[Curiosities] Multiverse

[Jeenyuhs]

What lies beyond the edges of the observable universe?

 

Is it possible that our universe is just one of many in an even larger multiverse?

Movies never tire of exploring these questions. The recent release of Doctor Strange in the Multiverse of Madness is a clear example of how science fiction stories are full of creative interactions between alternate realities. And depending on the cosmologist you ask, the concept of multiverse can be an interesting storytelling tool or something deeper than just a fantasy.

Humanity's ideas about alternate realities are old and varied: Edgar Allan Poe wrote a prose poem in 1848 in which he imagined the existence of "an unlimited succession of universes." But the concept of the multiverse really took hold when modern scientific theories attempting to explain the properties of our universe predicted the existence of other universes where events happen outside our reality.

"Our understanding of reality is by no means complete," comments Andrei Linde, a physicist at Stanford University. "Reality," he adds, "exists independently of us."

If they exist, those universes are separate from ours and unreachable and undetectable by any direct measurement (at least so far). And that makes some experts wonder whether the search for a multiverse can be truly scientific.

 

Will scientists ever know if our universe is the only one? We break down the different theories about a possible multiverse (including other universes with their own laws of physics) and whether there could be many versions of you out there.

 

 

What is a multiverse?

The multiverse is a term scientists use to describe the idea that beyond the observable universe, other universes may also exist. Multiverses have been predicted by various scientific theories that describe different possible scenarios, from regions of space on different planes than our universe, to universes in separate bubbles that are constantly bubbling up.

The one thing that all these theories have in common is that they suggest that the space and time we can observe is not the only reality.

 

Why do scientists think there might be more than one universe?

"We can't explain all the features of our universe if there is only one," says science journalist Tom Siegfried, whose book The Number of the Heavens investigates how conceptions of the multiverse have evolved over the millennia.

"Why are the fundamental constants of nature what they are?" asks Siegfried. "Why is there enough time in our universe to create stars and planets? Why do stars shine the way they do, with just the right amount of energy? All of those things are questions that we don't have answers for in our physical theories."

Siegfried says there are two possible explanations: First, that we need newer and better theories to explain the properties of our universe. However, he adds, it is possible that "we are just one of many universes that are different and we happen to live in the one that is nice and comfortable."

 

What are some of the most po[CENSORED]r theories about the multiverse? 

Perhaps the most scientifically accepted idea comes from what is known as inflationary cosmology, which is the notion that in the tiny moments after the Big Bang, the universe expanded rapidly and exponentially. Cosmic inflation explains many of the observed properties of the universe, such as its structure and the distribution of galaxies.

"This theory at first seemed like a piece of science fiction, albeit a very imaginative one," says Linde, one of the architects of cosmic inflationary theory. "But it explained so many interesting features of our world that people began to take it seriously."

One of the theory's predictions is that inflation could occur over and over again, perhaps infinitely, creating a constellation of bubble universes. Not all of those bubbles would have the same properties as ours; they could be spaces where physics behaves differently. Some of them might be similar to our universe, but all of them would exist beyond the dimension we can directly observe.

 

What are some of the other ideas?

There is another type of multiverse that is scientifically compelling, explained as a many-worlds interpretation of quantum mechanics, which is the theory that mathematically describes how matter behaves. Proposed by physicist Hugh Everett in 1957, the many-worlds interpretation predicts the presence of branching timelines or alternate realities in which our decisions play out differently, sometimes producing very different outcomes.

"According to Hugh Everett, there are actually an infinite number of parallel Earths and when you do an experiment and get the probabilities, basically all it shows is that you live on the Earth where that was the outcome of that experiment," explains physicist James Kakalios of the University of Minnesota, who has written about how physics interacts (or doesn't) in superheroes. Therefore, he continues, "on other Earths, there are different results."

According to this interpretation, different versions of you could be living the many different possible lives you could have had if you had made different choices. However, the only reality that is perceptible to you is the one you inhabit.

 

 

Where would all these alternate Earths exist?

They all overlap in dimensions that we cannot access. Max Tegmark of the Massachusetts Institute of Technology (MIT) refers to this type of multiverse as a Level III multiverse, where multiple scenarios play out in branching realities.

"In the many-worlds interpretation, you still have an atomic bomb, you just don't know exactly when it's going to go off," Linde comments. And maybe in some of those realities, it never will.

But the multiple universes predicted by some cosmic inflation theories, by contrast, would be what Tegmark labels a Level II multiverse, where the fundamental physics may be different across universes. In an inflationary multiverse, says Linde, "it is not even known whether, in some parts of the universe, atomic bombs are even possible."

 

Can we travel through the multiverse? 

Unfortunately, no. Scientists don't think it's possible to travel between universes, at least not yet.

"Unless a lot of the science of physics that we know is solidly established turns out to be incorrect, you can't travel to other universes," Siegfried says. "But who knows? A thousand years from now, I'm not saying someone might not discover something you never would have imagined."

 

Is there any direct evidence to suggest that the multiverse exists? 

 

Although certain features of the universe seem to require the existence of a multiverse, nothing has been directly observed to suggest that it actually exists. So far, the evidence supporting the idea of a multiverse is purely theoretical and, in some cases, philosophical.

Some experts argue that it may be a great cosmic coincidence that the Big Bang forged a perfectly balanced universe that is ideal for our existence. Other scientists think it is more likely that any number of physical universes exist and that we simply inhabit the one that has the right characteristics for our survival.

An infinite number of small alternative universes, or bubble universes, some of which have different physical laws or different fundamental constants, is an attractive idea, Kakalios confesses. "That's why some people take these ideas seriously, because it helps address certain philosophical problems," he adds.

Scientists even argue about whether the multiverse is an empirically testable theory. Some would say no, since by definition a multiverse is independent of our own universe and impossible to access. But perhaps we simply haven't discovered the right test.

 

Will we ever know if our universe is just one of many?

Maybe not. But multiverses are among the predictions of various theories that can be tested in other ways, and if those theories pass all the tests, then maybe the multiverse theory will hold up as well. Or maybe some new discovery will help scientists figure out if there really is something beyond our observable universe.

"The universe is not limited by what some blobs of protoplasm inhabiting a small planet can discover or prove," says Siegfried (referring to humanity). "We can say: this is not testable, therefore it can't be real, but that just means we don't know how to test it. And maybe someday we do figure out how to prove it, although we may never do so. But, in reality, the universe can do whatever it wants."