Thomas Young, in 1805 performed his famous double slit experiment and showed that light actually behaves like a wave. These results were baffling and created a turmoil in physics. Big question which everyone asked was : WHAT IS LIGHT? A WAVE OR A PARTICLE ? DOES LIGHT ACT AS BOTH PARTICLE AND WAVE?
The matter settled when Louis de Broglie gave a hypothesis. He said that light and every other moving object shows both particle and wave like characteristics simultaneously. But which one of the two characters dominate depends upon velocity (precisely momentum) and mass of the object. Here is de Broglie equation:
λ= wavelength, h= planks constant(6.626×10−34 ), p= momentum
In 1927, Davisson and Germer performed an experiment similar to Young’s but this time it was not with light but with electron. And like light, electron also produced an interference pattern. This showed that individual electron is also a wave ( However an electron act as particle when it is not observed. This is due to uncertainty principle. If not understood , it can be ignored)
Now the question. In water wave, a group of water molecules goes up and some of them goes down and thus forming crests and troughs, which is typical characteristic of wave. If an individual electron is a wave then what is it that is waving?
Erwin Schrodinger gave first guess. He said that electrons constituents smeared out in space and smeared electron essence does the waving. This was later disapproved. If electron smeared out in space then we will observe some part of its charge over here and some of its mass over there. But this is not true. When we observe electron, its mass and charge is concentrated at a single point.
In 1927 something unusual was said by Max Born. The wave is not a smeared out electron. It was something which was never encountered in science before. The wave which Max Born was talking about is a probability wave.
The probability wave envisioned by electron have regions of high intensity and low intensity. But the meaning which Max Born said about the shape of wave was totally different. He said ” the size of wave at a given point in space is proportional to the probability that the electron is located at that point in space”. Places where the probability wave is large are locations where the electron is likely to be found. The places in space where it is small is place where electron is less likely to be found.
Now let us look at double slit experiment with electron. When electron was forced to pass through each slit, it has a choice either to pass through right slit or through left slit. But the probability wave of electron through each slit has equal amplitude. So instead of electron passing through either one, electron pass through each of the slits, commingles its probability wave (?) and produce an interference pattern.Quantum mechanics once again was successful in explaining the behavior of micro particles. It always have. It explains what we see but it does not explain why we see. If not understood take a look at the video.