Search Talks

Optical frequency standards based on forbidden transitions of trapped and laser-cooled ions have now achieved significantly higher stability and greater accuracy than primary cesium clocks. At PTB we investigate an optical clock based on the electric quadrupole transition S1/2 – D3/2 at 688 THz in the 171Yb+ ion and have shown that the frequencies realized in two independent ion traps agree to within a few parts in 1016. Results from a sequence of precise measurements of the transition frequency are now available that cover a period of seven years. Combined with data obtained at NIST on the quadrupole transition in Hg+, this allows to derive a model-independent limit for a temporal drift of the fine structure constant. We prepare to observe two more optical transitions that will provide increased sensitivity to alpha variations: The electric-octupole transition S1/2 - F7/2of Yb+ at 642 THz offers a sub-hertz frequency resolution. The ratio of the 688 THz and 642 THz frequencies in Yb+ can be measured as a dimensionless number with a femtosecond laser frequency comb. Repeated measurements of this quantity permit to search for temporal variations of alpha with a sensitivity factor ≈7, the highest in any of the available combinations of optical frequency standards. Much higher sensitivity (of order 104) may be obtained in the study of the 7.6 eV nuclear transition between the two lowest states of Th-229. We have developed a concept for a highly accurate nuclear clock based on this transition and describe first steps towards the experimental realization. This work is supported by DFG, FQXi and QUEST.

At present a number of current or proposed experiments are directed towards a search for a `new physics\' by detecting variations of fundamental physical constants or violations of certain basic symmetries. Various problems related to the phenomenology of such experiments will be considered.

I will present a brief introduction to Big Bang Nucleosynthesis theory and observation. I will then discuss BBN as a probe of hadronic mass variation in the very early universe, including comparison with the observed Li7 discrepancy. Finally I want to explore the possibility of overproducing Li6 by three orders of magnitude in order to match reported observations.

Arguably, the most important issue in physics today is trying to unify the twin pillars of modern physics, quantum theory and general relativity, into a single theory known as quantum gravity. This introductory presentation will discuss various aspects of quantum gravity.

I will review the published quasar absorption line constraints on variations in the fine-structure constant, alpha, focusing on the apparent disagreement between those derived from Keck/HIRES and VLT/UVES spectra which have provided evidence for and against alpha variation, respectively. I demonstrate simple yet fundamental flaws in the UVES constraints which preclude reliable comparison with those from HIRES. I will outline our program to obtain a definitive UVES measurement. I will also present several new absorption line constraints on variations in the proton-to-electron mass ratio, mu. For the two molecular hydrogen absorbers from which previous authors found tentative evidence for mu-variation, we find robustly null results. A further two molecular hydrogen absorbers, including an entirely new system, also yield tight, null constraints. Finally, I present new, detailed comparison of a radio absorption system containing ammonia inversion and molecular rotational transitions which yields the strongest current astrophysical constraint on mu-variation, dmu/mu=[mu(z)-mu(lab)]/mu(lab)=[+0.74+/-0.47(stat)+/-0.76(sys)]x10^-6, at redshift z=0.685.