(freely edited by Zita)
Physical
Systems 1998-99
with E.J. Zita
Laser Tweezers Laser cooling of atoms and optical molasses Adaptive optics Aharonov-Bohm effect Quantum Hall effect Scanning Tunneling Microscope Atomic Force Microscope Nuclear Magnetic Resonance (NMR) Scanning Electron Microscope , ,
Modern Physics Paper Assignment Physics 232: Modern Physics Fall 1994
Due Dates: Topic--10 am, Monday, Oct. 10, 1994 Bibliography--10 am, Monday, Oct. 31, 1994 Paper--5 pm, Wednesday, Nov. 23, 1994 Rewrites--5 pm, Friday, Dec. 2, 1994
Grading: Content (6 points), Writing Style (3 points), Presentation (1 point) If you wish, you may rewrite your paper for a better grade. If you choose to rewrite, your paper grade will be 1/3 the original paper's grade plus 2/3 the rewritten paper's grade.
Miscellany: 5 - 10 pages, double spaced. Use typewriter or computer with laser printer. If you use a computer, I prefer 12 point Times font. Use 1" right, left, top, and bottom margins (ragged right ok) Number pages at right margin, 1/2" from top.
Assignment: Research a topic of contemporary interest in modern physics, and write a paper that explains that topic at the level of a Modern Physics student. Choose several sources so that you can really learn about your topic. A good paper conveys the reason why the topic is interesting, describes the physics using a few equations (starting from something in the textbook), and discusses current or recent developments. I would be concerned with a paper that could not be understood by one of your classmates, that is based on a single article, or that does not tell the reader why she should care about the topic.
The following list suggests some possible topics that would be at the appropriate level for your Modern Physics paper. You do not need to choose a topic from this list.
A focused laser beam can be used to manipulate objects such as paramecia and small styrofoam spheres. This technique has been used to study a variety of biological systems, including DNA and actin/myosin interaction in muscle cells.
Laser cooling of atoms and optical molasses
A beam of atoms can be slowed (and hence cooled) using a tunable laser. The laser is tuned so that a photon can be absorbed only if an atom is moving toward the light (because of the Doppler shift). This is used to make accurate atomic clocks, to look at transitions in single atoms, etc.
Atmospheric distortion is the main motivation for putting telescopes in space, but this is expensive and inconvenient. An earth-bound telescope can correct in some ways for the atmosphere's distortion if it is equipped with adaptive optics. Single particle interferometers (electron, photon, neutron, atom) Quantum mechanics states that a single particle can pass through two slits and interfere with itself. Some recent experiments have confirmed this disturbing prediction for photons, electrons, neutrons, and even whole atoms.
A beam of electrons passing through a pair of slits will form an interference pattern. This pattern will shift if a current is passed through a solenoid behind the slits. Aharonov and Bohm predicted that the shift would occur even if the electrons do not touch the magnetic field (so they feel no magnetic force), and this effect has been demonstrated experimentally.
Nuclear Magnetic Resonance (NMR)
NMR is used in chemistry to probe the structure of molecules and in medicine to produce high-resolution 3-d images of the internal organs without the use of x-rays. In NMR, a strong magnetic field is used to align the nuclear spins. When a perpendicular RF signal is applied at the resonant frequency of the nuclei, the nuclear spins flip over and then precess back into alignment. This resonant frequency is affected by the local electron density (i.e. the chemical structure near the precessing nuclei).
We have a 60 MHz NMR at Evergreen (a little one in the CAL lab?) and a 200 MHz NMR that quenched a few years ago and needs to be revived.
Suppose that you put a piece of semiconductor in a z-oriented magnetic field and send a current in the x-direction through the semiconductor. You wouldn't be surprised to learn that a voltage drop proportional to the current occurs in the x-direction, that's just Ohm's law. But a similar voltage drop occurs in the y-direction--that's the Hall effect. If the semiconductor is made really thin, increases in the magnetic field cause the Hall voltage to increase in steps. These steps are used to provide a quantum definition of the ohm.
The wavefunction of electrons at the surface of a conductor decays exponentially with distance from the surface. However, if a second conductor, say a thin needle, is brought near that surface, electrons can tunnel between the two conductors. A STM can map the surface of a conductor with atomic resolution by scanning the needle over the surface while keeping the tunneling current constant.
Like its cousin the STM, an AFM works by scanning a tiny conical stylus over a surface. The main difference is that the AFM does not rely on tunneling current to keep the stylus at a constant distance from the surface; rather, the AFM maintains a constant contact force, typically on the order of 10-9 - 10-10 Newtons. Thus, an AFM can be used to examine both conducting and nonconducting surfaces.
Velocity-selector Mass Spectrometer
We have one of these but where? Classic device - would be great to find and revive this mass spec.
Gas Chromatograph Mass Spectrometer
Fred Tabbutt has one of these- ask at Lab Stores how to get certified to use it.
Inductively Coupled Plasma
KV Ladd has one of these.
HP diode array - UV-visible spectrophotometer
We have 5-6 new ones, and a number of. They measure 200-1000 nm and are really easy to use. Can measure concentrations of colored species in solutions.
Cary 17 - UV-Visible-Near IR Spectrophotometer
Can measure concentrations of colored species in solutions, with 10x greater sensitivity than HP silicon diode arrays. Fred has used it to measure gas pollutants in air samples.
Spectrophotometer
We have one of these - ask at Lab Stores how to get certified to use this.
Fourier Transform Infrared Spectrophotometer
Dharshi has one of these
FRITZ is the original Fourier Transform Infrared Spectrophotometer.
Has a control computer set up in octal, is programmed bit by bit. The 456-K hard drive cost $5000. An antique with excellent optics. The interferometer has been interfaced with a Mac by Barlow's lab. Fred and Clyde looked at Bromine gas with this and were able to separate the IR absorbances of different isotopes.
X-ray crystallography
Student control the operation of this insturment by writing a program in LabView.
Scintillation counters
Betty Kutter and Jim Neitzel use these to count beta particles from radioactive samples. There's a radiophysics room down the hall from the CAL, across from the wet lab, where you could measure neutron activation and do elemental analysis from the gamma spectrum on activated samples - if we had a neutron howitzer!
We have one of these at evergreen.
Josephson junctions
Josephson junctions are small weak-links in a superconductor. Only a small current can be sent through a JJ before a voltage develops. Once that critical current has been exceeded, the JJ acts like the parallel combination of a resistor, a capacitor, and a voltage oscillator. These JJs are the basis for superconducting logic circuits and SQUID magnetometers.
SQUID Magnetometers
Superconducting quantum interference devices (SQUIDs) are made from two JJs in parallel. These devices are the most sensitive magnetic field sensors around. They are used to measure weak magnetic fields in a variety of applications, from geomagnetism to submarine detection to magnetoencephalography.
Superfluid helium 4 Imagine a strange liquid whose viscosity is sufficiently low that it climbs over the walls of a beaker to drip off the bottom. Even stranger, its thermal conductivity is high enough that it does not bubble as it boils away; rather it boils only from the top layer of atoms. This liquid is superfluid He4, a peculiar quantum phase that occurs when helium is cooled below 2.07 K.
Superfluid helium 3
Superfluid He3 is even more peculiar than He4 because of the way it is formed. When He3 is cooled below a critical temperature (only a few millikelvin), the He3 atoms undergo a Cooper pairing like the electrons in a superconductor. These pairs condense into a single quantum state that behaves similarly to superfluid He4, except the state of He3 is affected by magnetic fields.
Expansion of the Universe
Astronomical observations are consistent with the theory that the universe came into being about 10-20 billion years ago with an enormous explosion (big bang) and that the universe has been expanding ever since. However, some questions remain unanswered. Why is there a factor of 2 uncertainty in the age of the universe? Will the universe expand forever or fall back together in a big crunch? How were galaxies formed?
Dark Matter Through observation of the Doppler shifts in rotating galaxies, radio astronomers have discovered that 90% of the mass in the universe is nonluminous (or dark). This dark matter could be comprised of weakly interacting massive particles (WIMPs), massive compact halo objects (MACHOs), axions, or more exotic particles. Several experiments are currently underway to search for these dark matter candidates.
Einstein-Podolsky-Rosen paradox (quantum action at a distance)
Quantum mechanics predicts that a measurement on one particle can instantaneously affect a second particle distant from the first. Einstein said this is nonsense since nothing can travel faster than the speed of light. Find out which predictions have been borne out in experiment.
Paradoxes in special relativity
According to special relativity, the result of an experiment must be independent of the frame of reference from which it is observed. It is possible, however, to construct paradoxes which seem to result in contradictory results depending on the reference frame. These paradoxes can be resolved within the framework of special relativity.
General relativity
General relativity is based on the postulate that there is no experiment you could do in a closed lab that could tell whether your lab is accelerating uniformly or in a uniform gravitational field. This theory predicts a wide variety of effects, including black holes, gravity waves, gravitational redshifts, and light beams that curve as they pass near a massive object.
Magnetic fusion
Someday, we may generate power the same way the sun does: by fusing hydrogen into helium. This technique would have many advantages over current techniques: the fuel source is plentiful and cheap, and the waste product is a harmless gas. The trick is to get a sufficient density of the gas hot enough for long enough that it fuses together. One approach is to use a magnetic bottle (a Tokamak or Magnetic Mirror machine) to hold the gas for a long time at low density. Another approach is to use radiation pressure from lasers to compress a pellet of hydrogen to an extremely high density for a short time.
Fission reactors
Nuclear fission reactors produce electric power through the controlled fission of heavy nuclei. How do these reactors work, why can't they explode like nuclear weapons, and what are the practical problems with nuclear waste disposal.
Nuclear weapons
Unlike fission reactors, nuclear weapons work by the uncontrolled fission of heavy nuclei. Moreover, they are the only environment in which we have been able to produce artificially a sustained fusion reaction. Explore how these weapons work on a nuclear scale and learn why they are so much more destructive than weapons such as dynamite that are based on chemical reactions.
Gravity wave searches
One prediction of General Relativity is the existence of gravity waves. Unfortunately, gravity is such a weak force that the effect of a gravity wave on a terrestrial object would be minuscule. Nevertheless, several experiments have been built to search for these waves, including from enormous superconducting cylinders (Weber bars) to titanic Michelson interferometers (LIGO).
Proton decay experiments
Is matter stable, or is the universe fated to decay? This is the question asked by proton decay researchers. Proposed "grand unified theories" require that the proton decay. Several enormous experiments have been built to search for possible proton decay.
Neutrino astronomy
Supernovas are believed to emit most of their energy as neutrinos, but these neutrinos interact only weakly with matter. How weekly? Well, it would take many light-years of lead to stop 50% of the neutrinos passing through it. Nevertheless, neutrinos were detected during the first few moments of Supernova 1987A.
Magnetic monopole search
Although electrical monopoles exist, magnetic monopoles have not yet been found. Dirac has shown that the existence of even one magnetic monopole would explain the quantization of electrical charge. Cabrera claims to have detected a magnetic monopole in 1982 but has not seen one since.
Neutrino mass experiments
Neutrinos are neutral particles that are believed to be massless, but do they really have no mass? This is an important question because neutrinos, if they have mass, may account for a major portion of the missing matter (dark matter) in the universe. Learn about some of the sensitive experiments that have been conducted to measure the mass of neutrinos.
Solar neutrino problem
The sun forms neutrinos in addition to its other fusion products, but there is a factor of 3 discrepancy between the theoretical and experimental value of the number of neutrinos that are detected on earth. Learn about the experiments that have been conducted to measure the neutrino flux and possible explanations for the neutrino deficit.
Formation of antihydrogen
The Schr–dinger equation for a hydrogen atom is unchanged if the proton and electron are swapped for an antiproton and positron, but would this antimatter really have the same spectrum as normal hydrogen? Would it fall up or down in a gravitational field? There is no way to know until an experiment is done. Learn how antihydrogen is produced and stored and find out about antihydrogen experiments.
Parity violation
At first blush, the universe seems to be symmetric with respect to handedness (parity). Whether you look at an experiment directly or its reflection in a mirror, both show the same result; this is called "parity symmetry." There are, however, certain nuclear decays that violate parity symmetry; that is, you can tell whether you are observing the experiment directly or in a mirror.
CP violation
Is the universe symmetric if both handedness and charge are simultaneously changed? Well, no. It turns out that a particle known as a "kaon" violates this "CP symmetry" because the masses of the kaon and its antiparticle are slightly different.
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