Isotropy of Time Homework Help, Assignment Help, Online Tutor, Tutoring

Submit Assignment

* *

Payment Online Assignment Help

Homework Help

sfsf

Isotropy of Time

Isotropy means equivalence of direction; isotropy of time means equivalence of directions of time. However, we usually refer to only one direction of time, the forward direction, from present to future. Nevertheless, we can conceive of a backward direction of time, i.e. from present to past. Therefore, the question whether time is isotropic or not means whether time is reversible or not? Operationally, it means to find out what happens if we change t by – t?

Newton’s laws of motion do not change if we replace t by – t in it:

Since F does not explicitly contain t, it remains unaffected. Thus, the motion described by Newton’s laws remains same when we replace t by – t. What it means is the following: consider a physical process governed by Newton’s laws, as for example, motion of a particle thrown vertically upwards. It goes up, stops, and then falls back under gravity. Let us take a movie in reverse, we look at the process occurring backward in time. The entire process looks normal: particle goes up, stops and falls back exactly according to Newton’s laws. Thus Newton’s laws are reversible in time.

Processes in systems comprising of large number of particles do not show time reversibility. As a typical example, consider a glass which falls from a table and shatters into small pieces on the floor. If we run a motion picture of the above process in reverse, we see the pieces gathering together on the floor into a whole glass, which then jumps onto the table. This does not happen in nature. Thus, while each piece of glass follows Newton’s laws which are time reversible, the collective system breaks time reversibility. This is explained by II law of thermodynamics which prohibits a macroscopic system to move from less ordered to a more ordered state (law of increase of entropy).

Isotropy of time does not lead to any specific conservation principle in classical mechanics.

To sum up, in this section, we described the space-time symmetries and their consequences in understanding the nature of forces and conservation laws, within a given inertial frame. Now, we shall see how Newtonian space-time structure connects motion as observed from two different inertial frames.