Unraveling the Mystery of E=mc^2: The Surprising Truth About Light's Energy

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    Confusion E=mc^2
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Discussion Overview

The discussion revolves around the implications of the equation E=mc², particularly in relation to the energy of photons, which are massless particles. Participants explore the relationship between mass and energy, the concept of relativistic mass, and the nature of photons in the context of special relativity.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant asserts that E=mc² implies that light should not have energy since photons are massless, questioning the validity of this interpretation.
  • Another participant argues that if photons had a nonzero resting mass, their speed would be infinite, suggesting that the massless nature of photons is consistent with their energy.
  • A third participant introduces the equation E={m_0}{c^2} and discusses how energy and mass are equivalent, noting that a moving particle, even with zero rest mass like a photon, can still possess energy through its momentum.
  • Another participant clarifies the distinction between rest mass and relativistic mass, emphasizing that the correct formulation for energy includes momentum and does not solely rely on rest mass.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of mass and energy in relation to photons, with no consensus reached on the implications of E=mc² for massless particles.

Contextual Notes

There are unresolved aspects regarding the definitions of mass, the implications of relativistic mass versus rest mass, and the mathematical steps involved in deriving energy equations for massless particles.

Prasanna Suman
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"I believe E=mc^2 works." But!
Photon (Packet of energy as described by Einstein in 1905, "Photoelectric effect") is mass less particle. So, the relation suggests that light should not have energy, though it has.

Is this wrong, if yes why?
 
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It is nothing wrong. If a photon had a "nonzero resting mass", its "c speed traveling mass" would be infinite.

But it isn't.
 
[itex]E={m_0}{c^2}[/itex] where [itex]m_0[/itex] is the rest mass. Since energy and mass are equivalent, when a particle moves it gains mass, even if its rest mass is zero, like a photon.

For an object in motion there is, where p is the momentum, [itex]{E^2}={{m_0}^2}{c^4}+{p^2}{c^2}[/itex], so the photon's energy (with it's zero rest mass) is [itex]E=pc[/itex]
 
Last edited:
Jheriko said:
[itex]E={m_0}{c^2}[/itex] where [itex]m_0[/itex] is the rest mass.
No. [itex]E={m}{c^2}[/itex] where m is the relativistic mass.

You can understand that E is not [itex]{m_0}{c^2}[/itex], for a moving particle, from the correct equation you wrote then:

[itex]{E^2}={{m_0}^2}{c^4}+{p^2}{c^2}[/itex]

If it were [itex]E={m_0}{c^2}[/itex], substituting this value in the previous equation, you would have:

[itex]{{m_0}^2}{c^4} ={ {m_0}^2}{c^4} +{p^2}{c^2}[/itex]

that is: p = 0, which implies v = 0, so the particle is necessarily at rest.
 
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