What is space-time?

Space has three dimensions and time has before and after.


➥ A hundred years ago today Albert Einstein published his General Theory of Relativity—a brilliant, elegant theory that has survived a century, and provides the only successful way we have of describing space-time.

➥ Einstein’s theory of special relativity created a fundamental connection between space and time. The universe can be viewed as having three space dimensions — up/down, left/right, forward/backward — and one time dimension (generally seen ‘space-time’ as a combined dimension). This 4-dimensional space is referred to as the space-time continuum.

➥ If you move fast enough through space, the observations that you make about space and time differ from the observations that other person, who are moving at different speeds, make.


➥ Einstein’s theory of special relativity has had far-reaching implications, but it has left open certain questions that string theory hopes to answer. It has altered our understanding of time and space. It provides a theoretical framework that tells us how gravity works,

➥ String theory introduces many more space dimensions, so knowing how the dimensions in relativity work is a crucial starting point to understanding some of the confusing and amazing aspects of string theory.

➥ Albert Einstein’s theory of general relativity describes gravity as a geometric property of space and time. The more massive (heavier)  an object, the greater its distortion of space-time, and that distortion is felt as gravity.





➥ The reason for this space-time link comes from applying the principles of relativity and the speed of light very carefully. The speed of light is the distance light travels divided by the time it takes to travel its path, and all observers must agree on this speed.Sometimes,  different observers disagree on the distance a light beam has traveled, depending on how they are moving through space. This means that to get the same speed those observers must disagree about the time the light beam travels the given distance.


➥ You picture this for yourself by understanding the thought experiment depicted in given figure. Imagine that you are on a spaceship and holding a laser so it shoots a beam of light directly up, striking a mirror you’ve placed on the ceiling. The light beam then comes back down and strikes a detector.






➥ However, the spaceship is traveling at a constant speed of half the speed of light. According to Einstein, this makes no difference to you — you cannot even tell that you are moving. However, if astronaut Newton were spying on you, as in the bottom of the figure, it would be a different story.

➥ Newton would see your beam of light travel upward along a diagonal path, strike the mirror, and then travel downward along a diagonal path before striking the detector. In other words, you and Newton would see different paths for the light and, more importantly, those paths aren’t even the same length.

➥ This means that the time the beam takes to go from the laser to the mirror to the detector must also be different for you and Newton so that you both agree on the speed of light.
This phenomenon is known as time dilation, where the time on a ship moving very quickly appears to pass slower than on Earth. In some ways this aspect of relativity can be used to allow time travel. In fact, it allows the only form of time travel that scientists know for sure is physically possible.

➥ As strange as it seems, this example (and many others) demonstrates that in Einstein’s theory of relativity, space and time are intimately linked together. If you apply Lorentz transformation equations, they work out so that the speed of light is perfectly consistent for both observers.

“The important thing is to be able to make predictions about images on the astronomers photographic plates, frequencies of spectral lines, and so on, and it simply doesn’t matter whether we ascribe these predictions to the physical effects of gravitational fields on the motion of planets and photons or to a curvature of space and time.”  – Steven Weinberg