Gravitational lensing
Gravitational lensing
A blog on the universe and the cosmos above and the nature of it determined by great scientists.
Gravitational Lensing is the magnification and increased brightness of various objects and celestial events due to their light being obstructed by a gravitational field created by a massive object(s). This results in the light getting bent before reaching us.
- What is gravity according to General Relativity
- Bending of light and gravity
- How we use gravitational lensing
What is Gravity according to General Relativity :
Unlike the Newtonian model, Gravity is not any kind of force when referred with context to General Relativity. In fact, it is just the curvature that is created in the fabric of space and time by any mass that is distorted into it.
General relativity was put forward by Einstien in the year 1915. At that time, this theory was a brand new one and nobody understood it pretty well. But however, we today know that the predictions given by general relativity are precise and helpful until the predictions are made where the conditions of space and time are close enough to the point of singularity.
General relativity assumes the four dimensions of the space, namely:- 3 dimensions of space and 1 dimension of time together as a single fourth dimension which is called as the spacetime. It further denies Newton's equation famous equation regarding gravity that is Fg = Gm1m2 /r^2
According to Newtonian theory, Gravity is a force associated with massive bodies like planets, moons and stars. However, General relativity came up with a completely different aspect of gravity according to which gravity is simply defined to be the curvature that is generated in the 4-dimensional spacetime by massive objects.
Also, according to general relativity, the concept of geodesic was included in space. Geodysis are the straight paths followed by objects in the curved space-time. Therefore, according to it, the Earth, the Moon and all other planets and satellites orbited the sun in straight paths, however, the paths seem to be elongated due to the curvature that is created in the spacetime fabric because of the massive ass of the sun. If you want a better idea about spacetime fabric and curvature, then you can read this post. which talks in-depth about what spacetime fabric exactly is and the curvature of the universe itself.
General Relativity is a very important theory as it clearly points out through its idea of gravity that any object which gets close enough to the curvature will diverge around it. Also as of date, we know that this principle follows for all objects in the universe except for black holes which due to their extreme curvature cause a gravitational pull so strong that even the fastest entity in the known space (light) cannot escape it. However, this idea follows for all other objects in the universe including light.
In the picture below, space and time dimensions are taken together as a spacetime which is spread across. In this fabric of space and time, the earth creates slight curvature around it which causes the gravitational pull that earth exerts on other objects. Though the spacetime is considered to be a sum of the 3 dimensions of space and 1 of time which makes a 4dimensionsl fabric, it is often represented in a 2d form to make the concept of gravity according to general relativity seem simple.
Bending of Light and gravity :
Gravity, as described by Einstien's General theory of relativity, is simply the curvature which is created in the fabric created by the dimensions of space and time. When light passes close enough to any curvature that is created due to massive bodies, it bends and does not follow its usual straight path. This is called the bending of light.
The light was first understood to be an electromagnetic wave which travelled in space. Later in 1801, after the famous double-slit experiment was carried out by Thomas Young, it was understood that light behaved like a wave as well as like a particle. This is today known as the particle-wave duality and we today know that light behaving like a particle as well as light behaving like a wave is right.
Then after with Einstein's famous equation of E = MC^2 which showed that energy and mass are equal with 'C' being a constant of proportionality. With these few concepts taking together with the idea of gravity as described by General relativity, conclusions were drawn through theoretical as well as experimental basis that light will also bend its way while passing around any object which is massive enough to cause curvature in the space-time fabric and that is exactly what we observe today with the help of powerful astronomical telescopes.
Such a kind of bending in that path of light was first observed by an American Scientist called Robert Oppenheimer. He observed the light coming from a nearby star during a total Solar eclipse as otherwise, the brightness of the sun itself would have blocked the light coming to use from the neighbouring stars. These observations showed that the location of the star by taking into account the light received from it to be completely different from the actual position of the star This discovery was the among the first few ones to have a strong proof for bending of light through space due to the gravity of other massive objects. However, it is difficult to locate the exact position of the star as viewed from earth, the star would be seen in 2 different positions altogether as we perceive light to travel in a straight line across space while it isn't in such cases.
This leads us the imagine 2 different false positions of the stars as depicted in the second picture below. While this is tough to detect, astronomical satellites and observatories situated on earth are equipped with special instruments in order to detect the exact positions of the star.
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This is what is known as Gravitational Lensing. We see the light which comes from a distant star which is in between bent by various objects like clusters of galaxies, stars, planets and more which result into the path of the light getting curved because of the curvature that is created in the fabric of space and time. Gravitational Lensing is what allows us to view galaxies, supernovas and other astronomical events and objects which are situated at vast distances; as their light gets concentrated towards us because of the objects in between. Now there are a few types of gravitational lensing as follows:-
- Strong Gravitational Lensing
- Weak Gravitational Lensing
- Microlensing
Strong Gravitational Lensing :
Strong Gravitational Lensing is caused when the light emitted by an object is obstructed by objects which have a great amount of mass and really strong gravity. Large enough Black Holes are suitable examples for such a kind of strong Gravitational Lensing. In such cases, the gravity exerted by the obstructing object is so strong that it leads to the light being curved around it in a kind of circular shape like a border around it.
When a light source is lensed by the strong gravity of an object, that is when strong gravitational lensing comes into play; then at such times, the light source is divided into what is called as an Einstein Ring. Because of such a division of the light caused by the immense gravity, the light is seen to be spread over the black hole forming a kind of boundary around it. Such a phenomenon is also used to discover black holes.
An Einstein ring is a rare phenomenon where the real source of light, the large body which exerts the gravity to bend light ( mainly black holes ) and the astronomical telescopes and observatories on earth are completely aligned with each other. If you want to find more about it, then you could as well as visit this YouTube link which will help you understand the formation of Einstien Ring.
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The above picture represents a black hole which blocks the majority of light coming from sources behind it. If such large objects are observed with the help of astronomical telescopes, then they can help us find small and slightly bulged out borders along the black hole which tells us that the light is being bent over the black hole.
Also, sometimes, the strong gravitational lensing also causes an object or a celestial phenomenon to be observed in multiple places. For instance, a quasar can be observed at four different spots because of the gravitational lensing. That is, the gravity of the object obstructing the light emitted from the source is so strong that it rips off the light into multiple directions and then re-focuses it towards us and that results into we seeing the same phenomenon in different places. Such kind of phenomenon where multiple pictures of the same object are created due to the strong gravitational lensing of an object is known as an Einstien Cross.
Weak Gravitational Lensing :
Weak Gravitational Lensing is what occurs when the incoming light of an object is obstructed by another object lying in between in such a manner where it causes the light to deflect in weird shapes. In such a case, the exact mass of the object interfering the string path of light is not necessary to be very large to cause weird/elongated shapes of the light emitted by the object beyond.
Such a kind of weak gravitational lensing is observed on a much larger scale as compared to strong gravitational lensing. Such a kind of gravitational lensing tends to magnify the source of light and to increase its apparent brightness.
Microlensing / Micro-Gravitational Lensing :
Microgravitational Lensing, as its name suggests is slight disturbances served in the path of light. Such kinds of Gravitational Micro Lensing is used especially in order to detect exoplanets ( Plantes outside our solar system ). various exoplanets can be detected with the help of instruments which measure the slight disturbances in the brightness of light caused due to planets.
When any exoplanet(s) interferes between the light coming from the distant object and its curved path created by the large star or the host star of the planet, it causes differences in the apparent brightness observed back on earth. This method can be used to detect exoplanets.
How we use gravitational lensing :
Gravitational lensing is used in order to magnify the objects and other celestial phenomena which are located at distances of several light-years from us. This naturally occurring magnification of distant objects in space is detected by various instruments on observatories situated on Earth as well as with the help of various instruments that are fitted inside the artificial satellites orbiting the earth. These satellites provide us with more accurate data and pictures of the cosmos with an advantage of elevation which enables it to take large scale images of the universe.
Various observational telescopes like the Hubble telescope of NASA also use such techniques in order to detect galaxies which are situated at a great distance from us.
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