Motherships are there all the time. You simply don't see them. You can be right under one, and you won't see it. Because we have distorted light rays and we give you a view of what is above the mothership, from below, so you think you're looking right through it where you are actually looking around it. If this seems amazing, think about how light rays change when they go through water.
People accept the fact that they see things at an angle when they are under the water.
Bending light rays is no big deal, and we do it all the time. The mechanism we use is not a gravity field. It could scarcely be gravity, as the force required would compress us to the size of a pea, and of course we would not live. We use other means, something that doesn't affect us personally. Light is a particle ray, and is influenced by many particles that are similar to it, repulsed or attracted, and we simply utilize those elements, which are unknown to man.