## Archive for the ‘Physics’ category

### Stochastic resonance

9 January 2010

The topic of stochastic resonance came up today and I was lent a copy of Noise by Kosko.  SR occurs only with a non-linear system and a detector with a threshold (neurons are one example.)   The psychophysics paper Visual perception of stochastic resonance has an image (right) that shows this property very nicely.  The paper itself focuses on showing that the human brain’s visual system is nearly or as efficient at using SR as a computer to process information.

### Linac Coherent Light Source

5 January 2010

The Linac Coherent Light Source (LCLS) came on line in September of 2009, capable of producing x-ray laser pulses of wavelengths from 1 to 0.15 nm.  The source will be a laser pulsed photo-cathode in development (led by Howard Padmore) over the next 2 years.  The x-ray free electron laser (XFEL) pulses are $10^{-13}$ sec of $10^{12}$ photons.  This photon pulse is so energetic it destroys the sample viewed, but not before a snapshot is collected.  One technique for imaging proteins is to prepare an aerosol spray and inject a collimated stream into the electron beam.  A large set of diffraction images (up to 10 million) of the molecule in random orientations are generated.  The data is categorized, averaged, and fit together iteratively to form the best 3D diffraction representation.  The 3D electron density map can then be generated from the assembled diffraction volume.

### Snowflake formation

13 December 2009

Stellar dendrite snow crystal

The forecasts predicting snow for this weekend were wrong, but it reminded me of a question I had – why would snowflakes form symmetrically?   How does one individual branch “know” how the other 5 are growing?   It turns out that the local conditions (temperature, altitude, etc) govern the crystal growth, and that snowflakes are different because of their trajectories through the atmosphere. Any one snowflake sees the same conditions and will grow in a symmetric fashion (though the majority of snowflakes are actually irregular)

Kenneth Libbrecht is a physicist studying snowflake formation, and has a great website discussing this, with great micrographs of snowflakes.   He also has a paper The physics of snow crystals.

### CUDA articles

30 November 2009

Read several interesting articles on the parallel programming architecture CUDA, some of which emerged from the SC 2009 conference, and some from the nVidia website.

### Oblate spherical coordinates

29 October 2009

Elliptical coordinates are defined in 2D and can be rotated around the x or y axis to yield 3D oblate spherical coordinates or prolate spherical coordinates.

### Susskind classical mechanics

27 October 2009

Found a lecture from Susskind covering Poisson brackets.  (Note: this is lecture 9, and I believe lecture 1 actually follows chronologically, since the boardwork is leftover from lecture 9).

### QED homework 3

15 October 2009

Here is the third homework for the QED topics class.  M. sketched out the second quantization.

QedHomework_2009_10_15