Why can the sophons not enter the sophon-free room?

Question

In Death's End, we are introduced to sophon-free rooms where sophons are unable to enter. Given their small size, what is stopping them from entering the room when the door opens for someone to enter or exit?

Answer 1

There isn't a detailed explanation of the technology. However earlier in Death's End there is mention of 'blind regions' which destroy the quantum entanglement of the sophons, with speculation that they are artificially created:

All of these sophons soon entered blind regions and lost contact with home. The longest-lasting one managed to get seven light-years away. Additional sophons launched later met with the same fate. The closest blind region, only about 1.3 light-years from Earth, was the one encountered by the sophons that accompanied Gravity.

Once the quantum entanglement between sophons was broken, it could not be restored. Any sophon entering a blind region was lost forever.

Trisolaris remained mystified by the kind of interference the sophons received: Perhaps it was a natural phenomenon, or perhaps it was “man”-made. Scientists from both Trisolaris and Earth leaned toward the latter explanation.

The method of screening an area involves electrifying the walls but is described in terms of creating a sophon-free region, rather than a barrier to entry. It is inferred that this is a lesser version of the blind regions - so not physically excluding them at the wall but stopping them from being present or operating within the enclosed space.

“This is a blind zone for the sophons,” someone said to Cheng Xin. Only then did she realize that humans had finally achieved the technology to shield themselves from the ever-present listeners, though it was only possible within extremely confined spaces like this one.

Answer 2

The sophons are kept out by some manner of electrically induced force field, not by a physical impediment.

Everyone wore high-voltage protective suits because the metallic walls of the sophon-free room were electrified. In particular, everyone was required to wear protective gloves, lest someone tap a wall out of habit, in an attempt to summon an information window. No electronics could function within the force field, thus the room had no information windows at all. To help the force field stay evenly distributed, equipment within the room was reduced to a minimum. Only chairs were provided, and there was no table. Since the protective suits were requisitioned from electrical engineers, the meeting within the metallic room resembled an ancient pre-shift gathering on a factory floor.

The force field appears to be much the same as the one used to hold the droplets impervious.

Communications with the Earth during the last few years had given these officers some basic insight into the principles of strong-interaction materials. They knew that the surface of a droplet was held in a force field generated by mechanisms inside. This force field counteracted the electromagnetic force between particles, allowing the strong nuclear force to spill out. Without the force field, strong-interaction material reverted to ordinary metal.