Particle Vs Wave
To understand what is the Double Slit Experiment ‘the most important experiment in the history of quantum mechanics’, we need to start with the basics. It is important to understand the difference between a particle and a wave. A particle is an object that has a determined place in space and time. On the other hand a wave is able to be in multiple places at once. You are unable to say the exact position of a wave as it can be in any place in a designated area. Think of a rope and a bead threaded into the rope. The bead acts like a particle as you are able to tell its exact position on the rope but when the rope is swung up and down, the wave that is generated can be in any place at a specific time. A wave can also be described as an ocean wave, an ocean wave can be in multiple places at a time and therefore you can not name the location of a wave.
Double Slit Experiment
Bowling BallsThe double slit experiment involves shooting electrons at a projection screen and observing the results. But before we do the experiment with sub-atomic particles we should do it with normal size particles (visible with the naked eye), to see the difference, in this case a bowling ball. To set up the experiment, arrange a wall with two slits; just big enough for the bowling balls to fit through; in front of a projection screen.
Now randomly throw bowling balls towards the slits, most of them will bounce back but the ones that make it through will make a mark on the projection screen right behind the two slits. Depending on the angle at which the balls went through the slits will have an impact at the exact location where the bowling ball landed? All of them will hit directly behind or slightly left or right of the two slits. Two groups of marks will appear if the balls were removed from the projection screen. |
Going Quantum!
According to classical physics the same thing should happen even if sub-atomic particles were shot through the slits. But when this experiment has been conducted with sub-atomic particles like electrons, the results are completely different. Because the electrons are much smaller than the bowling balls we are unable to view them until they hit the projection screen. Once the experiment is complete the result derived is extremely odd, and formulates much of the information we know and don't know about quantum mechanics and the world as we know it.
ElectronsInstead of seeing two groups of electrons behind the two slits, we see a stripe pattern emerging. Vertical lines with the most electrons between the two slits and the rest spread out in line format. The least amount of electrons are the furthest away from the centre.
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WaveNow, the only way to understand this, is to theoretically conduct the same experiment but instead of using electrons or bowling balls we will use a wave. For simplicity, an ocean wave. When a wave hits and passes through the two slits, two individual waves are created. As the two waves interact with each other, they can do one of two things. They can either combine together to make a bigger wave or they can cancel each other out. Because of the angle of the waves when they pass through the slits, we see that the waves cancel each other out. As the waves hit the projection screen, we see the location/s of the wave where it hits strongest. Curiously, the harsh hitting locations of the wave, create a stripe pattern just like the one we received when we shot electrons in the two slits.
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One Explanation?
One of the explanations that physicists came up with included, the fact that as the electrons were shot randomly, so they might have bounced of each other as they passed through the slits to create the stripe pattern. When the experiment was redone they shot the electrons one by one into the slits, to ensure no interference between the electrons as they passed through the slits, yet and result was the same, with stripe patterns emerging.
Understanding the Line Pattern
To understand this line pattern we need to understand how it was made. As the electrons passed through the two slits, it hit every line that was emerging. This might be hard to understand as we have never even imagined one thing, in two places at once. But let me say this, as we observe the particle we only see it in one place as the act of observing collapses all of it other locations. Think about it like this, every place the vertical line is has a percentage probability. For example, in between the two slits the probability of the particle appearing is 50%, one line out, it is 30%, then 20% and so on. When the particle hits the projection screen and before we observe it the particle is in every single vertical line. But when we observe, it the particle uses the probabilities to determine its location, just like rolling dice.
Bizarre Solution?
So the answer is inevitable, the electron was shot as a particle, split into two waves of probability and turned back into a particle as it collided with the projection screen. This conclusion allowed the stripe pattern to emerge as after the electron passed through the slit and before it hit the projection screen it was able to collide with itself to create the stripe pattern.
Final Experiment
Physicists were completely confused about this phenomenon but had a technique that would settle the matter once and for all. They decided to place detection devices behind the two slits, to peek and to actually see which slit the electrons went through. The results discovered by this last experiment plunged classical physics down the drain, and started unraveling some of the mysteries involved with quantum physics. But of course, like any good scientific experiment it answered less than what it questioned.
Detection DevicesWhen the detection devices were implemented into the experiment and electrons were fired at the projection screen, the wave quality or the wave function of the electrons was destroyed. Therefore the stripe pattern that emerged with the waves was also gone, leaving us with two groups of electrons bunched behind the two slits. So basically when we tried to fool quantum mechanics by looking at the result before it reached the projection screen, the electrons stopped changing from a particle to a wave and started acting just like the bowling ball. Another technical way of saying this is "the act of observing the particle collapsed its wave function, destroying the stripe pattern."
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This experiment has proven the every particle has both a particle and wave function or state but will not disclose its hidden function unless forced to.