“‘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.
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:
- Biggyanchinta, October 2022
- Nature, 12 August 2025
- Scientific American, 8 August 2025
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