PBS Space Time Season 2016 Episode 29 The Quantum Experiment That Broke Reality

As one of the newer branches of physics, quantum mechanics can be intimidating to wrap your head around. Yet it’s undeniably powerful in terms of its explanatory power and technology. From computers to cryptography, there are few areas of modern life that haven’t been impacted by advances in quantum mechanics.

But just because we can apply quantum mechanics in these ways doesn’t mean that we fully understand it. In fact, the deeper scientists have dug into the quantum world, the more mysterious and otherworldly it has seemed. This episode of PBS Space Time delves into one particularly fascinating example of this: the quantum experiment that broke reality.

This experiment goes by many names, including the double-slit experiment and the wave-particle duality experiment. Its essence involves firing a stream of particles (usually electrons or photons) through two slits in a barrier, and watching how they interact with a screen on the other side. The screen ends up showing an interference pattern, as if the particles were behaving as waves that had diffracted through both slits, rather than just two concentrated streams that had been shielded by the barrier.

This aspect of the experiment alone challenges our understanding of particles and waves, but it’s only the beginning. Other variations of the experiment have produced even more confounding results. For example, if a detector is placed near one of the slits, which should allow scientists to determine which slit the particle actually went through, the interference pattern disappears and the particle behaves as a straightforward particle instead of a wave. How can this be? How does the particle “know” whether or not it’s being observed?

This is where interpretations of quantum mechanics come into play. There’s no one widely accepted way to explain what’s going on in quantum mechanics, and opinions range from the Copenhagen interpretation (which posits that particles don’t have definite properties until they’re observed), to the many-worlds interpretation (which suggests that every possible outcome of a quantum event happens in different universes), to the transactional interpretation (which argues that quantum effects aren’t actually weird, but merely the result of backwards and forwards causation).

All of these interpretations and more come into play as the episode explores the implications of the double-slit experiment. The implications are not just theoretical, either. As scientists continue to push the boundaries of quantum mechanics in laboratories, they’re bumping up against reality-breaking phenomena. For instance, the famed physicist Erwin Schrödinger proposed a thought experiment in which a cat was trapped in a box with a vial of poison that would be released if a quantum event went one way, and would remain sealed if the event went another. According to quantum mechanics, the cat would be both alive and dead until the box was opened and observed.

All of this might sound like philosophical noodling, but it’s not. Scientists who are working on building quantum computers, for example, must grapple with how to deal with the fact that the simple act of measuring a qubit (the basic unit of quantum computing) will irrevocably alter it. Understanding the implications of quantum mechanics becomes more than just a matter of intellectual curiosity – it becomes essential for driving technological progress.

Still, as the episode makes clear, understanding the quantum world is no easy task, and scientists are still struggling to piece together what’s going on beneath the surface. The concepts involved are by nature abstract and paradoxical, the results of quantum experiments often confounding. But even if we can’t quite get a handle on the realities of the quantum world, that doesn’t mean we can’t continue to explore, experiment, and discover. With each new piece of data, perhaps we can inch ever closer to unlocking the mysteries at the heart of quantum mechanics.