Can gravitation lensing massively shift the apparent location of stars?

Submitted by IPv6Guy t3_125z8l8 in askscience

So I look up in the sky and I see a star. Maybe the star is close to where I am looking, but the light has been slightly warped because of gravitation lensing from another star.

But how do I - or maybe an astronomer - know that the physical location of the star it isn't on the other side of the horizon from where I am looking? Isn't it possible I am actually looking at a black hole and the starlight has been warped enough so that I see it, but it isn't anywhere close to where I am looking?

As an analogy, I am sitting in the stands at a football stadium and I am staring at the 50 yard line, but I see the uprights and goalpost because there is a black hole on the 49 yard line (oops) that is deflecting/warping the light by 90 degrees and toward my eyes.

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Aseyhe t1_je7fact wrote

Light can be deflected by a large angle if it passes close enough to a black hole. In principle, light from a star on one side of the sky could indeed be deflected toward us by a black hole on the other side of the sky. See for example this simulated picture and notice how the galactic center on the right-hand side of the picture also appears to the left of the black hole, within the Einstein ring.

However, there is no concern that this effect could lead to seeing stars in the wrong places. As you can see in the picture, the black hole makes a distorted image of the whole sky. If we were able to resolve an individual star in that image, we would certainly also resolve the whole distorted image of the sky and infer that a black hole is there.

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Thoughtfulprof t1_jeat74g wrote

That simulated image is quite cool, but can you imagine what a night sky like that would have done to world religions?

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pando93 t1_je76nsa wrote

Definitely, and that is exactly the case in weak gravitational lensing, where the effect is small enough to not significantly distort the image. Also, this is exactly how this effect was first measured by Eddington!

Usually, what would happen is that we first know there is a massive object in the way to a far away galaxy or cluster, and then by doing some (mostly statistical) analysis, would learn about the details of the massive object through its effect on the observed galaxy.

The wiki page has some nice explanations.

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mfb- t1_je7qygw wrote

The amount of light that gets deflected by more than a tiny fraction of a degree is negligible, and a measurable deflection needs the lensing of other galaxies and good telescopes to notice it at all. All the stars you see with the naked eye are where you see them.

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[deleted] t1_je7z80h wrote

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mfb- t1_je88q1o wrote

There are no "absolute positions" anyway.

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Arquit3d t1_je8afeg wrote

The fact that a reference point can't be defined by our standards doesn't make the system static or absurd.

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More-Grocery-1858 t1_jegug9t wrote

What bakes my noodle is the thought that "true location" is a bit of a Newtonian concept. When you factor in the warping of spacetime, the whole universe is a little wibbly wobbly.

It's easy to think of stars and galaxies like they're all in a fishbowl and you're watching them from the outside, but that's not how we experience the universe.

In other words, the 'where' of where something is all depends on how you plan to get there.

From the point of view of the light beam hitting your eye, the star is exactly where it appears to be. From the point of view of traveling there slower than light, your path and the star's position would only converge sometime in the future.

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[deleted] t1_je9matw wrote

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aphilsphan t1_jea2ao4 wrote

The spirit of your point is correct. We can only see fairly close stars with our naked eyes, but we can see great big stars that are further than 50 ly. Rigel is like 800 ly away and even I can see it.

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