Abstract
What happens if you take a vortex loop - akin to a smoke ring in air - and tie it into a knot or a link? The possibility of such knottiness in a fluid has fascinated physicists and mathematicians ever since Kelvin’s ’vortex atom’ hypothesis, in which the atoms of the periodic table were hypothesized to correspond to closed vortex loops of different knot types. 

More recently, knottiness (hydrodynamic or magnetic helicity) of  fluids and plasmas has re-emerged as a  conserved quantity, offering the potential for  fundamental insights.  Progress in understanding its implications has however been hindered by lack of accessible experimental systems and explicit models. I will tell of how to make a vortex knot and link in water (in experiment), in the wave function of a superfluid (on a computer) and in light fields (in theory) and of what happens thence. In particular, I will talk about how linking coiling and twisting interplay across scales. 

Please Note:
The Colloquium Ehrenfestii takes place on Wednesday evenings starting at 19:30 hours in the main auditorium of the Oort building. Before the Colloquium, there is a common dinner in the canteen located on the ground floor of the Oort building. This dinner starts at 18:00 hours sharp and is FREE of charge, under the condition that one attends the colloquium and that one has made a reservation before noon on the Tuesday preceding the colloquium. 

The registration form is only accessible from within the University (to block spammers); if you would like to register for the dinner from outside the University, please send an email to fran@lorentz.leidenuniv.nl   The revolving doors to the Oort building  are accessible until 9 pm after that they are locked and one should instead use the sliding doors directly adjacent to the Huygens building: these can be opened with an electronic key (seek help from locals).