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What is the cause of gravity? Answered

A friend of mine MUSAB said that  two bodies attract each other because the electrons and protons of both their atoms attract each other, however, I don't think this is the case, I have heard from some where else that it is due to distortion in space time created by a mass and geodesics are formed and they bend towards the center of the mass due to which every other body which comes in its gravitational field tends to move towards its center and all that stuff I have studied in detail. So who is right? If he is wrong, then please also specify where is he wrong, I think that individual atoms can not attract each other because of charge because an atom, as a whole, is a neutral particle.

Thanks in advance.


Objects attract one another gravitationally with a force proportional to the product of their masses. Large objects are electrically neutral, and there is essentially no charge attraction between them.

Objects bump into each other (that is, not passing through like ghosts) because of the electrons in the atoms. Those electrons are all as close together as they can get (look up "Pauli exclusion principle" and "atomic orbitals"). If you try to push two atoms together, or through one another, they push back and you can't do it.

So gravity (masses attracting) is what pulls you down onto the floor, but atomic "repulsion" (electrostatic forces) is what keeps you from falling through the floor.

I have also heard about gravitons, and that objects of different masses continuously emit gravitons and whenever one of them hits another particle it causes it to move closer and thus the objects move more and more closer to each other. That is what I have heard and have no idea if this is all right or not, but I still don't understand how do gravitons get the bodies to move closer to one another and create the effect of gravity according to this theory?

You don't need to worry about any of the particle physics or general relativity aspects of gravity, in order to answer the question you posed. Newton described the force of gravity perfectly well for essentially any situation you are likely to encounter: objects falling on a planet, objects in orbit, planets around the Sun, the motion of galaxies and clusters of galaxies.

General relativity, and the hypothetical quantum extensions to GR, are only important if you want to (a) understand "why" gravity works, rather than "how"; or (b) understanding the behaviour of gravity in extreme situations, such as very (very) close to large masses.

Your question about how "gravitons" can cause an attraction between objects rather than repulsion, is exactly the same question of how photons can cause an attraction between charged objects. You need to understand much, much more quantum mechancics, and even basic physics, than you currently do. I encourage you to continue your school work in chemistry and physics, and you'll end up with enough background knowledge to move forward.

ok, I really did mean "why" rather than "how", I know how gravity works but I have trouble understanding why it happens in the first place. I know the theory of general relativity but I know a little about quantum mechanics, and therefore I can not understand the concept of gravitons and how they cause attraction between bodies, and may be I know the basic physics pretty well.

Gravitons are virtual particles -- that means they are exchanged as part of an interaction, but do not appear as free bodies (remember the "free body diagrams" from classical mechanics). Similarly, the photons which are involved in electrostatic forces are virtual.

Virtual particles can transfer momentum of either sign, depending upon the specific vertices of the interaction you're computing. If the momentum transfer is positive, then the resulting force is repulsive. If the momentum transfer is negative, then the force is attractive.

I can not understand, can you please tell in little more detail how can the particles transfer negative momentum?

Wups. My fault for reading too soon; sorry. Please ignore the first paragraph of my previous reply. I thought this was written by the original poster. I haven't seen any questions/comments from you, so I don't have any basis to judge your level of math training.

The only way to explain it is with mathematics. From the level of questions you've been asking, I am not sure that you have the necessary background.

Since virtual particles never appear externally to the interaction, that means that their exact momentum and mass are unknown (unknowable). To calculate the matrix element for the interaction, you have to integrate over all possible values of momentum and mass, even non-physical (such as negative) values.

For an attractive interaction (like a positive and negative charge, or two masses) the phase of the vertices at each end of of the interaction are such that when you do the integral, only the "negative momentum" states remain, leading to a net attraction.

Dr. Michio Kaku explains what is gravity.

Your answer is correct. When charged particles DO attract, the Coulomb force is many orders of magnitude higher than the gravitational force.

So to summarize your answer so that my friend could understand easily (he is not much of a in depth and technical thinker) is that we are not standing on earth because are atoms are attracting the atoms of the earth, but it is due to the force of gravity which is completely different than the the force of attraction due to charges.