A new way to think about space and time
In the early 1900s, a young patent clerk in Switzerland named Albert Einstein changed physics forever. His theory of relativity showed that space and time are not fixed and separate, as Isaac Newton had assumed. Instead, they stretch, shrink and bend depending on motion and gravity.
Relativity comes in two parts:
| Theory | Year | Main topic |
|---|---|---|
| Special relativity | 1905 | Objects moving at constant speeds, especially close to the speed of light |
| General relativity | 1915 | Gravity and acceleration |
Special relativity
Einstein based special relativity on two ideas:
- The laws of physics are the same for everyone moving at a steady speed. If you’re in a smooth train with no windows, no experiment can tell you whether you’re moving or still.
- The speed of light is the same for everyone. Whether you’re standing still or racing toward a flashlight beam, you always measure light at about 300 000 km/s.
The second idea seems impossible. If you throw a ball forward from a moving car, the ball’s speed adds to the car’s. But light doesn’t work that way. For the speed of light to stay the same for everyone, something else must change: space and time themselves.
Time dilation
Moving clocks run slow compared with clocks at rest. This is time dilation.
- At everyday speeds, the effect is tiny.
- At 87% of the speed of light, time passes half as fast.
- Astronauts on the International Space Station age slightly slower than people on Earth, by about 0.01 seconds over six months.
The twin paradox: if one twin travels near light speed to a distant star and returns, they’ll be younger than the twin who stayed home.
Length contraction
Objects moving very fast appear shorter in their direction of motion to an observer at rest.
The cosmic speed limit
Nothing with mass can reach the speed of light. The faster an object goes, the more energy it takes to speed it up further.
E = mc²
Special relativity also gave the world’s most famous equation:
E = mc²
- E = energy
- m = mass
- c = speed of light (300 000 000 m/s)
Because c² is enormous, a tiny amount of mass contains a huge amount of energy. This explains how the Sun shines (turning about 4 million tonnes of mass into energy every second) and how nuclear power plants work.
General relativity
General relativity describes gravity in a completely new way. Instead of a force pulling objects, Einstein said that mass and energy bend space-time.
Space-time combines the three dimensions of space with time. Picture a stretched trampoline. Place a bowling ball in the middle, and it makes a dip. Roll a marble nearby, and it curves around the dip. Planets orbit the Sun in the same way, following curves in space-time created by the Sun’s mass.
As physicist John Wheeler put it, matter tells space-time how to curve, and space-time tells matter how to move.
Predictions confirmed
| Prediction | Evidence |
|---|---|
| Light bends around massive objects | Starlight bent by the Sun was measured during the 1919 solar eclipse |
| Mercury’s orbit shifts slightly over time | Explained an old mystery Newton’s gravity couldn’t |
| Clocks run slower in stronger gravity | Measured with atomic clocks at different heights |
| Black holes exist | First image of a black hole’s shadow in 2019 |
| Gravitational waves | Ripples in space-time detected by LIGO in 2015 from colliding black holes |
Relativity in everyday life
GPS wouldn’t work without relativity. GPS satellites move fast (special relativity makes their clocks run slow) and are farther from Earth’s gravity (general relativity makes their clocks run fast). Overall, their clocks gain about 38 microseconds a day. Without correcting for this, GPS positions would drift by roughly 10 km every day!
Common mistakes
- Thinking "relative" means everything is a matter of opinion. The laws of physics and the speed of light are the same for everyone.
- Thinking relativity only matters in space. GPS and particle accelerators rely on it.
- Thinking Einstein proved Newton wrong. Newton’s laws are still excellent for everyday speeds; relativity extends them.
Try it yourself
- Stretch a sheet over a laundry basket. Place a heavy ball in the middle and roll marbles around it. How does this model gravity? What does it get wrong?
- Explain why GPS satellites need relativity corrections.
- If 1 gram of mass were fully turned into energy, how much energy would that be? (Use E = mc², with m = 0.001 kg.)
(Answer to 3: 9 × 10¹³ J, roughly the energy of a 21-kilotonne explosion)
Quick recap
- Special relativity: the speed of light is the same for everyone, so time and space change with motion.
- Moving clocks run slow; nothing with mass can reach light speed.
- E = mc² shows mass and energy are equivalent.
- General relativity: mass bends space-time, causing gravity.
- Relativity is confirmed by experiments and needed for GPS.