# GRE Subject Test: Physics : Introductory Concepts

## Example Questions

### Example Question #1 : Introductory Concepts

The difference in age for the twins in the Twin Paradox occurs during which key moment in the trip?

While approaching a black hole in space during the trip.

During the time returning to Earth traveling fast.

During the period of great acceleration during the changing of directions and return to Earth.

None of these

During the beginning of the journey traveling fast.

During the period of great acceleration during the changing of directions and return to Earth.

Explanation:

While moving clocks do in fact record time moving at different rates, the time dilation works both ways. This means that a stationary person will view a moving clock ticking slower, but at the same time, a person moving alongside the moving clock will see the stationary clock ticking slower. However, clocks experiencing great accelerations will be permanently changed, "losing" time relative to a clock not being accelerated. Thus, the age difference occurs during the portion of the journey when the traveler accelerates at a great rate in order to return to Earth.

### Example Question #2 : Introductory Concepts

A black hole is an object whose gravitational field is so strong that even light cannot escape. Assuming no change in radius, approximately how much mass would our Sun have to have in order to become a black hole?