In 3rd Density, spiritual potential is realized, etc. In densities higher than 3rd Density, co-existence can occur. Thus, it occurs that on the planet Earth, where 3rd Density humans are living and walking about, 4th, 5th, and higher Density incarnations are living and moving about, occupying the same space, as it were. This occurs because of a higher vibration rate of the sub-atomic particles. There will still be a 3rd Density Earth, on the same plane as the future 4th Density home of humans, as a portion of the Earth will be left in 3rd Density.
Much confusion reigns on the issue of density.
Humans, for instance, were used to the concept of matter and energy being discrete and separate. Matter could not be energy, and energy could not be matter. Then Einstein shook things up and humans, after much painful denial and agony, allowed their concepts of reality to change. Density understanding is just such a quantum concept leap. In all fairness, the concept of density differences is difficult for even your scientists to deal with.