Kind: captions
Language: en
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this video was sponsored by caseta by
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lutron
according to the general theory of
relativity gravity is not a force
there are no gravitational fields
gravity
is kind of an illusion and in this video
i will prove it to you by blasting off
into outer space
in three two one
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albert einstein said the happiest
thought of his life
was imagining a man falling off the roof
of a house
what made einstein so happy about this
wasn't shot in freuda
it was the realization that this man
while he was falling
wouldn't feel his own weight he would be
weightless
and anything he dropped on his way down
well it would remain stationary relative
to him
or moving in uniform motion
the whole situation would be just
like if you were in deep space
not near any large masses
with your spaceship at rest or coasting
along at constant velocity
here you would feel no weight
objects would remain stationary relative
to you
or if you give them a push they would
move in a straight line at constant
velocity
and you would be the very definition of
an inertial observer
you're not accelerating not in a
gravitational field
all the laws of physics apply in your
reference frame
meaning there is no experiment you could
do to distinguish
your inertial reference frame from any
other
now here comes the big leap einstein
looks at these two scenarios and says
they are equivalent not just similar
physically they are exactly the same
thing which means
man falling from roof is not in a
gravitational field there are no
gravitational fields
and he is not accelerating he is an
inertial observer
just like rocket man whoa whoa whoa
i mean okay i can see how both of these
observers
feel weightless but man falling from
roof is clearly in a gravitational field
i mean he's right
next to the earth and he's obviously
accelerating
his speed is increasing by 9.8 meters
per second every second a fact
that will become painfully apparent when
he crashes into the ground
i know that these two situations look
very different
but einstein's equivalence principle
tells us the one
thing to focus on the experience of the
observer
if they feel weightless then they are in
an inertial frame of reference
every bit as good as rocket man's out
drifting through deep space
imagine if rocket man coasting along at
constant velocity
and not paying attention comes upon a
planet a long way off in the distance
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an external observer might notice that
the path of the rocket bends ever so
slightly towards the planet
but inside rocket man would remain
oblivious
he feels no force experiences no
acceleration
as the rocket gets closer to the planet
it goes faster and faster
but rocketman still feels weightless for
him nothing has changed
so where on this journey would you say
the frame of reference changes from
inertial to non-inertial
an on-board accelerometer would never
even register a blip
he has continued on his inertial path
through space-time
so the logical conclusion is his frame
of reference is inertial up until the
instant he crashes into the planet
okay so how do you explain the curved
path of his rocket without
gravitational forces or gravitational
fields the answer
is curved space time first focus on
rocketman's observation that the whole
time
he felt like he was moving with constant
velocity in a straight
line he was moving in a straight line
through space-time
but space-time around massive objects
like planets is curved
so that's why his path appeared curved
to a distant observer
now this isn't as unusual as it seems
airplanes for example always try to fly
the shortest route between cities
essentially they just go in a straight
line but since the earth's surface is
curved the shortest path
doesn't look like a straight line these
shortest paths over curved surfaces
are called geodesics and we use that
same word
geodesics for the straight line paths
followed by inertial observers through
curved spacetime
here's another analogy imagine you and a
friend
are standing a thousand kilometers apart
on the equator
now you both set off due north over time
you will come closer together ultimately
bumping into each other at the north
pole
it's as though there was a force pushing
you together
but you didn't feel a force and your
friend didn't feel a force
gravity is just like that force it
doesn't actually
exist the real reason for you coming
together
was that you were both on straight paths
geodesics
on a curved surface
astronauts on the space station are
weightless
that means they too are inertial
observers traveling on a geodesic
but the earth curves space-time around
it
which is why their straight-line path
appears as a helix
it only looks like a circular orbit if
you forget the time dimension
don't forget we are all moving through
space and time
space time
this is the standard bent sheet analogy
for
curved space-time but i think this demo
is misleading
it allows you to fool yourself into
thinking you understand general
relativity
when the intuition you're actually
drawing on is just that
objects like to fall towards the middle
of a well
due to the gravitational force but in
general relativity
there is no gravitational force what you
should be thinking about is
objects traveling on a straight line
path through space-time
it just so happens that space-time is
curved around massive
objects so that straight-line path
doesn't look like a straight line
matter tells space time how to curve and
space time tells matter how to move
now let's go back to deep space what
happens
if you turn on the rocket thrusters and
accelerate
at 9.8 meters per second squared
well someone outside would see all
objects remain
stationary while the floor of the rocket
accelerates
into them
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inside the rocket everything would
appear to accelerate down to the ground
and you would feel a force pushing up on
your feet
the same force that's pushing up on you
as you watch this video
this situation feels exactly the same as
being at rest
on the surface of earth
because we are at rest on the surface of
earth
now i want to ask you are you watching
this video in an inertial frame of
reference
well i mean do you feel weightless
no so you are not an inertial observer
your situation is exactly the same as
someone accelerating
on a rocket ship in deep space and let
me be clear i don't mean
that being at rest in a gravitational
field is like accelerating in a rocket
i mean it is the exact same thing you
are accelerating and there is no
gravitational field
gravitational fields do not exist now i
know that sounds crazy
but come with me for a minute
this is you in standard newtonian
physics
we draw your weight force the force of
gravity
pushing you down and the normal force
from the floor pushing you up
we say these forces are equal and
opposite so there's no net force on you
so you are not
accelerating but in general relativity
gravity is not a force you have
no weight so the only force on you are
these normal forces pushing you up
so you are accelerating upwards but i'm
not
moving up relative to what i mean
relative to the flip chart and the floor
and basically everything in this room
but all of those things are in your
frame of reference which you know is not
inertial
relative to everything in my rocket ship
i'm not accelerating
what you need if you really want to
measure your acceleration
is someone in an inertial frame of
reference like the guy who fell off the
roof
and he would see you accelerating up at
9.8
meters per second squared i think
what this shows is that what an
acceleration really is
is it's a deviation from a geodesic
you can't follow a straight-line path
through space-time
because the floor prevents you from
doing that it applies a force upwards on
you
so you're accelerating up but if i'm
accelerating up
and so is everyone else around the world
and presumably the whole surface of the
earth
then shouldn't the earth be expanding
no it is possible for you to be
accelerating
even though your spatial coordinates are
not changing
i will show you one equation from
general relativity
this says that the second derivative of
your position with respect to time
is equal to your acceleration that's
just f
on m and if you were in flat space time
well this is exactly what you're saying
if you're accelerating your spatial
coordinates have to change
but you're not in flat space time and
this term is related to the curvature of
space time
and this is your velocity through time
squared
you don't have to worry about the
details here the point
is your position can be not changing
this
can be zero which means your
acceleration
must be exactly equal to this
this curvature term times your velocity
through time
squared so in curved space time
you need to accelerate just to stand
still
a lot of this may seem more complicated
than newtonian physics but
one classical mystery looks a whole lot
simpler in general relativity
which is why all objects fall at the
same rate
now i have made a number of videos on
this topic and
i would always give the standard
newtonian explanation the only force on
a free-falling body is its weight
gmm on r squared which equals its mass
times acceleration
you can cancel the object's mass on both
sides of the equation
hence all objects will have this same
acceleration
the mystery is why we could cancel these
two
m's the one on the left was
gravitational mass the property of an
object
that creates and experiences a
gravitational field
while the m on the right is inertial
mass a measure of
resistance to acceleration why should
these two
conceptually different properties be
numerically
identical scientists have spent a lot of
time and effort
experimentally testing down to around
one part in 10 trillion
that these two types of mass really are
the same
but in general relativity there is no
mystery
all objects appear to fall the same way
because they're not
accelerating they're just following
straight-line paths through space-time
until they encounter something that
stops them
like the objects in the rocket ship they
appear to accelerate at the same rate
because
they're not really accelerating it's the
floor accelerating
into them now a lot of this might seem
pretty far-fetched as it did back in
1915 when einstein proposed it
so he very cleverly came up with a
measurable prediction that scientists
could make to test his theory
imagine that this rocket ship is
coasting through deep space
if you shine a light beam across the
rocket ship
well it'll do exactly what you expect
light travels in a straight line
and hits a point on the opposite wall at
exactly the same height as the source
but now what if this rocket is
accelerating
well to an external observer they're
still going to see the same thing
light traveling in a straight line but
inside the rocket
during the time it takes for the light
to travel across the cabin
the rocket will have sped up so by the
time it hits the other wall
it'll hit a little lower than before
so in an accelerating frame of reference
light
deflects down here i am dramatically
exaggerating the effect
even if i were accelerating up at 10 g's
which would
probably kill me the deflection would be
on the order of the width of a proton
still it shows that an accelerating
frame of reference
will bend light so einstein reasoned
light must also bend when it passes a
large
mass but where do you find a mass large
enough
well the only obvious huge mass near the
earth is the sun
so the ideal experiment would be to look
at light passing just
next to the sun and see if it is
deflected
say light from distant stars the problem
was of course the sun is just so bright
you can't see stars right next to it
unless there is a total solar eclipse
which is exactly what happened in 1919
so arthur eddington set out
to take pictures of the stars right next
to the sun
during totality and what he found by
analyzing those pictures was that their
positions
appeared deflected by the precise amount
predicted by einstein's general theory
of relativity
the result was twice the deflection some
had calculated using a strictly
newtonian model
in general relativity has passed
virtually every test
put to it over the past hundred years or
so but there are still
more things to try
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a well-known and empirically validated
finding
is that accelerating charges radiate
electromagnetic radiation
so one conceptually simple experimental
test
would be to compare the behavior of a
stationary charge in a gravitational
field to
a free falling one if a more newtonian
picture of gravity is correct
well then the stationary charge should
not radiate electromagnetic radiation
but a free falling one is accelerating
and therefore it should radiate
in contrast general relativity sees the
free falling charge as
non-accelerating it's just going on a
straight line path through curved
space-time
whereas the stationary charge is
accelerating and therefore
it should give off electromagnetic
radiation
now thus far logistical challenges have
prevented anyone from actually carrying
out this experiment
but what you believe will happen reveals
what you truly think
about the nature of gravity do you think
that a freely falling charge will
radiate electromagnetic radiation
or not is gravity an illusion
this episode was sponsored by caseta and
so i used the sponsorship money
to have a rocket ship built
caseta by lutron makes these smart light
switches in the rocket ship
they also make remotes motion sensors
smart plugs basically
every smart switch you could use for
turning things in your home on and
off but installing a smart switch isn't
rocket science
just turn off the power to the switch
detach the existing wires
and reconnect them to the caseta smart
switch what's great about smart switches
as opposed to smart bulbs
is one switch often controls multiple
bulbs
so you can save money by replacing only
the switch and they connect to most
smart devices like alexa google
assistant
and apple home kit and you can control
them via an
app on your phone so if you forget to
turn off the lights in the rocket
no need to go back just turn them off in
the app you can also use the app to set
up schedules
turning the lights on when it goes dark
this gives you peace of mind knowing
your family will always come home to a
well-lit house
now we may not have personal rockets yet
but we have smart switches and the smart
choice for these devices
is caseta by lutron