Each time we make a decision, a different timeline appears. Every decision we make is a yes / no decision because we either do something or choose not to do it. In the quantum universe, every decision has the same meaning: so there is a reality where we chose yes and another reality where we chose no. Hence the huge Multiverse that contains all the possibilities.
A new reality can be generated by any possible quantum interaction. Some interpretations of quantum mechanics suggest that our entire universe is described by one universal wave function that continually divides and multiplies, creating a new reality with every possible quantum interaction. This is quite a bold statement. How did scientists get there? One of the earliest theories in the history of quantum mechanics is that matter has wave properties. This was first proposed by the French physicist Louis de Broglie, who argued that every subatomic particle has a wave associated with it, just as light can behave as both a particle and a wave.
Other physicists soon confirmed this radical idea, especially in experiments where electrons scatter across a thin foil before they hit their target. The method of scattering electrons was more characteristic of the wave than the particles. But then the question arose: what exactly is a matter wave? What does she look like?
Early quantum theorists such as Erwin Schrödinger believed that the particles themselves were smeared in space as a wave. He developed his famous equation describing the behavior of these waves, which is still used today. But Schrödinger's idea passed many experimental tests. For example, although an electron in flight behaved like a wave when it reached its destination, it landed as a single compact particle so it could not be physically stretched out in space.
Instead, an alternative interpretation began to gain popularity. Today we call it the Copenhagen interpretation of quantum mechanics and it is by far the most popular among physicists. In this model, the wave function - as physicists call the wave-like property of matter - does not actually exist. Instead, it's a mathematical simplification that we use to describe the probability cloud of quantum mechanics where we can find a subatomic particle the next time we look for it.
However, the Copenhagen interpretation has several problems. As Schrödinger himself noted, it is unclear how the wave function changes from a cloud of probabilities before measurement to simply nonexistent at the time of observation. So maybe there's something more significant about the wave function. Perhaps it is as real as all particles. De Broglie was the first to propose the idea, but eventually joined the Copenhagen camp. Later physicists such as Hugh Everett re-examined the problem and came to the same conclusions.
By realizing the wave function, we solve this problem of measurement in the Copenhagen interpretation, because measurement ceases to be a super special process that destroys the wave function. Instead, what we call measurement is actually a long series of quantum particles and wave functions interacting with other quantum particles and wave functions. If you build a detector and shoot electrons at it, for example at the subatomic level, the electron won't know it's being measured. It just hits the atoms on the screen which sends an electrical signal (made up of more electrons) down the wire which interacts with the display which emits photons that hit the molecules in your eyes, and so on.
In this photo, each particle has its own wave function, and that's it. All particles and all wave functions just interact as usual, and we can use quantum mechanics tools (such as the Schrödinger equation) to predict how they will behave.
But quantum particles have a really interesting property because of their wave function. When two particles interact, they not only collide with each other; for a short time their wave functions overlap. When that happens, you can no longer have two separate wave functions. Instead, you should have one wave function that describes both particles simultaneously. When the particles move apart, they still maintain this single wave function. Physicists call this process quantum entanglement - what Albert Einstein called "ghostly action at a distance."
When we follow all the steps in the measurement, we get a series of entanglements from the overlapping wave functions. The electron becomes entangled with atoms on the screen, which become entangled with the electrons in the wire, and so on. Even the particles in our brain are entangled with the Earth, with all the light coming in and out of our planet, down to every particle in the universe entangled with every other particle in the universe. With each new entanglement, you have a single wave function that describes all the particles that are connected. So the obvious implication of an actual wave function is that there is a single wave function that describes the entire universe.
This is called the "multiverse" interpretation of quantum mechanics. It got its name when we ask what is happening in the observation process. In quantum mechanics, we never know exactly what a molecule will do - sometimes it can grow, sometimes it can fall, and so on. In this interpretation, whenever a quantum particle interacts with another quantum particle, the universal wave function breaks down into many sections, with different universes containing each of the possible outcomes.
This is how the multiverse is created. As a result of the interaction of quantum particles, numerous copies of the universe have created that repeat over and over again. Each of them is identical, except for a slight difference in some random quantum process. This means that there are multiple copies of what you are reading in this article right now, and they are all exactly the same except for tiny quantum details.
There are also difficulties with this interpretation - for example, how does this division actually occur? But it's a radical way of looking at the universe and demonstrating just how powerful the theory of quantum mechanics is - what began as a way to understand the behavior of subatomic particles can control the properties of the entire cosmos.
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