WHAT IS GRAVITATIONAL LENSING ?

BY: Arjit Mishra

Infinitely dispersed this Universe has number of mysteries, to remove the counts from these mysteries we created a powerful Telescope renowned as Hubble Telescope, which not only just enable us to observe distant and complicated Astronomical events but also we can peep into Past and observe the Past time of our Universe.

It is not completely true to say that only we(Humans) have to make efforts and build powerful machines for solving the mysteries of this Universe, sometimes nature also help us to do so. Gravitational Lensing is one amongst the natural phenomenons which stands for the above given fact.

The concept of Gravitational Lensing completely depend upon Properties of Space. It help us to see those Galaxies, Quasars and Stars which are billion of Light years away from us but we can't observe them directly. It is Gravitational Lensing which reveal the concept of Dark Energy.
This Blog will reveal the basic understanding behind Gravitational Lensing along with How Dark Matter was discovered with the help of Gravitational Lensing.

"what is Gravitational Lensing? and how does it work?" before answering these questions I would like to tell you the concept of lenses in normal Life:

Atually Lenses have the tendency to bend the path of Light.In case we have a Convex lens on which a beam of Light is focused it will converge the beam of light at a particular point.but if we have a Concave lens it will diverge the beam of light.

However Gravitational Lensing works similar to a Magnifying Glass. Let us understand how a simple Magnifying glass works:

We could see anything around us only if it's Light is reaching our EYE.When we see an object with our eyes it appears of same size as it is actually but if we see it through a magnifying glass it appears bigger in size.It is because beam of Light coming from the object got converged by the convex lens present in the magnifying glass and if we extend this converged path of light in it's backward direction we would be able to see that object apparently Bigger than it's original size; this is because our human brain can only perceive Light as a straight line.

Now we should move on to Gravitational Lensing Phenomenon:

In 1915 Albert Einstein proposed his general theory of Relativity which truly define gravitational force. He proposed that every mass warps the space-time continuum, and this warping phenomenon shows direct proportionality to the mass of an object. which means higher mass object would distort space-time more as compare to smaller one and because of this smaller mass object feel attraction towards the curve formed by higher mass object bcoz of the shape of space-time at that position, and this attraction is so called Gravitational Force.

We know that Light always travel in a straight in vacuum, but if Einstein's general theory of Relativity was true then it is obvious that higher mass objects like: Stars, galaxies, clusters, etc. would also warp the space-time according to their masses.which means when light passes near to these objects,it also changes it's original path because of shape of space-time at that position indicating to a fact that Gravity will act as a Lens similar to a Magnifying Glass, if this theory is true then we should get a natural Magnifying Glass witch would magnify the far present Galaxies, clusters, etc. along with we would be able to observe those stars, quasars, etc. which are hidden behind another big galaxies.but these are only theoretical considerations therefore there was need for experimental verification.

It was the year of 1919, when Eddington decided to prove this theory. He observed sun in Total Solar Eclipse through Telescope at that time a star was present behind the sun which we can't observe as sun was present just before it resisting it's vision. But if Einstein was right and sun warps space-time due to it's mass then due to Gravitational Lensing phenomenon we can observe that star, but you won't believe that Predictions of EINSTEIN'S theory were true and Eddington observed that star.
Originally light from that star shouldn't get to Earth by any means but as sun has bend the space-time by some extent so when light from that star passes through the sun it also gets bended by some extent and due to this virtual position of star changes and we can observe the star (hereby we can't see the original position of star but its apparent position).

TYPES OF GRAVITATIONAL LENSING:

Strong Lensing: This occurs when Gravitational lens is powerful or more specifically the light source behind it is situated very close it. For example- if a galaxy is present behind the galaxy cluster which is situated very close to galaxy cluster then the Galaxy cluster present in front of us act as a  strong lens and the galaxy present behind it appears bigger in size as compare to it's original size as similar to magnifying glass.  As I have already said we can observe an object only if it's light is reaching us.  The light of hidden galaxy takes different paths to reach us due to strong lensing, therefore we can observe multiple images of same galaxy and if position of galaxy somehow changes the positions of multiple images though changes because these multiple images are of same galaxy but it can happen that all images don't change their positions at a same TIME because Light has taken different paths to reach us more specifically the image formed by short path taken by light changes first as compare to images formed by other paths.

Strong Lensing reveals one more wonderful phenomenon called Einstein Ring, when source of Light, observer and lens are present in a same line then light coming from source is uniformly bended by the lens which results a formation of bright light ring which we call Einstein Ring.

Weak Gravitational Lensing: We observe weak gravitational lensing when gravitational lens is not much powerful to propose multiple images of source or Einstein Ring. In this case source appears to be extra streched or magnified. If we know the size and shape of source then we can easily discover the properties of gravitational lens by calculating how much the light bend from it's original path.
Generally we don't know about shape and size of source but we know about their average properties which can be used. for example if we have images of 1000 galaxies and all are of spiral shape then we can say that far present this galaxy is of spiral shape. by using weak Gravitational lensing mainly we discover the properties of those galaxies and clusters which are working as weak gravitational lens.

Micro Lensing: This is totally different from strong and weak gravitational lensing because in this case gravitational lens is much smaller. In strong and weak gravitational lensing we observed that gravitational lens are either galaxy clusters or galaxy. but in micro lensing the work of gravitational lens is done by stars or exoplanets, by this type of lensing we can discover those far away planets or stars whose emitted light turns dim while reaching to us but with help of micro lensing this dim light turns out to be shining for some time let us know how?

As we know that gravity bends the light and lead to the formation of gravitational lens. micro lensing occurs when gravity of a star or planet focuses the light coming from the far away planet or star such that those far away star or planets appears to be shining for some time.

Let us now understand how Dark Matter was Discovered with the help of Gravitational Lensing:

NOTE: About 23% of our Universe is composed of Dark Matter, It is not alike our ordinary Matter and we can't see it but it exists.(I will make a separate blog about Dark Matter and Dark Energy )

We know that Gravitational Lensing takes pace when a mass comes between us and a Light source. for example a Galaxy cluster; it acts as a lens which results distorted magnified or multiple images of that Light source. Imagine the light of hidden galaxy needs to pass through galaxy cluster before reaching us which works as a gravitational lens. The amount of Light we have received can be used to calculate the mass of that galaxy cluster (whiz acting as lens) with the help of computer simulation.
As we know that how Gravity works,we prepare the same gravitational lensing effect on computer and enter the data of received Light, Now we enter the observed mass of galaxy cluster(causing gravitational lensing) and see if we get the amount of light on computer same as we get in real life but surprisingly we obtain that original mass of Galaxy Cluster is not sufficient to bend the Light to an extent same as an extent Light has reached us. In fact the original mass of galaxy cluster is only 10% of that mass which actually done the work of Gravitational lens. It means in computer simulation to make the same gravitational lensing effect we require the mass of 9 other galaxy clusters, this missing mass can be of faint baryonic objects like- Black Holes, Interstellar objects, dust and planets.but after observing many galaxies closely we discover that the masses of these objects too are not same as missing masses. which means it is true that In this Universe something is there which we can't see but it has it's mass and is responsible for Gravitational Lensing but question is what is this new entity and where it exists. After further studies we concluded that it is present everywhere in this Universe and as we can't see it we call it as Dark Matter, it do not interact with Light and therefore is not visible.

  "WE HAVE QUESTIONS BUT DARKNESS HAS ANSWERS TO THEM"

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WHAT IS SPACE-TIME ?

BY: Arjit Mishra

 Time we can't see it, can't hear it, but we can feel it or more specifically we can observe it.
                                        

 What is it ? is it somehow related to Space ?

Before Einstein Scientists used to believe that space and time both exist independently; Any change in Space doesn't effect Time. According to Newton "Time is Absolute" means in every place and every condition it remains same.
But Einstein has it's own definition of Time, He believed that Space and Time are connected to each other and do not exist independently. He defines Time to be Relative instead of Absolute, means it is never same for everyone, it passes differently for different conditions. In the relativistic context we can't separate Space and Time.

Moreover speed of propagation of time depends upon object's velocity and it's Gravitational Field.
If my Gravitational field and velocity is different from your's then Time will not be same for me and you.
An Increase in velocity and Gravity will decrease the speed of time propagation.
                             
                                     Let us understand this with the help of an Example :

As Saturn is bigger than Earth then it's Gravity is also bigger than Earth and because of this Time propagation speed will be less on Saturn as compare to that of Earth.
If a Human/Animal somehow able to live on Saturn then he/she/it would live longer than a person/Animal of same age on earth because Time would pass differently for both of them.

Similarly If we are travelling in a spaceship whose velocity is very high then time propagation speed would be slow as compare to outer world. This clearly indicate that Space and time are interconnected. The concept of Time dilation  and Length contraction is very amazing but I would talk about them on my later blogs.

According to Einstein speed of Light is fixed for any inertial frame. As we tend towards the speed of Light, Time propagation speed tends to Zero. Specifically at the speed of Light Time completely stops. To explain the events happening in space and time a Mathematical model is there which we call "Space Time Continuum".

            Before understanding Space-Time we have to understand Space and Time separately :

Everything present around us is a part of Space. To define the position of object in space we take the help of 3-D Graph which has X, Y and Z axis which depict the length, width and height/depth of an object and makes up the 3-Dimensional object.

While in the case of Time it would be difficult for us to define it, but we know that it always moves in one direction that's why we can say that it is our 4th Dimension.

If we combine the space and time then we have presented with 4-Dimensional Space-Time.

as space-time is 4-Dimensional and unfortunately we can't visualize 4-D , but we can visualize it through a 2-Dimensional Graph :

Here X axis defines the space and Y axis defines Time, since Space and Time are connected to each other Anything that moves in Space It also moves in Time.

Physicists observe this world frame by frame Let us know how ?

When we throw a stone into pond we can clearly observe the formation of ripples in it, This is a very common Event that we have observed at least once in a lifetime.

Now let us observe it frame by frame :

When we throw a stone instantly nothing happens but after 1 second one ripple is formed, after 2 seconds two ripples are formed and after 3 seconds formation of three ripples can be observed.

When we see it with full concentration a cone can be observed, Basically this cone is the Locus of light through Space-Time, If we reverse the direction of time then we could obtain the opposite cone which describes the events in past. This cone is specifically called as Light Cone. 

EINSTEIN SAID :

"For us Physicists, the separation between past, present and future is only an illusion, although a convincing one"

Which means past, present and future always exist, propagation of Time is just an Illusion.

A question must be arising in your mind that "why the world seems to moving ?" 

The answer this question is very philosophical, answer totally depends upon principle of video. when we play a video it seems to be moving but actually it is the combination of sequential images slide showed in such a way that the time interval between any two sequential images is very less. but actually there are number of images/Frames at a same time.

Now what is the Advantage of Space-Time Graph ?

Actually with the help of space-time graph we could determine the position and conditions of various celestial bodies and even Universe itself by drawing Bigger and Smaller graphs appropriately.

"Our virtues and our failures are inseparable, like force and matter. When they separate, man is no more."

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WHAT WAS BEFORE BIG-BANG ?

BY: Arjit Mishra

In 1929 Edwin Hubble was the first person who observed that our Universe is expanding constantly till then our Scientific data and Astronomical techniques have improved a lot.

Today without hesitating we can say that our Universe was born 13.8 billion years ago.

After detecting cosmic microwave background we can predict the conditions of Universe after 0.3 million years of Big-Bang.

We have created conditions like Big-Bang on the microscopic level in Large Hadron Collider to understand the Universe from the beginning.

Today our Universe is expanding, this statement directly indicate to the fact that there must be a time previously where all this is constricted to a point(which later expanded due to explosion).

If we go back into time we would reach a point whose temperature is very high, and this point is specifically known as point of SINGULARITY.

So, Singularity got exploded which created the Universe and this explosion is known as Big-Bang.

But a question must be arising in your mind that What was before Big-Bang ?

Some Scientists believe that Big-Bang was the first step of Construction which not only created this Universe but also time and other Physical laws which indicate there was no Universe, no time and no Physical laws. thus the question is wrong in itself because when there was no time before Big-Bang then discussion of this topic is hopeless.

This theory clearly says that there was nothing before Big-Bang. but this theory was not able to satisfy us because according to 2nd law of Thermo Dynamics "Something can't come from nothing"

which says there must be something before Big-Bang.

While another theory says that "Universe always existed" if it is true then what was happening till now and what will be happening was done many times before. If we believe this theory then our Universe should be in a state where all the Physical processes are completed but observations deny this fact and so the theory.

We know that density of Black Hole is very high and Big-Bang was happened from Singularity which was also very dense, Some scientists believe that this point of Singularity must be formed by conversion of Past Universe into Black Hole.

Our Universe is expanding constantly, but some people believe that some time it would expand to a great extent that an explosion would occur which would cause the formation of a new Universe and by this process only our Universe had formed. 

"Construction and Destruction is eternal" but this theory too has a problem that actually how

does the first Universe came into existence, means we have reached the same place where we have started.

There is another theory "Multiverse Bubble Theory" :

According to this theory there are countless Universes which are constricted in a bubble, either they collide with other ones or get parted into number of pieces. these collisions or parting cause immense explosions which forms another Universe, Hopefully Big-Bang also is an outcome of such aspects.

If this theory is right then there must be the formation of circular pattern in the cosmic microwave background due to collisions or parting. But till now we didn't detect such a thing.

Regarding Multiverse there is another theory which came from String Theory :

It says that at basic quantum level particles looking like strings or membranes exist which always vibrate. their different vibrational patterns give birth to different particles like: electrons, protons, neutrons, Gravitons etc.

According to String theory there are number of dimensions in which these strings operates; Due to different behaviour of such strings different Universes came into existence which interact each other with the help Gravity.

Dark energy which is still a mystery to us is an energy which describe us the Gravitational influence of another universe on our one. When two Universes collide Big-Bang occurs which form another Universe.

This theory has answer of our every question but irony is that most of the scientists consider it as a Philosophical theory because it talk about Dimensions and other sci-fi concepts whose existence cannot be defined from physics laws.

We cannot perfectly tell about what was before Big-Bang. But we have progressed a lot to answer this question. Today we are able to see the conditions of {10 to the power -43} seconds old Universe which is the time before 13.8 billion years.

The question that what was before Big-Bang is very complex. to get the answer we could only depend upon observational analysis and mathematical theories. but a problem persists that mathematical laws which work in general world loose their importance in Quantum World.

Physics laws do not work in bodies like Black Holes and so they do not exist before Big-Bang then if somehow we are able to define a condition of working for physics laws then we would be too close to our answer.

"EVERYTHING IS THEORETICALLY IMPOSSIBLE, UNTIL IT'S DONE"       

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DOUBLE SLIT EXPERIMENT ?

BY : Arjit Mishra

The World in which we are living has it's Mysteries beyond our expectations.

Do you know, your observation effects the various possibilities of this World.

You must be thinking I'm a sci-fi maniac but I will prove you wrong by Introducing Double Slit Experiment.

NOTE : Don't confuse your mind with Young's Double Slit Experiment given in class 12th Textbook.

That Experiment was done by the use of Light but I'd be Introducing a model which used matter like Electrons.

Before entering to the Quantum World I want you to be Introduced with "How Particles and Waves actually work ?"

PARTICLE : Marbles are of  particle nature, When we fire them through a narrow slit they form single straight line on the screen(placed after the slit). Similarly if we use two slits, Patterns of two straight lines would be formed.

WAVE : When we allow waves to pass through a single slit then where highest intensity waves strikes the screen a straight line is formed. But when we use two slits instead of one then something unexpected happens; When Tops of one wave meets with Bottoms of another, they cancel out each other and forms interference patterns on the screen. Let us understand this more briefly :

When tops / bottoms of two waves meets, the intensity of wave reach to it's maximum value that's why a bright line is formed on the screen and where top and bottom of two waves meet, they just simply cancel out each other which leads to formation of 'nothing' on the screen.

Now Let us enter the Quantum World

An Electron is a basic form of matter which can exist independently. When we place an Electron Gun in front of a single slit, and allow the electrons to pass through the single slit then same single line pattern can be seen as we have seen it in the case of marbles.

But the story changes when we fire electrons through multiple slits. Generally two single line patterns are expected to be formed on the screen but they represent the interference pattern(whiz shown by waves when we use them in double slit experiment). Firstly some of the physicists thought this would be occuring due to collision of electrons. To check this possibility they constructed an electron gun which fire a single electron at a time but again they were shocked by the outcome of Interference pattern. This was possible only if a single electron just  before entering a slit transform into waves and pass through both the slits and after passing again confirm their particle nature. Mathematically this was impossible.

To carefully observe and understand this phenomenon physicists set an observer machine which can detect the change in electron to a very small extent. When experiment was set up again the result turns the moral of scientists down, this time electrons form two single line patterns on the screen(whiz resemblance of particle nature).

when observer machine was removed electrons show Interference pattern and when it is placed near the electron gun they show particle nature. This behaviour of electrons directly indicate that they know the presence and absence of observer and change their behaviour accordingly.

CONCLUSION :

This poperty of electron is still a mystery for us.

even scientist tried every possibility to make no technical mistakes, for this cause they specially constructed an observer machine which do not emit any type of electro-magnetic signal but again the result was same as mentioned above.

Our Universe is made of electrons which indicate towards a simple theory that this world is acting differently when we are not watching it and act differently when we are watching it.

"WHAT WE KNOW IS A DROP, WHAT WE DON'T IS AN OCEAN."   

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WHAT IS 4th DIMENSION ? | WHAT IS TESSERACT ?

BY: Arjit Mishra

DIMENSION: A measurable extent of particular kind such as length, breadth, height. 

4th Dimension is basically a mathematical extension of the concept of 3rd Dimension.

FACT : The idea of adding a fourth Dimension began with Jean le rond d'alembert with his "Dimensions" Published in 1754.

Firstly we should go through 0th, 1st, 2nd and 3rd dimension :

0th dimension : In 0th dimensional  world there is no measurable extent, more specifically in this dimensional world object sticks to it's position like a point.

1st dimension : In 1st dimensional world there is existence of only one measurable extent, object would move only in a line.

2nd dimension : In 2nd dimensional world there are two measurable extents, length and breadth.

3rd dimension : In 3rd dimensional world  there are three measurable extents; length,breadth and height.

NOTE : From above explanation an observation can be made that "Every dimension is made with the help of it's preceding dimension."

4th DIMENSION :

Now 4th dimension must have be made with the help of it's preceding one whiz, 3-d .

This is the image of 4th Dimension from our 3-D perspective

Scientists on the basis of theoretical consideration made Time as the 4th Dimension.

The reason behind this is all the living things lives with time and become older with time.

hence time can be considered as a major element which effect every entity in the space.

This relation between Time and Space results into 4-Dimensional space-time.

NOTE : Space and Time coexist or they don't exist independently.

This is how you can Imagine a basic or Ground structure of a 4-dimensional object :

Above figure shows the basic structure of 4-d object(commonly known as tesseract). As shown it's structure changes every single moment.

Human life in 4-D :

Through this image we can conclude that with the help of 4th dimension we can physically interact with our every phase of life. 

But unfortunately humans are not made to observe and interact with 4th Dimension.

We Humans are 3 dimensional entities or more specifically in philosophical terms we are slave of time; we live in present and cannot go physical with our past and future.

Now a question must be arising in your mind that can we do time travel with the help of Dimensional Analysis?

The answer of this question is yes.According to the super string theory there are at least 10 Dimensions in the observable Universe and if somehow we can trigger 4th Dimension into 5th Dimension we would be able to time travel and this concept defies most of the time paradoxes.

"Time is our God and we don't exist without it"

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WHAT ARE GRAVITATIONAL WAVES? | LIGO AND IT'S WORKING?

In this mystical universe there are some entities which we can observe and understand, but beyond this fact some entities are also there which we could not observe and understand but yet they exist and can be predicted through mathematical equations.

From this undefined amount of treasury I present you the concept of Gravitational Waves :

In 1916 Albert Einstein first predicted the gravitational waves on the basis of his General Relativity.

On 14 september 2015 LIGO detected the Presence of gravitational waves which were radiated by merging of two black holes situated Billions of Light years away from Earth.

This discovery has created revolution in the history of science or more specifically provided another Pillar to Modern physics.

In 1915 Einstein published his general relativity in which he talked about 4-dimensional space-time concept which completely changed the meaning of Gravity. According to him every type of mass Distorts the space-time fabric.

If an object has huge mass It will distort the space-time fabric more and small mass object cause less distortion and this results to attraction of low mass objects towards huge mass objects; this attraction is what we call GRAVITY.

If that huge mass object start moving from its place, It produces ripples(disturbance) in space-time and this disturbance propagate around the UNIVERSE, which we call Gravitational Waves.

Above Example was just introduction. Let us know how Gravitational Waves are actually formed

When Two Neutron stars or black holes are nearby; they firstly revolve around each other and then merges to form single entity.

From the very moment they start revolving around each other repels also start forming in space-time and when they merges to form single entity, a huge amount of energy is released.

NOTE : Gravitational waves are created by two ways -

1. Merging of two massive objects.
2. Motion of a massive object.

These Gravitational waves travel around UNIVERSE at the speed if LIGHT.

Let us know how Gravitational waves were detected :

In 1974 two Astronomers discovered an Binary Pulsar  in which extremely dense and heavy stars were revolving around each other and this was the perfect system to test the theory of Gravitational waves.

After observing till 40 years It was found that both the stars were really attracting each other. Thus Einstein was correct and it means they have also created Gravitational waves bit till 2015 we were not able to detect them.

On the day of 14 september 2015, LIGO detected the distortion in space time field which was caused by merging of two Black holes situated BILLIONS of light years away from Earth(as mentioned earlier). After then we have detected them TWICE.

FACT : As Gravitational waves are ripples produced in space-time. If any source of  Gravitational waves is present near Earth we would have died years ago.

But as they emerge from very very distant source, before reaching earth their effect is very small that it effect earth as same as it's effect on 1000th part of nucleus present in an ATOM. That's why we can't feel this Effect.

WHAT IS LIGO AND HOW DOES IT WORKS  ?

LIGO : It is the Biggest Gravitational waves observatory in the world. In Technical terms it work as same as INTERFEROMETERS.

WORKING : From the central building a laser beam is fired which at base splits into two long perpendicular tubes and these tubes have same length and at the end of them a mirror is placed. when laser beam meets at base after reflecting from the mirrors. The light waves are aligned in such a manner that they cancel out each other which causes no detection at the Photo Detector(placed in central Building). Normally LIGO don't detect anything but when Gravitational waves arrives the earth, It distorts the space and because of that Length of both perpendicular tubes alters for some time.

Sometimes 1st tube gets expanded and 2nd one gets contracted and vice-versa, This happens till Gravitational waves completely passes the earth.

Instruments of LIGO are so powerful and accurate that if 1000th part of proton get effected they could detect it. Because of this the alignments of light waves are not same as before and they can't completely gets cancel out and thus same light reaches to photo detectors and the intensity of  light depends upon difference caused in the length of tubes.Measurement of this intensity provides measurement of Gravitational Waves.

NOTE : LIGO can only detect the Gravitational waves of massive events like merging of black holes or big bang itself.

Gravitational waves not only help us to understand gravity but also to understand the behaviour of unknown entities like WHITE HOLES, WORM HOLES, etc.                    

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