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Space-time

Space-time is the single four-dimensional framework of three space dimensions and one time dimension in which every event happens. Einstein's general relativity explains gravity as the curving of space-time by mass and energy, which bends the paths of light and objects and makes clocks run at different rates. These predictions have passed every test so far, from the 1919 eclipse to the GPS satellites in your phone.

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Clocks, Light and Relativity in Practice

Gravitational time dilation and special-relativistic time dilation both affect real clocks. GPS clocks lose about 7 μs per day because of their orbital speed and gain about 45 μs per day because gravity is weaker at their altitude, a net 38 μs per day that the system must correct.

In the 1971 Hafele–Keating experiment, caesium clocks flown eastward round the world lost 59 ± 10 ns and westward ones gained 273 ± 7 ns relative to the US Naval Observatory, in good agreement with predictions that combine both effects. In 1976 Gravity Probe A flew a hydrogen maser to about 10,000 km and matched the gravitational frequency shift to better than 2 parts in 10,000, and Viking radio signals passing near the Sun showed the relativistic time delay to 0.1%.

Info: The speed of light, c = 299,792,458 m/s, is an exact value, so a measured signal travel time converts directly into a distance — the principle GPS navigation relies on.
Full explanation — the complete reference version every reading depth is based on

What it is

To pin down any event you need four numbers: three to say where and one to say when. Newton treated space and time as separate and fixed. Einstein's special relativity (1905) showed they are united in a single fabric, space-time, and that moving clocks run slow. His general relativity (1915) added gravity: space-time curves near masses, that curvature is what we experience as gravity, and clocks deeper in gravity run slow too.

Info: GPS was not designed as a test of Einstein, but because its satellite clocks must be corrected for both kinds of time dilation, NIST describes it as perhaps the longest-running test of his theories.

Key ideas

  • Equivalence principle: inside a small, freely falling laboratory the ordinary laws of physics work as if there were no gravity — which is why astronauts in the orbiting International Space Station float.
  • Light follows the straightest available path through curved space-time, so mass bends light.
  • Clocks in stronger gravity run more slowly than clocks in weaker gravity.
  • The speed of light in a vacuum, exactly 299,792,458 m/s, is the same for every observer.
  • Accelerating masses — such as two black holes spiralling together — send out gravitational waves, ripples in space-time.

Worked example: GPS

A GPS satellite's speed makes its clock lose about 7 microseconds a day, while the weaker gravity of its orbit makes it gain about 45. The net gain is 45 − 7 = 38 microseconds a day. Light travels about 299,792,458 m/s × 0.000038 s ≈ 11.4 km in that time (our own calculation), so because GPS turns signal travel times into distances, an uncorrected clock would put positions kilometres out within a day.

Δtnet≈45 μs−7 μs=38 μs per day\Delta t_{\text{net}} \approx 45\,\mu\text{s} - 7\,\mu\text{s} = 38\,\mu\text{s per day}

Daily gain of a GPS satellite clock: the speed-up from weaker gravity minus the slowing from its orbital speed.

Common misconception: A common misconception is that space-time is a rubber sheet that objects sit on and roll into. That picture is only a two-dimensional analogy for curved space; real space-time curvature involves time as well, which is why identical clocks at different heights tick at different rates.

Where it connects

Space-time builds on Newton's gravity (Gravity) and the constant speed of light (Light). Extreme curvature gives black holes (Black holes), the stretching of space describes the expanding universe (Cosmic expansion), and relativity corrections keep satellite navigation working (Spaceflight).

How we know

  • Mercury: its orbit swings round by an extra 43 arcseconds per century that Newton's gravity could not explain; general relativity accounts for it.
  • Bending of starlight: Einstein predicted 1.75 arcseconds of deflection at the Sun's edge, twice the Newtonian figure of 0.875. The 1919 eclipse measurements agreed, though only to about 30%; radio measurements now agree to about 0.01%.
  • Clocks: from the Pound-Rebka tower experiments of 1960 to atomic clocks flown on airliners in 1971 and NIST clocks one foot apart in 2010, clocks higher up tick faster.
  • Gravitational waves: a binary pulsar found in 1974 loses orbital energy exactly as predicted, and LIGO first detected the waves directly in 2015.

Assumptions and limits

  • For everyday speeds and gravity, general relativity gives the same answers as Newton's laws; the differences show up in precise clocks, strong gravity and light bending.
  • The GPS figures (+45, −7, net +38 microseconds a day) are rounded values for the satellites' orbit.
  • General relativity has passed every test so far, but it predicts points of infinite density inside black holes, which suggests it is not the final theory.

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Sources and methodology

  • Einstein's special relativity revealed that space and time are united in a single fabric known as space-time. (awaiting scientific review)
  • According to special relativity, the faster you move relative to a motionless observer, the slower time passes for you; according to general relativity (1915), the more strongly you feel gravity, the slower time passes. (awaiting scientific review)
  • In Einstein's general theory of relativity, gravity is caused by changes in the geometry of space and time: space-time curves near masses. (awaiting scientific review)
  • The Einstein equivalence principle states that in any small, freely falling reference frame the non-gravitational laws of physics are the same as those of special relativity. (awaiting scientific review)
  • At the International Space Station's altitude Earth's gravity is still about 90 percent of its strength at the surface; the crew float because they and the station are in free fall around Earth. (awaiting scientific review)
  • The speed of light in a vacuum is exactly 299,792,458 metres per second. (awaiting scientific review)
  • In 1911 Einstein calculated that starlight grazing the Sun should be deflected by 0.875 arcseconds — the same as a Newtonian calculation — and in 1915 full general relativity doubled the prediction to 1.75 arcseconds. (awaiting scientific review)
  • Plates from the total solar eclipse of 29 May 1919 gave a grazing deflection of 1.98 ± 0.12 arcseconds at Sobral and 1.60 ± 0.31 arcseconds at Principe, in agreement with general relativity. (awaiting scientific review)
  • The 1919 eclipse light-deflection experiments had only about 30 percent accuracy. (awaiting scientific review)
  • Very-long-baseline radio interferometry has measured the Sun's bending of light in agreement with general relativity to about 0.01 percent. (awaiting scientific review)
  • General relativity accounts for the unexplained advance of Mercury's perihelion, whose modern value is 43 arcseconds per century. (awaiting scientific review)
  • The Pound-Rebka-Snider experiments of 1960–1965 measured the gravitational frequency shift of gamma rays rising or falling through a tower at Harvard University, to one percent accuracy. (awaiting scientific review)
  • In June 1976 Gravity Probe A flew a hydrogen-maser clock on a rocket to about 10,000 km and confirmed the predicted gravitational frequency shift, limiting any deviation to less than 2 parts in 10,000. (awaiting scientific review)
  • In 2010 NIST physicists ran two optical clocks at heights differing by about a foot (30 cm) and found they ticked at slightly different rates because of the difference in gravity. (awaiting scientific review)
  • In October 1971 caesium atomic clocks flown around the world on airliners lost 59 ± 10 nanoseconds on the eastward trip and gained 273 ± 7 nanoseconds on the westward trip relative to the US Naval Observatory, in good agreement with relativity. (awaiting scientific review)
  • For the 1971 round-the-world clock flights, relativity predicted a loss of 40 ± 23 nanoseconds eastward and a gain of 275 ± 21 nanoseconds westward. (awaiting scientific review)
  • Special relativity makes GPS satellite clocks fall behind clocks on Earth by about 7 microseconds a day, while the weaker gravity of their orbit speeds them up by about 45 microseconds a day, so they run about 38 microseconds a day fast and must be corrected. (awaiting scientific review)
  • Radio signals from the Viking Mars landers measured the relativistic time delay of signals passing near the Sun to 0.1 percent in 1976. (awaiting scientific review)
  • In 1974 Russell Hulse and Joseph Taylor discovered the first binary pulsar, which allowed general relativity's prediction that such a system loses energy as gravitational waves to be tested; the work won the 1993 Nobel Prize in Physics. (awaiting scientific review)
  • The orbital period of the Hulse–Taylor binary pulsar shrinks at the rate general relativity predicts for energy carried away by gravitational waves, agreeing to better than half a percent. (awaiting scientific review)
  • Gravitational waves were first detected directly on 14 September 2015 by both LIGO detectors, from two black holes merging. (awaiting scientific review)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
  • The Advanced explanation has not yet been reviewed for age suitability.