BoseEistein Condesation Essay Research Paper Everything in

Bose-Eistein Condesation Essay, Research Paper

Everything in the world around us is made up of particles. These particles can be placed into two categories called bosons and fermions. Bosons are particles that have integer spin values, such as 1, 2, 3 and so on.3 Examples of bosons are photons, which are quantum of electromagnetic radiation, phonons, which are quantum of vibrational energy, and most atoms. Fermions on the other hand are particles that have half-integer spin values, such as 1/2, 3/2, 5/2 and so on.3 Fermions are the most common particles, such as electrons, protons, neutrons, and even a few atoms. Since fermions form the elementary building blocks, fermions also make up individual atoms. When you put an even number of fermions together, what results is a composite particle with an integer spin number, or a boson. The significance of the spin numbers is that bosons tend to be uniform and in the same state. An example of this are the photons of a laser beam in the same energy state traveling in the same direction. In opposition to this, fermions are characteristically different. That is why electrons around an atom cannot have the same spin, hence the Pauli exclusion principle. This observation of the nature of bosons prompted Satyendra Nath Bose to develop some rules about photons, such as black body radiation. Einstein picked up this work by Bose later on, and suggested that some of the rules could be applied to other bosonic particles.4 Einstein ended up deriving a concept known as the Bose-Einstein distribution. He showed that if a sample of atoms were brought to a low enough temperature, that a large proportion would end up in the lowest possible energy state. Physically, the individuality of the atoms would disappear. The characteristics of the individual atoms, such as position and velocity would basically merge and become indistinguishable from each other. Having this large number of particles just sitting in the lowest available energy state is the formation of the Bose-Einstein condensate. It is this formation that causes a spike in the energy distribution of these atoms, right next to the origin in the figure below.3 Atoms have wave nature so they also have wavelengths, just as do photons, which is known as the de Broglie wavelength.3 The exact position of the atom cannot be known at low temperatures, only that it is in a general spot, known as a wave packet. This is the certain region in which the atom is expected to be found.4 As the temperature of the atoms is lowered further, the de Broglie wavelength increases and the size of the wave packet increases.2 The individual atoms can still be differentiated, but when the temperature reaches a low enough value, these wave packets overlap with neighboring wave packets. It is when the de Broglie wavelength of one atom overlaps one of another that the Bose-Einstein condensation occurs.5 This phenomenon results in a single macroscopic wave packet, composed of many individual wave packets, which behave as if they were a single atom.2 It took over 70 years for Bose-Einstein condensation to go from theoretical predictions, to actually succeeding in creating a Bose-Einstein condensate. It was not until the June of 1995 that Eric Cornell, Carl Wieman, Michael Anderson, and their colleagues were able to accomplish this historical deed.5 One of the main reasons that it took so long for this to occur was that the lack of refrigeration technology prevented even entertaining of such thoughts. It was in the 1970 s when technology had advanced enough for researchers to attain temperatures that were cool enough, but yet another obstacle surfaced. For Bose-Einstein condensation to occur, the gas needs to be cooled far past the point where the atoms would normally freeze into a solid. This caused some problems because a solid cannot form a Bose-Einstein condensate.3 For Bose-Einstein condensation to form, the gas needs to reach a metastable state. The trick in doing this is to make sure that the system is clear of impurities, such as dust, and to keep the gas at low densities. If there is nothing for the crystals or droplets to nucleate onto, gas can be cooled below the temperature that it liquefies and becomes solid. The low densities are needed because it lowers the chance of three-body collisions, which can cause molecules to form. When two atoms collide, they will not form a molecule, because there is nothing to cause them to stick. But, if three atoms come together, the third atom can take the energy of the collision, leaving the two atoms stuck together as a molecule. If this occurs, more molecules can accumulate to form snowflakes.3 Having to use low densities also results in some additional problems. To get a Bose-Einstein condensate, low temperature and high densities are needed.5 The difficulty in having lower densities is that it causes a need for even lower temperatures. But since having the lower densities is a necessity, cooling techniques had to be devised to reach the temperatures needed. Laser cooling was one such technique that was developed. The main function of lasers is usually thought of as something to heat or burn with. That is true, but laser beams also carry momentum. The force from this light is exceedingly small, but when compared to say the mass of an atom, it turns out to be significant. Temperature is related to the energy of the atoms, which is related to their movement. So by slowing the atoms down