Today we are going to talk about Space-Time whether its real or an illusion. So let’s get started.
So the question arises while understanding SpaceTime is that what is time? Is it related with space?
Before Einstein’s era, scientists believed that space and time are independent of each other. Neither of them is related in any manner. So any change in time doesnot affect space and also any change in space doesnot change time.
According to Newton, Time is absolute irrespective of any point in space. Which means if i spend 1hour on earth that means everywhere in the universe 1hour has passed whether anyone is on Mars, Jupiter or travelling with the speed of light or may be near a black hole.
Now lets understand Space and time separately, Space is everything we see around us. Each and every object is a part of space. To denote the position of an object in space, we use the 3-dimentional co-ordinate system.
Now time is something every one knows that it keeps moving forward. It is also called time dimension or the fourth dimension. And mixing it with 3-dimentional space, it creates a 4-dimentional space-time.
But despite of different characteristics, Einstein believed that Space and time are relative i.e they are related to each other. He said that time is relative and not absolute, which means that time passes with different speed depending on the situation and condition. The speed of time for an object depends on the velocity of the object and also on the gravitational field of the object. The speed of time slows down with the increase of the velocity and gravity for the object.
Lets understand this with two examples.
Let us take two planets our earth and Jupiter. As Jupiter is much larger than The Earth and also have a larger mass than earth, so the gravity on Jupiter will be more than that of our earth. So the speed of time on Jupiter will be slower than on earth. Hence, if a person travels to Jupiter and lives there then he will live longer than those people on earth as time will slow down for him.
Again time also depends on speed of the object, so let’s take two twin brothers. Now if one brother stays on earth and the other goes in a space mission travelling with high speed with almost the speed of light. Then what happens, the time for the brother travelling with the speed of light gets slower. Now when the brother returns back to earth after a couple of years, he will see that his brother has grown old while his age has just been increased by 2 years.
This phenomenon states that time and space are related with each other. And here the concept of Time dilation and length contraction comes into play. We will understand this topic in other videos.
Now according to Einstein, for every inertial frame of reference speed of light is constant. As we reach nearer to the speed of light, time slows down. If our speed is as fast as that of light then for us time would seem to stop and also space would seem to adjust itself to time. To explain this phenomenon a mathematical model was made which is called Space-Time Contenum.
Physics doesnot see the world as we see. For us every event in the universe is like a film which keeps moving forward but physics sees this film in frame by frame reference.
Let’s understand this with the help of an example.
If we throw a pebble in a pond then it creates ripples on water, for us the event goes simply as a stone is thrown in a pond and because of which ripples are created. But according to physics, it looks somewhat like this graph where initially no change occurred then after 1sec, 1 ripple has been created, after 2sec 2 ripple has been created and so on. So this graph makes a cone and now if we want to know the future event of ripples then we would just have to extend the cone.
This cone is the locus of light through space-time. Now if we reverse this cone then we get the past events of the cone. This cone is called Future light cone.
This cone helps us to understand our universe. A great example of this is the Big Bang theory. As we can understand how the past of any planet or event was and also we can know what the future of the planet or event will be by just calculating its position in the spacetime graph. And due to this Space-Time concept we can understand gravitation clearly.
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