Collision with Phantom Object?

Is it possible to detect a collision with an object that is phantom? (i.e., an object that I can pass through with another object that has a collider)

There are a lot of ways to do this. The simplest is likely just to set the IsTrigger property of the object you want to be passed through, and use an OnTriggerEnter function on that same object to tell it when to collide. Once you set the IsTrigger property, it will allow everything to pass though by default, so that you can use the script to control all of its collisions.