Paul A. Heckert's Blog

Posted by Paul A. Heckert

The Olympics are here again. You might not think that physics and the Olympics are related, but most sports involve physics in some way. The combination of human anatomy and the laws of physics determines the proper form needed to produce the best performance in any sport.

Gymnasts and divers doing various spinning maneuvers follow laws of physics such as conservation of angular momentum. Bringing their bodies close to the spin axis will cause them to spin faster; stretching out will slow their spin rates. The gymnastic iron cross maneuver on rings requires considerable arm strength because of the force vectors involved. Muscles pulling nearly horizontally must provide a significant vertical force.

Water produces more friction than air, so the new swimsuits that reduce the effect of drag in water might help some swimmers set new records. Attaining maximum speed with a swimming stroke requires maximizing the push in the water during the stroke portion and minimizing it in the return portion - all physics.

Many sports require throwing, hitting, or kicking a ball. Here the physics of projectile motion applies. For example, in the absence of air friction, the maximum range is attained with a launch angle of 45 degrees. Air resistance will affect that angle some.

Coaches and physical educators study kinesiology which is the science of physics applied to the motions of the human body. This knowledge, when properly applied, can help athletes achieve their maximum performance.

Posted by Paul A. Heckert

A few weeks ago I did the running leg of a triathlon relay. My wife, who did the bicycle portion thought the headwind slowed her down, but when running I was hoping for some wind to provide a cool breeze.

If you watched the Tour de France, or any other major bicycle race, you may have noticed that sometimes riders will deliberately ride close behind another rider. This practice, called drafting, is designed to save energy. By letting the front rider take care of the air resistance, the back rider can rest a bit. However in major marathons and other running races, the runners don't draft each other to save energy by blocking air resistance.

Air or wind resistance is more of a problem when bicycling than when running. Why?

The physics of air resistance is quite complex. The air resistance depends heavily on the shape of an object. Is it aerodynamic? Air resistance also depends strongly on the speed an object is moving.

Ignore the minor difference in shape. A bicycle racer is moving much faster than a competitive runner, so air resistance plays a bigger role. A bicycle rider must exert more energy fighting air resistance than a more slowly moving runner. Therefore drafting can save a bicyclist a significant amount of energy, but it will not significantly help a runner. The one time it can help a runner is when running into a strong headwind. The headwind increases the runner's speed relative to the air and therefore the runner's air resistance. Then a runner can save energy by letting another runner block the wind.

The basic physics of air resistance makes drafting common for bicyclists but not runners.

Posted by Paul A. Heckert

Neil Armstrong took his small step for man and giant leap for mankind 39 years ago on July 20, 1969. A lot of fuss will be made next year on the 40th anniversary, so I thought that I would beat the rush and celebrate the 39th anniversary.

Walking on the Moon must be the greatest adventure walk anyone has ever taken. I recall as a teenager that I stayed up quite late to watch Neil Armstrong take his first step on the Moon. The day after the landing radio stations played songs like "Everyone's Gone to the Moon" and "Fly me to the Moon". People were very proud of our national achievement and enjoyed the greatest vicarious adventure in history.

Debates on the pros and cons of manned space flight usually focus on the economic or scientific impact of what we will find or learn. I think however that the most important reason for manned (or womanned) space flight is the impact on the human spirit.

We all need seemingly impossible goals and challenges. When we meet our goals, we are ecstatic and proud. Our nation and world also need goals that will unite all mankind in pride when we achieve the goal. If we return to the Moon or explore Mars in person, and it accomplishes nothing else that could not be done by robotic spaceships, the impact on the human spirit of reaching such a goal will be enough.

President Kennedy said that we choose to go to the Moon because it is hard. That is still true today. We should continue to explore the Moon and beyond because it is hard.

Posted by Paul A. Heckert

I blogged about the good and bad physics in Indiana Jones and the Kingdom of the Crystal Skull. The really ugly physics has to be surviving a nuclear blast in a refrigerator. Is your refrigerator lead lined to protect food and occupants from radiation? Mine isn't. The one our audacious archeologist used was. The lining must have been thin. With walls no thicker than a normal refrigerator, most of the wall would be thermal insulation to keep food cold. The amount of radiation protection a lead lining provides depends on the thickness of the lead. A nuclear blast produces lots of radiation requiring a thick lead lining. I didn't do the math, but I'm guessing that the lead lining would not be thick enough to protect Indiana from a nuclear blast.

Nuclear blasts also produce a lot of heat. Refrigerators are insulated, but not that much. Any food, or occupants, are more likely to be cooked than still cold.

Then there is the landing. Indiana Jones was blasted into the air. We see the refrigerator and miraculously cool archeologist fall and bounce. Neither the refrigerator nor the ground appear dented. The impulse momentum law in physics says the change in momentum equals the force times the time. Falling and bouncing back represents a large momentum change. With no denting, the force was applied rapidly. Hence, the force has to be very large to change the momentum. The occupant of a refrigerator falling from a large height would not likely survive.

Of course most of us would not survive most of the scenes in Indiana Jones movies, so turn off the physics parts of your brain and enjoy the ride.

Posted by Paul A. Heckert

My last blog was about the good physics in Indiana Jones and the Kingdom of the Crystal Skull. Now here is some of the bad physics.

Near the beginning they were looking for a crate in a huge secret warehouse. Indiana Jones said it was magnetic and used gunpowder and lead shot pellets from shotgun shells to allow them to be attracted to the magnetic crate. Gunpowder is a mixture of sulfur, charcoal, and potassium nitrate. Shotgun pellets are usually made of lead. None of these materials are magnetic. They would not be attracted to the supposedly magnetic crate. However steel rifles, belt buckles, and other common steel items would be.

During the extended chase scene, there are two vehicles driving side by side and our heroes are fighting and standing with one foot on each vehicle. Later they go over several water falls and survive. If the two vehicles are traveling at the same velocity, an extremely coordinated person could stand with one foot on each, but as soon as one of the vehicles changes its velocity our heroes go tumbling. The opposing driver just needs to change velocities a little. A few people have survived trips over Niagara Falls, but most are killed. So, these events are of course highly improbable but do not actually violate the laws of physics. If everything happens perfectly right these events could happen.

Next the really ugly physics.

Posted by Paul A. Heckert

Indiana Jones and the Kingdom of the Crystal Skull was, like other Indiana Jones movies, fun but not realistic. Hollywood movies usually get physics completely wrong, so when I go to movies I usually just turn off the physics portion of my brain and enjoy the story.

There was one scene in this movie, however, where they got the physics right. In one of the many death and credibility defying escape/chase scenes the old professor was riding in the back holding the precious crystal skull. They went over a bump and his treasure flew vertically upward. When it fell back down it landed right in his lap. That may seem unrealistic but the physics was exactly right. It was perhaps the most realistic portion of the chase and certainly more realistic than a sixty something archeologist doing all those death defying stunts.

Why should the skull fall right back into the old professor's lap? A fundamental principle of two dimensional motion is that the vertical and horizontal motions are completely independent of each other. Vertical motions have no effect on the horizontal motion and vice versa. When you are driving in a car and throw something straight up, the change in its vertical motion has no effect whatsoever on the horizontal motion. The horizontal speed remains the same as the vehicle's speed, so relative to the car and passengers the object seems to go straight up and down. As long as the driver does not accelerate, brake, or turn it will fall back down right on top of you. (neglecting wind effects)

The skull therefore falls right back into the old professor's lap in a rare, for an Indiana Jones movie, realistically possible scene.

Posted by Paul A. Heckert

I recently read in an article that vending machines consume 3000 kilowatts per year. The article was about installing devices that turn off the electricity consumption when people aren't around. That sounds like a good idea, but the statement is technically incorrect.

Kilowatts is a unit of power consumption, so the time is already included. So any statement that specifies the amount of time for a watt or a kilowatt must be incorrect.

Energy is measured in joules. Power is the energy divided by the time and is measured in watts. A watt is a joule per second. So a machine, vending or otherwise, that has a power consumption of 3000 kilowatts uses 3000 kilojoules of energy every second. But saying it uses 3000 kilowatts every year makes no sense. It's like saying that a car drives a total distance of 60 miles per hour every year.

To me, 3000 kilowatts sounds high for a vending machine. The main power consumption would be a few light bulbs and a refrigerator to keep the drinks cold.

Perhaps it is 3000 kilowatt hours per year, which when divided by the number of hours in a year gives a power consumption of almost 350 watts, which could power a few light bulbs and a small cooling unit. Multiplying a power unit, kilowatts, by a time unit, hours, gives an energy unit, kilowatt hours. Physicists seldom measure energy in kilowatt hours, but power companies usually do. The total number of kilowatt hours used per year makes sense.

Errors such as this one in science articles make it difficult for technically minded people to figure out the real details of the story.

Posted by Paul A. Heckert

I just finished my daily run. I didn't get moving as early as I should have this morning, so it was about 90 degrees F by the time I finished. With the heat wave in the eastern US, it is difficult to run or do other outdoor exercise.

The reason hot weather exercise is so difficult boils down to basic physics, specifically the laws of thermodynamics. The first law of thermodynamics says energy is conserved. It can change form but can not just disappear. The second law of thermodynamics says that no machine or process can be 100% efficient. There is waste energy which is converted to heat.

Applying these ideas to the exercising or working human body tells us that muscular processes are less than 100% efficient. Working or exercising muscles generate waste heat. In the winter this warms us to the point that it is possible to run through snow wearing shorts. In the summer the waste heat makes it difficult to keep our bodies cool while exercising. Therefore basic physics means that runners and other outdoor exercisers have a difficult time cooling their bodies.

If like me you run or do other outdoor exercise in hot weather, familiarize yourself with the symptoms of heat stroke and take precautions to keep your body cool. You can't violate the laws of physics.

Posted by Paul A. Heckert

The recent Phoenix lander on Mars is the first successful rocket soft landing on Mars since the Viking landers in 1976. The other intervening missions either landed inside inflated airbags or failed while landing.

Shortly after the Viking landed, I went to a talk by one of the Viking experimenters. He started the talk with a slide of a laboratory housing the equipment needed to do his experiments on Earth. The equipment filled a good sized room, consumed vast amounts of power, needed ideal conditions to function, and so forth.

The assignment from NASA was to design an instrument that could make all the same measurements, run on the equivalent of a flashlight battery, fit into a tiny package, and survive both the shock of launch and rigors of interplanetary space. Every experimenter on the Viking mission and most similar space missions has had the same assignment. Space exploration requires miniature electronics to minimize launch weight.

When I went home to relax with some music after the talk, I pulled out a 12 inch vinyl disk, containing perhaps a half dozen songs, and played it on a stereo that filled most of my living room wall. Now we can listen to music on Ipods that can store more songs than most of us have ever heard, yet are so small they are constantly in danger of being lost.

Much, but of course not all, of the early motivation to miniaturize electronic devices came from the space program's need to minimize launch weight. Once NASA learned techniques to miniaturize electronics, others applied these techniques to design music players, laptop computers and other tiny electronic marvels.

When you listen to your Ipod or use your laptop, thank the space program for getting the ball rolling.

Posted by Paul A. Heckert

Mars, the blood red god of war, has dominated our evening skies for the past several months, but it has dominated our collective imaginations for much longer.

As the latest chapter in our long term quest to explore Mars, NASA just landed the Phoenix mission on Mars' polar ice cap. The focus of the Phoenix lander and indeed much of our Mars exploration program is to answer one question. Is there or has there ever been life on Mars? Phoenix is not equipped to detect life directly, rather it will study the possibility of liquid water.

Why should we care?

We live in a universe of unimaginable vastness. Are we alone in this universe, or is it teeming with life? Based simply on the vastness of the universe, most astronomers, including me, think that we could not possibly be alone. But, we have so far found not one shred of credible evidence that there is life someplace else in the universe. This question is therefore still unanswered.

It would be less lonely if Earth life has a companion out there, so we are looking. Mars is the closest moon or planet with a reasonable possibility of life. Jupiter's moon, Europa, is also a likely candidate, but it is farther away and more difficult to get to. So we start by exploring Mars.

We still don't know if life is something that forms easily or only very rarely. If we find evidence of life or past life on Mars, then life probably arises easily and is common in the universe. If we find no such evidence, then on to Europa. We are just trying to make our vast universe a little less lonely.

Posted by Paul A. Heckert

On a recent trip to Panama I worked alongside and conversed with a fellow named Enrique. I practiced my Spanish and he practiced his English, which was much better than my Spanish. Enrique is a construction laborer, who gets by on $14 a day. He was obviously intelligent but did not have the opportunity for an advanced education, and probably does not have easy access to all the modern communications and news media that we take for granted in the developed world.

Yet when Enrique learned that I teach astronomy, he asked about the Hubble Space Telescope and the things that we've learned from it. I was a bit surprised that even people in the third world, who by necessity are mostly concerned with basic survival, have heard of the Hubble Space Telescope. Perhaps there is a lesson here.

People are curious about the world and universe around us. Projects like the space telescope, which don't have immediate applications, still excite people because they help us learn about the universe. Applications of science are important, but basic science just for the excitement of knowing more about our universe is also important. It excites imaginations all over the world.

Beyond basic knowledge and curiosity, there are many justifications for the space program in general. One that the experience with Enrique points out to me is national prestige. People all over the world know about our space program and its successes. This applies to both the manned space program and to unmanned robotic spacecraft.

Successes in space excite people all over the world by providing new knowledge and in the process increase the national prestige of the country behind the successful project.

Posted by Paul A. Heckert

Is there anyone out there or are we alone in the universe? We don't yet know, but physics is an important tool in finding out. For questions about life, most people normally think of biology, biochemistry, and similar sciences. However the best ways to search for extraterrestrial life combine all areas of science.

The search for life in the solar system involves sending robotic probes to other planets, mostly Mars. After landing, the probes rely on biochemistry to search for life. However getting the probe to another planet involves quite a bit of physics, such as rocket propulsion and orbital mechanics.

Outside the solar system, searches for life have traditionally looked for radio signals from possible extraterrestrial civilizations. That uses the physics of electromagnetic radiation. Special relativity and the speed of light limit keeps us from sending probes to other stars.

The April 2008 Scientific American has a nice article about a new strategy. Author Nancy Kiang discusses work that she and coworkers are doing on strategies for detecting vegetation on extrasolar planets. We have detected water on extrasolar planets using spectroscopy. Why not try to detect lush vegetation on a planet with similar techniques. NASA satellites use this tool and the spectral signature of chlorophyll to map vegetation on Earth. Increasing sensitivity and resolution might allow us to try this on extrasolar planets.

The trick is that plant life on extrasolar planets may have different spectral signatures. The green pigments on Earth plants are most efficient for the Sun's energy spectrum. Planets orbiting stars of different temperatures will have different spectra of natural light. Therefore plants on these planets are likely to have a different color chlorophyll and different spectral signatures. Knowing the physics of blackbodies is crucial to this analysis.

Posted by Paul A. Heckert

I once read a definition of serendipity: when you want to go fishing, dig for worms, and strike oil. If this happened in real life, there are many who would bemoan the black goo and their inability to catch fish. Only a few would recognize the value of the black goo.

Sometimes scientific discovery is like this. Serendipity plays a large role, but the scientist must take advantage of the lucky break to make a discovery. Examples of serendipitous discoveries in science abound.

In 1820, Hans Christian Oersted performed an electrical demonstration for a class he was teaching, when a nearby magnetic compass started behaving strangely. Oersted could have declared the compass broken and tossed it in the trash. Instead he investigated further and discovered a long sought connection between electricity and magnetism.

In 1967 Jocelyn Bell-Burnell was a student worker on a large radio observation project. She noticed what she called "a bit of scruff" in her data. Her fellow students told her to ignore the scruff, finish her project, and graduate. She chose to ignore the well meaning advice instead of the scruff. For her efforts, she discovered pulsars, which turned out to be the neutron stars that had been predicted more than 30 years earlier but never found. After her discovery, it turned out that a few other astronomers had observed similar effects in their data, but ignored them.

Bell-Burnell found the equivalent of black goo in her data. Her reward for not going fishing and instead investigating the nature of the black goo was the scientific equivalent of finding oil - a major new discovery.

How many scientists missed out on an important discovery because they ignored the black goo or scruff in their data?

Posted by Paul A. Heckert

Few astronomical objects capture the public imagination like black holes. These exotic stellar corpses are so dense that their gravity prevents anything, even light, from escaping. It took a while for the idea to catch on. During World War I, Karl Schwarzschild's solution to Einstein's general relativity equations predicted the possibility of black holes and their event horizons, but few scientists took the idea seriously until the 1960s.

John A. Wheeler, who is usually credited with coining the name black hole, died last week at age 96. Despite providing the name and doing much to popularize the study of black holes, Wheeler did not buy the idea at first. At a scientific meeting in 1958, Wheeler disagreed with Robert Oppenheimer and said that such objects could not exist. Wheeler however eventually came to accept the possibility that such highly collapsed stars could exist and came up with the perfect name for them.

In his book, Black Holes and Warped Spacetime, Kip Thorne, a former student and colleague of Wheeler, describes how the term originated. They had been called collapsed stars by western researchers and frozen stars by Russian workers. According to Thorne, Wheeler pondered until he found the perfect name for these exotic objects. Wheeler then simply started using the name during a 1967 meeting, as if they had always been called black holes.

How much did finding this perfect name contribute to exciting the public imagination about black holes?

Posted by Paul A. Heckert

I recently read some news articles about the discovery of the smallest black hole. It is an interesting discovery, but the headlines should read the lowest mass black hole rather than the smallest black hole.

Reading the articles, I notice that no distinction is made between size and mass. The two words are used as if they are interchangeable terms. They are not. Size and mass are two distinct properties. We can measure an objects mass, which is in grams or kilograms, with a scale. We measure its size, which is in meters or similar units, with a ruler or similar device. An object with a larger mass is not always bigger in size. For example a 10 kilogram lead weight will be smaller in size than a feather pillow having a mass of 1 kilogram.

The astronomers who discover a black hole measure its mass from its gravitational effects. They then use the mass to infer its size, which we can not measure directly. It turns out that the size of a black hole is directly proportional to its mass. Hence, a more massive black hole will be larger in size, as long as we are referring to the size of the event horizon or Schwarzschild radius rather than the central singularity point.

Not all stars however have this property. For white dwarf stars or neutron stars the size decreases as the mass increases. Using the words size and mass as synonyms for these stars will result in many incorrect statements.

Language in physics is must be very precise. Words have very specific meanings that should not be interchanged. Science writers and journalists need to use the terms correctly to avoid confusing their readers.

Posted by Paul A. Heckert

I just read an article claiming that cell phone usage increases the risk of brain tumors. I have also read contradictory research reports claiming no connection. Contradictory claims for health research are common; so how does one evaluate health claims?

News articles often leave out crucial details. Look for in-depth articles that give necessary details, or better yet, the original article. Articles in peer reviewed medical/scientific journals are best.

The human body is extremely complex making epidemiological studies complex. Small effects are difficult to isolate causing many contradictory studies. For claims about health effects of cell phones, or anything else, don't rely on a single study. Look at many studies for overall trends.

When evaluating studies consider the following points:

• How large is the study sample? Larger samples are more likely to be accurate.
• Is there a control sample? Controlled double blind studies are the gold standard for medical research.
• Are variables properly isolated? A good study isolates or corrects for other variables so that only the tested variable changes.
• To claim an effect, there should be a correlation, the stronger the better. However that is not enough. Correlation does not prove causation. So to claim something causes a health effect, good or bad, there should also be a plausible mechanism. In the case of cell phones, they send their signals via microwaves. Microwaves are very low energy electromagnetic waves, but there is a very small possibility repeated exposure could cause cellular damage.
• How long did the study last? Very small effects, such as from cell phones, may take decades to cause problems.

We won't really be able to reliably evaluate the long term effects of constant cell phone exposure on brain tumors until cell phones have been around for several decades.

Posted by Paul A. Heckert

Arthur C. Clarke died today at age 90. Clarke was best known as a science fiction writer and I have enjoyed reading his science fiction since childhood. Clarke's science fiction combined excellent science with excellent story telling. Like many scientists, I prefer science fiction that is based on solid science.

Clarke's best known science fiction story may very well be 2001: A Space Odyssey. The book and movie, which was produced in collaboration with Stanley Kubrick, came out at time when 2001 seemed impossibly far in the future. Many science fiction books are based on good science, but the movie remains a rarity in that it is one of the few Hollywood science fiction movies with accurate science.

In addition to science fiction, Arthur C. Clarke did good science. His contribution to science that most affects us on a daily basis is the idea of communications satellites in geosynchronous orbits. At the time Clarke suggested the idea, it was more science fiction than science. The first satellite would not be launched into orbit for over a decade.

Like many brilliant ideas, it is elegantly simple. Launch a satellite to an orbital distance with an orbital period of 24 hours so that it stays above the same location on Earth. It is a relatively simple physics problem to figure out the required orbital distance. Satellites in geosynchronous orbits are very useful for communications satellites, because satellite TV and communications antennae do not have to move to track the satellite.

How many more of Clarke's science fiction ideas will someday become reality?

Posted by Paul A. Heckert

In the excellent book, Thinking Physics, one question asks if a battleship can float in a bathtub. Ignore the obvious fact that no real bathtubs are as large as a battleship. The question is really about the amount of water required for a ship to float.

Very little water is really required. According to Archimedes's principle, the buoyant force on a ship or other object in water equals the weight of the water displaced. If the weight of the displaced water exceeds the ship's weight, the buoyant force is enough to float the ship. Ships weighing more than the displaced water sink.

Reading that statement might lead one to think that there must be at least enough water surrounding the ship to equal the weight of the ship, but that is not the case. The weight of the water displaced, not the weight of the water surrounding the ship, determines if the ship will float. The displaced water does not actually need to be there, hence there does not need to be much water actually surrounding the ship.

If there were a battleship-bathtub combination allowing the ship to just barely fit inside the tub, a thin layer of water between the outside of the battleship and the inside of the bathtub would be enough to float the battleship.

The locks in the Panama Canal are a good example of this principle. On a recent trip, I watched a ship going into the Miraflores locks of the canal. Even ships that barely fit into the locks float on the very small amount of water surrounding the ship in the lock. Observations confirm the theory.

Yes it is possible for a battleship to float in a bathtub.

Posted by Paul A. Heckert

My last blog entry was about eclipses as an example of the predictive power of science. I was able to watch the eclipse however because another prediction was wrong. The weather report for my location predicted rain or snow but was wrong. It cleared up just in time to see the eclipse and the clouds moved back in when the eclipse ended.

In principle weather should be as predictable as eclipses. Weather also results from basic laws of physics. Weather however results from a much more complex system. We therefore do not have the computing power needed to accurately predict the weather. Accurate weather predictions would require a computer as complex as Earth's atmosphere.

This complexity makes weather a good example of a chaotic system. It is in fact the original example. In the early 1960's, Edward Lorenz started using computers to try to compute long range weather forecasts. It didn't work at the time, and still doesn't work, because our atmosphere is too complex. Very minor, almost immeasurable, differences in initial conditions can compound into significant differences. That is essentially the definition of a chaotic system and the origin of the term "butterfly effect". In a chaotic system, a butterfly in Brazil might cause a tornado in Texas.

This chaotic compounding of very small effects makes it impossible to accurately forecast weather. Science can predict things very accurately in principle, but in practice it does not work for extremely complex systems. That is the basic idea behind the mathematical field of chaos.

Posted by Paul A. Heckert

The lunar eclipse occurred on schedule last Wednesday night. It started and stopped just when the predictions said it would. Eclipses can be predicted correctly well into the future. That is part of the nature and power of science. Natural phenomena follow very specific laws of nature that allow us to predict what will happen. Mathematical statements of the laws of nature coupled with mathematical tools allow us to make specific quantitative predictions.

Before we understood these laws, nature seemed capricious. Our ancestors, with no understanding of how orbits work, looked at eclipses and saw random events, which they often attributed to capricious gods. They then performed various rituals to appease these gods in hopes the gods would return the Sun or Moon.

Scientists however try to understand natural events and their causes. This understanding allows us to predict natural events. As our understanding of natural laws has increased throughout history, our ability to predict natural phenomena has also increased. Our belief in the capriciousness of nature has correspondingly declined. Complex natural phenomena still elude prediction, but we at least usually understand their causes and are working on the predictions.

There are limitations to the power, but much of the power of science comes from the ability to accurately predict what will happen in a specific situation. Otherwise we are, like our ancestors, at the mercy of capricious nature.

Posted by Paul A. Heckert

Many years ago I watched a Star Trek episode with a group of science types. The crew was trying to get Enterprise's engines working again, otherwise the Enterprise and her good crew would crash into the planet they were orbiting.

Because we understood how orbits work, we laughed at the writers' lack of physics knowledge. Once something is orbiting a planet or star, it needs no engines to keep it in orbit. Earth doesn't have an engine to orbit the Sun and the Moon does not need engines to orbit Earth. These orbits have been stable for nearly 5 billion years.

Why then do satellites fall from orbit? Lately the possibility of a spy satellite falling from orbit has been in the news. Those of us who are old enough might remember the Skylab falling into the Pacific in the late 1970s. Occasionally other small satellites streak down from orbit.

The key is friction. Orbits remain stable as long as no frictional forces slow the satellite. Rather than ending abruptly; Earth's atmosphere just gradually gets thinner. In a low Earth orbit the atmosphere is very thin but not quite a vacuum. Hence there is a small amount of atmospheric resistance to slow the satellite. As the orbital speed slows, the orbit decays to a closer distance. The atmosphere is thicker, increasing the atmospheric resistance. The orbit decays faster and the satellite streaks down to Earth. Let's hope it doesn't hit a populated area.

If the Star Trek writers had written something into the script about atmospheric friction, we would not have made fun of them.

Posted by Paul A. Heckert

Einstein's famous equation, E=mc2, is arguably the most famous equation in the history of physics. At least one entire popular book has been written on this equation: E=mc2 by David Bodanis, Berkely, 2000.

The preface of this book begins by quoting a magazine interview with Cameron Diaz, in which the actress said that one thing she really wanted to know was the meaning of E=mc2. Apparently even Hollywood celebrities wonder about this equation.

This equation stems from Einstein's special theory of relativity, which he published in his miracle year, 1905. About a decade later Einstein published his general theory of relativity.

What does this equation mean? In simple terms it means that mass, m, and energy, E, are equivalent. Energy can be converted into mass and mass into energy. The equation tells us how much energy is equivalent to a certain amount of mass. Because c, the speed of light, is so large a very small amount of mass converts into a very large amount of energy.

Prior to this equation physicists had separate laws for conservation of energy and mass. These laws stated that the total amount of both mass and energy in the universe is constant. After Einstein's work, these laws were modified to a single law of conservation of mass and energy. Mass and energy can interchange, but the total amount of mass-energy in the universe is constant.

The American Institute of Physics has an interesting recording of Einstein himself explaining the meaning of his equation. If you want to hear it directly from the source, check it out. If you happen to see Cameron Diaz, let her know about it too.

Posted by Paul A. Heckert

One of the problems most scientists have with Hollywood science fiction is the sound of space ships swishing by. Hollywood directors always seem to think that space ships should make sounds. Sound in space is however impossible. Space is a vacuum and sound can not propagate through a vacuum. There must be molecules present to vibrate and carry the sound.

The one place that a space craft can make a sound is when traveling through a planetary atmosphere, while either landing or taking off.

Saturn's largest moon, Titan, is the only moon in the solar system that has a significant atmosphere. Because Titan has a significant atmosphere, sound can propagate. A space craft landing on Titan would make noise as it lands.

The Cassini mission to Saturn included the unmanned Huygens probe to the surface of Titan. The Huygens probe sent back the first pictures of the surface of Titan. It also sent back information on Titan's atmosphere.

Using microphones attached to the probe it was possible to reconstruct the sound of the Huygens probe flying down to the surface. To hear the recording click here, enjoy the pictures of Titan's surface, scroll to the bottom of the page, and click on "sounds of Titan during Huygens descent". The real space craft landing does not sound anything at all like the space ships swishing by in Hollywood movies.

Posted by Paul A. Heckert

On the last page of the February 2008 issue of Sky and Telescope Magazine, an article by Gil McFarlane describes a telescope project by his teenage daughter, Gina. She built a 3 inch telescope and showed her peers the planet Saturn. She then showed them the much more detailed Hubble Space Telescope pictures of Saturn and asked which they preferred.

The teenagers overwhelmingly preferred the real thing through a homemade 3 inch telescope to a picture through the Hubble. The real thing always beats pictures. Gina's conclusion: "If you want young people to become more interested in astronomy, show them the real thing."

Science educators, myself included, should listen to Gina. This principle doesn't just apply to astronomy. Kids can learn about various flora and fauna by reading a stuffy textbook. They will however become much more excited about biology if they go outside to observe real plants and animals. In geology or earth science classes, collecting and handling real rocks will do much more than reading a text.

Including real experiments and demonstrations, even those done with simple equipment, in a physics class will make physics much more exciting to students. Elaborate computer simulations or complex equations can illustrate important points, but students need to get their hands on real experiments.

Too few young people study science, at least in part, because they perceive science as both hard and rather dull. Making science come alive for students will help interest more young people in science. Science teachers and parents, follow Gina's advice. Show them the real thing!

Thanks for the reminder, Gina.

Posted by Paul A. Heckert

One of the first things that I noticed about doing astronomy is that given the same daytime conditions, clear nights are colder than cloudy nights. Clouds blanket Earth and trap heat, so there really is a basis to this observation. Winter nights are longer, so observing stars all night can mean 12+ hours in the cold.

As a working observational astronomer, I have spent many 12+ hour winter nights at the telescope. Sometimes I'm lucky and it is relatively warm, but I have observed all night on many nights cold enough to make even Al Gore support global warming.

To enjoy the winter night sky, dress warmly. Don't just throw a parka on over your T-shirt. Wear many layers under the parka.

I typically wear 3 to 4 pairs of long johns, some wind resistant pants, and then some ski pants. Heavy long socks also help the legs and feet. For my torso, I might wear a half dozen or so long and short sleeve T-shirts underneath a couple fleece or sweatshirt layers. Then I have a down vest, occasionally two, and my down parka. A hat is also crucial; don't leave your head unprotected. Your fingers and toes cannot be warm if your head is not. I can gain 20 pounds getting dressed for observing. If you are wearing too much, you can always save a layer or two for the later part of the night when it gets colder.

Don't dress like you would for skiing. Wear much more. When skiing, the exercise warms you. Astronomy is less active, so wear more to keep warm.

Proper dress can help you enjoy stargazing on even the coldest winter nights. Put on your layers, get out there, and enjoy the night sky.

Posted by Paul A. Heckert

In some places, people fire shots into the air on New Year's eve and other celebratory occasions. This custom may seem harmless and even be relatively safe in sparsely populated areas. In densely populated areas, it can however be very dangerous. Physics tells us why.

First the obvious. From the law of gravity, whatever is fired into the air must come back down, unless it is fired fast enough to escape Earth's gravity and go into space.

Now a quick quiz. When the bullet returns to the ground, how fast is it moving? Is it slower than, faster than, or the same speed as when it left the pistol?

The bullet is propelled upward with its initial muzzle speed. Gravity tugs it downward, so it slows. At the highest point in its path it slows to a zero velocity for an instant. Gravity is still pulling it downward, so it begins its descent.

The gravitational force acting on the bullet remains constant the entire time, so the upward and downward speeds are reversed but symmetric. From this symmetry, when the bullet returns to the level it was fired from, its downward speed equals its initial upward speed. Air friction may slow the bullet a little, but no more than when it travels horizontally. If you picked the third choice, congratulations, you were correct.

If someone happens to be standing where the bullet comes back down, the results can be tragic. I have occasionally seen news stories about someone being accidentally shot this way.

There are safer ways to celebrate the new year and other occasions.

Have a good 2008.