# Why cannot we observe gravitational lensing in the local Universe... **URL:** \ **Category:** Factual Questions\ **Created:** [September 6, 2018, 11:27pm UTC](https://boards.straightdope.com/t/why-cannot-we-observe-gravitational-lensing-in-the-local-universe/820918 "2018-09-06T23:27:19Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![mandala](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mandala/32/95_2.png) [@mandala](https://boards.straightdope.com/u/mandala)\ **Post date:** [September 6, 2018, 11:27pm UTC](https://boards.straightdope.com/t/why-cannot-we-observe-gravitational-lensing-in-the-local-universe/820918/1 "2018-09-06T23:27:19Z")
… of the kind we readily observe at cosmological scales, where big-ass galaxies create pronounced arcs and Einstein rings of far-in-the-background galaxies? I am sure Andromeda, with over a trillion stars, masks a fairish number of background galaxies, and has the gravitational heft to create these cool lighting effects. Or does it? Do we have to be a certain distance away from the gravitational pit to see these cool curves? ---
**Author:** ![etasyde](https://avatars.discourse-cdn.com/v4/letter/e/9f8e36/32.png) [@etasyde](https://boards.straightdope.com/u/etasyde)\ **Post date:** [September 6, 2018, 11:39pm UTC](https://boards.straightdope.com/t/why-cannot-we-observe-gravitational-lensing-in-the-local-universe/820918/2 "2018-09-06T23:39:25Z")
Gravitational lensing can be seen in light passing by Sol. And certainly by galaxy sized objects. Also, “local universe?” I mean, that usually refers to an unfathomably large region of space containing billions of galaxies, or is used to refer to the entire observable universe (as opposed to, say, a “split” universe in many worlds). Did you mean local star system? ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [September 6, 2018, 11:58pm UTC](https://boards.straightdope.com/t/why-cannot-we-observe-gravitational-lensing-in-the-local-universe/820918/3 "2018-09-06T23:58:12Z")
Graviational Microlensing > **[Gravitational microlensing](https://en.wikipedia.org/wiki/Gravitational_microlensing)** > > Gravitational microlensing is an astronomical phenomenon caused by the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit ... > [@](#): > > When a distant star or quasar gets sufficiently aligned with a massive compact foreground object, the bending of light due to its gravitational field, as discussed by Einstein in 1915, leads to two distorted unresolved images resulting in an observable magnification. The time-scale of the transient brightening depends on the mass of the foreground object as well as on the relative proper motion between the background ‘source’ and the foreground ‘lens’ object… > > Microlensing by an isolated object was first detected in 1989. Since then, microlensing has been used to constrain the nature of the dark matter, detect exoplanets, study limb darkening in distant stars, constrain the binary star population, and constrain the structure of the Milky Way’s disk. Using microlensing data, paper showing exoplanets present around most of the stars in the Milky Way: [https://www.nature.com/articles/nature10684](https://www.nature.com/articles/nature10684) > [@](#): > > Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10 AU (Sun–Earth distance) from their stars. We find that ~17% of stars host Jupiter-mass planets… Cool Neptunes (10–30 M⊕) and super-Earths (5–10 M⊕) are even more common: their respective abundances per star are ~52% and ~62%. We conclude that stars are orbited by planets as a rule, rather than the exception. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [September 7, 2018, 12:02am UTC](https://boards.straightdope.com/t/why-cannot-we-observe-gravitational-lensing-in-the-local-universe/820918/4 "2018-09-07T00:02:54Z")
> [@mandala](#): > > Do we have to be a certain distance away from the gravitational pit to see these cool curves? At the right distance[the gravitational lensing effect of the sun can be used as a huge telescope](https://en.wikipedia.org/wiki/Gravitational_lens#Solar_gravitational_lens). > [@](#): > > Albert Einstein predicted in 1936 that rays of light from the same direction that skirt the edges of the Sun would converge to a focal point approximately 542 AUs from the Sun.[31] Thus, a probe positioned at this distance (or greater) from the Sun could use the sun as a gravitational lens for magnifying distant objects on the opposite side of the sun.[32] A probe’s location could shift around as needed to select different targets relative to the Sun. > > This distance is far beyond the progress and equipment capabilities of space probes such as Voyager 1, and beyond the known planets and dwarf planets, though over thousands of years 90377 Sedna will move farther away on its highly elliptical orbit. The high gain for potentially detecting signals through this lens, such as microwaves at the 21-cm hydrogen line, led to the suggestion by Frank Drake in the early days of SETI that a probe could be sent to this distance. A multipurpose probe SETISAIL and later FOCAL was proposed to the ESA in 1993, but is expected to be a difficult task. ---
**Author:** ![mandala](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mandala/32/95_2.png) [@mandala](https://boards.straightdope.com/u/mandala)\ **Post date:** [September 7, 2018, 12:08am UTC](https://boards.straightdope.com/t/why-cannot-we-observe-gravitational-lensing-in-the-local-universe/820918/5 "2018-09-07T00:08:41Z")
> [@etasyde](#): > > Gravitational lensing can be seen in light passing by Sol. And certainly by galaxy sized objects. > > Also, “local universe?” I mean, that usually refers to an unfathomably large region of space containing billions of galaxies, or is used to refer to the entire observable universe (as opposed to, say, a “split” universe in many worlds). Did you mean local star system? By “local universe” I meant the region out to a few dozens of millions of LY away, as opposed to “cosmological distances” which is hundreds of millions to billions of LY away. More or less conformant with the general usage of these terms in the popular press. Also, would a single star system have enough heft to create pronounced lensing curves of far larger background objects? ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [September 7, 2018, 12:48am UTC](https://boards.straightdope.com/t/why-cannot-we-observe-gravitational-lensing-in-the-local-universe/820918/6 "2018-09-07T00:48:27Z")
> [@mandala](#): > > Also, would a single star system have enough heft to create pronounced lensing curves of far larger background objects? See the beginning of what Der Trihs quoted - the “focal length” of the Sun’s gravitational lensing of parallel rays of light from distant objects is 542 AU.