Einstein's relativity and modern physics.
Test your knowledge with these top questions!
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.
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.
LIGO's first direct gravitational-wave signal came from which cosmic event?
The signal's rising-frequency chirp traced the pair's inspiral, merger, and final ringdown.
In Einstein's elevator thought experiment, gravity can feel locally identical to what?
A sealed rocket accelerating through empty space can make loose objects appear to fall toward its floor.
Which motion describes an inertial reference frame?
An inertial frame neither speeds up nor turns, so a force-free object keeps constant velocity.
Gravitational waves from compact-object mergers travel as ripples in what?
LIGO detects the waves by measuring tiny, alternating changes in the lengths of perpendicular laser arms.
On a spacetime diagram, what marks the boundary of possible cause and effect?
Events outside a light cone would need a faster-than-light signal to connect with the event at its tip.
How does general relativity describe gravity?
Matter and energy curve spacetime, and that geometry guides how freely moving objects travel.
What does one point on a spacetime diagram represent?
A spacetime event joins a location with a time, like pinning both where and when something happened.