With the coming into view of a quantum mechanical picture of matter and of all that exists, many of the traditional ways in which physicists were thinking about nature had to change. In the pre-quantum world, Newton thought of objects as being made of particles and the particles had mass, they had size, they had motion. Then they had other attributes that later were added on, like electric charge. So all of these could be measured, so that in a Newtonian picture of physics, everything was made up of particles and the particles could all be measured, every aspect of them could be measured so that the object or the matter was identical to its observables. Everything you could think about, you could measure. Quantum mechanics changed that in a profound way. Quantum mechanics began to show us that whereas matter obviously exists, the things that are available for us to know about it are limited. For example, the position and the velocity of an object are not simultaneously knowable with arbitrary precision in the same direction. So in these ways, quantum mechanics gave us a sense of humility about our level of knowledge. Matter exists but what it presents to us is limited. There is only so much we can know about it. But there has been a very long-standing and in fact ongoing question in the field of quantum foundations as to what is actually going on? What is really happening to matter so that it behaves in the ways that we see? For example, there is this simple experiment that people often use to describe quantum mechanics. They talk about a double-slit experiment. In a double-slit experiment, you consider a situation where you have a source of particles. You have a wall, in the wall are let’s say two holes, you close one hole, you look back behind the wall and you have a detector. And from this source, you shoot off let’s say bullets. Eventually on this back surface, you see a pile up of incidents of hits. When you look at that distribution, you see a hump. Now you do the experiment again, you open this hole and close the other hole. Now from this one you do the same thing and you see the same thing. It’s just another distribution. Now the strange thing is that when you open both holes, you expect to see just the sum of the two distributions, in other words, they would add up together. But in fact you see a pattern that looks like an interference, that looks as if a wave went through it. In some places, what you see is less with both holes open than with one hole open. How is it that I open the second hole and some go away here? And this was the first hint that something very mysterious, something very strange was happening; that there seems to be a wave nature involved as well. Now the question is how do we then have a coherent, consistent ontology, which means that description of existence that is consistent with both particle nature and wave nature? And that’s been a subject of great debate in quantum mechanics. I would say at this point, there is not a definite answer and this is an area which is still needing a lot of work. Whereas we know enough about the quantum world to predict the results of experiments extremely well, we don't have a clear understanding of the underlying things that are going on.
