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Silencing the Bang - How does a gun silencer stop the loud sound of a gunpowder explosion? - John

As exotic as a gun silencer (or to use the more official term "suppressor") seems, it really is very similar in many ways to something we see every day: a car muffler. Both were invented by Hiram Maxim in the beginning of the 20th century. Maxim was a clever inventor who also created the first modern machine gun and tried his hand at building flying machines.

In both cases a silencer or muffler needs to take a high pressure shock wave that to our ears is a loud "bang" and lower the pressure before it gets to us. This is done by allowing the gasses inside the gun barrel (or exhaust pipe in the case of a car) to expand in a closed container.

A basic silencer that is screwed on to the end of a gun can be as simple as a large, empty can with holes at each end to allow the bullet to pass through. As the bullet travels through the silencer, the gas behind it expands into the can and the pressure is lowered. More sophisticated silencers may also have "baffles" that further suppress the sound by adding additional smaller chambers near the final exit hole.

Some of these devices also utilize water, liquid, a gel or grease inside to cool the hot gases and which will further decrease the pressure and sound. This is an effective approach, but often these materials are partly vaporized each time the weapon is fired and must be replenished after a limited number of shots. Some disposable silencers are designed to only work for a handful for rounds before they lose their effectiveness.

While silencers can lower the sound of high pressure gas coming out of the barrel of a gun, there are other sounds a weapon makes that it has little effect on. In particular, if supersonic rounds are used the bullet will break the sound barrier with a loud crack after it leaves the front of the silencer. For this reason sub-sonic rounds are often used with silenced weapons, but this reduces the range and effectiveness of the bullet.

While we often picture silencers as always being screwed onto the end of a gun, some are built right into the weapon. Many are not as effective as often seen in the movies and may not even lower the volume of a gunshot enough that the marksman can avoid wearing hearing protection. However, in many cases it is not necessary to lower the sound of the shot as much as change the character of it so that it is not easily identifiable as a gunshot. In an urban setting this allows the sound of the shot to blend it with the ambient noise.

In many countries and jurisdictions silencers are highly regulated. They are legal to own in the United States in most places, but require an expensive permit.

Stars or Galaxies? - When I look up at the night sky, how many of those stars are really stars and how many are galaxies? - John

First, let's start with defining the difference between a star and a galaxy, for those not familiar with these terms. A star is a giant ball of hydrogen gas massive enough support a fusion process that generates heat and light. Our local example is the sun. There are also dimmer white and brown dwarf stars and these are usually stars that have burned off enough of their material that they can no longer really support fusion.

A galaxy is a group of stars bound together by their gravity. A galaxy often takes the shape of a flattened, rotating disc (left). The stars are pulled into arms that give the galaxy the appearance of a whirlpool when viewed from above. Not all galaxies have this shape. Scientists speculate that galaxies with other shapes may be the result of a collision between two galaxies. Galaxies typically are composed of billions of stars. Scientists all speculate that most galaxies may have a supermassive black hole at the very center.

On a good, dark night if your vision is exceptional, you might be able to spot some 2,500 stars in the sky with your unaided eyes. However, only a handful of galaxies can be seen without binoculars or a telescope. There is, of course our own galaxy (the Milky Way) and if you live in the Southern hemisphere you may be able to spot the large and small Magellanic Clouds. In addition if you know where to look you might be able to find the great Andromeda Galaxy (M31), the Triangulum Galaxy (M33) and the Centaurus A Galaxy (NGC5128).

That's at best six compared to twenty-five hundred. And not all of those galaxies can be seen from one location on the Earth.

That, of course, doesn't mean there are not a lot of galaxies in the sky. They are just mostly too dim to be seen without a telescope.

A related question might be, are there more stars in own galaxy that galaxies in the visible universe? Current estimates put the number of stars in the Milky Way Galaxy at around 100 billion. That's a lot, but it's only a drop in the bucket when compared to the estimated number of visible galaxies: Over seven trillion.

And that's, only those galaxies we can, in principal, see with our telescopes. There may be trillions beyond the reach of our current equipment. In fact, many astronomers suspect the universe, and the number of galaxies, is infinite.

It's the Hair, Not the Humidity - I have a thermometer hanging on the wall that also has a humidity gauge. The humidity gauge is just a round strip of metal. How can a strip of metal tell how much humidity is in the air? - John

Humidity is a measure of water vapor in the air. We usually refer to it as relative humidity as it is measured as a percentage of the maximum amount of vapor the air can hold. If the humidity reaches 100 percent, it is sure to start raining as the water in the air will condense into droplets.

The device you have there is probably a mechanical hygrometer. It's not really the metal that is doing the work, but a small bundle of human hair (chemically treated to remove any oil). Hair, being organic in nature absorbs water in the air and stretches (That's why bad hair days are usually those with high humidity).

If your mechanical hygrometer is like others I've seen there is a needle that points to the level of humidity. On the shaft that turns the needle there is one spring that is spiraled around it pulling he needle back toward zero. The shaft also has a chain (or possibly some inorganic thread) wrapped around it that goes from the shaft to the center of the hair bundle. Attached to the other side of the center of the hair bundle is another, more powerful spring. The two springs pull the mechanism in opposite directions. What actually controls the position of the needle is the length of the hair. As the hair lengthens a tiny bit, it deflects under the pull of the powerful spring, and this, in turn, pulls the chain to move the needle more toward 100%

This is hard to describe, so hopefully the attached diagram will help.

Moon's Strange Orbit - Does the moon revolve the Earth directly above equator? If yes, does it mean that people in upper northern hemisphere will be see it on the horizon? - Anonymous

Our moon, unlike most moons in our solar system, does not follow a path directly above its planet's equator. Instead, our moon follows an orbital path very much closer to Earth's ecliptic plane. Earth's ecliptic is the path Earth follows as it orbits the Sun.

The earth's equator is tilted off its ecliptic by a little more than 23 degrees. This angle is what gives us the seasons as the northern hemisphere is more tilted toward the sun during the summer and away from the sun in the winter (The opposite is true for the southern hemisphere where the seasons there are reversed).

This tilt also explains why the moon traces a different path across the sky depending on the season. Like the sun, during the winter it is closer to the horizon. In fact, further north than the Arctic Circle the moon will not be visible for 14 days at a time as it passes out of sight behind the tilt of our planet for half of its orbit. Or course when it does re-emerge it rises and stays up for fourteen days (The same is true at the Antarctic Circle).

The fact that the moon orbits close to the Earth's ecliptic plane has been used as evidence against the theory that the moon was created at the same time the Earth. In this theory, most of the spinning material in the region of Earth was pulled together by gravity to form our planet, but some of pulled together to form the moon. If that was the case, however, we would expect out moon to be orbiting along the equator. The current leading theory as to the creation of the moon is that a body the size of Mars hit Earth throwing massive amounts of material into orbit. Over the course of the next century this material was drawn together by gravity to form our moon.

Please how old is the Earth? Biblically it is accurately about 6042 to 7000 years and scientifically it counts on millions. Should we believe in God's wisdom or mere knowledge of man? - Cheta A

There has been a dispute going on in some circles between some biblical fundamentalists, who argue that science is wrong about the age of the planet because the Bible says that the Earth is only 10,000 years old; and some scientists who claim that the Bible must be inaccurate because clearly the Earth is millions of years old. There are, however, a number of people who hold the views that these differences are not irreconcilable. Though I am not a theologian, I will endeavor to give you the highlights of some of these ideas.

Age/Day View - One of the major reasons that science doesn't seem to match up with the Bible is the creation story in Genesis that seems make the universe and the earth appear in only 6 literal days. This view says that the days mentioned in Genesis are not 24 hour days, but "ages." These "ages" might have lasted millions of years or even billions of years and may have also overlapped. Though some critics argue that a "day" in this context in the Bible must only be 24-hours long, others argue that this alternate interpretation is not really inconsistent with some Biblical understandings of the word "day."

At least one author, Israeli physicist and Genesis scholar Gerald L. Schroeder, argues that depending how you define "time" these days could be both 24 hours and millions or billions of years long. For more information on this idea check out his book The Science of God: The Convergence of Scientific and Biblical Wisdom.

Mature Creation - Another view is that the Earth and Universe were created in seven days 10,000 years ago, but they have been given a whole consistent history of billions of years. This idea isn't really inconsistent with other parts of the Bible. For example, Adam is created as an adult man without the usual 20 years or so need to grow from baby to mature human under the usual laws of nature. Perhaps the earth and universe were also created in a relatively short amount of time without the 15 billion years that might normally be needed for such a process.

If you accept this view then for theological reasons the world is 10,000 years old, but for purposes of science the world is some 4.5 billion years.

Some argue that this seems somewhat disingenuous of God to create a false history. However, it isn't any more dishonest then creating Atom, calling him a man, though he never went through the normal human creation process. Indeed in our own poor attempts to create worlds and universes inside computers (i.e. video games like "The Sims") we always apply this method and it never seems dishonest to us within that context.

This isn't meant to be a complete discussion on the subject - just a starting point. Debates over this subject have already filled thousands of web pages, so I recommend you take a look at what has already been written about these ideas across the internet.

Vital Vitamins - What is a "vitamin", and how can sunlight make vitamin D? - John

A vitamin is an organic compound needed by a human or animal in tiny amounts in order to stay healthy. Usually a compound is only called a vitamin when the animal is unable to make it by itself, but must get it by eating it. This means that some compounds are vitamins for some animals but not really for others. For example, vitamin D is not really a vitamin in the human diet because we create it ourselves when sunlight hits our skin. It is a vitamin for most fish, however, who must get it by eating algae (Or by eating other fish who have eaten algae). The algae in turn create when they float in shallow waters under the sun.

For many years scientists suspected that certain foods contained tiny amounts of some substances needed for health, but they didn't know what those substances were. For example, in 1749, the Scottish surgeon James Lind discovered that citrus foods helped prevent scurvy, a particularly deadly disease often suffered by sailors who did not get fresh fruit in their diet. As it turns out the sailors were not getting vitamin C - otherwise known as ascorbic acid - which is found in the fruits. Though Lind didn't exactly know what the missing ingredient was, he recommended eating lemons and limes to avoid scurvy, an idea which was adopted by the British Royal Navy and led to their nickname "Limies".

In 1881, Russian doctor Nikolai Lunin did an experiment where he gave one group of mice milk and the other group an artificial mixture of all the separate parts of milk known at that time: proteins, fats, carbohydrates, and salts. The mice that got the regular milk were fine, but those which got just the parts got sick and died. This told Lunin that there was something in the milk that science was unaware of that was needed for the mice to stay healthy. The first scientist to extract one of these micronutrients was Japanese researcher Umetaro Suzuki in 1910. He named his discovery aberic acid. It would later become known as vitamin B1.

A couple more facts about vitamins:

-The world "vitamin" is a blend of the words "vital" and "amine" where amine is a specific sort of organic compound. However, as other vitamins were found, not all turned out to be amines, but the name stuck.

-Often an animals will have to eat the vitamins they need every day because their bodies will not store the vitamins for any length of time.

Vitamin D is produced photo-chemically when ultra-violet light interacts with the substance 7-dehydrocholesterol. In the case of humans the creation of the Vitamin D takes place in the epidermis, the top layer of our skin, when light from the sun penetrates it and hits the 7-dehydrocholesterol our bodies put there. How much and how quickly you make your Vitamin D depends on how much sun light you get and the color of your skin. People with darker skin produce it more slowly than people with lighter skin.

For mammals with fur, who can't get sunlight to their skin at all, the Vitamin D is synthesized in oily secretions that are deposited onto the fur. As those oils sit on the fur and are exposed to the sun, the vitamin D is created. The animal then must lick the oils off and swallow them to get the Vitamin D into their systems.

Glowing Arthropods - Why do scorpions fluoresce under a UV light? - Warren

This is a subject that scientists don't know a great deal about, but let's start with some basic facts. Some materials when hit by a light with a wave-length shorter than humans can see will absorb that light energy and then radiate back light within the visible spectrum so when a person looks at the object, it seems to glow. This process is called fluorescence.

There is a substance in the epicuticle (sometimes called the hyaline layer) of the scorpion's exoskeleton that fluoresces when exposed to ultra-violet light. Nobody knows exactly what this stuff is but some scientists speculate it is a complex of mucosaccharides (a simple form of sugar) and proteins. Also ß-Carboline, a trytophan derivative, is known to play an important part.

Nobody really knows how the fluorescence gets there either. Baby scorpions aren't born with it and scorpions that have just molted don't have it. This has leads some people to suggest that it is either secreted by the scorpion over time, a side effect of the animal's exoskeleton as it is tanned by the sun or the result of chemical reactions as the new exoskeleton hardens. The fact that some scorpions that live their entire lives in dark caves and still fluoresce, however, leads some people to think that it unlikely to be the tanning process.

Finally we also don't know what advantage this gives the scorpion. Some have speculated that this property somehow helps the scorpion with their ultra-violet light sensitivity, but studies have shown that different levels of UV light seem to have little effect on the animal's behavior.

We do know that scorpions have had this characteristic for a very long time. This kind of fluorescence has even been seen in some of the fossils of ancient scorpions. We also know it is not unique to scorpions as some sow bugs, millipedes, centipedes, solfugids and a few beetles also will glow in ultra-violet light. We also know that with each molting the effect grows stronger so that older scorpions glow brighter than young ones. The amount a scorpion glows is also connected the particular species. Some glow brightly, others hardly at all.

Whatever it is, this characteristic has been a boon to scientists and scorpion enthusiasts. A small camping lamp can have its fluorescent bulb replaced with one that produces ultra-violet (or "black") light that will cause scorpions to glow a soft blue or green at a distance of one or two feet. This is a great aid in finding the small animals. Scientists can then easily use tongs to collect specimens and many new species have been found this way. A flashlight that produces UV light can also be useful when camping in scorpion habitats to check your sleeping back to make sure you are not crawling in with one of the tiny critters.

Air on the moon - Is it possible to channel a pipe from Earth to Moon and pump in some of earth's atmosphere so as to support free life? - Cheta A.

Construction of a pipeline from the Earth to the Moon would be a difficult and probably impossible construction problem. Though scientists think it might be feasible to build an elevator that would lift people and materials in earth orbit, the space station that the elevator would be connected to would rotate in sync with our planet so it would always be directly overhead. The moon orbits the earth once a month, however, while our planet spins every 24 hours, so a pipeline from earth to moon would quickly get twisted and tangled.

There would be little point in building one, anyway, in an attempt to pressurize the moon and give it a breathable atmosphere. The moon already has an atmosphere, (mostly created by out gassing from the underground chemical reactions) but the atmosphere is so thin it almost does not exist. The gravity of the moon, only one-sixth that of Earth is too weak to hold any significant amount of gas on the surface. Most of it drifts into space to be swept away by the solar wind.

Of course we still might have an interest in putting stations and maybe even cities on the moon. (These would probably be airtight and partly or completely underground. By putting a couple of meters of rock above the habitations you can protect life from the stray radiation that often bombards the lunar surface) If we do build underground cities we will need air for the inhabitants to breath. Rather than pipe it up, or even bring it up in large cargo spaceships, it would much more efficient to create it from materials already on the surface. There is plenty of oxygen and nitrogen (to major components of air) locked up in lunar rocks and soil. Getting these out of the rocks will require energy, but there is plenty of that on the moon. It gets lots of sunlight (no cloudy days) that can be turned into electrical power. For that reason NASA is thinking of locating the first lunar stations near the poles so they can get an almost continuous exposure to energy from the sun.

Using Magnets for Traveling Through Space - I'm wondering if it's possible to use the principles of magnetism for travel (besides Maglev). For example, could a ship with a highly focused electromagnet aim and pull itself to a planet's magnetic field, or to the heavy metal core of an asteroid? Could this same idea be used to create a flying car, by pushing or pulling off more than one point at the same time? Thanks - Maxwell

While magnets and magnetic forces are very important in present and future transportation designs, the type of arrangement you suggest - focusing a magnetic field toward a distant object to pull yourself toward it seems an unlikely mechanism to be used. The problem is that magnetic fields lose their strength very quickly over long distances. So if you attempted to build a ship using this principal to pull yourself toward to a distant object you would need an impractically large magnetic field requiring a tremendous amount of energy. You would also have the problem that your engine would be attracting every piece of ferrite material (those attracted to a magnet) within miles -the wrenches in you tool kit, you belt buckle, other ships near you etc... You vessel would soon be covered with loose ferrite objects.

A train using Maglev does not have this same difficulty. The train uses magnetism to float just above its rails (often less than an inch) so that distance is not a problem. By changing the poles on the magnets involved the train can be not only pushed upward by the magnetic field but also down the track to give the vehicle forward speed.

A magnetic flying car might be workable, but only if it was levitating above a special magnetic road. Like the Maglev train it would be limited to "flying" just a few inches above the ground.

Of course many engines used in transportation now use magnets to operate. Almost every electric motor uses magnetic fields to generate movement and some advanced space probes use magnetic fields to shoot particles out the of the back of the probe at high speeds to push the device forward.

The only example of a magnetic transportation system that I could find that was similar to your design was a satellite engine being developed in conjunction with NASA. While details of the design are limited, the engine would interact with Earth magnetic field to allow satellites to maneuver while in orbit. Last year a model of the engine exploded during testing, but the inventors of the engine think they have worked out the bugs and are hopeful that they will be able to try a test in space in the next few years.

Geostationary Satellites - Is it true that for a satellite to hold the same position over the earth it can only be over the equator? - John

The type of satellite you are talking about is called a geostationary satellite and the idea for it was first proposed by Herman Potonik, a Slovenen rocket engineer, in 1928. Most people connect the idea, however, with famed science fiction writer Arthur C. Clarke. Clarke wrote an article about the idea for Wireless World in 1945.

The speed with which a satellite in orbit circles the Earth is dependent upon how high above the Earth's surface it is. Objects in low Earth orbit circle the globe much faster than those in higher orbits. For example, the space shuttle orbits the earth at a height of between 115 and 380 miles and will circle the Earth about 16 times in a 24 hour period. If an object is placed in orbit at a much higher level, say 22,300 miles, it will circle the globe only once in a 24 hour period. This makes it the object a geosynchronous satellite orbiting at the same rate the planet turns.

However, unless the satellite is also in an orbit over the Earth's equator, it will appear to move back and forth in the sky along a north to south line during the course of the day. To be a geostationary satellite the object needs to be in a circular orbit directly over the equator at the height of 22,300 miles (This is sometimes refered to as the "Clarke orbit"). Only then will it appear to be fixed in a single location in the sky.

There are many uses for geostationary satellites including communications (for example, the dish television broadcast satillite I get my TV on) and weather observation. Since they do not move in the sky, geostationary satellites allow receivers on the ground to use a simple fixed antennal to point to them and pickup broadcasts. Because the satellites are over Earth's equator, however, any northern hemisphere location wishing to point an antenna at them must have a clear view of the southern sky. The opposite is true in the southern hemisphere.

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