Going against the flow
Heat can flow without temperature differences - but thermodynamics remains intact.25 June 2002
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Physicists have devised a way to dodge one of the most fundamental laws of nature: the fact that heat flows only from hot to cold. At face value, it implies they have invented a never-ending source of energy. Well they haven't - but it could provide a method for carrying out unheard-of chemical reactions.
By imagining two chambers containing particles, Souvik Das of St Stephen's College in Delhi, India, and colleagues say that they can make heat flow between two identical systems1. But the laws of thermodynamics say that this cannot happen, as heat must always flow from hot to cold, just as water must run downhill.
To make their claim, Das and colleagues invoke 'non-thermal', reservoirs, which do not have a meaningful temperature that can be measured with a thermometer.
This isn't quite as bizarre as it sounds. Lasers are very similar. An object's temperature is related to the way in which its energy is distributed among its constituent particles. A warm gas is made up of atoms or molecules with a normal distribution of energies - a few are cold, the majority is warm, and a few are very hot.
Just like the gas, the non-thermal reservoirs considered by Das' team also consist of particles with a range of different energies. But the distribution of energies is no longer normal. The fictional reservoirs contain an inordinately large number of high-energy particles and - as is the case with lasers - are not in equilibrium.
Das and colleagues find that as long as the two reservoirs are not in equilibrium, heat can flow between them - but a return to equilibrium would stop the flow of energy between the reservoirs. Because it costs energy to keep them in the non-equilibrium state, no energy is obtained 'for free' and the laws of thermodynamics stay intact.
Cranking out the heat
The heat flows down a theoretical 'conductor' made a chain of particles rather like ball-bearings. As in Newton's famous cradle, energy is transmitted from one particle to the next by collisions between them. Das' team shows that if the masses of these particles decrease from left to right, energy flows along the chain preferentially in this direction.
A heavy particle is relatively efficient at transmitting energy down the chain to a lighter particle, whereas the energy picked up from a reservoir by a light particle is mostly reflected back to where it came from. So the heat transport is asymmetrical, like the motion of a ratchet.
This 'heat ratchet' will not provide us with inexhaustible energy. But the researchers claim that it may be possible to build one anyway. If two chambers containing heat-generating chemical reactions (which are out of equilibrium while the reaction proceeds) can be connected by molecular-scale wires, it should be possible to see heat flow in the wrong direction.
- Das, S., Narayan, O. & Ramaswamy, S. A ratchet for heat transport between identical reservoirs. Preprint, (2002).