READ MORE: Evil doppelgängers, alternate timelines and infinite possibilities: the physics of the multiverse explained (Science Focus)
The multiverse has become part of mainstream culture with series like Loki and the hit indie film Everything Everywhere All At Once, but many physicists believe that multiverses could actually exist.
After all, it wasn’t too far back in history that we understood the world to be flat, or that the earth was the center of the universe, or that earth was the only planet. The more we learn about the outer limits of deep space through telescope probes like the James Webb, the more likely it seems that ours is far from the only universe.
Here are some of the leading multiverse theories, as compiled by freelance science journalist Robert Lea for Science Focus.
The Cosmological Inflating Multiverse
The basic idea is that our Universe is one particular patch of space-time that is homogeneous, the same in all directions and expanding in a well-defined manner. If you trace the evolution backward to the Big Bang, then you find an age for the Universe of about 13.8 billion years.
Physicists believe that other regions of the multiverse could be experiencing their own Big Bangs, and therefore their own expansions. This means that they are not able to aﬀect our Universe.
“That means some of these universes could have laws of physics that aren’t fit for the formation of large-scale structures like galaxies or stars. They may not even have the same fundamental particles.
Consequently, these universes aren’t variations of our universe and thus could not host any life at all, never mind some version of you or I.”
The String Theory Multiverse
String theory is a suggestion put forward by physicists to connect quantum mechanics and General Relativity. The underlying idea is that fundamental particles like quarks and electrons are actually a single point in one-dimensional strings, vibrating at diﬀerent frequencies.
“In order to be mathematically sound, string theory needs ‘extra dimensions’ to exist. These aren’t parallel dimensions like we see in science fiction. Instead, string theorists believe these extra dimensions are curled up within the three traditional dimensions of space. They remain invisible to us, as we evolved only to see in three dimensions.”
These extra dimensions could oﬀer a “way in” to the string theory multiverse.
However, so far the evidence is theoretical, not experimental. And, unfortunately, says Lea, we just cannot do any direct experiments to verify or falsify what goes on in other universes.
“Our inability to test these ideas is a double-edged sword. While the lack of ways to test a multiverse means we can’t prove its existence, it also means we can’t disprove it either.”
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The Black Hole Multiverse
Einstein’s General Theory of Relativity tells us that a large mass can curve space-time. The theory also says that the heart of a black hole has a singularity where the mass is so great that the space-time curvature becomes infinite and, consequently, the laws of physics break down. This is a concept that troubles physicists, but one hypothesis could do away with the singularity and replace it with an entire universe and in turn, a multiverse.
“Singularities are unphysical because they cannot be measured. That means their existence indicates that a theory is incomplete,” says theoretical physicist Dr. Nikodem Poplawski, from the University of New Haven. “In my hypothesis, every black hole produces a new, baby universe inside — on the other side of the event horizon — and becomes an Einstein-Rosen bridge, also known as a wormhole, that connects this infant universe to the parent universe in which the black hole exists.”
“These infant universes would be hidden from the occupants of their parent universe by the light-trapping surface of the event horizon, and once that event horizon is crossed there’s no going back,” Lea writes. “That, and the fact nothing can enter a white hole (which is still purely theoretical but allowed by General Relativity), means no interaction between parent and infant.”
However, if two black holes existed in the same universe, and each of these black holes created a new universe, then there is a possibility that these two sibling universes could merge, “just as two black holes merge to create one black hole,” says Poplawski.
As for the possibility of an alternate version of you existing beyond the event horizon of a black hole, Poplawski concludes that chances are not good. “There would be no ‘alternate you.’ At any time, an object can only exist in one universe,” he says.
The Many Worlds of Schrödinger’s Cat
In quantum physics, which deals with the physical laws of the subatomic, the term multiverse doesn’t exist, explains Lea. Alternate universes are instead referred to as ‘many worlds’ and are part of a radically diﬀerent concept than the multiverse.
The many-worlds hypothesis is used to explain how a quantum system can exist in seemingly contradictory states at the same time — called a “superposition” — and how these paradoxical states seem to vanish.
The most famous thought experiment for this is Erwin Schrödinger’s Schrödinger’s cat.
He wrote (in theory) that a cat is placed in a sealed box with a device containing a vial of lethal poison which is released only if an atomic nucleus in the box decays. Treating the box, the cat and the device as a single quantum system, each state — in this case, ‘dead’ or ‘alive’ — is described by a wave. As waves can overlap to form a single wave function, the cat can exist in a superposition of states. This means that in quantum mechanics the cat is both simultaneously dead or alive.
“Each flick of a light switch would create a near-infinity of worlds. One for each possible path of each photon as the light fills your living room, not just a world in which you didn’t flick the switch at all.”Robert Lea
This seemingly contradictory state persists only until the box is opened — analogous to making a measurement on the system — and the wave function collapses meaning the superposition is gone and the state is resolved. The cat is either dead or alive. Yet why measurement causes this collapse of superposition is still a mystery.
The many-worlds hypothesis does away with superposition altogether. Instead, it suggests that rather than the opening of the box collapsing the wave function, measurement causes it to grow exponentially and “swallow” the experimenter and eventually the entire Universe.
“This means each flick of a light switch would create a near-infinity of worlds. One for each possible path of each photon as the light fills your living room, not just a world in which you didn’t flick the switch at all.”
In terms of the Schrödinger’s cat thought experiment, the experimenter isn’t opening the box to discover if the cat is dead or alive. Rather, they are opening the box to discover if they are in a world in which the cat is dead, or one in which it lives.
“The objects, events and physical records of observers are different in different worlds. There is a world where the Eiﬀel Tower is in Los Angeles,” one physicist says. “All of the worlds — universes — are part of a single global universe. It looks just like this universe from the perceptive of our branch world.”
That sounds a lot like the premise for Everything Everywhere. Lea agrees, saying, “All of this makes the quantum version of the multiverse the one that most closely resembles pop culture, at least in principle. This is because it doesn’t just probably contain infinite versions of you, it definitely does.”
WATCH THIS: A Brief History of the Multiverse
Welsh astrophysicist Geraint F. Lewis, co-writer of Where Did the Universe Come From? And Other Cosmic Questions, explains the multiverse theory, including its origins and milestone developments, as well as what might come next.
“There are new universes born beyond our cosmic horizon,” he says. “Multiverse theories have proliferated, hoping to answer the deepest questions about what we, and the entire cosmos is.”
The hour-long video is separated into five parts: How Big is the Universe?, The Bubble Multiverse, The Cyclical Multiverse, Branes in the Bulk, and Many Worlds.
By thoroughly examining the history of the multiverse and what we know about it today, Lewis makes a complex topic easier to understand:
Only Faith Will Help Us Make the Leap to the Multiverse
By Adrian Pennington
The Multiverse — of Dr Strange, Loki and Everything Everywhere fame and other sci-fi tropes — is a nice idea. It leads us to think that we are not alone. It can be used to explain glitches in our behavior. Perhaps we can jump to another ‘me’ in a parallel universe and escape our troubles in this. Or go back in time to reverse a mistake. Or solve climate change.
But it’s a fallacy and despite the efforts of science to pull matter out of the dark there is zero proof of the existence of other universes outside of our own.
That is what Marcelo Gleiser wants to draw attention to in his opinion piece for Big Think.
There are two main inspirations for the modern version of the multiverse, he notes. Inflationary cosmology (the big bang) and superstring theory.
String theorists believe that extra dimensions are curled up within the three traditional dimensions of space and that these extra dimensions could provide a framework for understanding the multiverse.
The catch is that the only universe we can actually measure is ours. We cannot do any direct experiments to verify or falsify what goes on in other universes.
So, frankly, what’s the difference between using “science” to theorize the unproveable and theology, which explains the unexplainable by inserting “God”?
As Gleiser puts it, “Proponents believe the Multiverse can explain our origins without having to reference God. But the Multiverse is in no way falsifiable, and the arguments in its support are nearly identical to the arguments for God.”
So far, we have zero experimental evidence of strings, extra dimensions, or supersymmetry — “an extra symmetry of nature that predicts that each particle has a supersymmetric partner.”
Scientists might counter that maybe the supersymmetric particles are just too heavy to be seen by our current accelerators, while the extra dimensions are too tiny to be detected.
Maybe, but then we can’t ever actually falsify this theory; particles can always be too heavy and extra dimensions can always be too small for any machines that we build to detect.
“The same with the Multiverse,” says Gleiser. “By construction, these extra Universes exist outside our own and thus are not directly detectable. On physical grounds there is not much support for the string landscape and its Multiverse.”
And what about philosophically?
“God’s existence is not provable by observations. The Multiverse is not provable by observations. God explains the Universe. The Multiverse explains the Universe. The Multiverse, then, is a lot like God.”
In other words it’s a catch-all to explain the unexplainable; a salve for the impending extinction of the human species.
“The false assumption is that something that exists requires an explanation, whatever the cost of this explanation.”
The question Gleiser poses, then, is this: What is the price we must pay to have an “answer”? Is the price a supernatural cause, or an untestable scientific explanation? And in the end, does accepting either position make a difference? Does it offer a way out?
We should instead accept that not all questions need to be answered in order to be meaningful.