The Quantum Origin of Time
&gt; If you watched a video of two billiard balls colliding and bouncing away, you would be unable to tell if it was being run forwards or backwards.<p>I&#x27;ve seen this image used more than once to make the point, but now I think about it, I don&#x27;t think it works. It is only acceptable if we think of billiard balls as ideal elastic spheres. They are good illustrations of the concept of &quot;ideal elastic sphere&quot; in our material life -- but they aren&#x27;t, really, and that destroys the thought experiment.<p>Let&#x27;s run the tape at super-duper slo-mo, shall we? The two balls slowly approach, collide, rebound. But now at super-duper slow-mo we can see that, being actual collections of atoms, they behave somewhat like water-balloons. At the point of contact, there is deformation, flattening, and rings of compression waves radiate away, to meet at the other side of sphere, interfere, and come back. (Something of the sort happened to Saturn&#x27;s moon Mimas.)<p>So if we play the video in reverse, it is quite easy to tell the &quot;before&quot; from the &quot;after&quot;. In reverse, we have balls that, as they roll toward each other, develop increasing concentric ripples of deformation that start from the opposite sides and shrink rapidly to the contact faces, disappearing completely at the moment of contact, and two unperturbed balls separate.<p>The only way the billiard balls image works is if we instead use ideal perfect spheres of adamantium. Even balls of titanium or diamond would have some kind of internal elastic shape change that would exist only &quot;after&quot; and not &quot;before&quot; the collision.<p>Which is simply to admit that in the real world, there are no collisions that are actually reversible even in principle.