ScienceVerseLearning path: Space-time
The concepts to learn before Space-time, in order. Each step links to its page and, where one exists, its quiz.
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Your path
- GravityDirect prerequisite
The attractive force between any two masses. Newton's law of universal gravitation describes it as proportional to the product of the two masses and inversely proportional to the square of the distance between them. Near Earth's surface this produces a roughly constant downward acceleration of about 9.8 m/s^2 on falling objects.
- LightDirect prerequisite
Light is an electromagnetic wave that travels through vacuum at exactly 299,792,458 m/s and more slowly through materials. At a boundary it reflects (angle of reflection equals angle of incidence) and refracts according to Snell's law, n₁ sin θ₁ = n₂ sin θ₂. Visible light spans roughly 400–750 nm, and it also behaves as a stream of photons, each with energy E = hf.
- Space-timeYour goal
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.
Where to go next
Concepts that build directly on Space-time:
- Black holes: A black hole is a huge amount of matter packed into so small a space that, inside the boundary called the event horizon, nothing — not even light — can escape. Black holes form when the cores of very massive stars collapse, and supermassive ones sit at the centres of most large galaxies. Although they give out no light themselves, they have been detected through orbiting stars, X-rays, gravitational waves and, since 2019, images of their shadows.
- Cosmic expansion: Cosmic expansion is the stretching of space that carries distant galaxies away from one another, so that the farther away a galaxy is, the faster it recedes (Hubble's law). Traced back, it implies a hot, dense beginning about 13.8 billion years ago, whose afterglow we still detect. Since 1998 we have known the expansion is speeding up, driven by an unexplained 'dark energy' — and today's best measurements of how fast it is expanding do not agree, an open problem called the Hubble tension.