According to various studies conducted by physicists, quantum particle changes its behaviour in a double-slit experiment when it is being observed. Although, we can’t say for sure that whether the particles can be described by its particle behaviour or wave behaviour. That is why measurements are important.
So, let’s begin from the start. What is a double-slit experiment?
Till date, the double-slit experiment is one of the most famous experiments in physics. It reveals that the little particles of matter have something of a wave in them, and when someone observes it, the particle will change its behaviour. Let’s take an example;
Imagine there is a wall with two slits and you throw a tennis ball at the wall. Some of the balls bounce back, and the others travel through the slit. Now imagine there is another wall at the back, the balls that have travelled through the slit will eventually hit the ‘back wall’. If you highlight all those points where the ball has hit the back wall, you will notice that the size of the mark will be the same shape as that of the slits.
What do we conclude from this? In the double-slit experiment, particles like photons will travel through the slit one at a time. If the path is noticed, a photon will pass through one slit or the other, without any interference. However, if no observation is done, a photon will appear to have travelled through the slits together before interfering with itself, resembling a wave.
Back in 1978, John Wheeler, an American theoretical physicist proposed a series of experiments where he questioned that when a particle passes the slit, could it be considered to have the well-defined path through one or more slits. In the experiments, once the photons are emitted, the decision is made, whether to observe the photons or not. This is how the side effects of observing the photons are tested.
Let’s look at an example; what will happen if the decision to close or open one of the slit is made once the particle has dedicated to passing through one or both the slits? If you notice an interference when the back slit is open, this means that our decision to measure the path of the particle affects its previous decision about what path to take, or, to rule out the concept that the position of the particle is defined regardless of our measurements.
So, what do you learn from the double-slit experiment? This experiment proposes that electrons commonly called as particles, combine their characteristics of waves and particles. This is why is it called the wave-particle duality of quantum mechanics. The experiments also suggest that when you observe the movements of the particles, it has an effect on the system.