Just to make sure other readers are starting at the same point, hereís what heat energy is. There are all sorts of random jigglings and wobbles etc of the atoms, electrons, spins, etc in any material. These all require some energy above what a material would have in its lowest energy state. The higher the temperature, the more likely the system is to be found in some high-energy state, and the lower the temperature the more likely to be found in a low energy state. I donít know what to say that íheatí is beyond this heat energy. What more definition would you be seeking? In formal scientific usage, íheatí refers not to the total amount of that thermal energy but only to the transfer of thermal energy caused by a temperature difference between objects. However, I think that sort of formality is not what you were asking about.
So why does heat exist? I guess that amounts to asking why energy gets distributed around among all the little random microscopic vibrational modes rather than concentrated in a few large-scale modes, things like bouncing balls. The reason is that there are many more microscopic states of the world with the energy distributed into the tiny modes than there are with the energy concentrated in a few big modes. Nature seems to become progressively indifferent to what quantum state it is in (thatís called the 2nd law of thermodynamics), so the best bet on what youíll see is based on giving equal probabilities to different quantum states which have the known amount of energy, etc. The vast majority of those states take almost all the energy from the big visible modes and instead have it distributed in the little modes.
Why does heat feel hot? Some of the heat energy from a hot object flows into your hand, if you touch it. The reasons why your nerves are designed to respond to that are pretty obvious from an evolutionary point of view. As to why anything has any particular subjective feel, thatís way too deep a mystery for us.
Iím not sure if that answers your question, so feel free to elaborate on it if needed.
(republished on 07/27/06)