Down The Rabbit Hole of the Double Slit Experiment | Answers With Joe

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Joe Scott

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Shared October 5, 2015

The Double Slit Experiment started as a way to determine if light is a wave or a particle - but it uncovered mysteries that have baffled science to this day.


This subject is really complicated, so I borrowed footage from these videos. Go check them out because they are fantastic and will help shed further light on the subject (pun intended).

Veritasium Single Photon Interference

Veritasium original double slit experiment

TechLaboratories DIY

Dr. Quantum (from the movie What The Bleep Do We Know?)



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Also, you can find me on Periscope and Blab at @joescottwriter


In the late 1600’s it was pretty much settled science that light traveled as a particle.

There had been many experiments that seemed to back this up and it even had the support of Sir Isaac Newton.

But then a physicist named Thomas Young created the double-slit experiment. The thinking was, if light were traveling as a particle, it would create 2 parallel lines on the background, because the particles that are passing through the two slits would hit the wall behind those two slits.

Instead, he got an interference pattern. This can only happen if the light was traveling as a wave. But other experiments proved light was a particle. Somehow light functioned both as a particle and as a wave.

This was known as the wave-particle duality.

in the 1920's, physicists added another layer to the experiment when they fired photons through one at a time, amazingly still getting the interference pattern.

This proved that the photons were going through both slits at the same time and interfering with themselves.

This is because quantum particles travel in a wave function of potentials, only returning to the particle state when the wave function collapses.

Adding to the weirdness was another extension of the Double Slit Experiment - the Which Way variation.

In this experiment, they placed a tiny detector on one of the slits. Surprisingly, the pattern on the back wall of the experiment was a particle pattern. This means the wave function collapsed before it went through the slits. Why? Because it was being observed.

It was the act of observing the particle that caused its wave form to collapse. This cannot be explained to this day.

Yet another layer of weirdness is the Delayed Choice experiment conducted by John Archibald Wheeler.

In this, he added a second double slit that changes while the photon is in mid-flight. The results showed that by changing the nature of the second slit, it altered the behavior of the particle at the first slit.

In other words, this particle was being altered by things that hadn’t happened yet. Future events can change the present.

When Thomas Young created the double slit experiment, he had no idea the endless stream of mysteries this would create. Who knows what mysteries still await us...