Relative to humans, the mysteries of the universe are hidden in the two extremes of time and space, that is, extremely large space, extremely long time, extremely small space, and extremely short time. The reason is very simple. Because we live in this world, the smallest we feel about time is probably a second, and the longest is about a lifetime. The range covered by these limits of time is compared to the 13.8 billion years of the universe, and for a time as short as "attosecond" (10-18 seconds). In addition, the range of space that our eyes can see is also extremely limited, the smallest is probably millimeters (10-3 meters), and the longest is about tens of kilometers. This kind of space range is incomparable to the diameter of the universe of 94 billion light-years, and the size of atoms such as 10-10 meters. Therefore, most people will feel unfamiliar with super large or super small phenomena. It is mysterious and difficult to understand. The phenomenon of quantum is the phenomenon of the microscopic universe (the atomic world). "Quantum mechanics" flourished from the 1920s to today, for about 100 years. The quantum world has its own rules, which are very different from what we are familiar with. So, to this day, we still feel mysterious about most quantum phenomena. Quantum entanglement is one of them. Quantum entanglement is so mysterious that even a genius like Einstein still cannot completely understand it.
Albert Einstein was born in 1879 in Ulm, a small city in southern Germany, about 70 miles southeast of Munich. At the age of 26, he published his theory of special relativity, which surprisingly united time and space, shaking the physics community. Ten years later, he published his theory of general relativity, which showed that matter could warp spacetime, causing a global sensation. In recognition of his contributions to theoretical physics and his discovery of the photoelectric effect in quantum physics, the Nobel Committee awarded him the Prize in Physics in 1921. Although Einstein made significant contributions to quantum physics and was considered a pioneer in the field, he was never entirely satisfied with quantum mechanics' explanation of the world using "probability," because he believed the universe followed strict causal laws and mechanical principles. His famous quote, "God does not play dice," reflects his belief that explaining everything through "probability" stemmed from our incomplete understanding of all variables. That's why we have the concept of "probability" – "it might be like this" or "it might be like that", because there are some hidden variables that we are not aware of.
This "maybe this way," "maybe that way," "maybe here," "maybe there" is precisely the accurate description of the microscopic world of atoms and electrons by the theory of quantum mechanics. An electron, in essence, is a particle. We generally think of it as a particle, like a baseball thrown by a pitcher; although it's flying, it has a fixed position and speed at any point in time. However, quantum mechanics, at any given moment, can only say what the "probability" is of an electron being here, and what the "probability" is of an electron being there. The essence of reality is simply a matter of "probability." Before measurement, a particle doesn't have a "fixed state," only "possible states." The act of "measurement" collapses these "possible states," turning them into "fixed states." Quantum mechanics doesn't mean we don't know the electron's position, but rather that it gives the probability of the particle in any position before measurement. Only after measurement does the electron's position exist; only then do we know, "Oh, the electron is here." If two particles are entangled, regardless of their distance, measuring a particle doesn't just cause the particle to collapse. Instead, it causes a collapse of the entire system (the two particles). This "probabilistic" quantum theory (also known as the "wave function" in quantum mechanics) of Niels Bohr, a master of quantum physics and a member of the Copenhagen School, was first presented to the world's leading scientists at the Solvay Conference in Brussels in October 1927. This conference was extraordinary, bringing together the most outstanding minds in physics, including Einstein, Curie, Bohr, Werner Heisenberg, Schrödinger, Louis de Broglie, Dirac, Max Born, Wolfgang Pauli, Max Planck, and many others. A total of 29 scientists were present, 17 of whom were Nobel laureates. After proposing the "probabilistic quantum theory" and the idea that "existence is only known after measurement," Einstein was deeply troubled and restless. He felt uneasy that the theoretical framework governing the universe, as proposed by quantum mechanics, was not deterministic but rather "probabilistic." He believed that God would not determine the course of the universe by rolling dice. He also rejected the idea that particle properties (such as position or spin) only exist after measurement. In Einstein's view, existence is existence; what a fallacy if existence is true only after measurement! Bohr's claims were utterly absurd. He even asked Bohr at one point, "Do you really believe that the moon only exists when we observe it?" At that meeting, Einstein made “challenging quantum mechanics” his mission, sparking the legendary debate on the "meaning of quantum mechanics." Every morning at breakfast, Einstein would pose a new thought experiment to Bohr to prove that this "probabilistic" quantum theory must be wrong. Bohr also struggled with the weighty questions from the world's most profound thinker and often found it difficult to respond immediately. He would spend the entire day meticulously discussing with his colleagues, returning with impeccable answers only at dinner. The next morning, Einstein would appear with a new challenge. This intellectual contest permeated the entire meeting. Despite Einstein's relentless challenges, the mathematics of quantum mechanics was extremely rigorous. Despite Einstein's best efforts, no one at the meeting could find any loopholes to prove the quantum theory wrong. However, Einstein did not give up. To prove the incompleteness of quantum mechanics, he collaborated with two other scientists, Nathan Rosen and Boris Podolsky, at the Institute for Advanced Study in Princeton, New Jersey. Together, they delved into the theory of quantum mechanics, searching for flaws, and ultimately discovered what they considered a paradox: quantum mechanics predicted a situation that they believed could never occur. Their goal was simple and clear: to prove that quantum mechanics was not the final word. This is the famous EPR paradox, named after Einstein, Podolsky, and Rosen, and published in the physics journal *Physical Review* (47, 777, 1935). Due to Einstein's immense fame, the *New York Times* published a news article on May 3, 1935, titled "Einstein Attacks Quantum Theory." The report stated that he believed any satisfactory physical theory must meet two simple conditions: correctness and completeness. He argued that "quantum mechanics is correct, but not complete.".
So, what exactly is the EPR paradox? This paper gives an example, saying that according to quantum mechanics, the reason why two particles are entangled is because they must obey the "conservation law" of physics when they are produced. Therefore, when a photon spontaneously converts into a negative electron (which is what we know as an electron) and a positron, the spin directions of the two particles must always be opposite. If the spin of the negative electron is down, then the spin of the positron must be up, and vice versa. One up and one down cancel each other out. This is because the photons that generate them have no spin. But the key is that according to quantum mechanics, neither particle has a definite spin state until we actually measure them. They are both in a spin-up and spin-down superposition state at the same time. If we measure a negative electron and find that its spin is up, then at the same moment, no matter how far away the positron is, it must immediately change its spin to down. But here's the problem. The speed of information propagation in the universe is limited. Imagine stretching the distance between these two entangled particles until they are light-years apart. If we measure a negative electron and find that it has spin up, then the positron must immediately change spin down without any delay. Of course some people will say how is this possible, faster than the speed of light! How can the second particle know what happened to the first particle so far away? No signal can travel faster than the speed of light. Even light takes years to travel across a distance of several light years. Quantum mechanics, however, insists that this connection occurs instantaneously. This is the EPR paradox. Einstein used it to demonstrate the incompleteness of quantum mechanics, believing that there must be some hidden variables beneath the surface, carrying information in a way that quantum theory cannot describe. He called this strange effect "spooky action at a distance." Einstein's explanation of the EPR paradox is simple. He believed that when two particles are created at the same time, their properties are already predetermined. The spin of each particle is fixed from the beginning and stored in some hidden form. The only problem is we don't know what this hidden form is. When we measure any of these particles, they will only exhibit a predetermined state, not a random state. It's that simple. This view later became known as the latent variable theory. If hidden variables exist, then two particles do not need to communicate instantaneously across vast distances. In Einstein's view, quantum mechanics is wrong in saying that properties are determined at the moment of measurement; he always believed that quantum mechanics is incomplete because it has not yet revealed these hidden variables. Interestingly, even the founders of quantum mechanics did not yet, at that time, fully realize the importance of entanglement. It was Einstein who first highlighted this phenomenon in his attempt to overturn the theory of quantum mechanics. Later, entanglement was considered the strangest and most mysterious feature of quantum mechanics.
After the publication of the paper on the EPR paradox, Bohr immediately refuted the concept of hidden variables, arguing that they were unnecessary. According to Bohr, entangled particles do indeed influence each other instantaneously, at speeds exceeding the speed of light. He couldn't explain how this happened, but he insisted it was real. Einstein, until his death in 1955, never accepted quantum mechanics as a complete theory. He vehemently opposed it, firmly believed that something deeper was missing. But this didn't stop scientists from using quantum mechanics. The reason is simple: the mathematical calculations are flawless. Every prediction of the quantum mechanical equations matches experimental results time and again. Even if no one fully understands its deeper meaning, the calculations are always correct. Therefore, people use the knowledge of quantum mechanics to develop semiconductors, lasers, and computers, steering human civilization in the direction of quantum mechanics.
The Einstein-Bohr debate took a turn in 1964 when an Irish physicist named John S. Bell proposed a way to completely resolve the dispute. Bell himself leaned towards Einstein's views. He wanted to use the method of purely hidden variables to see if he could obtain the predictions of quantum mechanics regarding quantum entanglement. If he could, it meant that hidden variables existed; if not, it meant that hidden variables did not exist. His paper mainly used many "entangled spin-particle pairs" to conduct a special spin experiment and measurement. If hidden variables existed, meaning Einstein was right, the measurement results would satisfy "Bell's Inequality"; if hidden variables did not exist, meaning Einstein was wrong, the measurement results would violate "Bell's Inequality". In other words, Bell transformed the Einstein-Bohr debate into a measurable condition. The power of this "inequality" lies in the fact that it provides a direct experimental method to distinguish between Einstein's theory and quantum mechanics. When Bell first proposed his test method in 1964, it was purely theoretical. Nobody knew how to actually conduct such an experiment. And the technology simply didn't exist at the time.
This situation changed in 1972, when John F. Clauser, who had just received his PhD in physics from Columbia University in New York, went to the University of California to do "postdoctoral" research and designed the first practical test method for "Bell's inequality". His experimental results clearly showed that "Bell's inequality" was violated. This means that Einstein and his hidden variable theory were wrong. But there are still loopholes in the experiment. Because of this, it cannot completely rule out the possibility of hidden variables. In 1982, French scientist Alan Aspect improved the experiment and successfully plugged some of the loopholes. "Bell's inequality" is violated again. But even so, critics pointed to other possible loopholes. Therefore, this problem was not completely solved. Over the next few decades, Austrian scientist Anton Zeilinger became one of the leaders in experimental quantum physics. He and his collaborators have carried out groundbreaking work in the field of quantum entanglement and even demonstrated a form of quantum teleportation. Over time, through the efforts of Zeilinger and other research teams around the world, the remaining leaks were systematically plugged. The result is always the same: Bell's inequality is violated. The conclusion is inevitable: Einstein's hidden variables do not exist. Bohr was right all along. Ultimately, experiments that were originally intended to prove Einstein right instead proved him wrong. In the quantum world, reality is just probability. Various possibilities exist for particles when they are not observed. At this point, this protracted debate has finally come to an end. Yet quantum mechanics remains as strange and mysterious as ever, bordering on magic. However, it is from this magic that real technologies are born: quantum computing, quantum information science, quantum cryptography, and even quantum teleportation. All of these fields grew out of experiments testing Bell's inequality. For this reason, the 2022 Nobel Prize in Physics was awarded to Clauser, Aspect and Zeilinger to commend these three scientists for experimentally confirming the reality of quantum entanglement, ending one of the most profound debates in physics, and opening up opportunities for human development in quantum computing.
In addition, quantum entanglement poses a disruptive challenge to the traditional philosophy of "local realism." Before the invention of quantum mechanics, philosophers believed that "locality" was inevitable, meaning that no influence could exceed the speed of light (a key conclusion of Einstein's "special relativity"); anything we do here cannot instantly affect distant things. Another principle was "objective reality," that every object possesses definite properties, regardless of whether we measure them. These properties are independent of whether they are observed; they are objective and real. However, these two cornerstones have been shaken by the experimental results of quantum entanglement. Experiments have repeatedly demonstrated that nonlocality is real (although it lacks signal transmission capability), and any theory with local hidden variables—that is, any theory that simultaneously maintains locality and reality—cannot reproduce all the statistical predictions of quantum mechanics. Before observation, the existence of reality is agnostic. Quantum entanglement has been confirmed by the violation of Bell's inequality, which shows that the universe cannot be both "local" and "realistic." This completely overturns the traditional philosophical explanation of nature, which has been around for 2,500 years, that it consists of independently existing objects with definite properties and that these objects interact only locally.
This can be considered one of the most impressive experimental rebuttals to metaphysical worldviews in history. "Local realism" was not only challenged, but it also completely failed in the realm of quantum phenomena; our intuition about reality is not correct. Even Einstein, a staunch believer in "local realism" and the idea that "reality exists independently of the observer," couldn't escape the confines of his Local Realism, ultimately forced to ask, "Do you really believe the moon only exists when we observe it?"
Conclusion： Quantum entanglement allows humans to have a deeper understanding of the real mysteries of nature. It allows us to know that the deep truth is sometimes very different from our intuitive imagination. However, quantum entanglement is not a purely academic research, but has a very practical side, that is, quantum technology including quantum computing. Just like more than 20 years after the development of quantum mechanics, the semiconductor invented by Bell Labs in New Jersey in 1947 created today's popular computers, and thus led to unprecedented civilization. The quantum technology produced by research in quantum entanglement will profoundly affect the development trend of human civilization for hundreds of years to come. (2025-11-25 New Jersey).