Today, the term ‘space-time matrix’ is well known to scientists and laypeople alike. Until the early 20th century, however, almost everyone held the classical view that space and time are two absolute, independent entities. In 1915, Einstein’s general theory of relativity transformed that view forever. This article will explore how this radical transformation occurred.
For the vast majority of human history, almost no one conceived of space and time as mutable or interrelated. This is no accident. In our everyday lives, all events seem to occur in the context of three dimensional space and the one way arrow of time. Isaac Newton captured this view in his work The Principia when he wrote, “Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration.”
Newton’s sentiments aside, there does seem to be a profound connection between space and time. If your friend asks you to meet up at 8 pm, the next obvious question would be, where? Conversely, if your friend asks you to meet at a local restaurant, you would instinctively ask, at what time? Beyond these obvious links however, nothing in our everyday experience seems capable of altering space or time itself. The notion that space can curve like a fabric or the flow of time can be altered by a strong gravitational field seems bizarre and antithetical to common sense.
The first indication that the Newtonian theory of gravity was incomplete came from observations of the orbit of the planet Mercury. Astronomers noticed that the planet’s perihelion point, or location where Mercury is closest to the sun, gradually shifts over long spans of time. Nothing in Newtonian physics accounts for this anomaly. While mathematicians like Gauss had developed non-Euclidian systems of geometry by the early 19th century, concepts like spherical curvature seemed to apply to tangible bodies like the Earth and moon. No one seriously entertained the possibility that space itself might be curved.
When Albert Einstein published his Theory of Special Relativity in 1905, he purposely shied away from addressing gravity. Instead the theory focused on the speed of light as the maximum possible speed in the universe and its implications for the laws of physics. A decade later, Einstein published his Theory of General Relativity. One of its main themes, that gravity does not act as an invisible string but as a force field akin to electromagnetism, directly contradicted the Newtonian view of gravity as a "force acting at a distance.” This idea was vindicated by the solar eclipse of May 1919. Arthur Stanley Eddington, a proponent of Einstein’s theory, took photographs showing a shift in the predicted position of stars that became visible for a few minutes during totality, when the sun’s disk was completely covered by the moon.
It took many more years and experiments to demonstrate the time dilation effect predicted by General Relativity. Essentially, this idea states that the curvature of space affects the flow of time. In the absence of a strong gravitational field, time slows down (or dilates) from the perspective of a non-moving observer. In a thought experiment, two twins are separated at birth. One twin is placed on a rocket ship while the other is left on Earth. The rocket travels at close to light speed and returns to Earth a year later. Upon arrival back on Earth, the baby on the rocket ship would be one year old while his or her twin would be nearly 100. Although this sounds like science fiction, records from atomic clocks placed on moving aircraft compared to identical instruments left on the ground demonstrate that time dilation is real.
In spite of the progress science has made in understanding space and time, much more remains to be learned. When asked to define space and time, Einstein allegedly answered “space is what you measure with a ruler; time is what you measure with a clock.” At the end of the day, two mysteries (space and time) have been condensed into one (space-time).