I would like to have a small orb, producing light of proportions of a standard household bulb today. No big deal, right. They have these already for sale on Amazon as night lights.
I would, however, like to create a floating version of this, anchored above shoulder height of its owner, providing a moving, ambient luminescence.
Power- This light should be able to last for months on end. I envisioned something powered by possibly a small RTG, as it would supply constant power for long periods of time.
Staying Airborne- I would prefer for this light to be perfectly spherical, with no large protuberances. How could I make this lighter than air?
Cost- I'm sure its no difficulty making one of these expensive, and I originally intended only the wealthiest class to be able to afford them readily.
I don't feel committed enough to this small facet to change the time period from that which I was originally imagining: No more than 400 years into the near-future. Assume, for example, that aside from cheap-spaceflight, we are still bound to our solar system by failing to achieve FTL travel.
How do I make a glowing, floating, and long-lived light source that can be used in every day situations?
I would like to keep this as realistic as possible. Not too much handwavium.