# What is time?

Oct 14, 2016

Time is the fourth dimension.

#### Explanation:

Classically time is the progression of events from the past through the present to the future. The second is the fundamental unit of time.

There is the concept of time's arrow which implies that time can only flow in one direction.

Time is the parameter for laws of motion. Distance traveled is defined by the equation:
$s = u t + \frac{1}{2} a {t}^{2}$
Where t is time, u is the initial velocity and a is the acceleration of a body.

Einstein changed things with his theories of relativity. Special Relativity states that the speed of light is constant for all observers. This means that time runs slower for an observer travelling at speed to that of an observer at rest.

General Relativity adds time to the 3 dimensions of space to make 4 dimensional space-time. Time runs more slowly in curved space-time which is gravity. Gravity is not a force it is curved space-time. Einstein had a thought experiment where something accelerating is equivalent to something in a gravitational field.

Nov 7, 2017

illusion

#### Explanation:

time is fourth dimension - this is too popular on internet .
Time as a function of velocity.
Our calculations with velocity goes smooth until and unless we decide to think about time and since we are too curious to know about it let's talk about it .
As described by the newtonian concept time is absolute or universal everything in the world has same coordinates with respect to the time axis but then came physicist the father of modern physics Albert Einstein

this man was curious to know about whether it's time to understand time it take reference of light when he was young he thought that he will fly on light.

The basic idea of the theory of relativity is that time is not Universal its relative and what makes the time to slow and faster is velocity and gravity.

He derived the formulas of time dilation.

What we generally mean by time is what we see in our clocks but what do these clocks show. We know that the Earth rotates around its axis and revolution of Earth around sun we divided this all occurrence into segments and this is what we call seasons and time.

Let's take an example first for instance let there be a point P in deep space with two neutrinos a and b they both are trouble and the same direction the speed of a is 4 metre per second and b is 2 metre per second so after the interval of 1 second of each of these what will be the distance between the two the answer that we would give to it is 2 metres but this was what Einstein's theory of relativity suggest both neutrinos are not at same time instance when we measure the distance between the two.

So far I haven't written anything about time I've been talking about all the stuff Android that time is function of velocity.
But Time may be real or virtual . But there's something whose effect we do observe in our day to day life . That's what responsible for days and nights .
We have been programmed to accept what's told to us . But may our curiosity doubt the aptness of reality of everything , truly everything .

may the reality not be real

Nov 14, 2017

Time is the interval between two events in curved spacetime in the time dimension.

#### Explanation:

Isaac Newton describes time as a constant and every observer observed time as passing at the same rate. He describes velocity as relative to some universal standard. He also described gravity as a force.

Albert Einstein proved that none of these descriptions are actually true. Although they are a good approximation as long as you don't travel too fast or gravity isn't too strong.

Relativity regards time as a fourth dimension. Everything is travelling through four dimensional spacetime at the speed of light. A stationary object in the absence of gravity is travelling at the speed of light in the time direction!

To understand all of this fully requires an understanding of Einstein's field equations. I will present the field equations as they look elegant. In reality they are 10 second order partial differential equations which are very hard to solve.

${R}_{\mu \nu} - \frac{1}{2} R {g}_{\mu \nu} = \frac{8 \pi G}{c} ^ 4 {T}_{\mu \nu}$

Starting with the right hand side. It is a very small constant term as the gravitational constant $G$ is very small and the speed of light $c$ is large.

The $T$ terms says that mass, energy and momentum tell spacetime how to curve. The left hand side of the equations says that curved spacetime tells matter and light how to move!

In relativity there are two definitions of time. There is the time coordinate $t$ which is how fast a clock runs in the absence of gravity, mass or energy. There is proper time $\tau$ which is how fast an observer's clock runs. In principle every observers' $\tau$ is different!

The rate of $\tau$ slows due to velocity and gravity. Proper time stops when travelling at the speed of light in space, or in a strong enough gravitational field. Photons, which travel at the speed of light, don't experience time.

So, one way of thinking about time is that everything is travelling at the speed of light in four dimensional spacetime. The actual rate of time is the component of the motion in the time dimension. Motion and gravity, which is equivalent to acceleration, make time pass more slowly.