In 1678, Christiaan Huygens gave a famous speech in France, which triggered a famous debate that lasted for more than 300 years, “What is light?”.
In the end, wave–particle duality was recognized by most people, not only for light, but also for all particles in nature.
In the journey of human beings to explore what light is, Thomas Young devised the Double-Slit experiment to demonstrate the wave nature of light. Sometime in the first decade of the 1800s, Young performed his initial double-slit experiment with light, demonstrating how the interference of the light waves from the two slits led to the creation of a distinctive fringe pattern on a screen.
Geoffrey Ingram (GI) Taylor carried out an Double-Slit experiment in 1909 in which he demonstrated that interference fringes may be produced by even the weakest light source, which is comparable to “a candle burning at a distance just over a mile.” It is expected that this is like shooting out photons one by one, and there should be no interference fringes. Interference is the interaction of two waves. If only one photon passes through the slit at a time, who can it interfere with?
Since a photon is maybe just a concept, it is still possible to pass through the double slit in the form of a wave, is it still possible to have interference fringes?
German physicist Claus Jönsson used electrons instead of photons for a double-slit experiment in 1961. Electrons have mass and are out-and-out particles, but the result is still bright and dark interference fringes.
In 1974, several Italian physicists managed to emit only one electron at a time, and interference fringes still appeared. Is it possible that electrons can split into two parts and pass through the double slits, otherwise who would it interact with?
To understand this, scientists have come up with an idea. Can an observation instrument be added between the double-slit plate and the detection plate to see whether each photon/electron passes through from the left or the right. The idea was simple and straightforward, but the experimental results were unexpected.
Every time you look at the photon/electron, the interference fringes disappear. That is, when you look at the photon/electron, the photon/electron behaves like an individual particle. When you don't observe it, it behaves like a wave.
Can a photon/electron know if someone is watching it?
The scientists went a step further and did more experiments. Experiment after experiment brought more incredible results. I will continue this journey of discovery in follow-up articles. Teleportation, time travel, changing history, and predicting the future may not be the imagination of science fiction movies. These are what quantum mechanics will bring.
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.
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