>>12152477On classical microscopic level, we imagine particles to follow Newtons laws, which are symmetric in time. Imagine a jar filled with some peas. Shake the jar and the peas will bounce around. Record this and play it backward. The motion of the peas will look kinda the same, backwards and forwards.
However, macroscopic processes in many particle systems turn out not to be reversible.
For example, if a cup of coffee cools down, it wont just heat up again afterwards by absorbing the heat of the air around it, even though this would not violate energy conversion. This is because of Entropy, which makes thermal processes a one way street.
Boltzmann brains were already explained.
One reason why people think about them is because entropy needed to be reasonable low for the universe as we know it to exist. an alternate explanation is that the entropy was just low enough for one boltzmann brain to exist: you
