# GeeKay

Senior Members

118

1. ## Neutron Stars II: Gravity v Magnetism

Neutron stars have been described as the most powerful bar magnets in the known universe. This being so, is it possible for the magnetic fields of such compact objects to take precedence even over their gravity fields? An example: would a small object captured by a given neutron star wind up impacting upon one or other of its magnetic poles, rather than gravity pulling it down to impact anywhere else on the star's surface/photosphere? Many thanks.
2. ## Specific impulse & Velocity?

Many thanks for the explanations - brick analogies and all Yes, I think I finally understand the equation now. On a personal note, I suffered badly on the maths front at school. Partly the fault was my own; but I do contend that a substantial chunk of it was due to the teaching practices typical during those times, especially when it came to rote-learning techniques. This method has its uses, but for me back then, especially when it came to the teaching of maths, it was heavy on the 'how' and light on the 'why'. It wasn't until I did GCSE Maths as a mature student that I began to understand that maths is a language - and a wonderful language at that. At least I know that now, even if I remain a poor reader of numbers. I just wanted to say that. End of personal note.
3. ## Specific impulse & Velocity?

Regarding these "two pieces of information, force and time", I'd appreciate it very much if you could provide me with an example how a given final velocity may be calculated. Many thanks.
4. ## Specific impulse & Velocity?

The quote below is the most succinct description I've found on the net explaining the relationship between spacecraft velocity and performance: "The speed of a rocket is directly affected by 2 factors. The first is mass ratio, which is a ratio the weight of the rocket at lift-off compared to the weight of the rocket at engine shutdown. The second factor is specific impulse, which is the amount of thrust produced from each pound of propellant per second. The higher the mass ratio and the higher the specific impulse, the faster the rocket can go." Mass ratio I understand. Likewise, I can get my head around specific impulse - although I still have to strain it a bit. What I don't understand is how all this translates into spacecraft velocity. It's all very well to talk about exhaust velocity, but at risk of sounding fatuous, were I a petrolhead wanting to buy a sports car, I might be interested in fuel consumption (possibly) and knowing how efficiently the exhaust functions (well, maybe) but really, I'd love to know how fast the car goes. Unfortunately, concerning specific impulse and 'forward' velocity as it apples to spacecraft, I can find nothing anywhere that can join up these two dots. Many thanks. PS. Again, I apologise for the crassness of the question. But how can one find out if you don't ask?
5. ## Lowering the speed of light

I've just read an article on the BBC website (see link below) in which experimentalists have reduced the speed of light by remodelling photons. Does this mean that the frequency of light is also reduced? http://www.bbc.co.uk/news/uk-scotland-glasgow-west-30944584
6. ## Interstellar, the Movie

Yes, on reflection I have to agree. . . at least in part. There were some fine moments in the film, especially visually. But it did drag in places. I'm afraid I still don't understand the 'timeloop' sequence, though the tensions it created certainly added to the drama. Timeloops? Is this a reading of the multiverse concept?
7. ## A Milky Quasar?

Yes, I should have done my homework about quasars before popping the question. Three additional thoughts, though: (1) possible disruptive effects caused by the 'merging' of two SMBHs - gravity waves etc? (2) colliding galactic gas and dust clouds? (3) dark matter? I mention these colliding aspects, having ascertained that the closing velocity between the two galaxies is approx 110 km/s (or very roughly equal to the distance between the Earth and the Moon covered in an hour). This doesn't strike me as inordinately fast, at least not by cosmic standards. But it's still fairly brisk - brisk enough, I would imagine, to have all manner of effects at the 'local' level - the interstellar medium, planetary systems etc.
8. ## A Milky Quasar?

With regards to the future merging of the Andromeda Galaxy with our own Milky Way, would this event create a quasar? If so, would this in turn eradicate all existing life in both galaxies? (assuming, of course, that Earth isn't the only repository of life in our part of the universe). Many thanks.
9. ## Swimming on the surface of Titan

I'm no expert on this, but I'm inclined to agree with Moontanman about seemingly being able to fly around on Titan, using a pair of borrowed wings. I wonder if I read it in Starship Century - an article by Robert Zubrin, possibly?
10. ## Interstellar, the Movie

With regards to Interstellar (which as enjoyable as it was, induced in this viewer some time-dilation issues of its own, given its length) I found this interview in a recent Scientific American article, in which Kip Thorne explores some of the physics contained in the film. PS. I'm not sure if this link functions properly, but it's worth a try. http://blogs.scientificamerican.com/observations/2014/11/28/parsing-the-science-of-interstellar-with-physicist-kip-thorne/
11. ## Optical limits to eyes

Many thanks: l learnt a great deal about optics from this thread. So the upshot then is that the size of the pupil determines its capability to resolve incoming light within a given spectrum of wavelengths. And this limiting factor applies whether one is viewing an object with or without the aid of magnifying glasses, and so forth. So the only way a human being is able to 'view' infrared radiation, for instance, is to use an infrared scanner of some kind, an external device that detects infrared radiation - i.e heat - and converts this radiation into 'visible' light. Re depth of field: I well recall a childhood memory of my brother and I taking turns to watch incoming traffic through a pair of fairly powerful binoculars while travelling in our father's car. It made for quite a strange visual experience - comic-strip Special Relativity for beginnners almost. Not to be recommended while driving, however.
12. ## Naked-Eye Visibility in Space

Yes, I'm looking forward to the advent of the James Webb Telescope and although I'm not superstitious, I'll be tempted to cross my fingers when launch day finally comes. Enthalpy, thank you for your comments about the transparency of the atmosphere. Intriguingly, the Bortle Scale - or someone referring to it - did claim an upper naked-eye limit of +7 magnitude, albeit under 'exceptionable' viewing circumstances. If so, then I guess this degree of clarity would also apply to space itself.
13. ## Optical limits to eyes

If glasses do not increase the resolving power of the eye, what's the situation with binoculars, telescopes etc? Or is there a trade-off here between resolution and field-width? I'm sorry if this seems a stupid question.
14. ## Optical limits to eyes

Re. the Rayleigh criterion. Yes, I begin to understand now. This raises another question, though: if the eye's resolving power is determined by the size of the pupil, does this explain how glasses (or other forms of external lenses) can overcome this problem?
15. ## Optical limits to eyes

I understand that there's a definite relationship between the size of an eye and the wavelengths it can receive. This being so, just how small can an eye be, while still able to receive light in the visual spectrum? NB. Here, I'm referring to what might be described as 'camera' (or animal) eyes, rather than the compound eyes of insects. Many thanks.
16. ## Did a Meteorite Hit the Capital of Nicaragua?

Just a suggestion: could the alleged meteorite once have been a moon of the Pitbull asteroid, but which was pulled away by Earth's gravity? I've no idea if Pitball ever came close enough to pass within Earth's Hill Sphere, but I do recall a close flyby made fairly recently (2013?) by another asteroid, which definitely had a moon, albeit a very small one.
17. ## Naked-Eye Visibility in Space

Having recently chanced upon the Bortle Scale, I'm intrigued to know just how much increase there would be in stellar luminosity as observed from space - say, from the ISS. For the sake of argument, I'm discounting the effects of spacesuit visors etc. Many thanks. http://en.wikipedia.org/wiki/Bortle_scale
18. ## SETI@Home

Yes, I do support the SETI programme - though I am aware of its inherent dangers (ref. Stephen Hawking). We don't know for sure if an intelligent species has to first get through an 'ethical' bottleneck in order to become a space-faring species. The SF novels by Larry Niven are instructive here. Unlikely though it might be in actuality, it's just possible to conceive of an alien species with 25th century space technology and the morality of The Beano (a British children's comic). Personally, I'm inclined towards the view aired by Frank Zappa, who when asked a related question, responded by saying that he considered the universe to be composed mostly of stupidity. It's the less 'stupid' parts we're discussing here, I guess.
19. ## Gravity issues

So I gather then that a fairly largish object (an astronaut, say) could safely stand on that part of Phobos facing Mars and not be borne away by the Martian gravity field - or its tidal forces, I mean?
20. ## Gravity issues

it would appear that any spacecraft being inserted into a fairly stable and fuel-efficient orbit round Phobos (and possibly Deimos too) requires a non-Keplerian orbit, such as are described in the link below. http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/42478/1/12-3014.pdf
21. ## Gravity issues

Well, let's see: if one accepts that the mass of Mars is 639e21 kg and 1.0659e16 kg is that of Phobos, accepting too that the mean distance between the centres of these two bodies is about 12,7805 km, it turns out (or the calculations appear to indicate) that the gravitational attraction of Mars at this distance is 0.2094 Newtons. Meanwhile, an object orbiting Phobos at an altitude of (say) 31 km from the satellite's centre works out to a measily 0.00074 Newtons. If these figures are correct, this strongly suggests - to me at least - that no stable orbit is possible round Phobos. I did as a control apply the same set of calculations for the Earth and the Moon, and they checked out fine. Of course, the Moon is many times larger than Phobos, but then the distance between the Moon and Earth is in relative terms much greater than that between Mars and Phobos. Indeed, I understand that Phobos is in a dangerously low orbit, that it's not that far off its Roche Limit and that tidal stresses will eventually cause it to break up and form a ring round Mars and/or make a number of impressive dents in the Martian regolith. I hope I've not made too many assumptions - simple or otherwise - in my assessment of the situation thus far. But I do wonder if I've made a fundamental error somewhere along the line. Hence my query. Correction: a misplaced decimal point. The distance between the centres of Mars and Phobos is actually 12,780.5 km.
22. ## Gravity issues

According to my (often suspect) calculations, it appears that no object is able to orbit Phobos, the inner moon of Mars, without being continually pulled in by Mars' own gravity field. Even the situation with the outer moon, Deimos, is a close call in this respect. I wonder, though, if I've got my maths right. This being more than possible, I would welcome any help or advice. Many thanks. PS. I am aware that maintaining a freefall orbit round either moon would be problematic in any case, given the varying gravitational fields resulting from their irregular dimensions. I'm ignoring that for now.
23. ## SETI@Home

"I know Darwinian Natural Selection, but I don`t know how you are going to relate it with your explanation. Maybe you can clarify it?" Yes, that's relatively simple: although we Homo sapiens cannot make any direct comparisons, other than studying ourselves in this respect, it would appear that inquisitiveness is a normal component of intelligence. In this thread's context, inquisitiveness = a desire by intelligent extraterrestrials to make contact with other intelligent species. . . ourselves, for instance? Hope that clarifies things.
24. ## SETI@Home

Nicholas, you should really consider reading up on Darwinian natural selection - assuming, of course, you haven't already? It is at present the only known mechanism that offers a thoroughgoing description of how living organisms thrive and evolve (and become extinct) over time. You won't go far wrong with born explainers like Richard Dawkins or Steve Jones. For starters just google their names. . . Hope this helps.
25. ## SETI@Home

Yes, I should have qualified my final throwaway remark by rephrasing it as "living nature" instead of just "nature". One may still consider whether it is relevant to the above discussion. I did so myself, hence my tagging it with a question mark. Nevertheless, I still lean towards the adage "nothing ventured, nothing gained" serving as a convenient shorthand for the workings of Darwinian evolution. I could expand upon this point, but feel that in doing so would further increase the risk of my wandering away from the main themes contained in this thread and/or boring everyone rigid by stating the obvious, which is the sin of sins.
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