Transcript: The Doppler Effect is the shift of wavelength or frequency of a source of waves due to the motion of that source of waves.  Doppler Effect is most familiar in terms of sound waves.  As a source of sound, such as a siren, approaches you, the pitch or frequency increases.  As it moves away from you, the pitch or frequency decreases.  You can see that if a source of waves is moving towards you, the waves will be bunched up in the direction of motion, and so the wavelength will be shorter or higher frequency.  This is a blue shift in terms of light.  If the source of the waves is moving away from you, the waves are stretched out in the direction of motion away, and so the wavelength is increased or the frequency decreased.  This is a red shift in terms of optical light.  The Doppler Effect is a fundamental way of knowing about the line of sight motion of any astronomical object.  The amplitude of the Doppler Effect is given by a simple equation which says that the fractional wavelength shift of a source of radiation due to the Doppler Effect equals the velocity of that source relative to the velocity of light.

Doppler Effect

Transcript: German physicist Gustav Kirchhoff experimented with hot gases in the laboratory in the mid-nineteenth century and deduced three laws of radiation that apply to all astronomical objects in space.  The first is that a hot gas or sufficiently hot solid will emit with a continuous spectrum of radiation or a continuum.  The second is that a hot gas will emit only certain bright wavelengths of radiation called emission lines.  Each element emits its distinct and unique set of emission lines.  The third is that a cool gas, when placed between the observer and a hot source of continuous radiation, will absorb certain wavelengths of light also corresponding uniquely to each element.  These are called absorption lines.

Kirchoff's Laws of Radiation

Transcript: When a hot object like a star is surrounded by a cooler layer of gas or has an intervening cool material we see an absorption spectrum.  Always they’re at the smooth thermal radiation reflecting the temperature of the object itself, but in the case of an outer or intervening cool layer of gas the radiation is absorbed by the cooler gas exciting those atoms that then reradiate in all directions causing notches in the spectrum or lowering of the intensity at the exact position corresponding to the spectral transitions of the elements in that gas.  Thus the smooth thermal spectrum has superimposed on it an absorption spectrum where the wavelengths of the absorption features tell us about the chemical composition of the gas.  Fraunhofer in the mid-nineteenth century observed an array of dark absorption lines in the spectrum of the sun, and this was our first clue that the sun was made of hydrogen and helium.

Absorption Spectrum

Transcript: When a hot object like a star is observed directly, two types of radiation are seen at the same time.  One is the smooth continuous spectrum or thermal spectrum whose peak wavelength reflects the temperature of the object being observed.  For a star like the Sun emitting yellow light, this peak wavelength is in the visible spectrum, and the surface temperature of the sun is at about 5,700 Kelvin.  In addition to the smooth radiation of the thermal spectrum the hot or ionized gas also emits emission lines, sharp spectral features whose positions reflect the chemical composition of the gas.  These two things combined make up the typical emission spectrum of a hot gas in any astronomical situation.

Emission Spectrum

Transcript: Advances in technology have caused a surge in our ability to understand the universe.  After 30 or more years where the Palomar 5-meter Telescope was the largest useful telescope on the ground, we now have a dozen or more 8 to 10-meter class telescopes with which to study the distant universe.  Large optical telescopes of apertures 30 to 50 meters are planned.  In space NASA has completed the launch of its Great Observatories spanning the electromagnetic spectrum from x-rays and gamma rays through to infrared, through to optical and ultraviolet wavelengths.  Even astronomy observing on the Moon has been proposed.  Other forms of astronomy use less conventional means of detection.  A neutrino detector has been in place for a while in Hawaii, and an even more ambitious project is planned for the Antarctic plateau.  Finally gravity waves, perhaps the most exotic way to learn about the distant universe, are now in operation in the LIGO Project.  Astronomers have used as many ways to gather information about the cosmos as technology will allow.

Telescope Frontiers

Transcript: For all regions of the electromagnetic spectrum where radiation does not penetrate the Earth‘s atmosphere, astronomy must be done from space.  This includes gamma rays, x-rays, ultraviolet radiation, far infrared, and some kinds of millimeter waves.  Space astronomy is expensive compared to ground-based astronomy.  Typical launch costs are ten thousand dollars per kilogram, and so space telescopes are intrinsically more expensive then ground-based telescopes.  A small ground-based telescope of a meter or so may cost ten million dollars or a few million dollars; the largest optical telescopes cost a hundred million dollars or greater.  Even a modest-sized telescope placed in space will cost about a hundred million dollars, and the largest space observatories, NASA’s Great Observatories, all have cost over a billion dollars each.  Space astronomy is expensive, but to work in those parts of the electromagnetic spectrum or to completely cheat the distorting effects of the Earth’s atmosphere at optical wavelengths, space astronomy is the only way to move forward.

Space Astronomy

Transcript: The biggest optical telescope can see about 10 billion times fainter then the human eye.  This is the enormous advance of astronomy in the last 400 years since the invention of the telescope.  Where does this huge factor come from?  It comes from three different pieces.  First the aperture of a 10-meter telescope compared to the roughly 1-centimeter aperture of the eye is a factor of a thousand.  Second, the efficiency of optical detectors, CCDs, compared to the chemical detection of the eye is another factor of a hundred.  Finally the eye must register its information about every 30th of a second so that we can get the illusion of motion, whereas a telescope can gather light and integrate for something like an hour.  This is a factor of 105 or a hundred thousand.  Combining these three factors gives you a factor of 10 billion and means that compared to the naked eye, which can see an object at 6 magnitude, the largest optical telescopes can see objects at about 30 or 31 magnitudes, an incredible gain in light gathering power.

Light Grasp of a Telescope

Transcript: Interferometry is a technique that astronomers have developed to improve the performance of a telescope.  A single telescope has its resolution limited by the diameter of the primary mirror.  However, in principle it is possible to use two or more telescopes separated by a distance, point them at the same object, and combine their signals.  If the signals are combined coherently, that is timing measurements are used to combine the peaks and troughs of the light exactly, then you can simulate a telescope that has the aperture equivalent to the distance between the separate elements.  In this way a huge telescope can be simulated, although it will not have the collecting area of a huge telescope since the entire aperture is not filled in.  This technique was first perfected at radio waves where the challenges of coherence of combining the radio signals are not so severe.  Radio astronomers routinely use interferometers, the Very Large Array in New Mexico is a primary example, to attain angular resolutions that are fractions of an arcsecond, sometimes as small as a ten thousandth of an arcsecond.  In optical astronomy the challenge is higher for interferometry to work, but optical interferometers have also been developed that can achieve very high resolutions of about a milliarcsecond from the ground.

Interferometry

Transcript: Image processing is essential to modern astronomy.  Astronomical data is obtained in the form of digital information captured using a charged coupled device or CCD detector.  Astronomers use image processing for various purposes.  They use it to dig out faint signals in CCD images because objects as stars and galaxies are detected in the presence of a background from the night sky.  Even when the night sky looks totally dark with no moonlight there is residual brightness due to airglow.  Astronomers also use image processing to combine images taken through different astronomical filters to make a true color image.  When you see a color astronomical image it represents a result of one or more exposures taken through different filters, at least a red and a blue filter, sometimes more then two filters.  These are then combined and balanced digitally to give a true color representation where the blue objects look blue and the red objects look red.

Image Processing

Transcript: The most basic information about an astronomical object is the amount of light that it emits; this is photometry.  We learn more, however, when we can disperse the light from an astronomical object into a spectrum; this is called spectroscopy.  Spectroscopy gives us the opportunity to see the spectral transitions of the atoms contained within the object whether it’s a planet, or a star, or a galaxy, and this in turn gives us information on the chemical composition, information that cannot be obtained in any other way.  Thus, spectroscopy is a key process and part of the astronomer tools, and we are always trying to disperse the light from astronomical objects into a spectrum.  Making a spectrum is more intensive of light then just counting photons, and so very faint objects cannot be observed with spectroscopy.

Spectroscopy

Transcript: Astronomers measure the radiation from distant astronomical objects in several ways.  The most basic information is called photometry.  This is just a calibrated measure of the amount of light coming from an astronomical object.  The light is gathered by the telescope and is collected, and the number of photons is counted or inferred from the brightness on a CCD image.  Photometry is a measure of the light from an object defined over a specific range of wavelengths.  Astronomers typically use filters to define different pass-bands across the visible spectrum all the way from the blue end of the spectrum to the red end of the spectrum.  The absolute amount of light or photons per second is calibrated in terms of a known brightness astronomical object.  It could be the Sun, but the Sun is usually too bright to observe with a large telescope so astronomers typically use bright stars in the night sky.  Photometry is much of the basic information of astronomy.  What is the amount of light from a star or a galaxy or a quasar?

Photometry

Transcript: If you work out the angular resolution of a four meter telescope in optical waves of light it should be about two hundredths of an arcsecond.  The images obtained by a single four meter mirror are never this good because of the blurring effect of the Earth’s atmosphere.  Essentially turbulence in the upper levels of the atmosphere jumbles the light rays as they go through the atmosphere and blurs the image by a significant degree.  Most ground-based observatories have intrinsic seeing, or the level of spread of the image, of about a half an arcsecond to one arcsecond.  To achieve the full resolution of an optical telescope you must either go into orbit, and this was one of the prime motivations for the Hubble Space Telescope, or somehow cheat the atmosphere by distorting the mirror that collects the light to counteract the turbulence that blurs out the signal.  This is called adaptive optics.  Adaptive optics has been exercised several ways.  In some cases the primary mirror is thin enough that it can be deformed tens of times per second to adjust for the turbulence.  In other cases the secondary mirror does the same thing.  With these techniques the full resolution of ground-based optical telescopes can be achieved.

Adaptive Optics

Transcript: Essentially all modern telescopes used by research astronomers are reflecting telescopes.  They use mirrors to bend, deflect, and focus light to form an image.  The first reflecting telescope was perfected by Isaac Newton, and reflecting telescopes of larger and larger aperture have been produced ever since.  Edwin Hubble discovered the distant nature of galaxies and the expansion of the universe using the largest reflecting telescope at the time just outside Los Angeles.  For several decades the largest reflector was the 200-inch or 5-meter diameter telescope at Mount Palomar.  The Russians built a 6-meter telescope in the 1970s.  Currently the largest single mirror telescopes are just over 8-meters in diameter.  The advantage of a reflecting telescope relative to a refractor is that the mirror can be very large and thin because the light is only reflected from the front surface.  A reflector also reflects all the waves of light by equal amounts, and so the images that are formed have no chromatic aberration.  The two slight disadvantages of reflecting telescopes are that the mirror’s surface is exposed to the elements and must be occasionally recoated and the fact that since the light is focused in front of the mirror, some of the light must be blocked by a secondary or by a detector to gather the light.

Reflecting Telescopes

Transcript: The fundamental purpose of a telescope is to gather light.  There are two important ways of characterizing a telescope.  One is in terms of the light gathering power or how much radiation is gathered and then focused into an image.  The light gathering power depends on the area of a telescope and so goes up as the diameter or radius of a mirror squared.  Thus a telescope of twice the diameter has four times the light gathering power.  A telescope of three times the diameter, nine times the light gathering power.  The other fundamental attribute of a telescope is resolution or angular resolution.  This is the ability of a telescope to discriminate fine details in the image of a distant object.  The resolution is defined numerically in arcseconds as 250 thousand times the wavelength of radiation being used divided by the aperture of the telescope.  As long as the wavelength of the radiation and aperture are in the right units the answer will be in arcseconds.  This scaling means that the larger the telescope the smaller the resolution or the finer detail that can be seen on a distant object.  It also means that the shorter the wavelength of light the higher the resolution or the finer detail that can be seen on distant objects.  Thus optical astronomers want the largest possible telescopes and want to use the shortest possible waves of light to see fine detail.  Magnification is not a fundamental attribute of a telescope even though it’s written on binoculars and small amateur telescopes.  Magnification is not fundamental because blurring is of course also magnified when an image is formed, and when an image is magnified the image is spread over a larger area of the detector.  Resolution and light gathering power are the most fundamental attributes of a telescope.

Purpose of a Telescope

Transcript: The two processes by which light is gathered in a telescope are refraction and reflection.  In refraction, light, when it travels from a less dense to a more dense medium, is slowed down, and when the light ray arrives at an angle of incidence the wave is refracted or bent.  The amount by which the light bends depends on color or wavelength.  Blue light travels slower in a physical medium light glass then red light, and so the blue light is bent more.  Thus, when white light enters a prism it is dispersed into a little spectrum, the blue light bent more then the red light; that’s refraction.  In reflection, light incident on a reflecting surface bounces off that surface.  The basic law of reflection is that the angle of incidence with respect to a normal of the surface is equal to the angle of reflection.  This applies independent of the wavelength of light.  So when white light bounces off a mirror, light of all waves and colors is reflected back at the same angle, and the light is not dispersed into a spectrum.  Astronomers in history have used both refraction and reflection to collect, gather, and magnify light in telescopes.

Refraction and Reflection

Transcript: Astronomy is unusual among the sciences because there is almost no direct experimentation, unlike the situation for physics, and biology, and chemistry.  In astronomy we have almost never traveled to the places that we study with the exception of the Moon and robotically to some of the planets and moons in the solar system.  Almost everything we learn about the universe is a matter of remote sensing that is radiation that we receive from space.  This radiation allows us to understand the temperature, luminosity, and chemical composition of distant astronomical objects.  Remote sensing is a powerful tool, and our ability to use radiation from space beyond the visible spectrum has only been possible in the last fifty or sixty years.

Radiation from Space

Transcript: The early twentieth century was a time of innovation and revolution in both science and the arts.  Scientists used the quantum theory to delve deep into the structure of nature and produce new representations of matter.  Artists were doing the same thing, exploring new representations of the natural world in painting and in sculpture.  They were also taking different forms of representation beyond the traditional view; relativity was a part of this revolution in science.  These innovations occurred in the same place and at the same time.  The same broad cultural movement produced Einstein’s relativity, Picasso’s cubism, Schoenberg’s twelve-tone serial music, and James Joyce’s revolutionary novel Ulysses.  Some of these innovations were met with incomprehension and even hostility.  The first post-impressionist exhibition in Paris in 1910 was met by rage in the audience, and the first performance of Stravinsky’s Right of Spring the audience stamped, cheered, walked out, and yelled.  The early reactions to quantum physics were similar; many physicists could not comprehend a universe governed by chance.

Changing World Views in Science and the Arts

Transcript:  The quantum theory of matter imbeds the idea of probability in a fundamental way.  There is no certainty when it comes to talking about atoms and fundamental particles.  This is embodied most clearly in Heisenberg’s Uncertainty Principle where, when we know the position of a particle or atom, its momentum is not perfectly well determined; if we know its momentum, its position is not well determined.  And so in the quantum theory the wave-particle duality of matter and radiation leads to the idea of particles more as fuzzy entities whose probability distribution fades off to zero.  We can only localize particles within a probability distribution.  The same is true of photons which are not small bullets but actually wave packets with extent in physical space.  Probability is also fundamental in the idea of quantum processes.  Radioactivity is perhaps the best example of this.  We can have an ensemble or large number of atoms and be sure that within the half life half of the radioactive atoms will decay, but we can not say with any certainty which atom will decay at what exact time.  In this way the fundamental nature of the physical world is involved with probability.

Atoms and Probability

Transcript: In the quantum theory there is a profound connection between the observer and the thing being observed, and this leads to some interesting philosophical consequences.  Einstein was uncomfortable with the quantum theory, and he created some examples to illustrate its strangeness.  One of these is the idea of Schrodinger’s cat.  Schrodinger was a German physicist who worked on the theory in the 1920s and 30s.  In Schrodinger’s cat the idea is of a cat trapped in a box where you cannot see inside.  Inside the box is an apparatus with a radioactive source.  After a fixed period of time, thanks to radioactive decay, there is exactly a fifty percent chance that a decay will cause a hammer to fall smashing a flask containing a poison.  Thus, at the end of this period of time the cat has a fifty percent chance of having been killed by radioactive decay.  The question is what is the state of the cat at the end of the time interval but before you open the box to see what happened?  According to the quantum theory the only way to describe the state of the cat is as a fifty percent alive and fifty percent dead cat because the outcome is based on a random probability of radioactive decay with no way determining which outcome occurred until you see the result.  This strange outcome or a quantum probability as mapped into macroscopic situation is a fundamental illustration of the strangeness of the quantum theory.

Schrodinger's Cat

Transcript: The quantum theory imbeds in a fundamental way idea indeterminacy and imprecision.  Heisenberg’s Uncertainty Principle sets a fundamental limit on our knowledge of the physical world independent of the degree of observations or the quality of the measuring apparatus.  Einstein in particular was very uncomfortable with this aspect of the quantum theory which is called the Copenhagen Interpretation.  Einstein believed that there was a deeper and more fundamental theory that would allow physical quantities to be determined or predicted exactly.  So far, no such theory has emerged, and we have to except that quantum mechanics is an excellent theory for the description of microscopic matter.

Quantum Theory and Determinism

Limited in scope but pretty well informative. Took no time to get through on double speed haha

Short

6. Optics and Quantum Theory

Transcript: Neils Bohr was a Danish physicist who won the Nobel Prize early in the last century.  One of the towering figures of modern physics, he established an institute in Copenhagen that was a Mecca for physicists from all around Europe.  Bohr was an architect of the modern view of the atom within the quantum theory of a nucleus surrounded by electrons that could only take fixed energy levels.  Bohr believed deeply in the quantum theory and had violent arguments with Einstein who believed there must be a deeper theory of matter.  Bohr was also famous for a sense of humor.  When he was being interviewed after having won the Nobel Prize by a journalist from the New York Times, the journalist stopped on the threshold of his laboratory noticing a horseshoe that was pinned above the door.  “Surely Professor Bohr,” he said, “you of all people, a physical scientist, winner of the Nobel Prize, are not superstitious.”  Bohr looked at him and said, “I’ve heard that it works whether you believe in it or not.”

Neils Bohr

Transcript: Each element in the periodic table has a unique number of protons and electrons.  Each element also therefore has a unique set of energy levels for the atom.  That unique set of energy levels maps into a unique set of spectral features of either emission or absorption.  Thus the spectral lines from an element are a fingerprint unique to that element.  In astronomy spectral features can be used to detect tiny concentrations of different elements.  When the elements are mixed in a compound, the different spectral features show up in superposition, and the different elements can be deduced from the patterns.  This is an extraordinarily sensitive technique.  Concentrations of a few parts per trillion of rare elements can be seen in the spectra of gases in the universe.

Spectral Fingerprint

Transcript: Atoms produce sharp spectral features of emission or absorption corresponding to the specific energy states of the atom.  Each element has its own set of energy states.  Atoms that are grouped into molecules have a much larger set of possible energy states, in part because they share electrons and in part because they can vibrate and oscillate many different ways.  This large set of possible energy states produces a large set of corresponding spectral transitions.  Often, these blend together to produce broad bands rather than sharp narrow lines, and so we talk about the spectral bands of either emission or absorption caused by molecules.

Spectral Bands

Transcript: Atoms gain energy by collisions or by absorbing photons of radiation.  However atoms cannot absorb photons with any energy or wavelength.  The energy states of an atom are quantized so atoms can only gain energy according to wavelengths that correspond to the energy difference between electron energy levels.  This removal of waves of a certain value leaves dark features in the spectrum or absorption lines.  Since atoms that are raised in energy eventually reemit photons, why do the dark absorption lines not get filled in?  Usually this is because the atoms causing the absorption are being seen with the background radiating source.  As the photons are reemitted they are reemitted in all directions.  And so the dark absorption likes are not completely filled in, and we see a series of dark lines in the spectrum.

Absorption Lines

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