Is the momentum of a massive particle traveling at the speed of light zero?

In summary, the conversation discusses the question of whether photons are the only massless particles that travel at the speed of light. The standard model states that all massless particles must travel at the speed of light, and it is currently accepted that the photon and the gluon are both massless particles. However, there is still uncertainty surrounding the existence and properties of the graviton. The conversation also mentions an upper limit on the mass of a photon, but this does not necessarily mean that the photon has a non-zero mass. Overall, the conversation highlights the importance of experimental methods in determining the properties of particles, and the need for further research in this area.
  • #1
RAD4921
347
1
I know this isn't a deep discussion and the thread will be short but this question has been bothering me for a while and I hope the moderators won't terminate it.

Are photons the only massless particles that travel at the speed of light? Diagrams of the standard model do not reveal the answer to this and the physics forum is my last hope. Thanks
RAD
 
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  • #2
Currently it is accepted that the photon and the gluon are both massless particles. The graviton [if it exists] is predicted (quantum theory of gravity??) to be massless, but as yet no observations have been made and the related theories are still a bit 'shaky'. The evidence to support the massless photon is basically rock solid, the gluon somewhat less so I believe.

Edit: Oops I believe I forgot to answer your actual question. All massless particles must, acording to the standard model which obeys SR, travel at the speed of light due to the very fact that they are massless.
 
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  • #3
And to add any particle with mass as I understand it cannot travel at c, due to the energy required to achieve c.

Interesting side note and spoilered so as not to confuse the issue

https://www.physicsforums.com/showthread.php?t=152855
 
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  • #4
Schrodinger's Dog said:
Interesting side note and spoilered so as not to confuse the issue
[spoiler removed from quote]
Seems very interesting, I may have a read of that paper this weekend if I get chance.
 
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  • #5
i remember Meir pointed out a link to some study that put an upper limit on the (invariant) mass of a photon and it wasn't zero. it was something like 10-50 kg. it seems to me very dissatisfying that the speed of particles of light or E&M in general do not fly by at the same speed as the wavespeed of E&M but it might not be so rock solid.
 
  • #6
rbj said:
i remember Meir pointed out a link to some study that put an upper limit on the (invariant) mass of a photon and it wasn't zero. it was something like 10-50 kg. it seems to me very dissatisfying that the speed of particles of light or E&M in general do not fly by at the same speed as the wavespeed of E&M but it might not be so rock solid.

An upper bound like that just means that our methods of measuring the photon's mass can't measure a mass smaller than that. This means that the photon could have a mass as small as 0 kg or as large as 10-50 kg, and we just wouldn't be able to tell the difference with any equipment that exists today. The existence of a non-zero upper bound does not mean that the photon actually has a non-zero mass.
 
  • #7
Parlyne said:
An upper bound like that just means that our methods of measuring the photon's mass can't measure a mass smaller than that. This means that the photon could have a mass as small as 0 kg or as large as 10-50 kg, and we just wouldn't be able to tell the difference with any equipment that exists today. The existence of a non-zero upper bound does not mean that the photon actually has a non-zero mass.

Parlyne has brought up an extremely important point. In many cases, this is the limit of the resolution of the best experiment that we have today. This upper limit has continued to become smaller and smaller as our experimental method improves. It certainly does NOT mean that it has a mass, it is just that the experiment has not detected anything LARGER than that upper limit value.

The same is done with experiments measuring the possible variation in c (see Recent Noteworthy Papers thread). Each of these experiment will quote the upper limit in the variation of c, meaning that the experiment does not detect any various above and as low as that value. Again, this upper limit value continues to fall as the experiments become better.

Zz.
 
  • #8
Parlyne said:
An upper bound like that just means that our methods of measuring the photon's mass can't measure a mass smaller than that. This means that the photon could have a mass as small as 0 kg or as large as 10-50 kg, and we just wouldn't be able to tell the difference with any equipment that exists today. The existence of a non-zero upper bound does not mean that the photon actually has a non-zero mass.

i agree with everything you're saying (and i trust that the rest masses of photons are exactly zero), but just to point out that it would be more satisfying if there was some independent confirmation that they really are 0 (and that we can simply equate the speed of photons to the wavespeed of E&M and never need think of any qualification to that). it's sort of like we expect the speed of gravity to be the same as the speed of light and it was measured to be within +/- 20% of the speed of light which is still consistent with GR but it would still be nice if the coffin was nailed shut on it. [from a percentage POV, which is worse?]

edit: actually, thinking of it, i think now that we are more secure in the rest mass of photons being exactly zero from measuring the speed of photons as indistiguishable from c than the sloppy 20% range of error (s.d.?) regarding the relative speeds of gravity and E&M.
 
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  • #9
Hootenanny said:
All massless particles must, acording to the standard model which obeys SR, travel at the speed of light due to the very fact that they are massless.

not to disagree (because equivalence is a bidirectional arrow), but i like to think of cause and effect as the other way around. first postulate that photons travel at speed c (since that is what is observed to a high degree), then you get the result that their rest mass is zero.
 
  • #10
rbj said:
i remember Meir pointed out a link to some study that put an upper limit on the (invariant) mass of a photon and it wasn't zero. it was something like 10-50 kg. it seems to me very dissatisfying that the speed of particles of light or E&M in general do not fly by at the same speed as the wavespeed of E&M but it might not be so rock solid.

There is no way of measuring an "exact zero" experimentally. In this context it is important to notice that all experiments that expect a null result (mass of the photon, MMX, etc.) are represented by a series of experiments whose results "converge" towards zero. A "perfect" zero is not attainable due to the experimental error of the instruments involved.
 
  • #11
RAD4921 said:
Are photons the only massless particles that travel at the speed of light?

In special relativity, the momentum of a particle is expressed by a four-vector. This vector is 1) parallel with the particle's four-velocity, and 2) has length proportional to the particle's rest mass (EDIT: these are properties, not a definition). This implies that "travelling at speed of light" and "massless" are synonymous.
 
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  • #12
cesiumfrog said:
In special relativity, the momentum of a particle is expressed by a four-vector. This vector is 1) parallel with the particle's four-velocity, and 2) has length proportional to the particle's rest mass. This implies that "travelling at speed of light" and "massless" are synonymous.

While it is true that "travelling at speed of light" and "massless" are synonymous, you would be hard pressed to prove that this is a consequence of your above defintion of the 4-momentum. On the other hand, if you gave the correct defintion of the 4-momentum as in [tex]P=\gamma m v[/tex] with [tex]\gamma=\frac {1}{\sqrt{1-v^2/c^2}}[/tex] then...a massive particle traveling at c would have a momentum of...fill in the blanks.
 
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1. What are massless particles?

Massless particles are particles that have no rest mass, meaning they do not have any intrinsic mass even when they are at rest. They only have energy and momentum, and their speed is always equal to the speed of light.

2. What is the significance of the speed of light in relation to massless particles?

The speed of light, denoted by c, is a fundamental constant in physics and is the maximum speed at which all particles can travel. Massless particles, being particles with no rest mass, are able to travel at this maximum speed.

3. Can massless particles exist in the universe?

Yes, there are several known massless particles that exist in the universe, including photons (particles of light), gluons (particles that bind quarks together), and possibly gravitons (particles that carry the force of gravity).

4. How does the concept of massless particles relate to Einstein's theory of relativity?

Einstein's theory of relativity states that the energy and mass of a particle are equivalent, and the speed of light is the limiting speed at which energy and mass can be converted into each other. This theory has been confirmed by experiments involving massless particles, such as the bending of light around massive objects due to gravity.

5. Are there any practical applications of massless particles?

Massless particles, particularly photons, have numerous practical applications in technology, including in telecommunications, solar panels, and medical imaging. They also play a crucial role in our understanding of the universe, as they are responsible for carrying electromagnetic and strong nuclear forces between particles.

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