How High Up Is That Place? How Far In
The Future Is That Event?

(continued)

(continued)

E. **Spacetime**

We have indicated that, if space and time have the kind of
interrelationship indicated by Minkowski's diagrams, anything that travels
at a particular finite speed (the speed of light in a vacuum) turns out to
have the same speed in *any* frame of reference. Einstein's
discovery was the reverse. Knowing from various experiments that the
speed of light in a vacuum evidently was the same in every frame of
reference, Einstein worked out mathematically how space and time have to
change with one's velocity in interrelated ways so that the speed of light
in a vacuum does stay the same. Some of these changes turn out to be,
for the velocities we're accustomed to traveling, too small to notice but
drastically and obviously different for much higher speeds.

Minkowski diagrams illustrate that space and time are not separate things, but are different features of a whole. As Minkowski put it, "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." Space and time are related somewhat as vertical and horizontal directions in space are related, in a single continuum now known as "spacetime". There are differences. Note how the vertical and horizontal directions change in the same direction in our "earth-and-balloon" diagrams, while the changes in space and time directions are opposite in the Minkowski diagrams. And we obviously perceive space quite differently from the way we perceive time. Nonetheless, the situations are similar. "Past" and "future" turn out to be opposite directions in spacetime, just as "up" and "down", or "right" and "left" are. And which way is "future" turns out to depend on one's frame of reference, just as which way is "up" depends on where one stands.

We have only introduced some key concepts of relativity theory here without going into all the reasoning that led to them, or the many experiments that confirm them. If you would like to learn about the theory in more detail, the references at the end of this article offer plenty of insight.

*Next article:* **E=mc ^{2} - What's the Speed
of Light Got to Do with It?**

**References, Links, and Comments:**

*The Universe and Dr. Einstein* by Lincoln Barnett

A journalist's account of Einstein's theories, including Special Relativity.

*The Evolution of Physics* by Albert Einstein and Leopold Infeld

Traces the main ideas of physics from Galileo to modern quantum theory in
four chapters. Relativity theory is one of the main subjects of
chapter three. Remarkably, this book only has two equations in
it: 2 x 2 = 4 and 3 x 3 = 9!

*Relativity and Common Sense* by Hermann Bondi

Einstein's relativity theory is often considered revolutionary, through
Einstein himself simply considered it a logical conclusion of the ordinary
theory of his own time. Bondi takes the same point of view, and
demonstrates how relativity is simply an extension of common-sense ideas.

*Relativity: The Special and the General Theory* by Albert
Einstein

Einstein's original paper on relativity, "On the Electrodynamics of
Moving Bodies", made use of calculus, but in this book Einstein showed
a way to arrive at the same conclusions using a more direct approach.
Incidentally, if you've ever wondered what uses there are for algebra and
the Pythagorean theorem, this book shows one of them.

"On
the Electrodynamics of Moving Bodies" [exit federal site] by Albert Einstein

English translation of the original paper in HTML. Links at the bottom
of the page to PDF version and to zipped PostScript and LaTeX versions.

*The Priniciple of Relativity* by A. Einstein, H.A. Lorentz, H. Weyl,
and H. Minkowski, with notes by A. Sommerfeld

Original articles on relativity, in English or English translation,
including Einstein's first two papers on Special Relativity, and Minkowski's
address on "Space and Time", in which he introduced his diagrams
to show the geometric relationship.

*The Meaning of Relativity* by Albert Einstein

Advanced mathematical treatment of both the Special and General Theories of
Relativity and Einstein's attempts to extend them.

The Light
Cone: an *illuminating* introduction to relativity [exit federal site] by Rob Salgado

We've only touched on the relationship between space and time in this
article. Salgado explains the relationship in more detail,
illustrating it with computer-animated versions of space-time diagrams, and
takes you step-by-step from our knowledge before Einstein through Special
Relativity to General Relativity.

Prepared by Dr. William Watson, Physicist

DOE Office of Scientific and Technical Information

Last Modified: 04/01/2005

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