# I Reversibility of physical law

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1. Apr 26, 2017

### Higgsono

What does it mean for the laws of physics to be reversible in time? Does it mean that for every possible physical process, the same process can happen as it would do if we "played the tape backwards" so to speak? If a particle follows a path due to some physical law, Does it mean that if we were to reverse the momentum of the particle it would follow the same path backwards?

2. Apr 26, 2017

### PeroK

You could try watching this:

http://www.cornell.edu/video/richard-feynman-messenger-lecture-5-distinction-past-future

3. Apr 26, 2017

### Higgsono

Thanks! But when we reverse the path for the particle, does the particle's path have to be described by the same equation, or can it have another form?

What I mean is, suppose we have a function f that takes a state A to B. That is, f(A)=B. If we suppose that the particle now follows the same path backwards, is this path then expressed by the same function f, so that f(B)=A, or can it be another function g such that g(B)=A ?

4. Apr 26, 2017

### PeroK

I don't really understand your question. The simplest motion of a particle is uniform motion in a straight line, which we can take as the x-axis. This is described by:

$x = x_0 + vt$

Where $x$ is the position of the particle at time $t$, and $x_0$ is where it is at $t=0$.

This equation is time-reversible, as you can enter negative values for $t$, which will tell you where the particle was before $t=0$. In any case, there is nothing inherent in the mathematics that demands that $t$ must increase.