9 ms·

If you go on, the article addresses this: measuring the peak of a pulsar signal places you on an infinite set of spheres centered on the pulsar, because you don

If you go on, the article addresses this: measuring the peak of a pulsar signal places you on an infinite set of spheres centered on the pulsar, because you don't know "which" peak you measured (i.e. when it was emitted). Since the pulsar emits a peak every millisecond or so, these spheres are separated by one light-millisecond each. If you repeat the procedure with several pulsars, you should be able to find that there is only one point in space that lies in the intersection between a sphere of each pulsar.

cjslep 9y ago
And indeed this is how the golden record on the Voyager spacecraft potentially informs any aliens of the location of Earth.
nitwit005 9y ago
Ahh, I missed that it was a large set of spheres. Thanks.
hinkley 9y ago
It’s [a little bit] bullshit. The light emitted from a pulsar is only a sphere if you live in a universe that contains only you and the pulsar. Spacetime is curved by every mass in it. The distance light travels is subtly changed by what is between you and the source. If four observers cannot agree on what was your exact location at time n (without a perfect model of the entire universe) then how would you be able to determine your location by observing four other objects? Edit: You can get the right neighborhood, but if you’re moving at relativistic speeds it will be problematic. And if you’re in the middle of nowhere and not moving at relativistic speeds you’re probably gonna die anyway...
- hinkley 9y ago
  I guess my point is that if you’re going to call it Galactic Positioning, it better work someplace else than on the surface of the earth, and I am not seeing how. We are all so ‘close’ that you can find a near exact distance between telescopes but that’s different than plotting your current distance to Tau Ceti.
- raattgift 9y ago
  Er, can you characterize which post-Newtonian correction/s is or are of non-negligible order here for Milky-way millisecond pulsars? Where do they enter, if not extremely close to the pulsars themselves? I'm curious, because we use them to test, among other things, |\alpha_3| in the PPN formalism alpha-zeta notation. Along the way you necessarily get a very good characterization of the transforms available on local arrival times in different coordinate systems and can probe other PPN parameters. (cf. Kramer, sec 4 and 5.1 free version: http://arxiv.org/abs/astro-ph/0405178v2 http://arxiv.org/abs/astro-ph/0405178v2). More generally, do you object to systems of coordinates that are in-practice recoverable by a wide variety of observers? We sure aren't Eulerian observers of the Milky Way, but does it really seem parochial or idiosyncratic to take a Eulerian approach to its matter? > If four observers cannot agree on what was your exact location at time n Find four observers who see the CMB (and matter in the bulk) as isotropic and homogeneous, who measure the same temperature of the relic photons, and who have a direct line of sight (with improbably good telescopes) into the relevant part of the Milky Way, and they can agree very precisely on your location in a cosmological frame constructed like the standard one used in this tiny patch of spacetime. The tricky part is that the light travel times are long compared to chaotic movements of individual humans, and the choice of gauge has to be agreed and the observations shared. > If you're in the middle of nowhere and not moving at relativistic speeds Where in spacetime is the middle of nowhere? What's materially different for a relativistic observer moving through the same general curved spacetime (especially if "the middle of nowhere" is, say, a large region of extremely-close-to-Minkowski spacetime) as a non-relativistic one? While you're there, what's different for an accelerated observer in the same region? Are you saying that something more than a Lorentz transform would be needed?

cjslep 9y ago

And indeed this is how the golden record on the Voyager spacecraft potentially informs any aliens of the location of Earth.

nitwit005 9y ago

Ahh, I missed that it was a large set of spheres. Thanks.

hinkley 9y ago

It’s [a little bit] bullshit. The light emitted from a pulsar is only a sphere if you live in a universe that contains only you and the pulsar. Spacetime is curved by every mass in it. The distance light travels is subtly changed by what is between you and the source. If four observers cannot agree on what was your exact location at time n (without a perfect model of the entire universe) then how would you be able to determine your location by observing four other objects? Edit: You can get the right neighborhood, but if you’re moving at relativistic speeds it will be problematic. And if you’re in the middle of nowhere and not moving at relativistic speeds you’re probably gonna die anyway...

hinkley 9y ago
I guess my point is that if you’re going to call it Galactic Positioning, it better work someplace else than on the surface of the earth, and I am not seeing how. We are all so ‘close’ that you can find a near exact distance between telescopes but that’s different than plotting your current distance to Tau Ceti.
raattgift 9y ago
Er, can you characterize which post-Newtonian correction/s is or are of non-negligible order here for Milky-way millisecond pulsars? Where do they enter, if not extremely close to the pulsars themselves? I'm curious, because we use them to test, among other things, |\alpha_3| in the PPN formalism alpha-zeta notation. Along the way you necessarily get a very good characterization of the transforms available on local arrival times in different coordinate systems and can probe other PPN parameters. (cf. Kramer, sec 4 and 5.1 free version: http://arxiv.org/abs/astro-ph/0405178v2 http://arxiv.org/abs/astro-ph/0405178v2). More generally, do you object to systems of coordinates that are in-practice recoverable by a wide variety of observers? We sure aren't Eulerian observers of the Milky Way, but does it really seem parochial or idiosyncratic to take a Eulerian approach to its matter? > If four observers cannot agree on what was your exact location at time n Find four observers who see the CMB (and matter in the bulk) as isotropic and homogeneous, who measure the same temperature of the relic photons, and who have a direct line of sight (with improbably good telescopes) into the relevant part of the Milky Way, and they can agree very precisely on your location in a cosmological frame constructed like the standard one used in this tiny patch of spacetime. The tricky part is that the light travel times are long compared to chaotic movements of individual humans, and the choice of gauge has to be agreed and the observations shared. > If you're in the middle of nowhere and not moving at relativistic speeds Where in spacetime is the middle of nowhere? What's materially different for a relativistic observer moving through the same general curved spacetime (especially if "the middle of nowhere" is, say, a large region of extremely-close-to-Minkowski spacetime) as a non-relativistic one? While you're there, what's different for an accelerated observer in the same region? Are you saying that something more than a Lorentz transform would be needed?