In quantum level, the act of measurement affects the measured. This was one of the answer unfolded by 5th Solvay conference resulted by the concept of duality and Heisenberg uncertainty principle. 
It has transformed the concept of reality, though the world seems so real despite being so unreal and uncertain at the quantum level.
The Heisenberg uncertainty principle
Heisenberg uncertainty principle was published by German physicist Werner Heisenberg in 1927. It states that ” The more precisely the position is determined, the less precisely the momentum (speed) is known at that instant and vice-versa.
In simple words, you cannot know where a particle is and how fast it is moving at the same time. If you know the speed exactly, you cannot know where it is and vice-versa. This principle is true since its application has given results that are very accurate.
Explanation
According to the Heisenberg uncertainty principle, the measurement of position makes speed very uncertain so that the system is disturbed by the act of measurement. Let us observe an atom since observation is an act of measurement, we cannot observe the truth that underlies within an atom.
In other words, our observation makes the atom to take a stand but actually, the electrons are continuously in motion and thus we deviate from reality. We can also put it another way, the light’s particle nature transfers momentum to the electrons and other particles in the atom and makes them uncertain.
So, our observation is disturbing the system and the system we see is the disturbed one. The reality is, in fact, a puzzle.
Quantum Mechanical Picture of Heisenberg Uncertainty Principle
In quantum mechanics, every particle can be represented by a mathematical function called a wave function, which contains all the information about it. This wave function can be written as a sum of other basic state functions in which the particle can be.
The state functions give a certain value when operated by operators associated to a physical quantity (say position or momentum). The value is called eigenvalue and it is the value of the physical quantity, while the state function is called eigenstate.
According to this principle, the eigenstate of a momentum operator cannot be an eigenstate of position operator. So, we cannot have the value of both for the same state. Measuring both at the same time is impossible.
Why is Quantum World so Uncertain?
Not only position and speed but there are also other variables which are conjugate, one goes uncertain on another becomes certainty. For instance, the energy-time uncertainty can be stated as – a state having short lifetime cannot have definite energy. The root for these uncertainties is one.
The concept of duality or matter-wave give rise to these uncertainties. A quantum particle can behave as both particle and wave. It is spread out as a wave so that its position is not fixed but speed is. When you look at it (observe), the wave collapses giving a certain position leaving its velocity uncertain.
Wave and particle characteristics are mutually exclusive as is position – speed. If the position is instantaneously changing, we can determine the velocity or speed. Our approach to determine an object’s position will suddenly make it stop since there is no idea of position without making it stop. So, the speed will be indeterminate in this case.
The Certainty of the Classical World
Heisenberg uncertainty principle rules the quantum world but the classical world is not uncertain. The dual nature of matter is not effective in the classical world. This is due to the small value of the associated wavelength. As the particles get bigger, their wavelength gets diminished.
The wavelength for massive particles is less as if it vanishes. The vanishing of wave nature eliminates the uncertainties. The particle is no longer a wave and it cannot have uncertain positions.
We may perceive it on the basis of transfer speed by external wave. The external wave made for observation can affect the small particle. But, the massive particles are not necessarily affected. This eliminates the possibility of indeterminacy in speed while we measure the momentum.
Heisenberg uncertainty principle is a fundamental concept of quantum mechanics which makes quantum mechanics unique from classical mechanics. It is viewed mathematically rather than physically. So, our interpretation here is just an outline but the rigorous part lies in mathematical formulation and its interpretation.
