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10 Most Recent Questions and Answers (out of 33)

1594. Are smart meters bad for people’s health? Is this actually not knowable at this time? -- ED
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If by smart meters you mean the devices that monitor power usage and possibly adjust power consumption to periodically, then I don't see how they can affect health. Their communications with the smart grid are of no consequence to human health and having the power adjusted on household devices is unlikely to be a health issue (unless they cut off your power during a blizzard or a deadly heat wave).

The radiated power from all of these wireless communications devices is so small that we have yet to find mechanisms whereby they could cause significant or lasting injury to human tissue. If there is any such mechanism, the effects are so weak that the risk associated with it are dwarfed by much more significant risks of wireless communication: the damage to traditional community, the decline of ordinary human interaction, and the surge in distracted driving.


1573. If there was a taught cable, wire, or string 5 light years long and I pulled on one end, would the other end move instantly or would the wire, string, or cable lengthen as it tried to pull the other side towards it? — SB
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The cable would indeed lengthen when you pulled it. In fact, you would produce a wave of stretching motion that travels along the cable at the speed of sound in that cable. That's because you can't directly influence the cable beyond what you can touch. You can only pull on your end of the cable, causing it to accelerate and move, and let it then pull on the portion of cable adjacent to it.

Each portion of cable responds to being pulled by accelerating, moving, and consequently pulling on the portion of cable adjacent to it. There will be a long series of actions—pulling, accelerating, moving, and pulling again—that propagates your influence along the cable. A wave will travel along the cable, a wave consisting of a local reduction in the cable's density. It's a stretching wave. In that respect, the wave is a type of sound wave—a density fluctuation that propagates through a medium.

How quickly the density wave travels along the cable depends on how stiff the cable is and on its average mass density. The stiffer the cable, the more strongly each portion can influence its neighboring portions and the faster the density wave will travel. The greater the cable's mass density, the more inertia it has and the slower it respond to pulls, so the density wave will travel slower.

A cable made from a stiff, low-density material carries sound faster than a soft, high-density material. A steel cable should carry your wave at about 6100 meters/second (3.8 miles/second). But a diamond cable would reach 12000 meters/second (7.5 miles/second) because of its extreme stiffness and a beryllium cable would approach 13000 meters/second (8.0 miles/second) because of its extremely low mass density.

Regardless of which material you choose, you're clearly not going to be able to send any signals faster than the speed of light. It would take a density wave more than 100,000 years to travel the 5-light year length of your cable. And sadly, friction-like dissipation effects in the cable would turn the density wave's energy into thermal energy in a matter of seconds, so it would barely get started on its journey before vanishing into randomness.


1568. What does a radio wave consist of? Is it any gas? I want to know what is the material that is carrying the data? — S, India
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Unlike sound waves or ocean waves, radios waves do not travel in a material. Radio waves are a class of electromagnetic waves and consist of nothing more than electric and magnetic fields. Since they don't require any medium through which to travel, they can go right through empty space. That's why we're able to see the stars, after all.

The idea of a wave that travels through space itself was a rather disorienting notion to scientists in the late 1800s. They were used to the idea that waves are disturbances in a tangible material or "medium": fluctuations in the density of air, ripples on the surface of water, vibrations of a taut string. Having observed that light and radio waves are electromagnetic waves, they set about looking for the medium that supported those waves. They were expecting to find this "luminiferous aether" but they failed. In fact, the absence of an aether led in part to Einstein's theory of special relativity.

The structure of a radio wave, or any electromagnetic wave, is quite simple. It consists only of a fluctuating electric field and a fluctuating magnetic field. An electric field is a structure in space that affects electric charge; it pushes on charge and causes that charge to accelerate. Similarly, a magnetic field is a structure that affects magnetic pole. Remarkably, changing electric fields produce magnetic fields and changing magnetic fields produce electric fields. That interrelatedness allows the wave's fluctuating electric field to produce its fluctuating magnetic field and vice verse. The wave's electric and magnetic fields endless recreate one another. Although electric charge or magnetic pole is needed to emit or receive a radio wave, that wave can travel perfectly well for billions of light years without involving any charge or pole. It travels through space itself.


1546. How can light "travel" through a vacuum when there were no "particles" in the vacuum on which it could "transmit" its charge? — DC
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Light has no charge at all. It consists only of electric and magnetic field, each endlessly recreating the other as the pair zip off through empty space at the speed of light.

The fact that light waves can travel in vacuum, and don't need any material to carry them, was disturbing to the physicists who first studied light in detail. They expected to find a fluid-like aether, a substance that was the carrier of electromagnetic waves. Instead, they found that those waves travel through truly empty space. One thing led to another, and soon Einstein proposed that the speed of light was profoundly special and that space and time were interrelated by way of that speed of light.


1513. Can infrared lasers, thermal cameras, digital cameras, or optical fiber cameras be used to see through walls of homes or to monitor people's conversations? — CB, Connecticut
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I'm beginning to think that movies and television do a huge disservice to modern society by blurring the distinction between science and fiction. So much of what appears on the big and little screen is just fantasy.

The walls of your home are simply hard to look through. They block visible, infrared, and ultraviolet light nearly perfectly and that doesn't leave snoopers many good options. A person sitting outside your home with a thermal camera—a device that "sees" the infrared light associated with body-temperature objects—or a digital camera is going to have a nice view of your wall, not you inside. There are materials that, while opaque to visible light, are relatively transparent to infrared light, such as some plastics and fabrics. However, typical wall materials are too thick and too opaque for infrared light to penetrate. Sure, someone can put a camera inside your home and access it via an optical fiber or radio waves, but at that point, they might as well just peer through your window.

The only electromagnetic waves that penetrate walls well are radio waves, microwaves, and X rays. If someone builds an X ray machine around your home, they'll be able to see you, or at least your bones. Don't forget to wave. And, in principle, they could use the radar technique to look for you with microwaves, but you'd be a fuzzy blob at best and lost in the jumble of reflections from everything else in your home.

As for using a laser to monitor your conversations from afar, that's a real possibility. Surfaces vibrate in the presence of sound and it is possible to observe those vibrations via reflected light. But the technical work involved is substantial and it's probably easier to just put a bug inside the house or on its surface.

Since I first posted this answer, several people have pointed out to me that terahertz radiation also penetrates through some solid surfaces and could be used to see through the walls of homes. In fact, the whole low-frequency end of the electromagnetic spectrum (radio, microwaves, terahertz waves) can penetrate through electrically insulating materials in order to "observe" conducting materials inside a home and the whole high-frequency end of that spectrum (X-rays and gamma rays) can penetrate through simple atoms (low atomic number) in order to "observe" complex atoms inside a home. Still, these approaches to seeing through walls require the viewers to send electromagnetic waves through the house and those waves can be detected by the people inside. They're also not trivial to implement. I suppose that people could use ambient electromagnetic waves to see what's happening in a house, but that's not easy, either. Where there's a will, there's a way: stealth aircraft have been detected by way of the dark spot they produce in the ambient radio spectrum and the insides of the pyramids have been studied by looking at cosmic rays passing through them. Nonetheless, I don't think that many of us need worry about being studied through the walls of our homes.


1386. Is a CB radio also an AM radio?
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CB or citizens band radio refers to some parts of the electromagnetic spectrum that have been set aside for public use. You can operate a CB radio without training and without serious legal constraints, although the power of your transmitted wave is strictly limited. The principal band for CB radio is around 27 MHz and I think that the transmissions use the AM audio encoding scheme. As you talk, the power of your transmission increases and decreases to represent the pressure fluctuations in your voice. The receiving CB radio detects the power fluctuations in the radio wave and moves its speaker accordingly.

1312. Is it possible to track a person based on the fact that they are listening to a radio receiver? — BRAR, India
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While tracking a radio transmitter is easy—you only need to follow the radio waves back to their source—you might think that tracking a radio receiver is impossible. After all, a radio receiver appears to be a passive device that collects radio waves rather than emitting them. But that's not entirely true. Sophisticated radio receivers often use heterodyne techniques in which the signal from a local radio-frequency oscillator is mixed with the signal coming from the antenna. The mixing process subtracts one frequency from the other so that antenna signals from a particular radio station are shifted downward in frequency into the range the radio uses to create sound. This mixing process allows the radio receiver to be very selective about which station it receives. The receiver can easily distinguish the station that's nearest in frequency to its local oscillator from all the other stations, just as its easy to tell which note on a piano is closest in pitch to a particular tuning fork.

But heterodyne techniques have a side effect: they cause the radio receiver to emit radio waves. These waves originate with the local radio-frequency oscillator, and with other internal mixing frequencies such as the intermediate frequency oscillator present in many sophisticated receivers. Because these oscillators don't use very much power, the waves they emit aren't very strong. Nonetheless, they can be detected, particularly at short range. For example, it's possible for police to detect a radar detector that contains its own local microwave oscillator. Similarly, people who have tried to pirate microwave transmissions have been caught because of the microwaves emitted from their receivers. In WWII, the Japanese were apparently very successful at locating US forces by detecting the 455 kHz intermediate frequency oscillators in their radios—a problem that quickly led to a redesign of the radios to prevent that 455 kHz signal from leaking onto the antennas (thanks to Tom Skinner for pointing this out to me). As you can see, it is possible to track someone who is listening to the right type of radio receiver. However, the radio waves from that receiver are going to be very weak and you won't be able to follow them from a great distance.


1247. I work finding sites for cellular & PCS wireless telephone antennae. I would like to know how radio waves work and how they are able to carry voice and data information. What are these waves and do they exist naturally or do we set them up using electric charges? — PAB, Madison, WI
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Radio waves are a class of electromagnetic waves, specifically the lowest frequency, longest wavelength electromagnetic waves. Actually, the electromagnetic waves used in cellular & PCS transmissions are technically known as microwaves because they have wavelengths of less than 1 meter, but there are no important differences between radio waves and microwaves.

Like all electromagnetic waves, radio waves and microwaves consist of coupled electric and magnetic fields that sustain one another in stable structures that move rapidly through empty space. Because an electromagnetic wave's electric field changes with time, it is able to create the wave's magnetic field and, because its magnetic field changes with time, that magnetic field is able to create the wave's electric field. Since they consist only of electric and magnetic fields, these waves cannot stay still—they must move (although you can trap them between mirrors so that they appear to stand in one place as they bounce back and forth). While they contain no true mass, they do contain energy and an electromagnetic wave carries energy from one place to another.

Electromagnetic waves are created whenever electrically charged particles change speed or direction; whenever they accelerate. Since there are accelerating electric charges everywhere—thermal energy keeps them moving about—there are also electromagnetic waves everywhere. But the radio waves used in communications systems are generated deliberately by moving electric charges back and forth. When charges are sent up and down a radio antenna, these charges are accelerating and they form complicated electric and magnetic fields that include electromagnetic waves. Once launched, those electromagnetic waves propagate through space at approximately the speed of light.

To send information with radio waves, a transmitter makes modifications in one or more the wave's characteristics. In an amplitude modulation scheme (AM), the transmitter changes the strength or "amplitude" of the wave to convey information—like sending radio smoke signals. In the frequency modulation scheme (FM), the transmitter changes the frequency of the wave to convey information—like whistling a tune with a complicated melody.


1192. Could you explain the meaning of polarization in optics? Please try to associate it with water waves if possible, to help me visualize it, and avoid the use of electric and magnetic fields. — AM, Yavne, Israel
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I can't completely avoid electric and magnetic fields because polarization in optics is associated with a wave's electric field. I also can't depend entirely on water waves because they only have one (transverse) polarization. Still, I will try.

First, consider a wave traveling toward us on the surface of a lake. Suppose that this wave passes under a small boat and I ask you which way the wave is making the boat move. You would tell me that the boat is moving up and down. I would then tell you that the wave is vertically polarized because it causes objects that it encounters to move up and down rhythmically.

Unfortunately, pure water won't do for the next step because it won't support horizontally polarized waves. So let's imagine that some ecological disaster has turned the entire lake into gelatin. An explosion at the side of the lake now causes a wave to begin heading toward us on the gelatin lake, but this strange wave involves a side-to-side motion of the lake's surface. Now when the wave passes under the boat, the boat moves side-to-side rhythmically. In this case the wave is horizontally polarized because it causes objects that it encounters to move left and right rhythmically.

Now let's return to optics. When an electromagnetic wave heads toward us, its electric fields will push any electrically charged particles it encounters back and forth rhythmically. If we watch one of these charged particles as the wave passes it and observe that this particle moves up and down, then the wave is vertically polarized. If instead the charged particle moves left and right, then the wave is horizontally polarized.


1155. How can I build an AM radio?
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That's a very open ended question so I'll describe the simplest AM radio I can think of—a crystal radio. A crystal radio already addresses most of the issues of AM radio and more sophisticated AM radios just improve on its performance.

You need only four basic components for a crystal radio: an antenna, a tank circuit, a diode, and a high-impedance earphone.

The antenna is a long wire that projects upward into the electromagnetic fields of the passing radio wave so that electric charges begin to move up and down its length. The ideal length for this wire is a quarter of the wavelength of the wave you're trying to receive, but since that's hundreds of meters for a typical AM station, you'll have to settle for a shorter than ideal antenna.

The tank circuit is a coil of wire that's connected at each end to the two ends of a capacitor. In a typical crystal radio, one of these items—either the coil or the capacitor—is adjustable and forms the tuning element that allows you to select a particular AM station. The tank circuit is a resonant device—electric charges and current flow back and forth through it rhythmically at a specific frequency. If that resonant frequency is adjusted so that it coincides with the transmission frequency of an AM radio station, the small currents flowing in the antenna that's connected to the tank circuit will excite large movements of charge and current in the tank circuit.

The diode is also connected to the tank circuit. Its job is to extract some of the charge that oscillates back and forth in the tank circuit and to send that charge to the earphone. By allowing current to flow only in one direction, the diode samples the overall amount of charge moving in the tank circuit. What it passes to the earphone is a measure of how strong the radio wave is, which is actually the form in which the AM radio station is transmitting sound information.

The high-impedance earphone uses the diode's tiny charge deliveries to reproduce sound. The diaphragm inside the earphone moves back and forth as the amount of charge passing through the diode fluctuates up and down. Each time the radio wave increases in strength, the diaphragm moves in one direction. Each time the radio wave decreases in strength, the diaphragm moves in the other direction. Thus as the radio station varies the strength of its radio wave, the earphone's diaphragm moves back and forth and it reproduces the sound.


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