Echolocation


Ben Underwood (1992-2009)

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Salsichório global

[Certa ciência dedica-se a encher salsichas… mas com patente! Ah, as altas patentes: General, Major… e João Salsichão! “Receita pobre para produzir um monstro”. É preciso é encher a tripa de massa – e que alguém a pague! O Marketing encarregar-se-á de rotular de daninho e rudimentar o que é gratuito e espontâneo: a abundância. Ao fim e ao cabo, todas as espécies reproduzidas em cativeiro “assemelham-se cada vez mais a porcos” (inclusive os próprios porcos), e não só os peixes, as uvas, os caracóis a ração… Os médicos dizem que o intestino é “o segundo cérebro”. Então, de igual modo, o cérebro será o segundo intestino? Mente intestinal. Ditado grego: “O peixe, é pela cabeça que começa a feder” (a ranço). Talvez, por isso, o primeiro órgão a extrair durante a mumificação egípcia era a mioleira. Saía toda pelo nariz: “o possível, senão sufoco!”. Era uma piada filosófica (humor negro…).]

Tough head, fragile tail

«Prince Rupert’s drops are relatively simple to make; they’re little more than molten glass dropped into cold water, creating a solid blob with a long, thin tail.

Smacking the fat end with a hammer, pressing it with up to 20 tons of force, or even shooting it with a gun won’t do it a lot of damage.

To break it, however, you only need to tap its tail, which will cause the entire object to disintegrate into a shower of tiny shards.

There aren’t any records on the drops’ origins, but sometime around 1660 Prince Rupert of the Rhine reportedly gave a number of ‘glass bubbles’ to King Charles II of England as gifts, who passed them on to the Royal Society of London to conduct a few studies of their own.

The drops’ remarkable properties were put down to the rapid cooling of the outer surface of the glass, forming a hard shell that allowed the insides to cool and then contract a little slower.

It was this difference in layers – the ‘squeezing’ (or compressive forces) of the outer layer and the ‘pulling’ (or tensile forces) of the core – that was thought to explain both its toughness and fragile tail.»

Source

The Fluctuation Theorem

«In 1993 we discovered a relation, subsequently known as the Fluctuation Theorem (FT), which gives an analytical expression for the probability of observing Second Law violating dynamical fluctuations in thermostatted dissipative non-equilibrium systems. (…)

We take the assumption of causality to be axiomatic. It is causality which ultimately is responsible for breaking time reversal symmetry and which leads to the possibility of irreversible macroscopic behaviour. The Fluctuation Theorem does much more than merely prove that in large systems observed for long periods of time, the Second Law is overwhelmingly likely to be valid. The Fluctuation Theorem quantifies the probability of observing Second Law violations in small systems observed for a short time. (…)

The view that irreversibility is a result of our special place in space-time is still widely held [4]. In the present Review we will argue for an alternative, less anthropomorphic, point of view. (…)

One of the proofs of the Fluctuation Theorem given here, explicitly considers bundles of conjugate trajectory and antitrajectory pairs. (…)

In violation of the Second Law, heat energy from the surroundings can be converted into useful work to provide sufficient energy for the machine to run in reverse. This is an inescapable property of Nature which places a fundamental constraint on the operation of nanomachines. As you scale down machines they inescapably run in a mode: `two steps forward and one step back’. The ratio of forward to backward steps is given by the FT. This must also be the way that living sub-cellular organelles (machines) operate. (…)

It is logically possible that the Axiom of Causality could be replaced by its time conjugate: the Axiom of Anti-Causality. (…)

In an anticausal Universe, knowledge of its present state would enable us to predict the past but not the future.»

Denis J. Evans and Debra J. Searles, “The Fluctuation Theorem“, in Advances in Physics, 2002, Vol. 51, No. 7, 1529-1585.

 

«In other words, for a finite non-equilibrium system in a finite time, the Fluctuaction Theorem gives a precise mathematical expression for the probability that entropy will flow in a direction opposite to that dictated by the second law of thermodynamics.»