Introduction
The introduction of special relativity in 1905 was one of the first steps taken in blending abstract geometric concepts such as Lorentz's coordinate transformations, with physically meaningful, principles such as E = mc2. The constancy of the speed of light in all inertial reference frames meant that when you translate your measures of space and time intervals from one frame to the next, the new coordinates become a mixture of space and time units. The term spacetime was coined to express this intertwining of space and time in all descriptions of the physical world. As Hermann Minkowski once said of this,
"The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality." — Hermann Minkowski.
Minkowski, a mathematics teacher at the Swiss Federal Polytechnic school when Einstein was a student there, developed a variety of geometric techniques for intuitively coming to terms with Einstein's new theory of Special Relativity, and spacetime. As with all geometric discussions we may remember from our high school Geometry class, the three basic elements are a flat surface which mathematicians call a continuum, the point, and the line.
Four-Dimensional, Space-Time Continuum
A sketch of a spacetime diagram.
A sketch of a spacetime diagram.
What Minkowski realized was that the true arena for relativity was not 3 dimensional space with a dangling time coordinate tacked on as an afterthought, but time and space as coequal partners in a 4-dimensional continuum, which had its own geometric rules. Minkowski was the first to treat the motion of particles as trajectories or 'world-lines" in a 4-dimensional flat space that included 3-dimensions of space and 1 dimension of time: the 4-dimensional, space-time continuum.
This geometrical treatment by Minkowski was at first not received very enthusiastically by Einstein, but eventually he adopted many of Minkowski's methods in 1912 in the development of his new theory of general relativity. Minkowski and Einstein by the way weren't the first to recognize that the motion of bodies could be thought of in terms of the geometry in the fourth dimension. Lagrange, as long ago as 1796, had referred to classical mechanics as a problem 4-dimensional geometry.
Re-discovering the Fourth Dimension
The fourth dimension was also re-discovered by Charles Hinton in 1887 who published a book "What is the Fourth Dimension?" He speculated from various geometric analogies that 4-dimensional objects would appear as solid bodies in our 3-dimensional world. Atoms may actually be thin, 4-dimensional threads whose cross sections we can only perceive as microscopic, 3-dimensional bodies. He went on to speculate that,
"...it would probably be in the ultimate particles of matter that we should discover the fourth dimension, for [their] sizes in the three dimensions are very minute, and the magnitudes in all four dimensions would be comparable." — Charles Hinton.
Soon after this book appeared, H.G. Wells wrote the "Time Machine" in which the fourth dimension became time itself, not an independent spatial direction.
Time as Vital Coordinate
Many people today are still surprised to learn that time is the fourth dimension, thinking that somehow this concept is still theoretical. This is much like what happened during the 19th century when "atoms" and "molecules" were still considered by many to be only a theoretical construct. But let's think about this for a moment. A strictly three-dimensional world is pretty boring. Nothing happens in it. Suppose you tell your friend that you will meet her at the entrance to the Washington Monument. Your well intentioned instructions will help her narrow your location in the universe to a particular point on the surface of Earth, but unless you also say when to be there, the instructions are useless. Time is a vital fourth coordinate (or dimension) to our world. Without it, we would all be trapped in a perpetual Now, much like the frozen images captured on a photograph. It is obvious that only Space and Time taken together define the complete arena in which we live.
In fact, space and time form such an integral, cohesive framework for our existence that physicists since Albert Einstein refer to their combination as simply "spacetime."
What is Spacetime, Really?
Is it a "thing" that can be seen or felt, or carved-up? Popularizers love to describe spacetime as some kind of rubber sheet that can be ironed flat, or rolled-up into a ball. However, it is none of these things. Spacetime can no more be felt than can a thought, or the landscape of a dream. It is not a place in which we live, but a condition in which we exist. Unlike thoughts and dreams, however, it has a precise geometry and it is through this geometry that every action and event in the universe is defined.
In some ways, spacetime is to the physical world what "economics" is to money. You can no more divorce spacetime from the physical world than you can speak of money without invoking economics. Spacetime is vast. It extends well beyond the Earth and Solar System, encompassing the entire universe out to the farthest galaxy. Its indivisible time-like aspect also extends from the instant that the universe flashed into existence, through the present moment, and on into the future.
Where did Spacetime Come From?
Astronomers who study the universe have developed a detailed model of its evolution called the "Big Bang Theory." About 15 to 20 billion years ago, everything in the universe came into existence in an awesome explosion. What remains of the feeble light from the fireball of creation can still be detected by sensitive instruments as they peer into the depths of space. The magnitude of this event is truly mind boggling. Earthbound explosions begin with a bomb whose detonation sends debris flying out into a pre-existing 3-dimensional space. But in the Big Bang, not only did matter come into existence, but space and time as well!
Related EoC Articles
- Spacetime: Foundations
- Spacetime: Geometry
- Spacetime: Gravity
- Spacetime: Light Cones
- Spacetime: Special Relativity
External Links
- Charles Hinton's writings at ibiblio.org – "Ibiblio" Home to one of the largest "collections of collections" on the Internet, a conservancy of freely available information. It is a collaboration of the School of Information and Library Science and the School of Journalism and Mass Communication at The University of North Carolina - Chapel Hill.
- Charles Howard Hinton – Wikipedia.
- Further Reading - Learn More About Spacetime & General Relativity – Stanford University.
- Gravity Probe B - "Testing Einstein's Universe" – Stanford University.
- Hinton, Charles Howard (1853-1907) – The Internet Encyclopedia of Science, David Darling.
- Newtonʼs Gravity or Curved Spacetime? – NASA Gravity Probe B. (PDF)
- Spacetime – Wikipedia
- Spacetime: From the Greeks to Gravity Probe B – Stanford University.
- Spacetime Vortex: NASA's Gravity Probe B spacecraft has gathered all the data physicists need to check a bizarre prediction of Einstein's relativity.
Preview Image
An artist's concept of twisted space-time around Earth. (Source: Spacetime Vortex - NASA.)
Citation
Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Spacetime." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published January 6, 2008].
<http://www.cosmosportal.org/articles/view/138070/>
Contact Us
Send to a Friend
Comments
There are no comments.