Rotaxane machine - a nanomachine inspired by Maxwell's demon

Continuing my earlier post about Maxwell's Demon, recently Professor David Leigh of the University of Edinburgh's School of Chemistry and and colleagues have built a model that mimics Maxwell's demon - an atom-sized motor that could lead to man-made nanomachines.

The molecular model of the demon is a rotaxane (mechanically-interlocked molecular architecture consisting of a dumbbell-shaped molecule that is threaded through a macrocycle or ring-like molecule.). In this rotaxane machine (nanomachine), information about the location of the molecular ring is used to move the system from equilibrium. But gathering and then implementing that information costs energy... in other words the second law of thermodynamics remains valid.

Maxwell's Demon Becomes Reality :

“Our machine has a device - or ‘demon’ if you like - inside it that traps molecule-sized particles as they move in a certain direction.

“Maxwell reasoned that if such a system could ever be made it would need energy to work. Without energy, it might appear that the perpetual motion of the molecules could power other devices in the same way as a windmill, but Maxwell reasoned that this would go against the second law of thermodynamics.

“As he predicted, the machine does need energy and in our experiment it is powered by light. While light has previously been used to energise tiny particles directly, this is the first time that a system has been devised to trap molecules as they move in a certain direction under their natural motion. Once the molecules are trapped they cannot escape.”

Dreams of theory of everything

The ultimate aim of many scientists and theorists is to find a complete description of nature, maybe some simple mathematical equation that you could write on your hat!

This theory of everything would refer to gravity in the same mathematical form as the other forces - that of quantum theory. This would tie up all the basic forces of the universe into one coherent system. So far, though, there is no satisfactory theory of quantum gravity, but "superstring theory" - in which particles are regarded not as point-like objects but as modes of vibration on loops of "string" - seems to bring gravity into the quantum regime.

Steven Hawking predicts that such a theory will be discovered in the next 20 years. A new theory asserts that biology, not physics, will be the key to unlocking the deepest mysteries of the universe, such as quantum mechanics.

"The answer to the universe is biology -- it's as simple as that," says Dr. Robert Lanza, vice president of research and scientific development at Advanced Cell Technology. He details his theory in The American Scholar's spring issue, published on Thursday. Lanza says scientists will establish a unified theory only if they radically rethink their understanding of space and time using a "biocentric" approach. His article is essentially a biological and philosophical response to Hawking's A Brief History of Time, in which he questions how we interpret the big bang, the existence of space and time, as well as many other theories -- assertions that might ruffle the feathers of some physical scientists.

Read more at Will Biology Solve the Universe?

Non-Newtonian fluid

Non-Newtonian fluid is a fluid whose viscosity changes when the gradient in flow speed changes. Colloidal suspensions and polymer solutions like ketchup and starch/water paste are non-Newtonian fluids. A non-Newtonian fluid is a fluid in which the viscosity changes with the applied strain rate. As a result, non-Newtonian fluids may not have a well-defined viscosity.

A pool filled with non-newtonian fluid

Maxwell's Demon - violating the entropy

Maxwell's demon is an imaginary creature created by James Clerk Maxwell in 1867. It is a thought experiment meant to raise questions about the possibility of violating the second law of thermodynamics.

Suppose there is a room filled with a gas at some temperature. This means that the average speed of the molecules is a certain amount depending on the temperature. Some of the molecules will be going faster than average and some will be going slower than average. Suppose that a partition is placed across the middle of the room separating the two sides into left and right. Both sides of the room are now filled with the gas at the same temperature.

Maxwell imagined a molecule sized trap door in the partition with his minuscule creature (the demon) poised at the door who is observing the molecules (look at the picture above). When a faster than average molecule approaches the door he makes certain that it ends up on the left side (by opening the tiny door if it's coming from the right) and when a slower than average molecule approaches the door he makes sure that it ends up on the right side.

So after these operations he ends up with a room in which all the faster than average gas molecules are in the left side and all the slower than average ones are in the right side. So the room is hot on the left and cold on the right. Then one can use this separation of temperature to run a heat engine by allowing the heat to flow from the hot side to the cold side.

Thermodynamics says this is impossible, you can only increase entropy (or rather, you can decrease it at one place as long as that's balanced by at least as big an increase somewhere else).

So why wouldn't a setup like Maxwell's demon work? Well, any real "demon" that does this would not be a disembodied spirit receiving its information telepathically. To acquire information about the world you must be in physical interaction with it, and on the atomic and molecular scale you cannot ignore the quantum mechanical nature of the world. For instance, to be able to see the molecules the "demon" would have to absorb whole photons at a time, and any detailed version of the thought experiment will run into the uncertainty principle and the fact that an interacting "demon" will acquire the same temperature as the rest of the system.

The link between thermodynamics and quantum physics is even stronger: macroscopic entropy can only be computed correctly from the cumulative contributions from microscopic states if these are described quantum mechanically.

Information theory is about the concept of entropy abstracted away from physical systems, and applied to any context that deals with knowledge about the state of a system (signal processing, communication, data compression, encryption &c.). In fact, Shannon originally referred to information as "entropy" (with a minus sign). In recent times, there have been interesting developments in applying the ideas of information theory back to physics, notably in dynamical systems theory, aka chaos.

Real-life versions of Maxwellian demons (with their entropy lowering effects of course duly balanced by increase of entropy elsewhere) actually occur in living systems, such as the ion pumps that make our nervous systems work, including our minds. Molecular-sized mechanisms are no longer found only in biology however, it's also the subject of the exciting new field of nanotechnology.

So Maxwell's demon is an icon for our times, sitting at the crossroads of information and the physical universe.