Topics Physics and everyday science

What is the theory of relativity?

The theory of relativity is really two theories by Albert Einstein. Special relativity, from 1905, says the laws of physics are the same for everyone moving at a steady speed, and that the speed of light is the same for all of them. General relativity, from 1915, says gravity is not a pull across space but the bending of space and time by mass and energy.

What makes it interesting is how strange the consequences are, and how well they have held up. Moving clocks tick more slowly than stationary ones. Lengths and the order of some events depend on who is watching. Mass and energy turn out to be linked, which is the idea behind the famous equation E equals mc squared. Light bends around the Sun, and GPS satellites have to correct for these effects to stay accurate. Physicists also know relativity does not fit neatly with quantum physics, and that gap is still unsolved.

An episode on this topic, made by bre, whose hosts are AI and can be wrong, would start from the everyday puzzle that led to the theory and build up slowly, one concrete picture at a time. You can press Talk at any point and ask your own question, such as what exactly is bending, and a host will answer from what the episode covers.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. What does relative actually mean?Start with the older idea that motion is relative, like walking on a moving train, and why that was already part of physics before Einstein.
  2. The problem with the speed of lightLight's speed turned out to be the same for every observer, which clashed with ordinary intuition and forced a rethink of space and time.
  3. Special relativity and slowing clocksHow steady motion changes measurements of time and length, and why we never notice at everyday speeds.
  4. Mass, energy and E equals mc squaredWhat the equation says in plain terms: a small amount of mass corresponds to a very large amount of energy.
  5. General relativity: gravity as curved spacetimeWhy Einstein described gravity as the shape of space and time, and how this differs from Newton's picture while agreeing with it in most everyday cases.
  6. Tests, GPS and black holesStarlight bending, GPS clock corrections, gravitational waves and black holes: the evidence that the theory works, and where physicists know it is incomplete.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Tess. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, let's ask the question everyone's thinking. Relativity sounds like a thing you're supposed to nod at. So what is it, in one breath?

  2. TessAI host

    Two ideas from Einstein. Special relativity: space and time aren't fixed, they depend on how you're moving. General relativity: gravity is what curved space and time feel like.

  3. breAI host

    That's a lot of weight in one sentence. Let's start with a train. You toss a ball up inside a smoothly moving train, and it comes straight back down.

  4. TessAI host

    Right, and you can't tell you're moving without looking out the window. That part is old news. Galileo had it.

  5. breAI host

    So what did Einstein add that was new?

  6. TessAI host

    Light. Everyone measures it at the same speed, no matter how fast they're moving. Okay, but real talk, that should sound wrong. If I chase a beam, it should look slower.

  7. breAI host

    It should. And it doesn't. So something else has to give, and what gives is our assumption about time itself.

  8. TessAI host

    Which is where it gets fun. And I promise it ends somewhere useful, like your phone's map app.

Questions people also ask

What is the theory of relativity in simple terms?
It is Einstein's description of how space, time and gravity work. Special relativity says measurements of time and distance depend on your motion, with light's speed constant. General relativity says gravity is the curving of spacetime by mass and energy.
Is the theory of relativity proven?
Science does not prove theories absolutely, but relativity has passed many tests over a century, including starlight bending near the Sun, GPS timing and gravitational waves. It is very well established, though physicists know it must be reconciled with quantum physics.
What is the difference between special and general relativity?
Special relativity covers objects moving at constant speed and leaves gravity out. General relativity extends the ideas to acceleration and gravity, describing gravity as curved spacetime. It also gives the model used for black holes and the expanding universe.
Does relativity affect everyday life?
Yes, though you rarely notice. GPS satellites carry clocks that run slightly differently from clocks on the ground, and the system has to correct for both speed and gravity effects or its positions would drift. At walking or driving speeds, the effects are tiny.

Related topics

More: all 300 topics, physics and everyday science, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.