I hope you can bear with me as I warm up to my question regarding Brownian motion. I am currently studying physics on my own and am watching a series of lectures on Quantum Theory. Obviously I am just some guy with an interest in physics and I have no clue. Apparently, Brownian motion is the movement of atoms. We all know the story of Robert Brown's discovery and Einstein's explanation of it. But I am not satisfied with merely asserting that it is because the atoms absorb photons (that is, “heat”) and thus they vibrate faster. So I began thinking about the “Brownian motion” of a single isolated atom. I made some of my own assumptions in order to help me clarify my thoughts. I thought only electrons absorbed photons, and as a result the electron moves into a higher quantum energy state. I made the assumption that the electron/photon interaction has little effect on the nucleus of the atom because the nucleus is shielded from that interaction. I wondered how that could translate into the “increased vibration” of the atom? Why does the atom move in the first place when all that’s happened is that the electron has absorbed (destroyed?) a photon and moved up one or more quantum energy levels in a higher electron shell. I could not believe nor imagine a massless particle like a photon having enough kinetic energy to physically move an atom in space. It would be like hoping to move the Earth by smashing a grain of sand against it. Even at the speed of light the effect would be too small to notice. I think many of us learn at a basic level that Brownian motion is caused by atoms ricocheting off each other like some tiny billiard game, but what really causes it and what keeps it in motion? I can imagine the motion of mechanical kinetic energy being transferred chaotically like when you pour water into a glass, but that has to die down sometime due to friction and gravity. So I am left with only the additional of more heat as the cause of Brownian motion. I see only two kinds of motion relevant to this question, that of translational movement like a billiard ball hitting another ball and transferring kinetic energy, and the other movement is some sort of internal vibrational energy like the ringing of a bell. So to sum up what I want to know: How does heat make an atom move faster, and by movement I am assuming the only movement involved is some sort of ringing effect like maybe when you strike an immovable bell with a hammer, and not a translational movement in some random direction. If a vibrating atom wandering around somehow collides with another one, would not the kinetic energy eventually be consumed? How then is Brownian motion sustained in the absence of kinetic energy from outside the system other than heat energy?