Does Light Have Mass? Friend Debate

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Discussion Overview

The discussion revolves around the question of whether light has mass, exploring different definitions and interpretations of mass in the context of physics. Participants engage in a debate that touches on theoretical aspects, definitions of mass, and implications of light's properties.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that light does have mass, but emphasize the need to clarify what is meant by "mass," particularly distinguishing between inertial mass and proper mass.
  • One participant explains that the mass of a photon is considered zero when using the proper mass definition, as derived from the relationship between energy and momentum.
  • Another viewpoint suggests that while a beam of light has no proper mass, light propagating isotropically may be interpreted as having mass under certain conditions.
  • Several participants reference external resources, including FAQs and threads on the forum, to support their arguments and provide additional context on the definitions of mass.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether light has mass, with multiple competing views presented regarding the definitions and implications of mass in relation to light.

Contextual Notes

The discussion highlights the complexity of defining mass in different contexts, including inertial mass and proper mass, and the implications of these definitions for understanding the properties of light.

offtheleft
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funny little topic a friend and i were on. i think it does.
 
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offtheleft said:
funny little topic a friend and i were on. i think it does.
Yes. Light does have mass. But we need to qualify that by providing the definition of mass as I have just used it. The term mass (as used here) is defined as the m in p = mv. When defined as such this mass is more precisley know as inertial mass. There is another definition which is also used quite a lot. If you have a point particle and you measure the inertial energy to be E and the magnitude of the momentum to be p then the quantity m0 in the expression

E2 - (pc)2 = m02c4

is referred to as the proper mass of the particle. The proper mass is related to its inertial mass buy

[itex]m = \gamma m_0[/itex]

When it is said that the mass of a photons is zero it is because the energy of a photon is related to its momentum by E = pc. Substituting this value into the expression above imples that m0 = 0.

So if you want a definitive answer to your question you have to first state what you mean by the term mass.

Pete
 
offtheleft said:
funny little topic a friend and i were on. i think it does.

Please read the FAQ thread in the General Physics forum.

Zz.
 
ZapperZ said:
Please read the FAQ thread in the General Physics forum.

Zz.
The FAQ asserts that the relativistic mass is defined as [itex]m = \gamma m_0[/itex] whereas relativistic mass is defined as the m in p = mv. This is an important difference.

Pete
 
offtheleft said:
funny little topic a friend and i were on. i think it does.
An interesting thing is that a beam of light has no mass (I'm only referring to "proper" or "invariant" mass, which is what is usually called simply "mass"), while light propagating simultaneously and isotropically in all directions DOES have mass!
 
GRB 080319B said:
Search 'does light/photons have mass' on this website and you should find the answer in one of the multitude of threads on this question. Or you can go to this http://en.wikipedia.org/wiki/Photon#Physical_properties".

There is a new relativity FAQ on the notion of relativistic mass. Its located at

http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

There is also another FAQ on this topic at

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

Pete
 
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