Joachim Andersen

Accelerated Learning

The Theory of General Relativity: Why Gravity Is Curved Spacetime

A glowing gold sphere hovers over a golden thread grid that dips beneath it on a black marble table, beside an open antique pocket watch and small brass marbles, in a dark palazzo hall
Mass tells spacetime how to curve, and spacetime tells mass how to move.


The Theory of General Relativity is Albert Einstein’s explanation of gravity. Instead of a force pulling objects across empty space, gravity is the curving of space and time itself by mass and energy. Planets, light and even clocks simply follow the straightest possible paths through that curved spacetime, and those paths are what we experience as falling, orbiting and the passage of time.

The essence of The Theory of General Relativity is to better understand the universe and some of it’s key components, such as gravity, space and time.

One might also stretch this somewhat, in search of an analogy in order to apply it to challenges in your daily life: “What looks like a force acting on me might really be the shape of the environment I’m moving through. So, what shape is the environment that I’m in?”


Origin: The Happiest Thought

In 1905, Einstein’s special relativity showed that space and time are relative to the observer, but it left gravity out. In 1907, while working at the patent office in Bern, he had what he later called the happiest thought of his life: a person falling freely from a roof would not feel their own weight. Being in free fall and floating far from any planet are, from the inside, the same experience.

That insight became the equivalence principle, and it took Einstein eight more years of struggle, detours and difficult mathematics to turn it into a full theory. In November 1915, he presented the field equations to the Prussian Academy of Sciences, and he published the complete theory in 1916. The physicist John Wheeler later summed it up in one line: spacetime tells matter how to move, and matter tells spacetime how to curve.


Key Concepts

  • Gravity Is Geometry
    Mass and energy bend spacetime. An apple falls and the Moon orbits not because something tugs on them, but because they follow the natural paths of the curved space around Earth.

  • Falling Feels Like Floating
    Gravity and acceleration are locally indistinguishable. This is why astronauts in orbit are weightless: they are not beyond gravity, they are falling around Earth all the time.

  • Time Runs Slower Deeper Down
    The stronger the gravity, the slower clocks tick. This is called gravitational time dilation, and it means my feet age a tiny bit more slowly than my head.

    You can imagine that a heavy object (e.g. a planet, or a star such as our sun) creates a larger dent in the fabric of spacetime (much like a heavy ball on a stretched out carpet), and down in this dent/well of spacetime time moves slower (imagine time taking more time to reach down into the spacetime dent/well).

  • Spacetime Can Ripple
    When massive objects accelerate, they send waves through spacetime itself, moving at the speed of light. Einstein predicted these gravitational waves in 1916, but doubted they could ever be detected.

  • Newton Still Works, Mostly
    For everyday speeds and weak gravity, General Relativity gives almost exactly the same answers as Newton. The differences only matter where gravity is strong, distances are vast or precision is extreme.


Some Examples

  • Light Climbing a Harvard Tower
    In 1959, Robert Pound and his graduate student Glen Rebka sent gamma rays 22.5 meters up and down a tower at Harvard’s Jefferson Laboratory. Rays traveling upward lost a tiny amount of energy and rays traveling downward gained some, matching Einstein’s prediction. It was the first time gravity’s effect on light and time was measured in a laboratory, an experiment now known as the Pound–Rebka experiment.

  • The Satellites That Would Get Lost
    The clocks on GPS satellites run faster than clocks on the ground by about 45.8 microseconds a day because they sit higher in Earth’s gravity, while their speed slows them by about 7.2 microseconds. The net gain of about 38 microseconds a day sounds trivial, but without correction, positions would drift by around 11 kilometers every day. Every time I use a map on my phone, I’m relying on Einstein.

  • A Chirp From a Billion Years Ago
    On 14 September 2015, the two LIGO detectors in Washington and Louisiana picked up a signal lasting about 0.2 seconds. It came from two black holes, each around 30 times the mass of the Sun, merging more than a billion light years away and converting about three solar masses into gravitational waves. The ripple stretched the detectors by a fraction of the width of a proton, and the discovery earned Rainer Weiss, Barry Barish and Kip Thorne the 2017 Nobel Prize in Physics.

  • Clocks on a Step Stool
    In 2010, physicists at the US National Institute of Standards and Technology compared two ultra-precise optical clocks and raised one by about 33 cm. The higher clock ticked measurably faster. Gravity bending time is not just a cosmic phenomenon, it happens between the floors of a building, or between a desk and a shelf.


Applying The Theory of General Relativity

When I want to use this mental model outside physics, this is the simple process I follow:

  1. Question What Feels Absolute
    Space and time felt fixed for centuries until Einstein showed they bend. Try to ask which of my own “obvious” assumptions might only be true from where I stand.

  2. Look at the Shape, Not Just the Force
    When people or organizations keep drifting in one direction, look at the incentives, structures and environment that curve their paths, rather than blaming a single push.

  3. Change Frames
    Einstein’s breakthrough came from imagining the world from the point of view of a falling person. Try to deliberately look at problems from other vantage points, because the same event can look completely different from another frame of reference.

  4. Respect Small Errors at Scale
    The 38 microseconds a day that would ruin GPS remind me that tiny inaccuracies can grow into large failures when a system runs long enough or relies on precision.

  5. Use the Simplest Model That Works
    Engineers still use Newton to build bridges. Try to limit the complexity of your thinking to the problem at hand, only reaching for next (advanced) model when the simple one breaks (gently ramping up in model complexity).

  6. Let Bold Predictions Be Tested
    General Relativity earned trust by making risky predictions that could have failed, like the 1919 eclipse test I describe in Falsifiability. I strive to state my own ideas in ways that reality can prove wrong.


Pitfalls and Limitations

  • It doesn’t fit with quantum mechanics. General Relativity describes the very large and quantum mechanics describes the very small, but the two break down when combined, at the center of black holes or at the very beginning of the universe. A theory of quantum gravity remains one of the biggest open problems in physics.

  • Metaphors can mislead. The popular image of a ball on a rubber sheet helps, but it uses gravity to explain gravity and hides the fact that time is curved as well as space.

  • There are still puzzles. To match what we observe, cosmologists add dark matter and dark energy, which together seem to make up about 95% of the universe and which nobody has yet directly detected.

  • It’s easy to overstretch. “Everything is relative” is a misreading. General Relativity is built on laws that are the same for every observer, so don’t use it as e.g. an excuse for relativism in ethics or opinions.


Related Mental Models

The Theory of General Relativity help me better understand how gravity works through the dents they make in the fabric of spacetime, which helps me grasp gravity, space and time much more intuitively.

If stretching the analogy somewhat, it also reminds me what feels like a force can be the shape of the world I am moving through, and that even time depends on where I stand. By questioning what seems absolute, changing my frame of reference and respecting small errors, I can attempt to see situations more clearly and hopefully make slightly better decisions.



To learn more about mental models and philosophy,
check out: Wise: How to Attain a Deep Understanding of the World, History, Our Place in It, and How to Live Well



📚 Sources and References