Comment by causi
3 years ago
I've never really understood the statement that "most of the mass of the proton comes from virtual particles inside it." That being the case, why isn't the mass density of space outside the proton almost as great as it is inside the proton? Is the density of virtual particles greater inside, and if so, why?
Just ignore the "virtual particles" part. Most of the mass comes from the binding energy. As in the rest masses of the individual quarks is small compared to total mass of proton.
Virtual particles themselves can always be ignored as they are not physical. They're purely a computational method in some approaches. They don't exist in others at all. And even when they are part of the method what kind they are depends. Looking at momentum space? Your virtual particles can have any position. Looking at position? Your virtual particles can have any momentum.
Alternatively: Virtual particle just means that if you have a certain kind of field, what kind of "particles" you need to sum up to get that kind of field. The field itself is the physical thing. Viewing it mathematically as sum of virtual particles is just a mathematical viewpoint.
Is the binding energy made up of gluons?
It's not. Just like how if you push two identically charged plates towards eachother the potential energy in that system is not made of photons.
Sure you can describe the electric field in that case by a viewpoint where you sum virtual photons together to get said electric field. Whereas a non virtual photon is alltogether a different thing. You can actually describe a normal non virtual photon as a sum of virtual photons.
Point is that virtual particles are just a mathematical tool.
Actual real gluons do exist and they're analogous to the actual photon.
In case of electromagnetism the actual stuff is the electric field. With proton (so in quantum chromodynamics) it's the gluon field. It's called that because every particle has a field and every field a particle. It would be kinda like calling electric field a photon field. Same difference.
It's not made of anything, it's just energy.
To think of a proton as containing tons of gluons would be a mistake.
Additionally gluons are expected to be massless, they basically come into existence as needed.
It's a bit like the "energy" in an electric capacitor. It's a property of the system's state that is related to the interactions between the particles.
In a charged capacitor, there's a lot of electrons on one side, but very few of them on the other. When you close the capacitor, suddenly you get a lot of energy out of it.
Looks like it, quarks are bound together by gluons so as you go up the energy scale and 'see' more quarks the gluon energies dominate. In fact about 99% of the Proton's mass is in the form of this binding energy.
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“A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle.”
https://profmattstrassler.com/articles-and-posts/particle-ph...
The quarks that are bound in a way we call a proton are held together by the strong force at a certain distance from each other - that distance is what we consider the volume of the proton, with the quarks being "inside that volume". This system of 3 quarks has some amount of potential energy, and/or some amount of kinetic energy from the relative movement of the 3 quarks. The mass of this system is then given essentially by E=mc^2, with E being this kinetic energy.
There is a very good video of a lecture by Leonard Susskind that explains why energy and mass are interchangeable in this way if you want a more in-depth explanation:
https://www.youtube.com/watch?v=JqNg819PiZY
Fundamentally, "mass" is nothing other than "confined energy". Whenever you have a mechanism for confining energy (Higgs mechanism; or a force which binds and creates a bound state) the combined package has inertial "mass"; This means: It cannot move at speed of light (c), it can change its speed and it takes energy to change its speed (a "massless" particles does none of this).
The famous thought-experiment in the regard is Einstein's "photons-box": If you could confine a bunch of massless photons (which only have kinetic energy and momentum) inside a (massless) box made out of mirrors, (he argues) the combined package would have "mass", even though the constituents do not (and the emergent "mass" equals E=m c^2 !). In other words, "mass" is an emergent property of the confined ensemble. All of the forces (especially the strong force), create bound-states which are massive and are the exact analogues of this "photon-box".
So the mass of a proton (mostly) comes from the kinetic energy of its confined (by the strong force) constituents (the quarks and gluons).