From Canada's Perimeter Institute for
Theoretical Physics, welcome to Perimeter Inspirations,
classroom videos that investigate the
frontiers of science.
Join host and physicist Damian Pope of Perimeter's outreach
team, and leading scientists as they take you on an educational
journey of wonder and discovery.
It's a navigational tool used by millions of people every day.
With it drivers know where they're going,
pilots fly planes more safely, it helps construction workers
build straighter roads, farmers plant fields more efficiently,
because of it, golfers choose better clubs,
skiers find faster ways down mountains.
The uses for this innovative tool are virtually unlimited;
it's called the Global Positioning System,
better known as GPS.
So exactly how does this incredible tool work?
So image you're on a field and you wanted
to triangulate where you were.
Because you know the positions
of things in the distance, you can see
by measuring the direction to one object
to another object, you can figure out where you would
have to be in the field.
Well GPS is like that but it's larger.
It's in all three dimensions, not only where in the field
but how high and also it's measuring time.
You have four satellites that are sending you signals
and you're measuring accurately where the satellite appears to
be and when the signals were sent and from that you can
figure out where you are on the Earth's surface.
The Global Positioning System is a network of over 30
satellites orbiting 20,000 kilometres above us.
They move at a speed of 14,000 kilometres per hour.
Each satellite follows one of six orbits arranged so at
least four satellites are visible from any point on Earth.
The satellites constantly transmit signals easily
picked up by anyone with a GPS receiver.
Each signal contains information on where the satellite is and
what time the signal was sent.
Using this information the receiver calculates its
distance from the satellite.
This is done by multiplying the signal speed,
the speed of light by the time the signal traveled.
The receiver then repeats this procedure for three more
satellites narrowing down its location within a few metres.
To achieve this incredible accuracy the timing information
from the satellites must be extremely precise.
So inside each GPS satellite is an atomic clock,
the most accurate timing device ever created.
The GPS is so precise it must take into account
a number of subtle effects.
Some of these are predicted by Einstein's theory of relativity,
an idea that revolutionized our understanding of the universe.
This theory includes special relativity
and general relativity.
Special relativity is a theory of space, time and motion.
Key to it is the fact that motion alters time.
We see time running slower for a clock that's
moving relative to us.
Satellite based GPS clocks are moving past us at
14,000 kilometres per hour.
Because of special relativity they gradually fall
behind clocks in Earth based receivers at a rate of
seven microseconds per day.
After Einstein formulated the theory of special relativity
he went further ahead and he proposed his theory of general
relativity, which is not only a theory of space time but
it's a gravitational theory.
Key to Einstein's theory of general relativity is the
fact that gravity alters time.
Clocks further away from Earth where gravity is weaker run
faster than clocks closer to Earth where gravity is stronger.
The GPS satellites are located 20,000 kilometres
above Earth where gravity is much weaker than on the surface.
Due to general relativity satellite clocks run 45
microseconds faster daily.
When we combine the effects of special and general relativity,
satellite clocks run 38 microseconds fast every day.
This may not seem like much but if uncorrected all GPS
measurements would be off by 11 kilometres daily.
The effects of relativity on Earth are normally
one part in 10 billion.
And so that's an incredibly small effect and
you're used to thinking you can forget it for most things,
but when you're relying on how far light travels in a
particular amount of time, if you're wrong by even
microseconds then the distances that you're getting
wrong are that much larger.
To correct this error, engineers adjusted the atomic
clocks inside the satellites so on Earth they run slower
by 38 microseconds per day.
This compensated for the effects of relativity and
meant once in orbit the clocks ran accurately giving us the
powerful tool we have today.
So as you see what began as an abstract theory over
one hundred years ago, is now used in every day technology.
Every time you use GPS you are using relativity.
Whether you're heading out to meet friends,
trying to find the nearest movie theatre or looking
for a restaurant in another country, you'll always know
where to go using GPS.
So relativity is not just something to study in school,
it actually affects daily events in people's lives, your life.
It's very amazing that after they were developed,
they became such an important factor in designing GPS'
and navigation systems, and basically we would not be
able to do them if we didn't know much about relativity.
On one hand you might think of Einstein's theory of general
relativity, this theory that space and time are warped,
you live in a four dimensional space and time,
you know you might think that that's the part of physics
that's furthest away from day to day life and yet it shows up in
this fundamental way, in GPS technology which is crucial
to everyone's day to day life at this point.
Theoretical physics, closer than you think.