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Modern Physics research project ideas
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 , , and more
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 (from Grinnell College): 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.
Format: 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 margins are fine.) Number pages at right
margin, 1/2" from bottom. Include a title at the top of page 1, with all authors’
names, the program name, and the date.
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.
Reference your sources diligently, either with footnotes or with (Author, page).
Any time you use information in a sentence that is not common knowledge, there
should be a reference at the end of that sentence. Avoid paraphrasing. Either use
your own words, truly, or “quote” the author if necessary. Include a complete
bibliography at the end of your paper.
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.
Laser tweezers
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.
Adaptive optics
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.
Aharonov-Bohm effect
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 xrays. 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.
Quantum Hall effect
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 xdirection, that's just Ohm's law. But a similar voltage drop occurs in the ydirection--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.
Scanning Tunneling Microscope
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.
Atomic Force Microscope
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!
Scanning Electron Microscope
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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