Double slit experiment interpretation

Part 1 - Description of the Double Slit Experiment:

Introduction

The double slit experiment displays curious results which appear to contradict what we understand about reality. It seems to question causality and suggest that time can be rewritten and that something can be both a particle and a wave at the same time. The effect was first discovered in 1930 and as technology has improved over time, different and more complex variations of the experiment have been devised to try to explain the contradictions. But these have simply added to the contradictions and have also confirmed the results as not being erroneous by making ever more accurate experiments over greater distances to rule out any other improbable but not impossible explanations.

The variants of the double slit experiments are very well explained in several very good videos from PBS Spacetime on YouTube. They need to be viewed in order for them to make sense.

1- The Quantum Experiment that Broke Reality | Space Time | PBS Digital Studios

2- How the Quantum Eraser Rewrites the Past | Space Time | PBS Digital Studios

3- Quantum Eraser Lottery Challenge | Space Time | PBS Digital Studios

4- Quantum Eraser Answer | Space Time | PBS Digital Studios

Here is a very brief thumbnail description. Skip forward to Interpretation if you are already familiar with the experiments.

The Experiments:

As technology improved experiments were devised so that the photons could be sent through the slits one single photon at a time. They do arrive one at a time, being detected on the screen as single dots which at first seems to confirm the particle concept, but as more photons arrive they built up in a diffraction pattern again. (Diagram below) So the puzzle is not solved and the observations just become more strange. The light seems to be arriving as a particle but with the interference of a wave, despite only being a single particle with no other particle to cause interference.

Credit: Wikpedia

A laser shines a beam towards a blanking plate that has two very narrow slits. The resulting light pattern on a screen on the other side of the plate shows up as a diffraction pattern. This is interpreted as the result of interference between the two streams emanating from the two slits and would seem to prove that light is a wave. This is the first puzzle because the problem here is that light has also been proved to be a ‘photon’ particle from experiments on the photoelectric effect. If light consists of particles, then there should be just two slit shaped patterns on the screen.

Credit: Wikipedia

To try to understand what was happening experiments were designed to try to detect which slit the photon passed through but as soon as any photon is detected at a particular slit, it no longer forms a diffraction pattern, but just falls on the screen as if it has passed through the single slit it was detected at. This seemed to suggest that the act of observation was altering the result of the experiment.

Credit: Wikipedia -Thierry Dugnolle

Another technical improvement to the experiment in 1999 used crystals that can split the light into two beams of entangled particles. These are particles that have mirrored properties while being spatially separated. A crystal is placed after the slits so the beam can be split and sent to both the screen and also to detectors at a greater distance away. This added even more to the puzzlement because any detection of the particle at the later location still caused the diffraction pattern to collapse. This suggested that the action of detection even though at a later time and further away in space, (the delayed choice) was altering the diffraction pattern. To add to the confusion, if detectors are arranged so they can detect the particle, but not know which path it took, (erasing the information) the interference pattern is maintained.

Later experiments on entanglement using improved technology have even ruled out the possibility of any sort of signal being fed back at the speed of light. This was done by measuring the spin of the entangled particle and also that of its pair, in a shorter time than any signal could have travelled between them. Other experiments, first designed by John Bell in 1964, but only carried out in 2015 when technology was advanced enough, proved that the difference seen between the entangled particles was a genuine effect and not simply a measure of ‘handedness’ preset in the entangled particles. The effect of entanglement is apparently instantaneous and in theory the later detection could be at any distance in the universe. It should be noted though, that although entanglement seems to have instant effects over any distance it doesn’t actually break causality as no information can be transmitted. That is because the knowledge of the state of the distant detector cannot be known at the emitter or the screen. So any information contained by the pattern on the screen can only be inferred by knowing the state of the distant detector and that information can only travel at the speed of light.

Quantum Entanglement & Spooky Action at a Distance - John Bell's Experiment - Veritasium

Cosmic Quantum Bell Test - Physics Girl