Young's double slit experiment is a very common term and it explained the quantum world (wave-particle nature of matter). Niels Bohr is a well-known scientist and his work in this field is remarkable. Niels Bohr said “Everything we call real is made of things that cannot be regarded as real. If quantum Mechanics has not profoundly shocked you, you haven’t understood it yet” This statement is true about the understanding of quantum Mechanics. Here is the simple idea of what is act of measurement.

If we randomly shoot objects, let’s say marbles, through a slit than we will see a pattern on the back wall, where the marbles went through the slit and hit. The pattern is in a single line just like the slit. Let’s add another slit and shoot marbles through both slits, we will see another band on the wall similar to the first one. Now, instead of marbles let’s use waves and when waves pass through a single slit they give the same result as with marbles, but when they pass through both slit then instead of two bands on the wall, you will see an interference pattern on the wall (as shown in the picture). With matter like marbles we get two separate patterns on the wall and with waves we get an interference pattern on the wall with many bands.

Interference Pattern of many Bands on the Wall

So let’s move toward the quantum world instead of marbles and waves and we shoot electrons through a single slit. On firing one electron at a time through one slit we will get a single pattern band just like marbles in the first case but when we try to shoot it through two slits an interference pattern of many bands appears on the wall just like waves. An electron can be imagined as a tiny marble so it can only pass through one slit at a time but it doesn’t. How could pieces of matter can create bands like waves? It simply does not make sense at all. The single electron leaves as a particle and becomes a wave of potential, it goes through both slits and interferes with itself to hit the wall. Mathematically, it is a lot stranger. It goes through both slits, it goes through one slit and goes through none at all. There are all the possibilities.

Physicists were very sick of this and they tried to observe that which slit through an electron passes and hits the wall. They put a measuring device by one slit to see which slit through electron passes, but quantum world is far more mysterious than their imagination. When they observed, the electron started to act like a marble, it went through one slit and left only one pattern of band on the wall not an interference band of many. The very act of measuring or observing made the electron to choose one slit to go through not both. On observance the electron decided to act differently just like it was aware of being observed. This was the start of never ending quantum questions. What is matter, wave or particle? If waves, then waves of what? And what does an observer have to do any of this? The observer collapsed the wave function of electron by simply observing it.

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{Here is another article of quantum mechanics- Quantum Entanglement}
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