, conversely, anything which has energy must also have mass. Then, why are photons, bosons and gluons said to be massless?

Please start by reading the FAQ sub-forum in the Relativity forum.

Also note the flaw in the logic you are applying. An entity with a rest mass has an equivalent energy content. However, the reverse is not necessarily true. There is nothing that says that ALL energy must have a rest mass. This is different than saying that a clump of energy can be converted into an equivalent rest mass.

Zz.
 
Why is it so?
 
khil_phys said:
Why is it so?

Why is what so?

In any case photons with high enough energy (> 1.022 Mev) can be converted into electron-positron pairs (rest mass of each is .511 Mev) under the right conditions.
 
Another example is that if you had a perfectly mirrored box with a bunch of photons inside, they *would* contribute the rest mass of the ensemble. This may be initially quite mysterious, but consider analogy with an isolated atom versus a confined gas of atoms. The isolated atom has some rest mass. The confined gas of atoms contributes more rest mass to the ensemble than the sum of atom's rest masses. The difference is the the kinetic energy of the atoms - this also contributes to mass of the ensemble. Photons have kinetic energy but no rest mass.

As to why, there are really no good answers to such a question. If you assume special relativity is true, then anything whose speed is observer independent must have no rest mass.
 
Isn't saying a photon has 0 rest mass another way of saying it is never at rest?
 
in some sense yes
 

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