# How do we know that objects that appear in duplicate or triplicate, etc. due to strong gravitational lensing aren't actually multiple objects?

If we are looking at faraway objects, there is no parallax view, correct?

So isn't there a chance that an object that appears multiple due to strong foreground gravitational lensing is actually several distinct objects?

• Then how would you explain the lack of such near-identical objects in the remainder of your viewing field? Nov 3, 2021 at 12:44
• Well, yes, there’s a chance. For complicated cluster lenses and multiple images without much detail, the chance might be good, and some analyses do in fact do lens reconstruction while including and excluding some of the more uncertain candidate images. Jan 26, 2022 at 0:16

Perhaps this isn't the case for every scenario, but I can think of at least two instances where this can be determined:

Stars

In the case of stars, it's pretty straightforward to get the spectra of the objects and compare them. If they are identical, with maybe some gravitational redshift, then they're the same object.

Galaxies

For galaxies, the same kind of idea can be applied, but galaxy spectra can be a bit more complicated. Sometimes for these, the images can visually be compared and found to be the same object. But even if they've been distorted, the same basic principle can be applied; luminosity, general spectra, and other things should be roughly the same, and in the case of a cataclysmic event happening in a galaxy like a supernova, this will be observed in all of its copies, confirming that they are duplicates

A good point made by Hartmut Braun in the comments is the fact that a lot of these objects can be variable, be it a star itself, a binary system, or a variable object in the galaxy, and these variations can be detected sometimes with a time lag, and this can help prove that they are, in fact, the same object, since they will have the same periodicity.

• You might add that some sources tend to be variable. The the variation can be observed for all images usually with a time lag. Nov 3, 2021 at 11:27
• Good answer, but also a good point by @HartmutBraun.
– pela
Nov 3, 2021 at 11:36
• That reminds me of nasa.gov/feature/goddard/2021/… Nov 3, 2021 at 16:41
• youtu.be/ljoeOLuX6Z4
– Tim
Nov 4, 2021 at 8:34

A smoking gun is to show that the two pathways exhibit quantum interference. I don't know if this has been done successfully on astronomical targets yet, but I recall hearing about how ultra-fast radio detectors can enable this trick. By measuring a very short period of time, you can tap into the Time vs Energy duality and get an interference pattern even if the two paths have a timing difference of years. By adjusting the observation window time and seeing when the interference pattern appears, you can determine the time offset between the two paths.

In real images, you often don't see multiple copies of a galaxy; you see distorted smears and rings. This "funny mirror" effect makes it clear that you are not looking at an actual object!

• – uhoh
Nov 3, 2021 at 22:55
• Do have a reference for this idea? It sounds rather dubious. Nov 7, 2021 at 19:13
• @PeterErwin it may have been a SETI Seminar video, talking about advances in radio telescope detectors. But I don't remember specifically. Nov 8, 2021 at 14:40