About a century ago, in Berlin, a young clerk working at a patent office was formulating ideas that later revolutionized our view of the universe. His name was Albert Einstein. Einstein was on a quest to understand the fundamental workings of our universe and began by creating hypothetical real-life situations called thought experiments.
When he was just 16 years old, Einstein came up with a thought experiment that led to his big idea of spacetime continuum. It may sound daunting, but can be explained simply by his thought experiment.
At the time, physicists relied on the laws written by two physicists; Newton and Maxwell. Newton’s laws dictated the movement of objects when affected by forces such as gravity, and Maxwell’s laws codified electricity, magnetism, and light. Both Newton’s and Maxwell’s laws were widely accepted by physicists at the time and were very successful; both created large advancements in technology. Einstein, however, had a problem with these laws. According to his thought experiment, Newton’s and Maxwell’s laws could not coexist.
Einstein imagined running next to a light beam at its exact velocity; 671,000,000 m/h. Using reason and Newton’s concept of inertial frames (in which rectilinear velocities are simply added), Einstein assumed he could observe the light beam’s oscillating electric and magnetic fields, seemingly stationary. According to Maxwell, however, this would not be so. Maxwell firmly believed that the speed of light remains constant in any situation.
A very frustrated Einstein realized this contradiction and sought after a theory that could unify both Maxwell and Newton’s equations. It soon came when he brilliantly conceptualized yet another thought experiment. In this scenario, a man stands on a train station platform, looking onto the tracks. Suddenly, two lightning bolts strike the ground on either side of him, each equally distanced from his eyes. At the same time, a woman passes him in a train speeding at nearly the speed of light. The man on the platform sees both lighting strikes at once, however; the woman on the train experiences the strike closer to the front of the train a moment before the other. This is because the light from the “first” strike reaches her eyes sooner than the other, as a result of the train speeding toward its source. It has had a shorter distance to travel, allowing her to see it faster than if she were at rest. This introduced the idea of relative simultaneity. Because of movement through space, one observer was able to experience time differently than another observer. Space and time, therefore; go hand-in-hand.
Einstein’s train experiment is modelled in the Loedel diagram, which shows the effects of 4D spacetime in a block universe. The man on the platform is shown in green and the train passenger is shown in blue. You can see that the lightning strikes are both pinpointed on the same spot on the man’s timeline. On the woman’s timeline, however; the lightning strikes are found at different spots. This is because its placement has shifted as a result of the woman’s movement through space.
Einstein’s now widely accepted theory regarding the relationship between space and time is called special relativity. It corrected flawed mechanics by combining 3D space and 1D time into one 4D continuum. This theory was based on two postulates: the speed of light in a vacuum is invariant (Maxwell), and the laws of physics are invariant in all inertial systems (Newton). The logical outcome when considering both postulates was to come up with a 4D spacetime continuum.