Thread by @vishuguttal

Our new paper in @NaturePhysics led by the recently minted PhD from our lab - Jitesh Jhawar (@jitesh2412) & with Richard, Danny, Amith, Hari & Tim!

This is the FIRST experimental work of our 'theory-lab'. Obviously, this is exciting.

Thread. @iiscbangalore

@NaturePhysics @jitesh2412 @iiscbangalore Some background: We have seen collective motion in birds, mammals, fish, insects, microbes, etc - all fascinating patterns.

Each individual has only limited local information about surroundings. Yet they show these fascinating patterns. 2/n

@NaturePhysics @jitesh2412 @iiscbangalore So a question that many of us are interested is: what types of interactions produce these fascinating patterns?

This has been a question of substantial work over last few decades, and we provide some new insights here. 3/n

@NaturePhysics @jitesh2412 @iiscbangalore To answer this broad question, physicists and computer scientists have built mathematical and computational models since 1980’s.

They show that organisms don’t need complex rules to exhibit collective motion. 4/n

@NaturePhysics @jitesh2412 @iiscbangalore For example, in the classic Vicsek model (PRL 1995), particles follow a simple rule: move in the average direction of their neighbours. This simplistic rule produces a highly-ordered collective motion.

[Pic below is Fig 1 of Vicsek et al 1995 PRL]

@NaturePhysics @jitesh2412 @iiscbangalore Here is the link to read the paper (FREE):

rdcu.be/b2pgG

@NaturePhysics @jitesh2412 @iiscbangalore The first main result from our study is related to the above context -- whats the rule that fish follow?

We show that in a species called karimeeen (Etroplus suratensis)
(i) fish just copy the direction of a (nearby) random fish, or
(ii) they turn a bit randomly. 6/n

@NaturePhysics @jitesh2412 @iiscbangalore We call this rule a ‘pairwise copying’ -- which is relatively simpler than the Vicsek-class of models which assume that organisms average the direction of neighbours and turn towards them. 7/n

@NaturePhysics @jitesh2412 @iiscbangalore Ours is not the first paper to show that real organisms behave differently compared to Vicsek-like models.

In fact, fish school studies by @JHerbertread, @GTheraulaz etc also show a similiar simpl rule of interactions in other fish species. 8/n

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz That brings me to the second main result- also the title of the paper:

We show that schooling in this fish is a rare empirical example of a phenomenon well studied in non-equilibrium stat physics: ‘noise-induced phase transitions’.

But what is this? Let's dig in a bit 9/n

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz So this is what we do in terms of experiments and analyses to make the connection with the physics theory.

We use karimeen (Etroplus suratensis) -- a popular edible fish in western coast of southern India and put them in a fish tank.

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz We maintain shallow water, so that fish are effectively in a two-dimensional system. This makes tracking of fish movement relatively easy.

[This is from fig 1 of our paper.]

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz Here is a short video with 30 fish in the tank. Our experiments also has 15 and 60 fish.

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz From video tracks of how fish, we calculate "polarisation M" - which measures how well aligned are fish with each other.

We then plot this quantity as a function of time. Crucially, we retain all information -- not just mean but also how fluctuations are occurring over time.

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz From such a time series of the polarisation, we construct a stochastic differential equation!!!

I think this is one of the coolest part of the paper - because unlike most papers that intuitively derive a model or equation, here we let the data talk!

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz In simple words, this equation tells that

i. when fish are ordered, even random things that they do, like copy one other, doesn't change the overall behaviour very much

ii. when the fish are moving in a misaligned state, the fluctuations are actually high.

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz Therefore, when the fish are not well aligned with group members, the noise grows larger, eventually ‘kicking’ the group from one state -- random swimming -- to a different state -- schooling!!!

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz I will stop here!! There is quite a bit of technical stuff in Methods and Supplementary Materials.

If you are interested, you can send me an email to get the pdf OR check this link: rdcu.be/b2pgG

All codes/data are available via: doi.org/10.5281/zenodo…

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz If you read the entire thread and came this far, do like this specific tweet so that I know how many people read these threads :-)

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz An insightful comment:

https://twitter.com/GTheraulaz/status/1234537698419003392

@NaturePhysics @jitesh2412 @iiscbangalore Generous support by @DBTIndia @serbonline @IndiaDST.

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz This video (based on simulation of the model) makes the point on how noise is high in disordered state, and pushes the group towards higher alignment.

@NaturePhysics @jitesh2412 @iiscbangalore @JHerbertread @GTheraulaz

https://twitter.com/vishuguttal/status/1234548849173094401