<p>While I don't have my Astronomers license, I'll start the ball rolling for this <em>really interesting question!</em></p>
<blockquote>
<p>Is it theoretically possible to see a spiral galaxy head on, and side on, if there is a convenient gravitational lensing effect?</p>
</blockquote>
<h3>Photons via lensing</h3>
<ol>
<li>It certainly depends on what the definition of &quot;see&quot; is. Astronomers are famous for squeezing out every drop of information available from even a meagre drizzle of photons; Astronomy doesn't depend exclusively on high resolution images.</li>
<li>Gravitational lensing can bend light through arbitrarily large angles, see for example answers to</li>
</ol>
<ul>
<li><a href="https://physics.stackexchange.com/q/467144/83380">Could a trajectory around a large mass ever deflect by more than 180 degrees due to general relativistic effects?</a> (Physics SE)</li>
<li><a href="https://astronomy.stackexchange.com/q/49873/7982">Gravitational mirroring: Can we theoretically see the Milky Way using a telescope?</a> (here)</li>
</ul>
<p>This won't be a complete answer, but <em>certainly</em> it is possible to receive at least a stream of photons at Earth from both the edge-on and axial direction if at least one (or two) sufficiently strong gravitational lensing bodies happen to be in the right place.</p>
<h3>Photons via scattering</h3>
<p>Dust and (to some extent) gas can scatter light. It is not impossible that an extended object can be illuminated by axial light from an edge-on galaxy and scatter some of that towards us. The most likely scenario would probably involve some kind of high energy event or flare from matter falling into a supermassive black hole at the center of the galaxy; the time delay or &quot;echo&quot; between any direct light and the reflected light with a longer path would confirm that the two observed transient events were in fact from the same initial event.</p>

CC BY-SA 4.0
2022-11-26T03:53:06.440Z
A problem with high deflection angles in gravitational lensing is that they require small impact parameters (around $3r_s$ for 90°). You can only get that close if the lensing object is a "naked" black hole, not a whole galaxy or galaxy cluster. And when you're that close, the angle changes rapidly for a small change in impact parameter, so you get a lot of distortion of extended light sources, as shown in typical Einstein Ring images.
PM 2Ring
CC BY-SA 4.0
2022-11-26T03:58:20.780Z
@PM2Ring which is exactly why I have started off with item #1 "It certainly depends on what the definition of 'see' is. Astronomers are famous for squeezing out every drop of information available from even a meagre drizzle of photons"
uhoh
CC BY-SA 4.0
2022-11-26T01:35:24.027Z
2022-11-26T01:35:24.027Z
uhoh
<p>I was thinking this when I heard Professor Mike Merrifield say something to the effect of &quot;unfortunately we've never seen the same galaxy side on and head on, so we can compare notes on our theories about spiral galaxies&quot;, in a DeepSkyVideos' video (I forget which).</p>
<p>Perhaps with the right gravity well in the right place, it might be possible?</p>
<p>Do we have any examples of this? Is anyone looking?</p>
<hr />
<ul>
<li><a href="https://about.me/michael.merrifield" rel="nofollow noreferrer">https://about.me/michael.merrifield</a></li>
<li>playlist <a href="https://www.youtube.com/watch?v=0RiAxvb_qI4&amp;list=PLcUY9vudNKBNGAgplgACrGL9UlxyDHq7k" rel="nofollow noreferrer">Mike Merrifield - Sixty Symbols</a></li>
</ul>

CC BY-SA 4.0
2022-11-25T17:53:06.220Z
Not confident enough on this to put an answer, but I’m pretty sure it’s a no: gravitational lensing, while a pretty nonlinear effect, still can’t perform miracles like taking light from two completely orthogonal views of a galaxy and spitting them both out at you; these gravitational wells can only change what they have, and they can only have one or the other, because like us they are often pretty far from the source.
Justin T
CC BY-SA 4.0
2022-11-25T19:29:20.127Z
All a gravitational lens does is redirect light, it cannot redirect light that never arrived at the lens.  Any view of a galaxy it going to block the view of parts of it, so those parts would never make it to the gravitational lens.  But, with two different gravitational lenses in just the right places, it theoretically could happen.
Greg Miller
<galaxy><gravitational-lensing><spiral-arms>
Is it theoretically possible to see a spiral galaxy head on, and side on, if there is a convenient gravitational lensing effect?
Rabbi Kaii
# Is it theoretically possible to see a spiral galaxy head on, and side on, if there is a convenient gravitational lensing effect?

(Asked by: username_0 on 2022-11-25)

I was thinking this when I heard Professor Mike Merrifield say something to the effect of "unfortunately we've never seen the same galaxy side on and head on, so we can compare notes on our theories about spiral galaxies", in a DeepSkyVideos' video (I forget which).

Perhaps with the right gravity well in the right place, it might be possible?

Do we have any examples of this? Is anyone looking?

---

* <https://about.me/michael.merrifield>
* playlist [Mike Merrifield - Sixty Symbols](https://www.youtube.com/watch?v=0RiAxvb_qI4&list=PLcUY9vudNKBNGAgplgACrGL9UlxyDHq7k)

**Tags:** galaxy, gravitational-lensing, spiral-arms
> **Comments:**
> **username_1**: Not confident enough on this to put an answer, but I’m pretty sure it’s a no: gravitational lensing, while a pretty nonlinear effect, still can’t perform miracles like taking light from two completely orthogonal views of a galaxy and spitting them both out at you; these gravitational wells can only change what they have, and they can only have one or the other, because like us they are often pretty far from the source.  
> **username_2**: All a gravitational lens does is redirect light, it cannot redirect light that never arrived at the lens.  Any view of a galaxy it going to block the view of parts of it, so those parts would never make it to the gravitational lens.  But, with two different gravitational lenses in just the right places, it theoretically could happen.  

---

## 1 answers

### Answer by username_3 on 2022-11-26. Score: 3 | Accepted answer

While I don't have my Astronomers license, I'll start the ball rolling for this *really interesting question!*

> Is it theoretically possible to see a spiral galaxy head on, and side on, if there is a convenient gravitational lensing effect?

### Photons via lensing

1. It certainly depends on what the definition of "see" is. Astronomers are famous for squeezing out every drop of information available from even a meagre drizzle of photons; Astronomy doesn't depend exclusively on high resolution images.
2. Gravitational lensing can bend light through arbitrarily large angles, see for example answers to

* [Could a trajectory around a large mass ever deflect by more than 180 degrees due to general relativistic effects?](https://physics.stackexchange.com/q/467144/83380) (Physics SE)
* [Gravitational mirroring: Can we theoretically see the Milky Way using a telescope?](https://astronomy.stackexchange.com/q/49873/7982) (here)

This won't be a complete answer, but *certainly* it is possible to receive at least a stream of photons at Earth from both the edge-on and axial direction if at least one (or two) sufficiently strong gravitational lensing bodies happen to be in the right place.

### Photons via scattering

Dust and (to some extent) gas can scatter light. It is not impossible that an extended object can be illuminated by axial light from an edge-on galaxy and scatter some of that towards us. The most likely scenario would probably involve some kind of high energy event or flare from matter falling into a supermassive black hole at the center of the galaxy; the time delay or "echo" between any direct light and the reflected light with a longer path would confirm that the two observed transient events were in fact from the same initial event.

> **Comments:**
> **username_4**: A problem with high deflection angles in gravitational lensing is that they require small impact parameters (around $3r_s$ for 90°). You can only get that close if the lensing object is a "naked" black hole, not a whole galaxy or galaxy cluster. And when you're that close, the angle changes rapidly for a small change in impact parameter, so you get a lot of distortion of extended light sources, as shown in typical Einstein Ring images.  
> **username_3**: @PM2Ring which is exactly why I have started off with item #1 "It certainly depends on what the definition of 'see' is. Astronomers are famous for squeezing out every drop of information available from even a meagre drizzle of photons"  

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