Relativity is one of the most famous scientific theories of the 20th century, but how well does it explain the things we see in our daily lives?
Formulated by Albert Einstein in 1905, the theory of relativity is the notion that the laws of physics are the same everywhere. The theory explains the behavior of objects in space and time, and it can be used to predict everything from the existence of black holes, to light bending due to gravity, to the behavior of the planet Mercury in its orbit.
The theory is deceptively simple. First, there is no "absolute" frame of reference. Every time you measure an object's velocity, or its momentum, or how it experiences time, it's always in relation to something else. Second, the speed of light is the same no matter who measures it or how fast the person measuring it is going. Third, nothing can go faster than light.
The implications of Einstein's most famous theory are profound. If the speed of light is always the same, it means that an astronaut going very fast relative to the Earth will measure the seconds ticking by slower than an Earthbound observer will — time essentially slows down for the astronaut, a phenomenon called time dilation.
Any object in a big gravity field is accelerating, so it will also experience time dilation. Meanwhile, the astronaut's spaceship will experience length contraction, which means that if you took a picture of the spacecraft as it flew by, it would look as though it were "squished" in the direction of motion. To the astronaut on board, however, all would seem normal. In addition, the mass of the spaceship would appear to increase from the point of view of people on Earth.
But you don't necessarily need a spaceship zooming at near the speed of light to see relativistic effects. In fact, there are several instances of relativity that we can see in our daily lives, and even technologies we use today that demonstrate that Einstein was right. Here are some ways we see relativity in action.
1. Global Positioning System
In order for your car's GPS navigation to function as accurately as it does, satellites have to take relativistic effects into account. This is because even though satellites aren't moving at anything close to the speed of light, they are still going pretty fast. The satellites are also sending signals to ground stations on Earth. These stations (and the GPS unit in your car) are all experiencing higher accelerations due to gravity than the satellites in orbit.
To get that pinpoint accuracy, the satellites use clocks that are accurate to a few billionths of a second (nanoseconds). Since each satellite is 12, 600 miles (20, 300 kilometers) above Earth and moves at about 6, 000 miles per hour (10, 000 km/h), there's a relativistic time dilation that tacks on about 4 microseconds each day. Add in the effects of gravity and the figure goes up to about 7 microseconds. That's 7, 000 nanoseconds.
The difference is very real: if no relativistic effects were accounted for, a GPS unit that tells you it's a half mile (0.8 km) to the next gas station would be 5 miles (8 km) off after only one day. [Top 10 Inventions that Changed the World]
If you take a loop of wire and move it through a magnetic field, you generate an electric current. The charged particles in the wire are affected by the changing magnetic field, which forces some of them to move and creates the current.
But now, picture the wire at rest and imagine the magnet is moving. In this case, the charged particles in the wire (the electrons and protons) aren't moving anymore, so the magnetic field shouldn't be affecting them. But it does, and a current still flows. This shows that there is no privileged frame of reference.