<p>I think it is better to say that gravitational lensing is <em>geometrical</em> in nature.</p>
<p>Topology deals with how spaces are connected. It entirely ignores the metric, how distances are defined in the space. General relativity is a metric theory that describes of how mass and energy curve spacetime and give rise to the metric. It actually says nothing about the topology.</p>
<p>You can redirect light in an optical way using different indices of refraction, such as in ordinary glass lenses, total internal reflection, or mirages. That is not what a gravitational lens does: the light travels through vacuum.</p>
<p>You could as you say redirect light topologically if there was (say) a portal where light enters one surface and it is topologically identified with another surface sending it off in a different direction. But this is not what gravitational lensing does: the spacetime around heavy masses is curved but does not have any weirdness (it is locally connected).</p>
<p>What relativity predicts is that light follows the locally shortest path in spacetime (a null geodesic). This is a metric effect (note the word &quot;shortest&quot;, distance is involved). So it makes sense to call this geometric lensing: light is redirected by the curvature and distances in spacetime,</p>

CC BY-SA 4.0
2023-09-24T19:59:28.950Z
if path's, get  'split'  then that looks just as a higher dimensional effect.
Peter
CC BY-SA 4.0
2023-09-23T08:31:32.260Z
2023-09-23T08:31:32.260Z
Anders Sandberg
<p>If a large mass can adjust space geometry<br />
Like a blackhole or another star system<br />
Then is this effect optical in nature or topological?</p>
<p>I know those sounds close, but a mirror reflects light.<br />
A mirror does not act as a portal (if you know the game portals).</p>
<p>I came to wonder if, by some topological disorder of space itself.
The effects as seen in gravitational lensing might be a topological effect.
Though I don't know if we can already discriminate between optical vs. toplogical.<br />
(Perhaps based on theory or perhaps because of the things astronomers can see in gravitational lenses or even if people have ever tried to look for such effects)</p>
<p>With an extra dimension, topological mirrors would be possible (and thus could be proof of such). If so, then this would affect essential all 3d space (leaving out time as a dimension as it's not a dimension)</p>

CC BY-SA 4.0
2023-09-22T21:56:55.340Z
"Optical" implies electromagnetic interaction with photons; that's what happens in mirrors and lenses to redirect photons. So gravitational lensing is not optical; light is redirected because space itself is "bent"; so I guess that is topological in your classification.
antlersoft
CC BY-SA 4.0
2023-09-24T19:56:33.577Z
So we then see 'the same' space over different path's, space is duplicated then.
Also in a 4D space an object can apear in 3D space over a distance but be the same object in a higher dimension.  (as a thorus in a flatlander world, it apears as multiple objects but in a higher space its one space).
Peter
<gravitational-lensing>
Is gravitational lensing, optical in nature or topological?
Peter
# Is gravitational lensing, optical in nature or topological?

(Asked by: username_0 on 2023-09-22)

If a large mass can adjust space geometry  
Like a blackhole or another star system  
Then is this effect optical in nature or topological?

I know those sounds close, but a mirror reflects light.  
A mirror does not act as a portal (if you know the game portals).

I came to wonder if, by some topological disorder of space itself.
The effects as seen in gravitational lensing might be a topological effect.
Though I don't know if we can already discriminate between optical vs. toplogical.  
(Perhaps based on theory or perhaps because of the things astronomers can see in gravitational lenses or even if people have ever tried to look for such effects)

With an extra dimension, topological mirrors would be possible (and thus could be proof of such). If so, then this would affect essential all 3d space (leaving out time as a dimension as it's not a dimension)

**Tags:** gravitational-lensing
> **Comments:**
> **username_1**: "Optical" implies electromagnetic interaction with photons; that's what happens in mirrors and lenses to redirect photons. So gravitational lensing is not optical; light is redirected because space itself is "bent"; so I guess that is topological in your classification.  
> **username_0**: So we then see 'the same' space over different path's, space is duplicated then. Also in a 4D space an object can apear in 3D space over a distance but be the same object in a higher dimension.  (as a thorus in a flatlander world, it apears as multiple objects but in a higher space its one space).  

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## 1 answers

### Answer by username_2 on 2023-09-23. Score: 8

I think it is better to say that gravitational lensing is *geometrical* in nature.

Topology deals with how spaces are connected. It entirely ignores the metric, how distances are defined in the space. General relativity is a metric theory that describes of how mass and energy curve spacetime and give rise to the metric. It actually says nothing about the topology.

You can redirect light in an optical way using different indices of refraction, such as in ordinary glass lenses, total internal reflection, or mirages. That is not what a gravitational lens does: the light travels through vacuum.

You could as you say redirect light topologically if there was (say) a portal where light enters one surface and it is topologically identified with another surface sending it off in a different direction. But this is not what gravitational lensing does: the spacetime around heavy masses is curved but does not have any weirdness (it is locally connected).

What relativity predicts is that light follows the locally shortest path in spacetime (a null geodesic). This is a metric effect (note the word "shortest", distance is involved). So it makes sense to call this geometric lensing: light is redirected by the curvature and distances in spacetime,

> **Comments:**
> **username_0**: if path's, get  'split'  then that looks just as a higher dimensional effect.  

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