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841. How does a fax machine send written words over telephone wires? — AM, Halifax, CA
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The fax machine uses a row of optical sensors to detect dark and light spots on the original document. It scans the document one line at a time and enters the pattern of dark and light spots into a digital controller or simple computer. The controller or computer than encodes this pattern, together with enough information to correct minor transmission errors if they occur, as a series of numbers. The numbers are then sent through the telephone system in much the same way that computer information is sent through the telephone wires by a modem. The numbers becomes specific patterns of tones and volumes. While the electric currents flowing through the telephone system are meant to represent voice sounds, they can do a moderately good job of representing numbers instead. Because of various limitations on the currents that the phone wires can carry well, the fax system can only so much information each second. The receiving fax machine analyzes the tones and volumes it receives over the telephone wires and recreates the pattern of dark and light spots. It then uses one of several printing techniques to reproduce that pattern on a piece of paper. It recreates the document one line at a time.

842. You mentioned that time perception is different for different locations in the universe. Were could we find a place where one day is equal to one thousand years of time on earth? — AWG, Karachi, Pakistan
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The perception of time is different for observers who are in motion relative to one another. The issue is not how far away they, it's how fast they are moving relative to one another. If you were to observe a person who is traveling past the earth at almost the speed of light, you would notice that their watch is running extremely slowly. It might be as though you'd have to wait one thousand years for their watch to show that a day has passed for them. Yet paradoxically, they would make the same observation about you! You would see them aging slowly and they would see you aging slowly! The resolution to this apparent paradox lies in the differences in the perceptions of space that these differences in the perceptions of time. In this short answer, I can hardly begin to resolve the paradox. I'll simply point out that the mixing of space and time associated relativity are caused by relative motion not by relative position.

852. I have read about how black holes can emit X-rays and radiation. If they absorb light, why do they emit these other things? — BA, Fairbury, IL
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A black hole is surrounded by an imaginary surface called the event horizon. Nothing at all can escape from within this surface-not light, not X-rays...nothing! However, as matter falls into the black hole, and before it reaches the event horizon, the matter can emit any type of radiation it likes. The X-rays and radiation emitted "from a black hole" are actually coming from the area surrounding the event horizon, not from within that surface. As matter pours into a black hole, it often heats up so hot that it emits incredible amounts of radiation of all types so that black holes appear as very bright objects.

853. Is the jet stream flowing in the same direction in the southern hemisphere as it is in the northern hemisphere? — LS, Ashland, OR
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The jet streams flow eastward in both hemispheres. Their directions of flow are determined by the Coriolis effect, in which high-altitude winds that are heading away from the equator veer eastward because of their angular momentum on the spinning earth.

871. Einstein's famous equation E=mc2 says that mass is directly proportional to energy. Does this mean that an object that is suspended overhead has more mass than an object located at ground level? — ST, Denver, CO
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Yes, the mass/energy of a suspended object is greater than the mass/energy of that same object at ground level. The extreme example of this result comes with lowering an object slowly toward the surface of a black hole—as the object descends, its mass/energy diminishes until it reaches zero at the surface of the black hole.

872. What is a vacuum? Is it filled with charges with no mass? — AW, Karachi, Pakistan
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In principle, a vacuum is a region of space containing no real particles (no atoms, molecules, electrons, or other subatomic particles). Because the universe is filled with particles that pass easily through lots of matter (neutrinos, for example), it's very hard to obtain a true vacuum. But let's suppose that you could actually obtain a region of space with no real particles in it. That region of space would still contain large numbers of virtual particles at any given moment. These virtual particles are temporary quantum fluctuations of the vacuum; brief excursions of the quantum fields associate with various subatomic particles. These excursions are permitted by the Heisenberg uncertainty principle, which allows temporary violations of the conservation of mass/energy as long as those violations are extremely brief. While the presence of these virtual particles can only be detected indirectly, they are not massless. Except for their short lifetimes, these particles have characteristics similar to those of normal particles. In fact, if enough energy is used in the process of looking for a virtual particle, that virtual particle can be converted from virtual to real so that it can be detected directly. The energy of detection serves to "pay" for the mass of the particle so that it can leave the virtual realm and become a real, permanent particle.

899. Why does an artificial sponge absorb more water than a natural sponge? — JH, Angleton, TX
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Water is drawn into a sponge in part because of an attraction between the water molecules and the sponge's surface and in part because of water's tendency to minimize its own surface area. When you put a drop of water on a waxy surface, the water beads up. That's because water and wax don't bind well to one another and the water molecules pull toward one another instead. The water droplet tries as best it can for form a sphere, since a sphere has the smallest surface area that a given volume of water can occupy. These forces that pull water's surface inward are called surface tension.

But when you put a drop of water on real cellophane (a smooth form of cellulose), the water spreads out. That's because water and cellulose bind strongly to one another and the water will permit its surface area to increase somewhat if that increase allows it to attach to more cellulose. Similarly, water binds well with other forms of cellulose, including paper, cotton, and Rayon. I think that most artificial sponges are either cellulose or a close chemical relative of cellulose.

A sponge absorbs water by allowing that water to cling to an extensive surface that binds well with water. The water spreads out along that surface while trying to minimize the surface area of any water that isn't touching the sponge. The surface of a natural sponge interacts well with water (the sponge lives in water after all), but a natural sponge can't compete with modern technology. A company that makes artificial sponges can adjust the chemical structure of the sponge's plastic so that it binds nicely to water molecules; it can adjust the sizes of the holes in the sponge to attract the water as efficiently as possible with a given mass of plastic; and it can tailor wall thickness to give the sponge the right elasticity. Furthermore, some of the water is brought right into the plastic and that water softens or "plasticizes" the plastic. That's why a sponge is hard when dry and soft when wet—the water molecules are effectively lubricating the plastic molecules so that they can slide past one another.


906. How do record players and their needles work? - JW
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As a phonograph record turns, the needle of its playing arm slides through a narrow spiral groove on the record's surface. This groove is cut with a 90° angle at its bottom and both of its sides have undulations in them. As the needle slides through the groove, it rides up and down on these undulations. The needle's movement causes currents to flow in two separate pick-ups that are attached to the needle. One pick-up responds to needle motions caused by the right edge of groove and the other pick-up responds to needle motions caused by the left edge of the groove. The physical mechanism for converting needle motion into electric current depends on the needle cartridge—it can involve moving magnets, moving coils of wire, or squeezed piezoelectric crystals. Since the groove undulations represent air pressure fluctuations at the right and left microphones during recording, the currents from the two pick-ups represent those pressure fluctuations during playback. With the help of amplifiers and speakers, these currents are used to reproduce the sounds that were recorded at the two microphones.

914. How can one tell the difference between a gravitational red shift of light and a red shift caused by motion? Could the red shift of quasars be from gravity and not speed, therefore making the quasars closer than we think they are? - FG
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At astronomical distances, there is no way to tell the difference between the two red shifts. An object that is deep in the gravitational potential well of a very massive object experiences time slowly and its light appears shifted toward the red (low frequency and long wavelength) when it reaches us. The light from an object that is moving away from us rapidly also appears red shifted (low frequency and long wavelength), but this time it's due to the Doppler effect.

Quasars exhibit enormous red shifts and one explanation for those red shifts is that the quasars are located near the other side of the universe. If so, they would be moving away from us rapidly, along with their surroundings in the expanding universe, and their light would appear highly red shifted. Moreover, their light would have been traveling almost since the beginning of the universe so that we would be observing very ancient objects. However, it's also possible that quasars are much near to us and that their red shifts are caused by gravitational effects rather than relative motion. As far as I know, this possibility can't be ruled out and remains a concern amount the astronomical community.


932. What is gravity? We know Newton's formula but he did not answer what the true nature of gravity is. I hear talk about "gravitons" — is this real or just another elegant metaphor? — BC
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Newton's gravity has been superceded by Einstein's gravity; the gravity of general relativity. In this understanding of gravity, the accelerations associated with gravity result from a curvature of space/time around concentrations of mass & energy. The gravity of general relativity is responsible for such exotic effects as the bending of light by gravity and the existence of black holes.

But physicists are still not satisfied with the gravity of general relativity. General relativity is what's known as a "classical" theory of interactions—it does not include quantum physics and is thus considered to be incomplete. All the other classical theories of interactions have given way to quantum theories. For example, the classical theory of electromagnetic interactions, dating from the works of Oersted, Ampere, Maxwell and others in the 1800's, was replaced in the 1940's and 50's by quantum electrodynamics, through the works of Feynman, Schwinger, Tomonaga, and others. Each time that a classical theory is replaced by a quantum theory, the responsibility for the interactions themselves shifts from classical fields (e.g., the electric and magnetic fields) to quantized or particulate fields (e.g., photons). These sorts of quantum field theories, theories in which interactions between particles are mediated by the exchanges of other particles (the particles of the quantized fields) are the bases for all modern interaction theories except gravity itself. People are still trying to quantize gravity but so far without real success. The particles that mediate gravitational interactions have been named gravitons, but the full theory in which these particles operate is still uncertain.


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