Along with the Michelson–Morley experiment, the double-slit experiment is one of the most ground breaking experiments in the history of modern physics. For many years I’ve seen references to the double-slit experiment in books on quantum mechanics, but there was never enough detail about the experiment; especially about the photon detection methods used. The more I read about the double-slit experiment, the more questions I had. Finally, I discovered Through Two Doors at Once, which gave a complete history of the experiment and answered most of my questions. I learned that the experiment has evolved over time in ways that I never expected. Don’t be fooled by the title, this is a profound subject that touches on philosophy and on the nature of existence. This book is essential for anyone who is interested in quantum mechanics. Also, the narrator has a pleasant voice that is unusually clear and easy to understand for those of us with some degree of hearing loss.