Paul Halpern is a professor of physics at Saint Joseph’s University and the author of eighteen popular science books, including Flashes of Creation, The Quantum Labyrinth, Einstein’s Dice and Schrodinger’s Cat, and Synchronicity. He is the recipient of a Guggenheim Fellowship and is a Fellow of the American Physical Society.
Below, Paul shares five key insights from his new book, The Allure of the Multiverse: Extra Dimensions, Other Worlds, and Parallel Universes. Listen to the audio version—read by Paul himself—in the Next Big Idea App.
1. Curiosity drives us to think outside the box of observability.
“Pics or it didn’t happen,” goes the popular phrase. In our modern technological age, with cameras virtually everywhere, we long for pictorial proof of everything. Hence our fascination with the extraordinary photos taken by the James Webb Space Telescope of galaxies from the nascent years of the universe after the Big Bang. If only our instruments could pick up signals from all of space, we could systematically map it and perhaps develop a testable theory of everything. Many people who reject multiverse ideas argue that everything in science must be fully verifiable.
Complete measurability would be ideal, but it’s not going to happen. For one thing, the speed of light sets a strict limit on the extent of what we can detect. It divides an enclave called the observable universe from potential regions beyond that. The actual universe could be infinite, but only a finite region is directly detectable. That doesn’t stop us from wondering what is beyond those limits. In the absence of direct proof, indirect evidence could be very convincing.
In a similar way that volcanoes reveal Earth’s innards, perhaps effects on the observable universe might reveal aspects of the rest of it, including the possibility of other expanding universes with a greater multiverse. When we are in the higher floors of a stable high-rise building, we presume that it has a sturdy foundation beneath it, without having to visit that bedrock. Similarly for the cosmos, perhaps indirect evidence might suffice when direct observation is impossible.
2. Quantum reality is like an infinite hotel with a curious system of keys.
Quantum physics offers even more frustration to those aspiring for complete tangibility and testability in science. Compared to classical mechanics, in the quantum realm, physical observables such as position and velocity are no longer fundamental. Rather, as the brilliant scientist John von Neumann emphasized, what is key to the theory is the evolution of quantum states in an abstract domain of unlimited dimensions called Hilbert Space.
“In the absence of direct proof, indirect evidence could be very convincing.”
Mathematician David Hilbert, after whom that term was coined, reportedly used the analogy of a hotel with an infinite number of rooms as a way of grappling with the concept of infinity. The expression of quantum systems, according to von Neumann, depends on the choice of what an experimenter is trying to measure. In Hilbert’s Hotel, it is like offering guests a key to just one of the hallways that forbids them from exploring others. Then within that chosen hallway, they are randomly assigned a room. Similarly, in quantum systems, after the experimenter observes a certain physical parameter, such as position, the state collapses with a certain probability into one of its values. One might wonder why an observer’s choice should matter so much.
In the mid-1950s, young physicist Hugh Everett III developed one of the first scientific multiverse models to eliminate the role of observers. Instead of quantum states collapsing, they maintain all possibilities indefinitely. The catch is that observers persist in a medley of possibilities as well, with their consciousness bifurcating along parallel strands of reality. Such a notion, dubbed the Many Worlds Interpretation of quantum mechanics, has remained controversial since its inception.
3. Inflating space seems as straightforward as blowing bubbles.
Another type of multiverse model stems from a popular explanation of why the observable universe is so smooth and regular, called the inflationary model. According to that scenario, the presently detectable part of the cosmos passed through an ultrafast era of exponential expansion very soon after the Big Bang. The effect was like quickly stretching a wrinkled sheet in all directions, making it flat and uniform. Inflation thereby explains why, on the largest scale, the density of matter and energy has a remarkable evenness.
Physicist Andrei Linde developed a successful mechanism for explaining how in the intense conditions of the early universe, quantum fluctuations of energy fields might readily cause sectors of space to expand rapidly. However, as he soon discovered, triggering inflation is not a solitary phenomenon. Rather, it is extremely simple to generate, implying that it has happened very often and keeps happening—a situation called eternal inflation. Therefore, instead of imagining a single Big Bang, it is helpful to envision the primordial universe as a frothy bubble bath. One of those bubbles grew into our universe, but there might well be other universes out there. Astrophysicists are in an ongoing search for evidence of collisions between our universe and a neighboring one.
4. Telescopes and calculations reveal the observable universe’s weirdness.
Cosmologists’ understanding of the universe is based on Einstein’s masterful general theory of relativity. In an early version of that model, before researchers discovered cosmic expansion, Einstein introduced a stabilizing factor, called the cosmological constant. It acts to provide a kind of continuous outward pressure. In 1998, researchers found that not only is the universe growing, but it is also accelerating in its expansion. A tiny cosmological constant nicely models that accelerated growth. Around the same time, theorists found a way of calculating what the cosmological constant would be if it were caused by the energy of quantum fields emerging from the vacuum of the universe and found an enormously higher value. Why should the universe have a small detected cosmological constant if theory predicts a large one?
“In 1998, researchers found that not only is the universe growing, but it is also accelerating in its expansion.”
In the Anthropic Principle, proposed in the 1970s by Brandon Carter, conditions in the universe are limited by the fact they must end up producing intelligent observers; otherwise, we wouldn’t be here. One way of thinking about that idea is imagining a multiverse housing numerous alternative parallel universes. Of those, only a tiny percentage have the right factors to create stars, planets, and life, including a relatively small cosmological constant. Thus, according to that view, the cosmological constant is tiny in our universe because we are here. In most other universes, it is extremely large.
5. Meeting our alternative selves is not how science treats the multiverse.
What we don’t find in science, even in speculative fields, is the popular notion that each of us has near doubles with distinct personalities in other branches of reality. We can’t suddenly walk through a portal and meet our doppelgangers. While it is fun to imagine a team of versions of Spider-Man or Batman from other universes teaming up to defeat villains, such encounters have no basis in physics—even at its wildest.
According to theorists, true wormholes connecting our universe with others would require the mass of a galaxy and would obliterate Earth if created in its vicinity. No matter; they have become essential in cinema as plot devices enabling weird encounters with alternative histories. Scientific and cultural depictions stimulate each other but remain distinct.
To listen to the audio version read by author Paul Halpern, download the Next Big Idea App today:
