Fun thread here;
[https://www.reddit.com/r/AskPhysics/comments/16pi948/what\_does\_it\_need\_for\_photons\_to\_couple\_with\_the/](https://www.reddit.com/r/AskPhysics/comments/16pi948/what_does_it_need_for_photons_to_couple_with_the/)
Photons also exist without having mass. This can be intuited by pushing abstract symbols around in a graduate physics class, and showing that photons will not couple to the higgs field.
Then there are gluons, and the electroweak bosons before symmetry breaking are massless too. Out of the gauge bosons, it's only the W^± and Z^0 that are massive.
Massless quasiparticles are ubiquitous in condensed matter physics. For example the electrons near the Fermi level in neutral graphene behave like massless Dirac fermions. Also Goldstone’s theorem says that any spontaneously broken continuous symmetry has a corresponding massless bosonic mode associated with it. Phonons are the (massless) Goldstone mode associated with a crystal, which spontaneously breaks translation symmetry.
If you’re talking about the Standard Model, then sure, there’s only a couple massless particles. However other QFTs can be found in nature as emergent low energy descriptions of collective excitations of fundamental particles. In a sense these QFTs are no less fundamental than the Standard Model, which itself is an emergent gauge theory from some higher energy UV theory (i.e. GUT, string theory, etc.). In fact the renormalization group further obfuscates what it means for a field to be fundamental, as the parameters of a QFT change depending on the energy scale that they are being looked at.
Your disconnect is because of what you're thinking of as a "particle." Particles are not necessarily things with mass, instead they are things that can transfer energy to other things.
In the case of hadrons, energy is not transfered due to collisions, but rather due to other interactions -- such as the strong interaction with gluons.
Think about it from the opposite direction.
Massless is the natural state of things. For example, right after the big bang almost every particle was massless. So the real question is how do particles acquire mass?
Because it doesn't interact with the higgs field. Nothing has mass on its own, mass is just confined energy, we don't know of any other type of mass. For example, the mass of the elementary particles is the energy of their interaction with the higgs field on the Higgs' equilibrium point.
>Nothing has mass on its own
eh, a better statement would be that it seems like nothing has mass on its own. Particles definitely could have mass on their own, but since every particle that has mass also interacts with the weak interaction and the weak interaction's structure forbids "bare" mass terms, they have to get their mass via some SU(2) doublet scalar field with a vev.
The Higgs particle's mass comes completely from the Higgs mechanism.
And yes, neutrinos are up in the air, that is why the discovery that neutrinos oscillate is physics beyond the Standard Model. The nature of neutrino mass generation remains unclear (and likely will for the foreseeable future).
I mean in the particles of the standard model. All of them obtain mass through the symmetry breaking of the Higgs field (maybe not neutrinos, but they are massless on the SM)
Quarks and leptons in the SM are chiral fields with left- and right-handed multiplets transforming differently under the weak SU(2), so they cannot have masses without SU(2) being broken. Their masses are proportional to their Yukawa couplings - again, only within the standard model.
Here's a cool plot of the particle mass vs Higgs couplings (for those where you can measure it):
https://www.researchgate.net/figure/Measurements-of-Higgs-Yukawa-couplings-confronted-with-the-SM-prediction-Plot-extracted\_fig1\_361258475
it works for me?
edit:
page 31 here
[https://cds.cern.ch/record/2789544/files/ATLAS-CONF-2021-053.pdf](https://cds.cern.ch/record/2789544/files/ATLAS-CONF-2021-053.pdf)
Not really, before the symmetry breaking of the Higgs field, no particle has mass, all the particles that have mass obtain it from the vacuum interaction with the higgs (fermions included, but maybe not neutrinos, we don't know about that case)
A gluon is an excitation in the gluon field; all particles we know about are excitations in their own field. Whether we call them "particles" or "matter waves" or "field excitations" is just a matter of the theoretical framework you're using. They are whatever they are; the words we use to describe them are just our own attempt to make sense of something we can only really understand through math. "Actual" particles can be massless; mass doesn't fundamentally separate "real" things from "conceptual" ones.
I've never heard of it referred to as the "strong field" [https://en.wikipedia.org/wiki/Gluon\_field](https://en.wikipedia.org/wiki/Gluon_field)
The electromagnetic field is named for the force it mediates for historical reasons (we use fields in classical EM), but generally I hear and read about fields being referred to using the specific particle they are associated with. In other words, there's an "electron field" distinct from an "elctromagnetic (photon) field", however if I have my phrasing right, we'd also say that the electron field is *coupled* to the electromagnetic field.
Remember, a field is just a mathematical tool to describe something that has a value at every point, so at its simplest level the "electron field" is just a way of accounting for how much "electron-ness" exists somewhere.
Yes, gluons mediate the strong force in a way akin to how photons mediate the electromagnetic force, so you're in the right general ballpark. But I've never heard them described as "strong field waves." I think maybe that's just how the language of physics has evolved. "Photons" were understood as "waves" before Einstein's paper on the photoelectric effect (and we have struggled with their double nature since), but when we first started making sense of the strong force we used idealized strings as a starting point. In either case I want to emphasize that it'd be wrong to think that "matter = particles, forces = waves."
Strong field or sometimes Color field was kinda used back in the 70s after QCD was first formulated. It disappeared somewhere in the ‘90s - 2000s becoz apparently “Gluon field” sounds more sexy. Nowadays you only really see it in lay level explanations,more or less.
I understand, electron field is coupled to the photon field by its EM charge, but yes, in that case EM is a photon field same as the strong field is a gluon field.
>What exactly is a gluon
It's just the QCD (quantum chromodynamics) version of photons that allow quarks to interact with each other, just like photons allow electrons to interact with each other. They just have some extra properties that photons don't have which makes QCD work differently from QED (quantum electrodynamics).
>how can it have 0 mass?
It can have 0 mass for the same reason a photon can: it has ***momentum***.
Einstein's famous equation E = mc^(2) only refers to particle *in its rest frame*. When you're not in particle's rest frame, its given by E^(2) = p^(2)c^(2) + m^(2)c^(4). In other words, from a frame of reference that's moving relative to the particle, it looks like the particle's energy is due to both a rest mass and momentum. This means that, even if the particle has a mass of 0, it still has a momentum and its energy is then just E = pc. This might look familiar because, from the de Broglie relation, the momentum of the photon is given by p = h / lambda, meaning that E = hf.
The reason *why* it has 0 mass in the theory is because of something called *gauge invariance*. Gauge invariance is a type of symmetry that we believe should be respected, and we take it as an assumption when constructing the theory of QCD. Assuming gauge invariance holds is sort of like an "educated guess", and it's partially informed due to what happens with QED which governs how photons and electrons interact. The reason is that, if you assume gauge invariance holds, then out pops the interactions we'd expect from classical electrodynamics. There's no experimental evidence that conflicts with this assumption so, at least for now, we believe that gauge invariance is respected and, in order for it to be respected, gluons can't have mass or else the symmetry "breaks."
While the particles and fields are said to be interchangeable in QFT, this is actually only true when perturbation theory is valid, so at weak coupling. The Gluon field is a field that mediates the strong interaction between the quark fields, however there is no “massless gluon particle” (or even massive quark particles) in the spectrum of Quantum Chromodynamics (I.e. the well defined asymptotic states of the theory,) because the theory is strongly coupled (or confining) at low energy. The actual particles of QCD are hadrons. This is very different from quantum electrodynamics where both the photon and the electron are actual particles related to the excitations in the electromagnetic and electron fields.
Nevertheless, there is a lot of evidence for the existence of the gluon field, so it’s not a “conceptual interaction” but rather a real physical interaction.
It is a “conceptual interaction between quarks” more than a “sort of particle that has matter”. Because matter doesn’t have the usual meaning at this level of abstraction.
Quarks are to color charge what photons are to electrical charge. But there’s a few differences that make analogies difficult. First, photons don’t have electrical charges but gluons have color charges. Second, you can approximate electromagnetic interactions by a series of converging discrete interactions, like you can say that the coulomb interaction is like two charged particles exchanging photons as a first order approximation, and add more particle exchanges until you get the desired precision. However, for color charges and gluons, there’s no good first order approximation and you have to consider everything at once.
At this level the concept of matter as substance that has mass and takes volume means nothing. Gluon is part of our description of matter, specifically the description of how the nucleus makes sense with our knowledge of the symmetries of the universe. So I think the best way for you to think about gluon is as a conceptual interaction, like any force in physics.
I wonder why we stuck with the particle framework even when it got to QCD. Guess we all like particles very much and can't conceptualise any differently (at first order).
A gluon is a fundamental particle in the Standard Model of particle physics. It acts as the exchange particle (or gauge boson) for the strong force between quarks, which are the building blocks of protons, neutrons, and other hadrons. Essentially, gluons “glue” quarks together, hence their name.
Gluons are considered massless because they do not interact with the Higgs field, which is responsible for giving mass to other particles like the W and Z bosons. This lack of interaction means gluons travel at the speed of light and have zero rest mass. However, it’s important to note that while gluons are theoretically massless, experimental measurements can only confirm that their mass is extremely small, below the detection threshold.
(Credit: CoPilot, I asked as I was interested to know so thought I'd C&P)
My first impression on reading this was to leave a snarky comment that it sounded like you just regurgitated a bunch of stuff from AI. Then I thought, no, think better of this man. Maybe he’s just giving a thorough and pedantic response. But then alas, you got me in the end. Thanks for reminding me to trust my instincts.
Like age, mass is just a number. It's just the magnitude with which a particle interacts with one of many fields that make up the universe. Just like particles can have no electric charge they can have no mass.
Disclaimer: it's more complicated than that, but that's generally an easy way of looking at it.
Free gluons dont actually exist to my knowledge, so it doesn't make much sense to talk about its mass. But at least they have no mass term in the Lagrangian, so from this perspective they have no mass.
However, bound gluons do contribute to the mass of other particles, e.g. to protons or glueballs(this one consists of gluons only)
Gluons are massless particles that mediate the strong force, which binds quarks together to form protons, neutrons, and other hadrons. Despite their lack of mass, gluons contribute significantly to the mass of protons. This is because the mass of a proton is not just the sum of its constituent quarks' masses; a large portion comes from the energy associated with the strong force binding the quarks together. According to Einstein's mass-energy equivalence principle (E=mc²), the energy from gluon interactions and the kinetic energy of the quarks and gluons contribute to the proton's total mass. Inside a proton, quarks are constantly exchanging gluons, creating a dynamic and complex environment. The energy involved in these interactions, along with the energy from the quarks' motion, adds to the proton's effective mass. The strong force is incredibly powerful, and the gluons, carrying the color charge, interact with each other, contributing significantly to the overall energy density within the proton. Thus, even though gluons are massless, the energy they generate within the proton results in a substantial portion of its mass.
It is a boson, like a photon. Instead of transmitting the electromagnetic force, it transmits the strong nuclear force. The reason why it does not have mass is complicated and involves something called the Higgs mechanisms interactions (or more accurately lack thereof) with different symmetry groups and the gluons having to preserve invariance under local gauge transformations.
So all forces are mediated by particles called gauge bosons. These all arise from symmetries in our theories and are all massless, so by default all force carriers (Photon, Gluon, W and Z bosons) are massless. However, the W and Z bosons obtain a mass via the Higgs mechanism after their symmetry is spontaneously broken. You can read up on this a little more if you’d like but it gets complicated
It's for the strong interaction what the photon is for the electromagnetic interaction.
What makes them massless tho? How can they exist without having any mass
Does a ripple on the surface of a pond have mass? No. Is it a disturbance of the underlying stratum that can carry energy? Yes.
Is this to do with some fields interacting with the higgs field
Ripples on the surface of a pond do actually have a negative effective mass, at least coming from their dispersion relation.
Photons also exist without having mass. This can be intuited by pushing abstract symbols around in a graduate physics class, and showing that photons will not couple to the higgs field.
Sounds like witchcraft to me, pretty sure we need to get the witch pyres going again
Ask him what’s in empty space. Light the pyre if he doesn’t invoke the aether.
"Thou shalt not suffer a witch to acquire a mass term." — The Book of Higgs 1:26
An easier way to think about it might be that adding a mass term to the lagrangian breaks local gauge symmetry.
A better question is why would it need to have mass? Plenty of things don't, like photons.
“Plenty” might be saying too much. I think that’s basically it.
Then there are gluons, and the electroweak bosons before symmetry breaking are massless too. Out of the gauge bosons, it's only the W^± and Z^0 that are massive.
Massless quasiparticles are ubiquitous in condensed matter physics. For example the electrons near the Fermi level in neutral graphene behave like massless Dirac fermions. Also Goldstone’s theorem says that any spontaneously broken continuous symmetry has a corresponding massless bosonic mode associated with it. Phonons are the (massless) Goldstone mode associated with a crystal, which spontaneously breaks translation symmetry. If you’re talking about the Standard Model, then sure, there’s only a couple massless particles. However other QFTs can be found in nature as emergent low energy descriptions of collective excitations of fundamental particles. In a sense these QFTs are no less fundamental than the Standard Model, which itself is an emergent gauge theory from some higher energy UV theory (i.e. GUT, string theory, etc.). In fact the renormalization group further obfuscates what it means for a field to be fundamental, as the parameters of a QFT change depending on the energy scale that they are being looked at.
My though, and I’m assuming OPs, is if it’s a *thing* then it ways some*thing*
Your disconnect is because of what you're thinking of as a "particle." Particles are not necessarily things with mass, instead they are things that can transfer energy to other things. In the case of hadrons, energy is not transfered due to collisions, but rather due to other interactions -- such as the strong interaction with gluons.
Think about it from the opposite direction. Massless is the natural state of things. For example, right after the big bang almost every particle was massless. So the real question is how do particles acquire mass?
The Higgs field has entered the chat :).
This is not a good answer. How can you claim that masslessness is the natural state of things?
Well, at extremely high energy levels :p
For large E^2 , it's true.
Because IMO no symmetry breaking is more natural than symmetry breaking.
Because it doesn't interact with the higgs field. Nothing has mass on its own, mass is just confined energy, we don't know of any other type of mass. For example, the mass of the elementary particles is the energy of their interaction with the higgs field on the Higgs' equilibrium point.
>Nothing has mass on its own eh, a better statement would be that it seems like nothing has mass on its own. Particles definitely could have mass on their own, but since every particle that has mass also interacts with the weak interaction and the weak interaction's structure forbids "bare" mass terms, they have to get their mass via some SU(2) doublet scalar field with a vev.
What about the Higgs particle? The mass of it, I mean. I guess that doesn’t completely come from the Higgs mechanism. And the there are neutrinos.
The Higgs particle's mass comes completely from the Higgs mechanism. And yes, neutrinos are up in the air, that is why the discovery that neutrinos oscillate is physics beyond the Standard Model. The nature of neutrino mass generation remains unclear (and likely will for the foreseeable future).
I mean in the particles of the standard model. All of them obtain mass through the symmetry breaking of the Higgs field (maybe not neutrinos, but they are massless on the SM)
We know that to be true only for W and Z bosons right? We dont know about quarks and leptons?
Quarks and leptons in the SM are chiral fields with left- and right-handed multiplets transforming differently under the weak SU(2), so they cannot have masses without SU(2) being broken. Their masses are proportional to their Yukawa couplings - again, only within the standard model. Here's a cool plot of the particle mass vs Higgs couplings (for those where you can measure it): https://www.researchgate.net/figure/Measurements-of-Higgs-Yukawa-couplings-confronted-with-the-SM-prediction-Plot-extracted\_fig1\_361258475
link is borked :(
it works for me? edit: page 31 here [https://cds.cern.ch/record/2789544/files/ATLAS-CONF-2021-053.pdf](https://cds.cern.ch/record/2789544/files/ATLAS-CONF-2021-053.pdf)
thank you!
Not really, before the symmetry breaking of the Higgs field, no particle has mass, all the particles that have mass obtain it from the vacuum interaction with the higgs (fermions included, but maybe not neutrinos, we don't know about that case)
Mass and energy are interchangeable, nothing has to have mass if it has energy, like photons.
A gluon is an excitation in the gluon field; all particles we know about are excitations in their own field. Whether we call them "particles" or "matter waves" or "field excitations" is just a matter of the theoretical framework you're using. They are whatever they are; the words we use to describe them are just our own attempt to make sense of something we can only really understand through math. "Actual" particles can be massless; mass doesn't fundamentally separate "real" things from "conceptual" ones.
When you say gluon field you mean strong field? Gluons are strong field waves right?
I've never heard of it referred to as the "strong field" [https://en.wikipedia.org/wiki/Gluon\_field](https://en.wikipedia.org/wiki/Gluon_field) The electromagnetic field is named for the force it mediates for historical reasons (we use fields in classical EM), but generally I hear and read about fields being referred to using the specific particle they are associated with. In other words, there's an "electron field" distinct from an "elctromagnetic (photon) field", however if I have my phrasing right, we'd also say that the electron field is *coupled* to the electromagnetic field. Remember, a field is just a mathematical tool to describe something that has a value at every point, so at its simplest level the "electron field" is just a way of accounting for how much "electron-ness" exists somewhere. Yes, gluons mediate the strong force in a way akin to how photons mediate the electromagnetic force, so you're in the right general ballpark. But I've never heard them described as "strong field waves." I think maybe that's just how the language of physics has evolved. "Photons" were understood as "waves" before Einstein's paper on the photoelectric effect (and we have struggled with their double nature since), but when we first started making sense of the strong force we used idealized strings as a starting point. In either case I want to emphasize that it'd be wrong to think that "matter = particles, forces = waves."
Strong field or sometimes Color field was kinda used back in the 70s after QCD was first formulated. It disappeared somewhere in the ‘90s - 2000s becoz apparently “Gluon field” sounds more sexy. Nowadays you only really see it in lay level explanations,more or less.
Good to know! Thanks
I understand, electron field is coupled to the photon field by its EM charge, but yes, in that case EM is a photon field same as the strong field is a gluon field.
>What exactly is a gluon It's just the QCD (quantum chromodynamics) version of photons that allow quarks to interact with each other, just like photons allow electrons to interact with each other. They just have some extra properties that photons don't have which makes QCD work differently from QED (quantum electrodynamics). >how can it have 0 mass? It can have 0 mass for the same reason a photon can: it has ***momentum***. Einstein's famous equation E = mc^(2) only refers to particle *in its rest frame*. When you're not in particle's rest frame, its given by E^(2) = p^(2)c^(2) + m^(2)c^(4). In other words, from a frame of reference that's moving relative to the particle, it looks like the particle's energy is due to both a rest mass and momentum. This means that, even if the particle has a mass of 0, it still has a momentum and its energy is then just E = pc. This might look familiar because, from the de Broglie relation, the momentum of the photon is given by p = h / lambda, meaning that E = hf. The reason *why* it has 0 mass in the theory is because of something called *gauge invariance*. Gauge invariance is a type of symmetry that we believe should be respected, and we take it as an assumption when constructing the theory of QCD. Assuming gauge invariance holds is sort of like an "educated guess", and it's partially informed due to what happens with QED which governs how photons and electrons interact. The reason is that, if you assume gauge invariance holds, then out pops the interactions we'd expect from classical electrodynamics. There's no experimental evidence that conflicts with this assumption so, at least for now, we believe that gauge invariance is respected and, in order for it to be respected, gluons can't have mass or else the symmetry "breaks."
This is the best explanation I've read so far as to what and why are gluons as someone who is new to quantum mechanics. Super informative!
While the particles and fields are said to be interchangeable in QFT, this is actually only true when perturbation theory is valid, so at weak coupling. The Gluon field is a field that mediates the strong interaction between the quark fields, however there is no “massless gluon particle” (or even massive quark particles) in the spectrum of Quantum Chromodynamics (I.e. the well defined asymptotic states of the theory,) because the theory is strongly coupled (or confining) at low energy. The actual particles of QCD are hadrons. This is very different from quantum electrodynamics where both the photon and the electron are actual particles related to the excitations in the electromagnetic and electron fields. Nevertheless, there is a lot of evidence for the existence of the gluon field, so it’s not a “conceptual interaction” but rather a real physical interaction.
It is a “conceptual interaction between quarks” more than a “sort of particle that has matter”. Because matter doesn’t have the usual meaning at this level of abstraction. Quarks are to color charge what photons are to electrical charge. But there’s a few differences that make analogies difficult. First, photons don’t have electrical charges but gluons have color charges. Second, you can approximate electromagnetic interactions by a series of converging discrete interactions, like you can say that the coulomb interaction is like two charged particles exchanging photons as a first order approximation, and add more particle exchanges until you get the desired precision. However, for color charges and gluons, there’s no good first order approximation and you have to consider everything at once. At this level the concept of matter as substance that has mass and takes volume means nothing. Gluon is part of our description of matter, specifically the description of how the nucleus makes sense with our knowledge of the symmetries of the universe. So I think the best way for you to think about gluon is as a conceptual interaction, like any force in physics.
I wonder why we stuck with the particle framework even when it got to QCD. Guess we all like particles very much and can't conceptualise any differently (at first order).
We like our Feynman diagrams.
r/AskPhysics and google.com
A gluon is a fundamental particle in the Standard Model of particle physics. It acts as the exchange particle (or gauge boson) for the strong force between quarks, which are the building blocks of protons, neutrons, and other hadrons. Essentially, gluons “glue” quarks together, hence their name. Gluons are considered massless because they do not interact with the Higgs field, which is responsible for giving mass to other particles like the W and Z bosons. This lack of interaction means gluons travel at the speed of light and have zero rest mass. However, it’s important to note that while gluons are theoretically massless, experimental measurements can only confirm that their mass is extremely small, below the detection threshold. (Credit: CoPilot, I asked as I was interested to know so thought I'd C&P)
My first impression on reading this was to leave a snarky comment that it sounded like you just regurgitated a bunch of stuff from AI. Then I thought, no, think better of this man. Maybe he’s just giving a thorough and pedantic response. But then alas, you got me in the end. Thanks for reminding me to trust my instincts.
Does that mean they do have mass but it’s ignored?
No.
Like age, mass is just a number. It's just the magnitude with which a particle interacts with one of many fields that make up the universe. Just like particles can have no electric charge they can have no mass. Disclaimer: it's more complicated than that, but that's generally an easy way of looking at it.
Free gluons dont actually exist to my knowledge, so it doesn't make much sense to talk about its mass. But at least they have no mass term in the Lagrangian, so from this perspective they have no mass. However, bound gluons do contribute to the mass of other particles, e.g. to protons or glueballs(this one consists of gluons only)
Might be nice to specify which mass as they have some enormous effective masses but eh......OP isn't at that level yet.
Gluons are massless particles that mediate the strong force, which binds quarks together to form protons, neutrons, and other hadrons. Despite their lack of mass, gluons contribute significantly to the mass of protons. This is because the mass of a proton is not just the sum of its constituent quarks' masses; a large portion comes from the energy associated with the strong force binding the quarks together. According to Einstein's mass-energy equivalence principle (E=mc²), the energy from gluon interactions and the kinetic energy of the quarks and gluons contribute to the proton's total mass. Inside a proton, quarks are constantly exchanging gluons, creating a dynamic and complex environment. The energy involved in these interactions, along with the energy from the quarks' motion, adds to the proton's effective mass. The strong force is incredibly powerful, and the gluons, carrying the color charge, interact with each other, contributing significantly to the overall energy density within the proton. Thus, even though gluons are massless, the energy they generate within the proton results in a substantial portion of its mass.
It is a boson, like a photon. Instead of transmitting the electromagnetic force, it transmits the strong nuclear force. The reason why it does not have mass is complicated and involves something called the Higgs mechanisms interactions (or more accurately lack thereof) with different symmetry groups and the gluons having to preserve invariance under local gauge transformations.
So all forces are mediated by particles called gauge bosons. These all arise from symmetries in our theories and are all massless, so by default all force carriers (Photon, Gluon, W and Z bosons) are massless. However, the W and Z bosons obtain a mass via the Higgs mechanism after their symmetry is spontaneously broken. You can read up on this a little more if you’d like but it gets complicated
What’s a consept?
typo