LET'S ME START TODAY
   BY SHOWING YOU


THE LOOP DEMONSTRATION
YOU'VE ALREADY SEEN BEFORE.


I HAVE HERE,


JUST LOOK A WIRE
CONNECTED TO THE GALVANOMETER


WHICH TELLS US
WHETHER AN ELECTRIC CURRENT


IS RUNNING AROUND
IN THIS LOOP.


THE LASER BEAM,
THE SPOT ON THE SCREEN,


WILL MOVE WHENEVER
AN ELECTRIC CURRENT RUNS


IN THIS LOOP.


TO MAKE A CURRENT
RUN THROUGH THIS WIRE,


I TAKE 
THIS ORDINARY BAR MAGNET


AND MOVE IT INTO THE LOOP
LIKE THAT


OR OUT OF THE LOOP


OR MOVE THE LOOP
INTO THE MAGNET


AND BACK LIKE THAT.


THAT MAKES
A CURRENT FLOW.


THAT WAS ONE
OF THE GREAT DISCOVERIES


OF THE 19th CENTURY.


BY NOW, WE'VE MASTERED 
ALL THE FORMIDABLE MACHINERY


OF ELECTRICITY AND MAGNETISM.


WE KNOW
HOW TO DESCRIBE THEM,


SO LET'S DESCRIBE
HOW THAT EXPERIMENT WORKS.


A CURRENT FLOWS
THROUGH THE WIRE.


ELECTRIC CHARGES
INSIDE THE WIRE


ARE SET INTO MOTION,


WHICH MEANS SOMETHING
APPLIED A FORCE TO THEM.


THE FORCE
ON THE ELECTRIC CHARGE


CAN EITHER BE EQUAL TO
TIMES AN ELECTRIC FIELD,


IF THERE'S ONE PRESENT,


OR IT COULD BE EQUAL TO
THE CHARGE


TIMES THE VELOCITY
OF THE CHARGE,


CROSSED INTO
THE MAGNETIC FIELD.


EITHER WOULD MAKE
THE CHARGE MOVE.


THAT'S OUR EQUATION.


LET'S SEE HOW WE USE IT


TO DESCRIBE THE EXPERIMENT
WE JUST SAW.


THERE ARE TWO POSSIBILITIES.


ONE, THE LOOP
IS HELD STATIONARY,


AND I MOVE THE MAGNET
INTO THE LOOP.


THAT CAUSES
THE CURRENT TO FLOW.


IN THAT CASE,


THE CHARGES IN THE LOOP


WERE NOT THEMSELVES
IN MOTION,


SO IT CAN'T HAVE BEEN
THE BAR MAGNET'S MAGNETIC FIELD


THAT MADE THEM MOVE.


BUT SINCE THEY DID MOVE,


THERE THEREFORE
WAS AN ELECTRIC FIELD.


WE CONCLUDE
THAT A CHANGING MAGNETIC FIELD,


MOVING THE BAR MAGNET,


CREATED
AN ELECTRIC FIELD.


THAT WAS FARADAY'S
GREAT DISCOVERY


OF ELECTROMAGNETIC INDUCTION
IN THE 19th CENTURY,


AND THAT'S ONE EXPLANATION
OF THAT EXPERIMENT.


NOW, THERE'S ANOTHER
COMPLETELY DIFFERENT,


INDEPENDENT EXPLANATION,


AND THAT GOES THIS WAY.


SUPPOSE INSTEAD
I HOLD THE BAR MAGNET STATIONARY


AND I MOVE THE LOOP.


OF COURSE, EXACTLY
THE SAME THING HAPPENS,


BUT WE HAVE NO MOVING MAGNET,


NO CHANGING MAGNETIC FIELD.


INSTEAD, THE CHARGES
IN THE LOOP ARE MOVING


BECAUSE I'M MOVING
THE LOOP.


THEY HAVE THE VELOCITY


THIS VELOCITY CROSSED INTO
THE BAR MAGNET'S MAGNETIC FIELD


GIVES US A FORCE WHICH CAUSES
THE EXPERIMENT WE JUST SAW.


SO THOSE TWO 
DIFFERENT PHENOMENA--

LOOP STATIONARY
AND MAGNET MOVING


AND MAGNET STATIONARY
AND LOOP MOVING--


ARE ACTUALLY
TWO COMPLETELY DISTINCT,


INDEPENDENT PHENOMENA


THAT HAVE COMPLETELY
DIFFERENT EXPLANATIONS.


WHEN ALBERT EINSTEIN
SAW THAT, HE SAID,


"LOOK, GUYS,
YOU'VE JUST GOT TO BE KIDDING.


ANY YO-YO CAN SEE THAT
THOSE TWO ARE THE SAME THING."


SO IT WAS
THIS SIMPLE LITTLE EXPERIMENT


THAT WAS THE STARTING POINT
OF THE THEORY OF RELATIVITY,


NOT THE MICHELSON-MORLEY
EXPERIMENT,


NOT SOME EXOTIC EXPERIMENT

TO DETECT THE EARTH'S MOTION
THROUGH THE ENTER,


BUT THIS SIMPLE
LITTLE PHENOMENON


THAT OF COURSE
EVERYBODY KNEW ABOUT,


BUT WHICH DISTURBED NOBODY ELSE
EXPECT ALBERT EINSTEIN.


WHAT DISTURBED EINSTEIN


WAS NOT THAT WE HAD DIFFICULTY
EXPLAINING THESE PHENOMENA.


THIS EQUATION EXPLAINS THEM
PERFECTLY IN EVERY CASE.


WHAT DISTURBED HIM
WAS THE LACK


IN THE INNER PERFECTION
OF THE THEORY.


WHAT HE DID IN RESPONSE
WAS TO PRODUCE A THEORY,


THE SPECIAL THEORY
OF RELATIVITY,


WHICH HAD JUST THAT KIND
OF INNER PERFECTION,


AND IT'S THAT THEORY


THAT WE'RE IN THE MIDST
OF STUDYING RIGHT NOW.


HISTORIANS RECALL
THE TURN OF THIS CENTURY


AS A REVOLUTION.


NO MATTER THE FIELD,


FROM AVIATION
TO ANTHROPOLOGY TO ART,


DEPARTURE FROM THE NORM
WAS RADICAL ON ALL FRONTS.


AND THE YEAR 1905
STANDS AS A CRUCIAL LANDMARK.


THAT'S WHEN ALBERT EINSTEIN
ASKED THE WORLD


TO GET ON THE RIGHT TRACK,


TO THINK FOR A MOMENT
ABOUT WHAT A MOMENT REALLY IS.

TIME IS RELATIVE,
HE SAID,


AND A MATTER
OF SIMULTANEOUS EVENTS.


AND IN EINSTEIN'S RELATIVITY,


WHICH IS REALITY
NO MATTER HOW IT'S LOOKED AT,


SIMULTANEITY
IS A MATTER OF OPINION.


NOTICE, FOR EXAMPLE,


WHAT HAPPENS
WHEN A LIGHT IS SWITCHED ON


AS HENRY'S TRAIN MOVES BY.


THE LIGHT REACHES
THE FRONT AND REAR OF THE TRAIN


AT THE SAME TIME.


AT LEAST THAT'S HOW
HENRY SEES IT.


BUT CONSIDER
AN OUTSIDER'S POINT OF VIEW.


TO ALBERT,
AT REST ON THE GROUND,


THE REAR WALL
COMES UP TO MEET THE LIGHT,


AND THE FRONT WALL
MOVES AWAY FROM IT.


THESE TWO EVENTS
DON'T LOOK SIMULTANEOUS AT ALL,


AND THAT'S THE ESSENCE
OF THE TIME PROBLEM.


AND IN PHYSICS,

THE EXPLANATION OF THE PROBLEM
STARTS WITH TIME DILATION.


GIVEN THE CONSTANCY
OF THE SPEED OF LIGHT,


THERE'S NO DOUBT THAT
A SIMPLE LIGHT CLOCK, IN MOTION,


RUNS SLOW
BY A FACTOR OF GAMMA.


BUT IF A MORE CONVENTIONAL CLOCK
WENT ALONG FOR THE RIDE,


WOULD IT SLOW DOWN
BY THE SAME AMOUNT?


THE ANSWER IS TIMELY,


BECAUSE IF ONLY THE LIGHT CLOCK
SLOWS DOWN,


IT MIGHT BE POSSIBLE
TO COMPARE THEIR TIMES.


THEN IF THE TIMES WERE THE SAME,
THEY WOULD BE AT REST.


BUT IF THE LIGHT CLOCK
WERE SLOWER,


THAT WOULD BE
BECAUSE THEY'RE IN MOTION.


BUT IN MOTION
WITH RESPECT TO WHAT?


THE BASIC PREMISE
OF RELATIVITY


IS THAT THERE CAN BE
NO ABSOLUTE REST.


AND IF THOSE CONDITIONS
DON'T EXIST,


NO EXPERIMENT
CAN POSSIBLY DETECT THEM.


EVERY CLOCK IN THE UNIVERSE


MUST BEHAVE EXACTLY
AS A LIGHT CLOCK DOES,


AND THAT'S NOT JUST
A DISCOVERY ABOUT CLOCKS.


THAT'S A FACT
ABOUT THE NATURE OF TIME ITSELF.


TIME DILATION
PLACES THE RELATIONSHIP


BETWEEN TIME AND SPACE


IN AN ENTIRELY NEW LIGHT,


WHICH IS EXACTLY
WHAT WAS HAPPENING


IN ART AND LITERATURE


IN THE EARLY YEARS
OF THIS CENTURY.


IN ART AS WELL AS SCIENCE,


THE IDEA
OF TIME AND SPACE


WAS NOW FAR MORE
THAN A PASSING FANCY.


JUST AS EINSTEIN
WAS TRYING TO PLACE EVERYTHING


IN A NEW PERSPECTIVE,


VIRGINIA WOOLF URGED WRITERS
TO MOVE BEYOND


"THE FORMAL RAILWAY LINE
OF A SENTENCE."


NO WRITER INNOVATED MORE
WITH TIME AND SPACE


THAN JAMES JOYCE.


"LIKE EINSTEIN'S WORLD,"
WRITES CRITIC EDMUND WILSON,


"JOYCE'S WORLD
IS ALWAYS CHANGING


"AS IT IS PERCEIVED
BY DIFFERENT OBSERVERS


AT DIFFERENT TIMES."


AND SO IT WENT
AROUND THE CLOCK


WITH SCIENTISTS AND ARTISTS,


MUSICIANS AND MATHEMATICIANS,


POETS AND PHILOSOPHERS.


AND EINSTEIN'S
THEORY OF RELATIVITY


HAD GONE AT LEAST AS FAR
AS ANY CREATIVE IMAGINATION.


ON THE TRACK
OF RELATIVITY,


IT CHANGES THE WAY
THINGS ARE PERCEIVED.


THAT'S BECAUSE
EINSTEIN'S RELATIVITY


IS A THEORY OF MOTION


AS VIEWED
BY DIFFERENT OBSERVERS.


THE THEORY APPLIES EVERYWHERE,
EVEN BROOKLYN,


AND INCLUDES ALL OBSERVERS,


EVEN A GOOD SPORT
NAMED HENRY.


THEY SAY HENRY
HAD ONE HECK OF A FAST BALL.


WITH IT AND A DESIRE TO PITCH
FOR THE BROOKLYN DODGERS,


HE WAS ON HIS WAY
TO MAKING THE TEAM.


SOME SAID HENRY
COULD FLING A BASEBALL


AT NEARLY
THE SPEED OF LIGHT,


AND FROM HIS POINT OF VIEW,
IT DOES LOOK PRETTY FAST.


BUT FROM WHERE
HIS PAL ALBERT STANDS,


IT LOOKS EVEN FASTER.


COULD THE SPEED
OF THE BALL,


ADDED TO THE SPEED
OF THE TRAIN,


MAKE HENRY'S PITCH
LOOK EVEN FASTER


THAN THE SPEED OF LIGHT?


OF COURSE NOT,



BECAUSE EVEN ON A TRAIN
IN SPRING TRAINING,


NOTHING TRAVELS FASTER
THAN THE SPEED OF LIGHT.


SO WHAT'S
REALLY HAPPENING HERE?


BASEBALL STATISTICIANS
WORK IT OUT 


USING A SPACE-TIME DIAGRAM


WHERE FROM HENRY'S
POINT OF VIEW,


THE BALL SPEEDS ONE WAY
AND ALBERT SPEEDS THE OTHER WAY.


AND SINCE DISTANCE
IS ONE AXIS


AND TIME IS THE OTHER,



 Speed, say v prime,
can be seen in the slope of each path,
just as the speed of light
is given by the slope of its path.

On this graph,
light goes 1 unit of distance for each unit of time,
so  c = 1.

Meanwhile, henry's pitch has a speed
u sub x prime of .6.

While Albert, going the other way,
has a speed of -.6.

That's from henry's point of view.

On the other hand, as Albert sees things,
the speed of light remains the same,
but all other speeds are transformed
because all slopes are measured on tilted axes.

Of course, it's true that light still goes
1 unit of distance for each unit of time,
so c is still equal to 1.

But now, henry's moving forward
at a speed  v = .6,
and the baseball's speed, u sub x measures .88.

In the strange arithmetic of relativity,
no matter how you look at it,
when one speed is added to another,
the result is always less
than the speed the speed of light.

So, while Albert will surely agree
that henry throws one heck of a fast ball...

He can always throw a little light on the matter
and be sure it will get there first.


Even along the direction of relative motion,
components of velocity never add to speed
greater than the speed of light, and the velocity transformation
does affect components perpendicular to the motion,
because although distance remain the same,
time changes from one frame to the other.


Even when the theory's as beautiful as Einstein's,
in needs testing,
and there is a test of relativity
in a phenomenon called the decay of cosmic ray mu mesons.

A quarter of a century ago,
Dr.David Frisch of M.I.T.
and Dr.James Smith of the university of Illinois
filmed a classic experiment
proving the reality of the relativistic time dilation.

They did it by measuring the flux through the atmosphere
of a radioactive subatomic particle
called the mu meson.

Created at the top of the earth's atmosphere
by cosmic rays from outer space,
mu, mesons zoom straight downward at, nearly the speed of light.

But laboratory experiments,
such as the one done here by smith,
show that mu mesons disintegrate naturally
after an average life of only 2 millionths of a second.

That means that even going at near the speed of light,
they shouldn't make it very far through the atmosphere
before decaying.

Or, to put it differently,
when Frisch and  Smith observe 568 mu mesons
in their detector at the top of mt.

Washington, they calculate that only 27 per hour
ought to make it to the base of the mountain.

But when they cart their equipment
down to sea level,
they find almost as many mu mesons
as they had at the top--412 per hour.

Of course, Frisch and Smith
really aren't a bit surprised
the reason for theory of relativity.

Traveling at nearly the speed of light,
the mu meson's internal clock runs slow,
so that as viewed from earth,
they take much longer on the average
than 2 millionths oh a second
to disintegrate.

Or, to put it differently,
as viewed from the frame
of reference of the mu meson,
the earth's atmosphere,
rushing upward
at nearly the speed of light,
is Lorentz contracted
to such a degree
that the particle
can easily make it to the ground
in only 2 millionths
of a second.

A set of twins,
born in virtually
the same time and space,
are on the surface
very much alike.

And yet, on the inside,
may turn out as different
as night and day.

Being so different
and so alike at the same time
may be the ultimate paradox
of identical twins.

But in physics,
the twins' paradox
conjures up another image.

And, like Einstein's theory,
it being with an extraordinary
fact about clocks.

No matter how long
they've been around,
no matter whether they
ticktock, chime, or say cuckoo,
all clocks obey
the rules of relativity.

Naturally, these rules apply
to the human clock.

With a time-keeping device
called metabolism,
it starts running
even before the moment of birth,
and in a relativery
predictable fashion,
it keeps running
to the end.

But what would happen
if the human clock
were moving at nearly
the speed of light?

To find out,
consider there identical twins,
Albert and henry.

From the very outset,
their biological clocks
start out
in perfect synchronization.

Soon enough, however,
the best-laid plans
of mother and father alike
get out of sync.

Instead of an orthodontist,
Albert becomes a scientist,
open space itself.

Especially for identical twins,
the first parting
is such sweet sorrow.

Nonetheless, traveling
at nearly the speed of light,
henry has no trouble
leaving Albert behind.

This is henry's first long haul
for his new boss,
the interstellar
baking company.

Destination--
zog, 10 light-years away.

Assignment--take a load of ordinary white bread
and exchange it
for zoggie cakes,
those delicious little buns
that all the earthlings crave.

But that's not what's
so extraordinary.
if henry could compare
his clock to Albert's clock,
he would find Albert's clock
running slow.

And if Albert could compare
his clock to henry's,
he would find henry's clock running slow.

And that spells paradox.

How can each one
be slower than the other?

Fortunately,
in the theory of relativity,
there's no problem at all.

There won't really
be a problem
as long as the two of them
are far apart,
but what will happen
when they're not far apart,
say, when they get back together
for a family reunion?

Again, in the theory
of relativity,
that's no problem.
brother henry,
at nearly the speed of light,
will take about 10 years
to reach zog.

10 light-years away,
and then another decade
to haul his buns back to earth.

To Albert,
that's around 20 years.

But to henry,
the twin who's
actually en route,
the situation
seems different.

Since it's
Lorentz-contracted,
the distance to zog
seems like a milk run.

A few hands of solitaire,
40 winks,
and as far as
henry's concerned,
it's hello to the bright lights
of zog city.

Then equally quick,
because the distance
is again Lorentz-contracted,
it's back to earth
just as fast.
and so, when they do get to
compare those biological clocks,
while the result is amazing,
there's no paradox at all.

Henry, the twin
who zoomed away
and then came back,
has only aced
a couple of months,
but Albert,
the twin who never switched
inertial frames,
has aged 20 years.

And so, even among relatives,
time is relative.

Try to imagine, if you can,
a completely empty universe,
a universe with noting
whatsoever in it.

In such a universe,
it would make
no sense at all
to speak of time or space.

The very concepts
would be meaningless.

If there one single object
in the universe,
it would still be true.

Time and space
would have no meaning at all.

If there were two objects,
then you could image saying
they're getting closer together
or further apart.

Then you're
saying something
about the flow of time
and distance.

You might imagine
using some of them
to construct a device
that could measure
the flow of time
or measure
the distance between objects.

But it would still make
no sense at all in that universe
to say one object
is at rest
and only everything else
is in motion.

In such a universe,
the simple law of motion
it would be possible to imagine
is to say
that each body continues
in whatever state
of motion it has
until something
interferes with it.

That, of course,
is the law of of inertia.

But that has to
keep on making sense
in a universe in which light
can propagate through the void.

Now, if light
has some definite speed
as observed from
a particular frame of reference,
then there is such a thing
as a state of rest
in that frame of reference,
and the law of inertia
collapses.

Now, I don't know
if there are any other
logically consistent universes,
but in our universe,
that problem has been solved
by making the speed of light the sane to all observers
regardless of
their state of motion.

Knowing that,
and only that,
it's possible to deduce
that if there are
a pair of twins
and of them
stays behind on earth
and the other one travels
at high speed to a distant star,
turns around and comes back,
the traveling twin,
when they get back together,
will be younger than the one
who stayed behind.

I don't need to know anything
about how clocks work,
about quartz crystal
oscillators,
about human metabolism.

Those things
are completely irrelevant.

All I need to know
is the principle of relativity.
and the constancy of the speed of light.

From that Ican deduce
that the traveling twin
will come back younger
than the twin that was left behind.

That is an absolutely
breathtaking feat
of pure human logic.

That is the theory
of relativity,
and we'll get on with it next time.