What is spacetime?
Spacetime is the four-dimensional fabric that combines the three dimensions of space with time. Einstein's relativity showed that space and time are not separate or absolute: they stretch depending on how fast you move, and mass curves spacetime, which is what we experience as gravity.
Key facts
| Special relativity | 1905, Albert Einstein |
|---|---|
| Spacetime geometry | 1908, Hermann Minkowski |
| General relativity | 1915, Albert Einstein |
| Gravitational waves detected | 2015, LIGO |
| Clock at sea level vs Everest | Runs ~30 µs a year slower |
Space and time are one thing
In everyday life, space and time feel completely separate: where something is, and when it happens. In 1905 Albert Einstein's special relativity showed that observers moving relative to each other disagree about distances and durations, but agree on a combined quantity that mixes the two. In 1908 his former teacher Hermann Minkowski turned this into geometry: a four-dimensional spacetime with three dimensions of space and one of time.
Gravity is curved spacetime
In 1915 Einstein's general relativity went further: mass and energy bend spacetime, and that bending is what we feel as gravity. The Earth orbits the Sun because it follows the straightest possible path through spacetime curved by the Sun. Time is part of the bending: clocks run slower closer to a massive object. A clock at sea level loses about 30 microseconds a year relative to one at the top of Mount Everest.
Ripples in spacetime, gravitational waves, were first detected in 2015 by LIGO, from two black holes colliding 1.3 billion light-years away.
Special vs general relativity
| Special relativity (1905) | General relativity (1915) | |
|---|---|---|
| Deals with | Motion at constant speed | Gravity and acceleration |
| Main idea | The speed of light is the same for everyone; time and space stretch to keep it so | Mass curves spacetime; gravity is that curvature |
| Time effect | Moving clocks run slow | Clocks in stronger gravity run slow |
| Famous result | E = mc2 | Black holes, gravitational waves, GPS corrections |
The block universe
Relativity says observers moving differently can disagree about which distant events happen "at the same time". Many physicists and philosophers take this to mean there is no single universal "now": past, present and future all exist together in a four-dimensional block, and the flow of time is how we experience it from inside. Others disagree, and the question is still debated. Read is time real?
More: Einstein and time dilation · is time travel possible? · what is time?
Questions people ask
What is spacetime in simple terms?
Spacetime is the combination of the three dimensions of space and one of time into a single four-dimensional fabric. Einstein showed that space and time are linked, and that mass bends this fabric to create gravity.
What is the difference between special and general relativity?
Special relativity (1905) covers motion at constant speed and shows that moving clocks run slow. General relativity (1915) adds gravity, explaining it as the curvature of spacetime by mass and energy.
What is the theory of relativity?
Einstein's two theories describing how space, time and gravity work. They predict that time passes at different rates depending on speed and gravity, effects that GPS satellites must correct for every day.
What is the block universe theory?
The idea that past, present and future all exist equally in four-dimensional spacetime, so the passage of time is a feature of our experience rather than of the universe itself.
Is the space-time continuum real?
"Spacetime continuum" just means spacetime treated as a smooth, continuous whole. It is the framework of all modern physics and is strongly supported by experiments, from GPS to gravitational waves.
Last updated October 23, 2026.