Measuring sun's true direction

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I don't see a speed of gravity myself and I'm only trying to figure out the location of the sun relative its apparent position which is a 500 seconds distance apart. The speed of gravity enters the picture from the left of the difference in the two locations.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

So light from the Sun has a 500-second propagation time to Earth, and gravity has a near-zero propagation time. Different experiments set an upper limit to the propagation time of gravity, which it turn sets a lower limit to its speed. So you still need an experiment and some data to estimate the speed of gravity. -|Tom|-

The lower limit of this speed (of gravity) has been set by binary pulsars - according to the list available on this site. I have a question related to this observation.
How sure are we (or the discoverers of binary pulsars) that these things actually exists?
According to 'standard' models of how pulsars forms it would seem extremely unlikely that two stars would collapse to neutron stars and both radiating - become pulsars and both beams directed in our direction.
The point I trying to make is that there is a good chance that it might be something more in the line of a single pulsar (if they actually exist) that happen to have two (or more) radiating beams directed toward us. This too would be rare.
In such a case the lower limit of the speed of gravity could be anything again.


Rudolf

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>How sure are we (or the discoverers of binary pulsars) that these things actually exists?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Rarely do I answer such a question with "almost no doubt at all", but this is such a case. The arriving signals are so precise that we can easily see the unique signatures of orbital motion within them -- variations in distances of about two million kilometers, a large orbital eccentricity, an obvious inclination to the line of sight, a progressive movement of the nodes of the orbit, and even the advance of the periastron direction that relativity predicts. Moreover, when both stars are close to the line of sight at the same time, we see the added delay of the signal of the farther star as its signal is bent more by the nearer one, requiring the signal to take a longer path. Moreover, the data do not just show vague impressions of these orbital characteristics, but rather precise manifestations, allowing the determination of some characteristics to as many as 12 significant figures.

So I really see no room for doubt that two massive stars orbit one another in this system. There is, of course, still plenty of room to doubt that the pulse-generating mechanism is modeled correctly.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>According to 'standard' models of how pulsars forms it would seem extremely unlikely that two stars would collapse to neutron stars and both radiating - become pulsars and both beams directed in our direction.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

On the contrary, according to standard theory, when one star explodes and collapses, it dumps considerable matter onto its companion, destabilizing the companion and hastening its own explosion and collapse. And tidal forces would quickly align the axes of the two stars so that their emissions would be beamed in the same direction. -|Tom|-

Back on Earth the sun is seen 20sec of arc from where the sun gravity is centered(or so you said). Is this not an experiment that shows the speed of gravity? And can you post a link to data on this issue?

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>Back on Earth the sun is seen 20sec of arc from where the sun gravity is centered (or so you said). Is this not an experiment that shows the speed of gravity? And can you post a link to data on this issue?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

The 20 arc seconds shows the amount of the speed of light, not gravity. To measure the latter would require another experiment that measured the deviation of the direction of Earth's acceleration from the focus point of Earth's elliptical orbit (the true, instantaneous Sun direction). For example, planetary radar ranging is such a data type. Such data is available at the National Space Sciences Data Center. For a graduate student in celestial mechanics, I estimate it would take about one man-year to learn how to analyze that data type.

If you are looking for something you can verify for yourself in an afternoon, no such experiment exists. Most things at the frontiers of knowledge require extensive background and analysis. But any textbook that addresses the matter will confirm that Earth accelerates toward the Sun's instantaneous position, not its light-time-retarded position. That alone tells you that gravity must get here faster than light. -|Tom|-

I am not argueing the points posted above but rather the triangulation of the two locations of the sun. The real time location triangulated with the light or apparent location. This just needs the exact angles to determine the speed of gravity since the speed of light is assumed or given.