Are photons massless or practically massless

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    Massless Photons
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Discussion Overview

The discussion centers on the mass of photons and gluons, exploring whether they are massless or practically massless. It also touches on the implications of their mass on theoretical frameworks and the nature of their travel in a vacuum, particularly in relation to gravitational influences.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that photons are massless, while others note that their travel in a straight line can depend on the geometry of spacetime, especially near strong gravitational fields.
  • A participant mentions the experimental limit on photon mass being extremely small, referencing the magnetic field of the Sun.
  • Another participant discusses the poorer experimental limits on gluon mass, citing the Millikan experiment and the implications of not observing free fractional charges.
  • One participant introduces the concept of photons traveling along a "geodisk" in four dimensions, suggesting that their path appears curved in three-dimensional space due to the connection between time and space.
  • There is a discussion about the implications of a non-zero photon mass on theoretical frameworks, with one participant suggesting that a tiny photon mass could lead to a formulation of Maxwell's equations as Proca theory, while a gluon mass would disrupt the renormalizability of quantum chromodynamics (QCD).

Areas of Agreement / Disagreement

Participants express differing views on the mass of photons and gluons, with no consensus reached on whether they are massless or practically massless. The discussion includes both supportive arguments and challenges regarding the implications of their mass.

Contextual Notes

The discussion includes references to complex theoretical concepts and experimental limits, which may depend on specific definitions and assumptions that are not fully resolved within the thread.

bobsmith76
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Title says it all. Also, do photons in a vacuum travel in a straight line? I find that kind of strange. Also, are gluons massless or practically massless? How are gluons detected?
 
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Photons are mass-less. "Straight line" kind of depends on what geometry. Photons that travel through a region of space that is being influenced by a high gravitational force (like near a black hole or at the edge of a galaxy) follow a "straight line" that is not what Euclid had in mind.
 
bobsmith76 said:
Title says it all. Also, do photons in a vacuum travel in a straight line? I find that kind of strange. Also, are gluons massless or practically massless? How are gluons detected?

Please start by reading the https://www.physicsforums.com/forumdisplay.php?f=210" in the Relativity forum.

Zz.
 
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phinds said:
Photons are mass-less. "Straight line" kind of depends on what geometry. Photons that travel through a region of space that is being influenced by a high gravitational force (like near a black hole or at the edge of a galaxy) follow a "straight line" that is not what Euclid had in mind.

OP, do you understand what he said there at the end? If you don't need it explained, then I won't. If you do, let me know :)
 
The experimental limit on the photon mass is pretty good : a billionth of a billionth of an eV, from the magnetic field of the Sun. Now mind you, there is an excellent discussion in Feynman's lectures on gravitation. One thing he says is that it is pretty futile to ask whether something is so small that it is impossible to observe.

The experimental limit on the gluon mass is much poorer. From the Millikan experiment, and the failure to observe free fractional charges, we can set a limit on the gluon mass at about a tenth of thousandth of eV.

Please let me know if you want further details or reference.
 
1mmortal,

photons do not travel in a straight line in the Euclidean sense - I guess I don't understand that. Please illuminate.

humanino,

I think you've given me enough information for now. I'm not going to get anywhere unless I open the textbook, so for now I'm good.
 
Photons travel a "straight" line in 4 dimensions, called a geodisk. Because time and space are connected, and we cannot see this time dimension, in 3d it appears that photons are curving in space.
 
khemist said:
geodisk.

geodesic.
 
jtbell said:
geodesic.

I was wondering why firefox was correcting my spelling. Thanks :P
 
  • #10
And there you have it :)
 
  • #11
bobsmith76 said:
Also, are gluons massless or practically massless?
Afaik a tiny photon mass (which would turn Maxwell into Proca theory) could be formulated as a renormalizable QFT as well, whereas a gluon mass would spoil renormalizability of (non-abelian) QCD. That's a strong indication that gluons must be exactly massless.
 

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