### SPACE-TIME: summary of 'A brief history of Time"

## SPACE AND TIME (after Einstein)

## RELATIVITY :

**Albert Einstein**refused the idea of

**absolute time**. He proposed according with

**Maxwell’s theory**, all observers should measure the same speed of light, no matter how fast they are moving.

He gave the great relativity
equation,

**E = mc^2**

**where, E= energy**

m= mass of an object

c= speed of
light in vacuum

It means,

**one can convert mass into energy**. It was also proposed that nothing can travel faster than light in vacuum.
·

**As an object approaches the speed of light, its mass rises more quickly**, the mass will be infinite.
·
Only light or more waves that have no intrinsic mass
can move at the speed of light.

In other words, the theory of relativity
put an end to the idea of

**absolute time!**
·
Each observer could use radar to say where and
when an event took place by sanding out pulse of light or radio waves. Part of the
pulse is reflected back at the event and the observer measures the time at
which he receives the echo.

**The distance from the event = half the time taken for round trip × speed of light.**

So it is clear that, time is not completely
separate from and independent of space, but is combined with it to form an
object called

**space-time**. The theory of**relativity**does, however, force us to change fundamentally our ideas of space and time.
·
So one may describe the whole universe in terms
of a collection of overlapping patches. In each patch, one can use a different
set of three coordinates to specify the position of a point.

·

**An event is something that happens at a particular point in space and at a particular time. So one can specify it by***What is an event?***four numbers of coordinates.**Three coordinates for its position (**space coordinates**) and one coordinate for time (**time coordinate**).**So time is the 4**

^{th}dimension of universe.
·
It means to explain

**Universe**, three space dimensions are not enough, there should be one more dimension of time to specify when an event happened in the universe.
·
In

**relativity**, there is no real distinction between the space and time coordinates.
·
It is often
helpful to think of the

**four coordinates**of an event as specifying its position in a four-dimensional space called**. It is next to impossible to imagine a four dimensional space. Let’s take an example to visualize this thing –**__space-time__*When a stone hits water surface, the event can be explained in this manner,*

Here we are observing every
moments of the event. The ripples of the water are expanding or ripple spread out as a circle that gets
bigger as time goes on. And we are seeing that, if we take four coordinates
including time, the observation of the event forms a

**3D cone**.
·

**Similarly**, the light spreading out from an event forms a three-dimensional cone in the four-dimensional space-time. This cone is called the**future light cone**of the event as time is going from a point to the**future**. In the same way we can draw a**past light cone**, which is the set of events from which a pulse of light is able to reach the event.
·
This cones (past and future) of an event, let’s
say

**E,**divide space-time in three regions.**(I)**the**absolute future**of the event E, is the region inside the future light cone of E.**(II) t**he**absolute past**of E is the region inside the past light cone.**(III)****elsewhere**is the region where events can’t affect or be affected by event**E**.
·
If one knows what is happening at some particular
time everywhere in the region of space that lies within the past light cone of
E, one can predict what will happen at E. It means the past light cone of an
event is completely relatable to the future light cone of that particular
event.

### CONCLUSION:

- Time should appear to run slower near a massive body. More the mass increases, the speed of time decreases.
- Time runs slower near the observers who have some velocity compared to the observers who are in rest. More the velocity or speed of the observer increases, the speed of the time decreases.

**Read the Before Einstein part: http://www.theincredibletruths.com/2018/08/space-time.html**

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