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The concept problems will involve the most students (with their varying experience) if the issues are simple to grasp, foundational to relativity, and yet tough enough to challenge them...

**Is there absolute motion?**

If two objects are in relative motion with respect to each other, can't we conclude that at least one of them must be in some kind of absolute, true, or real motion?

Won't this hold true for any and all possible frames of reference?

**Is relativity "real"?**

Is relativity a theory of measurement, or a theory of existence?

Is it just a way to transform data from a distant/fast source into what makes sense to us locally, or does it actually describe the distant/fast situation?

**How does gravity get out of a black hole?**

How is there gravitational attraction outside of the black hole?

If it comes from inside the event horizon, how does it get out?

If from outside the event horizon, where is the source mass?

Does gravitational attraction pass through a black hole from one side to the other, or is it blocked?

**Why aren't photons allowed to carry flashlights?**

Why is there no inertial frame of reference for c?

Why don't velocities add up like ordinary numbers in simple addition?