Advances in fundamental physics through a partnership between theory and experiment

1. Advances in fundamental physics through a partnership between theory and experiment NASA ISS Workshop October 14, 2010 Peter B. Weichman BAE Systems AIT Burlington, MA BAE Systems Proprietary

2. Introduction • Given the very long road from formulation to flight, the true measure of the success of a NASA microgravity flight project has always been the surprising ground-based discoveries made along the way • As is standard in physics, experimental and theoretical discoveries go hand-in-hand, amplifying each other • New ground- and flight-based proposals emerge from old • Often, the new discoveries are actually required as part of the flight definition process • New ideas may be required to replace old untenable ones • Here, in pursuit of a brighter scientific future for all of us, I want to reemphasize the importance of the theoretical half of the partnership • Examples from past microgravity projects and proposals

3. Lambda Point Experiment (LPE) NASA Lambda Point Experiment (LPE, 1992): J. A. Lipa, D. R. Swanson, J. A. Nissen, T. C. P. Chui, U. E. Israelsson “Heat Capacity and Thermal Relaxation of Bulk Helium very near the Lambda Point,” PRL 76, 944 (1996). M. J. Buckingham and W. M. Fairbank, “The Nature of the Lambda Transition,” Progress in Low Temperature Physics III, 80 (1961).

4. LPE, CHEX, SUE, MISTE, … • Uncountable person-years of work (and several Nobel Prizes—van der Waals, Landau, Wilson,…) have gone into calculating critical exponents • Equally uncountable person-years of work have gone into ultra-high resolution measurements on 4He • Temperature resolution at the 10-12 K level around Tl is now the state of the art • Theory and experiment have achieved similar levels of accuracy for specific heat exponent a, and agree within error bars • A series of other flight experiments, underpinned by similar levels of theoretical effort, were proposed, some actually flown • Superfluid 4He finite size effects (CHEX) • Superfluid 4He critical exponent universality along l-line (SUE) • Liquid-vapor critical point in 3He (MISTE) • It is fair to say that, at these levels of precision, without agreement between theory and experiment, neither would be accepted!

5. Critical Dynamics Experiment (DYNAMX) • In a normal fluid, heat is transported by diffusion, leading to Fourier’s heat conduction law: • Leads to a nonequilibirum superfluid-normal interface if T is positioned to lie in the middle of the cell. • Interference at very low Q due to gravity-induced equilibrium superfluid-normal interface • DYNAMX proposed to study this interface under microgravity conditions, measuring its temperature profile as it moves through the cell, with its position controlled by precision adjustments to Q.

6. Dynamics of the Interface • Placing bounds on the interface wandering was one of many flight definition requirements • As it turns out, actual existence of a sharp, stationary interface is not obvious! • Boundaries between equilibrium phases are often rough, leading to log(L) smearing in 0g • Much like waves on the surface of a pond, the nonequilibrium SF interface turns out to have an acoustic mode that travels along its surface • A very intricate calculation (involving 5 different length scales and 4 different time scales), however, shows that the steady state mode excitation is weak enough to maintain a flat interface (good news for DYNAMX!) • Vibration (g-gitter) also drives the interface motion, and the same theory provided acceptable bounds on resulting interface wandering P. B. Weichman, A. Prasad, R. Mukhopadhyay and J. Miller, “Trapped second sound waves on a nonequilibrium superfluid-normal interface, Phys. Rev. Lett. 80, 4923 (1998).

7. A Proposed Resonant Cavity Experiment • How to confirm this interface second sound mode experimentally? • Inelastic reflection of an incident heat pulse off the interface: Some of the pulse energy is absorbed by the interface in the form of a surface-propagating pulse ripple • Reflection coefficient magnitude and phase computed as function of frequency, angle of incidence Q P. B. Weichman, “Second sound spectroscopy of a nonequilibrium superfluid-normal interface” PRL 96, 135301 (2006).

8. Other Fascinating Phenomena • Self-organized critical (SOC) State: “Heat from above” g-Q cooperation leads to fixedT(Q) = T(z) - T(z) • homogenous, vortex ring-mediated, SF transition as Q is varied • Superflow instabilities, and transition from uniform superfluid to interface state: • SF cooling by heating effect • Interesting singularities predicted in specific heat at constant heat current • predicted by V. Dohm et al.; CQ experiment proposed by D. Goodstein et al. • All in need of detailed experimental confirmation P. B. Weichman and J. Miller, “Theory of the self-organized critical state in superfluid 4He”, J. Low Temp. Phys. 119, 155 (2000).

9. Finally, something completely different: Microgravity Droplet Dynamics and Cloud Physics • Many deep (and practical) mysteries regarding rain droplet formation in clouds • Theory: Collision dynamics, over many orders of magnitude in droplet size, in turbulent air • Long time scales  need for mg lab experiments • mm-droplet sizes  nano-machine measuring devices • Similar questions arise in more “mundane” issues of “working” fluid manipulation in space H. Tang, R. V. Duncan, P. B. Weichman, NASA ISS fluid physics platform proposal (2008)

10. ISS 2010+ Towards a Brighter Future? gF 2005 AD