Illusion of Time: Twin Paradox, Time Dilation, Gravity, Time Travel

What is time? Time is an illusion! An ultimate quest in philosophy and science, I would say. Time is the ongoing sequence of events taking place. Time is relative, and flexible and, according to Einstein, "the dividing line between past, present, and future is an illusion". So, reality is ultimately TIMELESS. We have created or rather defined time to live in the space.  Space may not be the fundamental ingredient of the cosmos but rather a fundamental side effect that emerges from particles interaction.  That space is measured in time. Time is a distance travelled in space! Space comes along with time; it’s never ever existed without time. Time is measured in a vector scale in a space. 3-dimensional of space is—up-down, side-to-side, and back and forth. The fourth dimension added to it is time.  Time is used to describe events in 3-dimensional space.

Time in physics is operationally defined as "What  a clock reads".  Great Einstein told that time is not absolute; it’s a relative term and depends on which frame of references we are talking to. Each individual has his own personal measures of time that depends on where he is and how he is moving. For example, the person travelling in a train thinks he is in rest and rest of the world is moving but for an observer, standing in a platform, he is in motion.  Like on Earth we are still but for an observer, outside the earth, we are moving because earth is moving round the Sun. Both the observers are fully entitled to say that they are stationary and the other one is moving. This is the Special theory of relativity which talk about the inertial frame of reference.  Inertial frame of reference simply means which runs with constant velocity and no acceleration applies on it.

According to Einstein, there is no difference in the box that is on the surface of the earth and subject to the gravity of g and the box is accelerating in an outer space with velocity g.  There is no way for an observer to tell whether he is moving or standing still. The acceleration is balanced by the gravity in an opposite direction. The person in an accelerated box falls on the surface of a box and feels the effect of a gravity.

Einstein  said both the two things are equivalent! A person who is travelling in a space time, right side graph is the  journey is space time.  And by the principle, the person who is stationary on the earth but subject to gravitational attraction g, must also be travelling in a space time. Though, he is not moving!

Time is relative and depending on observer’s perspectives to see it from different frame of references. Time runs different for different people, in appearance.

General theory of relativity is the theory of gravitation. General theory of relativity have confirmed gravitational waves, gravitational lensing and an effect of gravity on time known as gravitational time dilation. Gravity affects the flow of time in General Relativity.

Einstein described gravity as the curvature of space time due to the object’s mass and density. That works very well in large scale cosmic world.

Time goes slow near gravity and at black hole it stops completely. In general relativity, bodies always follow straight lines in 4-dimensional space time, but they appear to us move along curved path in 3-dimensional space. The curvature of a space-time increases the straight-line distance and to keep the speed constant, time has to be increased.  It’s taking more time to cover the distance from A to B, now. Thus, time increases or runs slow near gravity. 

Einstein’s Special Theory of relativity is all about what's relative and what's absolute about time, space, and motion.  Special Theory of Relativity applies on Inertial frame of reference. The object which is not accelerating. For an acceleration, there is a General Theory of Relativity where gravity plays a major role. In an Inertial reference frame, object is either stationary or moving with a constant velocity in a straight-line path. In a circular movement, object starts acceleration because there is a centripetal force working outwards that changes the speed of an object.

According to Special Theory of relativity, Speed of light is constant everywhere. All observers will measure the same speed of light regardless of their motion.  A light year is the distance travelled by a light in 1 year. The speed of light in vacuum is approximately 3,00,000 km/sec. It is also the speed of all mass less particles travel in vacuum, including photons, electromagnetic radiation, and gravitational waves.

Length Contraction

When object approaches speed of light its relative mass increases but it doesn’t change its gravitational attraction. Gravitational attraction depends on the rest mass.

According to E==MC^{2 ,} when the object attained the speed of light, it needs an infinite amount of kinetic energy.  When object moves with speed of light, its mass increases to infinite which provides infinite amount of energy to an object.

E==MC² , Infinite mass is converted to an infinite energy!

When an object approaches speed of light, it’s length start contracting and as it reached to the speed of light it becomes a point. Length contracts in respect to an observer, in an alternate way it’s relative length is short. For a point like object, distance doesn’t exist. Distance become zero so the time taken is also zero. That’s why they say, you can travel past, present and future in some imaginary time machine!

In this process, momentum also reached to infinite as per the equation P==mv, where m is the mass of an object and v is the velocity.  M is the relative mass of an object, which is approaching the speed of light.

The momentum of a particle is a measure of the time required for a constant force to bring it to rest. When the object is moving with an infinite energy, it never gonna rest in any time. So, the momentum is also infinite, which always make it keep moving.
https://www.youtube.com/watch?v=a8Vssy0pEpI


To be continued