In 1905, a 26-year-old patent clerk in Bern, Switzerland, published a paper that changed our understanding of space and time. Ten years later, he extended his ideas into a new theory of gravity. Albert Einstein's theories of relativity revealed that time can pass at different rates, that space and time are woven together, that mass and energy are equivalent and that gravity is the curvature of spacetime itself. These ideas sound strange, but they have been confirmed by more than a century of experiments and are built into technologies we use every day, from GPS to particle accelerators. This explainer introduces the key ideas of relativity in plain language.
Before Einstein
In the seventeenth century, Isaac Newton described motion and gravity with laws that worked extraordinarily well. In Newton's view, space was a fixed stage and time ticked uniformly everywhere, the same for everyone.
In the nineteenth century, James Clerk Maxwell showed that light is an electromagnetic wave traveling at a specific speed, about 186,000 miles (300,000 kilometers) per second. This raised a puzzle: speed relative to what? Physicists assumed light traveled through an invisible medium called the "ether." But in 1887, Albert Michelson and Edward Morley performed a precise experiment to detect Earth's motion through the ether and found nothing. Light seemed to travel at the same speed in every direction.
Special relativity
Einstein's special theory of relativity, published in 1905, is built on two simple principles:
- The laws of physics are the same for everyone moving at a constant speed. There is no experiment you can do inside a smoothly moving train, with no windows, to tell whether it is moving or standing still.
- The speed of light in a vacuum is the same for all observers, regardless of how fast they or the light source are moving.
The second principle is deeply surprising. If you are on a train moving at 50 miles per hour and throw a ball forward at 20 miles per hour, someone on the platform sees the ball moving at 70 miles per hour. But if you shine a flashlight forward from the train, both you and the person on the platform measure the light traveling at exactly the same speed. For this to be true, something else must change: our measurements of time and space.
Time dilation
One consequence of special relativity is time dilation: a moving clock runs slower than a stationary one, as measured by the stationary observer.
Imagine a "light clock" in which a pulse of light bounces between two mirrors. For someone moving with the clock, the light simply goes up and down. For someone watching the clock move past, the light travels a longer, diagonal path. Since the speed of light is the same for both observers, the light takes longer to complete each tick for the outside observer. The moving clock appears to run slow.
At everyday speeds, the effect is incredibly tiny. But it becomes dramatic as speeds approach the speed of light:
| Speed as fraction of light speed | Time passes for the traveler compared with someone at rest |
|---|---|
| 10 percent | about 99.5 percent as fast |
| 50 percent | about 87 percent as fast |
| 90 percent | about 44 percent as fast |
| 99 percent | about 14 percent as fast |
Real evidence
- Muons are particles created when cosmic rays strike the upper atmosphere. They decay quickly, so most should disappear before reaching the ground. Yet many reach Earth's surface, because they travel close to the speed of light and their internal clocks run slow from our point of view.
- In 1971, scientists flew atomic clocks around the world on commercial airplanes and compared them with clocks on the ground. The differences matched Einstein's predictions.
- Particle accelerators routinely observe longer lifetimes of fast-moving unstable particles.
Length contraction and the speed limit
Special relativity also predicts length contraction: objects moving at high speeds appear shorter in the direction of motion to a stationary observer.
Another consequence is that nothing with mass can reach or exceed the speed of light. As an object speeds up, it takes more and more energy to accelerate it further. Reaching the speed of light would require infinite energy. The speed of light is the universe's speed limit for matter, energy and information.
E = mc²
Perhaps the most famous equation in the world comes from special relativity: E = mc². It states that energy (E) equals mass (m) multiplied by the speed of light (c) squared.
This means that mass and energy are two forms of the same thing, and a small amount of mass corresponds to an enormous amount of energy, because the speed of light squared is a huge number. One gram of matter, completely converted into energy, would release about as much energy as a large power plant produces in a day.
E = mc² explains how the Sun shines: in its core, hydrogen nuclei fuse into helium, and the resulting helium has slightly less mass than the original hydrogen. The missing mass is released as energy. It also explains the energy released in nuclear power plants and nuclear weapons.
General relativity: gravity as curved spacetime
Special relativity applies to objects moving at constant speeds. Einstein then spent ten years developing a theory that included acceleration and gravity. He published the general theory of relativity in 1915.
The equivalence principle
Einstein's key insight came from imagining a person in a closed elevator. If the elevator is at rest on Earth, the person feels their weight. If it is far out in space, accelerating upward at the right rate, the person feels exactly the same. No experiment inside the elevator could tell the difference. Einstein concluded that gravity and acceleration are equivalent.
Spacetime and curvature
In general relativity, space and time are combined into a four-dimensional fabric called spacetime. Mass and energy curve spacetime, and objects move along the straightest possible paths through this curved spacetime. What we experience as gravity is the effect of this curvature.
A common analogy is a heavy ball placed on a stretched rubber sheet. The ball creates a dip, and smaller balls rolling nearby curve toward it. The analogy is imperfect, because it uses gravity to explain gravity, and spacetime curvature includes time as well as space, but it captures the idea. The physicist John Wheeler summarized it this way: "Spacetime tells matter how to move; matter tells spacetime how to curve."
Predictions of general relativity
General relativity made several bold predictions, all of which have been confirmed.
Mercury's orbit
The orbit of Mercury slowly shifts over time in a way that Newton's gravity could not fully explain. General relativity accounted for the difference precisely.
Bending of light
General relativity predicted that light passing near a massive object would bend. In 1919, during a total solar eclipse, expeditions led by Arthur Eddington measured the positions of stars near the Sun and found that their light was bent by the amount Einstein predicted, making him world famous. Today, astronomers observe gravitational lensing, in which massive galaxies and clusters bend and magnify the light of more distant objects.
Gravitational time dilation
General relativity predicts that time runs slower in stronger gravity. Clocks closer to Earth's surface tick slightly slower than clocks higher up. This effect has been measured with atomic clocks separated by just a few centimeters in height.
GPS
GPS satellites must account for both special and general relativity. Their clocks run slower because of their speed and faster because they experience weaker gravity. The net effect is that satellite clocks gain about 38 microseconds per day. Without corrections, GPS positions would drift by several miles each day.
Gravitational waves
General relativity predicted that accelerating massive objects create ripples in spacetime called gravitational waves. In 2015, the LIGO detectors in the United States detected gravitational waves from two black holes merging about 1.3 billion light-years away, a discovery that earned the 2017 Nobel Prize in Physics.
Black holes and the expanding universe
General relativity predicts black holes, regions where spacetime is so curved that nothing can escape, and its equations describe an expanding universe, the basis of modern cosmology and the Big Bang model.
Relativity's limits
General relativity works extremely well for large objects and strong gravity, while quantum mechanics describes the behavior of particles at very small scales. The two theories are built on different principles and do not fit together at the centers of black holes or at the very beginning of the universe. Finding a theory of quantum gravity that unites them is one of the biggest challenges in physics.
Questions readers ask
What is the difference between special and general relativity? Special relativity deals with objects moving at constant speeds and the constant speed of light. General relativity includes acceleration and describes gravity as the curvature of spacetime.
Does time really slow down? Yes. Moving clocks and clocks in stronger gravity tick slower, as measured by other observers. The effect has been measured many times.
What does E = mc² mean? Mass and energy are equivalent, and a small amount of mass corresponds to a large amount of energy.
Can anything travel faster than light? Nothing with mass can reach or exceed the speed of light in a vacuum.
Does relativity affect daily life? Yes. GPS relies on relativity corrections, and relativity is essential for understanding nuclear energy, particle accelerators and astronomy.
In short
Einstein's special relativity showed that the speed of light is the same for everyone, which means that time and space are not absolute: moving clocks run slow, moving objects contract and mass and energy are equivalent through E = mc². General relativity described gravity as the curvature of spacetime by mass and energy, predicting the bending of light, gravitational time dilation, gravitational waves and black holes. Confirmed by countless experiments and used in technology like GPS, relativity transformed our understanding of the universe.