# Are some objects in the night sky just older images of other objects?

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• hjgfx697
hjgfx697
TL;DR Summary
In an infinite universe, could light deflected by gravitational lensing bounce around the universe from one massive object to another until it comes at us from another angle as well as the original direct angle?
It's a question I've pondered over for years and I've even asked a few people but I've never had a convincing answer. As per the summary, if gravitational lensing is spraying light all over the place like an automatic lawn irrigator, is there any chance some light will do a tour of the universe via other massive objects and arrive at Earth from a different angle? So we'd see the same object twice, one appearing to be much older and in a completely different place, having taken the long route? Could this happen many times from different angles? Or have I (as I imagine) got it all wrong?
To clarify, I've attached an artwork.
Perhaps there's a thread/website/video already answering this, but I don't have the correct vocabulary to find it!
Thanks!

PeroK
Gravitational lensing is generally not that strong. If you have an object directly behind a large mass however, you can get several images of that same object - such as in the case of the Einstein cross or even a spread out image as an Einstein ring.

Gravitational lensing from objects like galaxies is too weak for lensing of lensed images to be at all common. That means the bending angles in your sketch are absurdly large and I'm not aware of any examples in reality, but the concept isn't impossible.

Strong lensing near black holes can produce multiple images, though. Gargantua, the black hole in the film Interstellar has an accretion disc, which results in a single narrow glowing ring around the hole. The multiple filaments of light around the hole seen in the movie (from calculations done by Nobel prize winner Kip Thorne) are multiple images of that single ring caused by light from it orbiting the hole several times before reaching the camera.

As others have mentioned, your lawn sprinkler analogy is way, way exaggerated. Lensing is a very weak effect.

Compounding the problem, when the light from distant objects gets gravitationally bent, it tends to greatly distort the appearance of the objects.

Here is an example:

There's a central massive object that's lensing the more distant object so much that it's bent all the way around into a circle.

You're never going to mistake that near circle of an object for anything other than a highly gravitationally-distorted object - and any such objects are pretty closely associated with the mass that's doing the distorting.

A better analogy might be the distortions you see when looking through the front windshield of your car.

Your view of the road is 99.99% accurate. Because the glass has non-zero thickness, there is a bit of refraction happening, but it is quite small, and is strongly localized to the very edges of your windshield, where the glass curves the most. Because of the context and proximity, there is no way you'd ever mistake the small distortions at the boundary of your windshield for anything other than what they are.

Thanks for your replies and the references to Einstein crosses and rings - more for me to study! Okay, I think I understand better. However, saying that lensing is not so common can only be true from one vantage point, no? Ie. Earth. Because there aren't so many objects perfectly aligned with us. But I suppose that all light from every source must be lensed somewhere in the universe from other vantage points? Ie all light must pass by a massive object somewhere or other, it just doesn't pass by Earth. I realise now though that any light passing multiple objects would be extremely distorted.
My understanding came from images like this one, which are exaggerated for clarity and suggest that we would see perfect images of stars.

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hjgfx697 said:
However, saying that lensing is not so common can only be true from one vantage point, no?
There are plenty of instances of lensing, but I'm not aware of an instance of lensing of a lensed object. It could certainly happen - as you say, you just need three objects (four, counting Earth) coaligned, but that's not enormously common.

We can detect lensing by the Sun. I suppose that a lensed image that passes near the Sun is technically doubly lensed.

IIRC, because light via gravitationally lensed arc has taken the scenic route, two legs rather than direct, there have been cases where different arcs of same source have shown sufficiently different time-lapses to be 'useful'.

Clearly, deconvoluting *multiple* arcs is 'non-trivial' but, IIRC, it helped study a distant super-nova's progression, while those differing lag-times mapped the lensers' relative mass and geometry.

Here's an example...
https://physicsworld.com/a/gravitational-lensing-of-supernova-yields-new-value-for-hubble-constant/
quote:
The lumpy distribution of mass in the cluster created a complex gravitational field that sent the supernova’s light along several different paths towards Earth. When the supernova was first observed in 2014, it appeared as four points of light. As the four points faded, a fifth appeared 376 days later. This light was delayed by the longer path it had taken through the cluster.
/

sophiecentaur and Ibix
Very interesting article, thanks! An extra year bouncing around! Amazing.

Nik_2213
hjgfx697 said:
Very interesting article, thanks! An extra year bouncing around! Amazing.
Well, note that that's an extra year on a 9 billion year journey. That's what we mean by "weak". And I would not view it as "bouncing around" really - there were five curved paths from the supernova to us, and one was a slightly longer path. Think of taking the racing line round a corner versus a wider curve, rather than the racing line versus a series of wild pinball bounces.

Nik_2213 said:
As the four points faded, a fifth appeared 376 days later. This light was delayed by the longer path it had taken through the cluster.

hjgfx697 said:
Very interesting article, thanks! An extra year bouncing around! Amazing.
Ibix said:
Well, note that that's an extra year on a 9 billion year journey.
Maybe this will help to get a feeling of the distances involved.

That 1 part in 9 billion is the rough equivalent of walking around the Earth 375,000 times versus 375,001 times 375,000 times plus 1 mile.

I doubt you would be able to tell the difference by then.

Cheers,
Tom

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Tom.G said:
That 1 part in 9 billion is the rough equivalent of walking around the Earth 375,000 times versus 375,001 times.
That’s 1 part in 375000, not 1 part in 9000000000 …

Now, 1 mile in 375000 times walking around the Earth though … now that is quite literally walking the extra mile.

Tom.G
Thanks everyone for your explanations. I'm a slightly wiser man today than yesterday.

PeroK
hjgfx697 said:
I'm a slightly wiser man today than yesterday.
Would you say more or less than 1 part in 9 billion wiser?

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