We are still with relativity.. and the theory Einstein thought about.. and the part we discussed together before.. is in the first and second reference...
The idea Einstein thought about this time was "Space"...
Can we really determine the position of anything in "space"..?
Imagine a tourist standing on the deck of a Nile boat.. coming from Aswan to Cairo.. we try to determine his place so we say "he is 6 metres from the wheelhouse"..
But this cannot work because the wheelhouse is not fixed.. it moves with the boat, which moves on the surface of the Nile, which moves towards the estuary.. and if we try to know his place relative to the earth for example and say: "he is such and such a distance from the city of Qena", we will find this impossible.. because the city, together with the whole earth, is moving in space around the sun.. and if we try to measure his position relative to the sun.. we find the sun moving with the group around the centre of the star cluster, and the star cluster revolves around the centre of the galaxy, and the galaxy itself keeps changing its position...
Einstein had concluded that the universe in the end curves so as to represent a huge sphere or bubble.. this sphere is constantly expanding.. and this expansion means that the galaxy itself is shifting and moving from its place...
So it is impossible for us to determine the place of something in the universe.. we can only estimate its position "relatively".. meaning we say its place is such and such relative to such and such.. the tourist's position is such and such metres "relative to the wheelhouse"..
While his real place is impossible.. because everything is in a state of motion.. even if it appears to us stopped and still...
That is in determining the place.. of a stationary body.. so what about a moving body!!
Who among us has not looked out of the window of a car or a fast vehicle as a child at the shining moon, and followed it as it ran along with him beside the car?
While if the car stopped for a few minutes at a traffic light.. you see the moon in its place.. it does not run after the cars..
So in order to decide whether a body is moving or not.. we need a fixed reference.. a stationary pavement to stand on...
Consequently Einstein rejected the idea of absolute place.. and considered place to be variable and relative.. and in the best of circumstances we can determine this variable place.. by another variable place next to it...
And among what Einstein decided was to link time with motion..
Even while you are still in your place, the seconds and minutes pass over you and you move in time...
So there is motion.. meaning time and place are two limits that are not separate...
You move in the three dimensions.. length, width and height...
But there is also a "fourth dimension" that you move in...
Other than length, width and height, there is a dimension called "time"...
Fine, what is time?
If we go back to the first part we researched together.. which is in the first reference.. let us remember together that there is more than one world...
Every being has its own world with its own laws and colours.. there is the rabbit's world, which has only green and blue.. and the dolphin's world, which has no smells at all.. and the bat's world, which is all darkness.. and the human world, which has many colours and smells.. even among humans themselves.. women see colours that we almost never see..
Time is the same subject...
There is more than one time...
There is the time that passes over you while you are sitting with someone insufferable and heavy-blooded.. and in it a minute passes over you as if it were an hour...
And there is the time that passes over you with your beloved on a quiet, romantic holiday.. and in it the day passes as if it were a minute...
And there is the time in which you sleep exhausted and drained.. and you wake up after 8 hours feeling that you slept ten minutes.. and the half hour in which you sleep and wake up energetic feeling that you slept a whole day...
Each one has a different time.. and it differs from one being to another.. the human grows up and marries and has children and grandchildren while he is, for example, 50 years old..
And there is the time of cats.. in which a year equals 16 human years.. it has children and grandchildren in 3 years...
And there is the time of the "Mayfly" or the water nymph.. which matures and grows and mates and lays eggs and dies in a day or two...
And the personal time inside each one of us.. is internally variable and not homogeneous.. and you do not feel that it is moving in time towards the future..
You feel that you are moving from one present.. to another present.. you never look at the clock and say: "I have become five minutes into the future".. you say: "I am now such and such"...
And when you say to your wife "I will make you a glass of juice now".. "now".. you in reality do not bring it that instant on a jala jala system as if you were a magician snapping your fingers and a glass of juice appears in your palm....
You take time to make the juice.. and you bring it after five minutes.. and maybe ten.. but both of you have no problem at all.. even though you did not bring it "now".. you brought it after five or ten minutes in the future..
But relative to our inner time.. that is "now".. for you and for her.. this future is "present"...
We are living in a continuous presence.. an eternal present.. from "now" to "now".. and from "now" to "now" all the time...
And there is an erroneous saying attributed to the theory of relativity which says: "Relativity is your feeling of time when you are with your beloved for a minute.. and again when your hand is on a hot ironing board for a few seconds.. and this, in short, is relativity.."
And this talk is absolutely wrong.. because this is not the time Einstein meant..
This inner time of ours, which is connected to our sensations and our moods.. the one we talked about.. is not the time meant by relativity..
Einstein's and relativity's time is the external, objective time.. the one in which you turn into an "event" among many events in the universe.. planets moving and stars being born and dying.. the time in which you move..
And this is a relatively objective time.. in it we can know that it is day or night and set appointments and commit to specific timings..
The time known by the clock, the day, the month and the year.. the one our clocks run on.. all the clocks we use are based on the solar system..
The clock.. those sixty minutes.. they are the earth's rotation on its own axis by 15 degrees.. and the day is a complete rotation.. and the year is a complete circuit around the sun..
So time, by the common definition.. is transitions of the earth and the sun..
Time, by the common definition, is "motion in place"...!
Mere "motion" of the earth in space...
And since we have agreed that place is variable.. and that it is linked to time.. then in order to be precise about a body we say that it is "in such a place at such a time"...
So time and place are not separate..
There is a close bond between them that keeps them connected..
Fine, what is this bond called?
It is called "Spacetime"...
Fine, we move in place.. can we move in time?
In place we move backwards, up, down and forward.. can we do that in time??
Can we go back in time to the past.. and travel in it to the future?
And what is this "spacetime"??
We will continue that later in the rest of the subject of relativity, God willing...
------------------
References
-----------------
1 - https://goo.gl/QZB12B
2 - https://goo.gl/pS67Hb
3 - Ehrenfest, Paul (1920) "How do the fundamental laws of physics make manifest that Space has 3 dimensions?" Annalen der Physik 366: 440.
4 - Lorentz, H. A., Einstein, Albert, Minkowski, Hermann, and Weyl, Hermann (1952) The Principle of Relativity: A Collection of Original Memoirs. Dover.
5 - Kopeikin, Sergei; Efroimsky, Michael; Kaplan, George (2011). Relativistic Celestial Mechanics of the Solar System. John Wiley & Sons. p. 157.
6 - النظرية النسبية الخاصة لأينشتاين - د. حازم فلاح...