As quantum mechanics was developed in the 1920s and 1930s its statistical nature became more evident. Einstein was convinced that there were variables, that could not be seen, but that actually controlled the outcomes. Einstein conducted many debates with Niels Bohr and Max Born about the statistical, or probabilistic, interpretation of quantum mechanics. In 1935 Einstein, Boris Podolsky, and Nathan Rosen wrote a famous paper that explored what they considered to be the incomplete description of the world provided by quantum mechanics. They proposed an experiment that has now been done and confirmed the predictions of quantum mechanics.

In a 1926 letter to Max Born, Einstein wrote: “Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the ‘old one’. I, at any rate, am convinced that He does not throw dice.”

When a laser is sent through a microscope it creates a tiny spot of very intense light in the material on the slide. A tiny plastic sphere will interact with the light in such a way that it is pulled into the center of the light. The light can be moved around the slide, dragging the sphere with it. That is the essence of a “laser tweezer.” The sphere can be chemically attached to the end of a long molecule, such as DNA. When the other end of the DNA is similarly fastened to the surface of the slide, the sphere can be used to stretch the DNA, straightening it out, and measuring properties such as the force needed to stretch it. Proteins, enzymes, and other polymers can be used in place of the DNA and the forces they exert similarly measured. In addition, the tweezers can be used to sort cells, moving them to specific locations on the slide.

We’ll explore other mysterious consequences of quantum mechanics in the last chapter, “Unanswered Questions.”