Einstein's relativity and modern physics.
Test your knowledge with these top questions!
Who measures a spaceship's proper length?
Proper length means the length in an object's rest frame; motion along that length makes the measured value shorter.
Two ships approach at 75% of light speed each in Earth's frame. How fast does one see the other?
Einstein's velocity rule adds an extra denominator, keeping any two sublight speeds from combining past light speed.
A steadily moving rocket chases a light beam in a vacuum. What speed does the rocket measure?
Every inertial observer measures light in a vacuum at c, regardless of how the source or observer moves.
Why does an outside observer measure longer ticks from a moving light clock?
Both observers measure light at the same speed, so the diagonal route requires more travel time.
Which boundary marks where even light can no longer escape a black hole?
An event horizon is a boundary, not a solid shell; crossing it cuts off every path back to the outside.
Which feature can differ between inertial observers in special relativity?
Relative motion changes which distant events share one time coordinate, even though each observer's clocks work normally.
A spacecraft clock moves near light speed relative to Earth. How does Earth measure its ticking?
NIST atomic clocks measured motion-based time dilation at speeds comparable to a moving car.
A near-light-speed spaceship fires a forward laser. What speed does an outside observer measure?
Relativistic velocity addition keeps light in a vacuum at the same speed for every constant-speed observer.
Which effect makes GPS satellite clocks run faster than Earth clocks overall?
Motion slows GPS clocks, but weaker orbital gravity speeds them up more, so engineers correct the net gain.
Why do astronauts in orbit feel weightless while Earth's gravity still reaches them?
Orbit is continuous sideways free fall: the station and crew keep falling around Earth together.