- collapse of the wave function still presents a problem for deterministic physics
- solution is to not collapse the wave function, rather split reality
- many worlds hypothesis is allows for the existence of all quantum states, observation splits the worlds containing the states
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The many possibilities carried by quantum superpositions are spread out over space and time. However, Newtonian physics is an accurate description of ordinary experience. What is the relationship between the strange quantum world and the classical world of common sense? Clearly the difference occurs when we measure or observe a quantum system. Whatever the process, it occurs at that time. The ``how and why'' of this process is unsolved and many believe modern physics will be incomplete until it is resolved.
By the 1950's, the ongoing parade of successes had made it abundantly clear that quantum theory was far more than a short-lived temporary fix. And so, in the mid 1950's, a Princeton graduate student named Hugh Everett III decided to revisit the collapse postulate in his Ph.D. thesis. Everett's idea is known as the relative-state, many-histories or many-universes interpretation or metatheory of quantum theory. Dr Hugh Everett, III, its originator, called it the "relative-state metatheory" or the "theory of the universal wavefunction", but it is generally called "many-worlds".
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- macroscopic physics states that all variables are there, just hard to measure
- Copenhagen Interpretation states that variables are not there, randomness is fundamental
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In general, quantum theory predicts only the probability of a certain result. Consider the case of radioactivity. Imagine a box of atoms with identical nuclei that can undergo decay with the emission of an alpha particle. In a given time interval, a certain fraction will decay. The theory may tell precisely what that fraction will be, but it cannot predict which particular nuclei will decay. The theory asserts that, at the beginning of the time interval, all the nuclei are in an identical state and that the decay is a completely random process.
Even in classical physics, many processes appear random. For example, one says that, when a roulette wheel is spun, the ball will drop at random into one of the numbered compartments in the wheel. Based on this belief, the casino owner and the players give and accept identical odds against each number for each throw. However, the fact is that the winning number could be predicted if one noted the exact location of the wheel when the croupier released the ball, the initial speed of the wheel, and various other physical parameters. It is only ignorance of the initial conditions and the difficulty of doing the calculations that makes the outcome appear to be random. In quantum mechanics, on the other hand, the randomness is asserted to be absolutely fundamental. The theory says that, though one nucleus decayed and the other did not, they were previously in the identical state.
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- wave-particle duality is a manifestaion of quantum entities
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Wave-particle duality does not mean that a photon or subatomic particle is both a wave and particle simultaneously, but that it could manifest either a wave or a particle aspect depending on circumstances. Complementarity, uncertainty, and the statistical interpretation of Schrdinger's wave function were all related. Together they formed a logical interpretation of the physical meaning of quantum mechanics known as the "Copenhagen interpretation.
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- The Copenhagen Interpretation has three primary parts:
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- symmetry in quantum physics lead to the prediction of opposite matter, or antimatter
- matter and antimatter can combine to form pure energy, and the opposite is true, energy can combine to form matter/antimatter pairs
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A combination of quantum mechanics and relativity allows us to examine subatomic processes in a new light. Symmetry is very important to physical theories. Thus, the existence of a type of `opposite' matter was hypothesized soon after the development of quantum physics. `Opposite' matter is called antimatter. Particles of antimatter has the same mass and characteristics of regular matter, but opposite in charge. When matter and antimatter come in contact they are both instantaneously converted into pure energy, in the form of photons.
Antimatter is produced all the time by the collision of high energy photons, a process called pair production, where an electron and its antimatter twin (the positron) are created from energy (E=mc2). A typical spacetime diagram of pair production looks like the following:
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- since quantum events do not have a "cause", this also means that all possible quantum events must and will happen
- without cause and effect, conservation laws can be violated, although only on very short timescales (things have to add up in the end)
- violation of mass/energy allowed for the understanding of the source of nuclear power in the Universe, fission and fusion
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One of the surprising results of quantum physics is that if a physical event is not specifically forbidden by a quantum rule, than it can and will happen. While this may strange, it is a direct result of the uncertainty principle. Things that are strict laws in the macroscopic world, such as the conversation of mass and energy, can be broken in the quantum world with the caveat that they can only broken for very small intervals of time (less than a Planck time). The violation of conservation laws led to the one of the greatest breakthroughs of the early 20th century, the understanding of radioactivity decay (fission) and the source of the power in stars (fusion).
Nuclear fission is the breakdown of large atomic nuclei into smaller elements. This can happen spontaneously (radioactive decay) or induced by the collision with a free neutron. Spontaneously fission is due to the fact that the wave function of a large nuclei is 'fuzzier' than the wave function of a small particle like the alpha particle. The uncertainty principle states that, sometimes, an alpha particle (2 protons and 2 neutrons) can tunnel outside the nucleus and escape.
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- events in the microscopic world can happen *without* cause = indeterminacy
- phenomenon such as tunneling shows that quantum physics leaks into the macroscopic world
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The macroscopic world is Newtonian and deterministic for local events (note however that even the macroscopic world suffers from chaos). On the other hand, the microscopic quantum world radical indeterminacy limits any certainty surrounding the unfolding of physical events. Many things in the Newtonian world are unpredictable since we can never obtain all the factors effecting a physical system. But, quantum theory is much more unsettling in that events often happen without cause (e.g. radioactive decay).
Note that the indeterminacy of the microscopic world has little effect on macroscopic objects. This is due to the fact that wave function for large objects is extremely small compared to the size of the macroscopic world. Your personal wave function is much smaller than any currently measurable sizes. And the indeterminacy of the quantum world is not complete because it is possible to assign probabilities to the wave function.
But, as Schrodinger's Cat paradox show us, the probability rules of the microscopic world can leak into the macroscopic world. The paradox of Schrodinger's cat has provoked a great deal of debate among theoretical physicists and philosophers. Although some thinkers have argued that the cat actually does exist in two superposed states, most contend that superposition only occurs when a quantum system is isolated from the rest of its environment. Various explanations have been advanced to account for this paradox--including the idea that the cat, or simply the animal's physical environment (such as the photons in the box), can act as an observer.
The question is, at what point, or scale, do the probabilistic rules of the quantum realm give way to the deterministic laws that govern the macroscopic world? This question has been brought into vivid relief by the recent work where an NIST group confined a charged beryllium atom in a tiny electromagnetic cage and then cooled it with a laser to its lowest energy state. In this state the position of the atom and its "spin" (a quantum property that is only metaphorically analogous to spin in the ordinary sense) could be ascertained to within a very high degree of accuracy, limited by Heisenberg's uncertainty principle. |
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- an example of the weirdness of the quantum world is given by the famous Schrodinger cat paradox
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In 1935 Schrodinger, who was responsible for formulating much of the wave mechanics in quantum physics, published an essay describing the conceptual problems in quantum mechanics. A brief paragraph in this essay described the, now famous, cat paradox.
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- the paradox is phrased such that a quantum event determines if a cat is killed or not
- from a quantum perspective, the whole system state is tied to the wave function of the quantum event, i.e. the cat is both dead and alive at the same time
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One can even set up quite ridiculous cases where quantum physics rebells against common sense. For example, consider a cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat). In the device is a Geiger counter with a tiny bit of radioactive substance, so small that perhaps in the course of one hour only one of the atoms decays, but also, with equal probability, perhaps none. If the decay happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The wave function for the entire system would express this by having in it the living and the dead cat mixed or smeared out in equal parts.
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It is often stated that of all the theories proposed in this
century, the silliest is quantum theory. Some say the the only
thing that quantum theory has going for it, in fact, is that it
is unquestionably correct.
- R. Feynman
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- quantum mechanics is to the microscopic world what classic mechanics and calculus is to the macroscopic world
- it is the operational process of calculating quantum physics phenomenon
- its primary task is to bring order and prediction to the uncertainty of the quantum world, its main tool is Schrodinger's equation
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The field of quantum mechanics concerns the description of phenomenon on small scales where classical physics breaks down. The biggest difference between the classical and microscopic realm, is that the quantum world can be not be perceived directly, but rather through the use of instruments. And a key assumption to an quantum physics is that quantum mechanical principles must reduce to Newtonian principles at the macroscopic level (there is a continuity between quantum and Newtonian mechanics).
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- the uncertainty principle states that the position and velocity cannot both be measured,exactly, at the same time (actually pairs of position, energy and time)
- uncertainty principle derives from the measurement problem, the intimate connection between the wave and particle nature of quantum objects
- the change in a velocity of a particle becomes more ill defined as the wave function is confined to a smaller region
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Classical physics was on loose footing with problems of wave/particle duality, but was caught completely off-guard with the discovery of the uncertainty principle.
The uncertainty principle also called the Heisenberg Uncertainty Principle, or Indeterminacy Principle, articulated (1927) by the German physicist Werner Heisenberg, that the position and the velocity of an object cannot both be measured exactly, at the same time, even in theory. The very concepts of exact position and exact velocity together, in fact, have no meaning in nature.
Ordinary experience provides no clue of this principle. It is easy to measure both the position and the velocity of, say, an automobile, because the uncertainties implied by this principle for ordinary objects are too small to be observed. The complete rule stipulates that the product of the uncertainties in position and velocity is equal to or greater than a tiny physical quantity, or constant (about 10-34 joule-second, the value of the quantity h (where h is Planck's constant). Only for the exceedingly small masses of atoms and subatomic particles does the product of the uncertainties become significant.
Any attempt to measure precisely the velocity of a subatomic particle, such as an electron, will knock it about in an unpredictable way, so that a simultaneous measurement of its position has no validity. This result has nothing to do with inadequacies in the measuring instruments, the technique, or the observer; it arises out of the intimate connection in nature between particles and waves in the realm of subatomic dimensions. |
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- quantum physics is a science of possibilities rather than exactness of Newtonian physics
- quantum objects and quantities becomes actual when observed
- key proof of quantum superpositions is the phenomenon of quantum tunneling
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The fact that quantum systems, such as electrons and protons, have indeterminate aspects means they exist as possibilities rather than actualities. This gives them the property of being things that might be or might happen, rather than things that are. This is in sharp contrast to Newtonian physics where things are or are not, there is no uncertainty except those imposed by poor data or limitations of the data gathering equipment.
Further experimentation showed that reality at the quantum (microscopic) level consists of two kinds of reality, actual and potential. The actual is what we get when we see or measure a quantum entity, the potential is the state in which the object existed before it was measured. The result is that a quantum entity (a photon, electron, neutron, etc) exists in multiple possibilities of realities known as superpositions.
The superposition of possible positions for an electron can be demonstrated by the observed phenomenon called quantum tunneling.
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