ARE THERE OTHER UNIVERSES?

Sometimes in life we face tough decisions. Do you opt for Path A or Path B? This job or that job? Often we then wonder about the road not taken. But what if, in fact, we take all possible paths? Instead of making a ‘wrong’ choice we make every choice? Colin Stuart enters the multiverse.

Colin Stuart

It may sound counterintuitive, but time and again physicists and astronomers have summoned the spectre of multiple universes in an attempt to solve big mysteries. Perhaps the most famous example is the Many Worlds interpretation of quantum physics. It was devised by physicist Hugh Everett III one night in 1954 after “a slosh or two of sherry”.

Many Worlds has its roots in the Schrödinger’s Cat thought experiment, which goes something like this: there is a cat in a sealed box, the box also contains a vial of poison, a small hammer and a machine that measures the position of a sub-atomic particle. If the particle is in position A, nothing happens. If it is in position B, the hammer smashes the vial and the cat dies.

The problem lies in the fact that, according to the traditional interpretation of quantum theory, known as the Copenhagen interpretation, until a measurement is made all possible outcomes are melded together in a mysterious state known as superposition. In other words, the particle is simultaneously in both position A and B, and this means that the vial, too, is simultaneously broken and unbroken. By extension, the cat also is both alive and dead at the same time.

Schrödinger concocted this thought experiment to highlight the bizarre – perhaps even nonsensical – nature of this interpretation and its idea that only in the act of measurement are the particles somehow forced to ‘choose’ a position and seal the feline’s fate. The issue became known as ‘The Measurement Problem’.

Everett’s Many Worlds offers a solution by removing choice entirely. Instead it argues that reality fractures into two copies of itself. In one branch, the particle ends up in position A. It occupies position B in the other. There is never a choice resulting in the exclusion of one outcome.

The trouble is that Everett’s idea is very hard to test experimentally because it does not make unique predictions that distinguish it from the Copenhagen interpretation. Many Worlds also has its own existential problems, perhaps summed up best by physicist Max Tegmark’s version of Schrödinger’s Cat – the Quantum Immortality experiment.

Quantum Immortality

Instead of a cat in a box, you sit in front of a loaded gun that is hooked up to the device measuring the position of the particle. If the particle is in position A nothing happens, but if it is in position B the gun fires and you die.

If Many Worlds is correct then both outcomes could occur, just in different branches of reality. You die in one and live in the other. Now let us imagine the surviving version of you repeats the experiment. The universe would branch again, with one version of you surviving.

After 100 such attempts, you will die in 99 branches but live on in one. You could keep on repeating the experiment and it is guaranteed that a version of you will always survive. You are immortal. But again, how could you possibly test this?

Along with the Quantum Immortality experiment, Tegmark also categorised the different types of multiverse. The Many Worlds of Everett represent a level III multiverse in the scheme. The other categories reflect the fact that we do not need quantum physics to summon a multiverse; the idea pops up independently in other branches of astronomy too.

Everett’s Many Worlds offers a solution by removing choice entirely. Instead it argues that reality fractures into two copies of itself.

For example, we are unable to see the entire Universe. Instead we are limited to observing a small part of it that we call the observable Universe. Beyond our horizon, there could be many other ‘universes’, all sharing the same laws of physics because they were all created in the Big Bang, but completely disconnected from one another because there has not been enough time for light to travel from them to us, and vice-versa yet.

This is what Tegmark terms a level I multiverse. The fascinating aspect is that the bigger the multiverse, the more likely it is that there are other universes within it that are similar or even identical to our own. If the level I multiverse is infinite in size, this becomes a certainty.

To see how this works, imagine a room containing ten objects lined up on a table. There are more than 3.5 million unique ways to arrange those ten objects. But if you have more than 3.5 million rooms then it is guaranteed that some of them will be in exactly the same configuration.

Likewise, in an infinite universe there are going to be copies of the ‘room’ we call the observable Universe, with every atom in them arranged identically. A carbon copy of the Sun, the planets, the Earth and even you.

Others would be eerily similar, with only subtle changes. Perhaps you forged a different career or married someone else. Across these myriad patches of an infinite universe, every possible outcome is guaranteed to happen somewhere.

Astronomy also throws up the possibility of other universes that can have the same laws of physics but different constants of nature. The constants set the conditions in our Universe. They determine how fast the speed of light travels in a vacuum, what charge is held by an electron, how strong gravity is in relation to the other forces. The constants determine the structures that can form in the Universe. Change the constants, and things would be very different indeed.

Tegmark calls these the level II multiverse, and claims that they arise naturally from our ideas of how the Universe began.

Inflation and the multiverse

When it comes to understanding the origin of our Universe, the Big Bang is far and away the best theory we have. However, it is not perfect. There are some aspects of the cosmos that the theory cannot explain.

Take the horizon problem. Whichever direction we choose to view the Universe, it looks the same. Far-flung locations on opposite sides of the observable Universe are at the same temperature. To see why this is a problem, imagine two boxes placed alongside each other. One hot and one cold. Given enough time, the temperature of the two boxes will equalise. Crucially, however, to achieve this the boxes need to be close enough together.

Imagine that these boxes are now at two distant parts of the Universe. To reach their current far-flung locations, the boxes could never have been close enough to reach thermal equilibrium. There simply has not been enough time since the Big Bang for them to equalise. Astronomers say that they are beyond each other’s horizon.

Also, the Universe is too flat. Here, the word flat relates to the amount of matter and energy in the Universe, which deforms Einstein’s mathematical fabric of the Universe, the so-called space-time continuum. Being flat in this context simply means that there is a very specific amount of matter and energy in the Universe and that spacetime has no innate curvature on the largest scales. Practically speaking, it is the precise balance point between the Universe expanding forever and there being enough matter and energy to halt this expansion and pull it all back together.

Astronomy also throws up the possibility of other universes that can have the same laws of physics but different constants of nature.

There is a chance that the Universe was born this way, but the odds of it having no curvature at all are – pardon the pun – astronomical. Instead, it is far more likely that any initial curvature was ironed out by the expansion of the Universe.

However, similar to the horizon problem, there does not seem to have been enough time since the Big Bang for the Universe’s expansion to completely smooth things out.

The idea of cosmic inflation proposes a solution to these problems. It says that the infant Universe underwent an extraordinarily large growth spurt in an unfathomably short period of time. In just a trillionth of a trillionth of a trillionth of a second, it ballooned from far smaller than an atom to about the size of a grapefruit.

That may not sound too impressive, but it represents a growth factor of 1 followed by 78 zeroes. If you took an ordinary shipping container and expanded it by the same amount, it would suddenly be larger than the entire observable Universe.

It solves the problems by expanding the temperature of a tiny region to fill the entire observable Universe and provides the necessary extra expansion to smooth out any curvature. And it opens the door to other forms of parallel universes.

For the theory to work, Inflation has to do its thing and then switch off to allow the Universe to transition into the sedate expansion we see today. This was impossible in its original version, put forward by Alan Guth in 1980, but physicist Andre Linde tweaked the mathematics to make it possible. But it came at a cost.

For most of the Universe to make a graceful transition into the calmer state we see today, there must always be one region that has to ‘bud’ off and take the inflationary energy with it, creating another universe, which then experiences the same inflation, transition and budding. Astronomers refer to the idea as ‘eternal inflation’ because it spawns an infinite sea of bubble universes.

These bubble Universes would act like our erstwhile rooms. If there is an infinite number of them, there are guaranteed to be some identical in every way to our own. An infinite cascade of ‘yous’ finishing this article at exactly the same time.

If there is an infinite number of them, there are guaranteed to be some identical in every way to our own. An infinite cascade of ‘yous’ finishing this article at exactly the same time.

Following the level III Many Worlds’ multiverse that we opened the article with, Tegmark then includes a catch-all level IV description that answers the question: why does our Universe have the laws of nature that it does, and the forces that it does? Perhaps the answer is that every mathematically possible universe exists, we just find ourselves in the one that can support life as we know it.

As with all things in science, testing this hypothesis would require observable signatures that the theories predict, and that we could go and look for. Sadly, this is where many of the ideas fall down. So is it even scientific to be postulating ideas that are, in some cases, fundamentally untestable? Or have we strayed into the realm of metaphysics?

Even if we cannot prove the multiverse, take comfort in the fact that if the theory is right and all possibilities are played out somewhere in the bulk of space, then someone, somewhere knows the answers.

Colin Stuart is an astronomy author and speaker. Join his free astronomy club at colinstuart.substack.com.

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The Multiverse Hierarchy: https://arxiv.org/pdf/0905.1283