Enigmatic quantum theory

 

“‘There is actually no such thing as a quantum world. The quantum state exists only inside my head, something I use to do calculations. Quantum states at most describe certain information — they do not correspond very well to the real world.’”

Hearing these words, one might think a frustrated university student—annoyed by the incomprehensibility of quantum mechanics—is venting irritation. But to our astonishment, these remarks about quantum mechanics were recently made by Anton Zeilinger, the quantum physicist at the University of Vienna who received the 2022 Nobel Prize in Physics for proving experimentaly the principles of quantum mechanics.

Quantum theory is one of the most successful theories in twentieth-century science. From computer chips to complex medical life-support technologies, modern technology depends extensively on quantum theory. Whether explaining the intricate workings of microscopic biological cells or unlocking the mysteries of the universe, quantum theory works with remarkable success. So what does it mean to say such an effective theory has no real world behind it? If a Nobel-winning quantum physicist denies the existence of the quantum world, where do we turn?

Great scientists like Einstein and Feynman have issued various warnings about the perplexities of quantum theory. But did that stop quantum theory—or prove it wrong? The answer is no. For the past hundred years, quantum theory has only grown more powerful and dominant in the scientific world. The year 2025 is being celebrated as the Quantum Year of Physics, and for that reason scientists have begun reflecting anew on the fundamental question: What is quantum theory actually? Why is its relationship with physical reality so puzzling?

Not all scientists agree with Zeilinger in rejecting a real quantum world. Alain Aspect, the Paris-based physicist who shared the Nobel Prize with Zeilinger for similar quantum research, disagrees. Professor Aspect believes that the quantum world is fully real, though its behavior is as mysterious as nature itself.

The debate about quantum theory and physical reality intensified recently in response to a large survey published by the world-renowned journal Nature. The aim was to discover what quantum researchers themselves think about the quantum world — how they interpret its rules and underlying reality. A broad questionnaire was sent to 15,000 quantum researchers. All of them are active contributors to quantum-theoretical research, each with multiple recent publications. The same questions were also placed before many leading quantum experts who attended the centenary celebration of Schrödinger’s equation on Germany’s Heligoland Island.

Although fifteen thousand scientists were contacted, only eleven hundred responded and shared their views on quantum theory. Nearly fourteen thousand researchers did not even feel compelled to reply. Even so, this survey remains the largest ever of its kind.

The results show that researchers still have no consensus on how to interpret the reality behind quantum mechanics. And this once again demonstrates that quantum theory remains as enigmatic as ever.

Asia’s first Nobel-winning scientist in the sciences, C. V. Raman, once remarked—while explaining the difference between science and non-science—that in science, a precise question has only one correct answer. But in anything that is not science, a single question may have many correct answers. In quantum theory, we often see the possibility of many correct solutions to the same question. If we strictly follow Raman’s logic, we must conclude that quantum theory is not rigidly orthodox as a science. It leaves room for plurality.

Although scientists remain divided on how quantum mechanics should be interpreted, the rules for applying quantum mechanics are perfectly clear. For example, when predicting the possible outcomes of an event, quantum mechanics describes a superposition of multiple possible states at once. The most effective way to describe these quantum states is through the wave function. If one can construct a wave function solvable by Schrödinger’s equation, and solve that equation, then the probabilities of possible outcomes can be determined.

But when the outcome of the event is actually observed, the quantum world’s wave function of possibilities collapses into a single real state—so instead of a superposition of many outcomes, only one definite outcome appears.

Take Schrödinger’s famous cat as an example. When the cat is inside a box and we cannot see its real state, the wave function describing its life-status may be a superposition of both “alive” and “dead.” But once we open the box and look, we see only a single state: the cat is either alive or dead. This shows that observation causes the quantum wave function to collapse. Here the question arises: should we then assume that the quantum world is an invisible realm?

In Nature’s survey, researchers were asked about the wave function—whether this mathematical description of an object’s quantum state has any real physical basis.

Only 36% of scientists believe that the wave function represents something real in the physical world. Among them, only 17% believe it is entirely real, while 19% think it is partially real.

But 47% of scientists believe that the wave function is merely a mathematical tool used to solve quantum-mechanical problems, with no real existence of its own.

Another 8% think the wave function reflects a kind of personal preference embedded in the experimental outcome—meaning that through it, one can influence or control the results of quantum experiments!

Even more surprising: of the remaining 10% of scientists, 8% believe the wave function is something else, and 2% have no idea what the wave function really is.

 

 

Is there any conflict between our classical, everyday idea of objects and the quantum concept of objects? To put it simply: Is there a definite boundary between quantum objects and classical objects — a threshold beyond which a quantum object becomes classical, or a classical object becomes quantum? Scientists are divided on this question as well.

According to the survey, 45% of researchers believe such a boundary does exist (among them, 40% think the boundary is very definite, while 5% think it exists but is not sharply defined).

Another 45% of scientists believe that no such boundary exists between classical and quantum objects.

The remaining 10% are unsure.

 

One of the key features of quantum theory is its observer dependence. Quantum mechanics can speak about the probabilities of a particle’s behavior, but it cannot say with certainty what exactly will happen. To know with certainty, the particle must be observed. Einstein himself strongly objected to this idea. Frustrated by quantum mysteries, Einstein asked his biographer Abraham Pais:
“Do you really believe that the moon exists only when you look at it, and not at other times?”

Although Einstein raised this objection, quantum scientists agree that such questions do not apply to large objects like the Moon. But when it comes to the behavior of extremely tiny particles—whose properties must be described using quantum mechanics—we find that how a particle behaves depends on its observer.

This old question was raised again in the Nature survey. Here too, scientists could not reach agreement.

• 56% of scientists believe that the measurement of quantum events is observer-dependent.
• 9% believe that it is not enough for an observer to simply “look”; the observer must be conscious of what is happening—that is, must understand the full context of the event.
• 28% believe that no observer is required at all for the measurement of quantum events.
• 8% are unsure whether an observer is needed or not.
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Among the experiments on which quantum theory is built, the double-slit experiment is one of the most fundamental. It provides evidence that electrons exhibit both particle and wave properties. If a stream of electrons is directed toward a screen with two microscopic slits, and if a detector records the pattern created by electrons passing through the slits, we see that a single electron behaves like a wave—passing through both slits at once and interfering with itself.

But if we place detectors behind the slits individually—observing which slit the electron goes through—then the observed pattern changes. The electrons behave like particles, passing through a slit and landing in straight-line paths. Quantum theory accepts these results as true, and from this we embrace the wave–particle duality of nature.

In the Nature survey, scientists were asked a question about this experiment: When no one is observing them, do electrons pass through both slits?

The responses were striking:

• 48% of scientists expressed irritation at the question, saying that such a question is meaningless for quantum objects.
• 31% believed the answer is yes.
• 14% believed the answer is no.
• 6% were unsure how electrons pass through the slits, or whether observation has any role at that stage.

 

The fact that quantum scientists cannot agree even on the established phenomena of quantum theory is astonishing. This indicates an important divide among researchers. On one side are those who adopt a realist viewpoint—scientists who believe that the equations of quantum mechanics reflect an underlying physical reality. On the other side are those who follow an epistemic perspective—scientists who think that quantum physics deals only with information, even if that information does not correspond directly to physical reality.

On the question of how the quantum world can be explained in relation to the real world, scientists have split into multiple camps. The Nature survey asked researchers which interpretation they think best explains quantum events and interactions—that is, which attempt to link the mathematics of the theory with physical reality they prefer.

The largest group (36%) favored the Copenhagen interpretation. But when asked how confident they were that their preferred interpretation is the correct one, only 24% said they believe it is actually correct; the rest see it merely as a suitable or contextually useful tool.

 

Many researchers showed inconsistencies between their answers to one question and their answers to a related supplementary question. Does this mean that many quantum researchers simply use quantum theory without deeply thinking about its true meaning? An American physicist once gave this approach a name—“Shut up and calculate!”

So, what exactly is this popular interpretation of quantum mechanics—the Copenhagen interpretation? In quantum theory, the behavior of an object is described by its wave function. Erwin Schrödinger introduced the quantum wave function through his famous equation a hundred years ago. The wave function provides a mathematical representation of all possible states of a quantum system through the idea of superposition.

The key explanations of quantum mechanics were given by the founders of the theory themselves—Schrödinger, Heisenberg, and Niels Bohr. Because foundational research in quantum mechanics took place at the University of Copenhagen, this core interpretation became known as the Copenhagen interpretation. In this interpretation, the observer plays a central role. Fundamental particles exhibit both particle-like and wave-like behavior. Quantum tunneling and quantum entanglement are accepted concepts in this interpretation. This is one reason why it remains so popular.

But its major weakness is that, when we try to relate it to the real world, this interpretation does not seem fully “scientific.” It speaks only about probabilities, not certainties—it cannot say beforehand exactly what will happen. Yet for scientists who prefer the Copenhagen interpretation, this is the reality of nature. As a result, another more realist group of scientists emerged who believe that the wave function is merely a mathematical tool used to solve equations. The wave function cannot be seen in the real world, because whenever an observation is attempted, the wave function collapses.

This viewpoint is reflected in the Nature survey, in which 17% of scientists supported an epistemic interpretation. “Epistemic” here means that the wave function is constructed from the viewpoint of an observer—based on what the observer expects the possible outcomes to be.

Quantum scientists feel most uncomfortable when confronted with the idea that wave functions collapse simply because someone looks at them. In 1957, American physicist Hugh Everett proposed a solution to this discomfort: the Many-Worlds interpretation. According to Everett, a particle is, in a sense, present in multiple places at once. In one particular world, an observer measures one particular outcome. But the wave function never really collapses. Instead, it branches into many worlds, each world representing a different outcome. There is no doubt that these worlds are hypothetical. According to the survey, 15% of scientists favor the Many-Worlds interpretation.

As time goes on, scientists are presenting more and more new ideas. But it is clear that quantum theory is still incomplete. Even though quantum mechanics is one of the most experimentally verified theories in the history of science, its mathematics cannot describe gravity. Scientists hope that one day a more advanced form of quantum mechanics will emerge that will cover the shortcomings of the present version.

References:

1. Biggyanchinta, October 2022
2. Nature, 12 August 2025
3. Scientific American, 8 August 2025