We humans have done a remarkably thorough job of domesticating our world. We live on earth, but we have remade the environment around us to suit how we would like earth to be. We’re still subject to clouds and sun, rain and snow, and the earth’s changes in temperature, but we’ve invented ways to avoid the worst of those. In the heat of summer and the cold of winter, we shuttle from climate-controlled buildings via climate-controlled cars to other climate-controlled buildings; and in winter we cover our bodies with insulated materials to keep us warm when we’re in the climate-uncontrolled outdoors (no one has yet invented practical air-conditioned clothing for the summer, but we can still hope).
Likewise, most of us spend most of our time in domesticated landscapes. We walk on floors, stairs, sidewalks, lawns, and parking lots. We sit on furniture and sleep on beds. And these trappings of our domesticated world are often quite similar all over the world.
The world we came to take for granted from our earliest childhood was dominated by this domesticated environment and domesticated landscape, sculpted by humans to make humans as comfortable as possible. Thanks to our many determined accommodations, our planet has largely faded into the background, as a passive place things happen rather than an active agent in our lives. Only occasionally does our part of the planet become unstable enough to feel dangerous. In some places the earth itself moves underfoot, and some mountains sometimes spew out rocks and dust and flow with molten lava. But these catastrophes are rare. The most commonly unstable part of our planet is the atmosphere, stirred into motion by differences in pressure and temperature, bringing winds, and water falling from the air in drops and frozen crystals.
How does a young child experience these changes? Many are pleasant: a cooling breeze or drenching summer rain, the thrilling first snowfall of the winter. Even the less comfortable ones are merely inconvenient, no more remarkable to a child than an upset stomach or a scraped knee. Only in storms do the motions of the atmosphere become so dramatic that a child could experience them as the actions of a violent threatening being. High winds can tip over tall trees like an invisible monster, and hurl objects through the air as if in anger.
But for human children, the most emotionally evocative of these atmospheric effects is the thunderstorm. It brings with it winds and rain, but also the unpredictable flash of lightning and the deep, awesome voice of the thunderclap. When children fear thunder, they may have in mind the damage that thunderstorms can do, but the emotional heart of their fear is I think the sense of thunder as an angry and powerful cosmic being—announcing itself ominously from a distance, then growling louder and more deeply as it comes closer. As adults, we may have overcome these childish fears, but for most of us there is still something thrilling in the onset of a thunderstorm.
As children, we did learn about lightning and thunder—and some of what we learned was even true. At some point, I was told that thunder is caused by two clouds colliding—an explanation not so different from what Aristotle proposed in his Meteorologica. I must have been old enough to know something about lightning being caused by positive and negative charges, like a spark, because I interpreted that parental lesson in terms of positively charged clouds coming in contact with negatively charged clouds, and I have to admit that I held onto that idea well into adulthood. Like most children, I also learned to figure out how far away the lightning struck by counting the seconds between lightning and thunder and dividing by five.
But how many of us actually ever wonder what it is about a lightning bolt that causes the sound we call thunder? Much scientific thought has been devoted to this question over the centuries, and in broad terms at least we do now have an answer. But before we get to that, let’s make sure we’re clear on what a sound is.
We hear sounds when our eardrum vibrates, and that happens when the air around us vibrates. Those vibrations are caused by pressure waves moving through the air at—yes—the speed of sound, which is almost 800 miles per hour. In an air pressure wave, the molecules in the atmosphere are crowded together at the peak of the wave and spread apart (at lower pressure) at the trough of the wave. So sound is caused by anything that generates enough force to push against the molecules of the atmosphere and press them together. When you whack two sticks together, the collision between them pushes against the nearby air, producing the cracking sound you hear. More force generates more concussion, which produces a louder sound.
Storm clouds produce lightning because they accumulate a major difference in electrical charge between themselves and the ground or other clouds. Once this difference in electrical charge becomes great enough to overcome the electrical resistance of the atmosphere, streams of electrons feel their way towards the ground, eventually connecting with a return stroke of opposite charge coming up from the ground. This connection produces a long narrow channel about an inch thick in which massive amounts of electrical current can pass between the cloud and the ground. As the current begins to flow, there is enough electrical resistance within the lightning channel to convert some of the current’s electrical energy into thermal energy, rapidly heating up the air around the channel.
When the temperature within the lightning channel rapidly increases, the air around the channel becomes intensely pressurized (because the pressure of a gas is proportional to its temperature divided by the volume it occupies, according to the combined gas law). Then a moment later, the air expands faster than the speed of sound, producing a shock wave just like in an explosion. As that shock wave spreads out, it produces sound pressure waves that can be sensed by our ears. You can read about the details of how the sound of thunder varies depending on the shape of the lightning bolt and the atmospheric conditions in the references and external links cited in the Wikipedia article on thunder.
My sense from the little reading I’ve done on this subject is that there are still open questions about what exactly is happening from moment to moment within the channel of a lightning bolt, and how some of the electrical energy is transformed into the sound pressure wave that produces thunder. But the above should at least provide enough of an answer to satisfy the next inquisitive child you run across.
© Joel Benington, 2012.
If you walked up to people on the street and asked them why it’s colder in the winter, what do you think most of them would say? I bet they’d say that it’s colder because the sun is farther from the earth in the winter. I’ve never done an actual survey, but I’ve asked the question in my classes several times over the years, and that’s the most common answer I get.
Actually, the distance to the sun has practically nothing to do with the change of seasons. The earth revolves around the sun in a very nearly circular orbit, which keeps it pretty much the same distance from the sun at all times. Yes, earth’s orbit is just a tiny bit elliptical, so the distance from the sun does vary by something like 3% plus or minus, but that’s not nearly enough to produce the dramatic changes in temperature between winter and summer. In fact, winter in the northern hemisphere actually happens when the earth is that little bit closer to the sun in its orbit, which is the opposite of what so many people think. This relationship does change very slowly over time, however, so in another 10,000 years we will indeed be just a little bit farther from the sun during the northern hemisphere winter. Something to look forward to.
What does cause the change in seasons? Even if you haven’t thought much about this, you’ve probably noticed that the sun is lower in the sky in the winter, which also makes the days shorter. This is caused by the tilt of the earth’s axis relative to its orbit around the sun. In the northern hemisphere summer, the north pole is tilted 23 degrees towards the sun, which causes the sun to be higher in the sky from our perspective here on earth’s surface. In the northern hemisphere winter, the north pole is titled 23 degrees away from the sun, so the sun appears lower in the sky.
The light coming to us from the sun is a stream of photons. When those photons are absorbed by the material of the earth, their energy is transformed into thermal energy, which heats up the earth’s surface and atmosphere. Each photon contributes a bit of energy, so the earth heats up more when the density of photons striking its surface is greater.
When the sun is directly overhead, rays of sunlight strike the earth’s surface head-on, which means that the photons in a given patch of sunlight are spread over the smallest possible area of the earth’s surface. But the sun is only ever directly overhead if you’re in the tropics, and even then only at noon on two days of the year. The rest of the time and everywhere else, sunlight is always hitting the earth’s surface at an angle. The further that angle is from the perpendicular (i.e., the further the sun is down towards the horizon), the more the photons in sunlight are spread over a greater area of earth’s surface.
You see an example of this spreading out of the photons in a patch of sunlight in the lengthening of shadows towards the end of the day. A shadow represents the patch of sunlight that an object is intercepting. As the shadow lengthens, the area of earth’s surface that that patch of sunlight would have been spread across if the object hadn’t gotten in the way becomes larger and larger.
Of course, this illustration is only really accurate if the object is spherical—like a beach ball, for example—because only a sphere intercepts the same area of sunlight no matter what angle the sun is coming from. So if you have the time, you could set a beach ball on the ground in the late afternoon and watch the shadow gradually get larger as the sun gets closer to setting. But don’t do it on a windy day, because the beach ball might blow away. Or you could just imagine it.
Anyway, when the sun is lower in the sky in winter, the photons in sunlight are spread more thinly over a greater area of the earth’s surface, so there is a lower density of photons being absorbed by the material of the earth and its atmosphere. Fewer photons being absorbed means less thermal energy, so that part of the earth is colder. In short, the change of seasons is caused not by a change in the distance to the sun but by a change in the angle of the sun’s rays as they strike the earth’s surface.
© Joel Benington, 2012.
For humans, a big part of staying comfortable is keeping the temperature right. Too hot or too cold and our body freaks out. If the body’s core temperature gets just a few degrees above normal for too long, heat exhaustion can develop into heat stroke, producing dehydration, confusion, and eventually even death. Just a few degrees below normal produces severe shivering, poor coordination, and again confusion and even death. Like other mammals, we regulate our body temperature physiologically and behaviorally within a very narrow range, because the machinery of our body works best in that range of temperatures.
Where does the heat come from to keep out bodies around 37 degrees C? That’s almost always warmer than our surroundings, so our bodies must be generating heat or we would cool down. Like all living things, we have enzymes throughout our bodies that are constantly performing chemical reactions, and each chemical reaction releases a little bit of heat (the second law of thermodynamics says they have to, and they obey). So as long as we’re alive, the combination of all those little chemical reactions causes our bodies to keep pumping out heat.
To keep our core temperature constant, we need to let that heat dissipate away fast enough but not too fast. When we get too hot, we look for a cool place to be so there’s a steeper thermal gradient between our bodies and their surroundings, causing faster heat dissipation. Or we point a fan at our body to speed the flow of warm air away from it and to promote evaporative cooling.
When, on the other hand, we get too cool, we either seek shelter in a warmer place or put on warmer clothing. Ah, the snug comfort of slipping on a thick jacket on a cold day, or wrapping oneself in a warm blanket! Like our fear of burning ourselves in fires, our experience of snug warmth and our intuitive knowledge of how to get it date back long before we had the mental tools to understand how jackets and blankets could protect us against the cold. In fact, that experience of snug warmth dates back to before our earliest memories, to when our parents swaddled us lovingly on cold days, and held us close to the warmth of their own bodies.
But now that we’re all grown, we shouldn’t take these things for granted anymore, right? The basic idea of insulation is simple enough: if we let less heat flow away from our body, then more of the new heat that is generated in each second sticks around, which will raise the temperature around us, keeping us warm. We’ve got to let some of the heat flow away, or we’d get hotter and hotter till we died. But the colder it is around us, the less flow of heat we want, so the more insulation we need.
But what is it specifically about materials like wool and down that provide such good thermal insulation? Why is wool warmer than an equivalent amount of leather or rubber? Why is down so light and yet so warm? The key is the air pockets between the fibers of wool and between the tiny feathers in down.
Like all gases, the air around our bodies is a lousy conductor of heat. The molecules in a gas are far away from each other, and conduction of heat only happens when a faster-moving molecule collides with a slower-moving one, transmitting some of its kinetic energy to the other molecule. Since heat is simply molecular kinetic energy, this transfer of kinetic energy between molecules enables heat to flow through a body from where hotter areas to cooler ones. This does happen a bit in a gas, but much less than in liquids and solids, where the molecules are much closer together.
If the air around our bodies were completely still (and if we too kept completely still), then that air would act as a pretty good heat insulator, and we would stay nice and warm. But air being a fluid naturally moves around, and the movement of air carries heat away from our bodies by a process called convection. On a windy day, the atmosphere of warmer air near our bodies is carried away, and it is replaced by cooler air, thus speeding the loss of heat. That’s why the wind chill factor is a relevant measure of how cool the air feels on a cold, windy day.
But when we wrap our bodies in layers of wool or down, we surround ourselves with a huge number of tiny air pockets. The fibers or feathers between those air pockets block the movement of air from pocket to pocket, thus reducing the flow of heat by convection within the material. Convection can still carry heat away from the surface of a sweater or jacket, but the temperature is lower there than at the surface of our bodies, so there is less heat loss by convection than if we were naked or wearing less insulating clothing.
One famed property of wool is that it still insulates when wet, unlike cotton which only insulates well when dry. Why is this? When cotton gets wet, water fills up the tiny air pockets that otherwise would provide insulation. Because at the microscopic level water doesn’t cling as well to wool as it does to cotton, wool can get wet without its tiny air pockets being filled up with water, so it still reduces heat loss by convection.
Synthetic fiber insulation like Thinsulate and PrimaLoft works the same way as natural fiber insulation. The best insulation will pack as many tiny air pockets as possible into the smallest possible space with the least weight of material. There should be enough fiber between the air pockets to minimize convection, and yet enough space between those fibers to let moisture pass through the fabric to wick away from the body. Ideally, the fibers will also be made out of molecules that water does not readily cling to, so the air pockets don’t fill up with water when the fabric gets wet.
© Joel Benington, 2012.
We learn about our world so early—as little children. We find out what gives us pleasure and what causes pain. Even the most loving parents speak sharply to warn their children away from dangers. In this way, we all developed an emotionally charged idea of danger long before we had the mental tools to understand why dangerous things are dangerous.
Fire is one of those dangerous things we all learned about early, along with other hot things like stoves and ovens. Watch out—you’ll burn yourself! Keep away from that, it’s hot! From a respectable distance, fire is warm and cozy and so lovely to look at, but if you get too close it’s painful and destructive.
Most people’s understanding of fire pretty much ends there. We know what sort of things will or will not burn, and we have a sense of how fire behaves, and that’s all we really need to know about it. The chemistry and thermodynamics of fire have been thoroughly described by scientists, and most people once learned something about that in science classes, but relatively few people walk around with a very clear idea of it in their head. That’s fine for practical purposes, but if we don’t ourselves understand at least the basics of how fire actually works, then in effect fire is for us a kind of magic—producing familiar effects by mysterious means.
The science behind fire is discussed in the Wikipedia article on the subject. Basically, fire occurs whenever the temperature is high enough to oxidize some fuel. When wood burns, organic molecules that make up the wood react with oxygen in the air to produce carbon dioxide and water. Because the products of this reaction are held together by stronger chemical bonds than the reactants were, the reaction releases energy. That release of energy speeds up the motion of nearby molecules in the air, which further raises the temperature, because temperature is a measure of how rapidly molecules are moving.
Fire is called a chain reaction because the energy released through the oxidation of organic molecules helps keep the temperature high enough to permit the oxidation of other nearby organic molecules. If the release of energy stopped, the temperature would rapidly drop as heat flowed away. But if enough energy is released in each second, the temperature will stay high enough to keep the oxidation going, and so the wood will keep burning. That’s the chain reaction.
So how do you put out a fire? You can deprive it of fuel or of oxygen. Or you can get rid of the heat fast enough that the chain reaction stops, and that’s what water does.
Two things happen when water finds itself in a fire: the water’s temperature rises to boiling point, and then the water evaporates from liquid to gas. Both of these physical changes absorb energy—particularly the evaporation, which absorbs about six times as much energy as is needed to raise the temperature of water all the way from freezing point to boiling point. Any energy absorbed in evaporating water means less energy is left to help keep up the temperature of the fire. Toss in enough water, and the temperature falls below the threshold needed to sustain the chain reaction, and the fire goes out.
Water turns out to be an excellent liquid for lowering the temperature of a fire. It doesn’t combust like alcohol or gasoline, which is kind of important. But even in comparison to other non-combustible liquids, water has both a high specific heat and a high heat of vaporization. The specific heat is the amount of energy that must be absorbed to raise the temperature of the water, and the heat of vaporization is the amount of heat that must be absorbed to evaporate the water. Thus, water does a really good job of absorbing energy as its temperature is raised and as it evaporates, which makes it very efficient in putting out fires by lowering the temperature enough to interrupt the chain reaction.
© Joel Benington, 2012.
Most of what we know about the world around us comes from seeing. Vision is such a useful sense because photons emitted by the sun stream down to earth all day long and reflect off objects or are absorbed by them. Materials that reflect more photons appear lighter to our eyes, while materials that absorb more photons appear darker. Because photons travel at the speed of light, the stream of photons reflecting off an object gives us practically instantaneous information about it, even if it is far away from us. The only other sense that gives us information about far distant objects is hearing.
We can see because the retinas in the back of our eyes contain cells called photoreceptors, which can detect the presence of photons. Photoreceptor cells can sense photons because they contain molecules of retinal that change shape when they absorb a photon. The retinal molecules are bound to proteins called opsins, which change shape when the retinal changes shape. This triggers a cascade of molecular events in the photoreceptor cell that alters the release of neurotransmitter molecules by the photoreceptor, thus sending a neural signal to other cells in the retina.
Our opsins have evolved so photoreceptor cells are most sensitive to photons in the range of wavelengths emitted by the sun. What we call visible light has wavelengths from 400 to 700 nanometers (a nanometer is one billionth of a meter), because when retinal is bound to opsins, it doesn’t readily absorb photons with wavelengths below 400 nanometers or above 700 nanometers. Photons with wavelengths above 700 nanometers are in the infrared range, and they’re bouncing off objects all around us, but we can’t see them because they have no effect on the retinal molecules in our photoreceptors.
The different colors that we see are simply photons of different wavelengths within the 400-700 nanometer range of visible light. The colors in a rainbow from blue to green to yellow to orange to red correspond to photons having a range of wavelengths, from shorter to longer.
We can distinguish these different colors because the photoreceptors called cones come in three types containing three different opsin proteins. Those are called L, M, and S opsins because they interact with retinal so it preferentially absorbs photons having long wavelengths, medium wavelengths, and short wavelengths. Red light is absorbed best by cones containing L opsins, while green light is absorbed best by cones containing M opsins and blue light by cones containing S opsins. The visual circuits in our brains compare the neural activity triggered by these three different kinds of cones to distinguish between slightly different colors, like tangerine vs. pumpkin.
So why do we see these colors only during the day? Because cone photoreceptors aren’t sensitive to very dim light. The density of photons at night is so low that it has virtually no effect on any of the three different types of cones. Another type of photoreceptors called “rods” are the only ones responsive to dim light, and there’s only one type of opsin in rods, so there’s no way to compare the wavelengths of different photons. We can see, but only based on different intensities of dim light, so everything looks just like different shades of grey.
Of course all of that only applies to dim light at night—as bright as moonlight, for example. We can still see something like a neon sign at night in color, as long as our eyes receive a high enough density of photons to produce a response in our cone photoreceptors, so different wavelengths of light can be distinguished.
© Joel Benington, 2012.
Our sun is way hot. Its surface is hot enough to melt diamonds, and the temperature of its core is a mind-boggling millions of degrees Fahrenheit. Every second, it releases a billion times more energy than could be released by all of the nuclear weapons that have ever existed on earth. The sun is in fact an unimaginably massive hydrogen bomb that has been exploding for billions of years. Fortunately it’s almost a hundred million miles away from us, or all of the water on earth would be vaporized and we wouldn’t exist.
That, anyway, is the sun from a scientific point of view. But how often do we have that point of view in mind when we experience the sun in our daily lives?
As a human living on earth, I experience the sun as an intensely bright circle of light in the sky that floods my world with light and warmth. It is too bright to look at except when it is yellow-gold and half-masked by clouds on the horizon, just before it sets (and even then it’s dangerous to look at, so please don’t). Every morning it rises above the horizon and moves slowly and steadily through the sky from east to south to west, until it sets and returns the world to darkness. Its movements set the rhythm of our sleeping and waking, and determine when stores are open, when we eat our meals, and when our television shows are on.
That is the sun that I know first-hand. Contemplating it as it shines down on me and warms my body, I can more or less reconcile my experiences of it with the sun that scientists have described—probably less than more, actually, because I can’t really imagine anything as huge as the sun, as far away, or as hot.
My mind strains to grasp something even as large as New York State, which I drive through all the time. I can say how much further away from me Africa is than New Jersey, and how much further still I’d have to travel to go all the way around the earth. I can imagine a series of places I would pass through on my way. But frankly my mind slips whenever I try pull back from imagining this or that place, to conceive of the vast expanse of land and water that makes up the entire earth’s surface. And this unimaginable earth is by comparison just a tiny ball of molten metal covered by rock, a speck lost in the comparative immensity of the solar system.
So no—I may imagine I can imagine the sun as it really is in space, but I’m pretty much fooling myself. Even this one tiny corner of the universe is on far too vast a scale to fit in the limits of my first-hand imagining. So when I ask myself why the sun emits light, I am calling to mind what I know about an astrophysical abstraction, which only nominally relates to the old familiar sun that I have seen and felt my whole life. But with that understood…
Our sun formed billions of years ago when a diffuse cloud of matter was pulled together by gravity. The packing together of all that matter released energy, which raised the temperature of the Sun high enough to cause pairs of hydrogen atoms to fuse together to make helium atoms. A fusion reaction converts a little less than 1% of the mass of the hydrogen atoms into energy. Yes—matter can be converted into energy. The formula for that conversion is Einstein’s famous e=mc2, which says that energy equals mass times the square of the speed of light. The slight loss of mass when two hydrogen atoms form helium produces a corresponding increase in energy.
The energy of fusion is released in the form of a photon, which is an energy-carrying particle that moves at the speed of light. A photon released by one atom collides with another atom, energizing it and causing it to release another photon, which collides with yet another atom, and so on. Through this chain of events, energy radiates out from the core of the Sun, which is the only part that is hot enough to sustain fusion reactions.
At the surface of the sun, the density of material is low enough that a photon released by an atom is able to fly off into space without being absorbed by another atom. The release of photons at the surface is how the energy produce by fusion in the core dissipates away. All bodies release photons
in this way, in a process that is called thermal radiation. Hotter bodies release higher-energy photons, which have shorter wavelengths. Your body is only warm, so it releases lower-energy photons with wavelengths in the “infrared” range. We can’t see infrared light, which is why other people don’t appear as though they’re glowing, even though they are indeed releasing photons.
Light bulbs, though, are hot enough that they release photons with wavelengths between 400 and 700 nanometers (a nanometer is one billionth of a meter). That is the range of wavelengths that our eyes are able to see, so it’s called visible light. Most of the photons released from the surface of the sun are in that same range of wavelengths, so we’re able to see the light that the sun emits. How our eyes do that is the subject of the next post.
© Joel Benington, 2012.
I wrote my post on ten questions about the physical world on a whim, because it had been almost a month since my previous post, and I was backed up with exams at the end of our fall semester, and wanted something I could get out quick. But having done so, I figure I really ought to provide quick explanations to save people the trouble of hunting them up themselves. I’m also intrigued by the challenge of writing brief but reasonably complete answers to these questions.
Why is the sky blue? The reason it’s not black is because light reflects off the molecules of the atmosphere. Without that atmosphere, the only light we’d see in the sky would be what comes straight from the sun, stars, and planets, and the sunlight reflecting off the moon. The rest of the sky would be as black as at night, even though everything around us on earth would be lit up as bright as day.
But thankfully there is an atmosphere, and some of the sunlight passing through the layer of nitrogen and oxygen gas surrounding the earth hits the molecules of that gas and reflects off at an angle. We see the light that reflects at just the right angle to strike the light-sensing cells in our eyes. Enough light is reflected to light up every corner of the sky—not nearly as bright as the sun itself, but enough to make it appear far from black.
Why is it blue? The different colors we see are a result of how much energy is in different photons of light. Higher-energy photons look blue to us, lower-energy photons look red, and the colors of the rainbow between red and blue represent a range of energies from lower to higher.
Clouds look white to us because the tiny water droplets in them reflect all photons equally, so the light coming to us from clouds contains all the colors of the rainbow, and all colors taken together appear white. If the molecules of the atmosphere also reflected all photons equally, the whole sky would look white to us. But since higher-energy photons are more likely to reflect off the molecules of the atmosphere than lower-energy photons are, the sky looks blue.
Why do clouds look yellow and red at sunset? When the sun is close to the horizon just before it sets, the sun’s light is passing sideways through the atmosphere, which means it has to pass a greater distance through the atmosphere before it gets to our eyes. The atmosphere surrounding the earth is extremely thin relative to the diameter of the planet, so when the sun is right on the horizon, its light actually does pass through much more atmosphere than when it shines down from overhead.
As light passes through more atmosphere, more of the higher-energy photons are reflected in other directions, so the light that is left has a relatively greater number of lower-energy photons. The round orb of the sun that we see is made up of the photons that have come straight from the sun to our eyes. It turns more yellow and then orange as it sets because more of the higher-energy photons have been stripped out of the beam of light by reflecting off of molecules in the atmosphere. And since clouds reflect photons of all energies about equally, some of the yellow-orange light that strikes them reflects in our direction, making them look yellow-orange. They turn from yellow to orange to red as the sunset advances because more and more of the higher-energy photons are stripped out of the beam of light as that light passes more sideways through the atmosphere, as the sun sinks lower in the sky.
© Joel Benington, 2012.