*Peter Airey asked the Naked Scientists:*

Dear Chris

Q. What is the speed of propagation of a thermal disturbance in a medium?

For example, if I take a copper rod one meter in length at room temperature, and I plunge one end in boiling oil, how long before the tiniest change in temperature can be detected at the other end?

Every day experience with objects heating and cooling might lead us to believe that the answer is of the order of a few millimeters or centimeters per second.

Consideration of some simplistic particle-motion models of heat conduction might suggest a speed close to the speed of sound (particularly in the case of a gas).

A bit of googling turns up Fourier's equation for thermal conduction which, whilst apparently modeling reality fairly accurately, assumes a speed of propagation faster than the speed of light (instantaneous propagation, in fact). Further googling reveals that the more recent hyperbolic forms of Fourier's equation allow for a finite speed of propagation, and here my google trail turns cold.

So, snail's pace or speed of light or what?...... can you give some typical values for this speed in different media and materials (assuming that it is a material property)? and more importantly, describe in plain English the physical conduction mechanism(s) that justify the speed(s)?

Regards

Peter Airey

*What do you think?*