Quantum mechanics, philosophy and controversy

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Quantum mechanics has had many detractors including Albert Einstein and Erwin Schroedinger. Quantum mechanics has had a profound effect on philosophy. Determinism is a philosophical view that the universe is governed by determinism if given a specific state of the universe at a specific time, the future state of the universe is fixed as a matter of natural law. The philosophy of determinism was derived from science, from Newton's laws, and pre-Newtonian physics, in that the ability to predict future outcomes in the universe (such as future position of planets) was made possible by science. Quantum mechanics took away predictability and therefore was a blow to philosophy. However, the main founder of quantum mechanics, Niels Bohr, is said to have a philosophy of determinism similar to the rationalization by Immanuel Kant. This article will attempt, without going into religious implications which are personal matters, to explain the position of many physicists on quantum mechanics and the profound effect that quantum mechanics has had on philosophy.

Contents 1 Philosophical determinism 2 Consequences of the uncertainty principle 3 Erwin Schrödinger controversy 4 Comments by other quantum physicists 5 External links

[edit] Philosophical determinism

The 18th century saw many advances in the domain of science. After Newton, most scientists agreed on the presupposition that the universe is governed by natural laws that can be discovered and formalized by means of scientific observation and experiment. This position is known as determinism. However, while determinism was the fundamental presupposition of post-Newtonian physics, it quickly lead philosophers to a tremendous problem: if the universe, and thus the entire world is governed by natural law, then that means that human beings are also governed by natural law in their own actions. In other words, it means that there is no such thing as human freedom. If it is accepted that everything in the world is governed by natural law, then we must also accept that it is not possible for us to will our own actions as free individuals; rather, they must be determined by universal laws of nature. Conversely, if it is accepted that human beings do have free will, then we must accept that the world is not entirely governed by natural law. However, if the world is not entirely governed by natural law, then the task of science is rendered impossible: if the task of science is to discover and formalize the laws of nature, then what task is left for science if it has been decided that nature is not entirely governed by laws? Thus, there are extremely compelling reasons to want to accept both free will and determinism. However, the two seem totally irreconcilable.

Immanuel Kant, whose work dates towards the end of the 18th Century, attempted to reconcile the seemingly incompatible schools of thought known as empiricism (e.g., David Hume) and rationalism (e.g., René Descartes). According to the empiricists, the only possible knowledge of the world is the knowledge that can be obtained by means of perception (inductive reasoning). Thus, for the empiricists concepts are abstractions that we derive by mentally comparing several different perceptions and noting some quality shared by all of them: for example, we see a fire engine, a rubber ball, and a dress, we perceive some quality that is shared by these different objects, and we abstract this quality from the objects themselves in order to arrive at the concept of the color red. For Hume and the empiricists, this means that our concepts, such as the concept of cause and effect, are not actually legitimate properties of the world, but are rather mental constructs that we produce from repeated observation. Since we can never actually perceive cause and effect (because it is not an object, but rather a relation), we can never obtain certain knowledge of whether it actually exists. In other words, since we can't perceive it, we can never be totally sure that we are not just imagining it. For the rationalists, on the other hand, the situation is entirely the reverse, and the only certain knowledge is the knowledge that we derive by means of pure logic (deductive reasoning). The privileged model of certainty for the rationalists is mathematics. For the rationalists, we can never be certain of any knowledge derived from perception, since we are capable of perceiving objects that are almost certainly false in dreams. In other words, since there is no difference between an object perceived while we are awake and the same object perceived during a dream, we can never derive certainty from perception. Mathematics does not require any perceivable object in order to arrive at its proofs, because it works in purely logical relations between concepts.

Kant approached this problem most famously in his major work of epistemology, The Critique of Pure Reason (Kritik der reinen Vernunft, 1781). In order to reconcile these disparate views, Kant found it necessary to split the world into two completely separate aspects:

• 1. The world as appearance — that is, as it appears to us in our perceptions.
• 2. The world as a thing-in-itself — in other words, independent of all human perception.

For Kant, all scientific knowledge (which at that time included philosophy too) refers to the world of/as appearance. The world as a thing-in-itself is, according to Kant, not a "possible object of experience", and all human (conceptual) knowledge refers to the world as we experience it. By splitting the world in the world in this way, Kant was able to offer compelling solutions to some of the most historically difficult questions faced by philosophy. Most importanly, it allowed him to offer a solution to the question of free will versus determinism. Kant argued that, in the world of appearances, determinism is the rule. In other words, according to Kant, in the world of appearances, there is no object that is not governed by the laws of nature. However, this doesn't preclude the possibility that human freedom exists, with the proviso that it exists as a thing-in-itself. In other words, for Kant, human freedom is not a possible object of experience, but that doesn't make it any less real. Even though we can never perceive human freedom, the mere fact that we can will actions for which we can find no cause in the world of appearances is enough to make human freedom a reasonable assumption. It must remain an assumption, since we cannot have knowledge of something that is not an object of experience, but it is an assumption worth making, since it is what makes morality possible.

Thus, Kant was able to offer a coherent answer to the question of how it is possible for both free will and determinism to apply to the same world, but in order to do so, he found it necessary to split the world into these two totally separate aspects. This method made possible tremendous advances in philosophical thought throughout the late eighteenth and early nineteenth centuries. However, it also set strict limits on human knowledge. For Kant, we cannot 'know' freedom or any other thing-in-itself in a rigorously scientific way. Rather, freedom is more like a necessary assumption. We can only 'know' objects as they appear to us — our knowledge is only knowledge of the world of appearances. Any claim to have knowledge of objects as they are in themselves is an illegitimate use of the faculties of reason and understanding. This is why, for Kant, it is impossible to prove the existence of God, of the soul, or of human freedom: none of these are possible objects of experience. This is not just a historically specific problem that might be overcome as science advances and we learn more and more; it is constitutive of all human knowledge. In other words, even while Kant enabled great leaps forward in philosophical thought, he did so by introducing a concept of human knowledge as essentially limited, and essentially fallible.

Although twentieth century scientists left the question of human will to the philosophers, scientists themselves felt very firmly grounded in the idea that science could make predictions according to Newton's laws with regard to objects in nature. Therefore, determinism was still one of the fundamental axioms of scientific thought. Even with Einstein's theory of relativity, determinism was not seriously challenged. This was all about to change.

[edit] Consequences of the uncertainty principle

The Uncertainty Principle is a main theory in the physical science of quantum mechanics that explains the universe at atomic and subatomic scales.

The Uncertainty Principle was developed as an answer to the question: How does one measure the location of an electron around a nucleus?

In March 1926, working in Niels Bohr's institute, Werner Heisenberg formulated the principle of uncertainty thereby laying the foundation of what became known as the Copenhagen interpretation of quantum mechanics. Heisenberg had been studying the papers of Paul Dirac and Jordan. Heisenberg discovered a problem with measurement of basic variables in the equations. His analysis showed that uncertainties, or imprecisions, always turned up if one tried to measure the position and the momentum of a particle at the same time. Heisenberg concluded that these uncertainties or imprecisions in the measurements were not the fault of the experimenter, but fundamental in nature and inherent in quantum mechanics.

The term Copenhagen interpretation of quantum mechanics was often used interchangeably with and as a synonym for Heisenberg's Uncertainty Principle by detractors who believed in fate and determinism and saw the common features of the Bohr-Heisenberg theories as a threat. Within the widely but not universally accepted Copenhagen interpretation of quantum mechanics (i.e. it was not accepted by Einstein or other physicists such as Alfred Lande), the uncertainty principle is taken to mean that on an elementary level, the physical universe does not exist in a deterministic form, but rather as a collection of probabilities, or potentials. For example, the pattern (probability distribution) produced by millions of photons passing through a diffraction slit can be calculated using quantum mechanics, but the exact path of each photon cannot be predicted by any known method. The Copenhagen interpretation holds that it cannot be predicted by any method, not even with theoretically infinitely precise measurements.

Albert Einstein was not happy with the uncertainty principle, and he challenged Niels Bohr and Werner Heisenberg with a famous thought experiment (See the Bohr-Einstein debates for more details).

It is this interpretation that Einstein was questioning when he said "I cannot believe that God would choose to play dice with the universe." Bohr, who was one of the authors of the Copenhagen interpretation responded, "Einstein, don't tell God what to do." Niels Bohr himself acknowledged that quantum mechanics and the uncertainty principle were counter-intuitive when he stated, "Anyone who is not shocked by quantum theory has not understood a single word."

The basic debate between Einstein and Bohr (including Heisenberg's Uncertainty Principle) was that Einstein was in essence saying: "Of course, we can know where something is; we can know the position of a moving particle if we know every possible detail, and thereby by extension, we can predict where it will go." Bohr and Heisenberg were saying the opposite: "There is no way to know where a moving particle is ever even given every possible detail, and thereby by extension, we can never predict where it will go."

Einstein was convinced that this interpretation was in error. His reasoning was that all previously known probability distributions arose from deterministic events. The distribution of a flipped coin or a rolled dice can be described with a probability distribution (50% heads, 50% tails). But this does not mean that their physical motions are unpredictable. Ordinary mechanics can be used to calculate exactly how each coin will land, if the forces acting on it are known. And the heads/tails distribution will still line up with the probability distribution (given random initial forces).

Einstein was not adverse to quantum mechanics as a whole, but specifically with the uncertainty principle itself. As to other basic principles of quantum mechanics, Einstein whose own general relativity was firmly rooted in field theory said:

"The de Broglie-Schrödinger method, which has in a certain sense the character of a field theory, does indeed deduce the existence of only discrete states, in surprising agreement with empirical facts. It does so on the basis of differential equations applying a kind of resonance argument." (Albert Einstein, On Quantum Physics, 1954)

Niels Bohr himself appears to have taken Kant's view that there are two aspects of reality, what we can say about and what it is, when Bohr said:

"There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."

In other words, there may be a definite position of a particle contrary to the uncertainty principle, but the only way we mere humans can describe mathematically in a useful way what we see in the real world is to use Quantum Mechanics. This is because theories are simple models of complex systems. The universe is too complex to describe without simple models. Because quantum mechanics is useful and continues to provide sound mathematics when tested, it is a mainstream theory of the universe at the quantum level. Niels Bohr's comment was saying that he himself believed that in all probability the natural world was different than the explanation given by quantum mechanics which is similar to Kant's view.

Heisenberg wrote a conversation between himself and Einstein further debating their different viewpoints as follows:

• Heisenberg: "One cannot observe the electron orbits inside the atom. [...] but since it is reasonable to consider only those quantities in a theory that can be measured, it seemed natural to me to introduce them only as entities, as representatives of electron orbits, so to speak."
• Einstein: "But you don't seriously believe that only observable quantities should be considered in a physical theory?"
• "I thought this was the very idea that your relativity theory is based on?" Heisenberg asked in surprise.
• "Perhaps I used this kind of reasoning," replied Einstein, "but it is nonsense nevertheless. [...] In reality the opposite is true: only the theory decides what can be observed."-- (translated from "Der Teil und das Ganze" by W. Heisenberg)

Werner Heisenberg himself said, "`I myself . . . only came to believe in the uncertainty relations after many pangs of conscience. . . ." He knew what he was saying didn't make sense, but it helped measurements at quantum levels so much, he did it anyway. Richard Feynman, another major contributor to quantum theory said, "We have always had a great deal of difficulty understanding the world view that quantum mechanics represents. At least I do, because I'm an old enough man that I haven't got to the point that this stuff is obvious to me. Okay, I still get nervous with it.... You know how it always is, every new idea, it takes a generation or two until it becomes obvious that there's no real problem. I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem." He meant that he understood quantum mechanics very well, but that in 1982 some 50 years later, he still couldn't reconcile himself to it. That is why Einstein spent the entire rest of his life trying to disprove the Uncertainty Principle however there is no other theory to replace quantum mechanics that is so successful at the quantum level.

[edit] Erwin Schrödinger controversy

Later in life, the inventor of wave mechanics of quantum theory, Erwin Schrödinger began a campaign against the generally accepted quantum description of wave-particle duality and tried to propose a theory in terms of waves only. This led him into controversy with other leading physicists since he rejected mainstream quantum mechanical theory.

Sometimes Schrödinger's wave equation is erroneously said to give the exact location of the electron and doesn't need the uncertainty principle. Actually Schrödinger's wave equation explains the exact location of a wave. A wave not being a point particle has a natural integral probability distribution as a widespread disturbance. So Schrödinger's equation does in a sense give the exact location of the electron, however, only in its state of being a widespread disturbance, a wave. Schrödinger later in life tried to develop a theory that would show the electron is only a wave and not a particle at all and that fundamentally the atom is only a wave, thus making the uncertainty principle obsolete as it was only needed to show the uncertainty of the particle-like position of the electron and other subatomic particles. This was not a new theory. The idea that the atom could be explained mathematically as a wave was introduced in 1922 by Charles Galton Darwin, a physicist, in his paper at [1]. However, the consequences would be that planets, galaxies, human beings and atoms are completely described as waves of physical disturbance, some waves being massless as in the case of light and some waves being massive as the case of the subatomic particles of the atom. Einstein rejected such a theory when Schrödinger proposed it to him. Einstein followed intuitive lines of thinking which is why he rejected Heisenberg's uncertainty principle. There are difficulties in describing a single wave as having two polarities if the atom were a single wave and the idea of waves producing spin and magnetic moment seem hard to overcome. Schrödinger became so disenchanted with the idea of wave-particle duality that he was known to have said concerning it:

"Let me say at the outset, that in this discourse, I am opposing not a few special statements of quantum physics held today (1950s), I am opposing as it were the whole of it, I am opposing its basic views that have been shaped 25 years ago, when Max Born put forward his probability interpretation, which was accepted by almost everybody." (Schrödinger Erwin, The Interpretation of Quantum Physics. Ox Bow Press, Woodbridge, CN, 1995).

"I don't like it, and I'm sorry I ever had anything to do with it." (Erwin Schrodinger talking about Quantum Physics)

[edit] Comments by other quantum physicists

"This is the third of four lectures on a rather difficult subject -- the theory of quantum electrodynamics -- and since there are obviously more people here tonight than there were before, some of you haven't heard the other two lectures and will find this lecture incomprehensible. Those of you who have heard the other two lectures will also find this lecture incomprehensible, but you know that that's all right: as I explained in the first lecture, the way we have to describe Nature is generally incomprehensible to us." Richard P. Feynman, QED, The Strange Theory of Light and Matter, p. 77 [Princeton University Press, 1985]

"The discomfort that I feel is associated with the fact that the observed perfect quantum correlations seem to demand something like the "genetic" hypothesis. For me, it is so reasonable to assume that the photons in those experiments carry with them programs, which have been correlated in advance, telling them how to behave. This is so rational that I think that when Einstein saw that, and the others refused to see it, he was the rational man. The other people, although history has justified them, were burying their heads in the sand. I feel that Einstein's intellectual superiority over Bohr, in this instance, was enormous; a vast gulf between the man who saw clearly what was needed, and the obscurantist. So for me, it is a pity that Einstein's idea doesn't work. The reasonable thing just doesn't work." John Stewart Bell (1928-1990), author of "Bell's Theorem" (or "Bell's Inequality"), quoted in Quantum Profiles, by Jeremy Bernstein [Princeton University Press, 1991, p. 84]

"Thus the last and most successful creation of theoretical physics, namely quantum mechanics (QM), differs fundamentally from both Newton's mechanics, and Maxwell's e-m field. For the quantities which figure in Quantum Physics' laws make no claim to describe physical reality itself, but only probabilities of the occurrence of a physical reality that we have in view. … I cannot but confess that I attach only a transitory importance to this interpretation. I still believe in the possibility of a model of reality - that is to say, of a theory which represents things themselves and not merely the probability of their occurrence. On the other hand, it seems to me certain that we must give up the idea of complete localization of the particle in a theoretical model. This seems to me the permanent upshot of Heisenberg's principle of uncertainty." (Albert Einstein, On Quantum Physics, 1954)

[edit] External links

• The Center of Quantum Philosophy