Physics Tip Sheet #66, November 14, 2006
Highlights in this issue: attacking the Anthropic Principle, and a DNA-synthesizing shift register. The Anthropic Principle Under FireGlenn D. Starkman and Roberto Trotta Understanding why the fundamental constants (which reflect the strength of gravity, the speed of light, and other physical laws) have the values that we measure is one of the most challenging and important problems in physics. One popular way to explain the constants involves the anthropic principle. It's based on the argument that life is only possible under certain values of the constants, and that we wouldn't be here to measure them if they were even slightly different. Although it may seem like little more than circular reasoning at first glance, Steven Weinberg (University of Texas) managed to use the anthropic principle to calculate the cosmological constant with surprising accuracy back in the late 1980's, well before observations of the accelerating expansion of the universe gave us a measurement of the constant. Astrophysicists Glenn Starkman (Oxford) and Roberto Trotta (Case Western), however, take issue with anthropic reasoning in calculations of the cosmological constant. They claim that the parameters that go into the calculations, such as the number of sentient beings who try to measure the constant, are so poorly defined that anthropic arguments can lead to all sorts of values. They expect that similar problems hamper anthropic rationales for the other fundamental constants as well. The paper by Starkman and Trotta is just one in a recent series of assaults on anthropic principles in physics. In a paper published in the August 2006 issue of Physical Review D, Roni Harnik et al. described a universe with no weak force at all, which they believe could support life nevertheless (link.aps.org/abstract/PRD/v74/e035006). If true, it undermines anthropic arguments for the values of several fundamental constants. Earlier this year, Harvard's Abraham Loeb published a paper in the Journal of Cosmology and Astroparticle Physics (available on the preprint archives at lanl.arxiv.org/PS_cache/astro-ph/pdf/0604/0604242.pdf) showing that finding planets in dwarf galaxies would prove that habitable conditions could arise even if the cosmological constant were a thousand times larger than the one we measure, potentially eliminating anthropic arguments for the constant's value entirely. DNA Synthesizing Shift RegisterIlya Baskin et al. Physicists at Technion-Israel Institute of Technology have developed a scheme to construct long, custom strings of DNA by building them up piece by piece. The approach, which they call a molecular shift register, adds a single amino acid building block at a time to a DNA string. In order to attach the right amino acid at the right place, the group produced short lengths of amino acids that they call rules. A rule chemically recognizes a certain pattern at the end of a DNA chain that they are trying to extend. If the proper sequence exists, then the rule adds another amino acid to the chain. The researchers mixed several rules in a solution containing free amino acids and starting seeds of the DNA that they hoped to build. They then grew custom DNA molecules by cyclically warming and cooling the mixture. A single amino acid is added to the chain with each cycle. Other methods for building artificial DNA strands usually involve starting with complex strands of DNA that are several amino acids long, and subsequently stitching them together. Although this may initially seem like a quicker way to make DNA, it rapidly becomes unwieldy as the complexity and length of the strand increases. The researchers demonstrated the shift register approach by constructing moderate lengths of DNA, and show mathematically that the method should be much faster and more flexible than conventional DNA building techniques for long sequences. About APSThe American Physical Society is the leading professional organization of physicists, representing over 46,000 physicists in academia and industry in the United States and internationally. APS has offices in College Park, MD (Headquarters), Ridge, NY, and Washington, DC.
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