Heaven and Hell

Heaven and Hell respectively are higher and lower realms that are explained as being of "different wavelengths", with Heaven of a higher frequency, and hell of a lower frequency.

Overview
In traditional Abrahamic religious tradition, Heaven is the realm of God and his angels, though its existence is often interchanged with Paradise, or the final resting place of people that are under God's grace. Hell on the other hand, is traditionally the realm where the Devil and his angels are sent to be punished. Moreover, in many traditions, it is also the place where wicked souls are punished for all eternity, and is often interchanged with Sheol, a place that is separate from God, and a separation from God equals suffering.

In Toaru Majutsu no Index, due to the differing nature of the realms, humans of the world can neither detect Heaven nor Hell, even though both of them exist in the exact location, as is it's denizens cannot detect each other and their realms. According to Tsuchimikado Motoharu, only when the denizens of those realms go into the realm between Heaven and Hell, that is the phyiscal world of the living, that the denizens can detect each other. Motoharu continues by saying that their very presence in the human world will result in negative affects to the human world, describing it similarly to an object being lit by an infrared light and becoming hot because of it, or when glass vibrates when there is a high frequency sound. This is of course an inherent property of angels.