What is time? Most of the people always neglect this question. For them time is only a data measured by clock and its velocity is one second per second. For some people it is a factor that makes people old and steals their golden days. But some people who are little aware for them the definition of time is a different factor. For a theist it is creation of god and is eternal. For a philosopher the definition of time varies with their philosophy e.g. for Marxists it is circular and for anti Marxists it is linear. But physicist sees time as the most complex part of nature even studied.

I again want to ask what time is. From where was it created? What is its nature? Does time flow? Why man only becomes older not younger? I want to describe some answers of above questions in the basis of physics recent description.

According to Einstein’s new concept of space time, time is nothing but a dimension of nature so combined space and time dimension is often called space time dimension. So our traditional concept of three space dimension was revised by Einstein and gave a new ides. But a great problem is when we consider time as a dimension a baffling question arises. We can travel in both direction in space but not is time eg we can travel toward future not in past. Why? If time is a dimension why is differs from another three dimension?

The answer may as follow. We are animal whose evolution may be based on a condition in which time’s reverse direction is not possible. But all particles we observe also travel to future. We are not only being who find time running forward. Even particle traveling nearly equal velocity of light also find themselves traveling in future although for them time dilated tremendously.

This above condition can be explained by entropy theory, entropy is thermo dynamical tool that measures disorderness of nature and never decreases with time. Some may try to decrease entropy by make more orderness like build a house but by doing this they transfer a lot of energy to surrounding and entropy never decreases. It increases even in contracting phase of universe. Paul Davis another scientist use this concept as if universe goes expanding and contracting continuously then nothing will be different. If entropy is greater in big crunch then in previous big bang then time elapsed between big bang and big crunch goes increasing as shown in figure 1. So according to entropy theory if someone travels in past then entropy must decrease that violates the law of universe.

But rather then entropy theory some scientists argue differently like Richard Feynman argued that antiparticles are nothing but particle traveling in past. If everything remains same and only time get reversed then particle’s property become antiparticle’s property. So if particle travels in future then antiparticle must travels in past. Imagine you are traveling to Butwal and in Narayanghat you start to travel in past then after some time you will be in both place butwal and kathmandu. Similarly particle and antiparticle are nothing but part of energy traveling in both direction and annihilation is liberation of energy in end of journey. If Feynman was right then there must be equal numbers of particle and antiparticle but we observe particle more then antiparticle. Finally this theory was rejected.

To make time travel possible another hypothesis of tachyons was purposed. Tachyons are those particles that travel faster then light. Einstein’s relativity does not restrict particle initially traveling faster then light. Such particle has negative mass so infinite mass in speed of light and zero mass in infinite velocity. So the can never travel less then velocity of light. And light barrier is saved. And by this property they must traveling in past but again their psychological arrow of time will be in future.

Again I want to talk about special theory of relativity. In this theory Lorentz's transformation is used. We know in three dimensional co-ordinate systems the distance between two point is given by

ds

^{2}=dx^{2}+dy^{2}+dz^{2}But in four dimensional space the distance between two point is given by

ds

^{2}=dx^{2}+dy^{2}+dz^{2}-dt^{2}To make the time dimension equivalent to other three dimensions we multiply the time dimension by imaginary number as

ds

^{2}=dx^{2}+dy^{2}+dz^{2}+(idt)^{2}so time in imaginary system from our frame of reference behaves completely as other three space dimension. So real time dimension is in imaginary form and what we call real time is only part of imaginary time. In imaginary time travel in both directions is possible but our time is constrained version of imaginary time so we can travel in future and cannot remember it and remember past and cannot travel toward it.

**REFERENCES**

A brief history of time: Stephen Hawking

Time a traveler's guide: Clifford A. Pickover

The elegant universe: Bryan Green

The mind of god: Paul Davis

Time Arrows: Richard Morris