Special relativity and the Photon

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Discussion Overview

The discussion centers on the nature of mass in relation to photons and special relativity, particularly addressing the implications of an object's speed approaching the speed of light and how this relates to the concept of mass for photons. The scope includes theoretical considerations and conceptual clarifications regarding mass and speed in the context of relativity.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that if an object's speed approaches the speed of light, its mass appears to increase, leading to the question of whether a photon, traveling at the speed of light, would have infinite mass.
  • Others argue that this reasoning fails because photons do not approach the speed of light; they always travel at light speed, suggesting that they should be treated differently from sub-light speed particles.
  • One participant clarifies that photons have no rest mass and do not accelerate, thus their mass does not increase in the same way as objects with rest mass. They mention the concept of relativistic mass defined by energy, which allows photons to interact gravitationally despite having no rest mass.
  • Another participant reiterates that while the relativistic mass of objects with non-zero rest mass increases with speed, this does not apply to photons, which have zero rest mass.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between speed and mass for photons, with no consensus reached on the implications of mass in this context. The discussion remains unresolved regarding the interpretation of mass as it relates to photons.

Contextual Notes

Limitations in the discussion include the dependence on definitions of mass (rest mass vs. relativistic mass) and the implications of speed in the context of special relativity. There are unresolved nuances regarding how these concepts apply specifically to photons.

discord73
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If as an object's speed approaches the speed of light from an observer's point of view, its mass appears to increase, then shouldn't a photon appear to have an infinite mass?
 
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I think the crux of where this argument fails is that you're basing it on "approaching the speed of light". Obviously, photons never approach the speed of light, so the logic doesn't really apply to them. Basically, you have to treat photons different from all sub-light speed particles in that sense.

This is a crumby answer, I think, and someone else can probably come by and do much better, but that's my input for now.
 
My understanding is that the mass does not increase, because photons have no rest mass to begin with. Photons do not approach the speed of light c because they do not accelerate -- they always travel at c when measured in a vacuum, from the moment of emission. They do have what is considered a relativistic mass defined as m=E/c^2 according to Einstein's formula, but that is a different kind of mass. It does however allow photons to interact with other mass gravitationally. Since their energy can vary according to wavelength but is not infinite, their relativistic mass is finite and calculable.
 
discord73 said:
If as an object's speed approaches the speed of light from an observer's point of view, its mass appears to increase, then shouldn't a photon appear to have an infinite mass?
The "relativistic" mass of an object with non-zero rest (invariant) mass increases with speed, but this doesn't apply to photons, which have zero rest mass.
 

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