What is Gravitational lensing?

[Soundariya Raja]

I have just glanced this magazine and got curious about what is gravitational lensing

http://blogs.discovermagazine.com/d-brief/2017/06/07/19964/#.WYibwxWGO00

I am quite less knowledged in Astronomy, so explain me clearly!.

 

[scottdave]

In simplified terms, Einstein stated that gravity would bend space, so even light would follow the curved path near a massive object. For an observer, far away, it appears as if the light waves bend.

 

[Soundariya Raja]

how could it be so sure that the light is bent? because the observer can see it straightly where the light comes from(telescope or something).
Only when its seen laterally or in sidewise view it is possible to assure that light is bent.!

 

[infinitebubble]

Proven in 1919 and still true today of gravity bending light or gravitational lensing.

https://www.space.com/37018-solar-eclipse-proved-einstein-relativity-right.html

 

[scottdave]

how could it be so sure that the light is bent? because the observer can see it straightly where the light comes from(telescope or something).
Only when its seen laterally or in sidewise view it is possible to assure that light is bent.!

I'm not usually fond of going just with Wikipedia, but they have a nice animation of what something passing through the "lens" might look like. https://en.wikipedia.org/wiki/Gravitational_lens

 

[scottdave]

So this is my attempt at sketching out what is happening. I have greatly exaggerated the effect. As the light passes near a massive object, which has distorted the space, the light follows a curved path, near it then continues on. An observer would see images on either side of the front massive object.
https://flic.kr/p/WgNVyn

 

[Drakkith]

how could it be so sure that the light is bent? because the observer can see it straightly where the light comes from(telescope or something).
Only when its seen laterally or in sidewise view it is possible to assure that light is bent.!

We can compare the apparent relative position of stars in the sky as a massive object (such as the Sun) passes near our line of sight to the star. As the massive object comes very close to our LOS, the apparent position of the star changes slightly as the light is bent. The measured change in the apparent position matches predictions by GR.

 

[Soundariya Raja]

OH..WOW!.. THANK YOU SCOTTDAVE, DRAKKITH!.."FOR THE ANIMATIONS"
NOW I COULD UNDERSTAND THE WHOLE THING.

 

[mishima]

I've just had a little experience with it, since I did a little undergraduate research semester on it. But one thing that's not been mentioned, and what excites me the most about it, is that it enables an observer to actually observe behind an occulting galaxy. Just from line of sight considerations, it might be natural to assume that when a galaxy occults another galaxy from view, the more distant object is unobservable.

Not so due to the the effects of gravitational lensing.

The nearer galaxy acts as a lens to send information from the more distant galaxy to the observer. Quite amazing, really.

 

[mark!]

Proven in 1919 and still true today of gravity bending light or gravitational lensing.

https://www.space.com/37018-solar-eclipse-proved-einstein-relativity-right.html

True, but this lensing technique is also used to 'see' dark matter. But does ligt behave/interact the same way when it goes 'through' dark matter, unaffected whatsoever?

 

[rootone]

True, but this lensing technique is also used to 'see' dark matter. But does ligt behave/interact the same way when it goes 'through' dark matter, unaffected whatsoever?

Dark matter does not interact with electromagnetic fields, so light can pass through dark matter as though it's not there at all.
The light is not absorbed or reflected or refracted in any form.
Sufficient quantity of dark matter has a gravitational field though, and that field does interact with light, changing it's apparent direction as seen by a remote observer This lensing is the same as that produced by concentrations of normal matter

 

[pmwelec]

Isn't there some evidence that Eddington's calulations were incorrect?

 

[phinds]

Isn't there some evidence that Eddington's calulations were incorrect?

What difference does it make? Surely, you are not going to dispute General Relativity.

 

[stefan r]

how could it be so sure that the light is bent? because the observer can see it straightly where the light comes from(telescope or something).
Only when its seen laterally or in sidewise view it is possible to assure that light is bent.!

No, we only see light coming at us. (see lens) There is no sideways view of light.

Suppose you take a Sherlock Holmes style magnifying glass and place a sticker in the middle of the glass. You can still see parts of an object that are behind the sticker. Light that would not have hit our eye gets bent and now it does get to our eye (or telescope, camera etc).

A lot of telescopes have a blockage in the middle of the tube. Makes a good image anyway.