In a previous post titled What is Quantum Physics I introduced the subject and its principles: Quantum Physics, or Quantum Mechanics, is the theory that explains the motion of microscopic objects such as molecules, atoms, nuclei, and all the elementary particles of nature. This is the story of its creation: Before the beginning of the twentieth century physicists were able to discover almost all the laws that governed the observable phenomena at the macroscopic level. These laws, expressed in mathematical form, explained the two main realms of nature: 1) The motion of masses and the gravitational force that is exchanged between them. 2) The behavior of charged particles and the electromagnetic forces exchanged between these charges.
The first set of laws governing gravity and dynamics of masses is known as Classical Mechanics explained by Newton’s Laws of Motion. The second set of laws governing electricity and magnetism is known as Electromagnetism obeying Maxwell’s Equations. All natural phenomena such as heat, waves, etc. could be understood within these two theoretical frameworks. These two frameworks along with Einstein’s Theory of Relativity, which deals with objects moving with high velocities, are together known as Classical Physics. The important point is that in classical physics we are dealing with two distinct types of objects whose collective behaviors determines natural phenomena: Waves and particles. Sound and light are examples of wave phenomena; masses and point-charges are examples of particle phenomena.
As a result of technological advancements of late 19th and early 20th centuries experimental chemists and physicists were able to probe into the microscopic world of molecules and atoms. Scientists expected to find particles obeying Newton’s laws of motion; however, it turned out they were wrong, and in fact they saw that the concepts of classical physics were hopelessly inadequate in capturing the reality of the microscopic world. Let me give you just one example: Experimental findings gave a model of an atom, which is neutral, consisting of a very heavy, positively charged, nucleus concentrated in a very small region of space, within a length of about 10-14 meters. The negatively charged electrons rotate around this nucleus, hence keeping the whole atom neutral (This model is similar to our solar system though we will see that the underlying reality is totally different.) However, this kind of motion for electrons violates the most important tenet of electromagnetism: According to electromagnetism an accelerating charged particle, such as electron, radiates energy in the form of light, hence it keeps losing its energy as long as it is in accelerating motion. We also know from classical mechanics that rotation is an accelerating motion. This means that electrons rotating around the nucleus should radiate light and hence lose their energy until they fall onto the nucleus. But experiments have shown that nothing of the sort happens. Atoms, at least most of them, are stable structures; their electrons revolve the nucleus without radiating light; these electrons emit, or absorb, light only when they jump from one orbit to another orbit, and these orbits are not arbitrary; electrons can only occupy certain allowed orbits with discrete energies. Also, their jumps between orbits are not jumps through space; when an electron jumps from, say, orbit 2 with energy 20 to orbit 1 with energy 10 it does not fly through the space in between orbits; neither does it incrementally decrease its energy from 20 to 10 passing through 19, 18, …. This energy loss is radiated away in the form of electromagnetic radiation, photon. Electron’s jump is instantaneous and doesn’t take any time whatsoever: The electron is in one orbit and then suddenly shows up in another orbit. This inexplicable kind of jump is known as electronic transition or Quantum Jump. This example was one among the many experimental findings that needed a new physics in order to make sense.
Quantum Mechanics which was developed between 1900 to 1927 by the collaboration many physicists is the theory that explains the motion of microscopic objects. In other words, quantum theory was developed as a mathematical tool to make sense of and organize the strange experimental findings in the first decades of twentieth century. Double Slit Experiment is the cornerstone of quantum phenomena and it contains almost all the bizarre features of the quantum world. Quantum Mechanics is considered to be the most successful intellectual achievement of mankind since it has been able to explain all microscopic phenomena, and it is also the most experimentally verified theory in the history of science.
According to quantum theory the basic constituents of nature are neither wave nor particle. However, depending on the measuring instrument they can manifest either as wave or as a particle but not both at once, see The Complementarity Principle. Prior to the act of measurement the quantum system (particle is a misnomer but we have no better word) is neither a wave nor a particle, and it is also nothing else: It has no characteristics, no form and no properties, no position and no velocity in space. In fact, it is not a thing or entity anymore; it is a no-thing, a no-entity. Nothing can be said about it except saying that if we perform such and such a measurement on this no-thing there is such and such a probability to get such and such a numerical value for what is being measured. This bizarre feature of quantum phenomena is called stochastic behavior, that is the microscopic world is inherently indeterminate, see The Uncertainty Principle.
This indeterminacy is a matter of principle and not of the our ignorance nor of the inefficiency of our equipment. It is not that we don’t know the position of the “particle” in space; the particle has no position, or any dynamical property for that matter, prior to the act of measurement; the measurement process creates the very position that is to be measured. This means that the quantum world and the quantum objects cannot be thought or imagined in any possible way. Even the much used statement that “in quantum world a particle is in many places at once” is a false way of putting it because in the quantum world there is no such thing as particle; it is meaningless to speak of here and there, let alone of everywhere. Place has no place in the quantum realm.
The objection may rise, as it did for myself for quite a while, that this lack of knowledge about the nature of quantum objects is a lack on our part; perhaps the particle itself contains all this missing information but it is us who cannot access it, whether due to our state of knowledge at the present time or because nature somehow doesn’t like us to have that information!
But that is not true, for if it were our everyday world would not look like what it does. It is proven and experimentally verified that the quantum “particle” could not possibly have a position prior to measurement; if it did, whether we know that information or not, then we would not observe phenomena such as waves, colors of a soap bubble, etc. These phenomena can occur and be observed if and only if the underlying constituents do not have inherent properties such as a determinate positions or velocities. In other words, the missing information about the exact properties of quantum objects is not missing at all; it does not, and cannot, exist or else we would not be seeing what we are seeing right here right now. In other words, our observation of the form of appearances is possible only if that which appears is itself formless: Form is formlessness conditioned and partitioned. The set of experiments that have consistently proven this results are known as Bell Experiments and the theory that underlie them is known as Bell’s Theorem.
In the microscopic world what determines the place and status of quantum objects is the act of observation. To avoid mystical mis-interpretations I must add that observation here is not meant seeing with eyes or anything like that, anything depending on the consciousness of the experimenter. Observation in quantum mechanics refers to a complex process in which a macroscopic machine interacts with a microscopic object. Whether or not our consciousness is there in the room, whether or not we read the display of the machine that contains the result of measurement, it is always the internal mechanism of the instrument that by itself determines the state of quantum system and the possible outcome of the measurement. Human consciousness does not create reality, for it is itself already part of a created reality.
In future posts I will continue this subject and introduce you to Quantum Field Theory, QFT for short, in which the quantum world is no more seen as a collection of isolated particles and waves but as a field spread in space-time. In QFT particles are in fact the vibrations of the field. Quantum Field Theory was created by combining Quantum Theory and Einstein’s Theory of Special Relativity.