A full list is found at http://www.phy6.org/StarFAQsA.htm
and links arranged by subject are at http://www.phy6.org/stargaze/StarFSubj.htm.
396B Posssibility of Asteroid Hitting Earth (2)
396B Posssibility of Asteroid Hitting Earth (2)
305. About mountainsDear Dr. Stern,
I would be very grateful if you answer my following questions in details?
ReplyWhat strange questions! your first question--I would say "no," except that I really do not know what you mean. Stabilize against what?
Some things should be understood. Compared to the size of the Earth, mountains are not at all high. They provide less surface variation than the markings on a coin. What they do is provide evidence for forces inside the Earth which deform its surface--throw up volcano peaks, crumple sections of surface which are pressed or stretched sideways (just look at a relief map of Nevada, with its north-south ridges) or raise parts of continental plates as one plate pushes beneath another.
This last process is responsible for some high mountains--for the Himalayas, raised by the plate of India (which I have read started as an island south of the equator and migrated north) pushing beneath the Asian plate. In Alaska, Mt. Denali (or McKinley) is similarly produced as the Pacific plates pushes northwards beneath the American plate and lifts it. Both processes create earthquakes.
Mountains on Earth are just about as high as they can get: unless pushed up or held by pressure below, they would sag under their own weight and gradually flatten. Mars has a giant volcano, Olympus Mons, nearly 3 times higher than any mountain on Earth: but then again, Mars only has 0.39 times our surface gravity, and no oceans (measuring Earth mountains from the sea bottom shows greater height).
And how would Earth be without mountains? Depends. We have continents of lighter rock, which float on top of denser rock and poke out above the oceans. If that floating did not exist, Earth would be one big ocean and dry-land plants and animals would have no chance. How would Earth look without mountains? The icy satellites of Jupiter have all sorts of surface markings, e.g."ghost" craters from long-ago impacts. But unlike Moon craters, they are just markings on a flat surface ("palimpsests") because ice is weak and easily sags, even under low gravity.
306a. "Will the World end in 2012?"I came across your website while looking for answers to this question and wondered if you could help. Unfortunately a Google search on this subject brings up more coo-coo science from dooms-day authors than scientific facts. Fortunately I found your website and was able to locate your email address.
My eight year old daughter was told by a friend at school that, "the world was ending in 2012." Her friend cited something about reversal of the earths poles, increasing solar activity (solar max??), planetary alignment and the Mayan Calendar ending at 2012. I tend to put things like this in the Art Bell/George Noory category and move on, but this has really frightened my little girl so I'm seeking some guidance. More importantly, her question suggests that she has some interest in physics so I'd like to find a way to keep that spark alive while providing some reassurance.
Can you provide a simple way of explaining earth rotation, earth polarity and angular momentum that would help quell her fears while at the same time peaking her interest in physics? Also any information on the Mayan calendar would be helpful.
ReplyI hope your daughter learns a lesson from this--don't trust Google, which collects web pages by machine and cannot tell sane from crazy. And don't rely on rumors. Find out! Rumors and Google can provide hints where to look, but to make a judgment--of what makes sense and what does not--is one's own job.
Being 8 years old is a bit early for making such judgments, so I hope she has your help (and maybe her mother's and her teacher's) to sort out the facts. I myself am just a retired physicist, too far from you to be of much help.
Luckily, questions like hers have arisen before, and the more significant ones are answered on the web site. The key file is
which sorts the questions by subject. Your daughter's interests are particularly suited for section #7a ("Earth Rotation") and there to questions such as #26, #78, #165, #166 and #171 . Also section #7c ("Calendar and related items") and in particular the last two entries (#264 and #302) which relate directly to the Maya calendar and the year 2012 (as does one following immediately below here).
I hope your daughter (with your help) will be reassured and informed by what she reads. I also hope that she will become interested in science, and in particular astronomy! You can start her on the early sections of "Stargazers" and maybe begin with the paper sundial in section #2a there.
The reversal of the Earth's magnetic poles is a different subject altogether, discussed in "The Great Magnet, the Earth." It's not expected to happen soon, and while it has occurred in the past, you are not likely to notice it if it does, unless you are using a compass.
306b. Another question about the year 2012Hello, I've been doing a little bit of research on the December 21st 2012 theory lately, seeing as it is the only respectable end of the world theory out there. I believe I understand the principle behind it but I'm unsure about a few notes in it. It seems to state that we will be perfectly aligned with the Black Hole that is at the center of our universe, giving a 10 percent chance that our own poles will dramatically switch as the Earth slows down and starts to turn the other way. As I'm sure you already know of this theory, knowing your intelligence just from your other articles that I've read, I would like to ask you your own opinion on this theory, and would the change take effect on other planets as well, causing their poles to shift and sort of having a Galaxy wide end?
Nothing much will happen to the universe or Earth and planets in December 2012. It is the time when the Maya calendar, which is quite sophisticated and has long cycles, will end (I think) its 5th cycle and start its 6th. It's like your car's odometer going past 499,999 to 500,000.
The universe has no center, by the way. Our Milky Way galaxy has one, for sure, and a big black hole sits there (I have written about it) but I am not sure it has much influence on us. If its rotation axis were pointed at Earth, telescopes might see a lot of X-ray and radio noise from it, but we are too far for any large effect, also it probably rotates the way the galaxy does, and therefore we are looking at its equator. And Earth's rotation can't change significantly because of the conservation of its angular momentum..
For more, see top of this page and look under #264, #291b and #302, also (about the Earth's rotation) #165, #166 and #108.
307. Advice to graduating High School StudentI have just passed out my 11th exams (from the Gujarat state board in India) with maths, physics and chemistry as my main subjects, English as language and computer science as a side subject and I am thinking to take up aeronautics as my main subject (to be an aerospace engineer or designer) so which are good universities and which are good courses which you would suggest me to choose. I am a bit weak in chemistry.
Please give me some suggestions to improve it.
Your site have been really very helpful for my school studies and also in outer school examinations
ReplyI do not know enough about conditions in India to advise you. For instance: some very good universities exist in India, but they charge high tuition, have difficult entrance examinations, and much competition exists among those who wish to attend. Where you belong in this, I do not know.
One thing, perhaps. What will you be doing in the coming summer vacation? If you are interested in aviation, look for a technical job associated with it--perhaps at a local airport, at an airplane refitting or repair facility. If you can afford it, be prepared to work for minimal pay, or even no pay at all. That way you will be in contact with people in the profession. Choose carefully, and if you are lucky, you might find a good mentor.
One last note: whenever you write ANYTHING, make sure to re-read and edit your words more than once, to catch any mistakes in spelling (your message had a few) or in grammar. The way you write gives your correspondent the first impression about what sort of person you might be.
A similar letter, received earlier:I'm a 23 year-old girl. Work in astronomy and being an astronaut are an old dream dating back to my childhood. Since I was a little girl I wanted to be in space and when I became older dreamt of La Silla, and being an astronaut like Story Musgrave as my hero fascinated me. I was very young and I didn't know which major should I study in high school. I thought that aerospace engineering was the best choice for me and I intended to study this major. So I studied mathematics and physics in high school, although at that time we didn't have aerospace engineering for women in our country, but I was very serious in my choice. After finishing high school I participated in university entrance examination to fulfil my dreams but unfortunately it was very hard and I couldn't pass the exam for my selected major and I had to study industrial management, despite my other interest. Finally a year ago I finished my studying but it doesn't satisfy me. I studied a lot but I couldn't achieve my dreams because in my country there is a university entrance exam for entering the university which is very hard and unfair. I cannot forgive the government because of this carelessness and injustice.
Anyway, now I have the chance to follow my dreams. I want to study aerospace engineering or engineering physics but I have a concern and it is about my health. I afraid of passing the time and money and then reaping nothing. Now I have bachelor degree in industrial management and I work in a good company and I can follow my education in my major in Europe (I have admission from one of the best universities from Sweden) . I'm worried about this problem. I afraid to quit everything and follow my dreams but it doesn't have any result for me.
Now I have a question from you and I need your help and guidance. I'd like to know about the medical and physical examination for selecting the astronauts. I afraid that I lose everything because of these kinds of examinations. Could you tell me about this examinations? I know medical tests include audio and visual examinations, dental examination, electrocardiography, pulmonary function test, and analyses of feces, blood and urine but I do not know anything about the physical examination. Also I'd like to know whether or not the person has curvature in his/ her spine can be an astronaut or not. Please guide and help me. I'd appreciate if you kindly answer my questions and raise my ambiguities.
Please accept my thanks in advance and I'm looking forward to receiving your early reply.
ReplyYou may not like to read this--but in fact, you are asking a lot. In this world, many dreams end up being just dreams. As far as I recall, astronauts are selected from a very small group--either outstanding airplane pilots (usually military airplanes)--or people with outstanding professional records, usually in science. Many people have a dream like yours, but few are chosen. Also, almost all astronauts are US citizens, and those who are not, are chosen by their governments, in countries collaborating with NASA.
I should add that nowadays just a few space missions with astronauts are conducted each year, far fewer than what was once predicted.
I don't know what effect curvature of the spine has. May depend on how bad it is, how much it interferes with your normal life.
Aerospace engineering or engineering physics are more realistic goals, but take a lot of study and please remember, even there few jobs are exciting. I know a young man who dreamt of becoming a pilot (even though he studied computers), he studied flying, got his license and then found it was hard to get a good job. For a while he flew tourists to see the Grand Canyon and other small-plane missions (once even transporting convicts from one prison to another), but his company went bankrupt and now he is in Alaska, transporting passengers and cargo in small airplanes to distant native villages, or tourists who want to see some scenic areas. It is hard and uncertain work, and all but stops in the coldest part of the year.
In truth, the number of interesting jobs in aviation or space is small--much fewer than jobs for doctors, trained nurses, computer experts or school teachers. The same is true in astronomy--see "Advice to a would-be astronomer" on
I would not discourage you from asking around a university (and joining a flying club, maybe?), but you will need persistence, ability to work hard and luck. I hope you have them all.
308. Could a (heat resistant!) ship float on the Sun?Hello. I am Chris. I just stumbled upon your website when I was inquiring around the internet about the rotation of the earth. I found your website to be very interesting and informative. Thus, I would like to know your thoughts on this question.
This question may seem like an irrelevant and impossible excursion into science fiction, yet it is a question that has tickled my curiosity for quite some time. If we were to build a spaceship that could successfully reach the sun's surface, how would the ship behave on the sun's surface? Or, more simply, what would the consistency of the sun's surface be like? Would the ship fall right through the surface, or would it float on it like a rubber duck floating on water? Could it be something else? Although I heard that the sun is gaseous, wouldn't the immense gravity of the sun cause sufficient compaction of the gases to give it a more fluid consistency? I don't think anybody has a definite answer on the subject, yet I would like to know your thoughts on the matter. Thank you for your time.
Matter comes in 3 varieties--solid, liquid and gas (viewing here plasma as a special gas). Solid and liquid form well-defined surface boundaries, e.g. the ocean surface. Gas just dwindles away exponentially, e.g. in the Earth's atmosphere (discounting temperature variation) air density drops by half with every 5 km of altitude, approximately. So at 10 km it is down to 1/4 the sea-level density, 15 km to 1/8 of the density... I think you get the point (the process ends around 100 km, where molecular collisions become rare).
For mathematical reasons, physicists use the "scale height" H, the distance where pressure and density are reduced, not by a factor 2, but by a factor e=2.7128... , But the physics is the same. Light objects can definitely float in an atmosphere--consider helium balloons--but the altitude can vary, they do not rise to any outer surface, but find an altitude which matches their density.
Same with a balloon near the Sun--if it were not instantly evaporated by the heat. Gravity is about 30 times larger, but molecules are lighter (hydrogen) and a lot hotter, so H is larger. In
I estimate H=150 km. In http://web.njit.edu/~dgary/321/Lecture7.html the result is 270 km. Take your choice!
A balloon floating above Jupiter, Saturn etc. is subject to the same laws.
309. Reducing the fuel weight of the Space Shuttle?I have a question you probably could answer really easily. I know the escape velocity for the Shuttle is around 18,000 miles per hour. But the Shuttle is very heavy and the fuel tanks filled with propellant are very heavy too.
So if you had a craft that had some sort of electronic propulsion that did not require those tanks filled with heavy fuel, I gather the escape velocity might still be the same?
So if you had perhaps a craft with 2 or 3 different systems that were like 2 or 3 stage systems, the outlay of raw energy would be less but the required speed would still be the same. And basically, such a craft would be quite large enough–with using all the lightweight materials–to carry 6-20 astronauts (or a large family wanting to leave Earth for greener pastures).
ReplyYour scheme unfortunately will not work, Newton's laws of motion conspire against it. To understand how that happens, you will need read about them in "From Stargazers to Starships,"--sections 16, 17, various parts of 18, as well as 25.
The shuttle starts off with something like 2500 tons of fuel, NOT just because that fuel supplies it with the ENERGY needed for reaching escape velocity, but also, because it supplies it with the FORCE needed to accelerate it to that speed. By Newton's 3rd law, you cannot exert force without pushing against something. Forces come in pairs between two objects, if A pushes B, then B pushes A with equal and opposite force.
With the shuttle, "B" is the fast jet of burning gas: by pushing that jet backwards, the shuttle (or any rocket) is pushing itself forwards at the same time. It is like the recoil of a gun, which is another example of these laws in action.
Electric propulsion (sect. 33) is not very suitable here, it still would require something massive to be thrown backwards, and electric energy can't be stored as easily as chemical energy. As for "greener pastures"--no such thing exists, not for life like ours, depending on liquid water: Venus is too close to the Sun and hot enough to boil water, while Mars is to far from it, and water there would be frozen much of the time. Earth alone is in the "Goldilocks range" of distances: be grateful for its green pastures, forests and fields!
310. How do Rockets Land?I am not a young student, but a medical doctor. I have been teaching some stuff about space to my young daughter. However, I am unable to tell her how a rocket lands, reason being I myself don't exactly know.
Could you please enlighten me? How do rockets land? Do they land at all or do they all burn up? Why do we see pictures of astronauts jumping from parachutes?
Could you advise a good website which can satisfy my doubts?
ReplyA major problem of an airliner returning to Earth is how to get rid of its energy of motion--kinetic energy--while still getting enough "lift" from the motion of air across its wings. If your daughter ever sat by the window of a landing airliner, she would see all sorts of auxiliary extensions of the wing sliding into place at the back of the wing--increasing air resistance and slowing down the airplane, and at the same time creating extra "lift" to hold the airplane up. The lift is much less efficiently produced than in ordinary flight, but that is all right, this unusually slow flight lasts only a short time and then the airplane is rolling on the ground.
A satellite in low Earth orbit must move at 24 times the speed of sound (or faster), which means its kinetic energy, the energy of motion, is at least 24 x 24 = 576 times the energy of something moving at the speed of sound, which is already more than the speed of an airliner. Weight for weight, a satellite has about 20-50 times the energy of a rifle bullet, enough to melt it, even boil it away.
Getting rid of that energy safely is the main challenge in landing. The astronauts returning from the Moon (at even greater speed) could not save their spacecraft but had to abandon it, and return in a "capsule" designed to stand a lot of heating, and meanwhile creating a powerful shock wave ahead of it, containing very hot air whose glow dissipated the energy. Then in the thicker atmosphere, at low speed, they used a parachute, and splashed down into the ocean.
Like the Apollo Moon ship, many research rockets are abandoned when their job is done--it would be too hard to bring them back intact.
The space shuttle enters the upper atmosphere (which is very rarefied) sideways, with its bottom forward: the bottom has heat-resistant ceramic tiles, and creates a great shock wave (in the "Columbia" some tiles broke away and the heat destroyed the shuttle). By the time it reaches the denser atmosphere, it has slowed down to about half the speed of sound, and it can land like an airplane--still, much faster than a jetliner, requiring accurate computer control.
Burt Rutan's "Spaceship One" similarly used wings, first as brakes (turning them to create air resistance), then to land as an ordinary airplane. However, since it only reached about 3.5 times the speed of sound, this was not as great a challenge.
311. The Earth's Spin reduced by Global WarmingI have read an article on internet which says that due to continue warming of earth, the length of day is becoming greater. Although the change is very small but with the passage of time it may perhaps become appreciable. Reference: http://space.newscientist.com
My question is: does there exist a chances that a time will come when due to change, the rotation of earth will stop?
ReplyI could not find the item on the site you have mentioned, and the only thing I can guess about it is the following.
As you know, the water level in the oceans is rising. The main reason (at least now) is not melting of icecaps, but warming of the water, which like almost any substance expands when heated. Therefore, since most of the Earth surface is ocean, if the water rises by, say, half a meter, the effective radius of Earth might increase by something like 35 cm.
In a rotating object, expanding away from the rotation axis slows the rotation (think about a rapidly spinning ice skater, extending her/his arms out and almost stopping). Expansion of the Earth will therefore slow down the rotation. But only by a tiny amount, since 35 cm is maybe 1 part in 20 million of the radius of the Earth. It will never become appreciable.
312. Circumnavigation of the SunA question came up in my family and interests me. I am indeed a new stargazer who is very interested in the workings of our solar system, galaxy, and the universe itself. The question I would like answered is...
"Why can't humans circumnavigate the sun?"
ReplyNot only have humans done so, but you yourself have done so too. Every year the Earth carries you on such a trip!
There even exists a bumper sticker, something like "Living on Earth is expensive, but you get a free trip around the sun."
ResponseMy question had nothing to with being on planet earth. I meant it as traveling in a space ship. Like the ones they send out now.
Answer to the responseThere exists no good reason for such a trip. It is also a difficult thing to do: no one has yet supported a human in space for as long a year, the orbit must be chosen carefully to have a period of exactly one year (or else Earth won't be there on the return!), and there is not much a human can do in such an orbit that an instrument cannot do simpler and cheaper--observe not just light but ultra-violet, x-rays, energetic particles, magnetic field etc.
Such instruments have in fact orbited the sun aboard "Ulysses". The aim was not just orbit the sun, but do so above the poles of the sun, a region which cannot be observed from Earth. That is a difficult orbit to achieve: the spacecraft first approached Jupiter and used its gravity to rotate its orbital plane by nearly 90 degrees. It was primarily a European space mission, launched in 1990, it has accomplished interesting discoveries, and is still working. No human astronaut could so as much. More on Ulysses home pages, e.g. http://ulysses.esa.int/science-e/.
313. Are nuclear forces merely gravity at very close distance?Is it possible that when we are speaking of strong nuclear forces, we are actually speaking of a tiny gravity space for the nuclei?
Are there any pages on this? I don't even know what it might be called.
I am just a layperson, who cannot master the math in my quantum gravity book, but I think --what I can read is rather accurate.
It just "looks" like we may replace the phrase strong nuclear force with gravitational field (with quantum restriction (?)) as holding the particles in place. Would this also apply to the sub particles, --and might it be the sub particles which allow the escape of the alpha particle in the first place?
ReplyNuclear physics is not my field, so you might check what follows below with someone more familiar, and with a fresher education in physics-- I'm past 75!
The drawing in section Q8.htm uses a "gravity well" as an analogy to the energy well caused by the nuclear force. However, the two forces are far apart in strength: the nuclear force is much stronger than the electric force, while gravity is much, much weaker.
You might perhaps argue that nuclear particles could be so small, that distances between them are small enough for gravity to be quite strong. However, if the proton is small, its electric charge should be confined to the same tiny size, and then the electric repulsion between neighboring protons will always overcome their gravitational attraction.
Anyway, I suspect that protons cannot be that small, because quantum mechanics demands for them to be spread out over a certain wavelength. Also, when protons or nuclei collide, their "cross section" is of the order of a "barn" or a fraction thereof, suggesting a dimension of the order of one part in a thousand billion of a centimeter. It sounds tiny, but is still too large for strong gravity.
314. Changing the Earth's RotationI have a serious question
What is the energy required to either slow down of speed up the Earth's rotation by say, 1 msec? I have tried out the calculations but I think I am missing a parameter or two.
ReplyThe energy E of the rotation of Earth is (I believe) (I ω2)/2 (check the "Hyperphysics" web site) where I is the moment of inertia of Earth, and ω is the angular velocity (a Greek omega). The value of I can be found on
and is around 8 1037 kg/m2
The earth rotates in about 86000 seconds, so approximately
ω = 2π /86000 = 7.3 10–5 radian/sec
ω2 = 5.3 10–9
and the energy E is around
E = 2 1029 joule
Changing the period by 1 msec out of about 105 sec means a relative change around
dω/ω = 10–8
The energy change, found by differentiating the energy equation, is approximately dE = E 2dω/ω = 2 10–8 E = 4 1021 joule.
A million power stations of 100 megawatt each would produce about as much in a year. Of course, it is not enough to have the energy--the Earth must also grab hold of something outside it, to transfer the angular momentum. The Moon and Sun do slow down the rotation, by raising tides on the Earth. But they do so at their own slow pace, and discussing this process would take us too far off.
315. Why are planetary orbits eccentric?I was reading yours question/answer section wherein you clarified several doubts beautifully and interestingly.
Why would the orbit of any planet be elliptical? Any planet that orbits any star or other planet is being pulled by the gravity of the central object (star or planet). Shouldn't the orbit always be circle, as the force pulling the planet inside would want it to be?
Assume the planet encountered the star while moving somewhere in the space, and started orbiting it. Then due to its own initial motion, it has at first an elongated elliptical orbit. But that orbit would slowly become more circular as the speed of planet on the edge of ellipse would be reduced. If we go by the pendulum theory, as the planet is moving under the impact of gravity of the star, the distance it moves far from the star (the longer axis of the orbit) would eventually shorten and become equal to its shorter axis, resulting in a circular orbit.
In case of a planet orbiting another planet (like our moon), we may have to consider the motion of the center planet (the one being orbited) and that may reduce the speed with which the elliptical orbit becomes circle, but at the end the orbit should be circle.
I can't think of any other theory to convince myself with the truth that earth is still in an elliptical orbit, while orbiting an relatively stationary star.
If so, at some point of time, the Earth's orbit will become circle (considering for now that my ideas hold true). Then it's also interesting to think about what happens when there are not longer or shorter days on earth. All days measure exactly equal and there remains only one season through out the year.
Waiting to know the things I'm not aware of (not very good with Physics - didn't get a chance to study formally).
ReplyCopernicus thought orbits should be circles, because the circle had a perfect shape (or else, because he knew how to calculate with circles). But your question really relates to the way the solar system came to be. I don't have answers, but knowing some physics and astronomy (and having read a bit about the problem), I can guess a few things.
It is generally agreed that the solar system began as a cloud of dust and gas, pulled together by gravity (astronomers have observed such clouds). Planets were not encountered by chance, after they had already formed. At first the material of that cloud just moved (on the average) towards the center. A dust particle or gas molecule pulled towards such a center does have an orbit, but if it starts far away, this will be a very elongted oval. The width of that oval will depend on the distribution of the "sideways velocity" of dust particles (related to "angular momentum").
However, as particles and gas come closer to the center, they may collide and convert some of their energy to heat. Such energy loss slows them down, allowing gravity to clump them together. The clumps radiate heat into space and become very cold, so gas molecules and atoms hitting them are likely to freeze and stick (as may have happened in comets, whose motion remains very elliptical). Furthermore, if the cloud is not completely symmetric--extending further on one side than the other--more will circle in one direction than in the other (say more counterclockwise than clockwise, as seen from some distant star), causing the average of the cloud to rotate.
The loss of energy to collisions will prevent dust particles from returning to their starting region. It will also bring everything down to a rotating disk, explaining why all major planets are very nearly in the plane of the ecliptic. Any particle moving on a steeply inclined orbit will collide again and again with the disk, and its fragments tend to share the disk's motion.
You would think that in the end everything would move in circular orbits in a thin layer, like the particles with make up the rings of Saturn, which tolerate no ellipses. However, before that stage is reached, the clumping produces "planetesimals" and then larger "protoplanets." These will collide and stick together to form planets, before a smooth circular structure like the rings of Saturn is achieved.
Apparently the planets formed before all their orbital eccentricity (traceable to the elongated initial orbits) has completely disappeared, and so we have elliptical orbits. However, the eccentricity of planetary orbits is very small. I have seen a picture of the Earth's orbit superposed on a circle. The difference between the two is expressed by the varying thickness of the drawn line and is barely noticeable (however, the separation between the focus of the ellipse and the center of the circle is quite evident.
In any case, the major planets now orbit clear of each other, and the slight ellipticity of their orbits is quite stable. It might change with time due to the pull of other planets (e.g. Jupiter), but I know of no significant mechanism which would gradually make orbits more and more circular. The process which you propose resembles the gradual change of a satellite's orbit if its lower end (perigee) grazes the atmosphere, but there exists no significant "atmosphere" in interplanetary space able to do the same to the motion of a planet.
The length of days may perhaps change over the billions of years, because tides raised by the Sun take away rotation energy of the Earth. The final state would have one end of the Earth pointing sunwards, as is the case now with the Moon-Earth system. The process is very slow, however, and the Sun may run out of fuel, etc., before it happens. Seasons, in any case, are caused not by the daily rotation but by the inclination of the Earth's axis to the plane of the Earth's motion.
316. Forces on Comet-dwellersAssume a habitable comet in a highly eccentric orbit. If you were living on it, would you feel acceleration as it sped up close to the sun? My thoughts:
(i) No. the comet and you are just following curved spacetime. If there was a feeling of acceleration - would it not mean that the comet would break up? We don't feel acceleration as we orbit the sun (nor do astronauts in the space station orbiting the earth). Maybe we do feel it but we are so used to it that we can no longer notice it?
(ii) Yes. There is definitely acceleration - tangential as well as radial. If you have a large change in speed, how could you not feel it?
ReplyYou stand on the comet, somewhere far from the Sun, and let go of an apple. Ignoring now the feeble gravity of the comet itself, the apple would move in the same orbit and therefore just hang there. It will not "feel" any force impelling it to move to another location. And neither would you (if the icy chill, so far from the Sun, lets you feel anything).
Next suppose the same comet is near perihelion, following a hairpin orbit around the Sun. Sure, you are undergoing a large acceleration, moving you in a curved path. But so does the apple. If both of you sense the same force, you will maintain your relative positions, and so would all parts of your body. It will feel the same as no acceleration at all.
But wait! The apple may experience a different force (per unit mass) than the comet, if it is closer to the sun than the comet's center of gravity--or if it is on the side away from the sun, at greater distance. That difference, in fact, could even break up the comet itself, as has been observed (with Biela's comet). For more on that, look into the gravity gradient acting on our moon.
317. Nuclear reactors and bombsWhy does a nuclear reactor require a moderator to slow the neutrons down, but a nuclear bomb does not?
Also: Is the uranium in a nuclear bomb doped with an element that emits neutrons - in order to start the chain reaction? Or are there enough neutrons just flying around to trigger it (assuming that critical mass has been reached)?
ReplyA nuclear reactor uses slowed-down ("thermal") neutrons, while a nuclear bomb uses "fast" neutrons, emerging right after fission has occurred. The large spacing between "fuel rods" in a reactor makes sure it cannot explode like a bomb--though it may undergo "meltdown" if the nuclear reaction becomes uncontrolled (not supposed to happen, but a few times it has).
Slow neutrons are efficient in causing fission in U-235, but any such fission produces mainly fast neutrons, and if there is U-238 around, these are likely to be captured by it first (leading later to plutonium production). In a nuclear reactor, almost all these fast neutrons escape the fuel rod and wander around surrounding material, of a kind chosen not to absorb them well (such as "heavy water" or pure carbon), losing energy until they are slow enough to evade absorption. Then, if they re-enter a fuel rod, they can fission another U-235 atom. For these reasons, nuclear reactors can tolerate a fair amount of unenriched uranium containing mostly U-238, and the original "pile" of Enrico Fermi in 1942 used natural uranium with only 0.72% U-235. See
Bombs use highly enriched uranium or plutonium, with little or no U-238, and therefore can use fission by fast neutrons. Still, they need a "critical mass" for an appropriate ratio of surface area to volume. If they are too small, too many neutrons escape through the boundary and not enough are left to sustain the nuclear reaction (reactors have a somewhat similar problem). The reaction is started by explosively compressing the plutonium sphere to smaller size, and it will then spontaneously start a chain reaction, because plutonium 240 fissions and releases neutrons spontaneously at a small rate, although beryllium may also help speed up the process. See
May our world be safe from nukes.
318. Why doesn't magnetism affect electro-magnetic waves?I'm a 6th form student in the UK and it suddenly occurred to me while I was revising what we had learnt on waves and magnetic fields that, if EM waves, are indeed electro-magnetic waves, why is it that they are not affected by magnetic fields? And why do such waves not display the properties of magnetic fields in the fact that they loop from pole to pole?
Forgive me if this is a rather naive question, I do understand the principals behind EM radiation and waves quite well, its wave-particle nature, thus allowing superposition etc., but I'm just curious as to how there is such a distinction between an electromagnetic wave and a magnetic field. If one consists of the other then how is it not affected by the other, and how is it EM radiation can travel in a straight line for nearly infinite distances (gravity aside)?
ReplyElectromagnetic waves are linear--when several are added together, each preserves its identity and can be separated again. On the radio, or on TV, many stations can send their signals through the same region of space, and yet your receiver can pick out any of them and amplify it alone.
You could regard a steady magnetic field as a signal of zero frequency (taking forever to switch to its electric signature). It does not interact with any electromagnetic waves of non-zero frequency, at least in vacuum.
In a material medium the magnetic field may modify the electromagnetic properties of the medium, affecting the propagation of waves. For instance, the Faraday effect in transparent media may rotate the plane of polarization of an electromagnetic wave.
In a plasma (gas containing freely floating ions and electrons) many different kinds of modified electromagnetic waves are possible--depending on the frequency of the wave and how far it is from some characteristic resonances, which depend on density and magnetic field in the plasma. In particular, "Whistler Waves" (in space near Earth they are produced by lightning at frequencies like 3000 Hz) are indeed guided by field lines, sometimes bouncing back and forth from one hemisphere to the other.
319.   Is humanity changing the climate, or is it the Sun and the Earth's magnetism?Can you comment how climate change is affected by the earth's magnetic field, the sun's output, and other directly and not so directly related factors?
Where I can read about such theories? I have tried to understand the common reasoning about man being directly responsible for (however you label it) Global Warming, Climate Shift, etc. However, based on the sheer magnitude of such changes, I find that man-made causes are the most dubious direct factors. Among the other far larger factors, man's impact adds at most an assist or a catalyst to the equation.
It would seem to me that the earth's magnetic field might have a huge impact on how the sun's various and changing outputs are directed and distributed around our globe. Changes in the sun's output and changes in the earth's magnetic field seem to greatly affect global changes, with no input from mankind needed.
However, I have heard very little discussion of this highly suspect aspect of the discussion. It is almost as if the science of the sun and the earth, in areas where you appear to be an expert, have been curiously absent from the debate. Is this due to political pressure? Has the objective process of true scientific discovery started to buckle under the pressure of political correctness? Al Gore and Ponzi scheme carbon credits be damned!
ReplyI do not think magnetism has any connection to global warming. There is too little energy in the solar wind or its magnetic field, compared to the flood of energy we receive in the form of heat from sunlight.
Global warming is caused by molecules in the atmosphere, as described in
(you may like to browse that entire collection, too). Sunlight heats the ground, except for the small part reflected by clouds (something like 20%, I think), and some more scattered by dust and atomic processes. It reaches the ground, because air is transparent to visible light. Atoms can emit and absorb such light (though in general only in narrow ranges of wavelength), but atoms in the air are combined in molecules, which generally do not absorb.
But obviously, heat absorbed by the ground must return to space--otherwise Earth would just get hotter and hotter. In fact Earth radiates its heat by the same broad process as the Sun--the process which makes any heated object shine in electromagnetic radiation. With the Sun those are visible wavelengths, because of its 6000 deg temperature; luckily it is a small object in the sky, otherwise we'd quickly get fried. The Earth is only moderately warm, so it radiates in the infra-red. That is a weaker radiation, but it goes out in all directions, and can therefore balance the sun's input.
Molecules, as it happens, absorb and emit infra-red. They too are tuned to certain "frequency bands", not as sharp as atomic resonances but also depending on the substance. Water vapor, for instance, is a "greenhouse gas": it absorbs infra-red in its resonant frequencies, re-emits it, higher up it is absorbed again and re-emitted, and so on up to the high levels, which are dry and rarefied, allowing the radiation to escape to space. Were it not for water vapor, Earth may have been in a permanent ice age.
But there exist other wavelength bands which water vapor does not cover: carbon dioxide, methane and ozone cover some of these, and that is where human influence makes a difference. It is a serious effect. Put one drop of India ink in a glass of water and note how much darker it gets. Similarly, a moderate amount of an additional "greenhouse gas" can seriously slow down the escape of heat from the surface, by closing (or at least narrowing down) yet another wavelength window through which heat escapes. If that happens, the surface of the Earth needs to radiate more strongly to keep up with what arrives from the Sun. Climate gets hotter, oceans warm up and expand to flood seacoasts, and so forth.
I hope that made it clear. Enjoy my web sites!
By the way--the heat output from the Sun might conceivably vary too, which is why NASA has put very sensitive detectors of total radiation on some spacecraft (measurements from the ground are not accurate enough, the atmosphere absorbs and scatters too much of the light). These have found some tiny variations associated with sunspots and their cycle, but nothing to explain global warming.
320. Advice to home-schooling parentIn the process of searching for an Astronomy curriculum for my home-schooled high school senior, I came across your website.
I am interested in getting this curriculum for my daughter. Will she be able to complete the course work online? Or should I purchase the CD as well? I am not a very "technical" individual, but my daughter is! She desires to become an astrophysicist. I'm confident she'll achieve her goal because she has the passion for it as well as the understanding in math and science. Thank you for your help.
Could you please also provide a working link to the poems listed on your website, e.g. those by Bialik? I was not able to access them using the provided link.
ReplyThe central linking page is
You should be able to reach any of my files from there, including the poetry. The home page for the non-science pages is
As a home-schooling parent, you can buy a disk from April Dykes in Texas (see link at the end of big yellow box at the start of "From Stargazers to Starships"), but if you have a DSL web connection (or patience) you can also download the files to your computer (about 40 Mb) as "zip-archives" which automatically create the proper files after they are downloaded. The web addresses for this are on the "readfirst" page linked above. Of course, you can also copy them after that from your hard drive onto disks.
One thing you might want are solutions to problems of "Stargazers," made available only to teachers and teaching parents (the problems are listed at the end, in two sections). They will be attached to this message (but readers of this shouldn't expect them here!): please do NOT pass them any further (except to other homeschooling parents, who agree to abide by a similar restriction). They are not for your daughter--let her figure them out!
"Stargazers" is a course in physics and astronomy. Over the years it has grown far too big to be given in a regular classroom
(see http://www.phy6.org/stargaze/Scaution.htm )
but a determined teenager can cover most of it--depending on motivation and on competing interests, of course. Get her started on sections 1 to 11 of "Stargazers" and see how much time they take: if progress is too slow, some parts may need to be skipped or abbreviated.
YOU are the teacher. Therefore, even though your daughter may roam through the material at will, you should acquire an understanding of it, too, preferably ahead of her. The 46 lesson plans may help. Much more can be said, but homeschooling parents learn to improvise, and if your daughter has reached the senior level, I am sure you can do that well by now. Some thoughts:
---The questions and answers at the end of sections are a selection of what users have written (and my answers to them). They often clear up problems by viewing them from a different angle.
---The math course may help, if your daughter's math has gaps. It does have some problems to solve, too.
---The timeline (especially the one in "Stargazers") is useful for integrating history and science, and helping see the overall framework.
---If your daughter is studying Spanish as a foreign language, she may find it fun to read the Spanish translations together with the original English web pages. (Or the French, or Italian.)
Final thought: becoming an astrophysicist is hard and may be frustrating. Your daughter should read an exchange I had with a student in India, at about the same age, with the same question:
All the same, I wish you and your daughter every possible success.
321. Science of Clothing  Have any ideas about how to introduce some of the concepts which you discuss to 6th graders -- girls in particular? Any advice you can provide will be greatly appreciated. Thanks so much for sharing your knowledge on the internet.
(Later additional message) What I'm most interested in is the science of clothing: how can I apply angles, force, friction, color theory perhaps? All this, to show girls that there is science behind fashion.
Reply  There is science behind clothing, but it is mostly chemistry (dyes, fabrics...) and I happen to be a physicist. The physics involved is mainly related to heat insulation: why is a fluffy fabric so much better as a barrier to loss or entry of heat than a solid heavy material--bricks or metal, say? It has to do with the trapping of air and stopping it from circulating; air is a poor conductor of heat, but it can transport heat by circulating (as it does in the atmosphere). Wool and feathers have finer fibers than cotton, so they are most efficient in trapping air and in cold climate they are best.
On the other hand, cotton draws water and fluids, so for candle wicks and clothes in hot climate--to shield the sun but keep the skin cool--cotton is preferred. Desert Arabs (e.g. Tuaregs) wear a cotton shawl over their mouth: when you breathe out moist air, some of it is caught by the fibers and helps moisten the air you next breathe in, with water you otherwise may have breathed out and lost. And the Polynesians have their culture based on Tapa cloth, a felt made of fibers--the kind of fabric used before spinning and weaving were invented.
About colors, see
including maybe the section "experimenting with color"--if you have access to computers. You may need some help there. By the way, the color "orange" comes from the Indian name for the fruit (naranja), not from the name of the royal house of Holland. There exists a whole lore of color--Tyrian purple, mauve (first artificial dye), saffron, khaki, indigo and so on.
But I would rather use clothing to teach language: Muslin comes from Mosul in northern Iraq, Calico from Calicut in India (calico cats have patches of color like that cloth; and tabby cats resemble a cloth made in the Attabi quarter of Baghdad), Damask originated in Damascus, seersucker, pajama, dungaree, denim--let the girls search dictionaries on their own, also words like distaff and hose (pants in German are "hosen")... so much more.
Let them look through the book of Marco Polo and learn his story... being a merchant, wherever he traveled, he often noted what kind of fabric the city specialized in. In one place he was shown a fabric which could be put through a flame and not burn: I guess it was asbestos.
And then of course all those artificial fibers. But sorry, I am just a physicist, who specialized in space research. Ask someone from the textile industry!
322. Calculating a CollisionI saw your website and thought you have great answers to many questions. I hope you can help me figure out what I know is a probably a basic physics problem. I could be doing this wrong but I'd like to know. There was a story I read recently about a 1,500 lb wrecking ball that came loose, rolled downhill, and smashed into a car moving it 20 feet. The car weighs about 2,400 lbs, I would imagine. I was trying to formulate a way to calculate how fast the ball must have been rolling, using "slugs."
(A detailed calculaion followed).
Is this right?:
ReplyYour answer is probably not the right one, and in fact the problem cannot be solved without added information about the roughness of the surface. The same force will push the car much further if it is standing on smooth ice then in a plowed field!
You need to know the coefficient of friction k (my notation here, many tests use μ), the ratio (in this case) between the (horizontal) force of friction on a moving object and the (vertical) weight causing that friction. It is an observed fact that k is almost independent of speed, so whether you move fast or slowly, the friction force is the same. We assume so here--without doing so, the problem becomes even more complicated and more assumptions need to be made.
The value of k can vary anywhere from about 0.02 (car on smooth road, axles greased and turning freely, tires well inflated) to 1.00 (car with brakes applied tightly, to where it would not slide even on a 45-degree slope). Let me assume here k=0.2.
You must also assume something about the collision--most likely, an inelastic collision, in which the ball smashes into the car, after which the two travel together.
I am not familiar with calculation in slugs and feet--most of the world (especially technical people) uses the metric system, so excuse me if I solve a slightly different problem--a 700 kg ball hitting a car weighing 1100 kg, moving it 6.1 meters.
Say the ball started with velocity v, and after impact, car and ball continued together with velocity u. By the conservation of momentum (section #18b)
700 v = (700 + 1100) u = 1800 u
The kinetic energy of the (car + ball), after impact is
0.5 m u2 = 0.5 (1800) u2 = 900 u2 joule.
This is converted to heat of friction by dragging car and ball over a distance of 6.1 meters, against a friction force F, equal to 0.2 times the weight F' of the object being moved. We get F' = mg (section #18) by multiplying mass (1800 kg) by the acceleration of gravity, which is 9.8 m/sec2, so
F' = 1800 . 9.8 = 17,640 Newton
F = 0.2 F' = 3528 Newton
The work W performed (section #18c) equals the force F being overcome times the distance, and by conservation of energy, it should equal the initial kinetic energy 900 u2 (the final kinetic energy is zero). So
W = 21520.8 joule = 900 u2
u2 = 23.912
u = 4.89 meters/second
v = (1800/700) 4.89 = 12.57 meter/sec
which is about the velocity of a ball falling from a height of 12 meters. We ignore the fact the ball is rolling and assume it is sliding. Otherwise the calculation is more complicated, since the rolling motion takes up some of the kinetic energy. A ball rolling down a slope from 12 meter height is slower than one sliding (without friction) down the same distance, because some of the energy of gravity goes into the rotation of the ball.
You see how complicated real-life problems can become!
RsponseThanks! Yes, it is all about the friction, and I'll think in metrics from now on. Since the police report was in feet and pounds, I used slugs. Also thought it would be hard to calculate velocity since the car was slowing down after being hit by the ball. I very much liked this part of your explanation:
323. The Coriolis force and moreOn your website you state that the Coriolis force is too small to affect the rotation of water down a toilet, yet the water does flow in opposite directions in the Northern Hemisphere as compared with the Southern Hemisphere. What else would explain this phenomena?
The Coriolis acceleration I learned in school from Prof Wills at CCNY is described as a vector equation as follows:
Ac = 2 ω x V
where ω is the vector describing the angular velocity of the rotating earth, approx 70 micro-radians per second, V is the velocity vector and Ac is the coriolis acceleration. It is approximately 0.004 feet/second squared, which is small but perhaps large enough to start water flowing in different directions down a smooth toilet bowl.
I enjoy reading your web site just as I enjoyed reading the books of George Gamow in the 50s.
Why are all planetary bodies planets, stars, etc. round? Asteroids may be pear shaped but most bodies are round, why are some not shaped like a disk? Why does gravity exist at all? Did Einstein ever discuss that issue?
ReplyYour formula is correct, assuming V is the velocity vector in the rotating frame of reference (velocity in addition to the one resulting from being carried around by the Earth's rotation itself) and x denotes vector multiplication, which also depends on the sine of the angle between the direction of motion (vertical, for draining water) and the Earth's axis.
However, the rotation induced by the Coriolis force F = m Ac depends not just on F. It also depends on the variation of F between the two sides of the descending or rising fluid. If F is the same on both sides, the entire motion just gets deflected to one side. Rotation is produced because F is smaller on the point closer to the pole. Near the pole omega is almost parallel to V, making 2(ω x V) very small.
That is why hurricanes are affected--they are much bigger--and draining sinks are not. In Jupiter's red spot, which is still bigger, the effect is even more pronounced--and Jupiter also rotates faster than Earth. In fact Voyager's pictures of Jupiter show many additional swirls in its atmosphere.
Why are planetary bodies round? You would expect gravity to make them so, if they were all liquid or gas--any bump rising on the surface would be pulled down to its level. In a big planet like Earth, the force of gravity is strong enough to force the material into the shape of a sphere. Rocks on the surface can vary in elevation, but at a depth of 100 km (say), the weight of layers piled up above makes even rocks adjust their level as if they were fluid.
That is the reason no mountain on Earth reaches even 10 km: the weight of mountains pulls them down. On Mars, the highest volcano can rise to about 25 km, because gravity is weaker. Asteroids and moons up to about 500 km can maintain irregular shapes, but if they are larger, gravity causes them to become round. See also question 305 in this collection.
All this in the absence of rotation. Slow rotation makes the shape elliptical (oblate), a small effect on Earth, a larger one on Jupiter and Saturn. But a limit exists: speed up the rotation too much, and the planet breaks up to a disk, somewhat like the rings of Saturn.
See also reference 26 on the list on top.
Finally, why does gravity exist? We don't know any more fundamental reason, except maybe that if it did not, the universe and Earth would not exist, and we would not be here to ask questions. For us, and for Einstein too, gravity has been a fundamental force of nature, and we are still trying to understand its properties (see http://www.phy6.org/stargaze/Sun4Adop3.htm )
324. Why isn't the solar system stratified by density?I'm a law student (sigh) from Germany, and I spend way too much time thinking about physics
A question has been nagging me some time now.
I've been thinking through the formation of the solar system: ok, so we have this huge gaseous cloud compressed into a disk by it's own gravitational pull. Seeing that the disk is still rotating I assume a centrifugal force to be exerted on it which leads the elements it is comprised of to distribute according to their specific mass, lightest at the core, heaviest at the edges. Next step, they clump into pieces. OK, so far so good, plenty of hydrogen at the core, massive planets following and then the asteroid belts that for some funny reason don't want to stick together (maybe the pull of nearby Jupiter disrupted their clumping).
BUT THEN: the gas giants. I mean. Excuse me? Is my thinking that flawed? Isn't that gas supposed to be in the sun to give us a few more years of gentle warmth? Hydrogen and helium? (I imagine some kind of dialogue of the type "hey, what is such a cute and especially light couple doing out here at this time?") Could it be possible that it's the hydrogen scooped up from deep space AFTER the formation of the core solar system? I just don't get it. Where did I go wrong? What did I miss?
ReplyYou are asking intelligent questions, and it is not easy to answer them, especially without mathematics. If you go on with your studies, I wonder if you end up practicing patent law, where such questions sometimes arise.
Why doesn't the solar system separate--light stuff in the middle, heavy stuff outside? Let's look at a situation where this DOES happen: a very tall glass (tall enough so you need not worry about friction with the bottom) filled with a mixture of gasoline and water, and stirred into rotation by a long spoon, or maybe a paddle attached to a motor.
You know what will happen: the water which is heavier will gradually move outwards, and the gasoline which is lighter will be near the middle. In the rotating frame of the fluid, every bit of matter experiences an outwards-directed centrifugal force (sections #23 and #23a in "Stargazers"). If two equal globules rotate side by side, one of gasoline and one of water, the one of water is denser and senses the greater force, so it moves out while pushing the lighter gasoline inwards.
Now replace this by a solar nebula, rotating because of some systematic average motion it had when it started to pull together. No spoon or paddle is necessary--it just rotated when it came together, and there is nothing to stop it. The centrifugal force may be said to be balanced by gravity (or else, looking from a non-rotating frame, the centripetal force is supplied by gravity). If some material moves out, it slows down, but has then larger potential energy, so it regains its speed as is moves in again--as, say, Halley's comet does (see section on Kepler's 2nd law, section dealing with energy).. This holds for light and heavy material alike, for dust and gas, and in this motion, materials do not separate. The difference is that instead of glass walls, you have here the force of gravity, and it behaves differently.
At early times, however, you expect parts of the "solar nebula" to also have large random motions, producing many collisions between dust grains and gas molecules moving in different directions. Such collisions convert energy into heat, and help material concentrate in the middle, where the Sun is formed. The Sun also rotates, but its angular momentum (which measures its rotation) is rather small, compared to that left in the planets. (This process, by the way, is also discussed in the reply to question 315 above).
When the Sun started to glow, it evaporated much of the material which has collected around it, and those molecules were blown away--by light pressure maybe (this is not my area). The gas giants were distant and escaped great heating; comets and distant moons also stayed frozen. But Earth and Venus are lucky to keep their atmospheres; they may well have once had big hydrogen clouds around them, too, before the Sun blew them away.
These at least are my thoughts; you might ask an astronomer, who might have better information. My real field is the magnetic field of the Earth and ions and electrons moving in it.
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