by Bob Berman
Let’s saddle up and picture ourselves riding on a photon. In one second of zooming at light speed, we could circle our planet eight times. One hour at that speed would carry us to Jupiter. But reaching the nearest star, Proxima Centauri, would require that we remain in that saddle for 4.3 years. And, alas, the nearest spiral galaxy could be reached only if we continued at light speed for two and a half million years.
What about a much shorter commute? For example, zooming at light speed for just a thousandth of a second? That would let us commute from New York to Washington, DC. In one millionth of a second, we could cross three football fields. And in a billionth of a second we’d travel 11.75 inches—essentially a foot.
That’s a fun statistic, because it means that everything around you is seen the way it was as many nanoseconds, or billionths of a second, ago as it is feet away from you. You view a friend sitting across the room twenty feet away not as she is now but rather as she was twenty billionths of a second ago.
Since no image or information can exceed that velocity, we can never know what things are like now anywhere in the universe. Indeed we usually don’t even try to know. Instead we define now as “whenever an image arrives at our eyes.” We say, “Look how Jupiter and Saturn are passing one another in the night sky!” and no one bothers adding, “Or at least look at the way they were passing each other when the light we see now started our way an hour ago.”
The gap, or discrepancy, between the way things appear to our eyes and what the current reality is grows ever greater as we look farther away, and it equals the age of the visible universe when we peer 13.8 billion light-years away. This, then, is the boundary of observable reality, beyond which nothing can ever be seen or known.
There’s no way of getting around this limit. We view stars in the Whirlpool galaxy as they were thirty-five million years ago, and there is no possible method of obtaining news of their current state. Nor, watching us through some supertelescope at this moment, could Whirlpoolean extraterrestrials see anything but Earth thirty-three million years before the first appearance of humans. Probing us using invisible forms of light wouldn’t change anything. Anyone monitoring our radio and TV signals, or the infrared radiation given off by the heat of our bodies, would confront the same limitation, because these waves, too, travel at light speed. For the same reason, no technique or clever design could alert us that a laser beam or radio signal from an extraterrestrial civilization was en route to us until it actually arrived.
Probably the most intriguing of all aspects of light speed is a photon’s perception of totally frozen time. If you could walk a mile in its shoes (so to speak) you’d experience yourself as everywhere in the universe at once.
This happens because light travels in a fundamentally different way from the way we travel. We perceive ourselves as moving through both space and time. Actually, when we’re sitting still, we don’t move through space relative to objects around us, but we still must move through time at the rate of one day every twenty-four hours, even if we don’t want to. We age. As we walk, we travel through space and continue moving through time. Here’s the astonishing thing: as we go faster, we traverse more space, but our journey through time is reduced. Our time slows down from everyone else’s perspective. At just below light speed we are cruising through a lot of space but barely moving through time at all. The more you travel through one facet of space-time, the less you travel through the other. You can’t fully do both simultaneously. That’s what Einstein figured out, although most of us still don’t grasp the enormity of the concept a century later.
Light exists at the extreme end of this phenomenon. Its photons only move through space. They experience no time at all. Thus they cross the entire cosmos in zero time, which means that from their perspective, distance separations simply do not exist. If you aim a camera with a flashbulb out a window toward the sky, the moment you pop the flash the pulse of light has already arrived at the far end of the universe, from its perspective.
It’s very strange and unintuitive stuff. And yet light always moves at its famous constant speed no matter who does the observing. Light occupies a sort of higher, surer reality than the things we once thought we could count on, such as the interval between the ticks of a clock.
No wonder, then, that one of the first passages in Western religious scripture is “Let there be light” and at least one Eastern religion speaks about ultimate reality as a “clear light.” The scribes somehow sensed that light exists in a more secure realm than the mere spatiotemporal dimensions in which we spend our everyday lives.*
Someday we may figure out how to exploit the time-warping qualities of light speed. Imagine, if and when our propulsion abilities are up to the task, we could go anywhere in the cosmos while hardly aging at all.
The only problem with truly distant high-speed travel: the earth will be eons older when you return. Your descendants will have evolved: they will no longer be what we perceive as human. Your jokes won’t get a laugh. Records of your departure will have been lost thousands of years earlier. Your language will be unintelligible.
It’s a good news, bad news kind of situation. Without violating any of science’s laws, you’ve not only seen impossibly distant realms but also lived to witness eons of Earth’s history. On the other hand, you might have been wiser to have never made the U-turn.
CHAPTER 12
Microwaves Everywhere
Pop a bag of popcorn into the microwave and hit a button or two. When the “It’s ready!” beeps begin, you know the waves have stopped even as the popcorny aroma spreads throughout the house. And maybe some questions are flashing through your mind as insistently as the beeping. Are these microwaves safe? What actually makes those kernels pop? When people think about invisible rays zapping their bodies 24-7, they think of microwaves. Some people even use the word zap to describe what a microwave oven does: your mac and cheese isn’t hot enough? Just zap it in the microwave for a few minutes!
Microwaves were often lumped in the same category as the radio spectrum. The word microwave wasn’t even coined until 1931. But in the years before World War II, major technological uses for microwaves materialized, opening the door for their immense popularity. It was only then that the shortest-waved section of the radio spectrum earned its own discrete designation.
Since they are now customarily treated as an entity unto themselves, let’s invite them to join the party. Let’s define radio waves as each having a maximum length of one hundred thousand miles and a minimum length of a foot. Microwaves begin at that shortest point, with a maximum one-foot distance from crest to crest and a vibration rate of one trillion waves per second. When we reach the shortest end of the microwave spectrum, we find its waves spaced an eighth of an inch apart. Any closer together, and waves would earn membership in the infrared club.
Early on, within a decade of Hertz’s discovery of radio waves, researchers realized that whereas light waves diffract around obstacles, the shortest radio waves can easily focus on a target. Thus they are excellent at point-to-point communication. Unlike longer radio waves, which bend around objects, bounce off the ionosphere, and are good for widespread radio transmissions, the shortest waves are perfect for things like sending a signal from a plane to a control tower. They operate in a strictly line-of-sight manner. They also crisply bounce off metal objects.
German inventor Christian Hülsmeyer quickly exploited this capability. In 1903, he used what we now call microwaves to create the first-ever ship-detection apparatus, intended to help vessels avoid collisions in fog. This device, which he patented, reliably “saw” ships up to five miles away in zero-visibility conditions. Hülsmeyer could even determine the unseen ship’s bearing (direction), thanks to his device’s rotating parabolic receiving dish. But it could not determine range (meaning distance). Despite Hülsmeyer’s obtaining financial backing and performing an initial successful demonstration for the Holland America Line, interest in his device inexplicably sputtered, and withi
n a few years his company had to be dissolved.
However, others soon picked up the ball and ran with it. By World War I, several inventors had developed similar but improved systems that used triangulation to obtain both bearing and distance readings over short ranges. During the following decades, and particularly in the years just before World War II, thanks to increasingly intensive research conducted in Britain, short-frequency radio waves successfully detected not just ships but also incoming planes—and this microwave-utilizing technology was called radar, an acronym for “radio detecting and ranging.”
The creation of systems capable of putting out short pulses of radio energy, and the use of oscilloscopes to monitor delay times between the outgoing pulse and receipt of the echo, were the keys to determining both the position and distance of ships and planes—the basis of the radar systems that saved Britain.
Radar should have saved more than 2,400 lives on December 7, 1941, but human judgment got in the way. One of the first US radar installations was completed atop Kahuku Point, on Oahu’s northernmost tip. This facility detected the first wave of Japanese aircraft on their way to attack Pearl Harbor when the planes were still 132 miles to the west. That sleepy Sunday morning, the radar operator brought this seemingly impossible collection of hundreds of incoming planes to his superior’s attention. But because the system had been in operation for only two weeks, and because such an aggregation of aircraft was so unusual and seemed so unlikely, the supervisor dismissed the countless images on the oscilloscope as a fluke, or perhaps a flock of birds. The system worked perfectly as designed, but the critical warning was ignored.
After the war, further improvements to microwave technology resulted in the invention of Doppler radar, which allows the operator to determine the speed of an object moving toward or away from the beam of radio waves. This principle is illustrated by a speeding ambulance. As the ambulance comes closer, you’ll notice that the siren rises in pitch while the interval between each of its warbles is reduced. Then suddenly, the second after it passes you, those warbles seem stretched out and more widely spaced, and the pitch simultaneously lowers. The explanation for this effect, which applies to light as well, was first explained by the Austrian physicist Christian Doppler in 1842. It’s simple: although light always travels at a constant speed, its waves nonetheless either compress or stretch out depending on whether the light or the observer is approaching or receding. The wavelength change happens in all directions except sideways or tangentially, so that a radar gun will not be able to read a baseball’s speed if it’s traveling sideways to the instrument—from right to left, say. In the case of visible light, the effect is to make the light of approaching objects appear blue, because its waves are being crammed together, or shortened, and because blue light has shorter waves than red light. This effect is known as a blueshift.
All but half a dozen of the universe’s nearest galaxies are rushing away from our Milky Way, so they appear more red to us, the result of the famous redshift that occurs when a light source is flying away from us. Indeed, very distant galaxies, which rush away at a goodly fraction of light speed, appear more than merely reddened. Their images are shifted beyond the visible portion of the spectrum, and the galaxy only appears to us in the infrared part of the spectrum. This principle works just as well with light that was invisible from the get-go. When an object is approaching a radar antenna, the radio waves returning from it become increasingly compressed the faster it moves. Conversely, returning waves from objects moving away become increasingly elongated and have a longer wavelength and lower frequency. By measuring this frequency shift, you can pin down the speed of an object toward or away from the antenna.
These days, law enforcement uses Doppler radar to dispense traffic tickets. Coaches use it to time athletes’ running speeds and pitched baseballs, and meteorologists use it to study the detailed rain motions within a thunderstorm.
Synthetic aperture is yet another widely used modern type of radar. This involves the use of a moving radar device. We know that the larger the dish, the better the resolution, so one alternative to giant arrays of radar dishes is to mount a radar dish on a moving plane (say), which then keeps sending pulses and receiving the echoed signals from many “sending” and “receiving” locations. This in turn makes the device perform as if its dish were much bigger. This type of radar creates images of landmass formations and military targets in exquisitely fine detail, even if the objects are just a few inches across.
All these technological marvels really began with the magnetron tube, invented in the 1920s and continually improved through the early 1940s. The magnetron tube, which produces microwaves, is based on the same simple cathode-ray tube that fascinated the world in the latter half of the nineteenth century. In the twentieth century, until flat screens came along, it was an essential component of the picture tubes used in old-fashioned TVs. By introducing a powerful permanent magnet and cleverly spaced cavities into the tube, the electrons flying from the cathode are forced to change direction, which makes them emit microwaves. This shouldn’t sound too implausible if we remember that the motion of electrons is what produces every type of light there is, except for gamma rays.
Beyond all the marvelous radar breakthroughs, the microwave property that most changed our lives was discovered by accident near the end of World War II. That’s when an electronics genius named Percy Spencer enters our story.
He was born outside Boston in 1894, and his childhood wasn’t easy. His father died when he was eighteen months old, and his mother soon left him to be raised by an uncle. His uncle died when Percy was seven years old, leaving him an orphan. Spencer subsequently left grammar school to earn money to support himself and his aunt. Between the ages of twelve and sixteen, he worked from sunrise to sunset at a spool mill. Then he discovered that a local paper mill was soon to begin using electricity, a concept little known in his rural area, and he accordingly began learning as much as possible about it. His self-education was so thorough that when he applied to work at the mill, he was one of three people hired to install electricity in the plant, despite never having received any formal training in electrical engineering—not to mention never having finished grammar school. At the age of eighteen, Spencer decided to join the US Navy. He had become interested in wireless communications after learning about the wireless operators aboard the Titanic. While with the navy, he made himself an expert on radio technology by reading books, even while standing watch at night.
A quarter century later, Spencer had, astonishingly, become one of the world’s leading experts in radar-tube design thanks to his work with microwaves at a young defense company named Raytheon. As the chief of its power tube division, Spencer kept finding new ways to improve radar design and production. One of his breakthroughs enabled the company to increase its output of radar units from a dozen a day to 2,600 a day. His staff grew from fifteen employees to five thousand. By the end of World War II he had earned more than two thousand patents and was awarded the Defense Department’s highest civilian honor, the Distinguished Public Service Award.
Soon after the war ended, while inspecting one of his Raytheon laboratories, Spencer paused in front of a working magnetron tube. Suddenly he felt something soft and sticky in his pocket. His chocolate–peanut butter candy bar had begun to melt. Others had noticed this phenomenon before, but Spencer decided to investigate it full tilt. He brought in some popcorn the next day, and sure enough, his magnetron tube made it pop.
It turns out that microwave energy, though it bounces off metal, is easily absorbed by water. Thus anything with moisture in it—and this includes virtually all foods—will heat up in the presence of these invisible rays.
The first microwave oven Spencer and his Raytheon company produced weighed a third of a ton and was the size of a refrigerator. It was expensive, too, costing thousands of dollars in an era when a new car could be bought for one-tenth that amount. Only commercial kitchens and cruise ships found a use for it. N
onetheless, Raytheon hoped for more widespread adoption as they started marketing the oven under the catchy name of Amana Radarange.
As magnetron tubes got smaller and cheaper, so did microwave ovens. Home units were introduced in 1965, though they were not widely purchased until the 1980s, when plummeting prices made them irresistible. A microwave oven cost $495 in 1968, yet a sleeker model with more bells and whistles would set you back just $191 (in inflation-adjusted dollars) in 1986. Today, more than a billion microwave ovens are in use around the world.
Some wonder if food cooked in them is safe. The answer is an unambiguous yes. We know this for two reasons. First, in some places, including Japan, they’ve been in widespread home use for fully half a century. If any negative health effects were to arise, we would have seen them long ago; Japan actually has the highest longevity rate of any nation on the planet.
Second, if your food (or anything else) is hot, it simply means that its molecules have been sped up. During frying, broiling, baking, or any other cooking process, infrared radiation from a flame or an electric heating element produces the jiggling atoms. In the case of microwaves, exactly the same thing happens, but with two important and interesting differences.
Microwaves do penetrate the interior of food, but they still mostly heat from the outside in, whereas a frying pan vigorously heats the outside layers first before the heat works its way inward. The fact that microwaved food contains both hot and cool interior portions shows that it cannot simply work by an even process of convection from the exterior to the interior. Also, microwave ovens produce standing waves (persistent patterns) and repetitive microwave eddies within the oven, which are uneven. An internal “stirrer” tries to make the waves flow through the oven evenly, but this never works perfectly. The rotating tray helps, but nonetheless some parts of the food typically receive more waves than others, which results in uneven heating. Moreover, wet parts of the food heat more rapidly than dry parts. All these are culinary drawbacks, but none produces health risks, though they help explain why noted chefs rarely recommend using a microwave oven for your gourmet cuisine.