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Why is the strong Nuclear force the strongest?

  1. Feb 17, 2013 #1
    And the electromagnetic force stronger than gravity?

    That is what is written everywhere about the fundamental forces.

    It is simply because we don't generally see or observe matter (if at all such matter exists) having a mass to charge ratio high enough to make the magnitude of gravitational force comparable to electromagnetic force, that we say that it is stronger isn't it? Clearly there isn't anything that inherently makes Electromagnetic force stronger than gravitational force right? Or am I missing something... (same goes for all the other fundamental forces)

    I am only asking because I haven't seen any place where this is explicitly mentioned.
  2. jcsd
  3. Feb 17, 2013 #2


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  4. Feb 17, 2013 #3
    Haven't heard that gravity has charge, rather it's a phenomenon that every mass has -gravity.
    I don't know if you can compare them because the electromagnetic force is different.It's only happening when a charge is moving in time/space when you hold a piece of copper in your hands there is no em in it.It's just a piece of matter that has mass and certain gravity dependent on it's mass.
    magnetic properties of certain metals are also not the same as gravity they arise from different things.You can have mass that has both gravity and magnetic properties and you can have mass that has just gravity but no magnetic properties.
  5. Feb 19, 2013 #4
    I agree but that's not what I meant. So let me put it this way.

    Substituting all the values and constants in the expression for Coulomb's law, the electrostatic force between two protons is 2.304 x 10-8 N and likewise the gravitational force between them is 1.86 x 10-44 N. The ratio is 1.239 x 1036.

    So that means if the mass of each proton was 1.1131 x 1018 times of what it already is, then the electrostatic force would be equal in magnitude to the gravitational force between the two protons. Then we wouldn't say that one is stronger than the other would we? It just so happens that two protons don't have that much mass nothing else isn't it? Or would we still say so, in which case there is something about this "strength of forces" which I don't understand.

    The quantity of relative strength seems to be based on something called "Coupling Constant" and understanding that involved some mathematics which I still don't know about. But is it an inherent property of the force itself?
  6. Feb 19, 2013 #5
    This is how I've always understood 'gravity' in terms of relativity, hope this helps:
    The presence of mass warps the grid of space (rather, space-time), such that it is simply easier for matter to travel in the direction of other matter. Since matter is always moving around rapidly in many different directions at a subatomic level (unless it is at a temperature of absolute zero), it's movements in the direction of other matter travel greater distances with the same amount of energy, due to space-time being warped; the observed effect being the phenomenon of gravity.

    Now, this is not always stronger than the elecromagnetic force. The EM force of a single atom is stronger than that atom's gravitational force, but the collective gravitational force of many atoms (for example, the atoms making up the mass of a star) can overpower the EM force of a single atom on the subatomic level. When this occurs, nuclear fusion happens; gravity pushes the atomic nucleus of an atom past the repulsive electromagnetic field of another atomic nucleus, where it is within range of that nucleus's strong nuclear force (the force holding together the quarks of that nucleus's proton(s)), binding the nuclei together into a single nucleus - a heavier element. It's worth noting that electromagnetism is also contributing to fusion, and not just gravity; inside a star where fusion is taking place, the EM repulsion of other atoms is also what's pushing atoms into each other to cause fusion.

    I've always thought of the energy released from nuclear fusion as being all the stored photons of one (or perhaps both) of the atomic nuclei being 'given up' as the entropy of its momentum has to change to the same level of entropy as that of the other nucleus.

    The reason this "momentum" is stored up can be explained by relativity -- all motion is relative; so even objects which appear to be moving very slowly relative to each other could actually be moving at much higher speeds relative to another frame of reference. That "invisible momentum" is what's being released in nuclear fusion.

    The "big problem" between quantum physics and relativity is that measuring the "warp in the grid of spacetime" really only becomes possible on a macroscopic scale (the scale of celestial bodies such as planets and stars), so it becomes impossible to incorporate gravity into equations regarding interactions at the subatomic level. Since gravity is the result of the curvature of space itself and isn't mediated by a boson (like the other fundamental forces are), it doesn't fit into our subatomic physics models (which explain phenomenon in terms of interactions between subatomic particles -- no such interaction takes place with the phenomenon of gravity).
    Last edited: Feb 19, 2013
  7. Feb 19, 2013 #6
    @PiChiNietzsch Well they haven't found the graviton yet but haven't dismissed it either.So maybe gravity indeed can also be quantized just like the EM wave and others so then it becomes a part of the quantum physics of the particle world.

    @mishrashubham Ok I see were your going now.Well I think that it will not work that way because.Imagine the proton or the nucleus as a egg with a shell or outer layer being the electrostatic force and the shell with the egg together having mass.
    Now if you want top break that shell you have to apply a certain force to it to do that and if you have two eggs they both need certain amount of energy applied to them for them to break each others shell and mix together.
    Now we know that the mass of the two eggs is not enough to put them on one another and break the shell that's why we need a whole pile of eggs alot of them on top of one another so that the lower ones would start to break under the load.
    Something similar to what happens in stars just as you already pointed out.
    Ok Now let's increase the mass as you said and now one egg would be heavy enough to break the shell of the other and two eggs would be enough to break the shells and mix them.No I don't think so , increasing the mass also increases the size and the outer layer and probably the thickness of the outer layer , so basically your left right where you started you just have bigger eggs with stronger shells and they still need a force applied to them for them to break.

    I don't think we can change these values independently they are tied together because the proton is made up of quarks and gluons and the proton electric charge comes from the quarks that make it we can't make the proton bigger but even if we could i think the electrostatic force it exerts would also increase as long as the gravity it exerts from the mass it has or would have so the relationship between the gravity and electric repulsion wouldn't change just the values.
    But the best part is we can't change them, atleast not now, but I don't think even in the future.And by changing them it would be a different particle not a proton anymore.All we can change is the proton neutron balance or count to make different elements than the ones we started out with, and we are already doing that since WW2 in nuclear fission reactors and a little later the fusion in hydrogen bombs.
  8. Feb 20, 2013 #7
    I realise that this egg and shell picture is an analogy but could you elucidate what it is an analogy of ie. what do the eggs, shells and energy correspond to?

    I wasn't talking about actually changing the values of the masses or charges of subatomic particles. I was simply bringing up a hypothetical situation, where IF suppose the mass would have been something other than what it actually is in the real world, the forces would have been different, and my question being, whether the statement that "strong force is stronger than other forces" would still be a valid one in said situation.
  9. Feb 20, 2013 #8
    Well don't take the egg analogy as some holy grail it was just a picture i made up for a better understanding.
    Well the mass is the whole egg or in real life the whole proton the shell was meant to be the electrostatic force the proton has that repels the proton from another proton.
    I just said that if you could make a bigger proton somehow heavier it would also have more quarks in it (if it would be possible at all i believe it's not but anyway) so the electrostatic force would grow together with gravity and not much would change.
    Well if you are thinking of pure science fiction then we can make all kinds of stuff and ofcourse if I would be a science fiction writer the first thing I would do is eliminate by a magical hand the electrostatic force to make fusion possible at normal pressures and so on but that is like I said fiction not reality certainly not possible.

    Well in your situation yes the string nuclear force would not be so string anymore but there can be a million man made up situations and in everycase someone can think something different but the reality is only one, the truth is only one and it is impossible and unnecessary to deal with all these made up situations.Anyway if in reality the gravity would have different strength and all the other forces too there would be no reality in the first place everything would have been different since the very start if at all and I wouldn't be writing this to you nor you would be thinking of it.
    The universe is like a Rubik's Cube you can make different color of it but there is only one way to make it right.
    Last edited: Feb 20, 2013
  10. Feb 20, 2013 #9


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    I think that the two views you are looking at are equivalent. Two protons are attracted due to gravity and repelled due to EM with the EM repulsion being much stronger than the gravitational attraction. You can attribute that to gravitation being weaker than EM or equivalently you can attribute it to the mass being smaller than the charge.

    I prefer the mass < charge view over the gravity force < EM force view. The reason I prefer it is that there are cases where the gravitational force is stronger than the EM force, e.g. the gravitational interaction between the moon and earth is much stronger than their EM interaction. It is not because gravity has suddenly become a stronger force than EM, but because the mass of the earth is much greater than its charge.

    So the mass < charge view always works, at subatomic scales or interstellar scales, whereas the gravity force < EM force view only works at subatomic scales and you have to switch to the other view at bigger scales anyway. So I prefer the mass < charge view even though I think they are equivalent.
  11. Feb 20, 2013 #10
    Quite nicely said there @DaleSpam.
  12. Feb 20, 2013 #11
    People frequently think this...yet gravity can cause black holes and neutron stars [where electrons are forced into nuclei by gravity] while electromagnetism and the strong force cannot.

    Bu fundamentally nobody knows why there are such large 'force' differences observed today. What we think happened from the early big bang is that when all forces were 'unified', meaning they were combined in some way not yet fully understood, and were in a very high energy [hot] and very unstable environment, they changed into the separate entities we observe today...it's call spontaneous symmetry breaking. So all the four forces are related. In fact they are also related to space,time, and energy for example....all have a common origin. We have some insights about the forces viathe Standard Model of particle physics, where three can be shown [via quantum mechanics] to be related, but gravity still lies outside that understanding. Quantum gravity is the effort to achieve that understanding.
  13. Feb 20, 2013 #12


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    Electromagnetic forces hold that piece of copper together. Without them, there would be no solid material to hold (and no human either).

    Gravity has the same strength as electromagnetism between particles with a mass similar to the planck mass. In other words, all known particles are extremely light.

    For the comparison between strong and electromagnetic interaction, things are a bit different. You can describe their strengths with dimensionless numbers, as charges are quantized. For low-energy processes, the electromagnetic interaction has the famous fine-structure constant ~1/137 (increasing with increasing energy), while the strong interaction has some value close to 1 (decreasing with increasing energy). It is expected that they meet each other (probably together with the weak interaction) somewhere below the planck scale, but that is beyond the reach of current experiments.
  14. Feb 20, 2013 #13
    @mfb by the em in the copper in your hand i was thinking that the piece of metal doesn't have any external magnetic or electric field just as a piece of metal the inner fields of the atoms that make it in the first place that's another picture i'm not speaking about that.

    What do you mean by they meet each other below the planck scale that the forces equal out each other or what?
  15. Feb 20, 2013 #14


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    This is caused by the fact that there are positive and negative charges, which can cancel each other. If you would somehow remove all electrons from the copper at once, it would explode immediately.

    They have an equal strength, and they might merge to a single, more general force. See the graphs here, for example.
  16. Feb 21, 2013 #15
    So if there would be no electrons but complete positive ions in the copper piece why woudl it suddenly explode? Ions still have a positive electrostatic repelling force don't they?
  17. Feb 21, 2013 #16


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    Yes, that's why it would explode. They would repel each other very strongly.
  18. Feb 21, 2013 #17


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    The Standard Model has no explanation for the values of the coupling constants, and by extension, the strength of the fundamental forces. The values are just empirical. In other words, we don't know.
  19. Feb 21, 2013 #18


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    1kg of copper, without electrons, would accelerate a copper nucleus in a distance of 10cm with an acceleration of about 1027 m/s2, reaching ultrarelativistic speeds within less than a nanometer (neglecting the finite propagation time of the electromagnetic force here). The total energy content in the electrostatic repulsion would be 1 billion times the rest energy of the copper block and equivalent to the total energy radiated by the sun in ~1/4 seconds. This corresponds to 50 billion megatons of TNT - one billion times the yield of the strongest bomb ever built, and more than 100 times the energy of the asteroid which probably killed most of the dinosaurs.

    Yeah... so thank the universe that copper has always nearly as many electrons as protons. Otherwise, it would kill all life on the surface of earth.
  20. Feb 21, 2013 #19
    So the "strength" that we are talking about is related to the coupling constant (which is some inherent property of an interaction I presume) and not to charge/mass ratio right?

    Did you by any chance draw inspiration from this thread? ;D
  21. Feb 21, 2013 #20


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    The charge to mass ratio is buried in the coupling constants.

    [tex]\alpha_G=\frac{2 \pi G m_e^2}{hc}[/tex]
    [tex]\alpha = \frac{2 \pi k_e e^2}{h c}[/tex]
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