Deeply Hidden

Something Deeply Hidden by Sean Carroll

Disclosure: Advance reader copy made available via NetGalley in exchange for an honest review.

Something Deeply Hidden:

Quantum Worlds and the Emergence of Spacetime

by Sean Carroll

Physics necessarily conducts rather reductive science. Psychology emerges from biology, which in turn emanates from chemistry, which itself functions as physics writ large. Some physicists find it possible (and desirable) to explain this emergent narrative to a wide, non-specialist audience, a formidable task without employing the underlying numbers and equations.

The issue at hand concerns the nature of the world itself — world qua everything that exists, the universe and everything in it, the cosmos. Thinkers before Plato tried to determine the fundamental components of our world, and the urge to build upon this understanding with greater and greater specificity has continued to this day, creating a sort of physical refinery within the natural sciences.

At least since physics laid claim to the atom, scientists have been searching for ever more underlying substances and/or principles at work. The thinking goes that these more fundamental constituents are what really exists, while the rest simply emerges from these materials.

A computer or television monitor produces images made up of dots that combine to form a picture we see on the screen. The video then is “really” just a series of dots in a certain configuration.

The same holds true for physicists at the forefront of modern science. In fact, such has been the case for at least the last century, perhaps much longer, depending on one’s viewpoint. As far back as the 1800’s, John Dalton used the notion of atoms to explain certain features of elemental reaction. Today, physicists work largely in the realm of quantum mechanics, but the principles remain the same: to determine ever-deeper constituents of space, time, and matter.

Sean Carroll, of the California Institute of Technology, has written numerous books for lay audiences on these and other subjects from a physicist’s perspective. He writes clearly and with an essential dose of humility, an element glaringly missing in some treatments of the subject. Carroll sets out in Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime (Dutton, 2019) to explain the current state of quantum physics for interested non-physicists, as well as to elucidate some of the more speculative ideas that arise when moving from experimentation to  theoretical world-building.

While all books in the “quantum” category deal with a related batch of subjects, most of them attempt to answer or provide solutions to the tricky matter of measurement, which is inextricable from that locality and entanglement — or why particles seem connected even lightyears apart from each other. What Heisenberg and Bohr argued in Copenhagen about the collapse of the wave-function came to be accepted as Truth. This interpretation of the quantum problem of measurement does fit the math in many ways, but it fails to explain much else about the universe that further experimentation and equation-solving have brought to light. As Carroll himself writes,

“What exactly a measurement is, and what happens when we measure something, and what this all tells us about what’s really happening behind the scenes: together, these questions constitute what’s called the measurement problem of quantum mechanics. There is absolutely no consensus within physics or philosophy on how to solve the measurement problem, although there are a number of promising ideas.”

For his part, Carroll is a proponent of one promising idea called the Many Worlds Theory, an interpretation of quantum mechanics stating that each time someone makes a quantum measurement a new world branches off from the current timeline. If you’re like me, this explanation might strike you as a clear violation of Occam’s razor. It seems to introduce far more complexity than ought to be warranted. Carroll isn’t blind to this: “[A] proliferating tree of multiple universes...is arguably not very simple at all.”

Despite this, throughout most of the book, he propounds the Many Worlds Theory (also called Everettian, after the theory’s founder). He goes to great lengths not only to explain the theory in great detail, but to lay bare the difficulties this claim makes, as well as the problematic nature of any of the many theories attempting to explain the perennially thorny measurement problem in relation to the whole of the cosmos and its foundational aspects.

Ultimately, what we discover is that the Many Worlds Theory actually, at least in Carroll’s explanation, provides arguably the simplest interpretation of quantum mechanics. If nothing else, this ought to impress upon the astute reader how tricky the problem of measurement actually is. On the face of it, the very notion that a new “world” splits off each time someone takes a quantum measurement is anything but simple or straightforward. But, again and again, Carroll takes great pains to explain how this is almost certainly the case, amplifying the magnitude of uncertainty involved in this particular scientific and theoretical endeavor.

As we know, Newton laid out the foundations of classical mechanics and calculus, which explain to a high degree of accuracy most of what we experience in the world on a macroscopic scale. By the early 20th century, however, physicists like Heisenberg and Bohr, Schrodinger and Einstein, and many others, turned this idea on its head. While large objects obey the laws of classical mechanics, the world isn’t really classical: it’s quantum.

In other words, quantum mechanics describes everything we experience in the world, too, only to a much higher level of precision than classical, Newtonian physics. Carroll writes that the world is, in fact, quantum in every way, from top to bottom, even if we don’t always experience it as such. This notion is so antithetical to intuition that it’s hard to grasp, even for the physicists themselves; hence, the ever-present measurement problem.

Carroll’s explanation — in short — goes as follows: the universe and everything in it can be explained by something fundamental, namely, the Schrodinger equation as it describes the evolution of the wave function. He writes: “Schrodginer’s wave function works; you have a single function that depends on the possible positions of all the particles in the universe.” Sounds a lot like classical mechanics. Except he goes on: “It’s that simple shift that leads to the world-altering phenomenon of quantum entanglement.” And just like that, the world ceases to be Newtonian.

Most are familiar with Schrodinger through his famous cat experiment, in which a box contains one cat that is somehow both alive and dead, at least before we open the box to measure it. As soon as we measure, the cat is either alive or dead. In the Copenhagen interpretation, the measurement is what — more or less — causes the cat to be in that state. Something mysterious seems to be going on, there’s no doubt about that.

But Carroll says that the Everettian Many Worlds Theory solves this problem in a novel way. Instead of measurement causing the cat (standing in for the “wave function”) to be alive or dead (standing in for “collapse of the wave function”), measurement actually produces two distinct timelines, one in which the cat is alive and another in which the cat is dead. According to Carroll, while it’s far from certain or accepted in the physics community as a whole, much of the math makes this at least a little difficult to dispute.

Of course, my short appraisal here does not do justice to the theory (or include any of the equations). There is much more to be said on entanglement at great distances, black holes and their horizons, entropy, the emergence of space-time, and perhaps the biggest bugbear of all, gravity. Carroll makes each of these topics readily understandable to the reader willing to focus on the task. This does not mean they are easily understood, of course, only that Carroll’s text works admirably to make it so.

While we may not have a complete and full understanding of the universe, physicists labor away, crunching the numbers and building ever-larger particle colliders in the hopes that one day, quantum mechanics will include everything we are able to measure in the world without any spooky action at a distance. Either way, the search continues, and Carroll’s new book is a phenomenal place to get an in-depth look at the cutting edge, as well as all the progress made so far.