Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do we believe gravity to be a weak force...?

  1. Jul 5, 2015 #1
    So - this may sound like an easily answered question - but with push-back on my part I hope I can get to the real crux of what I'm asking, in any replies you might be kind enough to provide. I hope the answer is not, eventually, simple.

    The crux of my issue is this:

    Why do we believe gravity is a weak-force?

    Assumption/starting-point: that gravity is considered by physicists to be a "weak" (very weak) force.

    My arguments against this are:

    a) The gravity of all atoms - I believe - must effect every other atom in existence.

    i.e. the atoms at one end of the universe must surely have a gravitational effect on every atom at the other end of the universe, and removing "ends": every atom must effect, gravitationally, every other atom in existence.

    Furthermore; the atoms in every molecule must exert gravity on every other molecule in existence. (lessening as a square of the distance).

    The problem with proving this is that we just don't have any means (yet) to measure this.

    But surely it must be true (is my argument)?

    i.e. until we can measure the impact of one atom's gravity on another atom - and vice versa - (which as I understand it, is not possible because it's just way, way, way (way**32) too small for us to measure) we will never know what strength gravity has.

    b) Since light can travel effectively without limit (we can see the light as created shortly after the big bang via Europe's Planck telescope) - surely gravity must also have no limit.

    The significance of this is that if we consider that momentum is the resulting force from the atoms of an object moving through the EM/gravitational fields of all other atoms in existence (i.e. in this universe) [which struck me watching this Feynman talk , then surely all gravities impacting on all atoms in an object [arising from it's existence and therefore its default distance from all other atoms in existence] from must be measured and calculated before we can ascertain the "strength" of the gravitational forces at work.

    I'm not explaining this very well, and/or maybe I'm wrong in the initial assumption (that physicists think gravity is a weak force) but is it not "new thinking" to say that all atoms must exert gravity on all other atoms in existence? Is it already known/believed to be correct that gravity will end up being considered a strong force (an unbelievably strong force, with similarly infinite reach to light) but with almost limitless "dilution" owing to its effectively infinite reach?

    I guess I can rephrase to:

    Surely gravity is actually the stronger force than the other forces we know about; because the other forces we know about are limited to local effects; whereas gravity (and EM fields/field effects) have a reach that extends from one "end" of our universe to the opposite end, impacting every thing in between, in some tiny, unmeasurable way?

    Surely, looking at things in a universe-encompassing way: gravity is the one strong force we know about?

    Apologies if this is in the wrong place, or is easily answered. I thank you all so, so much for even entering into conversation with me about this, as an outsider/a newbie here.

    Namaste :)
  2. jcsd
  3. Jul 5, 2015 #2


    User Avatar
    2016 Award

    Staff: Mentor

    It depends on what you mean by "weak". Some ways of viewing things make gravity seem weak, others don't. There is no absolute sense in which any interaction is "weak" or "strong"; it depends on the context.

    This is certainly not the case. The very fact that we can't measure the impact of one atom's gravity on another, whereas we can certainly measure, for example, the electromagnetic impact of one atom on another, is one way in which gravity is a very weak force--compared to electromagnetism. If gravity were not weak in this context, we would easily be able to measure its impact.

    Why? There doesn't seem to be any logical reason why this must be the case. If you think there is a logical argument here, you'll need to spell it out.

    This is not a very fruitful way to look at momentum, precisely because we can't measure the force on an atom here on Earth due to atoms very far away--because those forces are too weak to measure. If they weren't weak, we'd be able to measure them. Since we can't, we can safely ignore them.

    I don't understand what you mean here.

    No, it's not. But "exert gravity" is not the same as "exert gravity that's strong enough to matter in practical terms". In practical terms, if we're just dealing with events on a local scale, we can ignore the gravity exerted by the rest of the matter in the universe. Only if we are looking on a global scale--for example, in cosmology--do we need to consider the gravity exerted by all the matter in the universe; and in cosmology, we do.

    In a different sense of "strong" from the above, yes. As you say, gravity and EM are the only long-range interactions we know of; and since EM has both positive and negative charges, on any large scale they cancel each other out, so that EM interactions on large distance scales are negligible, whereas gravity isn't, because it just keeps adding up as you consider larger and larger conglomerations of matter. So in cosmology, for example, gravity is indeed the only interaction we need to consider. But that doesn't mean gravity is "strong" in any absolute sense; it just means it's the strongest in that particular context.
  4. Jul 5, 2015 #3


    User Avatar
    Science Advisor
    Gold Member

    A cosmologist would likely dispute the notion of gravity as a 'weak' force. It is only weak by comparison to the other forces of nature, which, while nigh omnipotent on sufficiently small scales, are hoplessly feeble on cosmological scales.
  5. Jul 5, 2015 #4
    It seems to me you misunderstood what physicists mean when they say that Gravity is weak. A force's strength is not measured by its reach, but by its constant of proportionality. Consider the classical equations for Gravity and the Electrical Force. They are mathematically equivalent (both are inverse square relationships), if we were to take an object of mass 1kg and charge 1C and find the proportion of electric field strength to gravitational field strength we will find that FE/FG=k/G=1.35*10^20 . (Here FE is the electric force on a test charge and FG is the gravitational force on a test mass, with the mass and charge of the objects equal to each other. k is the electric constant and G is the gravitational constant).

    This means that 1C of charge will produce an electric field that is 1.35*10^20 times stronger than the gravitational field of 1kg of mass, at any given point. This shows how much stronger the electric force is than the gravitational force. There are, to the best of our knowledge, only 4 fundamental forces, and gravity is the weakest in the sense described above. Hope this helped :)
  6. Jul 5, 2015 #5
    Gravity is certainly weak in the context of that when you stand still on the floor you have the gravity of the whole of Earth pulling you down, yet this is balanced by the upward pressure of just a few sq cm of floor, (a result of electromagnetic force).
  7. Jul 5, 2015 #6
    Hi Jamie,

    "Why do we believe gravity is a weak-force? "

    Richard Feynman doesn't believe that.

    As already noted, gravity is very 'weak' and can be ignored in some situations, such as nuclear reactions [interactions within the nucleus of atoms] or the attraction between an electron and a nucleus.

    On the other hand, nothing beats the strength of gravity in other cases, say it's ability to crush from existence everything inside a black hole. And few if any would claim gravity is 'weak' regarding stars like our sun where all the explosive outward power of fission and fusion reactions generated cannot overcome the confining strength of gravity.
  8. Jul 9, 2015 #7


    User Avatar
    Science Advisor

    If you take two protons, the electrostatic repulsion between them will be some ##10^{40}## or so times stronger than their gravitational attraction. Gravity becomes significant at large scales only because the positive and negative electric charges cancel one another out, while gravity always adds.
  9. Jul 9, 2015 #8
    This post of mine is more vague than subtle, but some relation between gravity and the weak nuclear force may be of interest, as some tendencies in the quantum gravity that's expected (by many) to eventually let singularities fade out of cosmology may be of interest to the originator of this thread: In keeping the electrons in that (90% plus) piece of the universe's (visible?) mass that hydrogen atoms are said to comprise out of their nuclei's hair, the weak nuclear force seems to be keeping everything about as stable as it gets, on those numerous (understatement of the eon?) occasions when the electrons' probability wave (handily and wavily enveloping each atom) would otherwise let the protons snag them out of the cloud that the atomium's old-fashioned orbits have been reconfigured into during these last few decades. Because of this, and because he'd probably be able to make better sense of the relation between electromagnetism and relativistic gravity than I could, he may dig the thread on Rovelli's 2014 "Planck stars" paper. (in the paper itself, at arXiv.org > gr-qc > arXiv:1401.6562, the bit where the hypothetical astronaut's tidal-resistant protoplasm finds that the outside universe has evaporated during the few proper-time hours of her approach to the center of a Planck star's black hole is especially mind-blowing, and runs sort of counter to the general notion of generic "gravity" being at all weak.) I hope this isn't too far off the beam vis-a-vis the relativistic gravity that's been more at issue here, but, like Rovelli was saying, the two may be kind of related, depending on the scale.
  10. Jul 12, 2015 #9


    User Avatar
    Science Advisor
    Gold Member

    Shall we assume you have visionary insight on quantun gravity. I think not.
  11. Jul 12, 2015 #10
    Touche. Ouch!
  12. Jul 12, 2015 #11


    User Avatar
    Science Advisor

    Not really. The weak nuclear force is only weak because its force carriers (the W and Z bosons) have mass. At high energies, above the masses of the force carriers, the weak nuclear force isn't very weak at all. Also, the W and Z bosons are spin-1 particles, while the graviton has to be a spin-2 particle.
  13. Jul 12, 2015 #12
    Great, someone hep to force carriers and spin! Since Wikipedia (my window on the world) leaves the status of the graviton unclear as a boson and I'm a sucker for all Russian-dolls / kitty-In-the-keg cosmologies, let me just ask you if any consideration has ever been given to the possibility that gravitons might be things on the scale of, say, planets, thereby providing an overlap between our little realm and one on a seriously large scale. (I wouldn't dare start a thread with such a notion, but your "temporal proximity" leads me to risk it, particularly since the question seems a little less idiotic now that the Higgs boson is turning out to be so large.)
    Last edited: Jul 12, 2015
  14. Jul 12, 2015 #13


    User Avatar
    Science Advisor

    What? No. There's been a lot of work done on quantum gravity, and there are a lot of things that we do know. The spin-2 massless graviton is one of them.

    Now, to be clear, the spin-2 massless graviton is not necessarily a completely accurate description of what quantum gravity must be. Instead, it's an effective field theory. That is, at relatively low energies, whatever quantum gravity is must act like it is mediated by a massless spin-2 particles. This is the only way that it could behave like General Relativity.
  15. Jul 12, 2015 #14
    Is the Higgs boson so unexpectedly large?
    My impression was that it was discovered (at around 125 GeV) to be within the theoretically predicted range, after earlier CERN experiments had ruled out the range below about 115 GeV, and the Tevatron in the US had produced results indicating it unlikely to be above 140 GeV.
  16. Jul 12, 2015 #15
    What I meant was, "now that the expectedly large Higgs boson has been found to exist". Sorry for the confusion.
    Last edited: Jul 12, 2015
  17. Jul 12, 2015 #16
    Got it about the spin-2 particle being massless, which effectively rules out any relation to scaled-down planets. Thanks.
    Last edited: Jul 12, 2015
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Why do we believe gravity to be a weak force...?