One question about a photon and another about light

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    Light Photon
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Discussion Overview

The discussion revolves around the nature of photons, their momentum, and the concept of light as an electromagnetic wave. It explores theoretical aspects of mass, momentum, and the behavior of light, including how these concepts apply to both classical and relativistic physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how a photon can possess momentum if it does not have rest mass, suggesting a confusion between mass and momentum definitions.
  • Another participant introduces the idea of "relativistic mass" and distinguishes it from "rest mass," explaining that light as an electromagnetic wave encompasses various forms of light, including visible and radio waves.
  • A request for further elaboration on relativistic mass is made, indicating interest in deeper understanding.
  • There is a claim that not all photons travel at the speed of light, which raises questions about the consistency of this assertion.
  • One participant argues against the utility of the concept of relativistic mass, asserting that photons have no mass and always travel at the speed of light in a vacuum, while also explaining the relationship between energy and momentum for massless particles.
  • A clarification is provided that momentum for massless particles like photons is not calculated using the traditional formula p = mv, but rather through the relationship E = pc.

Areas of Agreement / Disagreement

Participants express differing views on the concept of relativistic mass and the behavior of photons, indicating that multiple competing perspectives remain unresolved regarding these topics.

Contextual Notes

There are limitations in the discussion regarding the definitions of mass and momentum, as well as the implications of light's behavior in different contexts. Some statements rely on specific interpretations that may not be universally accepted.

jalalmalo
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How can a photon have a momentum p, which if I'm not wrong is equal to mass X speed, when the photon doesn't have a rest mass?

what does it mean to say that light is an electromagnetic wave?

thnx
 
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Yes, the photon has "relativistic mass" or inertia, but "rest mass" is a completely different concept.

To say light is an EM wave? It means familiar (e.g., coloured) light is made out of the same stuff as radio waves (which are produced by shaking magnets, for example), and that (rather than always going in straight rays) light can be demonstrated to refract or interfere with itself.
 
thanx for your reply, could u please elaborate on relativistic mass? if u have time that is :)
 
So not all photons travel at the speed of light c, cause that would not make any sense unless I missed something
 
Relativistic mass is a misleading and unncecessary concept, it is much simpler to say that the photon has no mass. And of course it always travels at c in a vacuum.

Photons and classical EM waves carry a momentum that isn't given by p = mv, but they do transfer energy and momentum. The momentum of a massless particle is related to its energy by E = pc.

By the way, sunburn is caused by the momentum that EM waves carry.
 
jalalmalo said:
How can a photon have a momentum p, which if I'm not wrong is equal to mass X speed, when the photon doesn't have a rest mass?
That's where you're wrong: momentum only equals mass X speed for particles with non-zero mass traveling at (relatively) slow speeds. As confinement explained, massless particles (like photons) do carry momentum but it's not given by mv.
 

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