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  • Location
    Houston, Texas
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    Now that my kids are grown up and out of the house I will have time for some hobbies
  • College Major/Degree
    Physics, during the stone age
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    Spacecraft guidance, navigation, control
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    Aerospace engineer


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D H's Achievements


Scientist (10/13)



  1. That is his point. Nonsense. There is no point other than scientific curiosity in going to Mercury. There is no point at all in sending humans there, at least not in our lifetime. Getting to Mars is cheap. The delta V requirements for a one-way mission to Mars is less than a third, and maybe less than a quarter, of that for a one-way mission to Mercury. A round-trip mission to the surface Mars and back to Earth, while costly, is cheaper yet compared to a round-trip mission to the surface of Mercury and back to Earth. To make matters worse, that factor of 3 or 4 or more in delta V translates into a huge factor in terms of fuel. The amount of fuel needed is not a linear function of delta V. Fuel costs grow exponentially as delta V increases. Any resources on Mercury are much more easily obtained in the asteroids. Or for that matter, right here on Earth. The idea of mining asteroids (or the Moon, or Mercury) and sending the mined items back to Earth is, for now, science fantasy. The first materials that will be mined in space are things that are very easily obtained: water and other volatiles. There is no value in sending these materials back to Earth. Common metals mined in space such as iron and nickel will have zero value back on Earth. They're too common. Only the very rarest of materials might have value on Earth, but mining those falls into the category of "we haven't the foggiest idea how to do that."
  2. Actually, we don't see any 'serious proposals' to go to Mercury for the sort of operations you are proposing. The space settlement advocacy is the quintessential source of non-serious proposals of all sort. Space settlement on an extremely small scale, on close-by objects *might* happen in the next few decades. Space settlement on the scale advocated by those advocacy groups: It ain't gonna happen, not in my lifetime, probably not in yours. You have predicated your Mercury proposal on a large number of technologies that we don't know how to do from an economic or engineering perspective. Space mining? We don't know how to do that. Space manufacturing? We don't know how to do that, either. The same applies to large solar sails, solar thermal propulsion, getting people to and from Mercury. We don't know how to do any of them. All of these are at a very low technology readiness level. TRL 4 is where we can start saying we know how to do that. None of these is even close.
  3. The universe itself. It might have happened when the universe was very young. The tuning wouldn't take all that much because information content (entropy) was very low.The mechanism? Who knows? Saying that it had to be some god is a god of the gaps argument. Or it might be that the universe is infinite and those magic numbers aren't the same across the entire universe. We happen to live in a pocket universe that is amenable to life. The places that aren't? There aren't any beings there to ask this silly question about fine tuning. Or it might be that the universe is but one of a huge number of universes. Our universe is amenable to life. Others aren't, and there aren't any beings in those hostile universes to ask this silly question. This multiverse concept apparently is a leading contender amongst cosmologists. Think of it as the MWI interpretation of quantum mechanics gone wild. Or it might be that there is a "before the big bang", and that this "before" was another universe. This leads to a never starting, never ending cycle of universes. We happen to live in a cycle that is amenable to life. This is Penrose's view.
  4. This is extremely disingenuous quote mining. Susskind advocates for an infinitely large universe in which inflation made for drastically different conditions in different parts of this vast universe. Even if the vast majority of the universe is hostile to life, there are bound to be a huge (potentially infinite) number of pocket universes that are conducive to life. Rees is an advocate for the concept of a multiverse, Smolin advocates for an extreme form of this concept. If something isn't out-and-out impossible, it will happen somewhere in one of those infinite, potentially uncountably infinite, multiverses. Penrose rejects the multiverse concept as unscientific but instead advocates for a cyclical universe. In his mind, we are able to be here because this particular cycle is conducive to life. Cycles that aren't? There aren't any thinking beings in those cycles to ask such silly questions.
  5. This thread sure has been diverted off-topic. Note to krash: Antimatter is not unstable in and of itself. The problem is that antimatter and ordinary matter are markedly unstable when the two meet. It's time to go back to the original topic. The other way around, Moontanman. Physicists know that antimatter has the same inertial mass, including sign, as does ordinary matter. This means that if (and this is a big if) antimatter has negative gravitational mass, it would fall up, not down, due to Earth gravity. The equivalence principle says that inertial mass and gravitational mass are one and the same (including sign), but just because someone says that this is the case does not mean that it is the case -- even if that someone is Einstein. The equivalence principle has been subjected to a wide variety of tests. It now stands as one of the most precisely verified concepts in physics, but only for ordinary matter. Testing how antimatter behaves when subjected to gravitation has been highly problematic because of the proclivity of matter and antimatter to annihilate one another. The best results to date are that the gravitational mass of antimatter is somewhere between -65 and +110 times the inertial mass. That's not very good given that the factor is supposed to be exactly one. These results come from the ALPHA experiment at CERN. This experiment is currently being upgraded specifically for the purpose of studying how antimatter behaves under the influence of gravity. The update should be completed sometime next year, so we can expect results two or three years from now.
  6. Nonsense. I do understand where this nonsense comes from, but that doesn't stop it from being nonsense. A lot of schools now teach that the slug is the US customary unit of mass, and that pounds are a unit of force. This is nonsense. The slug is a non-standard unit used by some (but not all) engineers in the US. Other engineers are quite happy using the pound as a unit of mass and the pounds-force as a unit of force. The pound (Avoirdupois pound) is defined as 0.45359237 kilograms, exactly. The pound-force is defined as 4.4482216152605 newtons, exactly. Weight is legally and colloquially a synonym for mass in the US. Better said, mass is a synonym for weight. Weight is a much, much older word than is mass. English has two words for the same concept for the same reason that the animals that provides pork are called swine. The Norman invaders spoke French, and their French words eventually became English words. It took a few more centuries after 1066 for the descendants of those Norman aristocrats to invade the merchant class. Mass didn't become an English word until the 15th century. We in the technical community don't like it when the lay community pervert our own words. For example, "Evolution is just a theory." To avoid being hypocrites, we in the technical community should avoid doing the same to perfectly good English words such as weigh and weight. The supposed confusion between mass and weight is an invented controversy. As noted above, there is no confusion in everyday English. When your friend says he weighs 155 pounds (or 70 kilograms), fight back the urge to correct him. His usage is correct.
  7. That would have quite the feat of precognition on Newton's part. Linear algebra didn't exist in Newton's time. While Leibniz (but not Newton) did use determinants, it wasn't really linear algebra that he was using. Linear algebra got it's start with Vandermonde in the late 18th century. The vectors we use now in physics weren't invented until the late 19th century. The rather abstract extensions such as Hilbert spaces -- those are 20th century developments.
  8. Yes, it is, swansont. You apparently are thinking of the pound-force, swansont. The pound is a unit of mass. You don't. What you need to do is to be careful with units, particularly when working with English units. The lb in your 0.075 lb/ft^3 is the pound Avoirdupois, a unit of mass. The lb in your 109.165 lb/ft^2 is incorrect. That should be 109.165 pounds-force per square foot, or 109.165 lbf/ft^2. The pound (lb or lbm) is a unit of mass, the pound-force (lbf) is a unit of force. In the metric system, one uses F=ma. That doesn't work with force expressed in pounds-force, mass in pounds, and acceleration in feet per second squared. One must instead use the more generic form of Newton's second law: Force is proportional to (rather than equal to) mass times acceleration. Mathematically, F=kma. A force of one pound-force accelerates a one pound mass 32.174 ft/s^2, so k=1/32.174 with this choice of units. This means your expression "pressure = density x gravity x height" needs a bit of modification to work with English units. With pressure in lbf/ft^2 and density in lbm/ft^3, the correct expression is [imath]P=\frac 1{32.174} \rho g h[/imath]. Since g=32.174 ft/s^2, your expression simplifies to [imath]P=\rho h[/imath] if you express pressure in lbf/ft^2 and density in lbm/ft^3.
  9. I guess you're talking about the Voyager satellites. They didn't have atomic clocks. As far as I know, the only satellites outfitted with atomic clocks are the GPS satellites. Suppose some spacecraft outfitted with atomic clocks goes to the far reaches of the solar system and then returned. On return, the spacecraft clock would probably show a different time than Earth clocks. The spacecraft goes far from any gravitational sources, so general relativistic time dilation would make their clocks tick faster. However, it also went away from the Earth at significant speed (at least initially), so special relativistic effects would have made their clocks tick slower. Which is the dominant effect depends on how fast the spacecraft moves relative to the Earth and how much time it spends far away from the Sun. Atomic clocks at mean sea level all tick at the same rate. The geoid, which is what mean sea level is, is an equipotential surface of the gravitation force and the centrifugal force due to Earth rotation.
  10. That's an ornithopter, not a helicopter, and it flew a couple of years ago. Here's the helicopter that won the Sikorsky prize:
  11. Because they don't. Richard Feynman: "You don't like it, go somewhere else! To another universe! Where the rules are simpler, philosophically more pleasing, more psychologically easy."
  12. I was going to stay out of this thread, but then I saw that someone downvoted md65536. Here's a +1 to counteract that -1, md65536!
  13. Correct. Remember, though, that those masses are in a sealed container. You can't see them. All you can see is the container. If your oscillating masses are balanced, you won't see anything happen. If they aren't, you'll see the container move around a bit but it's mean (average) motion will still be zero. There is no difference between the empty space scenario and the freefall scenario. They are one and the same per the equivalence principle. No, there isn't. You can keep deluding yourself all you want. There's nothing here, and with that, I am done with this thread.
  14. The answer to your question is no. Forget about gravity for a bit. Imagine putting your device in a sealed container and in deep space, far from any gravitational source. Now power it up. What do you think will happen? Keep in mind the law of conservation of momentum. Now let's put it in orbit about the Earth. Power it up. Nothing happens, same as in deep space. I smell something a bit more foul.
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