# Why have scientists accepted that Brownian motion is caused by collisions with water molecules?

Colloid (sol) particles have a diameter between 1 and 1000 nanometers, approximately. If a sol particle, 275 nm in diameter, were compared to a water molecule, 0.275 nm in diameter, it would be 275 / 0.275 = 1000 times the diameter of a water molecule. Since volume is a cubic function of diameter, the sol particle would be 1,000,000,000,000 (one trillion) times the sol particle's volume and therefore, one trillion times its mass (Patterns in Fluid Flow Paradoxes., 1980; pp. 73-74 and Simple Harmonics, 2015: pp. 16-17, by G. Hamilton).

Sir Isaac Newton would roll over in his grave if he heard that modern scientists accept that which defies his principles relating to momentum exchange physics.

Furthermore, Brownian motion has been related to thermodynamics. Motion created in fluids by heat results in convection currents with gently curved paths, not abrupt sharp zigzag changes in direction.

• Don't read Newton. Read Einstein (1905). His paper and thesis treated this very model and that's what explained the drunken walk of pollen grains and colloidal suspensions alike in experiments that followed. Many 'apparent' paradoxes in physics - either missing or wrong information. Jun 29, 2017 at 20:00
• -1 for your pretentious way of asking a question. Before assuming that all scientists are stupid, please consider the possibility that there's something that you may have misunderstood.
– pela
Jul 3, 2017 at 11:20

## 2 Answers

What makes you think that you can assume that the density of a colloid particle is comparable to that of a water molecule? In other words, why do you think that the density of bulk water is the same as the density of a water molecule? Why not actually compute the mass of such a particle and compare it to the measured mass of a water molecule; you get a very different mass ratio (circa $10^9$ rather than $10^{12}$).

Then take into account the speed of water molecules (hundreds of meters per second) and the speeds for the colloid particles that can be observed in a optical microscopic (circa $\mathrm{\mu m/s}$); that makes the ratio of speeds on order of $10^8$.

So, yeah, an energetic water molecule ought to be able to make a visible difference in the motion of a colloid particle.

Then you add the success of the statistical math Einstein presented for the RMS displacement as a function of time and the theory looks really solid.

Brownian motion describes the dynamics of a particle experiencing random forces. This dynamics is well described by the Langevin equation and does not lead to a zig-zag motion for very short times. This is evident in the velocity-autocorrelation function which decays to zero over a time span covering numerous collisions.

You observe the zig-zag motion when you record the positions of the grain at times larger than the decay time of the velocity autocorrelation function. In the original experiments, the position was recorded very 30s, much much larger than the time scale of the microscopic dynamics. On this time-scale, the motion is indeed the purely random motion of a random walk. Nonetheless, it is still caused by the collisions of the grain with the water molecules.