When an observer has a high velocity (comparable to the speed of light), laws of physics are a little bit different. The observer sees the object smaller than a motionless observer: that’s what we call “length contraction”. We can measure the length of the object in a high speed referential using the following equation:
$$L = \frac{L0}{\gamma}$$
Here, \gamma$ describes how fast the referential is.
Twin paradox:
Let’s suppose that two 20 years old identical twins sign up for a special relativity experience: one will board a fast spaceship for a trip to a star (located 4 light years from Earth), while the other will happily stay on Earth. At the end of the trip, the astronaut twin will be 4 years younger. How is that possible?
We learned in the previous simulation that an observer with a very high velocity will see objects smaller than a motionless observer. This principle also applies on time: it passes slower in a high-velocity referential. Therefore, the astronaut twin won’t feel the same time interval as the one seen by the twin on Earth.
We call it “twin paradox” since it depends on the referential (the observer) we are considering. In the astronaut twin referential, the spaceship does not move; instead, the Earth is going backward at the same speed of the spaceship. Following this logic, the twin on Earth should be younger than the astronaut twin!
We can solve this paradox by observing the direction and speed switch when the spaceship reaches the star. To do so, the spaceship must undergo an acceleration; this is the solution to the paradox.