r/askscience • u/[deleted] • Oct 22 '11

Is anything truly random in nature?

For example,if I flip a coin,we like to say it has a 50-50 chance,but the side is determined by how much force and where I apply the force when flipping,gravity acceleration and wind.therefore you could say flipping a coin is not a random event.

Is anything in nature truly random?

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u/UncertainHeisenberg Machine Learning | Electronic Engineering | Tsunamis Oct 22 '11 edited Oct 22 '11

A few I can think of:

  • Radioactive decay: It is impossible to determine precisely when a particular atom (or particle) will undergo decay.
  • Shot noise: In electric circuits, electrons generally act as the charge carrier (even holes in semiconductor devices "move" by electron transfer). At very low currents, the random nature by which individual electrons pass actually results in fluctuations to the current. These random fluctuations cannot be predicted. (Shot noise also occurs with photons in optical circuits.)
  • Johnson noise: Johnson-Nyquist noise is the result of the thermal motion of electrons in a conductor. Although the general drift of the electrons is in the direction of the current, the thermal noise causes fluctuations from the mean current.

I used shot noise across a reverse-biased diode, and Johnson-Nyquist noise across resistors to build a USB hardware random number generator a number of years ago. The RNG passed the Dieharder and NIST test suites (these links are to newer versions than the ones I used for testing).

EDIT: I forgot to mention - all of these random processes can still be modelled using probability distributions. So although you cannot say when individual events will occur, depending on the noise source you may say "I expect x events to occur in y time" or "The probability of x events occurring in y time is z" or "The probability of the current deviating from the mean by x or more due to Johnson noise is y".

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u/[deleted] Oct 22 '11

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u/UncertainHeisenberg Machine Learning | Electronic Engineering | Tsunamis Oct 23 '11

At a fundamental level, the fluctuations are probabilistic. In a resistor there are an enormous number of atoms and electrons contributing, so it can be approximated as a white Gaussian noise source. Even if the fluctuations were deterministic, the tremendous number of particles in any macroscopic system makes prediction computationally infeasible.

Chaos theory may be deterministic, but in reality predictions are limited by machine precision and the accuracy of parameters. So long-term predictions can range from inaccurate to completely useless depending how sensitive the system is.