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What causes particles to move

  1. Sep 27, 2011 #1
    Is this known? Is there a cause known at all? Does it just happen? Do they move because of energy, but the amount of energy a particle has is just a description of that particle, in other words, you can't pop open the particle, look inside and find out what causes the particle to move. If energy causes the particle to move, then I don't understand how mass and energy could be the same thing.
  2. jcsd
  3. Sep 27, 2011 #2
    Newton's second law, particles keep going at whatever speed they start at unless acted on by an external force. Relativistically, there is no difference between moving at a constant speed and not moving. The only thing of importance is relative velocity between objects. Likewise, kinetic energy is also dependent on the observer.

    So nothing 'causes' them to move, in the sense that if one is already moving there is no reason why it should stop moving. To move in the first place they need some force to act on them. Perhaps I am misunderstanding your question. Mass and energy are not quite the same thing. I think it would be better to say mass is one kind of energy. You can have energy not related to mass, i.e. kinetic energy.
  4. Sep 28, 2011 #3
    are you saying God set them in motion during the Big Bang and they've been moving ever since?
  5. Sep 28, 2011 #4
    Well, basically yes. Obviously they collide with each other and influence each others motion in various ways, but all their energy came from the Big Bang. Well, actually if you want to go down that road things get a bit hairy, some say that there is negative energy associated with the existence of space and this balances the positive energy associated with the existence of matter, and so it all cancels out overall, meaning you don't need any energy to create the universe in the first place. This is where the energy driving the expansion of the universe is coming from; the creation of extra space itself. Is this really the point of your question though? Generally one considers the motions of particles on a somewhat shorter timescale...
  6. Sep 28, 2011 #5
    Ok, one more question. Energy is just a description of the particle, right? In other words, you can't smash the particle open and say, ah ha, look, that's energy, right? I think energy is the ability to move a mass, so ability sure sounds a lot like a description.
  7. Sep 28, 2011 #6
    Well, energy is not a description of a particle, it is a property that particles have. Actually, it is a property that whole systems of particles have. An individual particle alone in the universe only really has energy associated with its mass, because it doesn't move relative to anything and can have no potential energy either.

    There is energy associated with their mass and their motion, so you CAN "smash them open" and say aha, look that is what energy my particle had, relative to me (say), before I smashed it up. It is not a separate physical object though, if that is what you mean...
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