In the 1800's physicist Thomas Young showed that light had a wave property, since it created an interference pattern like waves in water when it went through two slits in a barrier.
We've created animations of important experiments and presented them in chronological order to help those of us who are non-scientists understand the basic concepts of quantum physics. (During this beta-test phase of building the website, we don't yet have the soundtrack for some of the animations.)
In the early 20th century, Einstein explained that light travels as particles called photons. So it was assumed that the photons were colliding with each other after going through the slits to somehow create the interference pattern (also known as a wave or fringe pattern), which gave an illusion that light was a wave. To test the theory that photons are particles, physicists experimented with firing one photon at a time at the slits. They expected to see a particle pattern on the screen, like they were firing little pellets as illustrated in this animation:
However, the physicists were very surprised to find that the experiment still resulted in an interference pattern. So they put detectors on the slits to see what was happening and got a particle pattern again! And when they removed the detectors, they were back to an interference pattern:
Since measuring / observing the photons caused the wave property to "collapse" to a particle, the phenomena became known as the "measurement problem." It was a problem for a lot of physicists wedded to Newtonian physics because for quantum physicists, the experiment showed that consciousness was involved in the process of creating our reality. As Nobel Prize winning physicist, Eugene P. Wigner said, "...it will remain remarkable, in whatever way our future concepts may develop, that the very study of the external world led to the conclusion that the content of the consciousness is an ultimate reality." To combat that assault on a Newtonian concept of an objective physical universe, opponents claimed that observation of the photons couldn't be causing the wave function to collapse, instead, the detectors must be physically affecting the photons.
Finally, the question of whether the detectors were physically causing the wave function to collapse was settled by experiments like the 1999 Delayed Choice Quantum Eraser experiment. It was performed by a team of physicists led by Dr. Marlan Scully, currently a professor at Princeton, Texas A&M and Baylor universities. (See the article about a recent visit with Dr. Scully.) The experiment showed that the wave property couldn't possibly be collapsed into a particle by some physical effect of the detectors. That's because there were no detectors between the slits and the screen and the "which-path" detection was made after the photons were already registered on the screen.
As quantum physicists have said for almost a hundred years, there is no physical action required to cause quanta to act like particles. Merely having the information about which path a quanta took is the determining factor as to whether the quanta behaves as a probability wave or a particle. In other words, information / data / knowledge is paramount. (Which is one of the reasons that we're interested in physicist Tom Campbell's digital virtual reality model that is featured in our THEORIES section.)
Following is an animation to show how the results were obtained in the Delayed Choice Quantum Eraser Experiment. (If you'd like to see just the full graphic without any explanation, click here.)
Having settled the debate about the measurement problem, the Delayed Choice Quantum Eraser experiment brings up another issue that can be said to be both amazing and very perplexing! It's the appearance that the experiment demonstrates an ability to reach back in time to affect the state of quanta in the present.
To help us understand his experiment, Dr. Scully's colleagues referred us to a two year long debate about retro-causation carried by the American Journal of Physics. The debate resulted in this article in which the author, who had been arguing with Dr. Scully conceded that he had been wrong and that Dr. Scully was right. Unfortunately, we don't completely understand the article, so if you do, please comment with your explanation!
Perhaps the ultimate version of the Double Slit Experiment for showing the involvement of consciousness is the one done by Dr. Dean Radin, who is featured in our Consciousness section. Please go there to learn more about the IONS Double Slit Experiment.
NOTE: We started a project to encourage someone to run the Double Slit experiment in a way that will settle a very important debate about the role of detectors, consciousness and data. Learn more about it on the Projects page.