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What causes one billiard ball to move another?`

  1. Dec 29, 2011 #1
    I put this in the atomic physics rather than classical because i want to know what happens on the atomic level that causes one billiard ball to move another. bryan greene said that if took all the empty space out of the empire state building that it would be the size of a grain of rice. so what forces another ball to move another? i'm thinking it's because the protons are surrounded by negative electrons and when the negative electrons approach the other negative electrons that that they repel each other. am i right?
  2. jcsd
  3. Dec 29, 2011 #2
    Brian Greene is notorious for being misleading. I would suggest you read his works with an understanding that he has simplified and stretched physics so much to be interesting and comprehensible to the lay man, that he verges on being misleading if not flat out wrong. There is very little "empty space" in the atoms of the empire state building because the fields and wavefunctions fill up what a hundred years ago people considered empty. You could not shrink an object simply but removing the supposed empty space in its atoms, because the fields and wavefunctions are pushing against shrinkage.

    You are right that contact forces between objects are fundamentally quantum electrodynamic in nature. But it's a little more complex than negative electrons repelling negative electrons. Unless purposely ionized, atoms are electrically neutral. The positive nucleus balances out the bound electrons so that the atom looks like it has no net charge to an outside observer. If you push one atom close to another atom, they no longer look neutral to each other, but this electromagnetic interaction (the Van der Walls force) is typically net attractive, not repulsive.

    So what makes billiard balls bounce of each other? The answer is the Pauli exclusion principle, which states that no two identical fermions (like electrons) can occupy the same state at the same time. As you bring the atoms closer, you are bringing the electrons closer and closer to occupying the same state, which is forbidden. So it takes more and more energy to approach this forbidden state, which is felt as a repulsion. Photons are not fermions, so they do not obey the Pauli exclusion principle. As a result, two laser beams traversing the same point in space do not bounce off each other.
  4. Dec 29, 2011 #3
    Great explanation. Thanks. I really appreciate your help.
  5. Dec 29, 2011 #4
    I've always thought, probably wrongly, that the repulsion in an elastic collision was due to dipole moments set up when the electrons are pushed slightly out of their normal state.
  6. Dec 30, 2011 #5
    No, induced and permanent atomic dipole interactions (as well as quadrupoles, etc.) are the basis for the family of Van der Waals forces, and they are dominantly attractive, not repulsive. You can draw a little diagram to convince yourself of this (the negative electron of one atom slightly repels the negative electron of the other atom to the other side of the atom, leaving the positive nucleus slightly bare, and opposites attract). The Pauli exclusion principle keeps an atom/molecule from collapsing in on itself. Most solids are giant crystal molecules. When an object comes in contact with another object, their surface atoms start to overlap, but Pauli won't allow this, so the molecules start to collapse, but Pauli won't allow this either, so they spring back, pushing back on the first atom, repelling the object.

    Think of an axe hitting a piece of wood. If the axe has enough energy, it can cut through the wood, but it must split bonds and push away the atoms to do so, which requires energy. The axe can't just slip through the so-called "empty space" inside the atoms. If the axe does not have enough energy to split bonds and push the atoms apart, it will bounce back. You can think of the molecular bonds as springs. When an object strikes another object, it is like it is trying to push the molecules apart so it can slip through. It succeeds a little, but then the "springs" pull the molecules back together, pushing the object back away. When two objects strike, they actually deform and then spring back. If the energy is high enough, the "springs" will be stretched until broken, and atoms will go flying everywhere. This is what happens when you throw a rock through a window. So a combination of Pauli exclusion principle an inter-/intra- molecular attractive binding forces within the object lead to a net repulsion when objects collide.

    It's analogous to how helium balloons float. Gravity only pulls things down, so why do balloons go up? Because gravity pulls the equivalent volume of air harder (because it is more massive) than the helium balloon. The air is pulled to where the balloon starts. They can't both occupy the same spot, so the air pushes the lighter balloon upwards out of the way. In this way the combination of an attractive force (gravity) and an exclusion principle leads to a net repulsion of the balloon from the earth.
  7. Dec 30, 2011 #6
    Interestingly, the Pauli exclusion principle only applies to fermions. Bosons are systems with integer spins. So any atom with an even number of fermions, such as helium-4, is a composite boson. If you can get such an atom into its ground state through cooling, it will not obey the Pauli exclusion principle. This is the principle behind Bose-Einstein condensates. This also means that two baseballs made out of supercooled helium-4 will not collide and bounce of each other. They will go right through each other, or even coalesce.
  8. Dec 30, 2011 #7
    Sorry, I don't get that. I would imagine the force required to push an electron "to the other side of the atom" would be enormous. I can imagine its probability density function (or whatever the term is) being skewed in that direction and that is what I mean by a dipole moment. That skewing would be a shift from a minimum energy state, which is what I can imagine causing a repelling force.
    In any case the effect would be the same on both atoms so even in what I consider the unlikely event that the electrons were pushed out of the way, the nuclei would repel each other, wouldn't they?
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