Quantum Mechanics
From SkepticWiki
Although classical physics is adequate to explain virtually all phenomena one will ever directly experience in one's life, certain phenomena cannot be explained by classical physics. Encompassing theories have been developed to explain these phenomena. In the world of the very fast, Einstein's theory of relativity provides a more accurate description of the real world than classical Newtonian mechanics. In the world of the very small, quantum mechanics provides the correct physics for phenomena on the atomic scale. In many respects, quantum mechanics presents the physics that underlies physical reality at its most basic level.
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[edit] Historical Development
In the late 19th century, physics was aware of a paradox regarding the spectrum of light emitted from an object (a so-called black-body) at a given temperature. Classical theory required that this black-body radiation should show equal amounts of energy being radiated at all possible wavelengths. This would necessarily result in an infinite amount of energy being radiated at shorter and shorter wavelengths, which is both physically impossible and also not what is observed. This paradox was known as the Ultraviolet Catastrophe.
In 1899, Max Planck devised a mathematical formula that provided the correct spectrum for black-body radiation, seemingly solving the paradox. The formula rested on an unusual hypothesis -- that the radiated light could not be of arbitrary size, but had to be composed of discrete lumps of energy that Planck dubbed 'quanta'. The energy of a quantum of light is equal to its frequency multiplied by a constant now known as Planck's constant. If high frequency UV light only comes in quanta with high energies, the Ultraviolet Catastrophe cannot occur. Planck's constant is so small that these individual lumps are essentially indetectable at the human scale.
In 1905, Einstein used Planck's idea of the quantum to explain another current mystery: the photoelectric effect, in which light shining on a metal surface ejects electrons in a way impossible for classical physics to explain. Hypothesizing that the light comes in quanta (now commonly called photons) allowed Einstein to correctly explain the observed properties of the photoelectric effect. It was for this work on the photoelectric effect that Einstein received his Nobel Prize.
From these tentative beginnings, quantum mechanics exploded into prominence in the 1920's when much of the groundwork of the theory was rapidly created by physicists such as Schrödinger, Heisenberg, Bohr, and Dirac. The advanced mathematics required to express the quantum theory was very unlike classical mechanics, but it still rested on cold hard equations and axioms.
In the 1950's, quantum theory and electromagnetic theory were successfully united by Schwinger, Tomonaga and Richard Feynman. Quantum Electrodynamics (or QED) was an incredibly successful theory, predicting certain physical quantities with unheralded precision.
In the 1970's, QED was itself united with the theory explaining the weak nuclear force by Glashow, Salaam and Steven Weinberg. The electroweak theory provides the heart of what is now known as the Standard Model of elementary particle theory.
[edit] Major Principles
[edit] Quantization
Certain physical quantities like charge and angular momentum cannot take on arbitrary values, but are quantized (i.e. come in discrete lumps). In bound systems (such as an electron in an atom) this means that electrons cannot be just 'anywhere', but they must exist in certain quantized states, often labelled by quantum numbers corresponding to different values of energy, angular momentum, etc.
[edit] Heisenberg Uncertainty Principle
There is an inverse relationship between the uncertainty in a particle's position and its velocity. If one measures an electron's position very accurately, then one knows very little about its velocity. As quantum mechanics is traditionally understood, this is not a limitation on human measurement, but a statement that the velocity of the particle simply does not exist. A similar relationship also exists between the uncertainties in energy measurements and time measurements and other conjugate variables. The mathematical relationship involves Planck's Constant, which again is so small that these unusual properties are indetectable at the human scale.
[edit] Wavefunction Collapse
As the Heisenberg Uncertainty Principle demonstrates, the position and velocity of a particle are not well-defined variables as they are in classical mechanics. In quantum mechanics, a particle can be described by a wavefunction, which encodes all of the known information about the particle. The square of the value of the wavefunction at any point in space yields the probability of finding the particle at that location. Other mathematical manipulations of the wavefunction yield information about the momentum, energy or other properties. Commonly discussed wavefunctions are those of the electron in a Hydrogen atom, pictured here for different values of the quantum numbers n, l and m.
If one measures the position (or other property) of a particle, the instrument does not read out a blurry number corresponding to the wavefunction. It yields a definite result. This has the effect of potentially changing the wavefunction. A crude example might be a particle kept inside a box, with the square of the wavefunction equal throughout the volume. A measurement of position (to a given accuracy) might reduce the wavefunction to a small sphere centered on the location indicated by the measurement. This is the concept of wavefunction collapse in standard quantum mechanics -- a measurement can alter the wavefunction, typically changing it to a state corresponding to the measurement. Since the wavefunction is a mathematical construct that describes the particle, the collapse of the wavefunction is not so much a physical process as it is a necessary change in the description of the particle made on the basis of new information gathered from the measurement ("the act of observing disturbs the observed"). During the development of quantum mechanics, the collapse of the wavefunction was highly controversial and it continues to play a role in the formulation of alternative interpretations of quantum mechanics.
[edit] Randomness
Perhaps the most disturbing consequence of quantum mechanics, at least to folks who prefer a predictable universe, is that it's impossible to predict exactly where a known particle will be at a specified time t in the future. A particle's wavefunction can only tell you the probability that the particle will be within some specified area at time t. This lack of definite predictability is not merely a limitation of the instruments used to measure where the particle is; it's a fundamental consequence of the laws of the universe. Even if you somehow, magically, could know the exact position and velocity of every particle in the Cosmos at time t, you still could not predict exactly where each particle will be at time t + 1 second. There is an inherent randomness that exists on this scale of the very small. It is only because the objects we interact with on an everyday basis are such enormous aggregates of these particles that the probabilities appear to "smooth out" into something predictable.
Einstein was one of the folks that was uncomfortable with this aspect of quantum mechanics, once going so far as to say "I am convinced that He (God) does not play dice."
[edit] Schrödinger's Cat
Erwin Schrödinger came up with the following thought experiment to explain the notion of wavefunction collapse:
- Place a cat in a box.
- Into the box with the cat, place a sealed vial of poison gas.
- Suspend a hammer precariously over the sealed vial.
- Attach a triggering mechanism to the hammer that will cause it to smash the vial if it detects the radioactive decay of an atom.
- Place an atom next to the detector which has a half-life of one hour -- that is to say, there is a 50% chance that the atom will undergo radioactive decay some time in the next hour. This means there is a 50% chance that the hammer will fall, and the poison gas will kill the cat, in the next hour.
- Close the box and wait one hour.
- After the hour is up, without looking in the box, is the cat still alive, or is it dead from the poison gas?
Obviously, we don't know if the cat is alive or dead without opening the box. As a problem in quantum mechanics, we would say that the state of the cat is uncertain -- it is neither alive nor dead until we open the box and look inside, at which point the wavefunction collapses and the cat's state is known.
Schrödinger considered this a thought experiment for demonstration purposes only. It is absurd to think that the act of opening the box and observing the cat was what caused the cat to live or die.
[edit] Tunnelling
Under certain circumstances, the wavefunction for a particle can predict a zero probability for that particle to be at some location a certain distance from its last known position, but a non-zero probability for that same particle to be at another location farther away. The point of zero-probabilty usually corresponds to some 'solid' object or 'barrier' that, in conventional thinking, the particle shouldn't be able to penetrate; e.g. if the particle is an electron, the zero-probability point might correspond to an electrical insulator. Since there is a non-zero probability of the particle appearing on the other side of this 'barrier', it sometimes will happen that the particle actually does appear there. In that case, the particle is said to have tunnelled through the barrier.
Some semiconductors, such as the tunnel diode, rely on this phenomenon of quantum tunnelling for their operation. The important thing to remember, though, is that the equations which predict tunnelling behavior only occur on subatomic scales. As the mass of the objects involved increases, the possible tunnelling distances decrease. While an electron could reasonably be expected to tunnel across a distance of one Ångstrom, a bacterium could not, and a full-grown human being definitely could not. Therefore, quantum tunnelling is not a viable explanation for any alleged teleportation of macroscopic objects.
[edit] Pauli Exclusion Principle
Some elementary particles have a property called spin, which acts like an intrinsic angular momentum. Spin is also quantized and can either come in integral or half-integral quantities. Particles with integer spin (e.g. the photon) are known collectively as bosons. Particles with half-integer spin (e.g. the electron, proton and neutron) are known as fermions. The Pauli Exclusion Principle states that no two fermions can be in the same quantum state. Roughly speaking, two electrons cannot be in the same place at the same time. Bosons can exist in the same state, a fact which makes lasers possible. Laser light contains bosons (photons) all in the same state.
[edit] Relationship to Paranormal Phenomena
Quantum Mechanics is often used by pseudoscientists and purveyors of the paranormal as a justification for their beliefs. Such efforts are often based on the misguided premise that because strange happenings occur at sub-atomic levels, then equally strange things - putative psychic phenomena - might be observable at the macroscopic level of everyday reality. The results of quantum mechanics are indeed unusual, counterintuitive and bizarre. However, the results of quantum mechanics can seldom be interpreted in a way at all favorable to paranormal phenomena.
In 1979, physicist John Archibald Wheeler was invited to speak at the American Association for the Advancement of Science. He chose to lecture on a topic on which he had frequently expounded - quantum theory and observation. Much to Wheeler's chagrin, he found himself sharing a platform with speakers who had pro-paranormal views. He was concerned that the audience would think that all the speakers condoned each other's views. He closed his speech thus:
"And let no one use the Einstein-Podolsky-Rosen experiment to claim that information can be transmitted faster than light, or to postulate any "quantum interconnectedness" between sparate consciousnesses. Both are baseless. Both are mysticism. Both are moonshine."[1]
Wheeler wrote two appendices to the transcript of his speech including one titled Drive the pseudos out of the workshop of science. In both, Wheeler attacks what he saw as the flagrant abuse of his and many physicists' 'honest' work. He refers to the "buzz of absolutely crazy ideas put forth with the aim of establishing a link between quantum mechanics and parapsychology - as if there was any such thing as 'parapsychology'."[2]
James E. Alcock wrote with regard to this:
"What is important is not that one can line up experts who take a very dim view of the paraphysicists' attempts to provide a quantum mechanical basis for [psychic phenomena], but rather that... paraphysicists are engaging in wild speculation in their assumptions that the eventual explanation of [QM enigmas] will support the notion of paranormal processes."[3]
The following modern physical concepts have been used to justify the existence of paranormal phenomena:
[edit] Einstein-Podolsky-Rosen (EPR) Paradox
This concerns a pair of particles which must always have opposite values of specific properties, and the particles are moving in opposite directions. The paradox states that it is impossible to determine the value of the property of one particle until it is measured and that, in that very instant of measurement, the other particle will have the opposite value, however far away it is.
It has been argued that the paradox implies that information can be transmitted almost instantaneously i.e. faster than the speed of light. Whilst relativity theory states that energy cannot move faster than light, parascientists have argued that perhaps information is arriving before anything else which determines a change in a physical property. The suggestion is that this might account for psychic transferences of information between people or people and locations (as in remote viewing, for example).
Critics of this idea have stated that while the act of measurenment appears to determine the value of a property, it is not the observer's knowledge which alters the outcome of a real measurement. There is therefore no suggestion that anything to do with 'mind over matter' is taking place[4].
[edit] Heisenberg Uncertainty Principle
This has been used to explain the so-called 'experimenter' and 'sheep-goat' effects of parapsychological theory; different test results are obtained by researchers depending on whether they are believers or skeptics. When the former and the latter participate in an experiment by observing the events, it is claimed that there is a tendency to determine a different set of values to each other.
Critics of this idea have noted casual and superficial misuse of the principle which is more complex than many popular translations might suggest.
- Tao of Physics (QM is a rediscovery of ancient wisdom)
- Nothing is Real
- Observer Created Reality
- Bell's Theorem Revisited
- "Quantum healing"
[edit] References
- "A Quantum of Common Sense" from SkepticReport
- The Straight Dope's rhyming description of Schrödinger's cat
[1]Cited by Martin Gardner, 'Quantum theory and quack theory', The New York Review of Books, May 17, 1979, XXVI(8), p39-40
[2]John Archibald Wheeler, Letter to AAAS, 'A Decade of Permissiveness', 1979
[3]James E. Alcock, Parapsychology: Science or Magic? A Psychological Perspective, 1981, Pergamon Press, p114
[4]Scientific American, July 1978, p78
