Do multiverses exist? An astrologer explains why it depends on how you define ‘truth’.

Curious Kids is a series for children of all ages. If you have a question you want an expert to answer, send it to CuriousKidsUS@theconversation.com.

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Do multiverses exist? If so, what do they look like? How do you get there without wasting time? – Emily, age 9, Pune, Maharashtra, India

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The concept of multiverse – an imaginary collection of possible universes – is one that science fiction fans love to explore. But is there diversity?

In order to answer the question of whether various things are real, we must first agree on what it means for something to be real. As an astronomer who studies cosmology – the great history and structure of the universe – and the philosophy of physics, I have thought about this question many times in my work.

The 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 loud). So “real” can be defined as something you can perceive with at least one of the five senses.

But that would leave out many things that are also real. The microwaves that heat your food are real, but you can’t see them directly – it’s just their effect, the heated food. So some real things you can “see” indirectly only through the evidence they leave behind. The presence of dinosaurs is another example – you can only see their remains.

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

Multidimensional quantum mechanics

The answer that most researchers would give as to whether you can perceive many species with your five senses is probably not. But there are many real things that are not real in this sense, like microwaves. So can we see any indirect evidence of diversity, such as the effects it might have on the physical world?

The short answer is yes, sort of.

Physics is another way to understand the behavior of very small things, such as atoms and subatomic particles. Scientists call the laws that govern how these tiny particles work quantum mechanics. In quantum mechanics, there is no certainty as to what the outcome of an experiment will be. You can simply write down the probability – that is, the probability – of something happening.

It’s like rolling a dice: You are not sure what number you will get, but you can say that you have an equal chance of getting one, two, three, four, five or six on the dice. However, if you knew enough information about the dice – such as their exact shape and weight, the wind patterns around them and the correct way you threw them – you could predict which side they would land on. It might take a big computer simulation to crunch the numbers, but it’s possible.

Now imagine, indeed, very small dice. Even if you had a very powerful computer, you would not be able to predict which side this little dice would land on. That’s because it’s governed by quantum mechanics, where you can’t predict outcomes with absolute certainty. You can only predict the probability.

Many and varied countries

Quantum Mechanics is almost immutable – not everything has an equal chance of happening. We can predict the probability of each event occurring, but not the actual outcome. In the case of quantum dice, all we know about it is that it has a 1 in 6 chance of landing on any face.

One way scientists have explained this amazing property of quantum mechanics is that every possible situation happens. But when it does, it creates another universe. This is called the many-state view of quantum mechanics.

In the case of our quantum dice, multi-world theory would say that there is a 1 in 6 chance of rolling each number because six universes are created each time we roll the dice. Although we live in one of them – say, a world where three dice appear – five other universes are also created where the dice appear as one of the other numbers.

In this picture of quantum mechanics, the universe conforms to all conditions. Of course, we can’t make a quantum mechanical dice and roll it – just interacting with the dice would destroy its quantum structure.

Does this mean that quantum mechanics is proof that multiplicity is real? I would say no. Although it is a fascinating way to think about quantum mechanics, it is only one explanation, not an irrefutable proof of diversity.

Theory of variables and strings

Another relevant aspect of diversity is its role in string theory. String theory argues that the fundamental particles that make up matter are themselves composed of vibrating strings of energy. Think of an elastic band that vibrates through each part.

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 can have different values. The same is true of most things in the universe, such as matter. That suggests a space of different possible universes, each with different conditions – different.

So far, there is no clear evidence of multiple models based on string theory. These universes would probably not be connected, otherwise they would not be considered a separate universe – just a part of us. So even if they exist, we may not find direct evidence of their existence.

However, there may be indirect evidence for the existence of a multiverse. For example, string theory can help scientists predict the results of very powerful experiments in our universe. It can also make predictions about how matter works on very small scales. If these predictions turn out to be true, that would be proof of string theory. And if string theory can be real in our universe, this indirectly means that many species can be real.

Although there is no clear evidence in our universe for the concept of strings, who knows what the future may hold.

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