The key thing is that the brain is not tethered within the cerebrospinal fluid, so it does bump around and get damaged. This conversation reminds me of the conversation we were having recently about helmets. There's a conversation about whether helmets help prevent concussions and so on. It's incredibly equivocal. They are not recommended right now as a prevention strategy that has very compelling evidence to say that helmets do prevent concussions. That's because of the physics of collisions.
There are three ways in which that could happen through helmets, theoretically. One is by absorbing energy, through a viscoelastic deformation. Currently, we don't have materials for which there is compelling evidence to say they can absorb enough energy to prevent a concussion. Two is by dissipating energy over more space, which is a great thing that helmets do, transferring a potentially focal injury into a more diffuse injury, preventing fractures but not preventing concussions in more diffuse injuries such as those. Three is by dissipating energy over time. Just making that impact last longer on a helmet could theoretically be very helpful because with that you would reduce the peak force and peak acceleration. At this point, we don't have compelling evidence or knowledge of materials in the padding of the helmet that would dissipate the energy over enough time to have that effect.