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Particularly given the fact that heat can propagate through a vacuum in the form of infrared radiation. But is the modern theory of heat based on the notion that heat in the matter(regardless of its phase or density) is essentially infrared photons being exchanged by molecules(or atoms in the case of a pure element or in fully ionized plasma)? Nor when it comes to friction, like metal on metal for example, you have valence electrons repelling each other which leads to an increase in the emission and absorption of infrared photons. Is this how heat actually works according to modern QED theory?

EDIT: Part of why I asked this question is from reading Richard Feynmann's book "QED: the strange theory of matter and light" many years ago.

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  • $\begingroup$ Heating is a way to change the energy of system, similar to how performing work is a way to change the energy of a system. Is does not really make sense to ask "is heat infrared" any more than it makes sense to as "is work infrared." But to the extent the question makes sense, the answer should be no. $\endgroup$
    – hft
    Feb 6 at 23:56
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    $\begingroup$ If you are asking about blackbody radiation, the spectrum is well known... The peak of the spectrum can be in the infrared range for some temperatures, but it can also be not in the infrared range for other temperatures. It depends on how hot the body is. $\endgroup$
    – hft
    Feb 6 at 23:57
  • $\begingroup$ FYI: There is nothing special about infrared. Have you ever felt the warmth of sunshine on a bright, clear day? Approximately half of the warmth that you felt (half of the heat you absorbed) was delivered by the Sun's visible light. The main reason why we associate infrared with thermal radiation is, most of us don't ever stand near any object that's anywhere near as hot as the Sun. $\endgroup$ Feb 7 at 5:14
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    $\begingroup$ Voting to reopen. A perfectly clear question with a straightforward answer. $\endgroup$
    – gandalf61
    Feb 7 at 9:05
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    $\begingroup$ I believe that the point of the OP is the following: all mechanisms for heat boil down, at the electron level, to an electromagnetic interaction, which in QED corresponds to the exchange of virtual photons (mechanical heat) or the radiation and later absorption of photons (radiative heat). His questions is then: can we say that heat reduces to the exchange of infrared photons? Tell me if I got you wrong, Mr X. $\endgroup$
    – Albert
    Feb 8 at 17:32

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In thermodynamics, heat is defined as "thermal energy transferred between two systems or bodies due to a temperature difference between them". Informally, we often use the term heat to refer to the thermal energy contained within a system or body - although in thermodynamics this is more properly called "internal energy".

In either case, heat is not the same as infrared radiation. Infrared radiation is one way of transferring energy between bodies, but there are other ways. Energy can be transferred by other wavelengths of radiation apart from infrared - microwaves, for example, transfer energy to food in a microwave oven. Energy can be transferred by electromagnetic induction. Or energy can be transferred directly between two bodies that are in contact with one another, without involving any radiation at all.

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    $\begingroup$ Fundamental definition of temperature is average kinetic energy of the molecules/atoms. The OP confuses heat with temperature by saying "heat in the matter" .... so we could say all heat in the matter is the kinetic energy of the molecules/atoms. $\endgroup$ Feb 7 at 15:02
  • $\begingroup$ “for example, transfer heat to food in a microwave oven.” Microwave ovens do not cook by means of heat $\endgroup$
    – Bob D
    Feb 8 at 6:04
  • $\begingroup$ @BobD There is no fundamental difference between heating the surface of something with infrared radiation and heating its interior with shorter wavelength, more penetrating microwaves. The difference between a microwave and a conventional oven is that a conventional oven heats food mainly by convection and conduction, and a microwave oven heats mainly by radiation. But these are just different mechanisms for the transfer of heat. $\endgroup$
    – gandalf61
    Feb 8 at 6:34
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    $\begingroup$ Technically the mechanism for microwave cooking is not traditional heat transfer, which is energy transfer due solely to temperature difference, but rather electromagnetic work performed on water dipoles giving them rotational KE that is randomized into translational KE. $\endgroup$
    – Bob D
    Feb 8 at 7:45
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    $\begingroup$ @PhysicsDave the net transfer of radiant heat between two bodies is proportional to the difference in their temperatures raised to the fourth power, is it not? $\endgroup$
    – Bob D
    Feb 8 at 16:37
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"Heat" is the thermal energy of particles, the energy of vibrating or colliding atoms or other particles. As Wikipedia puts it, "heat is thermal energy that is contained in the system or body, as it is stored in the microscopic degrees of freedom of the modes of vibration."

Heat can be transferred in different ways. One way is when two objects of different temperatures touch: their vibrating atoms will transfer energy from the hotter (faster vibrating) ones to the cooler.

Heat can also be transferred through electromagnetic radiation. In that case, the radiation covers all possible frequencies, but it peaks at a frequency determined by the temperature of the object. For an ideal (black body) object, the spectrum of the radiation is well known.

At everyday temperatures, that radiation is in the infrared; at higher temperatures it can be in visible light, for example when a metal bar is heated red hot. At extremely high temperatures the radiation will be at ultraviolet or even x-ray frequencies. This happens for example when matter falls into a black hole and gets accelerated to extremely high velocities - temperatures of millions of degrees. Conversely, at very low temperatures, objects emit radio waves. The CMB, at a temperature of 2.7K, peaks at just over 100 GHz.

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    $\begingroup$ ""Heat" is the thermal energy of particles, the energy of vibrating or colliding atoms or other particles." That is incorrect. Heat is energy transfer due solely to temperature difference. What you are describing is iinternal energy. $\endgroup$
    – Bob D
    Feb 7 at 2:33
  • $\begingroup$ @BobD Is he describing internal energy or temperature? Temperature ... is a better term. $\endgroup$ Feb 7 at 14:56
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You use the term "heat in the matter" ... what you should say is temperature ... and then you should get the correct answer that it is all vibrational/kinetic energy of the atoms/molecules.

In a body at equilibrium for every atom that releases an IR photon another atom absorbs it ... so NET IR is zero. Yes there some of this going on in the body ... I believe these are referred to as phonons.

When atoms push each other it does so with electron repulsion which science describes as a force relayed by the EM field. Forces in the EM field are well known but not understood ... science uses the term "virtual photons" .... which means we don't know how the EM field does it but the forces are very real.

Because light and IR is a trendy topic now many of the answers will shy away from the fundamental/boring definition of heat as the average kinetic energy of the molecules/atoms.

Heat transfer can also occur by phase change ..i.e. evaporation... also a boring topic.

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