The Alan Machine
Preface
and for Alan Ricardo.
How to read this book
This book is about a machine that does not exist. We call it Alan, and we use it to ask a question physics takes very seriously: can entropy be reversed, and if it could, what would happen to time?
You do not need a physics degree to read it. The chapters are written for anyone who is curious. When the math helps, it shows up in boxes marked Going deeper, which you can open or skip. The story works either way.
Some chapters have a companion paper, written for specialists, with the full argument. You will find the papers near the end of the book, and each chapter tells you when one exists.
Each part ends with an interlude: a short pause for a question that physics can inform but not answer, such as whether time really flows or whether a machine can think. After the main text, Building Alan looks at real technology, from today’s chips to quantum computers, and asks how far each one is from the limits the chapters describe. Those pages are updated every year.
Along the way, we keep five kinds of statement apart. Unmarked claims are established physics, with references you can check. A claim that rests on a newer source, such as a preprint or a manufacturer’s figure, says so in words: it is reported, not yet established. Boxes marked Extrapolation follow from established physics, but only if their stated assumptions hold. Boxes marked Speculation hold ideas that are conceivable but that no evidence supports yet. Boxes marked Philosophy hold questions that physics cannot settle, with the best arguments on each side. A thought experiment is only useful if you always know which of the five you are reading.
Every technical term links to its plain-language definition in the Concepts appendix at the end.