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Introduction to general relativity
General relativity (GR) is the geometrical theory of gravitation published by Albert Einstein in 1916. It unifies Einstein's earlier special relativity with Sir Isaac Newton's law of universal gravitation. This is done with the insight that gravitation is not due to a force but rather is a manifestation of curved space and time. In general relativity, spacetime is non-Euclidean, or curved. The need for curvature arises from the equivalence principle and a child's simple question: "What keeps the people on the other side of the world from falling off?". In other words, should not the inertial paths on the other side of the Earth take objects away from the planet? Instead, all free-fall trajectories in the vicinity of a massive object will draw objects towards it.
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Space & Time News
December 5, 2025
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Nov. 16, 2025 Dark matter may be invisible, but scientists are getting closer to understanding whether it follows the same rules as everything we can see. By comparing how galaxies move through cosmic gravity wells to the depth of those wells, researchers found ...
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Nov. 10, 2025 New research from UBC Okanagan mathematically demonstrates that the universe cannot be simulated. Using Gödel’s incompleteness theorem, scientists found that reality requires “non-algorithmic understanding,” something no computation can ...
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Dec. 4, 2025 A network of powerful ground-based telescopes captured rare starspot-crossing events on TOI-3884b, revealing cooler patches on the star’s surface and rapid changes tied to its rotation. By ...
Dec. 4, 2025 FROSTI is a new adaptive optics system that precisely corrects distortions in LIGO’s mirrors caused by extreme laser power. By using custom thermal patterns, it preserves mirror shape without ...
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Dec. 3, 2025 For thirty years, SOHO has watched the Sun from a stable perch in space, revealing the inner workings of our star and surviving crises that nearly ended the mission. Its long-term observations ...
Dec. 2, 2025 Some white dwarfs in rapid binary orbits are far hotter and larger than theory predicts. Researchers found that powerful tidal forces between them ...
Nov. 29, 2025 The discovery of strange, ultra-red objects—especially the extreme case known as The Cliff—has pushed astronomers to propose an entirely new type of cosmic structure: black hole stars. These ...
Nov. 29, 2025 Nearly a century after astronomers first proposed dark matter to explain the strange motions of galaxies, scientists may finally be catching a glimpse of it. A University of Tokyo researcher ...
Nov. 26, 2025 Michigan State University astrophysicists are closing in on one of space science’s biggest mysteries: where the galaxy’s most energetic particles come from. Their studies uncovered a pulsar wind ...
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