Quantum Phenomena Essay
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Quantum theory (including subfields such as quantum mechanics and quantum electro and thermo dynamics) is at least to say, is a relatively new field of physics, having been born just over a century ago. It can be described as one of the most powerful theory ever to be proposed in the history of physics, even today our knowledge on this subject is regularly updated with new ideas and as we progress further into this subject we begin to discover basically, the many secrets of our universe.
In the early 1900s, mankind had basically been able to answer nearly all of the questions concerning physics with the knowledge gained from giants such as Newton. However 10% were unanswerable, they simply could not figure out what was going on and why certain things did not obey rules set down by classical physics. In order to overcome this problem, scientists had to propose some new radical ideas regarding the relationship between matter and energy. Surprisingly a selection of these many dreamt up ideas worked and so quantum theory was born.
As said by one of the forefathers of this theory, Erwin Schri?? dinger (famous for his thought experiment), “I do not like it, and I am sorry I ever had anything to do with it. ” One from the “Great Dane”, Niels Bohr, “Anyone who is not shocked by quantum theory has not understood it. ” And lastly one from the Richard Feynman, “I think it is safe to say that no one can understand quantum mechanics. ” Generally quantum phenomena can only be observed in atomic and sub-atomic scale, it is rare to see such observations on a large scale.
Of the various interpretations of quantum theory that have been proposed, most physicists accept the Copenhagen Interpretation, however this does not mean it does not have any faults or lead to any paradoxes. This brings us onto Schri?? dinger’s cat, a thought experiment which criticises the Copenhagen Interpretation of quantum mechanics, stating that the interpretation would just collapse when applied to objects on a large scale, in his case, a feline mammal. If atoms can exist in a state of superposition then is it reasonable to say that larger bulkier objects can since it is also made from atoms?
He points of that such as theory would state that a cat can be dead and alive at the same time, which seems impossible. Now we introduce the observer effect, which makes this problem a little bit disturbing, many have talked about the act of measurement as the moment that causes the collapse of superposition, leading to an outcome. Furthermore some scientists even believe that human consciousness is necessary to trigger the collapse, resulting in questions about the roles of consciousness in our universe.
For example, if we turn our back on an object, does that object even exist anymore; was it just a part of our imagination, a virtual reality? However, we are not here to discuss about such philosophical subjects, as I mentioned before, there are some special cases where certain objects can be seen with one’s naked eyes obeying the rules governed by quantum theory. Once such example is the rather strange state of matter, BEC or Bose-Einstein Condensates, named after the two people who predicted the existence of such matter.
It is regarded as an example of a superfluid, we will talk more about those later. The basic formation of a superfluid involves cooling down a sample such as helium gas (helium IV) just above -217oC or 2. 17K to form helium II. Note that the temperature required for such a formation is just above absolute zero, 0K or 217. 15oC. Absolute zero is the theoretical temperature in which particles have a minimum energy. However at these conditions we can observe odd and strange phenomena that are governed by the rules of quantum mechanics.
To cut a rather long story short, when helium IV is cooled down to such a low temperature, every single atom will occupy the lowest energy level resulting in a very strange state of matter due to the fact that every single atom will be identical. The Indian theoretical physicists Satyendra Nath Bose in the 1920s, whilst studying the new idea of light being made from discrete packets (now we know as quanta or photons), proposed some rules which decided whether two photon should be treated as the same particle or differently. This is now known as Bose-Einstein Statistics.
Einstein had several roles to play in the events leading up to the proposition of the existence of BEC. Firstly he used his influence to allow Bose’s papers to be published in certain journals. Secondly but also most importantly, he used Bose’s rule in the context of atoms, seeing that photons and atoms are relatively the same thing. When these rules were applied to atoms in gases, for most temperatures, the behaviour according to him was pretty much the same as Bose’s prediction. However when it was applied in near absolute temperatures, near 0K, strange phenomena were predicted to happen.
So puzzled was Einstein that he himself was unsure if his predictions and calculations were correct. Both scientists were unable to live long enough to observe the production of BEC, although superfluids were able to be made, it was not until 1995 when the world’s first condensate was made. Made by Eric Cornell and Carl Weiman, both of whom with Wolfgang Kettle, won the 2001 Nobel Prize in Physics for their works on condensates. In order to achieve this feat, they had to cool a gas of rubidium atoms to 170 nanokelvin, equivalent to -273. 14999983oC.
Also it should be noted that BEC can only be called a ‘true’ BEC when bosons, particles that carry a force, are supercooled, otherwise when other particles are used, the BEC can also be called a super atom (due to the fact that all the atoms are identical thus losing their individuality, all forming a single ‘blob’). As I have mentioned earlier, BEC can be considered as a superfluid. One very special example of a superfluid is helium II. The boiling point of helium gas was found out to be 4K, therefore when cooled, helium I (normal liquid helium) can be seen boiling and bubbling away.
However as the temperaure moves towards absolute zero, 0K, we notice a change in properties. As it nearly approaches 2K, all movement stops, and the helium becomes very still. At approximately 2. 17K, helium I becomes helium II, a superfluid. This point is known as the lambda point due to the shape of the above graph, which looks similar to the greek letter lambda (? ). Here the helium I show some remarkable properties, such as at this point, the viscosity or ‘treacliness’ of helium duudenly drops by a factor of a million, soon becoming zero.
It also has zero entrophy, the measure of randomness of molecules in a system, and infinite thermoconductivity. It can perform two strange feats which can actually be seen with the naked eyes. One is when a beaker is lowered onto a container of helium II and then partially withdrawn, a thin film of helium II (a single atom thick) will form over the entire surface of the beaker. Then depending on the level of helium inside and outside the beaker, it will try and flow into the beaker until both levels are equal.
A similar behaviour can be observed when a beaker of heium II is withdrawn completely from a bath of the same substance, it will creep up the sides of a container and try to ‘debeaker’ itself via flowing over the top of the beaker and down the sides until it combines to form a drop of liquid at the bottom of the beaker, dripping back into the bath. Apparently in both cases, it is an example of the helium’s futile attempt to reduce its own energy level, which is theoritically already at its lowest. The other feat I wish to mention is known as the fountain effect or the thermomechanical effect.
If a capillary tube is placed in a bath of helium II and then heated, it will cause the liquid to flow up the tube, thus forming a fountain. It actually takes only a small amount of thermal energy to cause this effect, even radiation from visible light is enough to heat it up. One interesting application of superfluid was to trap and slow down the speed of light. In one experiment, performed by Lene Hau of Harvard, the speed of light was managed to be reduced to only 17 metres per second and momentarily stopped via the use of superfluids.
Another phenomenon which occurs near absolute zero temperature is the existence of superconductors. This only happens with certain materials, characterized by zero electrical resistance and the expulsion of a magnetic field. Superconductivity can occur in a variety of materials such as simple elements (e. g. tin and aluminium), metallic alloys and some semiconductors. However it cannot occur in noble metals and most ferromagnetic materials. There are two types of superconductor, Type I (conventional superconductors), materials that only exhibits superconductivity at near absolute zero conditions.
Secondly in 1986, there was the discovery of high temperature superconductors (Type II). This allows certain materials to undergo superconductivity at a higher temperature than conventional type I, around 77K, the boiling point of liquid nitrogen. The picture below shows a magnet levitating above a Type II superconductor; this is due to the fact that the superconductor can exclude the magnetic field of the magnet, resulting in the formation of an electromagnet that repels the magnet.
The basic principles of superconductors can be explained by the visualization of electrical current as a sea of electrons, basically a fluid, which moves across an ionic lattice. Electrical resistance is caused by collisions between the ions and the electrons in the fluid. However, in superconductors, instead of a fluid scattered with individual electrons, it is filled with bound pairs of electrons, known as Cooper pairs.
Quantum theory dictates that this fluid becomes a superfluid, resulting in a change in property, such as it cannot exhibit electrical resistance. The characteristics of superconductivity only appears when the termperature of the material is cooled below its critical temperature (Tc), generally Type I temperatures range from 20K to 1K. The graph below shows the relationship between temperature and resistivity of a material, we can see it must be a Type II superconductor due to its relatively high critical temperature.