The phenomenon you're referring to is known as time dilation, which is a fundamental aspect of special relativity. According to the theory of special relativity, the flow of time can appear to be different for observers who are in relative motion to each other.
In the scenario you described, where observer A is traveling away from observer B at near light speed, it is correct that observer A's time will appear to slow down relative to observer B. This is due to the principle of time dilation, which states that as an object approaches the speed of light, time appears to slow down for that object as observed by a stationary observer.
However, it's important to note that special relativity is based on the idea of relative motion. From observer A's perspective, they are at rest and observer B is moving away from them at near light speed. From observer B's perspective, they are at rest and observer A is moving away from them at the same speed. Both observers would consider themselves to be stationary while the other is moving.
The key point is that the principle of time dilation applies equally to both observers. So, from observer B's perspective, it would indeed appear that observer A's time is dilated or slowed down. However, since both observers are in relative motion, they cannot directly compare their own experiences of time dilation. Each observer can only measure the time dilation experienced by the other observer.
In summary, both observers would perceive the other's time to be slowed down due to their relative motion. This apparent contradiction is resolved by understanding that the observed time dilation is dependent on the relative motion of the observers and that there is no absolute reference frame to determine which observer is "moving" and which is "stationary."