Events - Daily
April 08, 2013 >>
|Monday, April 08|
Naama Brenner (Technion) "Universal protein distributions in cell populations"
The protein content of a cell is a primary determinant of its phenotype. Yet, large fluctuations in protein numbers are observed among genetically identical cells grown under identical conditions. Recently, cellular protein variability has been a hot topic of research, with microorganisms providing a popular model system. Despite much work and much progress in the field, a general integrative picture is still lacking.
This seminar will report recent experimental results obtained in collaboration with the groups of H. Salma, E. Braun and A. Libchaber, and their theoretical implications. We have shown that protein distributions measured under a broad range of biological realizations collapse to a single non-Gaussian curve under scaling by the first two moments. Moreover in all experiments the variance is found to depend quadratically on the mean, showing that a single degree of freedom determines the entire distribution.
Our results imply that protein fluctuations do not reflect any specific molecular or cellular mechanism, and suggest that some buffering process masks these details and induces universality. Non-Gaussian universal distributions with qualitatively similar features to those we measured were observed in various complex physical systems at or near criticality. This analogy calls for an understanding of cell populations in a broader context with an emphasis on many-body interactions and long range correlations.
Joseph Henry Room · 12:00 p.m.– 1:00 p.m.
Condensed Matter Seminar - Philip Kim, Columbia University - Bloch, Landau, and Dirac: Hofstadter’s Butterfly in Graphene
Electrons moving in a periodic electric potential form Bloch energy bands where the mass of electrons are effectively changed. In a strong magnetic field, the cyclotron orbits of free electrons are quantized and Landau levels forms with a massive degeneracy within. In 1976, Hofstadter showed that for 2-dimensional electronic system, the intriguing interplay between these two quantization effects can lead into a self-similar fractal set of energy spectrum known as “Hofstadter’s Butterfly.” Experimental efforts to demonstrate this fascinating electron energy spectrum have continued ever since. Recent advent of graphene, where its Bloch electrons can be described by Dirac feremions, provides a new opportunity to investigate this half century old problem experimentally. In this presentation, I will discuss the experimental realization Hofstadter’s Butterfly via substrate engineered graphene under extremely high magnetic fields controlling two competing length scales governing Dirac-Bloch states and Landau orbits, respectively.
Jadwin A06 · 1:15 p.m.– 2:30 p.m.
High Energy Theory Seminar - IAS - Philip Schuster, Perimeter Institute - “(Why) Is Helicity Lorentz-Invariant?”
Massless particle states carry integer or half-integer spin about themomentum axis, or "helicity". Lorentz symmetry allows for helicity states to mix under boosts (like massive particle polarizations); such mixing is not understood theoretically and is not obviously well constrained by experiment. This possibility is historically known as "continuous spin"and this talk provides an informal introduction to the subject. We introduce a gauge field theory description of CSPs, and review evidence that CSPs can interact with matter via scattering amplitudes that approach familiar scalar, electromagnetic, or gravitational ones in a high-energy (and/or non-relativistic) correspondence limit. Such interactions appear thermodynamically safe. We discuss potential signatures of matter coupled to a photon-CSP and related consequences for stellar cooling. We close by identifying some directions for discovering a full interacting theory of CSPs or proving that such theories can't exist.
Bloomberg Lecture Hall - Institute for Advanced Study · 2:30 p.m.– 3:30 p.m.