Transcript 05-1-Intro to QM

A few introductory words of explanation about this transcript. This transcript includes the words sent to the narrator for inclusion in the latest version of the associated video. Occasionally, the narrator changes a few words on the fly in order to improve the flow. It is written in a manner that suggests to the narrator where emphasis and pauses might go, so it is not intended to be grammatically correct. The Scene numbers are left in this transcript although they are not necessarily observable by watching the video. There will also be occasional passages in blue that are NOT in the video but that might be useful corollary information. There may be occasional figures that suggest what might be on the screen at that time. 101 AvatarIntro CHAUCER: Do you two know how scientists identify the substances in distant stars? Do you know how we measure the composition of the sun for example? KEVIN: Sure, one major tool is spectral analysis isn’t it? CHAUCER: Yes indeed. DIANA: I am familiar with the results of spectral analysis, but I am a bit unsure of how the spectra arise to begin with. Chaucer, is that part of our material for today? CHAUCER: Yes it is Diana, in fact it is the starting point. Let’s use Professor Peabody’s WABAC Machine, since we don’t have one of our own, and look at a little science history. Jeeves? 102 Spectra JEEVES: Traveling to the 1750’s we find that scientists were putting different substances in flames and passing the resultant light through a prism. They found that the hot gases given off by the burning materials emitted different colors of light or spectra.. Ordinary table salt, for example, generated a "bright yellow" spectra.

Furthermore, not all the colors of the rainbow appeared - there were dark gaps in the spectrum, in fact for some materials there were just a few patches of light. By the 1820's, they recognized that spectra provided an excellent way to detect and identify small quantities of an element in a powder put into a flame. Meanwhile, the white light of the sun was also being examined closely. And in 1802 it was discovered that the solar spectrum itself had tiny gaps - there were many thin dark lines in the rainbow of colors. But the reason for the spectral lines in the light and the relationship to each substance was a real mystery. 105 BlackBody Radiation Traveling forward from that era to a little over 100 years ago, scientists were examining the colors of light given off by solid heated objects, they discovered that these hot solids gave off continuous spectra and that the overall color of the light revealed the temperature of the object. Now this was important because scientists realized that this discovery made it possible to measure the temperature of an object from a distance…they could measure the temperature of the sun for instance! During these discoveries, they also noticed that some objects absorbed light extremely well … almost perfectly in fact. They were called Black Bodies because they absorbed virtually all of the light that shone upon their surfaces. These same objects also radiated almost perfectly, and as noted before, the temperature of the black body object determined the distribution of colors or wavelengths in the emitted light. This curve shows how much light of each color is emitted by a cool object. As you can see there isn’t much light and what light there is mostly lies out past the red end of the spectrum in the infrared. The figure on the right will show the different colors added together as we progress. Right now only red light is visible. This curve shows how much light of each color is emitted by a medium temperature object. And the most light is emitted in the orange-yellow-green wavelengths. So we will add orange, yellow and green to our cauldron of light on the right. As you can see the combination so far looks yellow. This third curve is for a really hot object – lots of light and with most of it being emitted toward the blue end of the spectrum. And as you can see our cauldron now is pure white in the center where all the colors overlap. A heated black body follows this color path as the temperature rises.

A good example of black body radiation is the heat inside a kiln. Inside the kiln, electromagnetic radiation -- light and heat -- exist in the form of standing waves – waves that, like the vibrations of a guitar string are attached at both ends. The ends of standing waves, like the ends of the guitar string, do not move – they are anchored to the sides of the kiln. And many, many waves exist with varying wavelengths or color. At low temperatures, the primary color inside the kiln is in the infrared and we cannot detect it with our naked eye. But as the temperature rises, the kiln begins to glow red, and as the temperature continues to rise, the dominant color changes to orange, then yellow, then bluish white. The distribution of energy in the light shifts to SHORTER wavelengths as the temperature rises. 110 Ultraviolet Catastrophe But there was a problem; the scientists expected the distribution of emitted light to continue to INCREASE at wavelengths toward the ultraviolet end of the distribution. It didn’t. Instead there was LESS AND LESS light given off as they went further and further into the ultraviolet. This was called the “ultraviolet catastrophe,” but it wasn’t a catastrophe at all…it was the beginning of something remarkable. A truly great scientist named Max Planck soon figured out a way to explain the observation. He concluded that the energy contained in the standing waves inside the kiln did not and could not possess just ANY AND ALL different amounts of energy. Instead, the QUANTITY of energy these standing waves possess had to be limited to a few specific discrete values of energy for each color. A standing wave of blue light for example can have energy equal to 0 electron volts …or 3 electron volts or 6 electron volts or 9 electron volts (and so on) In general … E = nhv Energy = any whole number… n times Planck’s Constant… h times the frequency of the light… v Which for blue light is any number times 3 electron volts

BUT NOTICE …BLUE-LIGHT STANDING WAVES CANNOT CONTAIN ENERGY EQUAL TO… 1 electron volt …or 2 electron volts …or 4 electron volts. Realizing that energy could have only discrete values was the beginning of Quantum Mechanics. The energy is said to be QUANTIZED…and n …from the equation above …is called a quantum number. 115-PhotoElectric Effect Planck’s conclusion that light energy is quantized was quickly used by Einstein to explain another puzzling phenomenon. It was known that shining a light upon a metal plate can release electrons from the plate, but the light has to have a certain wavelength before even a single electron is released. We can shine brighter and brighter light on the plate forever, but if the color isn’t right, the electrons stay home. Einstein concluded that the light striking the plate had to be coming in discrete bundles and unless a single bundle had enough energy to free an electron from its captivity, it would remain trapped. And as Planck had suggested, the wavelength and frequency of the light was a measure of the amount of energy each bundle carried. So while blue light packets might be able to free an electron, red light packets could not – no matter how many red packets hit the plate. 120-Duality But, wait a minute. That sounds awfully like light is a particle and not a wave, and there are mountains of data showing light behaving like a wave – diffraction, refraction, interference, etc. Which is it… a particle or a wave? It is both. These light packets are extremely tiny…let’s call them photons (from photos -- the Greek word for light) . And when you try to explain the behavior of things this small, you have to resort to some really unusual ideas – the ideas contained in Quantum Mechanics.