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I wouldn't call those "why" so much as "what" questions: What causes earthquakes, what causes tides. Regardless of how one looks at those questions, there is some point at which the answer becomes axiomatic. In other words, "because we said so". Geology uses a lot of physics and chemistry in its models. At some point a geologist will answer some question with "I don't know the answer. You'll need to go ask a physicist." (If the answer is "You'll need to ask a chemist", poke deep enough at that chemist and she will eventually end up saying "go ask a physicist.") We physicists have no deeper source to turn to. We could turn to metaphysics, but the problem with that is (a) the divorce decree between physics and philosophy was finalized hundreds of years ago, and (b) we want our axioms to be testable. Newton and others had a deep problem with the instantaneous action at distance that pervades Newtonian physics because it allows violations of causality. One could argue metaphysically that a causal universe must by necessity have a finite speed limit. A little more metaphysical handwaving and voila, only massless particles such as photons can (and must) travel at this universal speed limit. One minor problem here: These metaphysical arguments are not testable. At some point it becomes better to stop the infinite regress of "why" questions with axiomatic statements that are testable.

The Apollo program was a large drain on the federal government. The nuclear space concepts such as NERVA would have made the huge expenditures on the Apollo program permanent. Congress and the President did not want NASA to continue at those high level of expenditures forever. They had other things to fund: the Vietnam War, the Great Society programs, etc. Apollo, along with everything that went with it, was political toast once the primary mission was accomplished. Space exploration is not going to solve the population growth problem. The ability to put millions of people per year into space is a long ways into the future. The current population growth rate is about 75 million people per year. The growth rate is expected to decline but still be positive, and by a huge amount, for a long time. By the time space exploration becomes available to the masses we will either have solved the population growth problem on our own or the problem will overwhelm us (and there will be no space program anywhere). The proper recourse here is to do something positive about the supposed problem, not to flee it. We should be looking for those killer asteroids (we are) and if we see one, do something about it. The killer asteroids are big, fairly easy to see. Smaller ones are harder to see but are not a threat to civilization as we know it. I've never been fond of the space exploration as a population relief mechanism or space exploration as a disaster avoidance mechanism arguments. One reason for space exploration is that we are curious apes. We explore. Humans and our ancestors have been explorers for millions of years. Another reason is that we are greedy little apes. Space is full of exploitable resources, but we do have to drastically reduce the cost of access to make exploiting those resources make sense economically. Since space exploration is largely government funded, it is hard not to veer off into politics. We talk about science and engineering concepts here because most of us don't grok politics. Politics is not rocket science. That's why rocket scientists do so lousy in understanding politics. The private sector has been allowed to pursue this for a long time. A good fraction of the satellites in geosynchronous orbit are privately-owned (non governmental) satellites and were launched on privately-owned launch vehicles. What is new is that companies such as Virgin Galactic / Scaled Composites are looking to bring paying passengers into space (but for the most part not into orbit). Another new development is embodied by SpaceX. He is bringing the entrepreneurial spirit into the launch business. The companies that currently do do private launches once had that entrepreneurial spirit, but they appear to have lost it long ago from too much feeding at the government trough.

You need to specify what you mean by "mass", alpha. There is inertial mass, active gravitational mass, and passive gravitational mass. Newton implicitly assumed inertial and gravitational mass are one and the same. Einstein made this assumption explicit in the equivalence principle. In terms of Newton's laws, inertial mass is the factor m in a = F/m; the mass term M in a=GM/r2 represent active gravitational mass. I intentionally reformulated Newton's laws to hide passive gravitational mass. Newton's second law is F=ma; his law of universal gravitation is F=GmM/r2. If inertial and passive gravitational mass are not the same, the m term in Newton's second law is inertial mass for non-gravitational forces but is passive gravitational mass for gravitational forces. A particle and its antiparticle have the same inertial mass, both per theory and per experiment. Some very weird things would happen if the inertial mass of an antiparticle was negative. Suppose we place a charged particle and its antiparticle at rest with respect to one another. The particle with positive inertial mass would be repulsed by the other particle, but the particle with negative mass would be attracted to the other particle. The negative mass particle would chase the positive mass particle. We would see signs of this weird behavior in particle colliders if antiparticles had negative inertial mass. We don't see that behavior. Instead, we see behavior that is consistent with particles and antiparticles having the same (positive) inertial mass. Determining whether antiparticles have negative passive gravitational mass is a bit harder to test. A number of experiments have been proposed recently based on the recent acquired ability to make cold anti-hydrogen. Note that these proposed experiments are not looking for negative mass (negative mass is a crackpot notion); they are instead looking for tiny violations of the equivalence principle.

No, we don't. Just because you do not want to read the truth does not mean you get dictate the terms of the debate here. Ummm, no. The light we see now emitted by some object 13 billion years ago has been traveling for 13 billion years. This does not mean that the object was 13 billion light years away from us 13 billion years ago, nor does it mean that the object is 13 billion years away from us now. What it means is that the object was only 3.35 billion light years away from us 13 billion years ago and that it is now 29.7 billion light years away (comoving distance). The above values were calculated assuming a flat universe, a Hubble constant H0 of 71 (km/s)/Mpc, and a mass density ΩM of 0.270. Those values for H0 and ΩM are from the seven-year long WMAP study. Different values will yield different results, but qualitatively they will be similar. The object was much closer to us 13 billion years ago than than that 13 billion years appears to suggest, and it is much further than 13 billion light years away from us now.

I did a senior thesis in college as well. I expressed in interest in computational physics. My advisor wrote on the board [math]x_{n+1} = 4 \lambda x_n (1-x_n),\qquad x_n, \lambda\in(0,1)[/math] and asked me said to tell him as much as I could figure out about this by the end of the semester. This was back in 1977; that factor of four is typically omitted from the logistics equation (with lambda varying from 0 to 4 instead of from 0 to 1). He didn't tell me that this seemingly simple equation had a name nor did he tell me that this seemingly simple equation was all the rage in computational physics. In fact, he asked me not to do a literature search. He wanted me to find out what I could on my own.

A snarky way to look at it: The liberal arts haven't made any advances in teaching techniques in the last 2000+ years. We on the other side of the great divide between the two cultures have. A less snarky way to look at it: One consequence of the scientific revolution was that it forced the technical side of the two cultures to reevaluate the techniques used to train future mathematicians, scientists, and engineers. The well-honed and well-honored techniques used to train future leaders of industry, diplomats, lawyers, etc. simply did not work all that well for the burgeoning world of science. One of the key techniques that has worked for 2000+ years in the liberal arts world is the concept of a reading list. Students are given reading lists (lists that grow ever more voluminous as one progresses up the academic food chain) of writings of the great minds. This concept simply does not work for technical education, at least not at the introductory levels. The reading list concept is employed to some extent in technical education, but only in a modified form and only for highest level undergraduate courses and higher. We don't read the great masters for one thing. We instead read very recently published technical papers, most of which are written by great bumblers rather than great masters. Another difference is the size of the reading list. Ours is downright tiny compared to what graduate students in the liberal arts are expected to read. You guys are expected to read thousands of pages a week. We struggle to get through a very small number of five to ten page technical papers in that time. There's a dirty little secret that underlies much of the technical education at the elementary, high school, and lower undergraduate levels. The scientific knowledge presented as so beautifully consistent, so nicely compact, and so very well thought out was not always that way. The initial developments of almost any technical body of knowledge were overly verbose and more than a bit bumbling. Teaching that verbosity and bumbling around just gets in the way of the primary goal of technical education, which is to teach the underlying concepts. It is that pretty picture that technical educators want students to understand, and thus those underlying concepts are presented in the form of a pretty picture. Developing that pretty picture takes quite a long time. The main reason educators in technical fields use the reading list technique at the graduate level is because the pretty picture has not yet been developed.

There is no logic hidden under the conspiracy theories. Only illogic coupled with a driving desire to be safe. This desire for safety above all else is IMO a rather harmful one. If you want safety and stability, the best place to look is a police state. If you want rights and freedom you have to be able to accept some amount of uncertainty, instability, lack of safety, and antisocial behavior.

That's the appeal: If only we could expose and get rid of these known corrupted few then we will be safe again. If on the other hand, a very small group of wackos anywhere in the world can conspire amongst themselves and cause a great deal of harm, or even worse, a single lone wacko can do so without conspiring with anyone, then we are never safe. This is particularly so if our various spy and law enforcement agencies have these longstanding chinks in their armor. Many of those chinks are intentional constraints; we as a society do not want to give free reign to these governmental agencies. The only way to attain true safety is to forego our freedoms and let those agencies have free reign -- i.e., we need to become a police state if we want to be truly safe. In the minds of some it is better to believe in a conspiracy theory, no matter how ludicrous, than it is to consider the alternatives. The alternatives are just too hard to bear.

That is exactly what I was talking about in post #14. Some can see the beauty in math and science, and if you can see that beauty learning about math and science is anything but boring.

You are looking at it wrong. First off, I dispute your contention that someone in junior high can add anything of value to the humanities and social sciences. You can teach someone in junior high to appreciate a Picasso painting, a Rodin sculpture, a Mann novel. There is beauty in math and science, too. Some can see that beauty, some apparently cannot. To those who can see that beauty, just learning can suffice. It isn't a struggle or boring. It's beautiful.

Perhaps. Then again, if Marat had said it in those words my response would have been different. As is, the opening post is an attack on the sciences -- and this is a science forum.

Yes, undergrads in the sciences and engineering most likely are not going to contribute something new to their field, at least on their own. That doesn't mean they can't do anything creative. Problem solving requires and instills creativity. Nor does that doesn't mean they cannot participate in research. There are plenty of opportunities for undergraduates to participate in research in the sciences and engineering. Personally, I found those basic humanities courses to be the most boring, the dullest, and the easiest, of all my college classes. First off, what does this statement have to do with your thesis that basic science is duller than basic humanities? Secondly, it is a false statement. A student at Harvard can graduate with honors, and even high honors, without submitting a thesis in biology, chemistry, engineering, just to name a few. Had you said "highest honors", your statement would have been closer to the mark, but still incorrect. An undergrad physics student can write a thesis, and getting an A will knock out a lower grade. It is just that the physics department at Harvard that does not absolutely require a thesis to obtain "highest honors". It is not the only one.

There are no paradoxes infesting relativity. There are some things that appear to be paradoxical but are not. The point of these paradoxes is to illustrate that you need to think differently to understand relativity. Once you learn to look at things right the paradoxes disappear.

Correct. g varies from 9.780 m/s2 at the equator to 9.832 m/s2 at the poles. This is because the Earth is rotating. It has nothing to do with magnetism. Earth gravity, g, is defined as the local free-fall acceleration as observed by someone at rest on the surface of the Earth. This makes g at some location on the Earth equal to the scale weight of some object at that location divided by the object's mass. From the perspective of an observer at rest on the Earth, an apparent centrifugal force due to the Earth's rotation acts to slightly counter the downward force due to gravitation. Physical geologists distinguish gravitation and gravity. Local gravity, g, accounts for both gravitation and this centrifugal force. This centrifugal acceleration various from 0 m/s2 at the poles to 0.0034 m/s2 at the equator. Note that gravity is about 0.0052 m/s2 higher at the poles than at the equator. Centrifugal acceleration accounts for about 65% of this difference. The remaining 35% results from the Earth's equatorial bulge (which is also caused by the Earth's rotation). A person at sea level at the North Pole is 22 km closer to the center of the Earth than is someone at sea level at the Equator. Newton's law of gravitation says that for a point mass, gravitation due to the point mass is inversely proportional to the square of the distance from the mass. While the Earth is not a point mass, this 22 km difference in proximity to the center of the Earth does indeed account for the remaining 35% of the difference in g.

The homework is well past due (this thread is three weeks old now), so the answer can be given. Something orbiting the Earth such as an artificial satellite or the Moon is not outside Earth's gravity field. The best way to look at an orbiting body is that it is perpetually falling toward the Earth. I missed insane_alien's post #15. That is just about spot-on. This is something a 17 year old in AP physics should be able to answer.

That "at this moment" is the key concept. None of the technologies proposed in this thread is ready for prime time. Many never will be; that's just the nature of the research game. The hope is that at least one of them will be sometime in the future. I am not against doing research, nor am I against incorporating the fruits of a research project into an operational system. Either stance would be rather hypocritical of me; doing research is about as good as it can get. What I have been railing against in this thread is that some think our "space program is a joke" because it is not using propulsion concepts that are currently at the cutting edge of research. That isn't how the R&D game works. Some of what is now a cutting edge propulsion concept will be used in the next generation space program (20 to 50 years in the future). Some of those ideas we now think are hot will turn out to be absolutely useless. One last point: Some of the new technology in the next generation space program doesn't exist right now. Take a look at a 20+ year old Popular Science magazine where they tried to predict the future. The further back you go the more likely it is that they completely missed the boat. Paying attention to facts and reality counts even for flying to low Earth orbit. Fail to do so and facts and reality will bite you hard.

Excellent posts! I agree with all of you!

Those institutions charge their users/subscribers very low rates. The institutions do not get very low rates. They get very high rates. Paying for the paper journals that form a big part of a college / technical institution's library and paying for site-wide electronic access to journal articles is a huge portion of those institution's library budgets.

No, he didn't. Whether its safety of a nuclear power plant, a chemical plant, or a rocket, there is a certain lingo that is used to convince others that safety factors have been addressed. This is not it: "... should instill plenty of safety margin in the nuclear core of each thruster." The use of "should" and safety don't mix. His third scram mode, venting the fuel mass, doesn't quite mix with launching from ground. Given that the very top of the page says "Your independent source for nuclear advocacy," that should be viewed as a distinct possibility. More likely, though, is that the author is either exaggerating or misunderstands what has been done in the past. It's hard to tell; there are no references in his article. I've looked. I've looked and looked. I have found things like http://www.lanl.gov/orgs/pa/science21/NuclearRocket.html, which states "A gas core nuclear rocket still remains to be built and tested." I have found many similar statements regarding gas core reactors. What I have not found is one scientific paper that describes even an experimental gas core reactor. I have found plenty of white papers on the concept. Another thing that I have not found is one scientific paper by Anthony Tate, the author of the web pages in question. You are advocating a technology that is 20 or more years away from fruition. There is nothing wrong with that per se so long as you recognize (a) that the technology is 20 or more years away from fruition and (b) that it may never, and most likely will not, come to fruition. Regarding that parenthetical remark: Most advanced research projects never do come to fruition. It's the nature of the game. The gas core nuclear rocket is one of many possible technologies that are currently at a low level of technology readiness. Multiple papers at the recent Advanced Space Propulsion Workshop, which focuses on propulsion technologies in the Technology Readiness Level (TRL) 1-2 range, discussed gas core nuclear rockets. Most of the topics discussed at that workshop will never see the light of day. The hope is that at least one will. Which one, there is no telling. Nobody has the crystal ball that will let us see which of these myriad technologies will become the propulsion technology of the future. This is the heart of the problem. You are claiming that this specific technology, which no one has built yet, is the future. Technology organizations such as NASA fund lots of basic research efforts at a small level. Most of those projects never go anywhere. Some projects show limited hope and remain at that small funding level for quite some time. A small handful of those basic R&D efforts advance to the next stage. The proportion of the initial R&D proposals eventually make it to deployment / operational use is rather small, particularly so when the initial concept is at a very low TRL. It generally takes a long time for basic research to make it from the concept stage (TRL 1) to operational use (TRL 9). To illustrate, consider SpaceX again. They are using chemical propulsion, something that has been at TRL 9 for decades. SpaceX was founded in 2001. They will start flying cargo missions to the International Space Station in 2011. That is ten years from startup to operational use. That it took them only ten years is absolutely astounding -- and that is for a technology that was already at TRL 9.

Oh, please. Did I say anything like that? No. Have I said we should stop researching advanced technologies? No. So please do stop putting words in my mouth and using fallacies. I did read the article. It is just badly written sci-fi, and no, gaseous core nuclear reactors have not been made. All that has been done is paper studies and computer models. Gas core nuclear rockets might well be the next big thing. It will be a long time before such a spacecraft is ever used for people or even hauling cargo. And it will be an even longer time before it is used for launch. That picture was ludicrous. Nuclear power has some powerful opponents. NASA currently uses Radioisotope Thermoelectric Generators on its deep space missions that go beyond Mars (solar panels just don't cut it past Mars). The amount of plutonium in those RTGs is small - less than 10 kg. Yet every launch of a probe equipped with an RTG is met with protestors. Ramp that up to the scale needed for a GCNR and it wouldn't just be wacko groups protesting. Entire nations would get in on the act. The article did not address safety issues associated with gas core nuclear rockets at all. It instead said that people have died using chemical propulsion systems (and by implication, the GCNR proponents don't have to worry about safety). Ignore the safety issue: Nobody knows how to build one. To repeat what I said before, nobody has built a gas core nuclear reactor, let alone a gas core nuclear rocket. That means that GCNRs, if they ever are used, will not be used for at least 20 years. It takes a long time to develop new technologies and bring them up to the state of technology readiness where they are deemed ready for prime time. So what are we supposed to do for the next 20 years? Wait for the magic bullet to suddenly arrive? That would be the death of the space program, and it would be the death of any research toward finding that magic bullet. Various countries in various forms have tried this experiment before. Great Britain, for example, at the urging of their space scientists, made it illegal for British government agencies to have anything to do with human spaceflight. The end result: Britain has one of the weakest space programs amongst the Commonwealth nations. The space scientists who convinced Parliament to ban British involvement in human spaceflight had to move out of the country or find other careers. It is only in the last year or so that Britain has begun to relax its ban. Putting space exploration on hiatus until the magic bullet arrives that takes us to nec209's stage 2 is a guarantee that that magic bullet will never arrive. We need to keep moving forward with the technology that we have now and research candidate new technologies at a low level -- and that is exactly what the space programs around the world are doing.

What NASA and the DoD are currently researching, and have been researching for looong time, are scramjets. Whether this will ever have a return on investment, who knows? Regarding your nuclear light bulb that you raised back in posts #67 and #69 -- Some movies are "so bad they're good." "Plan 9 From Outer Space", "Twisted Brain", ... That webpage you cited in post #69 was right up there with those movies.

I used those as a comparison, Moontanman to illustrate that specific impulse is not the be-all and end-all in propulsion. You are acting as if specific impulse is the only thing that counts. It isn't. An air-breathing rocket would be a *huge* breakthrough. Cheap access to low Earth orbit is the first and absolutely essential step toward cheap access to space. Example: The delta-V cost from low Earth orbit to low Mars orbit is about 2/3 of the delta-V cost of just getting into low Earth orbit in the first place. Air-breathing rockets are one way to attain that cheap access to low Earth orbit. Air-breathing rockets are chemical propulsion. I told you exactly what those opportunities for improvement were in my previous post. I suggest you re-read it. Stop looking at specific impulse as the one and only metric for "improvement". (Besides, air-breathing rockets have a huge specific impulse, and they are chemical propulsion.) The ultimate metric is cost, e.g., dollars and cents.

There's little point in arguing with cypress anymore given that he has been banished from the site.

Specific impulse is not the be-all and end-all. Look at ion propulsion. It has a huge specific impulse. Short of an incredible breakthrough, it cannot be used for launch. Look at solar sails. With no propellant whatsoever, it has an infinite specific impulse. The tiny force (tiny, tiny, tiny force) exerted by even a very large sail limits the utility of solar sails. There is a lot to be said for low Isp fuels that require much less intensive processing and much less expensive / less massive storage. This is particularly true for fuel used on-orbit. If you want fuel depots in space (and NASA does), the economics and safety that result from easily-handled, easily-stored, high thrust, but low-Isp fuels can be rather compelling. There is a lot of room for improvement in operational costs. A lot of NASA's costs stem from the huge marching armies in Florida, Houston, Huntsville, Bay Saint Louis, etc. needed to plan, prepare for, operate, and recover from launch. Cut down on these marching armies and their multiple expensive facilities and you have done a lot to reduce the cost of access to space. This is the SpaceX route: Build a simpler, more reusable vehicle and keep the ground operations down to a minimum. There is still room for an immense breakthrough in chemical propulsion in the form of air-breathing rockets such as scramjets. Although this is still science fiction in a sense, a lot of work has been done in this arena. While specific impulse isn't the be-all and end-all, the potential for a *huge* specific impulse of an air-breathing rocket makes this the most active future propulsion research area currently funded by NASA and by the DoD.

You don't need to explain anything. You are wishing upon a star, a star that doesn't exist. You are not qualified to judge the quality of our space program. I am. This is my job. Part of my job is looking at the big picture with respect to space exploration. Chemical propulsion is what we know and what we use. There remains significant room for improvement in chemical propulsion. Ion propulsion and plasma propulsion have some promise. Fission propulsion has a lot of promise but also an immense amount of political baggage, enough political baggage to make it rather dubious that it can be employed. The rest? Pipe dreams, or worse. My advice: Stop dreaming and get real. The space programs we have today are not the stuff of science fiction movies. So what? They are real. There is a lot of exciting stuff going on right now. If you would just take off your sci-fi glasses you would see. You refuse to see, so I am done with this thread.