GeeKay

Nevertheless, I still wonder if a 'uniform' gravity field is possible under such conditions. It seems that Newton's field equations and General Relativity - in particular the transference of rest-energy into energy of motion with changes in altitude - require a gravitational tidal pull of some kind. This in turn would appear to be dependent upon a rotund mass curving space/time about itself. Another way would be to see it in terms of a conic section - an infalling object entering ever more constricting regions of space/time. This funnel-like geometry then is responsible for the gravitational differential, hence the tidal effect. Would this apply to a flattened geometry, however - one without a conic section? No conic section, no tidal pull? No tidal pull, therefore no G? Or would gravity behave very differently in such circumstances? Ref. Einstein's Universe (esp. the chapter 'Shells of Time') by Nigel Calder: BBC publications (1979)

Thanks for the helpful clarifications. Yes, I do understand that in the 'real world' of gravitational attraction the sphere offers the least surface area for a given volume of mass. However, I'm trying to step out of the real world for a moment in order to understand more fully the relationship between gravity, geometry and matter. Change the geometric arrangement of matter, for instance, and it seems that this alters the curvature of space/time, hence gravity itself. Thus, the most extreme example in this 'thought experiment' would be to imagine how a large uniformly flat expanse of matter would affect local space/time. Without a conic section being present (such as a section of a sphere), how could there be a tidal pull? If there is no tidal pull, does this mean an absence of gravitational attraction, period? My apologies for pursuing this line of thought, but I feel this is just the kind of question any astute eight-year old child would ask. At present I would be unable to answer it - that is, without tying myself up in knots in the process.

If gravity is what happens when mass curves space and time, what would be the situation were a given mass remotely non-spherical - i.e. a perfectly flat bar or sheet, for example? Would there still be a localised curvature of space and time? If so, how does gravity's tidal pull fit into this arrangement?

Yes, I understand now. Soon after I'd made my howler (viz the above KE release 'rivalling that of the Sun') I revised it down to 9.0e x 18 joules, or a tad more than 2,000 megatons, but remained in doubt about it. So many thanks for all the corrections, and in particular the explanations concerning the calculations - the finer points of E = mc2 included!

The concept that prompted this 'thought experiment' (inspired by a magazine article) had ten tonnes each of matter and antimatter undergoing a head-on collision, with an impact speed of some 30,000 kps. The total energy release turns out to be about 400,000 megatons - but 'total' only if one leaves out the kinetic energies of the impact itself. This left me wondering whether antimatter, as distinct from standard matter, is a special case when it came to kinetic energies. Also, my maths isn't to be trusted. According to my back-of-the-envelope calculations, two such ten-tonne objects impacting at 30,000 kps produces, in kinetic terms, an energy release far in excess of that produced by the matter/antimatter merger. Indeed, at 1.21e 26 joules, it appears to exceed that of the Sun itself. . . which is clearly preposterous!

Query: if two objects containing equal quantities of matter and antimatter were made to collide, would it be necessary to add the kinetic energy produced by the impact velocity to the energy release resulting from these two pieces of matter/antimatter coming together in the first place? If so, I'm left wondering how one can add still further to a full 100% of rest energy into radiant energy. Confused!

Yes, I should have said something about what I meant by 'advantageous'. My main concern is fuel consumption, followed by journey time. I'm less concerned about the rate of acceleration, other than it remaining constant. Another qualification: I cited the Earth and the Moon to keep things as simple as possible. However, what I really had in mind was an interstellar voyage that has a spaceship venturing out from the Solar System at a constant acceleration of 0.33 m/s2 to reach a nearby (and wholly fictitious) dwarf star, whose mass is half that of the Sun. If my calculations are to be trusted, and assuming the distance between the two stars is 2,120 AU (again this is the stuff of fiction), it appears that the gravitational null point lies about 1,400 AU outwards from the Sun - or roughly two-thirds of the distance to the destination star. (NB. this fraction appears to hold true, regardless of the distance between the two stars). Now, is it more fuel efficient for the spaceship to make its so-called 'mid-course' turnover here, or at the true (geometrical?) mid-point between the two stars: namely at 1,060 AU? Here, though, it's worth pointing out that the Sun's escape velocity would be approximately 1,300 m/s while that of the other star is a tad over 900 m/s - assuming my maths is correct, of course. I'm sorry to have laboured this to death, but one comes across 'mid-voyage turnovers' quite a lot in science-fiction novels and as such, I'm intrigued as to what this means exactly. Gravity fields are akin to gradients, after all, and as anyone who's ever ridden a bike knows, there's definitely a calorific difference is when it comes to cycling up a steep hill and freewheeling down the other side!

I'm not sure if this is the correct forum to ask this question, given that it's more concerned with science fiction than science fact. Still, it does bring into play Newton's field equations. So here goes: assuming a spaceship or probe of some kind is travelling between (say) the Earth and the Moon and it happens to have a suitably advanced propulsion system that enables it to maintain a continual thrust throughout the voyage; this being so, would the craft make its mid-voyage 'turnover' at the halfway point between the Earth and Moon? Or would it be more advantageous to perform this manoeuvre at the gravitational 'null point' between these two bodies - that is to say, where their gravity fields cancel out? Or is the answer, as I suspect, rather more complicated than that?

Well, I'm also a bit uncertain about what flows from the differences between lux and lumens etc. I also gather that measuring light intensity is still fraught with unresolved problems, or at least ambiguities. Still, my B&M senso front light certainly slings out a lot of photons, however one counts 'em. My question is what happens on a much larger scale when all that intensity is squeezed (if that's the right word) into a narrow beam of light, one exhibiting no visible disc of any kind. I accept that any -17 magnitude starlike light source is beyond our human experience. Venus at -4.4 mag is the brightest natural object of this type we have in the heavens, and staring up at that certainly doesn't cause the eyes to water! And so, trying to apply this line of thought to a starlike object many times brighter than Venus is always going to be exceedingly difficult to imagine. What about the Moon then? If a star were as bright as the Full Moon (-12.5 mag approx), would that be hard to look at directly? Many thanks.

I understand from Wikipedia that the apparent magnitude of the Sun, as viewed from Earth orbit, is about -24.74. This being so, and allowing for the well-known fact that one would risk permanent eye damage by staring up at the Sun for any length of time (other than during sunrise and sunset), would it be possible to safely gaze at a point source of light if it were shining at a magnitude of -17? I gather this magnitude is around a hundred times brighter than that of the Full Moon, but still extremely dim by Solar standards. Nevertheless, as anyone who has had a 40 lux LED dynamo lamp directed straight into one's eyes (my lamp and my eyes, as it happens) they will know just how squintingly bright modern bicycle lighting can be. Therefore, I'm left wondering whether the same effect would apply to a starlike object with the above luminosity, namely -17 mag. I really would welcome any suggestions to help me over this baffling question.

I would much prefer to call it the Big Boom. The vowel in 'bang' (as in Big Bang) is not really long enough for a process that began some 13.7 billion years ago. . . assuming it did, of course.

Many thanks, Airbrush, and again for the link. I hadn't realised dwarf stars came in so many different masses and colours. I shall resist positing the existence of Snow White stars, deeming such comments to be beneath the dignity of this forum. Incidentally, I did manage to resolve the ambiguity concerning a perceived gravitational tidal pull exerted by (high mass) dwarf stars - Sirius B, for example. Fortunately, even for such a star sporting a surface gravity of around a third of a million gees, any notional tidal pull was calculated to be negligible - a fraction of a G, in fact. . . fortunately in this case, because the SF writer concerned (see above) turned out to be none other than the late great Arthur C Clarke. Apart from brown dwarfs then, I guess non-pulsar neutron stars must be in terms of absolute magnitude among the faintest stars currently in the cosmos. I will stick my neck out here and suggest anything upwards of +30 mag?

Airbrush: I am genuinely surprised to learn that white dwarf stars are much more dense than the red dwarf variety. I'd assumed the colour difference was due (for the most part) to age and temperature. Ophiolite: I admit to being a bit hazy about how a star is actually defined. However, I'm referring to those compact (Earth-sized?) dwarf stars, which as far as I know, radiate light etc mainly via gravitational compression, rather than through fusion. It's these fainter stars my original question about absolute magnitudes was trying to address. My mention of Jupiter in this context turned out to be something of a red herring - my apologies.

Does anyone know what the faintest recorded magnitude of a given star happens to be - that is, in terms of absolute magnitude (as opposed to apparent magnitude)? I understand that Jupiter's absolute magnitude is +26. This being so, have there been stars observed that are intrinsically fainter than this planet? On a semi-unrelated point, do dwarf stars (of whatever hue) exhibit fierce gravitational tides near their surface? I ask this because I vaguely recall reading an old SF short story that had a spaceship performing a low altitude flyby past a red dwarf, during which the astronauts experienced no adverse effects from any such notional tidal pull. They wouldn't have found it such a breeze had they tried it on with a neutron star or a stellar-size black hole, of course. But then these two classes of stellar objects weren't around when this story was written. Even so, dwarf stars are no slouches when it comes to size and compression and as such, I just wonder if the author - whose name escapes me at present - got his Isaacs right.

Simone - I don't as yet pretend to understand a great a deal about your aptly named 'Dark Glove' theory (for some odd reason, the name makes me think of the Glove in the Beatles' 'Yellow Submarine' movie). That said, I do very much like the idea of matter 'losing a dimension' inside a black hole. It seems to make a weird kind of sense too, especially given the non-Euclidian geometry in regions of intense gravity. . . not to mention quantum mechanics as well? This is aesthetics speaking, though, not mathematics, I'm bound to admit.

Royston: many thanks for the comments and for the link - which I've now saved in my favorites for further investigation. immatfaal, what is EMR? (A wild guess: 'Energised Magnetic Radiation' - surely not??) About momentum: at one point in Robert L Forward's 'Dragon's Egg' novel, a meteor (or rather its plasma) strikes the eponymous neutron star at one of its magnetic poles. The force of impact is such that it triggers a brief but locally devastasting gamma ray outburst. I can remember reading this and thinking that if an incoming object the size of a grain of sand can produce this effect, what would happen if an asteroid, or a planetoid with (say) the mass of Vesta, were to impact upon the surface of the aforesaid neutron star, and with the same terminal velocity - in this instance, around 0.3 c. The thing is, Forward was an astrophysicist as well as a SF writer. So he knew his stuff. I just wish I knew enough of it myself, but - hey - that's part of the challenge.

If human beings are negatively charged, does this explain why we find it so hard to get along with each other sometimes?

Ah, yes, I guess you're right, there. Shame really. Still, it probably helps to make the universe a safer environment from the point of view of life on Earth (and elsewhere?) - a lot safer than having asteroid-primed GRBs popping off every other minute. Oh, well, back to the drawing board.

Yes, in the case of neutron stars, I can well understand why any infalling material - whether cometary or asteroidal - would be accelerated to a substantial fraction of the speed of light, this regardless of initial velocities. I guess too in view of Roche limits having been exceeded, that the kinetic energies resulting from such an impact would be much the same, even if (as seems highly likely) an infalling comet or asteroid had by this stage been reduced to plasma. Another thought: would an accretion disc orbiting a neutron star likewise be reduced to plasma? If so, would the resulting radiation output cause the star to 'behave' like a pulsar, albeit temporarily - hence the GRB effect? Or is this taking things too far? Moontanman: yes, I fully agree with your comment. It's just that I was trying to distinguish between mass and velocity, that's all. In doing so, I was basing this notion on the observable fact that the orbital velocities of comets tend to be be higher than those of asteroids. Thus there may be no difference in terms of explosive impact between (say) a high-velocity low-mass comet and a comparatively slow-moving high-mass asteroid. Sorry about labouring the point.

If there are trans-stellar planets, and I can see no reason why they shouldn't exist, I would imagine many if not most of them would be the result of gravitational close-encounters between passing star systems. Just a thought.

All other things being equal, I guess any differences in explosive impact between an asteroid and a comet must come down to mass and velocity. Finding yourself stopping a high-velocity snowball might be no less painful than being struck by a slow-moving pebble. Yes? No? As for any associations between GRBs and cometary material impacting upon neutron stars, well, like the White Queen in 'Alice Through the Looking Glass', I too can believe in six impossible things before breakfast. Seriously, though, if professional astrophysicists across the world can raise such a possibility about GRBs - as they did about the 2010 Christmas Burst - who am I (with my humble GCSE in Maths) to disparage their line of enquiry?

Mathematic - many thanks for the comment. Yes, I'm inclined to agree with you there. However, as so often seems to be the case whenever I post a question in forums such as these, it didn't take me all that long afterwards to find myself stumbling upon answers of sorts while hunting around elsewhere on the net. Here, I refer to the so-called 'Christmas Burst' of 2010. Back then two explanations were offered up for the cause of this extraordinarily lengthy gamma ray outburst. For me, the most telling of these two explanations pointed to a cometary impact upon an otherwise quiescent neutron star located within our own galaxy. Since then this notion has given way to the other explanation, then doing the rounds: namely that the cause for this GRB was due to a considerably more distant supernova explosion. Even so, the fact that the astrophysicists were willing to entertain a (large) cometary impact as a possible cause for this GRB does leave me wondering whether such strikes are as benign as is so often thought. Unfortunately, I simply don't have the necessary number-crunching abilities to sweat the answer out for myself - at least not yet! In the meantime, I offer the link below as supporting evidence for my query. http://www.universetoday.com/91406/did-a-neutron-star-create-the-christmas-burst/

I am currently writing an SF novel whose primary setting is a neutron star (this is essentially a projected sequel of the two novels 'Dragon's Egg' and 'Starquake' by Robert L Forward). The question that's bothering me at present concerns the likely effects a serious of asteroids would have were they to be directed down on to the neutron star itself. I am aware that the effects any such impacts might have could be extremely minimal, or else restricted to the star's magnetic poles. Nevertheless, I would welcome any views or opinions this question might raise.

I am in most instances highly supportive of the BBC's Horizon documentaries - their occasional quirks not withstanding. I am also aware of the Beeb's need to present its science-based material in ways that are readily comprehensible to its target audience. Having stated this, I'm bound to admit that I found the recent Horizon prog about the Milky Way's SMBH to be dull, uninformative and (in places) cringingly pretentious. Possibly it counts as the worst Horizon prog I can ever recall watching. As an example of what a missed opportunity it turned out to be, I can only compare it to the marvellous BBC documentary about the Antikythera Machine. End of gripe.

Well, it turns out that 'surface tides' in the above calculator is referring to the tidal force at a black hole's event horizon - e.g. establishing whether an astronaut could cross it without being spaghettified.