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The light coming from a distant galaxy or star can be distorted en route to Earth; this distortion is caused by the curvature of space when a massive object is present. My question is, how can we tell whether these doppelgangers are from the same galaxy (especially in the case of a star)?

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  • $\begingroup$ Do you mean how the location of the source of the lensed light can be determined? $\endgroup$
    – LocalFluff
    Commented Apr 15, 2015 at 12:14
  • $\begingroup$ I think the OP means how do we know that multiple images of the same source ARE in fact the same source, and not just 2+ sources that look similar. If so, the answer is that unless the spectra are of very bad quality, they will be sufficiently different that we know if they origin from the same or from different sources. The will look almost exactly the same, and if some transient event happens (like a supernova going off), there will be a time lag because the light takes paths of different lengths through the Universe. $\endgroup$
    – pela
    Commented Apr 15, 2015 at 12:23
  • $\begingroup$ I once read about Einstein's Cross and thinking the stars alignment is coincidence until the article said they are duplicates due to gravitational lensing hence how to tell apart? $\endgroup$
    – user6760
    Commented Apr 15, 2015 at 12:56
  • $\begingroup$ how to tell a lensing is there if a point source's arc is not obvious and its shape is not distorted? $\endgroup$ Commented Apr 16, 2015 at 1:44
  • $\begingroup$ @questionhang good question $\endgroup$
    – user6760
    Commented Apr 16, 2015 at 2:11

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In the case of multiple images of a background, distant object, the answer is relatively simple. You take a spectrum of the multiple images or parts of an extended lensed image and you see whether the spectrum looks the same, and in particular whether the redshift of the multiple images are the same.

Gravitational lensing affects light of all wavelengths equally, so regardless of the path taken, the spectrum should be unaltered, except that the different paths take different amounts of time to travel from the original object to us. So, if the background source, or its spectrum, are time-varying, then the lensed images could also appear different. However, distant galaxies do not change their redshift on such short timescales and so this should still be the same regardless.

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