SPACE-TIME.| What is Space-Time? Complete Explained.

Time, we cannot see it, cannot hear it but still we feel it every day and every moment. So what is this? Is it related to space in some way? Prior to Einstein, scientists believed that both space and time exist quite independently. Therefore, any change in space does not affect time. According to Newton, time is absolute, that is, it remains the same for everyone everywhere and in every situation throughout the universe. Meaning that if 2 hours have passed for me then only 2 hours must have passed for everyone in this universe. Whether it is on the planet Jupiter, close to a black hole or traveling in a vehicle that has very high speed. But Einstein's thinking was slightly different. He believed that both space and time are connected to each other, due to which both do not exit independent. He also believed that time is relative, that is, it does not remain equal for everyone. Time passes at different speeds under different circumstances. If we look at the relativistic contact, we cannot separate space and time.

Actually the speed of passing time depends on the velocity of an object and its gravitational field. If my gravitational field and velocity is different from your gravitational field and velocity then the speed of passing time will be different for both of us. As the gravity and velocity increase, the speed of time slows down. Let us understand this with some examples, because the size of the planet Jupiter is larger than our Earth. Its gravity is also higher than our earth. Due to this, the time on Jupiter will be slower than on Earth. That is, if a person starts living on the planet Jupiter then he will be alive for more days than humans living on earth because time will be slow for them there.

Similarly, if we are traveling in a vehicle whose speed is very high, then time slows down for us. Let us understand this with an example too. Suppose there are two twin brothers, Ram and Shyam. Ram gets a government job while Shyam becomes an astronaut. Now for a space mission to go away from Earth in the evening, Shyam meets his brother before leaving and then leaves in the vehicle. Because the speed of this vehicle is very high, the time for Shyam sitting inside it will be slower than that of his brother Ram on earth. Now when Shyam returns to Earth after his 5 years, he will see that his brother is old, while his age has increased to only 5 years. We can explain these two examples only in one way and that is that both space and time are interconnected. The concept of time delaytion and length contraction is quite interesting which we will understand in more detail in the other episode. According to Einstein, the speed of light is fixed for every internal frame of reference. As we get closer to the speed of light, time slows down for us. If our speed becomes equal to the speed of light, then time will stop at all for us. To explain the events occurring in space and time, a mathematical model was created which we called space-time continuum. Today in this episode we will learn about space-time. So let's start today's episode.

Before understanding space-time, it is necessary to understand both space and time separately. If we talk about space, then everything around us is only part of space. To represent the position of an object in space, we use a 3d graph, which has X, Y and Z axis.

Those denoting length, width and height or depth. That is, it has become our 3rd dimension. Now if we talk about time, it will be difficult for us to define it. But we know that it always keeps moving forward every moment, we consider it as the fourth dimension. Now if both space and time are combined then four dimensional spacetime will be created. Einstein said that both space and time are linked together as space-time and both do not exist without each other. But is it really true? Let's verify this through this example. Suppose, you become friend with a girl on a social media site that you have never seen before. But after chatting for days, both of you agree to meet each other. To meet her, you have to tell her both the space and time, such as the Taj Hotel on Wednesday at 4:00 pm. If either of these two things you tell is missed then it would be almost impossible for her to find you. For example, if you meet her at Taj Hotel somewhere, she does not have information about which day and when to meet at Taj Hotel. In the same way if you tell her that if you meet her at 4:00 pm on Wednesday, then she will not know where to meet. That is, it is impossible to understand an event without combining space and time. As I already mentioned, space-time is dimensional. Because it is not possible for us to visualize that dimension, we represent space-time only through 2 dimensional graphs.

Where the x-axis represents the space while the y-axis represents the time. Because space and time are interconnected, any object moving in space is moving in time also. Before proceeding further, let us have a look at the basics of the space time graph. Suppose a train is going from Delhi to Mumbai. If the movement of this is represented as a space only, then the graph will become something like this.

Now if its movement is represented only as time, then the graph will become something like this.

But both of them are not giving complete information. So when we create a space time graph, it will become something like this.

Physicists do not see this world the way we ordinary humans see it. For us, the world is like a movie that goes on continuously, but the physicists see it frame by frame. Let us know how, when we throw stones in a pond, we see ripple in the water.

This is a common event that we and you must have seen at some point in your lifetime. But if you see this event frame by frame, it will look something like this.

When we threw stones, nothing happened instantly. After 1 second a circle was formed, after two seconds two circles were formed and after 3 seconds three circles were formed. Now if you look at it carefully, it looks like an cone.

In this graph, we have shown only a change of 3 seconds. But if you want to see the change in its future, then only draw this cone further. This is basically the locus of space-time through the light. If we reverse the direction of time in this, then this cone will be reversed and we will get the past tense of this event.

We call this cone the light cone. Einstein had said, "For us physicist, the separation between past, present and future is only a illustration, although a convincing one", that for physicists, the past, future and present are always there. The passing of time is only our illusion. Now you must be thinking that if the past, future and present are always present then why do we see this world always moving. The answer is based on the principal of the video. When we watch a video, we see it moving. But what really happens is that, it is a combination of many images which are presented to us very quickly. Now take this flipbook only.

There are many pictures in it, in which a picture of some footballer remains. If we look at this page separately, there are only paused pictures in it. But if we bring these in front of you quickly, it will give you a video-like feel.

You will feel as if you are watching a video of a football match. Even at this time, the pages exist in exactly the same position in their place. But now your viewpoint has changed. This world is exactly the same for physicists. I hope by now you have understood space-time. Now the question arises that what is the benefit of making a space-time graph or diagram. Actually its advantage is that by making a small event of a planet, we can also tell about its future and also about its past. The best example of this is the Big Bang. By making a space-time diagram of the nature of today's universe, we can easily tell how it would have been in the past, then how it will be in the future.

In the same way, based on the space-time diagram of the present of any planet, we can easily tell where that planet would have been 10 years ago or where it will be after 10 years from today. So isn't this space-time diagram interesting. Due to the concept of space-time, we have been able to understand the force of gravity better today. You may have heard that time slows down for those sitting in a vehicle close to a black hole. But have you ever thought about how this happens? Actually it can be easily understood from the concept of spacetime. If you know the answer to this question, then tell me through the comment and if you want me to expand it, then also tell me through the comment.

Friends, that's all for today's episode. Hope you liked the episode. Do not forget to share it. If you are new on this website then do subscribe / bookmark it. So friends meet with another topic of science in the next episode. Till then Namaste.