Uncertainty principle
Bohr and his colleagues argued that photons, electrons, etc. do not have either position or momentum until they are measured. This theoretical position grew out of the discovery of uncertainty, and was not just some personal preference on what to believe. Bohr said that we know nothing about something like a photon or electron until we observe it. In order to observe such a small thing we need to interact with it somehow. In everyday life it is possible to do something like walking alongside an automobile while marking down the times it crosses points on a grid drawn on the pavement. Perhaps the weight of the automobile itself will depress little levers in the pavement that turn off clocks attached to each of them and record the automobile's weight. In the end we would have a clear record of where the car was at various times, and also could compute its direction of progress and weight. We could then know, at any time on the clock, both its position and its momentum (its velocity multiplied by its mass). We would not even imagine that the force required to move the little levers would have any influence on the progress of the car. We would also not imagine that the automobile had no location or trajectory between the points on the pavement where there are levers, or that the car exists in a kind of three-dimensional blur during those times and only settles down while it is depressing a lever. The world that we are familiar with does not reveal these strange kinds of interactions.