Our Western culture is so caught up in rushing back and forth, and so divorced from the cycles of nature, that most people are not attuned to the subtle experiences of spirit presence that surround us all.

We can feel vibrations on our body, almost down to zero Hz (cycles per second), up to several hundred. Our ears perceive sound from approximately 10 Hz to 20,000 Hz (when we are young). Our skin senses heat that lies in the infrared frequency band, just below the visible spectrum, ie., from around 300 billion to 400 billion Hz.  Our eyes perceive light frequencies from approximately 429 billion to 750 billion Hz. Those are the sections that make up what we call 'the physical world'. However, the physical world is only a very limited section of vibrations compared with all the other vibrations in the universe. Humans have only recently learned to produce machines that can tune into radio waves, television waves, and X-rays. These all existed before we were able to detect them. Scientists working in the Spirit world (which some call the Etheric world) tell us that their world is just as solid as our world, but on a different frequency - just above what most people's senses can perceive.
(Diagram and information taken from Arthur Findlay's "The Way of Life," p. 24.)