Are massless particles truly massless?

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Discussion Overview

The discussion centers on the nature of massless particles, particularly in the context of mass-energy equivalence and the role of energy in defining mass. Participants explore the distinctions between intrinsic mass and the energy associated with particles, as well as the implications for understanding gravity and acceleration.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that most of the mass of matter arises from the energy of strong force interactions between quarks, which have intrinsic mass.
  • Others argue that while quarks have intrinsic mass, other particles, like photons, are massless but still possess energy, leading to questions about how to categorize particles as massless.
  • One participant questions whether energy resists acceleration in the same way mass does, suggesting a potential relationship between energy and mass in the context of gravity.
  • Another participant emphasizes that anything moving at the speed of light in a vacuum is considered massless, while particles that do not move at that speed have mass.
  • A claim is made that the invariant mass of a photon is zero, indicating it has energy but not mass, contrasting with the invariant mass of an electron, which is consistently defined across reference frames.
  • One participant challenges the idea that particles with energy should be considered to have "effectively mass," stating that this is incorrect and contributes to confusion.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of mass and energy, with no consensus reached on whether energy can be equated with mass or how to categorize massless particles.

Contextual Notes

Participants highlight the complexity of the relationship between mass and energy, noting that definitions and interpretations may vary, and that the discussion involves nuanced technical arguments that remain unresolved.

Christofer Br
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Most of the mass of matter comes from energy of strong force interactions between quarks. However the quarks still have intrinsic mass. Other particles have no intrinsic mass but still have energy. So according to mass-energy equivalence, these particles should still have effectively mass, to my understanding. If so, how can we label a particle as massless or not if they all have mass due to energy they carry? Doesn't mass of quarks come from energy too?
 
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Christofer Br said:
Most of the mass of matter comes from energy of strong force interactions between quarks. However the quarks still have intrinsic mass.
Exactly. They are not massless particles.

Other particles have no intrinsic mass but still have energy.
Yes, a photon is massless but is subject to gravity

So according to mass-energy equivalence, these particles should still have effectively mass, to my understanding. If so, how can we label a particle as massless or not if they all have mass due to energy they carry?
Because they don't HAVE mass, they have energy.

Doesn't mass of quarks come from energy too?
Quarks are not massless.
 
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So what it boils down to is gravity acts on both energy and mass, but they're not the same thing, there's no "duality" there? Does energy resist acceleration in the same sense that mass resists acceleration (F=m*a)?
 
Christofer Br said:
So what it boils down to is gravity acts on both energy and mass, but they're not the same thing, there's no "duality" there?
That is correct, sort of. Gravity is the curvature of space-time and that "curvature**" is a result of the stress-energy tensor of an object.

Does energy resist acceleration in the same sense that mass resists acceleration, giving it a finite value (F=m*a)?
Yes, energy "acts like mass" in that sense. If you weigh a box of gas and then heat it up, it will weigh more.

** "Curvature" is a common term for this, but really it's straight lines (formally geodesics) in space-time geometry, which is pseudo-Riemann, not Euclidian.
 
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Simple distinction: Anything moving at the speed of light (in a vacuum) is massless. Otherwise it has mass.
 
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Calculate the invariant mass of a photon and you'll get zero. It has energy but not mass.
Calculate the invariant mass of an electron and you'll get 511 keV - in every reference frame. We call this the mass of the electron.
 
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"according to mass-energy equivalence, these particles should still have effectively mass,"
That is wrong, which is the basis for your confusion.
 
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