How we determine what lies light years away 

A blog on the universe and the cosmos above and the nature of it determined by great scientists.

 

The simple answer as to how we can see light years away from us is using reflection of light and radio telescopes. The light emitted by various stars or the one released at the time of big bang gets reflected when it crashes into any object. When this reflected light reaches earth, its radiations are detected by Radio telescopes and an estimate of the object's distance from earth is then calculated.

credit : Wikimedia commons

                                  

 

 

 

•  The concept of light year
• Reflection and Radio telescopes
• Science behind the Recent discoveries

 

 The concept of Light year : 

 

The space is unimaginably vast. On earth, we use units like meters and Kilometers to measure distances. But on a vast scale like space, considering the same units is not much helpful. Units like kilometers and meters are  sufficient to measure almost any distance on earth but in space, if we consider the same units then the measures we calculate will be unimaginably big and space related calculations will become very difficult.

                    For measuring large distances of space, an equally large unit of measurement must be invented. Thus in 1838, a German scientist Friedrich Bessel came up with an unit called as light years. Light years is the distance traveled by the light of sun in one year. The sun continuously emits light which is dispersed in all directions. 

 

 

 

                   The light emitted by the sun  spreads everywhere and the one which falls on various celestial  objects is reflected back. This is the same process because of which the moon seems to be illuminated even though it does not have its own light. However, the light which does not collide with any other celestial object continuous to move forward forever until it is absorbed or reflected. 

                    While all this process is taking place, the speed of light always remains constant, that is 299, 792, 458 m/s   .This way, the distance a light ray can travel in a time span of one year is called as one light year in astronomical terms. One Light year is approximately  9.461e+15 meters. When the light travels great distances and somehow crashes to a object and is reflected ( not if the object is a black hole ), sometimes this reflected light reaches earth and we are able to study it.

Reflection and Radio telescopes : 

1.  Construction and working of radio telescopes
2.  Problem of information of Radio telescopes 

 

 

 

Construction and working of radio telescopes :

 

                    As mentioned above, sometimes this light gets reflected by astronomical objects and reaches back to earth. On Earth, we have built huge Radio telescopes which help us identify these radiations coming from the outer space. By studying the radiation further, and by knowing how far it is from, scientists can give a very good estimate as to how far the astronomical object to which the light collided and got reflected is from earth.

                   The way these Radio telescopes function is very interesting and confusing as well. So I am not covering their functioning in depth in this post. If you want a post on Radio Telescopes covering very minute details as well then you can comment in the comment's section below saying so. I will consider it and write a post on the topic.

                   Explaining in very simple language, the Radio telescopes are built of the tree main components namely dish, receiver, recorder and the cabin section. 

• Dish : Dish is the huge disc shaped part that we can see on the top of the ground. This huge structure can be controlled and can be rotated in all directions that helps it to collect more of the radiations of the light rays reflected towards earth. The dish has an antenna like part at its center. This antennae is the main part which collects the radio waves, while the dish helps the structure of the antenna to remain intact by giving enough support to it. Also the Dish is designed in such a way that any radio wave falling on the dish is also reflected towards the antenna.
  

 

• Receiver : Receiver is the part where the received radio wave are further sent to. One of the main functions of this system is to convert very weak signals, that is signals with very very low frequencies into waves with wavelengths strong enough to study them.

 

Credit : Wikimedia commons

 

 

• Recorder  and Cabin : As the name suggests, this place is where the intensity of the radio waves is measured. While, the cabin is the part with highly specialized machinery helpful to study these waves. The recorders are also stored in the Cabin section.

In this manner, the scientist are able to determine by the distance the radio wave associating with the reflected light ray has come from and this is the very distance as to how far the celestial object is from earth. However, with this technology being improving since its discover, it can only help us to tell how far the object is and not its exact position.

                   

 Problem of information of Radio telescopes :

 

                     The reason why the radio telescopes are only  able to tell us the distance the celestial object is from earth but not its exact location in space is very logical and convincing. Let us suppose that the the celestial object detected is 2 light years away. So for the light ray to reach the object, get reflected and to return to earth it takes approximately 4 light years ( as it has to first travel 2 light years to reach the object and then again 2 light years to come back to earth ).

                    So for the light ray to hit the object, get reflected and return to earth , it takes 4 light year. And though light is the fastest thing known to humans, it is not so fast that it can cover the distance of 4 light years in a few seconds. In fact as discussed above, the distance light can travel in 1 year with its constant speed is called a light year, so definitely even light cannot cover 4 light years in very less time.

                    Though we don't consider the time light takes to return to earth, we still know that the time required will be large. And in this large amount of time, the astronomical object it had hit will not remain in its same position when the light had hit it. Either the object is in motion or because of the space continually expanding, the distance between the object and the earth is spacing out and thus we cannot know the exact position of the object. I have written a separate post on the topic of the dimensions of space and the expanding cosmic web of the universe where I have explained these concepts in easy to understand yet in dept. SO make a note to read this article.

 

 

 

 

Science behind the Recent discoveries :

 

                    Scientists recently discovered that the visible universe is around as big as 92 billion light years in diameter. But with this, there has been a very common question in everyone's head, that is that the scientists also say that the universe was formed about 13.7 billion years ago. So if the universe was formed 13.7 billion years ago, then how can it be 92 billion light years in diameter ?

                     Well, I am going to write a separate and in detail post on this topic but for giving a brief information, this is possible and not any paradox as people think it is. When the Big Bang happened, there was tremendous amounts of light and energy released. If we reposition our today's earth on the same position but in the early stages after big bang, then we can see that at that time, for us the observable universe was very small and close to none.

                     The reason is that the light that had been emitted during the big bang had not reached us till then. An approximation from 'Fermilab' tells us that the observable universe then was about 13 million light years. But however, the light to come back to us took about 14 billion years. That is because the space is continually expanding and there is more space created between any two objects on the universe's scale at every moment.

 

                   Today we can see up to 92 billion years in diameter, or to be precise, the observable universe today is about 92 billion light years in diameter and this is because the light emitted by the big bang 13 billion years ago has finally reached earth. But the space is expanding due to which it takes more time for the light to hit the object and get reflected and reach back to earth. And thus we can see the universe at about 92 billion light years ( 46 Billion light years radius ) in diameter even though the universe is 13.7 Billion years old.

 

 

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