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#51
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Identifying galaxies within 1 MPc
Am Thu, 04 Jan 2007 07:50:23 -0500 schrieb Yousuf Khan:
What's a good maximum range for a pulsar beam? Hundreds of light-years? Thousands? Tens of thousands? Etc. http://www.atnf.csiro.au/research/pulsar/psrcat/ |
#52
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Identifying galaxies within 1 MPc
Yousuf Khan wrote:
Erik Max Francis wrote: One issue we haven't mentioned is despite being good beacons, pulsars aren't perfect. Far enough away, even though in the Galaxy, and they'd be hard to detect, so if you were far enough from our solar neighborhood, even you were in the (feeble at that distance) beam of those reference pulsars you might not be able to detect it. What's a good maximum range for a pulsar beam? Hundreds of light-years? Thousands? Tens of thousands? Etc. From skimming papers on pulsar distances on Google, it looks like the effective range is ~6 kpc, or perhaps ~2 kly. -- Erik Max Francis && && http://www.alcyone.com/max/ San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis I'm the woman whose / Three wishes came true -- Lamya |
#53
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Identifying galaxies within 1 MPc
In article ,
Erik Max Francis writes: From skimming papers on pulsar distances on Google, it looks like the effective range is ~6 kpc, or perhaps ~2 kly. Dropped a decimal the 6 kpc \approx 20000 LY. I agree with the number, which is of course based on present Earth technology. I think the point of the Pioneer and Voyager plaques was that any advanced Galactic civilization would have a comprehensive map of pulsars throughout the Galaxy and know their behavior over long spans of time. _Given that information_, the probe's origin could be located both in space and time. -- Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA (Please email your reply if you want to be sure I see it; include a valid Reply-To address to receive an acknowledgement. Commercial email may be sent to your ISP.) |
#54
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Identifying galaxies within 1 MPc
Steve Willner wrote:
Dropped a decimal the 6 kpc \approx 20000 LY. I agree with the number, which is of course based on present Earth technology. Right. I zigged, when I should have zagged. I think the point of the Pioneer and Voyager plaques was that any advanced Galactic civilization would have a comprehensive map of pulsars throughout the Galaxy and know their behavior over long spans of time. _Given that information_, the probe's origin could be located both in space and time. I think the greater point about the plaques is that no one expects anyone else to find it, let alone decipher it. Its representation, along with the golden record put on the _Voyager_ probes, and in addition to that the unrelated Arecibo message, were really for our own entertainment and as a publicity stunt rather than as a serious attempt to communicate with extraterrestrials. For the probes, they'll be drifting in interstellar space and will be incredibly unlikely to ever be found even by superadvanced civilizations. For the Arecibo message, it was beamed to a distant globular cluster, far enough away that they'd be essentially unable to trace back the source, and globular clusters consist of old population II stars, not exactly conducive to technological intelligences. -- Erik Max Francis && && http://www.alcyone.com/max/ San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis To be refutable is not the least charm of a theory. -- Friedrich Nietzsche |
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Identifying galaxies within 1 MPc
"YK" == Yousuf Khan writes:
YK Do the pulsars' axes also precess, like the Earth's does? Or are YK they just too massive and spinning too fast for that? Maybe. Others have already pointed out forced precession, in which a torque exerted by a nearby object causes the precession (analogous to how the Moon and other objects in the solar system can cause the Earth's rotational axis to precess over time). There's also free precession, caused by an asymmetry of the object. One might think that this should not happen to a pulsar, given that its interior is liquid and should damp any asymmetries, but there's at least one report of this possibly being detected in PSR B1828-11 by Stairs et al. -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
#56
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Identifying galaxies within 1 MPc
John Schilling kirjutas: On 10 Jan 2007 14:50:25 -0800, "bbbl67" wrote: On Jan 5, 2:36 am, Erik Max Francis wrote: And, has been pointed out, if there's motion of the internal fluids inside the rotating neutron star, that could also result in precession and all kinds of lower-order rotational behaviors. Now, I know the interior of a neutron star has been described as a fluid. But I find it hard to believe that something so dense that its neutrons are touching could be a fluid. Can the neutrons slip past one another, somehow? Of course. What would stop them? In order for a thing to *not* be a fluid, something has to hold each particle in a particular spot, and that something has to be the strongest force at work in the system. In ordinary solid materials, a particular arrangement of the electromagnetic force does this just fine. In a neutron star, you've got A: neutrons, which are B: compressed to such a density that gravity overwhelmes electromagnetism. If even electromagnetism applied, which it doesn't because neutrons are electrically neutral. But you also have something that counteracts the gravity and stops the neutrons from being compressed to decreasing volume and black hole. Neutrons must repel each other. And this means that the best way to minimize the repulsion is occupying specific positions in space in a well-packed framework. Which would be solid. Any change of shape would increase the mutual repulsions somewhere. And unless there's something funky going on in quantum gravity that we haven't figured out yet, gravity is not a force for holding things in a particular spot. Gravity is just fine with having particles flow along equipotential surfaces, and actively *wants* particles to flow downhill. |
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Identifying galaxies within 1 MPc
On Fri, 12 Jan 2007 09:03:29 -0800, chornedsnorkack wrote:
John Schilling kirjutas: On 10 Jan 2007 14:50:25 -0800, "bbbl67" wrote: Now, I know the interior of a neutron star has been described as a fluid. But I find it hard to believe that something so dense that its neutrons are touching could be a fluid. Can the neutrons slip past one another, somehow? In order for a thing to *not* be a fluid, something has to hold each particle in a particular spot, and that something has to be the strongest force at work in the system. In ordinary solid materials, a particular arrangement of the electromagnetic force does this just fine. In a neutron star, you've got A: neutrons, which are B: compressed to such a density that gravity overwhelmes electromagnetism. If even electromagnetism applied, which it doesn't because neutrons are electrically neutral. But you also have something that counteracts the gravity and stops the neutrons from being compressed to decreasing volume and black hole. Neutrons must repel each other. And this means that the best way to minimize the repulsion is occupying specific positions in space in a well-packed framework. Which would be solid. Any change of shape would increase the mutual repulsions somewhere. That's not a bad thought. A bunch of well lubricated ball bearings being packed into a sphere just barely big enough to fit them will pack into a rigid formation. However, this gets into the difference between a "solid" and a "liquid". A liquid like water is nearly incompressible because the molecules are packed in almost as much as possible--but there's enough random motion to keep them "jiggling" and thus able to easily slide past each other. Similarly, a neutron star might theoretically compress into a rigid "solid", but with any sort of temperature the neutrons will "jiggle" enough to be a fluid. Isaac Kuo |
#58
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Identifying galaxies within 1 MPc
On Fri, 12 Jan 2007 11:37:41 -0600, kuo wrote:
: And this means that the best way to minimize the repulsion is occupying : specific positions in space in a well-packed framework. Which would be : solid. Any change of shape would increase the mutual repulsions : somewhere. : :That's not a bad thought. A bunch of well lubricated ball bearings :being packed into a sphere just barely big enough to fit them will ack into a rigid formation. Neutrons aren't analogous to ball-bearings, though. They're fuzzy, without well-defined borders. -- "The truths of mathematics describe a bright and clear universe, exquisite and beautiful in its structure, in comparison with which the physical world is turbid and confused." -Eulogy for G.H.Hardy George W. Harris For actual email address, replace each 'u' with an 'i' |
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