# Steven Lade

### From Santa Fe Institute Events Wiki

To the extent that labels work any more in these sorts of fields, I'm a theoretical physicist. I'm undertaking a PhD at the Nonlinear Physics Centre at The Australian National University under the supervision of Prof Yuri Kivshar. Most of the group works on nonlinear optics -- solitons, optical lattices, nonlinear vortices, etc. really nice nonlinear dynamics in other words -- or phenomena with similar equations of motion like Bose-Einstein condensates.

I, meanwhile, have been looking at a class of models called ratchets. In short these are systems periodic in space and time which achieve a nonzero average velocity through breaking of spatial and/or temporal symmetry. For example, a particle in a sawtooth potential (which is flat on average) under the influence of an additional oscillating force (zero on average) which through the combination of these forces moves on average in one direction. Such a ratchet could be implemented in lots of different areas, but historically the one people were focusing on were (processive) molecular motors like myosin-V and kinesin which also move without any net bias telling them which way to go.

This got me onto molecular motors. I'm not really looking at them in a ratchet sense any more. What I have is access to some single-molecule time series for their motion (nm in space and us in time!), trying to convert this data into an effective equation motion in order to uncover details about its dynamics and mechanical structure, and comparing against simulations another collaborator is doing.

Is this complex systems? Probably not. I'm looking at everything in a rather reductionist manner. In fact, despite having attended some other complex systems events, I still don't really know what complex systems are. That's one reason I applied to come to the summer school! The other reason is that, as you can hopefully tell from the above, I'm interested in interdisciplinary work. These sorts of events, and Santa Fe in particular, seem to be where people of a like mind tend to hang out, so let's meet up and do some collaborating! -- even if it's not necessarily on complex systems.

Outside of work, among other things I'm keen on outdoor activities, mainly bushwalking (which you'd call hiking or backpacking, I think). I'd like to do some while at Santa Fe, it sounds like a great place for it. I also try to do some campaigning on climate change and sustainability, and even recently managed to make this overlap with my scientific interests, giving part of a public lecture on nonlinearities and feedbacks in climate.

See you in June!

Website: http://wwwrsphysse.anu.edu.au/nonlinear/people/StevenLade.shtml

## My answers to Dan's questions

**1. What are your main interests? Feel free to include a "pie in the sky" big idea!**
Applications, usually interdiscplinary, of nonlinear dynamics, for example in biology (molecular biology, ecology) and climate. The idea, even if unlikely, of broad mathematical laws in biology.

**2. What sorts of expertise can you bring to the group?**
I studied as a physicist and electrical engineer. I can tell you about

- anything you like from first year physics and maths, with more gaps at later year levels...
- some topics in nonlinear dynamics
- basic concepts about feedbacks and control theory
- from a physicist's viewpoint about Fokker-Planck/Langevin/stochastic differential equations
- molecular motors (again as a physicist)
- a little about the Hodgkin-Huxley equations in neuroscience (from a previous summer school).

Other topics, not so related, include: Fourier and Laplace transforms, (electrical) circuit analysis, complex analysis.

I have some programming ability, particularly MATLAB, and a little C and Mathematica.

I also can contribute hiking experience!

**3. What do you hope to get out of the CSSS?**
Knowledge of cool stuff about complex systems! Some interesting topics on the 'Research' part of the Santa Fe website that I'd like to learn about include: self-organised criticality, q-entropy, universal scaling laws in biology. I've also heard a little about Maximum Entropy and Maximum Entropy Production, which I'd like to know more about.

I am looking forward to meeting the other students and forming collaborations! I would love to get out of this through conversations with the students and faculty some topics for future research, either for myself or collaboratively.

**4. Do you have any possible projects in mind for the CSSS? (Recall that you will all be working in groups on at least one project with the goal of presenting your progress on the last day and finishing up a paper by sumer's end.)**
Anything in physics, biology (from molecular biology to ecology) or climatology sounds good.

One currently poorly-formed project idea: coarse models of feedbacks in climate. Better understanding of feedbacks is right at the current edge of climatology, and as a result weren't comprehensively included in the last IPCC (Intergovernmental Panel on Climate Change) report. For an interesting recent summary of "policy-relevant tipping points" is by Lenton et al. and another version of their key diagram by Lindesay. A number of these may feed back on each other. The idea would be to combine all these into a network of feedbacks (one node for each tipping point mechanism) and calculate a likely tipping point for the entire system. For other papers on tipping points see Schellnhuber and Smith et al..

Have no idea how practicable this is.

A second idea: I believe our physical intuition is linear. Push something twice as hard, and we should get twice the output, right? Wrong, if the system is nonlinear. Explore examples where this 'linear thinking' has misled us, e.g. fisheries (the collapse thereof) and other 'limits to growth', potentially tipping points in climate change. Maybe even some kind of psychological study?