# Size of a proton as a function of relative strength of color force?

by Spinnor
Tags: color, force, function, proton, relative, size, strength
 P: 1,368 If we could set the strong force to be ten times weaker (compared with the electromagnetic force) how much would the radius of the proton change if any? In the limit that the strength of the strong force goes to zero does the proton radius approach some limit? Thanks for any help!
Thanks
P: 1,948
 Quote by Spinnor If we could set the strong force to be ten times weaker (compared with the electromagnetic force) how much would the radius of the proton change if any? In the limit that the strength of the strong force goes to zero does the proton radius approach some limit? Thanks for any help!
If the strong force strength was zero there would be no proton (infinite radius).
 P: 1,368 If that is true then if we could slowly reduce the strength of the strong force then matter would undergo a "phase" change where quarks were no longer confined? How would a single hydrogen atom in a box change as the strength of the strong force went to zero? Two up quarks, a down quark, and an electron could still form a charge neutral bound system in some limit where the strong force goes to zero? An excited state of such a bound system could emit a photon or a gluon? Thanks for any help!
 P: 916 Size of a proton as a function of relative strength of color force? I am having a weird feeling coming from your questions... A zero strong force, as stated above would mean the destruction of nuclei .... A proton would not be a good choice of a system, because what would keep the quarks together? What keeps them together, although they have charges, is the strong interactions... and the same is true for nuclei as well... Without that interaction, they would break up in their constitutes ... What is the limit that this can hold? probably depends on what you want to say... For example, you get an idea of the relation between strong forces and electromagnetic, by checking a nuclei... A rough calculation can be that you know that the radius of a nuclei is of magnitude ~1fm, and you just say you'd like at that level the electromagnetic repulse of the protons to be equal to their strong attraction.... then I guess you'll get a "ratio" between strong and electromagnetic interaction strengths.... then by lowering the strong interaction you'd raise the radius, in the limit of zero strong interaction, you'll get that there won't be a bound state.... I guess there can be more explicit calculations taking more into account... How could they make a neutral bound system? that is very unlikely to happen, and if it does I don't think you can have a stable state... In the way that you can't excite it (it would break everything appart). But if you send the strong interaction to zero, then there won't be gluons for sure... Photons will exist because you have charges If I am somewhere wrong, let me know...
 P: 1,368 If we had four charged particles of charge -1, -1/3, 2/3, and 2/3 won't they minimize electrostatic energy and form a bound system (allowing the strong force to get very weak)?
 P: 916 they could.... but as I said even if this happened, it would be terribly unstable.... the fact that in order to have a complete cancel out of the charges you'd have to put them in a very specific way to get the "minimized energy" which of course I am not even sure if it's enough for a bound state... To get a complete cancelation of charges you'd have to put in a point A the -1, -1/3 (impossible) and shoot around with enough momentum the 2/3, 2/3... and that's a classical way of seeing it
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 Quote by Spinnor If we could set the strong force to be ten times weaker (compared with the electromagnetic force) how much would the radius of the proton change if any?
The size of the proton will increase very rapidly as you decrease the strength of the strong force. But as long as the coupling constant is positive, the proton will be bound because of the phenomenon of quark confinement.

Actually the strength of the strong force depends on the distance scale. The "coupling constant" that defines the strength of the force increases as the distance scale you are examining increases. For instance at a separation of ##10^{-18}## meters the strong force is much weaker than it is at ##10^{-16}## meters. At about ##10^{-15}## meters the coupling constant, by a certain measure, actually blows up to infinity. This length scale where the coupling constant blows up sets the size of hadrons, including the proton: the proton's radius is about ##10^{-15}## meters.

Now, when we think about "making the strong force ten times weaker," what we presumably mean is that at a fixed length scale--say, ##10^{-18}## meters--we decrease the coupling constant by a factor of 10. This will delay the blow-up of the coupling constant to a larger length scale, and thus will make the proton larger. The dependence of the radius ##r## on the coupling constant ##g## [measured at a fixed length scale] is quite drastic; I think to first order it should look like

$r \sim e^{c/g}$

for some constant ##c##. The exponential means that ##r## increases quite rapidly as ##g## decreases. As ##g \to 0##, ##r \to \infty##.

 Quote by Spinnor If that is true then if we could slowly reduce the strength of the strong force then matter would undergo a "phase" change where quarks were no longer confined?
No. Quarks will always be confined as long as ##g > 0##. And if you set ##g = 0## then there is no strong force. Confinement has a certain length scale, though--basically, the maximum distance color charges are allowed to separate--which is approximately equal to the proton radius and which will increase as the proton radius increases.

 Quote by Spinnor How would a single hydrogen atom in a box change as the strength of the strong force went to zero?
At first, the radius of the proton would increase without incident. Once the proton radius became comparable to the Bohr radius of the electron wave function things would get interesting. I guess the system would become quite complicated to analyze.

 Quote by Spinnor Two up quarks, a down quark, and an electron could still form a charge neutral bound system in some limit where the strong force goes to zero?
Yes, in the limit where the strong force is extremely weak [i.e., proton radius much greater than Bohr radius], we could mostly neglect the strong force compared to the electromagnetic force. Then we'd have to analyze the hydrogen atom as an electromagnetic bound state of four particles of charge 2/3, 2/3, -1/3, and -1. This system is definitely bound electromagnetically.

 Quote by Spinnor An excited state of such a bound system could emit a photon or a gluon?
Yes, but gluons are still confined which essentially sets a maximum distance such a gluon could travel before being absorbed (which could be much greater than the radius of the "hydrogen atom" if the strong force is very weak).
 P: 916 is there quark confinement without strong interactions? Also what kind of state would that be? they are totally asymmetric
Mentor
P: 16,385
 Quote by The_Duck The dependence of the radius ##r## on the coupling constant ##g## [measured at a fixed length scale] is quite drastic; I think to first order it should look like $r \sim e^{c/g}$ for some constant ##c##. The exponential means that ##r## increases quite rapidly as ##g## decreases. As ##g \to 0##, ##r \to \infty##.
It's even stronger, I think: g2 rather than g.
P: 1,368
 Quote by The_Duck ... At first, the radius of the proton would increase without incident. Once the proton radius became comparable to the Bohr radius of the electron wave function things would get interesting. I guess the system would become quite complicated to analyze. ...
Thank you for the details! Interesting!
 P: 1,368 Maybe a follow up question. Let the strong force be small enough so that the following question makes sense (assuming it can make sense), Let there be three dipole antennas which occupy nearly the same space with the same orientation. Let these dipole antennas have oscillating color currents of the strong force, red, anti-red, blue, anti-blue, green, and anti-green, with adjustable phase and amplitude (this can't be done but is only a thought experiment to maybe understand the color forces better). When say blue color charge was maximum at one end of the antenna there would be an equal amount of anti-blue color charge at the other end. With proper phase and amplitude modulation can we produce the eight types (is it nine if color is not confined) of dipole gluon radiation? Thanks for any help! Edit, gluons carry color charge so the above won"t work? 2nd edit, so color and anti color charges must "bleed" off the antennas?
 P: 1,368 So if say a red charge jumps from the red charged end of the red/anti-red antenna to the green or blue charged end of the green/anti-green or the blue/anti-blue antenna Feynmans diagrams for the color force tell us the red charge can emit a gluon and change color. And like wise for the oppositely charged ends. Charges flow from end to nearby end of the antennas? Thanks for any help!
 P: 916 I'm trying to understand your question... can you please simplify it? what do you mean by charged antennas?
 P: 386 Compare proton with neutron. A neutron is bound by electromagnetic interaction alone. One particle of charge +2/3, and two particles of -1/3 each. Just like a helium atom only with 1/3 the charges. Considering the masses of quarks, what would the size of neutron be in absence of strong force? What is the binding energy? A proton is not bound by electromagnetic interaction, because it can split up into a particle charge +2/3 and a bound system of +2/3 and -1/3 totalling +1/3. These repel at long distances. If strong interaction were of negligible strength compared to electromagnetic then a proton would exist because of colour confinement but would be huge compared to neutron. Whereas a neutron would be only slightly smaller than it would be in complete absence of colour force. How much does the size of neutron and proton differ now, at the present strength of colour force?
Thanks
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 Quote by snorkack Compare proton with neutron. A neutron is bound by electromagnetic interaction alone. One particle of charge +2/3, and two particles of -1/3 each. Just like a helium atom only with 1/3 the charges. Considering the masses of quarks, what would the size of neutron be in absence of strong force? What is the binding energy? A proton is not bound by electromagnetic interaction, because it can split up into a particle charge +2/3 and a bound system of +2/3 and -1/3 totalling +1/3. These repel at long distances. If strong interaction were of negligible strength compared to electromagnetic then a proton would exist because of colour confinement but would be huge compared to neutron. Whereas a neutron would be only slightly smaller than it would be in complete absence of colour force. How much does the size of neutron and proton differ now, at the present strength of colour force?
Nope, the neutron - like the proton - is bound by the strong force. The proton and neutron are about the same size.
P: 1,368
 Quote by ChrisVer I'm trying to understand your question... can you please simplify it? what do you mean by charged antennas?

Take Maxwell's classical theory of electromagnetism and change it so we get a similar theory (if possible) using the fundamentals of the color force, three types of charge, red, blue, and green and their opposites anti-red, anti-blue, and anti-green. We would presumably have three conserved currents. Changing color currents give rise to radiation? Such an imaginary classical theory of a very weak strong force would allow us to think of gluon dipole radiation produced by oscillating color currents. My guess was we might have 8 (or 9) types of gluon dipole radiation by proper modulation of the color currents. A big difference with classical electrodynamics is that photons have no charge while gluons have equal amounts of color/anti-color charge. This makes the thought experiment a little harder.
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 Quote by dauto Nope, the neutron - like the proton - is bound by the strong force. The proton and neutron are about the same size.
Yes, both are bound mainly by colour force.
However, electromagnetic force mainly works to contract the neutron against the colour force, and it mainly works to expand the proton against the colour force. What is the resulting size difference?
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 Quote by snorkack Yes, both are bound mainly by colour force. However, electromagnetic force mainly works to contract the neutron against the colour force, and it mainly works to expand the proton against the colour force. What is the resulting size difference?
There is also a difference between protons and neutrons coming from the fact that up and down quarks have different masses as well as different charges. Both electromagnetic and quark mass difference effects are of order 1% or less. So presumably any size difference between the proton and neutron is of this order. For example, these effects are responsible for the mass splitting between the proton and neutron which is about 0.1% [this is a little smaller than you might expect because there is an approximate cancellation between the quark mass effects and the electromagnetic effects]. Lattice QCD calculations of hadron properties are starting to take into account these "isospin breaking" effects, which had previously been neglected because they were dominated by other errors.

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