"Greg Egan - Foundations 1 - Special Relativity" - читать интересную книгу автора (Egan Greg)

Foundations
by Greg Egan

1: Special Relativity
Copyright ┬й Greg Egan, 1998. All rights reserved.



Anyone who reads science fiction will be familiar with some of the remarkable
predictions of twentieth-century physics. Time dilation, black holes, and the uncertainty
principle have all been part of the SF lexicon for decades. In this series of articles I'm
going to describe in detail how these phenomena arise, and along the way I hope to shed
some light on the theories that underpin them: special relativity, general relativity, and
quantum mechanics. The foundations of modern physics.
These articles are meant for the interested lay reader. If you can follow high
school algebra and geometry, and aren't afraid to take in a few new concepts тАФ which is
the whole point, after all тАФ nothing here should faze you.


Spacetime

The idea that we inhabit a four-dimensional spacetime is a very natural and intuitive one.
It's only because we take the duration of objects so much for granted that we tend to
gloss over it and refer to them as three-dimensional. Since most of the Earth's landscape
changes slowly, factoring out time from our mental models and paper maps is a very
pragmatic thing to do, but it's this unchanging space that we imagine for convenience
that's the abstract mental construct, not spacetime. Spacetime is simply what we live in,
all four dimensions of it.
Drawing a diagram of spacetime comes almost as naturally as making any other
kind of map; every historical timeline is halfway there, and placing a timeline for
Germany next to one for France, then sketching in the movement of armies between the
two, is as good a spacetime diagram as anything you'll find in particle physics. Of
course, a spacetime diagram in ink on paper has only two useful dimensions, so it
generally only shows time plus one dimension of space (though one more can be added,
using the standard techniques for drawing three-dimensional objects). Fortunately, many
problems in special relativity involve only one dimension of space; for example, a
spacecraft flying from here to Sirius would almost certainly travel along a straight line.
 Egan: "Foundations 1"/p.2




Figure 1 is a spacetime diagram for such a flight. Distances are in light years and
times are in years. For the sake of simplicity, the slight тАЬproper motionтАЭ of Sirius relative
to the Sun, and any orbital manoeuvres and planetary take-offs and landings by the
spacecraft, are ignored. The spacecraft accelerates at the start of the journey, shuts off its
engines and cruises for the middle stage, then decelerates at the end, bringing it to a halt
just as it arrives. (There's no special reason for all three stages to cover equal distances;
this is just one possible flight plan of many.) Given that the distance to Sirius is almost
nine light years, it's reasonable to treat stars and spacecraft alike as mere specks, tracing