Quantum Physics Basics

Teacher: [Thunder Cid]




Brief Summary:

1.Introduction
2.Origins
3.Experiments




Introduction

Does the thought of quantum physics send a chill down your spine, just like the words calculus, differential equations, and -gasp- organic chemistry? You may not even think that quantum physics is a serious science, like the more familiar Newtonian physics. Just relax! I'm sure you're comfortable with regular physics, which describes the way that matter interacts with other matter, i.e. gravity, velocity, etc. If not, maybe you should check out my regular Interactive Physics site. Anyway, quantum physics is just the physics of the incredibly small. While Newtonian physics can suitably describe the orbit of the planets or the energy transformations during a game of pool, quantum physics describes how electrons surround the nucleus of the atom and other subatomic actions. At this point, you may be thinking that there's not that big of a difference between these two sciences. Hey, both explain how matter interacts with other matter, so what's the big deal? The difference is that the common laws of physics begin to deteriorate on small scales. For example, Nippendenso (Japan Electric) built a car that's only half a millimeters long. One could easily mistake it for a grain of rice if not for its gold color. At the scale of 1 to 1000, physics is already changing. Oil would now gum up the engine, and the tires wouldn't have enough traction to move the car.




Quantum physics tries to explain the behavior of even smaller particles. These particles are things like electrons, protons, and neutrons. Quantum physics even describes the particles which make these particles! That's right; the model of an atom that you were taught in high-school is wrong. The electrons don't orbit like planets; they form blurred clouds of probabilities around the nucleus. Protons and neutrons? They're each made of three quarks, each with its own 'flavor' and one of three 'colors'. Lets not forget the gluons, the even smaller particles that hold this mess together when they collect and form glueballs (not a very original name). Why weren't you told about this already? Were you fluent in calculus when you took general chemistry? The quantum model of the atom is much more complex than the traditional model, so most teachers save that stuff for college. (But this doesn't mean that you can't have a basic understanding and impress your friends!) The reason that quantum physics needs complex math to explain the behaviors and properties of small particles is that the world of these subatomic particles is a very bizarre one, filled with quantum probabilities and organized chaos. For example, the exact position and velocity of an electron is very hard to find because attempts to "see" it involve bouncing other particles off of it. By doing this, you've just changed the electron's velocity, so your data is useless. What quantum physics does is give us the statistical probability of the electron's location at any one moment. By learning how these particles act, scientists can better understand the matter which makes up the universe, and the way it behaves (or misbehaves). Quantum physics even plays a part in blackholes, where regular physics is thrown out the window and then some!




Origins

The roots of quantum physics reach far into the past. Even Isaac Newton, the father of classical physics, played a part in the development of quantum physics. He didn't know it at the time, but one of his most famous arguments was a matter of quantum physics.
Newton tried to explain the behavior of light in terms of particles, which he called corpuscles. He was the founder of the physics of particles after all, so why shouldn't light be treated like particles, just like the planets. The Dutch physicist Christiaan Huygens, however, tried to describe light in the terms of waves. Although the wave and particle theories of light were both sound, there was one obvious problem with Huygens' wave theory: when light is obstructed, it creates a shadow with well-defined edges. If light was a wave traveling through Huygens' "ether", it would flow around the edges of the obstruction, blurring the shadow. (This is not the case because the wavelength of light is small enough to create sharp edges, but this was not considered at the time.) Because of this flaw, and the fact that Newton was the physics hotshot of his day, the particle theory was accepted. With quantum physics, though, both of these theories are right.
The wave theory did not come up again until an English scientist named Thomas Young devised an interesting experiment to test it. This experiment, which is explained under Important Experiments, proved Newton's particle theory of light to be wrong. The experiment was ignored by most scientists because of Newton's greatness. Augustin Fresnel, a Frenchman, however, adopted this idea and worked to create a wave explanation of light. His work also included explaining why a thin film of oil creates such a colorful reflection. He noticed that a film of oil is bumpy and uneven, so it reflects the light at different angles. He theorized that if color was a product of the wavelength of light, and since waves can mix in ways to either strengthen or dampen the product's wavelength, the colors produced must be a product of the light bouncing off the uneven surface and interfering with the other reflected light waves. If light was made up of particles, it couldn't do this. By the nineteenth century, it had become accepted that light was made of waves.
 Knowledge of the atom had also increased since the time of Newton. John Dalton, an early 19th century scientist, had created a model of the atom that was indestructible, and with different sizes and shapes for each element, as the Greeks suggested. At this time, however, the existence of atoms was doubted by the majority of scientist. That is, until a young German theorist named Albert Einstein published a revolutionary paper.
One of his papers discussed the way light (photons) and electricity (electrons) interact, which was one of the first questions of quantum mechanics. The most important paper, however, established without a doubt the existence of atoms. Einstein used a seemingly unrelated scientific discovery which had been made nearly a century before by Thomas Brown, a British botanist. While examining a grain of pollen floating on water, Brown noticed that it was randomly shaking. The scientific community was puzzled by "Brownian Motion", but not very concerned. Einstein showed that this motion was caused by the random movement of the water particles.
In 1897, an Englishman named J.J. Thomson made another important discovery. Cathode tubes, an old scientific curiosity (which, because of his discovery, became more of an obsession) consisted of a glass vacuum tube with a small amount of gas in it and electrodes on each end. When current is applied, electricity flows through the gas, producing light. The type of gas used effected the color of light emitted (which, surprise surprise, has to do with our old friend, the quanta). A common application of cathode tubes today is neon lighting. In addition to creating light, though, cathode tubes gave off cathode rays. By using magnetic fields, Thomson was able to manipulate the rays. More of this experiment is described in Important Experiments. To sum up the experiment, Thomson discovered that the cathode rays were negatively charged particles which were being knocked off the atoms. He dubbed these particles electrons and claimed a Nobel Prize in physics for his discovery.
Thomson unfortunately over-looked the color issue. It was known that different elemental gases always created different colored light, but everyone was at a loss to explain why. As we know today, each element has a different number of electrons which are arranged differently. Light is produced when the atom of gas absorbs some energy, which excites some electrons into a higher energy level. Electrons don't stay excited very long, though, and soon fall back into their original energy levels. This means that they must give off the energy which they absorbed, which comes out as light. Depending on the difference between the two energy levels, which depends on the element, the light can have a lot of energy (blue - ultra violet), some energy (yellow - green), or very little energy (red - infra red). The quanta comes into play because there are set differences between energy levels, which means that the electron can't give off one and three quarters of an energy level's energy.
Thomson also overlooked something else: where's the positive charge? Because atoms are known to be electrically neutral, there must be a positive charge to cancel the electrons' negative charge. Thomson solved this dilemma with his Plum-Pudding model. He theorized that the negative electrons were embedded in a sphere of positive charge, much like raisins in the English snack plum-pudding. This model is wrong because it couldn't explain how the electrons got knocked off the atom if they were inside the positive charge. His experiments did, however, allow Thomson to discover ions, or atoms with a charge (made by adding or removing electrons).
In 1909, Ernest Rutherford and his two colleagues, Hans Geiger (more commonly known for his work with radioactivity, and hence the Geiger Counter) and Ernest Marsden created a very important experiment. Their experiment consisted of shooting the newly discovered alpha particles (positive particles made of two protons and two neutrons emitted during radioactive decay) at a piece of gold foil. They set up fluorescent screens to detect the particles, dimmed the lights, and let the alpha particles fly. What they discovered was that the majority of the alpha particles went right through the foil, occasionally being deflected a bit. The real surprise came when some of the alpha particles ricocheted right back at the source. Rutherford compared this to shelling a piece of tissue paper and being hit by your own reflected shots.
To sum it all up, Rutherford theorized that the alpha particles must be occasionally hitting other positive particles. By the statistics gathered, he found that the positive charge must be concentrated in the center of an electron shell. The startling thing is that there is a lot of empty space between the electrons and the positive nucleus. If the nucleus was the size of a pin-head in the center of St. Paul's Cathedral, the electron cloud would be around 100 meters away in the dome. Through these observations, Rutherford created a new atomic model in 1911. In his model, there is a cloud of electrons with a small, concentrated nucleus made of protons in the center.
Rutherford's model is much like our current generic atomic model, minus the neutrons (discovered through more experiments). Like all atomic models in the past, Rutherford's model had just one flaw. The electrons in his model were doomed to fall out of their orbit around the nucleus and collapse into the nucleus, annihilating the atom. To make a long decade short, Neils Bohr fixed Rutherford's problem with a little imagination.
In his atomic model, the electrons were located in different energy levels, where each energy level was further from the nucleus (which now had neutrons as well as protons) than the previous. For example, if the first energy level was one inch away from the nucleus, the second would be two inches away, the third would be four, the fourth would be eight inches away, and so on. Bohr's energy levels cannot have a energy between two levels though. The electron is much like a ball resting on the stairs. If you give it enough energy, it will go up to the next step. If not enough energy is provided, it will not go halfway between the steps, for there is nowhere for it to rest there. Any excess energy is given off as light. Also, if the ball goes up a step, there will be room on the lower steps for it, and since the arrangement with the lowest energy is always the most stable, it's pretty likely that the ball/electron will give off some light and fall back down to it's old step. Bohr's model solved the problem of Rutherford's decaying orbits, but it was an incomplete solution. Unlike balls on steps, electrons can't just sit there. There had to be some movement to oppose the attraction between the positive and negative charges. This movement is solved by the quantum atomic model.
Before scientists could get to the quantum atomic model of an atom, though, they had to find the right math. The mathematics that they were searching for comes from a surprising source: a German physicist named Max Planck. Planck was the first to use the quanta, which he more or less stumbled upon in 1900. The quanta is the smallest amount of energy possible which can be emitted or absorbed by matter. The cause of the quanta comes from Bohr's model of a stair-case of atoms. When an atom absorbs energy, it's electrons become excited and move up an energy level or two. Like I said before, though, the electrons can't absorb one and a half energy level's worth of energy. Conversely, the electrons can't fall one and a half energy levels and give off that much energy. It has to be an integer amount.
It is very surprising that Planck discovered this fact. Planck had been working long and hard on an electrodynamics problem, and as a last resort, he used another scientist's thermodynamics equations to help him solve the problem. At the time, the connection between electrodynamics and thermodynamics had not been discovered, so there was no reason for Planck to be doing this, but he did. And he did it wrong. The thermodynamic equations he was using had several steps. The first step involved dividing the energy up into small chunks (quanta, anyone?). After doing this, the mathematics could juggle the different chunks of energy, and then the last step put it all back together into a final solution with just one piece of energy. Planck, however, decided that the mathematics was working well halfway through the process, so he didn't bother doing the last step and didn't put the energy back together. Planck didn't apply the equations consistently either, yet somehow he got the right answer. No, he didn't look in the back of the book, he was just incredibly lucky and discovered the quanta.
The correct mathematical work was later done by Erwin Schrödinger, an Austrian scientist. His math was based upon the work of Planck, Einstein, and others, such as Werner Heisenberg. Heisenberg's key idea during the 1920's was the Heisenberg Uncertainty Principle. He stated that the position and the velocity of a subatomic particle cannot be known simultaneously. Because of this, a margin of probability and randomness is introduced into the whole equation. This did not go over well with any of the scientists, especially Schrödinger and Einstein. Hence the quote at the bottom of the page.
Schrödinger incorporated the H.U.C. Principal into his model so that the electrons randomly 'pop' around the nucleus, provided they stayed within their own energy level. This provides the necessary movement to prevent the electrons from falling into the nucleus. The electrons 'pop' in a random but predictable way. Each energy level has a different shape (called shells), which is formed by the highest probability where the electron will be. When two atoms join to form a molecule, their outer energy shells overlap and merge together. This is the current atomic model, which is based on the quantum theory, and it's much more interesting than Thomson's plum pudding.
The evolution of quantum physics first started with questions about light and the atom. It grew as more complete atomic models were developed, and it finally appeared as a science when Schrödinger developed his mathematical atomic model. Like all other sciences, quantum physics had humble beginnings, and grew to be the sophisticated science it is today. It has continued to expanded, and quantum theory is no longer limited to explaining the workings of light and atoms, but also black holes, quantum gravity, negative energy, and more.




Experiments

Thomas Young's experiment consisted of a light source shining on a obstacle with a small slit in it . If a wave hits such a slit, it spreads out the way shown. The next two slits on the next obstacle create two more such waves, which are close enough to interfere with each other. If light truly was a wave, then this interference would either increase or decrease the intensity of the light when it hit the screen on the right .
If light were just a particle, and you were able to send just one photon through , then there would be no pattern on the screen, just a single point of light. However, it has been found that even if just one photon is sent through, it creates the same interference pattern, although dimmer. If the light is measured, or observed, in between the screen and the second barrier, no interference pattern is formed. Instead, there is the most intense light in between the two slits, which gets dimmer as it progresses away .
This phenomenon is one of the basic principles of quantum physics, the Heisenberg Uncertainty Principle. If light is not being observed, it acts as a wave, but if it is being observed, it has to behave itself and act like particles.
Thomson's experiment with cathode ray tubes which allowed him to discover electrons was rather simple. He took a cathode ray tube, or a glass tube with very little gas left inside of it, and sent a current through it. As expected, it created a glow from one electrode to the other. Next, Thomson took a magnet, which deflects the flow of electrons (called cathode rays), and put it next to the cathode ray tube. This deflected the beam slightly. He realized that the cathode rays were being deflected by the negative end of the magnet, so the cathode rays must be negatively charged. Because of the flow of electricity through the tube, these negative particles were being knocked off the atoms in the gas. This led him to name them electrons, which (to him) where negatively charged particles embedded in a sphere of positive charge, which made up the atom.
Rutherford's experiment with alpha particles (which are positive) and gold foil led to the discovery of the protons. In this experiment, he used a piece of radioactive material as a source of alpha-particles. Today, a particle accelerator would also do the job. He used lead shielding to aim the alpha particles towards a thin sheet of gold film. Around the gold film was a series of detectors, which emitted a small flash of light when hit by an alpha-particle. In a very dark room, Rutherford and his colleges recorded the flashes and their locations. The majority of the alpha particles passed through the gold foil but some were deflected. The shock came when they found that some were deflected back at the source. From the statistics gathered, Rutherford theorized that atoms were made up of a cloud of electrons surrounding a very small positive nucleus, which was what the positive alpha particles were being deflected by.




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