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Comment by nitwit005 | Hacker News Reader

> That pulse was emitted long ago from the distant pulsar and then spread outward through space. If you knew the exact time was when it was first emitted and the exact time you had received it, you’d be able to deduce how long it had been flying through space. Dividing by the speed of light would provide you with a measurement of your distance from the pulsar.

Am I missing something here? It doesn't seem like you can know the time emitted if you don't know your position.

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Agreed that the explanation is a bit lacking. This uses the phase difference between the signals of multiple pulsars, as indeed the individual pulses are not marked with a timestamp like in GPS.

Imagine you have two pulsars each with a 1ms signal, that don't move relative to you and emit signals that are in sync at their source. If the tops of both signals overlap at your location, you are in one one of many possible circles that are centered around the line between the pulsars, where the difference in distance to the pulsars is N times 300 km (distance light travels in 1 ms). By adding additional pulsars, you reduce your possible position to a set of points, and finally a single point.

To generalize this to a set of pulsars with different timings and for whom you don't know the exact location, you need to know the phase difference for all signals at a given reference point in space and time. The same procedure will then give you your current position and time relative to the reference point.

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.

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

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

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...

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.

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).

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?

You'd calibrate the timing of the pulsars before launch.

Which means that you would have to keep track of each pulse, if you have a comparatively inaccurate clock. Does not seem practical.

EDIT: On second thought, using an accurate clock would be a tiny fraction of the cost of the whole system, so it's a solved problem.

TIL using multiple pulsars to create a galactic positioning system "is a solved problem."

There’s a washing machine sized pulsar detector bolted to the International Space Station that is successfully locating itself with pulsars.

https://www.nasa.gov/sites/default/files/atoms/files/session...

Though it's useful, you don't even need an accurate clock. You can observe the relative phases of several pulsars to recover your clock.

You can know the current time if the decay rate of the pulsars is characterizable and you have a reference rate->time (eg: on date X pulsar was 1.45266363mhz and decays 1.6ns/hr)

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