Tip:
Highlight text to annotate it
X
Relativity is just a method for two people to agree on
what they see if one of them is moving.
And since we all move about pretty regularly, we can find many examples
of how useful relativity is in everyday life — even if we don’t call it by name.
One miracle of modern life is the Global Positioning System (GPS).
It is pretty amazing that a GPS can pinpoint your location
anywhere on Earth to within a few yards.
And this magic depends entirely on the existence of two dozen satellites
12,000 miles above the Earth ... and a little Relativity.
Briefly, here is how it works.
The GPS receiver gets a timing-signal from several of these high-flying satellites,
and using Einstein’s Theory of Relativity, it calculates its distance from each satellite.
Throw in a little triangulation and out comes your location.
Simple in concept, but to do this successfully,
the timing-signals must be accurate to within a few billionths of a second
so that the distance calculations can be accurate to a few yards.
But with all this motion going on,
time and distance must be reconciled carefully.
Without Einstein’s version of Relativity, the accuracy
of the Global Positioning System would drift by more than seven miles every day.
But of course relativity was not a new concept with Einstein.
The problem of how two people reconcile
their observations about the world if one of them
is moving has been addressed for centuries.
Let’s ease our way into Relativity with some common experiences ...
If you are traveling in a car on a smooth,
straight stretch of highway,
there is no sensation of motion at all.
You can easily read a book,
pour a drink, or flip a coin ...
and everything looks and feels the same as if the car were sitting still.
That’s because relative to the car — you, the book, the drink and the coin are not moving.
Notice that this works only if the car is not changing direction or speed.
If the car turns, or accelerates,
pouring that drink is a real challenge.
But constant motion feels just like sitting still.
And if you want to know what it feels like to move
at thousands of miles an hour, just look around.
Because of the Earth’s spin, we zip along
with our time-zone at a speedy 1000 mph.
And because of its motion around the Sun,
the Earth carries us through space at about 67,000 mph.
And because of the motion of our solar system about the center of our galaxy
we are moving at more than half a million miles an hour.
But it is not enough to ask: “How fast am I moving?”
We must ask: How fast am I moving relative to some other thing. �