<p>Perhaps this isn't the case for every scenario, but I can think of at least two instances where this can be determined:</p>
<p><strong>Stars</strong></p>
<p>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.</p>
<p><strong>Galaxies</strong></p>
<p>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</p>
<p>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.</p>

CC BY-SA 4.0
2021-11-03T11:27:23.453Z
You might add that some sources tend to be variable. The the variation can be observed for all images usually with a time lag.
Hartmut Braun
CC BY-SA 4.0
2021-11-03T11:36:08.747Z
Good answer, but also a good point by @HartmutBraun.
pela
CC BY-SA 4.0
2021-11-03T16:41:13.853Z
That reminds me of https://www.nasa.gov/feature/goddard/2021/rerun-of-supernova-blast-expected-to-appear-in-2037
asdfex
CC BY-SA 4.0
2021-11-04T08:34:03.657Z
https://youtu.be/ljoeOLuX6Z4
Tim
CC BY-SA 4.0
2021-11-03T04:07:20.410Z
2021-11-03T16:09:20.953Z
2021-11-03T16:09:20.953Z
Justin T
<p>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 <em>years</em>.  By adjusting the observation window time and seeing when the interference pattern appears, you can determine the time offset between the two paths.</p>
<p>In real images, you often don't see multiple copies of a galaxy; you see distorted smears and rings.  This &quot;funny mirror&quot; effect makes it clear that you are not looking at an actual object!</p>

CC BY-SA 4.0
2021-11-03T22:55:44.250Z
`+1` see also [Have interference effects (in space) ever been observed by a single instrument, as opposed to interferometry?](https://astronomy.stackexchange.com/q/47285/7982) and just for fun [Are astronomers waiting to see something in an image from a gravitational lens that they've already seen in an adjacent image?](https://astronomy.stackexchange.com/q/30243/7982) and perhaps [Has a gravitational microlensing event ever been predicted? If so, has it been observed?](https://astronomy.stackexchange.com/q/43234/7982)
uhoh
CC BY-SA 4.0
2021-11-07T19:13:17.403Z
Do have a reference for this idea? It sounds rather dubious.
Peter Erwin
CC BY-SA 4.0
2021-11-08T14:40:45.507Z
@PeterErwin it may have been a SETI Seminar video, talking about advances in radio telescope detectors.  But I don't remember specifically.
JDługosz
CC BY-SA 4.0
2021-11-03T22:45:52.190Z
2021-11-03T22:45:52.190Z
JDługosz
<p>If we are looking at faraway objects, there is no parallax view, correct?</p>
<p>So isn't there a chance that an object that appears multiple due to strong foreground gravitational lensing is actually several distinct objects?</p>

CC BY-SA 4.0
2021-11-03T12:44:50.043Z
Then how would you explain the lack of such near-identical objects in the remainder of your viewing field?
Carl Witthoft
CC BY-SA 4.0
2022-01-26T00:16:38.793Z
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.
ntessore
<observational-astronomy><general-relativity><gravitational-lensing><eclipse-non-earth-related>
How do we know that objects that appear in duplicate or triplicate, etc. due to strong gravitational lensing aren't actually multiple objects?
Kurt Hikes
# How do we know that objects that appear in duplicate or triplicate, etc. due to strong gravitational lensing aren't actually multiple objects?

(Asked by: username_0 on 2021-11-03)

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?

**Tags:** observational-astronomy, general-relativity, gravitational-lensing, eclipse-non-earth-related
> **Comments:**
> **username_1**: Then how would you explain the lack of such near-identical objects in the remainder of your viewing field?  
> **username_2**: 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.  

---

## 2 answers

### Answer by username_3 on 2021-11-03. Score: 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.

> **Comments:**
> **username_4**: You might add that some sources tend to be variable. The the variation can be observed for all images usually with a time lag.  
> **username_5**: Good answer, but also a good point by @HartmutBraun.  
> **username_6**: That reminds me of https://www.nasa.gov/feature/goddard/2021/rerun-of-supernova-blast-expected-to-appear-in-2037  
> **username_7**: https://youtu.be/ljoeOLuX6Z4  

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### Answer by username_8 on 2021-11-03. Score: 5

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!

> **Comments:**
> **username_9**: `+1` see also [Have interference effects (in space) ever been observed by a single instrument, as opposed to interferometry?](https://astronomy.stackexchange.com/q/47285/7982) and just for fun [Are astronomers waiting to see something in an image from a gravitational lens that they've already seen in an adjacent image?](https://astronomy.stackexchange.com/q/30243/7982) and perhaps [Has a gravitational microlensing event ever been predicted? If so, has it been observed?](https://astronomy.stackexchange.com/q/43234/7982)  
> **username_10**: Do have a reference for this idea? It sounds rather dubious.  
> **username_8**: @PeterErwin it may have been a SETI Seminar video, talking about advances in radio telescope detectors.  But I don't remember specifically.  

---

