Are photons massless or practically massless

In summary, photons are mass-less and travel in a "straight line" that is influenced by high gravitational forces. However, this "straight line" is not the same as what is traditionally defined as a straight line in Euclidean geometry. Gluons are believed to be massless, as a tiny mass would affect the renormalizability of QCD.
  • #1
bobsmith76
336
0
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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  • #2
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.
 
  • #3
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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  • #4
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 :)
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
khemist said:
geodisk.

geodesic.
 
  • #9
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.
 

Related to Are photons massless or practically massless

1. What is the mass of a photon?

Photons are considered to be massless particles, meaning they have no rest mass. However, they do have energy and momentum, which can be thought of as equivalent to mass through the famous equation E=mc^2.

2. How is the mass of a photon measured?

Since photons are massless, their mass cannot be directly measured. Instead, their energy and momentum can be measured through experiments such as the photoelectric effect or Compton scattering, and the results can be used to calculate the equivalent mass.

3. Are all photons truly massless?

According to the theory of special relativity, all photons are considered to be massless. However, there are some theories such as quantum field theory that suggest photons could have a very small mass, but it has not been proven or observed.

4. How does the masslessness of photons affect their speed?

Since photons have no rest mass, they must travel at the speed of light in a vacuum, which is about 299,792,458 meters per second. Their speed is also constant, meaning it does not change with different observers or reference frames.

5. What are the implications of photons being massless in the field of physics?

The masslessness of photons has significant implications in physics, particularly in the theories of special relativity and quantum mechanics. It also plays a crucial role in understanding the behavior of light and electromagnetic radiation, as well as in applications such as laser technology and solar panels.

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