Department of
Physics and Astronomy
Research in Physics and Astronomy
This section provides a brief overview of current research activities in the Department of Physics and Astronomy at Penn, particularly as they apply to undergraduates. Penn has traditional strengths in Condensed Matter Physics and Elementary Particle Physics. More complete descriptions are provided on the departmental research page.
The Department of Physics and Astronomy is now engaged in a dramatic initiative to make Penn a recognized leader in Astrophysics and Cosmology, building on established excellence in theoretical Cosmology and neutrino Astrophysics.
Condensed Matter Physics: Penn maintains a leadership position in Condensed Matter Physics. Experimental and theoretical efforts are concentrated on both ``hard'' materials such as metals and semiconductors, and ``soft'' materials such as liquid crystals and colloids. Electronic phenomena (conductivity, optical properties, etc.) in macroscopic, three-dimensional media are now well understood, but the analogous phenomena in restricted geometries, such as at the surfaces of solids, in ultrathin wires, or in submicroscopic ``quantum dots'' are still puzzling and intriguing. The properties of these ultrasmall structures are frequently controlled by the quantum mechanical behavior of the electrons within the structure. Using such techniques as electron beam lithography and atomically precise crystal growth it is now possible to fabricate and control ultra small, high quality electronic devices, in which electrons are geometrically confined to move in two, one and even zero dimensions. These structures exhibit exciting new physical properties and hold promise for important technological applications. Such advances open up a new realm of fundamental Physics in which it is becoming possible to manipulate matter on an atomic scale.
Penn is also applying a variety of theoretical and experimental tools, including x-ray diffraction, the scattering of visible light, nonlinear optics, and modern optical microscopy and laser tweezers, to study soft materials such as foams and emulsions, concentrated colloidal suspensions, biomembranes, liquid crystals and polymers, extending all the way to the molecules and cells that make up the human body. One project aims to understand and use diffusing light to characterize highly scattering media such as colloidal suspensions, foams, and even human tissues. A diffusing light field is built from photons that travel through ``foggy'' media in a manner similar to the way heat flows through materials. By measuring the small amount of light that is transmitted through this type of material, it is possible to learn about the microscopic structures within the medium including their internal motions. This may lead to a new technique for noninvasive medical imaging.
Astrophysics: With the addition of new faculty, the Department's research and educational programs in Astrophysics are presently undergoing rapid expansion. The research focuses primarily in extragalactic Astrophysics, large-scale structure and galaxy formation. The department also includes leading theorists working at the interface between Particle Physics and Cosmology, exploring the extraordinary phenomena that occurred during the first instants after the big bang.
In Extragalactic Astrophysics and Cosmology, research is being done on the origin of the universe and of galaxies. The questions that are being asked include: How old is the universe, and how did it originate? What do we learn about the global structure of the universe from the relic cosmic blackbody radiation that was emitted when the universe was much hotter than today? How did the universe go from an early phase where the distribution of matter was very close to homogeneous, to the present highly inhomogeneous state where matter has concentrated into galaxies and clusters of galaxies? When did the first galaxies form, and what did they look like? How are galaxies grouped into clusters of galaxies? How will the universe continue to evolve in the future? Penn has been a pioneer in the development of the inflationary theory of the universe, a major improvement on the hot big bang picture. Much of our current research deals with the physical processes important for the formation of galaxies, the evolution of the intergalactic gas, and the interpretation of observations of faint distant galaxies in terms of the theory. We also study tools, such as gravitational lensing, to measure the distribution of matter in galaxies and clusters of galaxies. Experimental research at present is in radio astronomy, focusing on observations of clusters of galaxies and gravitational lenses, and the cosmic microwave background, exploring the primordial inhomogeneities that led to galaxy formation.
Penn continues its world-renowned research efforts to study neutrinos emitted by the Sun and other astrophysical sources. Neutrinos are elementary particles (like electrons or quarks) that are electrically neutral and have very small mass, if any. They are produced in the nuclear fusion reactions taking place in the center of the Sun that generate all the solar energy, and from there they are emitted in all directions moving at (or nearly at) the speed of light. These neutrinos reach the Earth at a rate of about 60 billion per second for every square centimeter of area, but they are difficult to detect because their interactions with matter are very rare. Worldwide, five experiments have detected solar neutrinos; Penn has played a key role in three of these experiments. The rate of solar neutrino detection measured in these experiments is only about one half that expected from the theory for the structure of the Sun and the rate of the nuclear reactions in its center.
Two new solar neutrino experiments are under construction by Penn physicists. They are attempts to understand the apparent deficit of solar neutrinos. One of these projects, the Sudbury Neutrino Observatory, will try to determine whether the sun emits neutrinos as expected, but some of the neutrinos undergo a quantum mechanical transformation to a type that would not have been detected to date. Such a transformation would mean that neutrinos have a non-zero mass, a very important result for elementary particle physics, for our understanding of the Sun, and perhaps for cosmology. The detector will become operational in 1998.
Elementary Particle Physics: The Elementary Particle Physics group has a major participation in the Collider Detector at Fermilab (CDF) where the collisions of protons and anti-matter protons at the highest energy in the world are studied. The Penn group is instrumental in designing critical micro-electronics of the experiment that interface the sensitive detectors with high speed computers. The CDF experiment is now famous for its discovery of the sixth quark, the top quark, in which Penn scientists played a major role. Searches for evidence of new particles will begin in about two years after the present accelerator and detector upgrade is complete.
Penn scientists are also involved in the study of the interactions of beauty quarks that might explain the source of the matter-antimatter asymmetry in the universe. (That is, why is it that ``stuff'' in our universe is mostly made out of matter, rather than antimatter, even though the laws of Physics treat them identically?) This Physics is related to time-reversal invariance, where certain interactions in Physics seem to ``know'' the difference between backwards and forwards in time.
Computational Physics: Computational Physics investigates the use of hardware and software to extract information about simulated or real physical systems. Modern computational techniques are of growing importance in the fields of Biophysics, Astrophysics, Surface Physics, Chemical Physics, and many other fields. Computers are increasingly used both to investigate hypothetical systems (simulation), to visualize complex interactions, and to extract information from large collections of data which come either from real instrumentation or simulations (``data mining''). Penn faculty are designing a computer architecture which links large clusters of computers at a number of universities using ultra high speed communication techniques. The resulting super-meta-computer can be used in a variety of ways, but is particularly well suited to problems which require both massive processing and massive amounts of data.
Medical Physics: Faculty from several areas of the department are using their expertise and experimental tools to attack important problems in medicine. The use of diffusing wave spectroscopy for noninvasive medical imaging has already been discussed. Optical methods are now being explored as a means to detect and image specific anatomical structures such as tumors, and strokes. In a different vein, the possibility of proactively using visible light to cure disease represents another exciting research frontier of biomedical optics. In this procedure, called photodynamic therapy, light sensitive drugs deposit preferentially in patient tumors, and are then activated intra-operatively by laser light. The light activated drugs create free radicals which in turn, locally destroy tissue.
The department's expertise with Computational Physics is being applied to medical problems. In a collaboration with Childrens Hospital of Philadelphia we are developing the ability to provide real-time analysis of functional Magnetic Resonance Imaging (MRI) images of the brain. The goal of the research is to provide fast parallel processing over high speed networks so that functional MRI can be used in real time in a clinical setting. MRI installations at hospitals can determine not only brain anatomy, but also brain function by using functional MRI. For both the research, and the eventual clinical application, it is important to provide rapid feedback to the clinical site. The ideal would be to reconstruct the image and deliver it to the clinical site before the patient leaves the MRI magnet. Any problems with a particular exposure could then be rapidly corrected.
We are also collaborating with doctors at the Hospital of the University of Pennsylvania (HUP)
to design and test
new prototype electronic circuits for the positron
emission tomography (PET) at HUP. The improved
electronics will increase the rate capability and improve image quality of the PET scanners.
Last updated October 30, 2003
Charles Kane, kane@physics.upenn.edu