Reachable with the speed of light from what inertial frame?

Also, there's been some interesting progress regarding information about the inside of the black hole possibly leaking out in the Hawking radiation.

The speed of light is the same in all inertial frames.

Distances are not.

If something is in imagination, where exactly does it exist? What plane and what dimension? Where is it situated and what's the extent of it?

"It's all in your head" might be one answer but that's the question, what plane or dimension is it? and why do we not see it anywhere externally?

Funny, just today I read about things like waves or energy being arguably on a "different plane of existence" than their medium/embodiment :

http://www.av8n.com/physics/black-box.htm#sec-plane-of-exist...

(Which made me realize how little I know about eastern mysticism...)

Did atoms exist before they were discovered, or not until after they were discovered?

This question could be applied to a number of things historically, and may even be in effect going forward.