an I ask you something?” a young man inquired after learning that I am a physicist. “About quantum mechanics,” he added, shyly. I was all ready to debate the measurement postulate and the pitfalls of multipartite entanglement, but I was not prepared for the question that followed: “A shaman told me that my grandmother is still alive. Because of quantum mechanics. She is just not alive here and now. Is this right?”

As you can tell, I am still thinking about this. The brief answer is, it’s not totally wrong. The long answer will follow in chapter 1, but before I get to the quantum mechanics of deceased grandmothers, I want to tell you why I’m writing this book.

During more than a decade in public outreach, I noticed that physicists are really good at answering questions, but really bad at explaining why anyone should care about their answers. In some research areas, a study’s purpose reveals itself, eventually, in a marketable product. But in the foundations of physics—where I do most of my research—the primary product is knowledge. And all too often, my colleagues and I present this knowledge in ways so abstract that no one understands why we looked for it in the first place.

Not that this is specific to physics. The disconnect between experts and non-experts is so widespread that the sociologist Steve Fuller claims that academics use incomprehensible terminology to keep insights sparse and thereby more valuable. As the American journalist and Pulitzer Prize winner Nicholas Kristof complained, academics encode “insights into turgid prose” and “as a double protection against public consumption, this gobbledygook is then sometimes hidden in obscure journals.”

Case in point: People don’t care much whether quantum mechanics is predictable; they want to know whether their own behavior is predictable. They don’t care much whether black holes destroy information; they want to know what will happen to the collected information of human civilization. They don’t care much whether galactic filaments resemble neuronal networks; they want to know if the universe can think. People are people. Who’d have thought?

Of course, I want to know these things too. But somewhere along my path through academia I learned to avoid asking such questions, not to mention answering them. After all, I’m just a physicist. I’m not competent to speak about consciousness and human behavior and such.

Nevertheless, the young man’s question drove home to me that physicists do know some things, if not about consciousness itself, then about the physical laws that everything in the universe—including you and I and your grandmother—must respect. Not all ideas about life and death and the origin of human existence are compatible with the foundations of physics. That’s knowledge we should not hide in obscure journals using incomprehensible prose.

It’s not just that this knowledge is worth sharing; keeping it to ourselves has consequences. If physicists don’t step forward and explain what physics says about the human condition, others will jump at the opportunity and abuse our cryptic terminology for the promotion of pseudoscience. It’s not a coincidence that quantum entanglement and vacuum energy are go-to explanations of alternative healers, spiritual media, and snake oil sellers. Unless you have a PhD in physics, it’s hard to tell our gobbledygook from any other.

However, my aim here is not merely to expose pseudoscience for what it is. I also want to convey that some spiritual ideas are perfectly compatible with modern physics, and others are, indeed, supported by it. And why not? That physics has something to say about our connection to the universe is not so surprising. Science and religion have the same roots, and still today they tackle some of the same questions: Where do we come from? Where do we go to? How much can we know?

When it comes to these questions, physicists have learned a lot in the past century. Their progress makes clear that the limits of science are not fixed; they move as we learn more about the world. Correspondingly, some belief-based explanations that once aided sense-making and gave comfort we now know to be just wrong. The idea, for example, that certain objects are alive because they are endowed with a special substance (Henri Bergson’s “élan vital”) was entirely compatible with scientific fact two hundred years ago. But it no longer is.

In the foundations of physics today, we deal with the laws of nature that operate on the most fundamental level. Here, too, the knowledge we gained in the past hundred years is now replacing old, belief-based explanations. One of these old explanations is the idea that consciousness requires something more than the interaction of many particles, some kind of magic fairy dust, basically, that endows certain objects with special properties. Like the élan vital, this is an outdated and useless idea that explains nothing. I will get to this in chapter 4, and in chapter 6 I’ll discuss the consequences this has for the existence of free will. Another idea ready for retirement is the belief that our universe is especially suited to the presence of life, the focus of chapter 7.

However, demarcating the current limits of science doesn’t only destroy illusions; it also helps us recognize which beliefs are still compatible with scientific fact. Such beliefs should maybe not be called unscientific but rather ascientific, as Tim Palmer (whom we’ll meet later) aptly remarked: science says nothing about them. One such belief is the origin of our universe. Not only can we not currently explain it, but also it is questionable whether we will ever be able to explain it. It may be one of the ways that science is fundamentally limited. At least that’s what I currently believe. The idea that the universe itself is conscious, I have found to my own surprise, is difficult to rule out entirely (chapter 8). And the jury is still out on whether or not human behavior is predictable (chapter 9).

In brief, this is a book about the big questions that modern physics raises, from the question whether the present moment differs from the past, to the idea that each elementary particle may contain a universe, to the worry that the laws of nature determine our decisions. I cannot, of course, offer final answers. But I want to tell you how much scientists currently know, and also where science crosses over into mere speculation.

I will mostly stick with established theories of nature that are backed up by evidence. All of what I am going to say, therefore, should come with the preamble “as far as we currently know,” meaning that further scientific progress might lead to revision. In some cases, the answer to a question depends on properties of natural laws that we do not yet fully understand, like quantum measurements or the nature of space-time singularities. If so, I will point out how future research could help answer the question. Because I don’t want you to hear just my own opinion, I have added a few interviews. And at the end of the book, you’ll find a brief glossary with definitions of the most important technical terms. Terms in the glossary are marked bold when they first appear in the text hereafter.

Existential Physics is for those who have not forgotten to ask the big questions and are not afraid of the answers.