Do photons have mass experiment?

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The discussion centers on the nature of photons and whether they possess mass. It explores a thought experiment involving a box accelerating in space, suggesting that light's path appears bent due to gravitational effects, leading to the question of whether photons are attracted by mass. Participants clarify that while photons have zero rest mass, they carry momentum, which complicates the concept of mass in relativistic terms. The conversation also touches on the idea of invariant mass in systems of multiple photons, indicating that while a single photon cannot have zero momentum, collections of photons can exhibit invariant mass under certain conditions. Ultimately, the dialogue emphasizes the nuanced understanding of mass and momentum in the context of modern physics.
  • #31
RobertsMrtn said:
Another question is - can they be said to have kinetic energy? The standard formula for kinetic energy being 1/2mv^2 which of course becomes 1/2mc^2

In Special Relativity, the kinetic energy of a particle is defined as KE=mc2-m0c2 where m0 is the rest mass. That is zero for a photon, so its KE equal to its energy, KE=mc2=hf.

ehild
 
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  • #32
RobertsMrtn said:
Correct me if I am wrong but I don't think that there is such a thing as a photon which is at rest.
Indeed. In order to define an object "at rest" in special relativity, we must first construct an inertial frame and then define an object "at rest" simply as one whose velocity relative to this frame vanishes. As a consequence of one of the fundamental postulates of special relativity, there exists no inertial frame in which light has vanishing speed hence the notion of "at rest" in the above sense does not apply to light.
 

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