We can intentionally move mind.heart energy within this body and we can change the temperature/color of this mind.heart stream with intentional thought...why not outside this body and mind.heart stream? Some of my teachers have moved my heart and untied internal knots of energy with just a smile or a word that reflected the temperature/color of their mind.heart stream. We share the temperature/color of our mind.heart stream via sound and light waves all the time throughout the course of this life. We project our internal energy with our very appearance, sound, and via the light in our eyes.
We're so caught in our sense of materiality and our mistaken sense that the body/mind.heart stream is separate from what we sense to be "external" that we forget that materiality is merely dense energy and the mind-stream is less dense energy...two relative ends of the density spectrum. Metta cultivates a light-nesss of energy within the mind.heart stream and interacts with the denser energy of the material body that we call "ours"...why would it remain trapped within and only benefit others by the effects it has on our body and mind.heart stream when we know that the body/mind.heart-stream is receptive to every other kind of non-physical energy in the world, including sound waves, radiation, countless forms of light, smell, etc... The Sun's radiation passes right through the body - the outer layer of the body doesn't serve as a shield nor do buildings shield the interior of the body from the sun's radiation. Heat and cold are energies that effect body and mind.heart stream. Even words are energy, as we've seen in this electronic space many times. The energy of my thoughts here on your screen is inseparable from the electronic energy that carried it into your eyes on a beam of light.
So, if the quality of the mind.heart-stream energy can be shared from one person to another in close proximity or through energy mediums such as sound and light, why not via intention which is also a very powerful energy? Some will likely argue that sound and light involve eye and ear receptivity on the part of the recipient, but there are ways of knowing and streams of receptivity that don't depend on our gross physical senses...likely most here have had at least some limited experience with other kinds of receptivity that didn't depend on sight, sound, smell, touch, or physical presence. We're all appearing inseparable in this sphere of phenomenality...any opinion we have on the limits of mind.heart energy is an opinion limited by the extent of our awareness of the Whole. Perhaps the combination of intention and thoughts/feelings of metta can be conducted through time/space via directed awareness, yet another kind of energy. There are some very interesting scientific research results that are causing us to rethink the effects of intention on material and distance. Perhaps there is some other energetic medium surrounding us that we're unaware of that serves as a conductor for intention with focused attention. Are we really so sure that we've discovered all means of conductive energy within the last several hundred years?
I'm generally not a fan of "faith" but in the practice of metta I have provisional faith based on what I know about the nature of energy. Like everyone else, I don't know anything...but our premodern ancestors from all over the globe all spoke of the ability to communicate over distances through the mind.heart-stream. Since we're living in a thick, carnal time that makes awareness of less dense forms of energy difficult, I'll take their world that energetic communication is possible. Better to err on the beneficial side, imo.