At the beginning of the 19th century a French scientist, the Marquis de Laplace, thought that there must be some scientific laws that would predict everything happening in the universe if the state of the universe was fully known at a given time. This is true in particular for the trajectories of the sun and planets. If one knows their positions and velocities at one time, Newton's laws allow us to calculate the same parameters at any time in the future. Determinism was true in this case, but Laplace thought it applied to everything else, including the human behaviour, and this was shown later on to be wrong. If Laplace had been right, based on the knowledge of the present state of the universe, the scientific laws would have been able to allow the scientists to predict the state of the universe in the future, but also to know everything in the past. Newton's laws allow us to predict with accuracy the future positions of all the bodies such as the sun, the planets and the stars in the solar system. Laplace's theory did not work for anything else, especially to predict the future behaviour of human beings. The determinist idea was also rejected by the Church that felt that it infringed on God's power.
Lord Raleigh and Sir James Jeans showed that any hot body, such as a star, should radiate an infinite amount of energy. This was a direct deduction of the laws known at that time that said that a hot body emits the same amount of electromagnetic waves (radio waves, light, X-rays) at all frequencies. As there are an unlimited number of frequencies, the total energy emitted would be infinite. This is obviously wrong.
In 1900, the German physicist Max Planck proposed a theory according to which light, X-rays, and all other waves, can only be emitted in certain fixed and limited amounts that he called QUANTA, and now we define a quantum of light as a photon. This amount of energy, moreover, is directly proportional to the wave frequency. Although photons of different frequencies are the same, photons of different frequencies carry different amounts of energy. The smallest quantity of electromagnetic energy emitted by a black body at a given frequency is a photon of that frequency, the photon energy being higher at higher frequencies. At very high frequencies the emission of one quantum require more energy that the black body contains and, consequently, no photon of that frequency will be emitted. In conclusion, at high frequencies the number of quanta emitted decreases and the rate of loss of energy of a hot body is finite.
In 1926 Werner Heinsenberg went further and killed definitely Laplace's deterministic theory. In order to be able to calculate the future position and the velocity of a particle, one must measure these two parameters with accuracy at a given time. To do these measurements one usually sends a light beam on the particle; some of the light-waves will be scattered, and this will give us the particle position. However, the accuracy cannot be better that the wavelength of the light used. To obtain an accurate measurement of the particle position one must use light of very short wavelength, but then its frequency and energy will be higher and the position and velocity of the particle will be more disturbed. According to Plank theory the smaller amount of light is one quantum, but even this single quanta will disturb and change the position and velocity of the particle in an unpredictable way.
In short, the more accurately one measure the position of a particle, the less accurate is the measurement of its velocity, and vice versa. Heisenberg found that the uncertainty in the position of the particles multiplied by the uncertainty of its velocity multiplied by the mass of the particle can never be smaller than a certain value known as Planck's Constant. This constant does not depend on how the measurements are made, or on the particle type of light used. This law is known as Heisenberg's Uncertainty Principle.
However the effect of the quantum theory, like the effects of the theory
of relativity, are very small and do not affect us in our everyday life.
But in the case of measurements of particles, these two theories cannot
be ignored. Moreover the limit imposed by the uncertainty principle does
not depend on the method used to measure the position and the velocity of
a particle. It is a basic and general physical law of the world.