When you observe or measure a process in classical physics it almost never really alters the experiment. For example, if you have an Carnot engine and measure the volume and pressure of a gas in some a cylinder while the process is running you can do it without disturbing the process, meaning that if you "ran the laws of physics backwards" or ran the process backwards in time for the same amount of time you ran it forwards it will end up at the same place it started at.
However, in Quantum Mechanics if you in any way observe a particle during an experiment it can ruin the whole experiment. For example:
If you sent a particle through a ring and then stopped the process and ran it backwards without ever observing it, the particle would go back to where it came from but if you observed the particle in any way, (even after the experiment) it would not matter whether or not you looks at it forward or backward in time, because the probability that particle's path might fluctuate to the left is the same for either case. How does this work?