A GUIDED JOURNEY
Does a Multiverse Exist?
The Case Against an Infinite Multiverse
by Elie Feder and Aaron Zimmer, cohosts of the Physics to God podcast
What if our universe isn’t the only one? Imagine an endless collection of universes, each with its own rules and possibilities. Some might look like ours, while others could be so strange that we can hardly imagine them. This idea is called the multiverse.
For a long time, the multiverse was just the stuff of science fiction or Marvel movies. But in recent years, some leading scientists have begun to take it seriously. Why? Because the universe we live in looks surprisingly special, and the multiverse seems like the only way to explain it without God.
This brief essay will explain what the multiverse is, why some scientists believe in it, and whether it really offers the answers they hope for.
Some points we’ll discuss include:
For all these points in more depth, watch or listen to the complete Season Two of the Physics to God podcast on YouTube, Spotify, or Apple Podcasts. Or, read our longer article, “Is the Multiverse Real?”
What is Fine Tuning?
Hidden behind the everyday world are a set of special numbers called the constants of nature. There are about 25 of them, and together they decide how everything in the universe works. One constant fixes the mass of the electron, another fixes the strength of gravity, and so on.
For a long time, no one knew why these numbers had the values they do. They looked completely random. Richard Feynman, one of the greatest physicists of the 20th century, called this “one of the greatest mysteries in physics.”
Then scientists made a startling discovery. The numbers aren’t random at all. They’re fine tuned. This means that if you changed some of them even slightly, atoms wouldn’t form, stars wouldn’t shine, and life could never appear. As physicist Stephen Hawking said, if you change the constants, “the universe would be qualitatively different, and in many cases unsuitable for the development of life.”
This discovery created a new puzzle. Why are the numbers so perfectly set? Is it just an incredible stroke of luck? That seems nearly impossible, since the odds are unimaginably small. Another possibility is that the numbers were chosen by something intelligent. But that points directly to the idea of God, and many scientists were very uncomfortable with that conclusion.
So they began searching for a different answer.
How a Multiverse Explains Fine Tuning
The multiverse is the idea that there are infinite universes beyond our own. In each universe, the constants of nature could be different. If that’s true, then most universes would be lifeless and chaotic. But every once in a while, one universe would have just the right numbers for atoms, stars, and life.
If so, it would be no surprise that we live in one of those rare “lucky” universes. After all, no one could exist in the billions of universes where the numbers were wrong. Only in a fine tuned universe could anyone even ask the question.
At first glance, this seems like a clever way to explain fine tuning without God. But the multiverse runs into serious problems, the biggest one being something called the measure problem, which we’ll get to later. Before we do, let’s look at the three key ideas the multiverse needs to explain fine tuning without God. The first two are straightforward, but the third idea is much more surprising.
First Idea: Infinite Universes
If there’s only one universe, the chances of getting numbers as perfect as ours are unbelievably small. For the multiverse to explain fine tuning by chance alone, there would have to be infinitely many universes.
Think of it like a lottery. If you buy only one ticket, you’ll almost certainly lose. If you buy a million tickets, your chances improve. And if you somehow buy an infinite number of tickets, winning is guaranteed.
To support the idea of infinite universes, some scientists extend the Big Bang with a theory called eternal inflation. It says that in certain regions of space, the rapid expansion that began at the Big Bang never stopped. Instead, it keeps bubbling off into new universes.
If eternal inflation is true, it would give us infinitely many universes. That sounds like it should explain fine tuning, but as we’ll see, it’s not enough.
Second Idea: Each Universe Has A Different Set of Numbers
An infinite number of universes isn’t enough to explain fine tuning. To see why, let’s go back to the lottery analogy. Imagine you buy a million lottery tickets, but every ticket has the exact same numbers. Your chances of winning are no better than buying one ticket. To really improve your odds, each ticket needs to have different numbers.
The same is true for universes. If every universe had the same constants of nature, the multiverse wouldn’t explain anything. To avoid that problem, multiverse scientists suggest that each universe has different values for the constants, like each ticket in the lottery having a different set of numbers.
But here’s the problem. Everything scientists know so far shows that the constants really are constant. They never change. So why would anyone believe they’re different in other universes? To simply say “they must be different because how else could our universe be so precisely fine tuned” isn’t legitimate. It’s just assuming, without any justification, that God can’t be the answer.
So how would they know that the constants vary? The leading answer comes from string theory, a bold attempt to explain all of physics. String theory says our universe has extra hidden dimensions curled up so tightly that we can’t see them.
Here’s one way to imagine it. Picture an ant crawling on a garden hose. From far away, the hose looks like a straight line, so you’d think the ant can only move forward or backward. But up close, the ant can also crawl around the hose in a circle. That extra direction was “hidden” until you zoomed in.
Similarly, string theory suggests that our universe has tiny, curled-up hidden dimensions. There are a lot of different ways that those dimensions could curl up—about 1 with 500 zeros after it—and each way would create a universe with its own set of constants.
Putting the first two ideas together, eternal inflation gives an infinite supply of universes, and string theory provides the different numbers. While neither theory has been proven, both are popular among scientists. Together, they paint the multiverse as a giant cosmic lottery with infinitely many different tickets. If that’s true, it wouldn’t be surprising that one universe turned out just right, and naturally, that’s the one we would find ourselves in.
Multiverse of the Gaps
At first, it might seem that an infinite multiverse with different numbers in each universe solves the problem of fine tuning. But even scientists who believe in the multiverse admit it’s not that simple.
Here’s why: if you use the multiverse to explain absolutely anything, no matter how unlikely or bizarre, then it doesn’t really explain why our universe is the way it is. It becomes a “multiverse of the gaps,” a catch-all explanation that plugs every hole in knowledge without actually explaining anything.
Imagine seeing something incredible—like dragons flying through the sky, unicorns grazing in a field, or even a booming voice from the heavens claiming to be God. If you believe in an infinite multiverse, you could always shrug and say, “Well, that happens in one universe, and I guess we’re in that one.” Since this move can explain away even contradictory evidence, it makes it a theory of the gaps that’s impossible to test or disprove.
Because of this, even multiverse scientists agree that simply saying “everything happens somewhere” isn’t enough. That’s why they add one final piece to the puzzle.
Third Idea: We Should Live in a “Typical” Universe
Even if an infinite multiverse exists, it doesn’t mean anything goes. To avoid becoming a “multiverse of the gaps,” the theory needs one more big idea: we should expect to live in a typical universe—one that’s normal for intelligent life.
Here’s the logic. Out of infinite universes, only some can support observers like us. Since we’re observers, of course, we’d find ourselves in one of those life-friendly universes. But not all life-friendly universes look the same. Some might have only one galaxy, some might have billions, and some might include bizarre features like unicorns or dragons.
So if we’re using chance and probability to make predictions, what should we expect to see? The answer is a universe that’s typical for intelligent life—not super rare or wildly unusual, but average. That’s important because it keeps the multiverse from explaining absolutely everything. It can only explain a typical universe.
This idea is called the principle of mediocrity: we’re not special, just ordinary among all possible intelligent beings. As physicist Brian Greene put it, “Life may be rare in the multiverse; intelligent life might be rarer still. But among all intelligent beings…we are so thoroughly typical that our observations should be the average of what intelligent beings inhabiting the multiverse would see.”
This principle gives the multiverse its only real prediction: that we live in a typical universe. And just as importantly, it makes the theory falsifiable. If we found that our universe was extremely rare or unlikely, that would count as evidence against the multiverse.
But here’s the problem. At first glance, our universe doesn’t look typical at all. It’s unbelievably large, with more than 100 billion galaxies. Do we really need all that for intelligent life? One galaxy should be plenty.
In fact, you don’t even need a whole galaxy to get intelligent observers. All you’d need is one brain that randomly popped into existence, complete with false memories of a life it never lived. These so-called Boltzmann brains sound crazy, but in an infinite multiverse, even that would happen—and it would happen far more often than universes with billions of galaxies.
The bottom line is that multiverse scientists have to defend the counterintuitive claim that our enormous, complex universe really is the normal way to produce intelligent observers. That’s already a tough sell. And it gets worse. Next comes the biggest challenge facing the multiverse theory.
Infinities and Measures
When scientists try to figure out whether our universe is typical, they run into a huge problem. This is because an infinite multiverse doesn’t just have every kind of universe once—it has an infinite number of identical copies of every kind. Physicist Alan Guth put it this way: “Anything that can happen will happen; in fact, it will happen an infinite number of times.”
That creates a major headache. If there are infinite universes just like ours, but also infinite universes with dragons or unicorns, how can you tell which type is more common? There’s no sensible way to compare two things that are both infinite!
Here’s an analogy. Imagine an infinite row of marbles, some gold and some silver. If both colors go on forever, there’s no way to know which color you’d probably pick.
To get around this, multiverse scientists try inventing rules called measures. A measure is basically a way of organizing the infinite universes in your head so you can “count” them and decide which ones are typical. It’s like saying that every gold marble in the infinite row should be followed by ten silver ones. While there are still an infinite number of both, now you can say that silver is ten times more common than gold.
So even though an infinite multiverse really does contain an infinite number of universes with dragons, unicorns, and Boltzmann brains, multiverse scientists are hoping to find just the right measure that makes our universe more likely than all those bizarre alternatives.
Making Up the Rules: The Three Problems with Measures
Let’s look at three major problems with using measures to make our universe seem typical in the multiverse.
First, measures are made-up rules. They don’t come from any law of physics, like inflation or string theory. They’re invented just to make the multiverse theory work. Think of it this way: imagine you have infinite gold marbles and infinite silver marbles. You can make up any rule you like—maybe 10 silvers for every gold, or maybe 37 golds for every silver. There’s nothing in the marbles themselves telling you which rule is correct. In the same way, creating a measure for the multiverse is completely artificial. It’s just a way to make the theory fit what we see, not something that comes from nature itself.
Second, none of the obvious rules actually work. The measures that seem most natural to scientists end up predicting that we should live in a universe very different from ours. In other words, the simplest rules give the wrong answer.
Third, even if scientists someday discovered a measure that did make our universe look typical, there’d still be a deeper question: who or what picked that special rule in the first place? As physicist Paul Steinhardt put it, “Even if they someday succeed, they will need another principle to justify using that measure instead of the others, yet another principle to choose that principle, and so on.” In the end, trying to specially design the “right” measure doesn’t really explain anything. It just pushes the problem back further and further. And it leaves us right where we started—with the idea that something intelligent set things up on purpose.
Because of these three problems, measures don’t rescue the multiverse. Instead, they reveal that an infinite multiverse simply can’t predict that we should be typical. And remember, without that third idea that our universe is typical, the multiverse becomes just another “theory of the gaps” that doesn’t explain anything at all.
That’s the measure problem, and it’s devastating for the multiverse theory.
Is the Multiverse Really Science?
Even scientists who believe in the multiverse admit it sounds wild—almost like science fiction or philosophy. Still, they argue that it should count as science, just like quantum mechanics, even if it feels strange at first.
But is the multiverse really science? For an idea to be scientific, it has to be tested through experiments and observations. The scientific method means you come up with ideas, test them, and see if they match reality.
The problem is, the multiverse can’t be tested. We can’t observe or measure other universes. Even the theories that lead to it, like string theory and eternal inflation, have never been experimentally proven. And because the multiverse suggests that anything that can happen will happen somewhere, but it can’t tell us what would most likely happen, it can’t make real predictions that we can verify. That means the multiverse doesn’t qualify as science.
Of course, the idea of God isn’t science either. It’s philosophy—good philosophy—because God is what the evidence of fine tuning naturally indicates. The point is, you can’t rescue the multiverse from all its problems by calling it science when it doesn’t meet the basic requirement of being testable.
When all is said and done, the multiverse doesn’t solve the mystery of why our universe is so perfectly set up. The multiverse remains a wild idea—closer to bad philosophy than to real, proven science. If we follow the evidence, the best explanation for our fine-tuned universe is still what it has always been: God intelligently set the constants of nature just right to produce a universe as complex and amazing as ours.
