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Are multiverses real? If so, what do they look like? How do you get there without disturbing time? – Emily, age 9, Pune, Maharashtra, India

The idea of a multiverse – a hypothetical collection of all possible universes – is one that science fiction fans love to explore. But does the multiverse actually exist?

To answer the question of whether the multiverse is real, we first need to agree on what it means for something to be real. As an astrophysicist who studies cosmology – the large-scale history and structure of the universe – and the philosophy of physics, I’ve thought about this question more than a few times over my career.

The most immediate definition of “real” might be that you can see and touch it. My lunch is real in this sense, because I can taste it and you can hear me chewing it (hopefully not too loudly). So “real” might be defined as something you can perceive with at least one of your five senses.

But that would leave out a lot of things that are also real. The microwaves that heat up your food are real, but you can’t directly perceive them – only their effect, heated food. So some real things you can “see” only indirectly by the evidence they leave behind. The existence of dinosaurs is another example – you can see only their fossils.

So, you can ask two versions of the question of whether the multiverse is real. One: Can you see, hear, touch, smell or taste it? Two: Even if you can’t, is there any evidence of its effects?

Quantum mechanics of the multiverse

The answer most researchers would offer to whether you can perceive the multiverse with your five senses is probably not. But there are lots of real things that aren’t real in this sense, such as microwaves. So can we see any indirect evidence of the multiverse, such as the effects it might have on the observable world?

The short answer is yes, sort of.

The multiverse is one way to understand the behavior of very, very small things, such as atoms and subatomic particles. Scientists call the rules governing how these very small objects behave quantum mechanics. In quantum mechanics, it’s never certain what the outcome of an experiment will be. You can only write down the chance – that is, the probability – of something happening.

It’s like rolling dice: You can’t be sure what number you’ll get, but you can say you have an equal chance of getting one, two, three, four, five or six on top of the dice. However, if you knew enough information about the dice – such as its exact shape and mass, the air patterns around it and the exact way you threw it – you could predict exactly what side it would land on. It might take a big computer simulation to crunch the numbers, but it’s possible.

Now imagine really, really, really small dice. Even if you had a very powerful computer, you wouldn’t be able to predict which side this super small dice would fall on. That’s because it’s governed by quantum mechanics, where you can’t predict outcomes with complete certainty. You can predict only probability.

Many worlds and the multiverse

Quantum mechanics is only somewhat random – not everything has an equal chance of happening. We can predict the chance of each scenario happening, but not the actual outcome. In the case of quantum dice, all we could know about it is that there’s a 1 in 6 chance of it landing on any face.

One way scientists have interpreted this strange property of quantum mechanics is that each possible scenario actually does happen. But when it does, it creates another universe. This is called the many-worlds view of quantum mechanics.

In the case of our quantum dice, the many-worlds view would say there’s a 1 in 6 chance of rolling each number because six universes are created every time we roll the dice. Although we stay in one of them – say, the world where the dice comes up three – five other universes are also created where the dice comes up as one of the other numbers.

In this picture of quantum mechanics, universes branch off with every scenario. Of course, we cannot really make a quantum mechanical dice and roll it – just interacting with the dice would destroy its quantum nature.

Does this mean quantum mechanics is evidence that the multiverse is real? I would say no. While it’s a fascinating way to imagine quantum mechanics, it’s just one interpretation, not undeniable evidence of the multiverse.

The multiverse and string theory

Another relevant aspect of the multiverse is its role in string theory. String theory argues that the fundamental particles that make up matter are themselves made of vibrating strings of energy. Think of an elastic band vibrating inside each particle.

String theory also argues that the universe has more than three dimensions. Different string theories predict different numbers of extra dimensions. This means physical constants such as the speed of light and the charge of electrons could have different values. So could the amount of stuff in the universe, such as matter. That suggests a landscape of possible different universes, each with different conditions – a multiverse.

So far, there isn’t definite evidence of a multiverse based on string theory. These universes probably wouldn’t connect to each other, otherwise they wouldn’t count as separate universes – just part of our own. So even if they do exist, we may never get direct evidence for their existence.

However, there could be indirect evidence of the existence of multiple universes. For instance, string theory can help scientists predict the results of very high-energy experiments in our own universe. It can also make predictions for how matter behaves on very, very small scales. If these predictions turn out to be true, that could be evidence for string theory. And if string theory is possibly real in our universe, this indirectly means the multiverse may also be real.

While there hasn’t been any definitive evidence in our own universe for string theory, who knows what the future may hold.

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This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Zachary Slepian, University of Florida

Read more:

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Zachary Slepian does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.