Thomas Young was a notable British polymath who lived around the turn of the 19th century. His famous contribution was his double-slit experiment. While I myself am no physicist, the gist of this original 1804 experiment was that he fired a light at a board with two slits in it and noted how the light hit a solid board behind the first board. If light were just made of particles (which it is: photons) then you would expect to just see two solid strips of light on the second board where photons passed through the slits and hit the board behind. What he found, instead, was an interference pattern implying that light was actually a wave.
But it is also a particle. This was the beginning of the idea of wave-particle duality or the concept that every particle can be described either as a particle or a wave.
As our technology has gotten better and the scientific questions we ask have gotten tougher, hundreds of versions of the original double-slit experiment have been performed and, in many ways, form some of the basis of quantum mechanics. Not only do we know that photons behave as both particles and waves, but we know that electrons and atoms of solid matter also behave as both things.
Around 2007, Stefano Frabboni and his team ran the double-slit experiment with an electron beam that they were able to reduce to such a low frequency that they could essentially fire a single electron at the double-slit board. That they found interference on the back board is nothing short of mind-blowing. Wave interference occurs when you have waves coming from multiple sources running into each other, like the wakes from two different ski boats on the same lake. The only reason they could have measured interference in this instance is if the electron passed through both slits simultaneously. Which, of course, the electron could do as a wave but not as a particle.
Mind-blowing.
The implication that particles of matter can exist in multiple states until measured is utterly mind-blowing.
You, I'm sure, are aware of the Schrödinger's Cat thought experiment. It's a fairly gruesome thought experiment that expresses this idea of superposition, or the ability for a particle or quantum system (or cat, I suppose) to exist in multiple states until measured. The idea is that the cat is both alive and dead until you open the box and it must be one thing or the other.
I read a book called The Holographic Universe (Michael Talbot, 1991) probably 18 years ago that first introduced me to a handful of these quantum physics concepts. It's definitely a pop physics book and I highly recommend it to anyone who is interested in these types of theories but doesn't have the math background to be a research physicist.
One of the concepts he throws out in the book is the idea of the universe being this spinning, fiery quantum soup right behind your back until the moment you turn around to observe it. Then it instantly must choose a state, solidifying into a beige living room with a coffee table (or whatever). To this day I catch myself trying to catch the quantum soup right on the periphery of my vision.
Physicists continue to struggle with these questions. Is there even such a thing as objective reality? Hard to know. The nature of this question is almost impossible. How do we observe the unobserved? Is the cat alive or dead?
You have to open the box to find out.
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