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Learn Relativity

Understand why moving clocks run slow and how mass curves spacetime.

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Relativity is Einstein's theory of space, time and gravity, and it comes in two parts. Special relativity, published in 1905, starts from two postulates: the laws of physics are the same for all observers moving at constant velocity, and the speed of light is the same for all of them. From those follow time dilation, length contraction and E = mc^2. General relativity, completed in 1915, explains gravity as the curvature of spacetime by mass and energy. With Learnr you get a relativity course built around your background, delivered as short interactive lessons.

How to learn Relativity, step by step

Understand why relativity was needed. Start with Galilean velocity addition, then learn how Maxwell's equations predicted a fixed speed of light and how the Michelson-Morley experiment found no ether. That tension is the reason Einstein's postulates matter.
Derive time dilation with a light clock. Picture a photon bouncing between two mirrors on a moving train and use the Pythagorean theorem to derive the Lorentz factor. Deriving it once yourself makes the formula feel inevitable instead of mysterious.
Draw spacetime diagrams early. Plot time against distance with light rays at 45 degrees. Use them to resolve the relativity of simultaneity, the barn-and-pole puzzle and the twin paradox, which are all about whose frame you are in.
Learn the invariant interval and Lorentz transformations. Observers disagree on time and distance separately but agree on the spacetime interval. Practice transforming events between frames, and use muons reaching the ground from the upper atmosphere as a real test.
Move to energy, momentum and E = mc^2. Learn the energy-momentum relation and why nothing with mass reaches light speed. Apply rest energy to nuclear reactions and particle accelerators, where relativistic effects are everyday engineering.
Take a conceptual tour of general relativity. Learn the equivalence principle, geodesics in curved spacetime, and the evidence: Mercury's orbit, light bending seen in 1919, GPS clock corrections and gravitational waves detected by LIGO in 2015. Short Learnr reviews help these ideas stick.

What you'll learn

State Einstein's two postulates and explain how they lead to time dilation
Calculate time dilation and length contraction using the Lorentz factor
Draw and read spacetime diagrams, including the relativity of simultaneity
Explain mass-energy equivalence and relativistic momentum
Describe the equivalence principle and the evidence for general relativity

What's inside your Relativity course

A sample path Learnr builds for Relativity. Your real course adapts to your goal and level.

1. Galilean relativity and the problem with light
2. Einstein's postulates and the Lorentz factor
3. Time dilation, length contraction and simultaneity
4. Spacetime diagrams and the invariant interval
5. Relativistic energy, momentum and E = mc^2
6. The equivalence principle and curved spacetime

How long does it take to learn Relativity?

Special relativity at an introductory level takes about 3 to 6 weeks at 3 to 4 hours a week. A conceptual tour of general relativity adds a few weeks more. A mathematical treatment of general relativity with tensors is a graduate-level course that takes months after a strong physics and math foundation.

What you need before you start

Algebra, square roots and the Pythagorean theorem for special relativity, plus basic mechanics. The full math of general relativity needs calculus, linear algebra and differential geometry.

Common mistakes to avoid

Thinking time dilation is an illusion or a clock malfunction. Moving clocks genuinely record less elapsed time, as particle lifetimes and atomic clocks confirm.
Forgetting that every inertial observer sees the other's clock run slow. Paradoxes resolve once you track which observer changes frames.
Picturing curved spacetime only as a rubber sheet. The analogy shows bending of space but hides the curvature of time, which drives most everyday gravity.

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Relativity questions, answered

Can I learn relativity without advanced math?

Special relativity needs only algebra and the Pythagorean theorem, so it is very approachable. General relativity can be understood conceptually without math, but its equations require differential geometry.

What is the difference between special and general relativity?

Special relativity covers observers moving at constant velocity with no gravity. General relativity extends it to acceleration and gravity, describing gravity as curved spacetime.

Does relativity affect everyday technology?

Yes. GPS satellites must correct for both special and general relativistic clock effects, or position errors would build up quickly over a day.