We often think of scientists working in black and white: creating stringent experiments to test their hypotheses in controlled environments. But it turns out that Albert Einstein, one of the most influential scientists and thinkers of all time, worked in more conceptual terms. Einstein took a unique approach to his experiments, which he called “Gedankenexperiments,” the German term for “thought experiments.” Einstein used this process for creating his world-changing theory of relatively. If Gedankenexperiments sounds abstract, that’s because it purposefully is. In fact, in some cases, these thought experiments are actually impossible to conduct in real life. Here are four thought experiments Einstein used to lead to some of his greatest findings:

Chasing a beam of light

Einstein’s most famous Gedankenexperiments started early in his scientific career when he was still a young student. When Einstein was only 16 years old, he started thinking about what it would be like to chase a beam of light through space. Einstein’s line of logic started by thinking that if you could catch up to the light, you could see it frozen in space; however, he realized that it is impossible for light to freeze in space because that goes against its nature and it would cease to be light.
Einstein used this thinking to lay the foundation for the theory of relativity when he realized that because light must constantly be moving and can’t be slowed down, there had to be another change in the equation. Time could have different rates at different places in the universe, which would cause the beam of light to look different depending where you were relative to the light.
Another way to think of it is to consider what it would be like to watch someone chase a beam of light. To you as a bystander, it might look like the person was nearly caught up to the light, but to the person actually chasing the light, it could appear that they were never even close. This, Einstein concluded, is because the person’s brain and clock were running much slower than the beam of light. The light could never freeze, but it could appear differently depending on the speed of its chase.
Einstein himself credits this thought experiment to some of his best findings. He said, “The germ of the special relativity theory was already present in that paradox.”

Standing on a train

Another piece of the groundwork for the theory of relativity came when Einstein considered what would happen if lightning struck both ends of a train at the same time. If you were on the train, you would see the lightning closer to you first because the train is moving towards it and it has a shorter distance to travel. However, someone watching the train from the outside would see both lightning bolts striking at the same time, which proves that time moves differently depending if you are moving or standing still, matching Einstein’s belief that time and space are relative and don’t exist simultaneously. The train thought experiment relates to Einstein’s chasing light Gedankenexperiments and solidified the basis of his famous theory.

Falling elevator

Another of Einstein’s more famous thought experiments would be impossible to physically test, but it led to some of his greatest work. Einstein deduced that gravity and acceleration are the same thing by considering what would happen if you were in a box or elevator and couldn’t see anything outside the elevator. If you suddenly dropped to the floor, you wouldn’t be able to tell if the elevator cable had broken and gravity was pulling you down or if the elevator was going faster and acceleration was pulling you upwards.
Since these two actions produce the same result, Einstein concluded they must be the same thing and that within a local system it is impossible to differentiate between physical effects from gravity and from acceleration. Linking this to his previous findings, Einstein concluded that gravity can affect time and space, which led to his theory of relativity.

Rocket ship twin

Furthering Einstein’s belief of the relative nature of time and space, he considered what would happen if twins were born at the same time, but one twin was immediately put in a spaceship and sent to travel at nearly the speed of light.
Since time moves more slowly the closer you get to the speed of light, the twin in the rocket ship would age much more slowly than the twin on earth—the space twin could possibly be preschool age when the twin on earth is preparing to graduate from high school. This thought experiment shows that time can move at different speeds for long periods of time.
Einstein’s unique Gedankenexperiments set the stage for some of the largest modern scientific discoveries and changed the way we think about experimentation and the world around us.