From “Rick and Morty” to the more recent “Fuck Multiverse” and “Doctor Strange 2: Runaway Multiverse”, the romantic concept of the multiverse has been a favorite element of various works. The protagonist travels to other parallel universes, encounters various possibilities, and realizes his unfulfilled wish.

Sadly, there doesn’t seem to be such a good thing in reality. There is only one world that the eyes see, and everything happens according to the original script, there is no chance to pass through or start over.

However, the descriptions of these fantasy works are not as wild as you might think. The imagination required to create sci-fi works is actually an important skill for scientists. Since the mid-twentieth century, the term “multiple worlds” has appeared in serious quantum physics discussions.

“Multiple worlds” is one of many interpretations of quantum phenomena. Why do pragmatic physicists resort to such illusory claims? In the end, they are also desperate.**It all starts with the problems that quantum physics brings to them.**

**Page Contents**

- Before the quantum age, the world of physicists
- The weirdness of the double slit experiment
- New Concept in Quantum Era – Wave Function (Wave Packet)
- The wave function is not an entity, but a probability distribution of an object – Copenhagen Interpretation
- Doubts about the Copenhagen interpretation
- All possibilities persist—Multi-World Interpretation
- Finding the Multiverse
- Epilogue

**Before the quantum age, the world of physicists**

Before the quantum age, the methods used by physicists to explain everything in the world were “classical mechanics” and “electromagnetism”.

- “Classical Mechanics” is an advanced version of “Newton’s Laws of Motion”, explaining “how a particle (matter) with mass moves”
- “Electromagnetism” is the basis of all electromagnetic wave-related technology (your cell phone signal), explaining “how massless energy travels in space as waves”.

“Classical Mechanics” and “Electromagnetism” divide the world into two different problems: “particles” and “waves” to explain, each other’s well water does not cause floods, and “nearly perfect” explanations for all common phenomena in daily life, however, After in-depth research, there is a phenomenon, but there is a contradiction, this phenomenon is**Double-slit experiment**。

## The weirdness of the double slit experiment

If you let the light beam pass through a slit, it will reflect a pattern brighter in the center and darker on the sides on the rear screen. The amazing thing is that if the experiment is changed to two slits, the pattern on the screen will not be equal to the sum of the patterns of two single slits, but will become stripes of light and dark intervals. This pattern can only be created by waves, as the peaks and troughs cancel each other out, resulting in darker sections.

The double-slit experiment has become evidence that light is a wave, which belongs to the category of “electromagnetic” interpretation, and the subsequent derivation also proved that light is a kind of electromagnetic wave.

It would have been nice if the story stopped here, but then it was observed**“Through electrons or neutrons into the double slit, you will get similar results as when light enters the double slit.”**

This problem is a bit big, because the motion of particles such as electrons or neutrons must be explained by waves in the same way as light in the double-slit experiment. The dichotomy of traditional physics “particles and waves” seems to be invalid.

## New Concept in Quantum Era – Wave Function (Wave Packet)

In response to these observations, physicists began to use “wave mechanics” to explain the double-slit experiment, that is,

Schrödinger equation。

**“Wave Dynamics”**is to treat all objects as**“Small Wave”**that is,**wave function**, and write down how it evolved over time.The original particles now look like pieces in the picture below**“Wave Pack”**moving in space with the Schrödinger equation.

These “wave packets” successfully explain how particles interfere with each other after passing through the double slit, forming bright and dark fringes.**But the problem is that no one can see these wave functions (wave packets). When observing, only electrons hit the screen through the slits.**

Although Schrödinger’s equations and calculation results are in full agreement with the experimental results, why is this correct? What does the wave function (wave packet) represent? But there is no good explanation.

**The wave function is not an entity, but a probability distribution of an object – Copenhagen Interpretation**

In order to explain this problem, the “Copenhagen Interpretation” was first proposed by Heisenberg’s group, who believed that:**The wave function represents the probability distribution of where an object appears, while the Schrödinger equation regulates how the probability distribution changes over time.**

When we intervene in observation, the wave function will randomly collapse to a specific value according to this probability distribution, and this value is the physical quantity we observe.

In the case of double slits, the wave function after passing through the slit produces the interference phenomenon that the wave will have. The screen at the rear collapses the wave function so that an exact point of light appears. As for where the light spot will appear, it is completely probabilistic, and the probability is dominated by the wave function. Where the peaks and troughs cancel out, there is a small chance that there will be few blips; and vice versa. In the figure below, you can see that the positions of individual particles appear to be random, but as the number of particles in the experiment increases, the probability distribution of the wave function begins to emerge.

Editor’s note: The Copenhagen interpretation holds that the wave function covers all the possibilities for an object to fall anywhere, and each possibility has a probability value. The Schrödinger equation, on the other hand, calculates the change in probability for each possibility.

When multiple identical objects repeatedly experience the same event (e.g. electrons continuously entering a double slit), you will see evidence that the wave function controls the motion of the object (the last graphic on the screen).

As for why the “single object” moves to a certain position, and how the “single object” actually moves, it is basically unknowable, everything is determined by the wave function, so the Copenhagen interpretation uses “collapse” , to refer to the situation where the other possibility disappears.

## Doubts about the Copenhagen interpretation

Today, about a hundred years later, this interpretation has become mainstream, but some people in the academic community at the time did not pay for it.

It is because of this saying**Directly embracing probability, the physical world is completely determined by the random process of wave function collapse**we can only know the appearance of the wave function; the second is**The vague but central role of the word “collapse” gives people a feeling of hard-boiled answers.**in addition,**It seems unclear why the role of the person or the instrument as the observer is so important.**

To point out the absurdity, Schrödinger brought out his world-famous cat.

Since the radioactive decay of atoms is also described by a wave function, we can use radioactive atoms to build a device that can kill a cat, and then keep the cat and the device in a box. Over time, the states of atoms are in a probability distribution of decay and non-decay, so cats are also in a “probability distribution of dead and alive”. The atomic and cat wave functions cannot be collapsed until the observer opens the box.

This experiment is fundamentally different from the lottery draw. Although the lottery also seems to be a matter of chance, each lottery ball is a macroscopic object that can be described by classical mechanics. Therefore, the position of each numbered ball has been determined long before the lottery, but no one can accurately predict it.

However, atomic decay is the realm of quantum. The initial development of quantum theory originated from small-scale worlds such as the photoelectric effect and atomic spectroscopy.**Experimental observations in these fields cannot be generalized by classical mechanics, but can only be explained by the probabilities of the wave function.**

And Schrödinger’s goal is to link tiny quantum objects (atoms) with macroscopic creatures (cats), trying to illustrate how uncomfortable a physical world dominated by probability distribution and collapse can be.

In the decades after the Schrödinger equation and the Copenhagen School clashed, there was still a lot of discussion about what observation and collapsing really are.**Many subsequent studies, under the framework of Copenhagen, proposed the details of repair, and many people gradually accepted the probabilistic collapse.**

Since then, another group of people has tried to come up with a quantum world that does not require the collapse of probability, including Einstein and others who do not like God to throw dice.**They believe that particles have always had definite positions and trajectories, but they have not evolved in the way we imagined.**there are unknown physical mechanisms behind it, and Copenhagen’s worldview is only a statistical result, not a complete picture.

Such interpretations are collectively referred to as**Hidden variable theory**, there are many different versions throughout history.But in**Bell’s theorem**After the relevant experiments of , the local hidden variable theory is almost completely ruled out. The theory of hidden variables, which is still valid today, claims that the wave function, like an electromagnetic field, fills the entire space and can guide the trajectories of particles in specific ways.

## All possibilities persist—**Multi-World Interpretation**

These new theories, although somewhat devoid of collapse, still sound cryptic. In the 1950s, an American physicist Hugh Everett III proposed a completely new solution in his doctoral thesis:

“Everyone, stop arguing, all the probabilities that can occur in the wave function do happen, but all the possibilities exist at the same time as non-interlaced world lines.”

Take the cat as an example, when you open the box, the wave function of the cat is not collapsed to a single dead or alive state, but the original world line is divided into two, and one of them sees the dead cat. and you who saw live cats. Thus, the wave function never needs to collapse to the single state we see.

In other words, there is no so-called “non-quantum” “observer” in this view to make the wave function real. All the atoms, cats, and people in the world are included in the wave function of the entire universe.Everett’s original paper title did not mention multiple worlds, but called it**global wave function theory**(Theory of the Universal Wavefunction). The wave function does not describe the probability distribution of observations – the wave function is the ontology, which evolves the various world lines according to the Schrödinger equation.

Perhaps because it was too avant-garde, when his paper was published, there was hardly any discussion, and no one even took the time to question it. Everett eventually left academia with regret and went to work at the Pentagon. Fortunately, after more than ten years, his idea was finally realized with the efforts of several supporters.**the many-worlds interpretation**” the name flourished.

As difficult as it may sound at first, it turns out that this interpretation is actually no more absurd than the original collapsed interpretation.

It can also explain all experimental phenomena, and the overall wave function can fully follow the predictions of the equation rather than probabilistic collapse,**There is no need to introduce an observer outside the quantum world to collapse the wave function to a single state.**Many physicists see this as a more concise way of thinking. Today, the multi-world interpretation has accumulated a lot of voices and supporters.

**Finding the Multiverse**

Do multiple world lines really exist? It may not be easy to find the answer. If Everett’s claim is true, that all people and all instruments are part of the overall wave function, then no one can stand outside everything and see all the possibilities in the overall wave function, or do Experiments to verify the existence of multiple worlds.

But aside from researchers in quantum theory, there is a group of people who also take the idea of a multiverse very seriously. In cosmology, a theory predicts that our observable universe is just a small bubble, inside many other bubble universes, a multiverse in effect.These universes are constantly expanding, and this theory is called**eternal inflation**。

In contrast to the many-worlds interpretation of quantum mechanics, eternal inflation is a scientific theory that needs to be verifiable. Logically, any signal from other cosmic bubbles can’t outpace the expansion and never reach our observable universe. But in the early stages of expansion, collisions between bubbles can leave a thermal footprint on the cosmic background radiation landscape. About a decade ago, scientists found four statistically significant collision traces in observations from the Wilkinson Microwave Anisotropy Probe (WMAP).

So why haven’t we set off for other universes? Although it is an exciting attempt in analytical methods, more observations are needed to make better judgments. After WMAP, the Planck satellite (Planck) also brought back images of background radiation with three times higher resolution, but whether the multiverse really exists is still inconclusive.

**Epilogue**

Looking back at history, it often took decades for different theoretical models to differentiate themselves as the results of quantum experiments surfaced. The double-slit experiment was completed in 1801, but the birth of the many-worlds interpretation was 150 years later. Just like quantum physics in the early twentieth century, inflation theory and the multiverse are both areas that have a lot of room for development today. It may be a while before we see these sci-fi content become textbooks.

Regardless of the outcome, the possibility of infinite bifurcations in the overall wave function, as well as the expanding multi-bubble universe, demonstrate the fascinating aspect of scientific research—both scientific and sci-fi texts stand together at the highest point of the human imagination. front end.

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