Historic Dispute : In his classic debate with Albert Einstein, was Niels Bohr correct in his approach to interpreting the world in light of the newly discovered field of quantum mechanics?

Viewpoint: Yes, Bohr's interpretation of the world in light of quantum mechanics was correct, and new applications of his interpretation are being determined with the passage of time.

Viewpoint: No, while the physics community came to accept the arguments of Bohr, some of the questions raised by Einstein remained unsatisfactorily resolved.

One of the great triumphs of human thought was Isaac Newton's formulation of his laws of motion and his law of universal gravitation. These laws laid the foundation for the development of classical mechanics, the branch of physics that describes the motion of slowly moving objects under the influence of external forces. An "object" might be a ball rolling down a hill, or the Moon orbiting Earth.The power of Newton's laws was that they were predictive: given an object of known mass and velocity, and given an environment such as an inclined surface or a gravitational field, it was possible to predict the subsequent motion of that object. Continuing refinement of the methods and techniques used to solve classical mechanical problems led to highly precise predictions of motions.

The careful reader, however, will notice a crucial caveat in the definition of classical mechanics above. The objects in question must be slowly moving, and in this context, "slow" means relative to the speed of light. It is now well known that classical mechanics does not predict the motion of rapidly moving objects or particles correctly, and that classical mechanics in general does not provide a full description of the physical universe as we currently observe it. Does this mean classical mechanics is wrong? Certainly not; it means that classical mechanics is valid only within certain limits, and those limits happen to encompass everything we experience in our daily lives. We do not move at half the speed of light, nor do we observe phenomena at the atomic level without specialized equipment.

For Newton and the scientists of his day, the principles of classical mechanics fully described the world as they perceived it. In the nineteenth century, however, problems began to emerge. James Clerk Maxwell and Michael Faraday achieved the next great advance in physics after Newton's laws, the mathematical description of electricity and magnetism as complementary elements of a unified concept, sensibly called electromagnetism. Accelerated charged particles were discovered to emit electromagnetic radiation (light), and this led to severe problems with the "classical" model of atoms, in which electrons orbited the nucleus of the atom like little moons orbiting a planet. In classical mechanics, orbiting particles are constantly accelerated, so it seemed like the radiating electrons should quickly lose all their energy and crash into the nucleus. Obviously that didn't happen in real atoms, so physicists quickly realized that the laws of classical mechanics simply didn't work when applied to atomic particles.

These and other problems led to the defining development of twentieth-century physics: quantum mechanics. The great problem prior to quantum mechanics was that matter and light had been treated as fundamentally different phenomena: matter was interpreted as classical particles, while light was treated as waves. In quantum mechanics, this distinction is blurred: matter and light each exhibit characteristics of particles and waves. This concept does not exist within the limits of classical mechanical theory. Development of this concept by the great physicists of the day—Max Planck, Albert Einstein, Louis de Broglie, Niels Bohr, Erwin Schrödinger, and Werner Heisenberg, among others—led to the mature theory of quantum mechanics that underpins modern physics.

Quantum mechanics emerged at the start of the twentieth century, and it was a concept that instigated a radical change in our understanding of the universe. As with any profound concept, its implications rippled from hard science into broader philosophical issues.

In particular, quantum theory introduced a probabilistic view of the universe. Previous formulations of mechanics and the structure of atoms employed a deterministic viewpoint, in which the characteristics of the particles and systems in question were described with definite numerical values. Two physicists, Erwin Schrödinger and Werner Heisenberg, proposed a radically different model. Instead of specifying the precise location of, for example, an electron, Schrödinger described it in terms of its probable location. Put in terms of a simple statement, Schrödinger's argument was that we cannot say "The electron is in this location at this point in time," but we instead say "It is most likely that the electron is in this location at this point in time." Heisenberg continued this idea with his famous uncertainty principle, which states that complementary characteristics of a particle, for example position and momentum, cannot be simultaneously determined with absolute precision. If we know the location of an electron with high certainty, its momentum is uncertain, and vice versa. The classic mental image of this is the process of measuring the position of an electron. How do we do it? We can only observe things by shining light on them. But photons (quanta of light) carry momentum sufficient to move an electron. The classical analogy might be like trying to determine the location of a billiard ball by striking it with the cue ball—we hear a click, so we know roughly where the balls struck, but now the target ball is moving off somewhere else. By analogy, when we shine light on the electron to determine its location, we introduce an uncertainty into its location by the very act of performing the experiment.

This was the crux of the great debate between Albert Einstein and Niels Bohr, and it is one of the finest examples in human intellectual discourse of the ideas and motivations that guide the evaluation of radical new theories. Einstein resisted the probabilistic description of the atom, probably because of deeply held religious convictions that dictated to him a sense of order and absoluteness about the universe. Einstein's famous "God does not throw dice" statement cuts to the center of his reservations about a "fuzzy" universe in which a fundamental uncertainty governed the basic nature of particles. Bohr, on the other hand, was more comfortable with the ideas of Schrödinger and Heisenberg, and he and Einstein debated these ideas at length. The details of these ideas are discussed in the articles that follow.

The essential lesson of the Bohr-Einstein debate is that science, however rigorous and dispassionate, remains a thoroughly human process. Some scientists are deeply religious, while others are atheists—just as might be found in any other profession or culture. Some scientists are conservative, while some are liberal. Although scientists are trained to judge evidence objectively, it is all too easy for these and other personal beliefs, or the career-long development of a given idea, to affect a neutral, objective evaluation of the da ta. This is not to say that Einstein was religious and Bohr was not; that would be a gross oversimplification. The split between Einstein and Bohr centered on profound philosophical differences, and because they were two of the most brilliant minds of the twentieth century, they were each able to construct detailed arguments supporting their points of view. The debate was also respectful and civil; given Einstein and Bohr's respective achievements, there was no need for acrimony. Their debates about the nature of the atom and, by extension, of the underlying structure of the universe, remain one of the essential examples of rigorous testing and cross-examination of a new and controversial idea.

—JEFFREY C. HALL

Viewpoint: Yes, Bohr's interpretation of the world in light of quantum mechanics was correct, and new applications of his interpretation are being determined with the passage of time.

The Effects of Quantization

Max Planck (1858-1947) proposed that rather than having continuous values (like any height if you ascend a ramp), energy could only come in discrete packets (like specific heights such as when you climb stairs). He called these packets quanta.

One person who immediately saw the validity and consequently the applications of Planck's quanta was Albert Einstein (1879-1955). He applied the concept of the quantum to the solution of yet another problem in physics at the time, known as the photoelectric effect. He saw that if, as Planck stated, energy had to consist of discrete packets, then the energy of the light was more important than intensity. As energy is directly related to the frequency, a beam of higher frequency (blue) and low intensity would contain the necessary energy to remove the electrons. On the other hand, a beam of lower frequency (red), no matter how intense, could not remove the electrons. Further, he reasoned, if the energy of the light was more than required to actually remove the electron, the remainder of the energy would be transferred to the electron that was removed. This was experimentally shown and the quantum gained acceptance.

Niels Bohr (1885-1962) was a quiet, introspective Danish scientist. He was fascinated by the discoveries of Planck and Einstein, both of whom he held in great regard. He was intrigued and dissatisfied by the model of the atom proposed by Rutherford, under whom he worked. Upon the advice of a colleague, he studied the most recent results in the field of spectroscopy, the study of the frequencies of light emitted or absorbed by substances (mostly atoms or simple molecules). He noticed that unlike for white light (where all frequencies or colors are observed blending together), atoms and molecules had discrete spectra. In other words, they emitted or absorbed only very specific frequencies of light. Bohr brilliantly put this information together with Planck's quantum to arrive at the first working model of the atom. He theorized that the electron that orbited the nucleus could not simply assume any energy value. He realized that the solution lay in having orbits of specific energy values in which the electrons could remain. If energy was added to the atom/molecule in the form of radiation or a collision, the electron could be "bumped" into a higher energy level. It would eventually lose that energy (in the form of radiation, seen in the spectrum of the atom/molecule), but the electron could not continue to spiral into the nucleus. The electron would return to its original (and lowest) energy level. This explanation satisfied all the physical requirements. It provided a stable model for the atom, and it explained the emission/absorption spectra of atoms and simple molecules. It was a landmark discovery, heralded because of Bohr's ability to determine an equation for the energy levels that gave almost exact values for the energies of the hydrogen atom. Unfortunately, Bohr could not make his model work for any atoms other than hydrogen and one form of helium.

Up to this point, Einstein was a great supporter of the quantum theory, so named because it used the fact that energy in atoms was limited or quantized. Further revelations, however, caused Einstein to change his mind in an almost complete turnabout.

Probabilistic Nature of Quantum Mechanics

Bohr had been able to make deterministic calculations of the atom (that is definite numerical values), in spite of the fact that his success was limited. Shortly after Bohr presented his model, great work was done in another field that had a direct impact on the direction in which quantum mechanics would develop.

Prince Louis de Broglie (1892-1987) provided yet another piece of the puzzle when he stated that if light could possess the properties of a particle, matter should possess the properties of a wave! This dual nature of matter and light came to be known as wave-particle duality.

Erwin Schrödinger (1887-1961) was now able to formulate his wave-equations that, to date, are the most accurate representation of the atom. His model of the atom describes the location of the electrons in atoms using a probability function rather than a defined orbit. He essentially postulated that while it was possible to know where the electron was most likely to be, it was impossible to know exactly where it would be. Werner Heisenberg (1901-1976) delivered the final blow to the deterministic scientists. He declared that it was impossible to determine two complementary qualities of an atomic system exactly. If one was known exactly, the other could not be known except in the most general terms. This limitation came to be known as the Heisenberg Uncertainty Principle. It was the uncertainty principle that made Einstein turn completely away from the quantum mechanical theory of the atom. Until the day he died, he refused to accept quantum mechanics as the complete solution of the atom.

What, then, was the nature of the disagreement between Bohr and Einstein? Essentially, until Schrödinger and Heisenberg, science was deterministic in nature. Physical quantities could be determined with great accuracy, and the behavior of a system could be predicted if the nature of the system was known. Hence, the laws of classical physics allowed for the determination of the acceleration, momentum, velocity, and position of a ball to be known if it was thrown in the air. Einstein pushed back the borders of classical physics with the Special and then General Theories of Relativity. He could not, at that time, perform actual physical experiments that would demonstrate the validity of his ideas, but he constructed what are known as thought experiments. In a thought experiment, an "imaginary" experiment was performed that HAD to conform to the known laws of physics. In other words, just because it was not performed in a laboratory did not mean that the physical laws could be ignored. Einstein's theories of relativity were indeed hailed as extraordinary because his thought experiments were subsequently proven with the advent of further developments in technology. Einstein's use of Planck's quantum to explain blackbody radiation was validated by the observed energies of the blackbody. Hence, Einstein accepted the quantization of energy with no hesitation.

However, Schrödinger moved away from the concept of absolute values with the introduction of the wave equation. The wave equation essentially described electrons using a probability function. Unlike Bohr, who assigned electrons to orbits having quantized energy values, Schrödinger deemed it necessary to calculate the probability of finding a certain electron in a particular region. The wave equation described the shape of the region where it was most probable that the electron could be found.

It is most likely that Einstein understood that the basic nature of this new field would no longer be deterministic. He accepted and extended the work of Satyendranath (S. N.) Bose (1894-1975) on statistical mechanics (the modeling of the average behavior of a large number of particles) to gases. The Bose-Einstein statistics was the first indication that exact behavior of each and every particle in a large assembly of particles was not necessarily a possibility. Rather, the behavior of the particles was averaged according to statistical probabilities. In spite of this awareness, Einstein was not satisfied with Schrödinger's wave equations. Bohr, on the other hand, immediately saw the necessity of the application of probabilities to describe wave functions.

It was while working with Bohr that Heisenberg formulated the uncertainty principle. While it is a simple mathematical formulation that limits the extent to which two complementary physical quantities can be determined, its implications were immediately understood by both Bohr and Einstein. The uncertainty principle was, in effect, a statement that the observer and the observed could not remain distinct at the atomic level.

What does this mean? If a ball is thrown into the air, the act of observing its motion (using vision or any measuring instrument) does not disrupt the motion of the ball, nor does it change its behavior in any way. The same is true of a car in linear motion, of an asteroid in circular motion, or a planet in elliptical motion. In all of classical physics, observation of a system has no measurable effect on the system and does not affect its subsequent behavior. However, as we approach the infinitesimal atomic systems, this observation is no longer true. It is not possible to "see" atomic systems visually. It is necessary to use sophisticated instruments, such as an electron microscope or an x-ray diffractometer. Such devices use energy, or light, or high-speed small particles (in some ways these are all synonymous) to study the system. The energy/light/high-speed small particle reflects off the system and delivers a signal that is read by the device.

However, the energy/light/high-speed small particle, in sending back information, also disrupts the system! (The closest analogy might be trying to determine the trajectory of a baseball by throwing another baseball at it.) If an effort was being made to determine the momentum and the position of an electron in an atom, then Heisenberg maintained that if the momentum of the electron was determined with some precision, the position was only vaguely known. Similarly if the position was determined with some precision, then the momentum was only vaguely known. If the position was determined precisely, it was not at all possible to determine the momentum and vice versa.

Bohr realized that this limitation was inherent in the nature of quantum mechanics. Einstein maintained that if such a situation existed, quantum mechanics could not be the final solution to the atom and was most likely an intermediate solution.

Bohr-Einstein Debates

Einstein's abrupt turnabout was a blow to the scientific community headed by Bohr, who firmly believed that Heisenberg's revolutionary statement of the nature of the atom was correct. When they met at the Solvay conference, a place where the great minds of the time all assembled to discuss the astonishing developments of the past few years, Einstein and Bohr had the first open debate about the validity of the quantum theory.

In order to refute the contention that only one of two complementary variables could be determined exactly, Einstein designed a thought experiment. The experiment consisted of a single electron being passed through a plate containing a slit and striking a photographic plate. Einstein maintained that once the electron had exposed a spot on the photographic plate, there was no doubt in its position or its path. He thus maintained that quantum mechanics dealt with the average behavior of particles and hence the uncertainty arose due to the averages. Bohr revised Einstein's experiment to include a second plate between the plate with the slit and the photographic plate. The second plate contained two slits that lay on either side of the original slit. Bohr showed that if a strong beam of electrons was passed through such a system, an interference pattern was produced on the photographic plate. Then he suggested reducing the intensity of the beam until it was so weak that only one electron at a time could go through the series of slits. He agreed that the first electron, as Einstein had suggested, would strike a particular spot, but if other such "single" electrons were allowed to pass through, they would eventually produce the same interference pattern! Thus, indirectly, the first electron DID have an effect on the behavior of the subsequent electrons. Bohr used this experiment to indicate that the system could not be completely independent of the method of observation.

Three years later, at the next Solvay conference, Einstein proposed yet another thought experiment called the photon in the box. He designed a system consisting of a box containing a clock and some photons (light particles). The box had a small hole. The clock, at a certain time, would allow the release of a single photon. The mass of the box before and after the escape of the photon would allow for calculation of the mass of the photon (exactly). Once its mass was known, then the energy of the photon could be determined exactly using Einstein's own equation, E = mc². Bohr took one night to come up with a response to Einstein. He indicated to Einstein that in order to determine the mass of the box, it would be attached to a spring scale. The initial measurement was accurate without question. However, once the photon escaped, the box would move due to the mass change. According to Einstein's own theory of relativity, such a movement would cause a change in the rate of the clock in the box. The longer you took to read the mass (spring scales take time to settle), the less certainly you knew the time. The more quickly you measured the time, the less certainly you knew the mass. Since the mass is directly related to the energy, the energy of the photon and time are complementary values and this again reduced to an equation of the form of Heisenberg's uncertainty principle. After this exchange with Bohr, Einstein did not publicly debate this issue with him. While he accepted that Bohr's logic was valid, he could never bring himself to accept the possibility that the Universe could be anything other than deterministic.

Deeper Meaning Behind the Bohr-Einstein Debate

The crux of the issue for Einstein was that he believed that the universe is ordered and has a logical nature. He believed that based on physical observations, one should be able to predict the nature of a system. Most importantly, he felt that the scientist was the observer and the system was the observed.

If Heisenberg was right, however, it meant that there was no order in the universe, and there was no predictability. Probability is about chance, not certainty. It meant that a system could ultimately not be known because in the final analysis, trying to analyze the system caused the system to change. In other words, the observer was part of the system, along with the observed. Bohr realized that it was possible to determine the values for two complementary parts of system; it was just not possible to determine them simultaneously. The observer determined what the observed would be by choosing the system and once chosen, until the course of the measure was completed, it could not be changed.

Deep down, in spite of his professed disbelief in any religious system, Einstein realized that Heisenberg's uncertainty principle affected his moral and religious core. Christianity, Judaism, and Islam, all religions that follow the Old Testament, essentially believe in that which is self-evident. Order exists in the universe, the order comes from a source, and the source is God. If science showed that the fundamental nature of the universe was not ordered, but random (based on probability), what did that say of God? Quantum mechanics touched on the very sensitive core of a number of scientists who professed atheism, yet who were not capable of truly accepting it in their work. Even if they didn't believe in God, they believed in the concept of order and a source (by any other name). Einstein's true dilemma was his inability to accept that the science he loved pointed to a world that was not ordered. Could such a world have been created by God? Creation, by its very nature, supports order. As long as Einstein's science supported this order, he accepted it. The moment the science veered from the path of order, Einstein could not accept it. His faith was deeper than he himself thought.

This is not to say that Bohr was without faith. However, he did not hesitate to carry his scientific convictions to the logical conclusion. If that meant that the concept of an orderly creation of God was disproved, then, so be it. However, the eastern philosophies are much broader in their way of thinking and it was to these philosophies that the great quantum scientists, such as Bohr and Schrödinger, turned. The Hindu philosophy believes in the created as part of the creator, not such a different philosophy from that derived in quantum physics.

An interesting anecdote is offered by Léon Rosenfeld (1904-1974), a contemporary and a close friend and supporter of Bohr. Rosenfeld met with Hideki Yukawa (1907-1981) in Kyoto, Japan, in the early 1960s. Yukawa's work with the meson and the complementary concepts of elementary particle and field of nuclear force derived largely from Bohr's insight. When Rosenfeld asked Yukawa whether the Japanese had difficulty convincing their scientists of the validity of the concept of complementarity, Yukawa's response was "No, Bohr's argumentation has always appeared quite evident to us. You see, we in Japan have not been corrupted by Aristotle."

Science today has more than validated the concept of complementarity, and has found numerous other examples of complementary quantities that exhibit the limitations of the uncertainty principle. In spite of his quiet voice, and soft demeanor, Bohr's science and strength of conviction have firmly established themselves in a world where quantum mechanics is a part of daily life. There is little doubt that Bohr's interpretation of the world of quantum mechanics is here to stay until, perhaps, the next great scientific revolution.

—RASHMI VENKATESWARAN

Viewpoint: No, while the physics community came to accept the arguments of Bohr, some of the questions raised by Einstein remained unsatisfactorily resolved.

Between 1927 and 1936 a series of debates took place between Niels Bohr (1885-1962) and Albert Einstein (1879-1955) over the details of quantum physics and the nature of reality. While the physics community came to accept the arguments of Bohr, some of the questions raised by Einstein remained unsatisfactorily resolved. Bohr's view became the standard used by physicists, and all experimental results since have confirmed quantum theory. Yet Bohr's interpretation leads to many uncomfortable consequences in the philosophy of physics.

At the very least Albert Einstein's criticisms of quantum theory were important in that they forced Bohr and others to face uncomfortable paradoxes inherent in the Copenhagen interpretation of quantum physics, and refine and clarify their arguments. Bohr himself wrote that "Einstein's concern and criticism provided a most valuable incentive for us all to reexamine the various aspects of the situations as regards the description of atomic phenomena." Yet the popular opinion among the scientific community at the time was that Einstein's objections were naïve, and that he was struggling to understand the new physics. Over time there has grown a greater appreciation for Einstein's opposition to Bohr's interpretation, and their debates are now considered some of the most important in the history of science.

Duality and Uncertainty

Two of the ideas essential to the Copenhagen interpretation are the ideas of wave-particle duality and the Heisenberg uncertainty principle, developed by Werner Heisenberg (1901-1976) in 1927. Particles, atoms, and even larger objects possess the strange property of exhibiting both wave and particle-like properties, depending on how they are observed. This can create some strange situations. For example, if Thomas Young's (1773-1829) famous two-slit interference experiment (which first showed the wave-like nature of light) is modified so that only one photon of light is passed through one at a time, bizarre quantum effects can be generated. If the experiment is allowed to continue as normal then the expected wave interference pattern is built up one photon at a time. The quantum wave function that represents the photon has the same chance of passing through either slit, and so in effect interferes with itself to produce the pattern. However, if a detector is placed at one of the slits then the photon "particle" either passes through the slit or it does not, and suddenly the interference pattern vanishes. Setting up the experiment to detect the photon as a particle produces a result that suggests it is a particle, whereas setting up the experiment to look for wave properties (the interference pattern) gives results that exhibit wave properties.

The uncertainty principle states that it is impossible to measure certain pairs of properties with any great accuracy. For example, you cannot know both the position and momentum of a particle within certain well-defined limits. The more accurately you determine the position of the particle, the more uncertain the value of the momentum will become, and vise versa. There are other uncertainty pairs of properties aside from position and momentum, such as energy and time.

Einstein's criticisms of Bohr's quantum theory exploited the strangeness of wave-particle duality and attempted to circumvent the uncertainty principle, and thereby show quantum theory to be incorrect. However, while Einstein may have failed in these attempts, in doing so other fascinating questions were raised.

Bohr developed a theory based on the experimental evidence that contradicted classical Newtonian physics. He believed he had discovered a consistent interpretation of quantum mechanics. Bohr suggested it was meaningless to ask what an electron is. Physics is not about what is, but what we can say to each other concerning the world. Since we can only really understand classic physics concepts, such as position and momentum, the role of quantum theory was to provide a mechanism with which these could be communicated. However, that in no way implies that objects at the quantum level actually have these values. In Young's experiment we can either leave the wave-particles alone, and observe an interference pattern, or take a peek at the wave-particles as they go through the experiment, as in doing so lose the pattern. The two situations are not contradictory, but complementary.

The Solvay Conferences

Taking Bohr's viewpoint, the world has no independent existence, but is tied to our perceptions of it. Einstein feared that physics as defined by Bohr was becoming wholly statistical and probabilistic in nature, and strove to keep some level of reality in physics by attempting to find an underlying determinism that was not explained by quantum theory. In 1927, during informal meetings at the fifth Solvay Conference in Brussels, Einstein attempted to offer logical arguments against the wave interpretation of quantum theory. He proposed a number of imaginary experiments to Bohr, attempting to outflank the uncertainty principle, however, Bohr managed to resolve all of Einstein's objections.

In 1930, at the sixth Solvay conference, Einstein attempted to attack the uncertainty principle again. He asked Bohr to imagine a box with a hole in its side which would be opened and closed by a clockwork shutter inside the box. The box contains a radiation source which emits a single photon through the shutter which would be opened for a short time. Einstein argued you could weigh the box before and after the photon was emitted and the change in mass would reveal the energy of the photon, using the equation E=mc². You could then read the time the photon was emitted from the clockwork inside the box, thereby determining both energy and time to an accuracy that would defy the uncertainty principle.

Many commentators have noted that Bohr's reply is difficult to follow, due to the condensed form in which he gave it, but nevertheless ingeniously showed that the uncertainty principle is still supported, and is a function of the measurement of the change in mass. Einstein accepted Bohr's reply as correct.

Is Quantum Theory Complete?

Einstein then modified the photon-box thought experiment in an article with R. C. Tolman and Boris Podolski (1896-1966). After the photon was gone, they suggested, you could either measure the mass change, thereby determining its energy, or read the time of its release from the internal mechanism (but not both) according to the principles of quantum theory. You could then predict either the time it would arrive at a certain location, or its energy at that location (but not both).

However, either of these can be done without disturbing the photon. The point, which was not made clear in the paper, was that if you can determine either property without disturbing the photon then they are not mutually interfering properties, and the process must actually have an exact time of occurrence and the photon must have an exact energy, even if we cannot observe them together. This would mean that quantum theory could not provide a complete description for the world.

Bohr's response was that the setup is not one, but two, experiment systems. The weighing device and the clock work shutter are two separate systems and so there are two different quantum phenomena to be measured. Therefore quantum uncertainty is saved.

Einstein's most serious attack on quantum physics was published in 1935 in a paper with Podolski and Nathan Rosen (1909-1995). The EPR paper, as it came to be known, suffered from an argument that was not as strong as Einstein first intended, and it was unclear in places. Bohr's reply was also confused. Nonetheless, most scientists at the time thought that Bohr had got the better of Einstein, and Bohr's interpretation became the orthodox interpretation. However, lying in wait in the EPR paper was the germ of an argument that would take 50 years to be resolved.

Imagine a single stationary particle that explodes into two halves. The conservation of momentum implies that the momentum of one particle can be used to deduce the momentum of the other. Or, symmetrically, you could measure the position of one particle, which would reveal the other particle's position. Like Einstein's previous thought experiment, the key is that you can therefore deduce something about a particle without observing it. However, this time the experiment definitely remains one quantum system.

Quantum theory suggests that until either particle is observed it is a probabilistic wave function. This wave function will have a wide range of possibilities, only one of which will be revealed when the particle is observed and the wave collapses. It does not, however, have an actual position or momentum until observation. This causes problems if the two particles in the EPR experiment are separated by a large distance. When both are observed simultaneously they must both give the same result as per the conservation of momentum. If the particles have no actual values until observation, how does one particle "know" what value the other wave function has collapsed to? They would appear to need faster-than-light communication, which Einstein referred to as "spooky forces" at a distance. Einstein's solution was to argue that there were hidden variables underneath quantum mechanics which gave the particles an actual position and momentum at all times, independent of observation. Bohr rejected Einstein's argument by saying that the particles must be viewed against the total context of the experiment, and that a physical property cannot be ascribed to a particle unless the situation is a meaningful one. This reasoning still allows this spooky conspiracy between the two particles, and it appears that even Bohr was unsatisfied with his answer. Certainly many commentators since have suggested that Bohr's published answer to the EPR paper does not make much sense.

Aftermath

After 1936 the public debate over quantum physics between Bohr and Einstein ceased. The Copenhagen interpretation remained dominant, and is happily used by physicists who ignore the bizarre philosophical implications simply because it works so well. However, the EPR argument was too powerful to disappear completely, and later developments brought it back into the spotlight.

In 1965 John Bell (1928-1990) published a highly mathematical paper that suggested a method for determining experimentally between Bohr and Einstein's interpretation of reality. Bell's inequality theorem examined the logic that governs the process of measurement in two-particle systems. The paper established a theoretical limit to the level of correlation for simultaneous two-particle measurement results. However, if quantum physics was correct then experimental results should sometimes exceed Bell's limit.

It was not until the early 1980s that sufficiently accurate experiments were carried out to test Bell's inequalities, and therefore determine between Bohr and Einstein's views of quantum physics. The most important experiments were carried out by Alain Aspect and his colleagues. Their experiments, confirmed by many others since, produced observations which cannot be predicted by a theory in which the particles in the EPR experiment do not influence each other. This appears to have settled the argument between Bohr and Einstein in Bohr's favor, and has some profound consequences for the philosophy of physics.

If Einstein had been basically correct, then quantum effects would be the result of the behavior of real particles lying "underneath" the quantum level. There would also be no faster-than-light signaling between the particles, which is usually referred to as locality. However, the work of Bell, Aspect, and others strongly suggests that Einstein was wrong, and that as Bohr suggested either there is no underlying reality behind quantum physics, or there are non-local effects that violate Einstein's theory of relativity.

Einstein was right to question the Copenhagen interpretation, and it should still be questioned. That is, in many ways, the nature of scientific inquiry. The philosophical problems that come with the Bohr's theory of quantum physics are considered too great for many thinkers, and in many quarters there is a profound "unhappiness" with the Copenhagen interpretation. There have been many other attempts to reinterpret quantum physics in a manner that removes these problems, such as the many-worlds interpretation, the many-minds interpretation, and non-local hidden variables, so Einstein was by no means alone in his philosophical objections. It is hoped that these alternative interpretations will one day be tested experimentally, just as Einstein's has been, but for now they remain different only in the philosophical, not physical, consequences they predict.

While many contemporaries saw the objections of Einstein as the reactions of a conservative unable to cope with the rapidly developing theory, many historians of science have come to regard the debate as one of the most important in the philosophy of science. It is important to realize that Einstein was not trying to discredit quantum physics per se, rather he disagreed with the philosophical direction Bohr's interpretation was taking.

The questions raised by the EPR paper have only recently been tested experimentally. Hidden variables and locality have now been shown to be incompatible, but there is still some room for debate, and the possibility of non-local hidden variables has not been ruled out. Indeed, in attempting to resolve Einstein's objections to the Copenhagen interpretation, many more questions, with some extreme philosophical consequences, have been raised. Bohr's interpretation of physics may have many challenges ahead in the years to come.

—DAVID TULLOCH

Further Reading

Beller, Mara, Robert S. Cohen, and Jurgen Renn, eds. Einstein in Context. Cambridge: Cambridge University Press, 1993.

Bernstein, Jeremy. A Comprehensible World: On Modern Science and Its Origins. New York: Random House, 1967.

Bohr, Niels. Atomic Physics and Human Knowledge. New York: Science Editions, Inc., 1961.

———. Essays 1958-1962 on Atomic Physics and Human Knowledge. Great Britain: Richard Clay and Company, Ltd., 1963.

Cohen, R. S., and John J. Stachel, eds. "Selected Papers of Léon Rosenfeld." Boston Studies in the Philosophy of Science 21 (1979).

Davies, P. C. W., and J. R. Brown, eds. The Ghost in the Atom. Cambridge: Cambridge University Press, 1986.

Fine, Arthur. The Shaky Game: Einstein, Realism and the Quantum Theory. Chicago: The University of Chicago Press, 1986.

Gamow, George. Biography of Physics. New York: Harper & Brothers, Publishers, 1961.

Maudlin, Tim. Quantum Non-Locality and Relativity. Oxford: Blackwell, 1994.

Murdoch, Dugald. Niels Bohr's Philosophy of Physics. Cambridge: Cambridge University Press, 1987.

Petruccioli, Sandro. Atoms, Metaphors and Paradoxes. Cambridge: Cambridge University Press, 1993.

"The Sciences." New York Academy of Sciences. 19, no. 3 (March 1979).

Snow, C. P. The Physicists. Great Britain: Morrison & Gibb Limited, 1981.

KEY TERMS

ATOM:

Originally believed to be the smallest indivisible particle of which matter was composed, it is now known to consist of protons, neutrons, and electrons, in addition to at least 40 other subatomic particles.

ELECTRON:

A negatively charged subatomic particle. Its mass is approximately one thousandth that of the proton. In a neutral atom, the number of electrons equals the number of protons. The electron is found in an area around the nucleus determined by the orbit it occupies.

HIDDEN VARIABLES:

Proposed as a means of putting reality back into physics. Rather than a wave-particle merely being a statistical probability pattern until observation, it has been suggested that there are hidden variables not explained by quantum theory so that a particle has actual properties independent of observation.

LOCALITY:

The local area around an object is the area that light could travel away from it from a given time. You can imagine it as an expanding sphere that grows over time. Objects outside that sphere are non-local, as, according to Einstein's theory of Relativity, nothing can move faster than light. Therefore, if locality holds true no object outside the local area can influence the behavior of an object.

PHOTOELECTRIC EFFECT:

The observation that light of great intensity but the wrong wave length had insufficient energy to remove an electron from the outermost energy level of an atom. If light of the correct energy was aimed at the atom, the electron received sufficient energy to be removed. Increasing the intensity increased the number of ejected electrons. Increasing the energy of the light caused the ejected electrons to have higher energy and hence velocity.

QUANTUM:

A discrete quantity or amount, from the Greek word quanta. Coined by Max Plank in 1900 in his explanation of black body radiation, the idea that energy was quantized, rather than continuous, was at first resisted, but later developed into a new theory of physics at the atomic scale.

QUANTUM MECHANICS:

The most current model that describes the atom. Quantum mechanics uses wave functions to describe the region of greatest probability of finding the electrons in an atom.

UNCERTAINTY PRINCIPLE:

The realization that if a system is defined by two complementary variables, the product of the uncertainties of the two variables has a limit of a very small number (usually Planck's constant). In other words, if two variables defining a system are complementary, the determination of one with certainty results in the inability to determine the other with certainty.

WAVE FUNCTION:

A mathematical solution to the wave equation developed by Schrödinger. The wave function mathematically contains the limitations originally set out by Bohr to describe the energy states of electrons in an atom.