related post: fire
Heat and Light
I always get confused when thinking of infrared radiation and heat. I tend to think of these as the same, but they are not. This post is a refresher course for me for when I start to have trouble with these concepts…
Heat is not a substance, nor is it a form of energy, strictly speaking. Rather, it is the transfer of energy. More specifically, it is the transfer of kinetic energy. More specifically still, heat is the mechanism by which the transfer of molecular motion travels from a faster moving thing to a slower moving thing.
Here are the official definitions…
In modern terms, heat is concisely defined as energy in transit. Scottish physicist James Clerk Maxwell, in his 1871 classic Theory of Heat, was one of the first to enunciate a modern definition of “heat”. In short, Maxwell outlined four stipulations on the definition of heat. One, it is “something which may be transferred from one body to another”, as per the second law of thermodynamics. Two, it can be spoken of as a “measurable quantity”, and thus treated mathematically like other measurable quantities. Three, it “can not be treated as a substance”; for it may be transformed into something which is not a substance, e.g. mechanical work. Lastly, it is “one of the forms of energy”. Similar such modern, succinct definitions of heat are as follows:
- In a thermodynamic sense, heat is never regarded as being stored within a body. Like work, it exists only as energy in transit from one body to another; in thermodynamic terminology, between a system and its surroundings. When energy in the form of heat is added to a system, it is stored not as heat, but as kinetic and potential energy of the atoms and molecules making up the system.
- The noun heat is defined only during the process of energy transfer by conduction or radiation.
- Heat is defined as any spontaneous flow of energy from one object to another, caused by a difference in temperature between the objects.
- Heat may be defined as energy in transit from a high-temperature object to a lower-temperature object.
- Heat as an interaction between two closed systems without exchange of work is a pure heat interaction when the two systems, initially isolated and in a stable equilibrium, are placed in contact. The energy exchanged between the two systems is then called heat.
- Heat is a form of energy possessed by a substance by virtue of the vibrational movement, i.e. kinetic energy, of its molecules or atoms. The kinetic energy and heat may formally be equivalent, but they are not identical.
So, back to infrared radiation…
Is infrared radiation and heat the same thing? Moreover, if we understand IR as part of the electromagnetic spectrum, could we say that heat is therefore EM radiation, like light?
Infrared radiation IS a form of heat. That is, IR will transfer kinetic energy, and this transfer of kinetic energy is what we call “heat.”
What does this mean? Well, it means that the range of frequencies in the spectrum of electromagnetic radiation that we refer to as “infrared” are such that they transfer kinetic energy (motion).
The first question that arises when we state things this way, is: do other frequencies in the spectrum of electromagnetic radiation transfer kinetic energy as well, and why or why not?
The answer is no. The other frequencies in the spectrum of electromagnetic radiation do NOT transfer kinetic energy. But why?
Because of wave harmonics.
Infrared radiation can be felt. It is (invisible) Light that warms the skin. When we get close to the stove, a fire, or feel our skin warming on a sunny day, we are experiencing the effects of the same infrared radiation. This is why I tend to think of heat and light as the same thing. Because this particular Light warms the skin. But note that this is true only of the light in the lower (less energetic, longer wavelength) part of the visible portion of the spectrum – at the red end. Notice that this is the lower (energetic) end of the visible portion of the spectrum. We cannot “feel” the light in the even lower, longer (radio) ranges or the much higher, shorter, more energetic (gamma) range. From this we can see that although IR warms the skin, and IR is Light, not all “Light: warms the skin. Radio waves do not warm the skin and gamma rays do not warm the skin. This fact confounded scientists for centuries (hint: it should confound you, too. If it doesn’t, stop here and explain why this is so – the answer, by the way, was given in the previous paragraph).
Another way of observing the above is to notice that increasing the energy level (kinetic energy) of the Light, does not increase the heating effect of the Light on an object.
Let me say that again, more precisely:
Infrared radiation (red Light) from the sun transfers kinetic energy (heat) from the sun to our skin. Ultraviolet radiation (purple Light) from the sun does not. But UV Light is more “energetic”. The question, then is: Why is it not “hotter?”
-Heat vs heat transfer – are these the same? From the wikipedia definition above, heat is the transfer of kinetic energy, so it is misleading to speak of the transfer of the transfer of something. What we are doing is confusing kinetic energy with heat when we start to make this kind of distinction between the thing and the transfer of the thing. In this case, the thing in question is atomic and molecular motion.
-Kinetic energy is this motion – the wiggling of atoms and molecules – and heat is the passing on of this “motion” from one object to another. Both kinetic energy and heat increase as the wiggling of atoms and molecules increase, but heat increases only to the extent that this increase in wiggling (kinetic energy) causes an increase in wiggling elsewhere (to another object).
-Temperature is the measurement of the amount of this wiggling (kinetic energy). This wiggling sounds familiar, for Light is itself a wiggle. So why is it that the UV wiggle does not increase the wiggle (temperature) in our skin as much as the IR wiggle? Why does Light with more kinetic energy not produce more heat? Why is the kinetic energy of this Light not transferred?
The wiggle of IR reinforces the wiggle in the molecules of our skin, sort of like how microwave Light reinforces the wiggle in the polarized water molecule.
The key is that the wiggle is best transferred when the speed (temperature, or kinetic energy – referred to as “color” in the visible spectrum) of the wiggling matches between the source of the radiation (Light) and the receiver of the radiation. Another way of saying this is this is that IR is a particular speed (wavelegth) of Light that makes molecules wiggle. Sometimes, this particular (range of) wavelength of Light is called “heat waves,” “radiation,” and/or “infrared radiation.”
In all of these cases, we are referring to Light (electromagnetic radiation) of a certain range (the infrared range), which is to say of a certain wavelength, or “energy.” It just so happens that this particular range of wavelengths, or energy, harmonizes with molecules and increases their ever present wiggling.
So, the wiggling is kinetic energy and certain frequencies of Light transfers this wiggling to molecules. But other frequencies do not. This is why it is not the Light that is the heat, but that some Light can transfer its kinetic energy (wiggle) to objects. This form of Light is called infrared radiation, or heat waves.
But why is that only certain frequencies of Light make this transfer? Why don’t higher frequency wavelengths transfer more heat, and lower less?
Harmonics. Infrared radiation (the lower energy Light in this example) wiggles at a similar speed (temperature) as the molecules in our skin. Therefore, the wiggling in the Light and the wiggling in our skin can harmonize such that the wavelengths of the Light and the wavelengths of our skin add to each other.
Wavelengths of skin? Yes, whenever an electron moves it generates electromagnetic waves in the field that permeates the universe (I like to refer to all EM as Light, but traditionally people speak of light as a particular range of EM, just as IR (heat radiation) is another particular range of EM ). Another way of saying this is that when an electron moves, it emits Light. Light behaves as a wave such that two waves can cancel or reinforce. When we feel our skin get hot from the Light of the sun, what is happening is that the Light that is emanating from our skin and the Light that is penetrating our skin from the sun, reinforce. In other words, we shine brighter, hotter.
We shine? Yes, everything in the universe (except dark matter) emits Light (EM) because everything in the universe wiggles and every electron that wiggles creates ripples in the electromagnetic field which permeates the entire universe. These ripples are called electromagnetic radiation, or Light, as I have described.
[Additionally, not only does all non-dark matter radiate Light, but all non-dark matter in the universe radiates the entire spectrum of Light. Right now, your nose is emitting radio waves and gamma rays and everything in between. However, each piece of matter in the universe has a peak radiation frequency. That is to say that the wavelength of light that your nose is most strongly emitting falls somewhere between radio and gamma. more on this here]
So the upshot of all of this is that infrared radiation transfers its wiggle to molecules because it wiggles with a wavelength that is in a certain range that the molecules respond to. This is why higher energy Light from the sun (like UV Light) does not transfer heat to our skin – the harmonics are not right. UV Light is, in a sense, invisible to our skin in the same way it is invisible to our eyes. It wiggles too fast for our skin to “see” or “hear”
The Wiggle is All
It is interesting to note that the transfer of wiggles is a concept that applies to every event and phenomena of the universe. All of our senses are merely ways that the wiggles of the universe are transferred to our bodies, whether the source of the wiggle be described as Light (molecular wiggles felt by our eyes), sound (molecular wiggles felt by our ears), taste (molecular wiggles felt by our tongue), smell (molecular wiggles felt by our nose), or touch (molecular wiggles felt by our bodies). In each of these phenomena, kinetic energy is passed from one object to another through physical touch.
This is an interesting concept to keep in mind, particularly when thinking of forces (post on forces coming soon).
Everything that happens in the universe, every interaction, is a form of one thing touching another. There is no such thing as force. Not the way we tend to think of it: as something separated from, independent of, matter. Force and matter are the same phenomenon, like space and gravity. Einstein showed that gravity is not some force that acts in space, but that gravity is space itself. Likewise with electromagnetic radiation. Light is not something that moves through space; rather, it is the movement of space itself.
These are difficult abstractions to work through, but the movement of physics in the twentieth century has progressively stripped down our distinctions such that it is nearly impossible to draw a line between matter and space and energy at all. At the deepest level, we have seen the equivalence of these. To those who would challenge the idea that the concept of all the forces can be reduced to material interactions may want to refer to Richard Feynman’s theory of Quantum Electrodynamics, the most successful, most throughly tested, and most empirically accurate theory of the physical universe ever conceived by man. It is the pinnacle of scientific thought, and of science itself. It is hard to overstate the facts, but I will leave off here by pointing out to the reader that it is this theory that describes the forces as the exchange of sub-atomic, virtual particles between one another. As such, inertia itself is transferred. And that’s all. Should we ever conceive or grasp what inertia is we will have perhaps finally understood the universe in its essence.
[side note: the transfer of something from one body to another? Does that sound familiar?]
Now, you may say that this is all metaphorical, to say that this thing is actually another, and you would be absolutely right. Metaphor is exactly that: an expression that equates two seemingly different things. In other words, mathematics. The equations of physics that describe the universe are metaphors that tell us that this equals that. E=MC2 is the mathematical metaphor that tells is that matter IS energy. We know that space is not separate from time, and yet we speak of time as if it flows through space. This is not correct. Neither is it correct to speak of light and radio as if these are two different phenomena. They are not. One just has more kinetic energy but they are, in fact, the same phenomena. The joy of physics and mathematics – and religious contemplation – is merely the joy in discovering the multifaceted unification of the variety of experiences and phenomena in the universe. In the religious sense, this experience begins with the discovery that you are not your mind; it is the discovery that to experience your true self is to experience the ever present communion with God. The progress of physics thus mirrors the progress of the spirituality that grows only by seeing the ever more connectedness of the universe as one vast, single multifaceted phenomenon. This should not come as a surprise, for both of these journeys are of men and the journeys of men have but one end.
But wait, we started this meditation on heat and light by distinguishing them and declaring that they are not the same phenomenon. And surely taste and sight are not the same phenomenon. Well, yes and no. In the same way that red and blue are not the same, that is. But there is another sense in which red and blue are the same. That is to say, that they are of the same nature. The larger point in this meditation is really about levels of abstraction. We can go in either direction and make distinctions between red and blue, or we can go in the other direction and draw them together under one classification. In either case, these are just abstractions, or ways of thinking about these phenomena.
Thus the idea of the grand, singular, multifaceted phenomenon, aka, The Universe. After all, seen from a perspective outside of time, every event and phenomena in the history of the universe can be seen as a single event – in the mind of God, for example.
But now we have wandered far off the topic, so this will have to continue in another post.
See you then.
- Heat is the transfer of energy between substances of different temperatures.