Why, the scientific individual measurement of love in a quantifiable state, of course!
No, really, what is it? Edit
It goes like this. In this world, love is not, in fact, an endless and forever thing. In this world, love can be stolen, drained, and several magical artifacts and at least one species run on it. Due to this knowledge, it is hypothesized that every individual only has a set amount of love within them at any given time. The leading authority on the matter, Princess Cadenze Mi Amore, has assembled various magics and technology for the express purposes of studying them, and has made several discoveries on the power of love, as it were.
How is it measured? Edit
The main method of observing ratings of output energy is measuring intensity and lack of intensity. Due to it not being observable in a physical state and due to increased and frequent fluctuations, there is no way to attach a number based system to the amount of Sparklejoules in any one being. However, due to the way they are observed, they are quantified by a color based system with addendum to their being to describe the amount of or lack of intensity. Lighter or brighter colors are stronger and produce more Sparklejoules, while darker and dimmer colors are lower in count and intensity. In short, someone cannot have a Sparklejoule count of, say, two, but instead their count would be measured as "Blue" or "Light blue" or "Dim Blue", depending.
Can it grow or shrink? Edit
Indeed! If a collection of unfortunate events should occur, it is possible to severely reduce a Sparklejoule count. Likewise, it is possible to grow love back to it's original state, but it has not been directly observed to go above the allocated previous maximum.
What does this all mean? Edit
That changelings are not, in fact, running on abstract concepts. Likewise, artifacts like the Crystal Heart or the amulet Cadence destroyed are likewise not connected to something purely emotional. They are draining a specific, set number of energy from a being. This is why Shining Armor, after being drained of love by Chrysalis, was unable to maintain his shield. It is directly connected to his body, and affects his physiology.
Is this useful? Edit
...Probably not, but it's cool.