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Why are all protons identical? 
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#1
Dec3011, 02:12 PM

P: 77

Since the proton is not an elementary particle, why do all protons have the same size gluon string connecting the three quarks? As evidenced by all protons having the same rest mass of 938MeV and size of 10^15m. Couldn't protons have varying numbers of gluons tieing the quarks together?



#2
Dec3011, 03:50 PM

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PF Gold
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There is no bound on the number of gluons in a proton, just as there are no bounds on the number of virtual photons that bind the electron and proton of the hydrogen atom. The ground state of the bound system is obtained when nature sums over all possible configurations of multiparticle states. These virtual gluons are never observed, so it isn't really meaningful to try to count them or determine an average number of them. 


#3
Dec3111, 10:12 AM

P: 159

I think it could be different, but we don't recognize any particle with uud as a proton,, it will be a particle with a different mass and spin and a different name..
this is very different from the case of the hydrogen atom where excitation is still a "hydrogen atom". my guess is that the names came from before they new about quarks and only had mass and spin to distinguish things. 


#4
Dec3111, 10:16 AM

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Why are all protons identical?
Good idea to relate this to the much simpler problem for the hydrogen atom.
In the hydrogen atom the electromagnetic field is usually treated classically resulting in energy eigenstates for the electron. The ground state for all hydrogen atoms is identical! Now forget about the complication with the quantized quarks and gluons and the inifinitly many degrees of freedom in the quantum field theory (QCD) for a moment. The proton is the groundstate of the uud> valence quark system  and all these ground states are identical by similar reasons as in the hydrogen atom. The first excited state with spin 3/2 is no longer called proton but Δ^{+}. The key question is: why are there eigenstates to the Hamiltonian H_{QCD} with discrete spectrum (H_{QCD}  E_{n})n> = 0 with n> corresponding to vacuum> with E_{vacuum} = 0, pions>, nucleons> = proton> & neutron>, ... This is related to the Clay Millennium Prize Problems http://www.claymath.org/millennium/YangMills_Theory/ 


#5
Jan112, 02:36 AM

P: 450

2) Is not the key question somewhere related to our comprehension of what "vacuum" is? 3) Could it be that protons are identical because each of them is automatically defined by some "eigenstate" of something more fundamental (a real flow of energy)? So I stop here and whish you all an happy new year 2012. 


#6
Jan112, 05:12 AM

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P: 5,451

1) I don't know
2) yes, definitly; the QCD vacuum itself is not less complicated than its excitations 3) I don't know what you mean; in the context of lattice QCD most of these masses can be calculated quite accurately, so it's QCD and nothing else ;) 


#7
Feb112, 01:44 PM

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#8
Feb112, 03:26 PM

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A proton is a proton is a proton. If it were different, it would not be a proton.



#9
Feb112, 06:15 PM

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PF Gold
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#10
Feb112, 06:20 PM

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#11
Feb2112, 12:47 PM

PF Gold
P: 765

I used to think what you said in your opening statement was true too, until I read this article: Rhody... 


#12
Feb2212, 01:03 AM

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PF Gold
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A quantum system is defined by a set of quantum fields along with their interactions. These fields correspond directly to the free particle states that exist when we ignore the interactions. For the proton, the most important fields describe the quarks and gluons, since the strong interaction is much stronger than the weak and electromagnetic interactions. From the free quark states, we can write down a multiparticle state, uud, that has the correct quantum numbers of the proton. In the free theory, the mass of the proton is just given by adding up the masses of the quarks, which gives a number that is much smaller than the observed proton mass. This discrepancy gets fixed when we turn on the interactions between the quarks and gluons. The uud state is replaced by the superpositions of states which are composed of uud plus arbitrary numbers of gluons and quarkantiquark pairs. The energy eigenstates of the interacting system are linear combinations of these states and the proton is the lowest energy, or ground state combination. The other eigenstates are the resonances. As is usual in quantum mechanics, the square (actually complex modulus squared) of the coefficients in the linear combination gives the probability of finding the system in a particular quantum state. So if we do a measurement on the proton, there is a small probability to find it in the uud configuration, but most of the time it will be in one of the "zillions" configurations. 


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