Today, I am going to write about one of the fascinating experimental findings of the contemporary world. An experiment, that has led us to ever-more fascinating Quantum physics. Richard Feynman regarded it as the "Central Mystery" of quantum mechanics.
A Little Back Story
Before Thomas Young came up with the Double-Slit experiment light was thought to be a particle, even Newton considered light to be made up of particles. The quanta of light consist of small particle called Photons.
What is Double Slit Experiment?
Light from a single light source is made to pass through a pair of slits. Each slit, diffracting the light into individual sources of light. A screen is placed at the other end to receive the light. The light impacts observation screen. A diffraction pattern is observed on the screen.
Interpretation of diffraction
Wave: If light exists as waves, the light waves will have interference under the principle of superposition, creating bands of light (constructive interference) and dark (destructive interference).
It is inferred by the experiment that light behaves like both wave and particles.
Double Slit Experiment with One Photon at a Time
So far light's particles i.e. Photons has collectively acted as both wave and particles. What if we let only one particle of photon cross at a time? Calibrate apparatus so it let only one Photon enter at a time. The Idea is that if there is one Photon passing it will pass through one of the slits at a time and skip the other one. So we are letting a stream of particles pass through, at a rate of one Photon per second, and recording the position of impact on the screen by particles. At first, the Impacts appear to be random but as time passes, the same diffraction pattern can be seen as was observed by the light source.
Here is where is starts to get crazy, In order to create an interference pattern there has to be something to interfere with. The Idea is that, although a single particle is getting through the slits it co-exists in probability state and passed through the both slits because we do not know which slit it passes through. It is like Schrodinger's cat being both dead and alive.
This is the fundamental truth about Quantum Mechanics, quantum objects can exist in a probability state. I.e. at any given time a single photon could go through the left slit or the right slit of bounce back hitting the barrier. While we do not know its state it can be in all the possible states as per the probability of the existence of that state. It is 33% through the left slit, 33% through the right slit and 33% hitting the barrier. However, this multiple states of co-existing positions have to go back to the particle form. In this experiment, it occurs when the particle or photon hits the screen.
This experiment has been conducted with different particles; electron, neutron, even atoms, and molecules. All get the same result. Fundamental particles in the quantum mechanics do exist in multiple states.
When scientists noticed the diffraction pattern appear as per wave-particle duality even if there is just one particle per unit time they decided to record the particle's probability state of and measure the accuracy of it.
In order to do so, they set up a detector on one of the slit to measure it. This was called the Which-way experiment. The result with the detector is shocking. This time, photons behaved like a particle.
Two lines, two slits with no interference and thus no wave. What just happened! The experiment was conducted again with detectors being there but turned off and this time interference pattern reappeared.
By detecting, the result of the experiment changes drastically. The wave-form collapse before it entered the slit. At this point, the photons acted exactly as a particle would behave. It is bizarre that somehow the act of observing of the photon collapsed the waveform. How was the Photon "aware" of being observe? Somehow it goes back to the origin of quantum physics and the Heisenberg's uncertainty principle.
These are some very bizarre series of a puzzle that we have no answer to yet. How does conscious observation change nature of matter? The truth is we do not know!
PS It gets weirder still ... John Archibald Wheeler proposed another kind of Double-Slit experiment called Wheeler's delayed choice experiment. This experiment indicates that the nature of particles still changes with respect to things that haven't even happened yet. But for the sake of simplicity and my limited knowledge let us leave it for some other day...
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