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I understand that the Shapiro time delay effect is one of the four classic solar system GR tests.

I understand the EM waves have to pop back from a planet beyond the star and then pass next to the sun coming backwards.

Question:

  1. has anybody ever done the same test interstellar, so with another solar system? Alpha Centauri is only 4.37 light years away, so it sounds doable. (It might be harder to time for the planet orbitals in that solar system, but it sounds still doable, just like with the Shapiro effect, the EM wave has to pop back from a planet from beyond Alpha Centaury, we know the planets' orbit in that solar system too, so we can calculate the right position when the EM wave gets there to pop back from it.)
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    $\begingroup$ How would you get the light back from 4.37ly away? There aren't any giant mirrors out in space... $\endgroup$ – enumaris Apr 24 '18 at 16:25
  • $\begingroup$ Dear enumaris, just like with the Shapiro effect, it has to pop back from a planet from beyond Alpha Centaury, we know the planets' orbit in that solar system too, so we can calculate the right position when the EM wave gets there to pop back from it. I will add this to my question. $\endgroup$ – Árpád Szendrei Apr 24 '18 at 16:30
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    $\begingroup$ There are no planets known around Alpha Centauri. $\endgroup$ – Rob Jeffries Apr 24 '18 at 17:53
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    $\begingroup$ Just an FYI: the "radar equation" has signal strength that falls off as $1/r^4$. Current interplanetary radar already maxes out our high gain antennae, and you have to have a really good idea of where the target is beforehand to even see it in all the signal processing. It's not a "blip" on a fluorescent screen. $\endgroup$ – JEB Apr 24 '18 at 23:07
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In principle yes, but I don't think that the Shapiro time delay is measurable interstellar/intergalactic. To do this a radar signal would have to pass a potential well (a Star or a galaxy) and then to be reflected back to the source and detected there. Both, reflection and detection don't seem to be within the scope of present technology.

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