1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravity and nuclear forces

  1. May 18, 2009 #1
    I don't understand why it is that the nuclear forces only apply at subatomic distances. If it is the case that they only apply at subatomic distances then surely that must be because the force has decayed over distance much the same as gravity does.

    Thus my hypothesis that en masse the nuclear forces create gravity.

    This is my first post here and i didn't really know where to put this question so feel free to move it.

    I've probably just spoken a load of nonsense but i just pondered this and i hoped you guys might have some thoughts.
  2. jcsd
  3. May 18, 2009 #2


    User Avatar
    Science Advisor

    Gravity and e-m forces have an inverse square dependence on distance. The nuclear forces fall off much more rapidly with distance, so they don't have any effect outside of atomic nuclei.
  4. May 18, 2009 #3


    User Avatar
    Homework Helper

    Well, the nuclear force a.k.a. the strong force is based on the color charge of quarks. The force is very strong inside the nucleus, where you have different colored quarks all mixed up together. But the quarks group themselves into colorless particles (protons and neutrons), which, since they're colorless, don't exert much of any strong force on each other. You could, roughly speaking, say that all the strong force exerted by the three quarks in a proton is "used up" in holding those three quarks together, and there isn't any left over to attract anything else. (Although a little bit does "leak out," just enough to hold together the protons and neutrons in a single nucleus. That doesn't account for gravity though.)

    It's similar to how oppositely charged protons and electrons group themselves together into neutral atoms. In that case you could roughly say that all the EM force is "used up" holding the atom together. The little bit that "leaks out" is called a van der Waal force.
  5. May 19, 2009 #4
    Just some related questions if someone could please help.
    Does gravitational "force" get used up? (In quotes to check if it is a force?)
    Does magnetic "force" get used up?
    More generally can any sort-of force get used up?
    Is net force different to used up force? (example of net "force": in bubble in middle of planet we'd be weightless).
  6. May 19, 2009 #5
    No gonegahgah.

    The four fondamental interactions (gravity, em, weak and strong nuclear) do not get "used" up like you would imagine a spring getting worn out.

    These interactions are created from the behaviour of matter and energy, and therfore will always be around, no matter what.

  7. May 19, 2009 #6
    Thankyou for your responses. I have learnt something new, especially about the attractions of quarks.

    I have evolved my question, however:

    Does gravity actually get 'used up' as it converts into kinetic energy?

    If so, then as the planets orbit the sun there must be a lot more gravity heading off into interstellar space than is converted to kinetic energy when pulling in the planets.

    Could it be possible that on an atomic level the nuclear forces act in a similar way as they pull each other together (the force radiates out in all directions, but only attracts particular particles)? I suggest all the different nuclear forces are all radiating outwards (like gravity) and on an accumulative scale can produce an attraction between large bodies.

    By scale the distances between atoms could be compared to the distances between stars, and at intergalactic distances a single star has almost no gravitational impact whereas a galaxy does.

    I don't expect to be anywhere near being correct about this, so please help me to understand.
  8. May 20, 2009 #7
    Hi there,

    You are talking about two different subject: gravity and energy (not the same). Gravity has the ability to induce energy to objects. This energy can take different behaviour, such as kinetic energy (energy associated with the speed of an object).

    Eventhough gravity can induce energy, it NEVER GETS USED UP!!! Thank gravity for that, otherwise, there would not be alot left after the billions of years of existence.

  9. May 20, 2009 #8
    Sorry, I didn't mean used up in the sense of all gone.
    I meant does it get consumed in its travel through things?
    ie Like sushi trains, where sushi passes in front of customers and some gets taken off and eaten.

    So writing it differently.
    1. Does gravitational "force" get consumed?
    2. Does magnetic "force" get consumed (by? paramagnetism, diamagnetism)?
    3. Does the quark strong force get consumed? by other quarks?
    4. Is net force different to consumed force?
  10. May 20, 2009 #9
    Hi there,

    1. No
    2. No
    3. No

    None of the fundamentals interactions get "consumed. They have been around since the beginning of time, and will stick around for a long time ahead.

  11. May 20, 2009 #10


    User Avatar
    Homework Helper

    oy... j_rankin and gonegahgah, I think you may have taken my very rough analogy too literally. As fatra2 has been explaining, forces do not actually get used up over time. I was talking about getting "used up" over space, but still that's probably confusing and may give you the wrong impression...

    Here is perhaps a better explanation: within an atom, the electromagnetic force holds the electrons in orbitals around the nucleus. Now, if a positively (for example) charged particle is somewhere inside the atom, between the nucleus and some electrons, it feels a strong repulsion from the nucleus and a strong attraction to the electrons, which means there is a fairly powerful electromagnetic force acting on it. But if you move the same particle some distance outside the atom, it is now repelled by the nucleus but attracted by the electrons. Since the charges from the nucleus and the electrons balance out to zero, the attraction and repulsion also balance out to zero, and the particle feels no net electromagnetic force while it is outside the atom. (Unless maybe the electrons happen to be on the close side of the nucleus at some moment, then their attractive force will be slightly stronger than the repulsion from the nucleus at that moment... that's the van der Waal force)

    Similarly, inside a proton, you have a red quark, a green quark, and a blue quark. They attract each other due to the strong force. If you could somehow have a small blue (for example) particle and put it in the middle of the nucleus, it would feel a very strong attraction to the red and green quarks and a repulsion from the blue quark. Thus there would be a very strong color force (a.k.a. "strong force") acting on this hypothetical blue particle. But if you put the same blue particle outside the proton, the attraction to the red and green quarks would balance out the repulsion from the blue quark, and the blue particle would feel no net color force. One could say that the proton as a whole appears colorless, and a colorless particle does not exert any color force.

    The technical term for what I'm describing, at least in the electromagnetic force, is charge screening. I'm not sure if there's an equivalent term for the color force - it is much more complicated and less well understood, and there are some very complex quantum effects involved... anyway, you should probably forget everything I said about forces being "used up" ;-)
  12. May 22, 2009 #11
    given that all matter is attracting all matter, should all matter become evenly distributed at some point in time, as gravity stabilises motion? if not what is preventing this from happening?
  13. May 22, 2009 #12


    User Avatar
    Homework Helper

    Well matter actually is pretty evenly distributed throughout the universe, on the largest scales... but actually, since all matter attracts all other matter gravitationally, you would think that eventually it would all collapse together. This has been labeled the "Big Crunch".

    The fact that the universe is expanding. Currently galaxy clusters are flying away from each other, and a lot of people are trying to figure out whether there's enough matter in the universe (and whether it's dense enough) for gravity to eventually overpower the expansion. It's hard to tell because the expansion and the gravity are very closely balanced.

    Then again, recent discoveries show that the universe seems to be expanding faster as time goes on (obviously this works against gravity). Physicists attribute this to "dark energy" but it's still quite a mysterious phenomenon - nobody has a really satisfying explanation for what "dark energy" is.
  14. May 22, 2009 #13


    User Avatar
    Gold Member

    Hmm. What if you imagine a globe/ballon. And you have a bunch of points completely evenly distributed over the surface in a grid fashion. The distances between every point and neighbour is the same. Now perturb each position by some amount so that the average distribution statistics match the galaxial distribution of our universe for some localized region on the surface.
    If the attraction between the dots at this distance is small, compared to say, an inflation of the balloon/globe, such that in some time T the distance any dot would move due to interactions with its neighbors is minimal compared to the increase in distance due to the expansion, would the system ever really collapse?

    I know with no expansion it would, breaking the symmetry pretty much guarantees that given enough time it will either collapse all points into one (if some nonconservative forces exist) or some steady state, orbits or some such state would happen, but the dots would all gather.

    Seems like there would be some comparisons of the relative interaction strength and the rate of expansion that you could match up with our current distribution and expansion rate. I'm sure someone is doing this research, and has been for 50+ years.

    I'm sure this is probably delving into some dynamic differential manifold theory or something.

    Maybe I'll program something.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook