https://www.youtube.com/watch?v=Xc4xYacTu-E
hey Vsauce Michael here down here but which way is down and
how much does down way well down raise about a hundredth of a gram per cubic
centimeter it is light and airy which makes it a great source of insulation and
buoyancy for water birds but if you let go of down it falls down so that's
which way down is it's the direction gravity is pulling everything in now for
someone on the other side of the earth my down is their up but where are
falling things going why do things fall are they being pushed or pulled or is
it because of time travel first things first let's turn the Sun into a black hole
we can do that using universe sandbox 2 this simulator will blow your mind I
love it in fact I love it so much I put a code to keep the game for free in the
current curiosity box if you're not subscribed to the box yet you are missing
out okay look for the purposes of this video we want the solar system and here
it is notice that everything's moving pretty quickly around the Sun that's
because we currently have the game set so that every second that passes for us
is 14 days almost in the game if I change this to one second we're looking at
the solar system in real time you'll notice that it almost looks like it's
frozen even though the earth is traveling around the Sun at about 30 km/s it
barely appears to be moving that is how vast space is anyway let's go back to
14 days I like that motion now look at the Sun it is not currently a black hole
but we can change that what we need to do is compress the Sun so let's lock its
mass so that it doesn't change while we make its radius smaller let's make its
radius as small as we can and oh where to go what's still there it's just
become a black hole pretty spooky but now let's look at the rest of the solar
system all right zooming out and huh nothing's changed I mean something's
changed it's colder and darker but nothing's flying off into space or getting
sucked in you see by shrinking the Sun we didn't change the direction of down
for the planets they're always being pulled by gravity towards its middle and
making it smaller didn't move where the middle was but also the strength of
that force pulling them to the middle of the Sun stayed the same that gives us
a clue as to what down is the clue is the other thing we didn't change mass
mass is a measure of how hard it is to accelerate something to change its
motion now right now these two balls have zero motion relative to me slapping
around this hollow plastic ball is pretty easy but doing the same to this solid
steel ball there's a lot harder now gravity and weight have nothing to do with
this gravity acts downward not against my horizontal slapping of course gravity
does contribute to friction but friction works against me when I start moving
the ball but works with me when I stop the ball and the steel ball is harder to
stop than the plastic ball the difference is mass the steel ball is more
massive it's more resistant to having its motion changed mass is an intrinsic
property it does not depend on what's around or change from place to place it
can sometimes be thought of as the amount of matter something has your mass is
the same regardless of where you are on the moon on earth in the middle of
intergalactic space floating around but all of this said mass does seem to care
about what's around mass loves company things with mass and/or energy are attracted
together by a force that we call gravity the feeling of gravity is just you and
the earth being attracted to one another now every portion of an object with
mass attracts other portions towards it the average of all this pulling is an
attraction between centers of mass giant things like Earth exert an obvious
pull but everything does even a baseball these two baseballs are attracted
together by their own gravity z' except their masses are so small the forces
minuscule and it can't overcome friction or push air out of the way they're
never gonna come together but if you put two baseball's one meter apart in the middle
of empty space where no other forces could act on them they would literally
fall together and collide it would take three days to happen but it would Isaac
Newton found that the strength of the force bringing two things together is
equal to the product of their masses divided by the distance between their
centers of mass squared times big G the gravitational constant if you make one
of two objects more massive or move them closer together the force will be
stronger and this force of attraction is what we call weight so mass is
intrinsic whereas weight depends on what's around now a weird thing happens
when you weigh yourself on most scales weight is a force but scales display
pounds or kilograms which are units of mass what's going on is that a scale is
activated by a force any force it doesn't have to be caused by gravity the
scale then displays what amount of mass near the surface of the earth would be
attracted to the earth with the force it's detecting now since scales tend to
be used on the surface of the earth by people on which the only force acting is
gravity they tend to not be very far off but they can be easily tricked and
they further lead to the confusion between mass and weight notice that weight
is mutual you are pulled down by earth with the same force that you pull up on
earth according to a scale I weigh 180 pounds on earth [Music] and the earth
weighs 180 pounds on me but because the Earth's mass is so much greater than my
own and because the more massive something is the more it resists being moved
our equal and opposite weight forces accelerate me a lot more than the earth if
you drop a pencil from a height of 6 feet the pencil doesn't just fall to the earth
more precisely they both come together they're attracted to each other by equal
forces but the same force moves the pencil a lot more than the earth when you
let go of the pencil the earth is literally pulled up to the pencil by the
pencils own gravity a distance equal to about 9 trillion the width of a proton
that same force moves the pencil the remaining distance which is still pretty
much six feet at the height of the International Space Station's orbit you and
earth are attracted about 10% less than when you're on the surface about eight
point eight times your mass but not zero for this reason weightless astronauts
in zero gravity are neither weightless nor in zero gravity their weight force
fails to bring them and earth together because they move horizontally so
quickly that they fall just as fast as Earth's surface curves away from them
and even though they're experiencing 90 percent of the gravity you and I are
feeling right now that's why they don't just fly away there are no forces called
g-forces to resist their weight since everything around them is falling to its
resistance to your weight force stress deformation that is needed for you to
feel weight what astronauts in orbit actually lack is apparent weight likewise
a helium balloon has weight I mean it's made out of matter it clearly has mass
so it's attracted to the earth let's try to measure its weight force that's
eight okay it has negative apparent weight that's because it's attraction to
the earth is weaker than the buoyant forces from the air around it that push it
up now while it moves up it pushes air molecules down but they transfer that
force widely not just directly down onto the scale buoyant forces are caused by
the fact that whenever you are immersed in a fluid like water or air molecules
lower down are at greater pressure that are being pressed by the weight of all
the molecules above them and are closer to earth so they're pulled to it with a
stronger force now having greater pressure means they pack a bigger punch when
they collide with things so horizontally those collisions cancel out but
vertically the stronger collisions from below went out providing lift a buoyant
force this even happens on your own body across its surface area air lifts you
with the force of about one Newton which is equal to the weight force of an
apple so if you weighed yourself in a vacuum you would weigh about this much
more but that's not all Earth's spin causes it to bulge at the equator so the
closer you are to it the further you are from Earth's center of mass and the
less your actual gravitational weight will be down is always changing I mean
where is Earth's center of mass it would always be the same as Earth's
geometric middle if Earth's composition was uniform but earth contains pockets
of massive rock at different depths water mountains it's got moving changing
insides and air and seasonal ice and though they're far away gravity extends
forever from everything so the moon the Sun the planets all of them pull on you
negligibly but truly you weigh about a millionth of your weight less when the
moon is directly above you this chunky shifting balance of material on earth
and ever where else in the universe means that down is always changing on top
of that Earth's spin skews what you consider the direction of down away from
its center of mass because the push you get from Earth's spin seems to slightly
lift you reducing your apparent weight and bending down towards the equator the
net result is an apparent weight reduction at the equator of about half of a percent
if a scale guesses your mass must be 200 pounds at the poles it'll guess that
you're 199 at the equator the 9.8 multiplier used so often in physics is
calculated based on how these factors affect someone at 45 degrees latitude all
of these influences on the direction of down result in a total vertical
deflection that's only ever at most a few arc seconds anywhere on earth that's
not enough to be felt but changes in direction and strength can be used to
study the shape of the seafloor determine what's under you or even help you
discover ancient buried rooms point is all of our downs aren't a bunch of
radially symmetric lines down is an uncombed mess now since solids don't flow
they can have shapes that don't pay much mind to this but water can flow so
ignoring influences like wind and tides the surface of oceans and lakes and
puddles is always perpendicular to down if water could pass through land or if
earth were submerged in water gravity would be the same everywhere along its
bumpy surface such a surface is called a geoid and can be drawn at any altitude
if you wanted to build a table that completely enclosed the earth it would have
to have rolling undulation z' nearly 100 meters at some points in order to be
level so that a ball placed anywhere on it wouldn't roll here is Earth's G I exaggerated
a thousand times you'd weigh about a hundredth of a percent less a few grams
here then you would say here where gravity is a bit stronger point is the
strength and direction down is variable by location and changes over time so
down is a fluctuating vector easy enough except why should matter attract
matter in the first place Isaac Newton was able to describe attraction but not
explain it humanity got closer however when Albert Einstein introduced his
general theory of relativity Einstein thought a lot about the fact that
everything falls to the ground at the same rate no matter how massive something
is when dropped it will accelerate towards the earth down gaining about 9.8
meters per second for every second that it falls that means that a hammer
that's quite massive and a not so massive feather when dropped from the same
height will hit the ground at the same time okay what just happened was in
error in order to fall through air a thing has to push air out of the way but
if it has a large surface area and a low weight force it will have a lot of air
to move but will be able to move that air very quickly in a vacuum things do
fall at the same rate regardless of mass this was famously demonstrated by
Apollo 15 commander David Scott on the moon that's weird right I mean if a more
massive object is pulled with a greater force shouldn't it fall faster well
Newton's explanation was simple larger masses are attracted with greater forces
but will also require more force to be accelerated the same as a less massive
thing something a hundred times more massive might require a hundred times the
force but it will be pulled by gravity 100 times more so everything falls to
earth at the same rate what a fun coincidence right maybe not Einstein realized
that there's another way for things to appear to fall together of their masses
imagine a feather and a hammer floating in space in a room if the room is
suddenly accelerated up at 9.8 m/s^2 the feather and the hammer will hit the floor
at the same time furthermore whether it's the room coming up to meet them or
gravity being suddenly switched on neither object will feel any force pushing
them there's no way to tell which of these happen this is Einstein's famous
equivalence principle he once admitted that his greatest thought ever was that
of a man falling off of a roof while falling the man would not feel any forces
on him even though he's speeding up freefall is indistinguishable from floating
alone in space from having no forces on you from not being moved what if
gravity isn't a force at all what if things fall not because they're being
pushed or pulled but because they're not being pushed or pull to see how this
could be we need to talk about straight lines what I have here is a retractable
ID badge holder this is a great way to test for straight paths because the
string is always kept taut the card I have behind has two lines drawn on it and
if while I pull the string out it always stays between those two lines I will
know that I never turned while I pulled it because any turn will translate into
a different angle between the lines on the card and the string now if I put two
of these on a flat table and pull them out always ensuring that they go
straight ahead they will never meet they will be forever parallel but now let's
put them on a sphere a curved surface again I pull both strings forward making
sure that they always are pulled out straight no turning wait they came
together well they didn't turn look maybe there's some kind of weird force that
pulled my hands together and just like gravity I didn't feel it but it happened
no what happened was not the result of a force it was just a natural result of curvature
you might be thinking wait a second are those really straight lines I mean they
don't look that straight to me also what if they've just moved along latitude
lines then they've never come together and those look pretty darn straight but
they're not a straight line never turns and although latitude lines look
straight at first glance following one requires turning to find straight line
paths on surfaces whether they're flat like this or curved I love the written
text now you can use an actual ribbon but I have found that a strip of paper
works even better let's take a look at this path right here it's straight at
first but then it curves now if two people are traveling along this curve and
they want to stay together the person on the inside will have to cover a
shorter distance than the person on the outside since both sides of this strip
of paper cannot change their lengths they'll help us find a straight path if
the strip of paper can lay flat we'll know that we have found a straight line
and as you can see the strip can lay flat and follow the straight part of this
path but when it comes to the curve in order to follow the path now the strip
well it has too much material on the inside and that material bunches up and
leaves the plane therefore we know that this part of the path is not straight let's
use the ribbon test to find straight lines on the surface of a cone well from
the looks of it aligned directly from the base to the tip seems like it would
be straight and sure enough yeah the ribbon lays flat on that path but what
about a ring around the cone nope doesn't work shorter distances around nearer
the tip of the cone mean that there's too much ribbon up at the top so doesn't
lay flat let's see what else is there though besides this well if I start here
and just allow the ribbon to lay flat huh I get a little curvy looking shape
like this I say curvy looking because while to someone say at the base this
path might seem to go up slow down change direction and then fall down faster
and faster since a ribbon on such a path is flat it's actually for inhabitants
on the cones surface perfectly straight if we trace the ribbons path on to the
cone we can see this clearly because a cone can be flattened a straight line on
a curved surface is called a geodesic here is a geodesic on a sphere the
Equator is one here's another a line of latitude is not a geodesic it's not a
straight line to see why let's try to follow it with the ribbon you know what I
have to keep kind of lifting it yeah see distances around the sphere becomes
shorter as we go up so there's too much material on the ribbon up here and it
leaves the surface this path contains turns and in order to turn a force has to
act on you if no forces did this is the path you would take notice that the
ribbon begins moving due east but then falls south Falls Einstein realized that
curvature could cause things to be seemingly attracted to one another without
needing to invent the existence of forces like gravity but attraction only
happens if things move along the surface if they stay still they well they
don't come together so for something at rest how does falling begin I mean the
thing has to move in this direction but it's at rest right well yes but it's
only at rest in space and that's not the whole story up down forward backward
and left-right are all you need to describe where an event occur but a complete
description will also need to describe when together these four dimensions form
the setting in which everything in our universe happens space-time since we can
talk about a falling pencil using just one spatial dimension up and down we can
use a piece of paper to model space-time for it okay so we've got up and down
but we have to add another direction the pencil moves in time now if no forces
act on the pencil it won't move through space it will only get older and as you
can see if all it does is get older it won't fall if space-time was flat when I
let go of the pencil it wouldn't go anywhere but now let's allow the earth
which is massive to manipulate space-time into say a cone now with no forces
acting on it every part of the pencil follows a straight line but on a cone as
we saw earlier such a path will look like this it will fall this is because
distances around the cone are shorter higher up time runs faster further from a
massive object but to go straight not turn every part of the pencil must cover
an equal distance in space-time like this only when the pencil hits the earth
does the repulsion of their mutual electrons provide a force pushing the pencil
off a geodesic for the earth time is a series of slices from this evolution the
pencils force-free geodesic is why it falls not a push or pull just the pencils
natural tendency to follow a straight line until something acts on it now we
only used one dimension of space and one of time because visualizing our
universes three of space and one of time would take us beyond the limits of
what could be shown on paper or screens but math can take us there general
relativity allows us to calculate how much mass and energy curved space-time
and has been used to explain things that Newton's older theory of falling as
the result of forces couldn't like anomalies in the orbit of mercury which
orbits nearest the Sun and is therefore most affected by the sun's grip on
space-time many other experiments have confirmed general relativity's picture
of the universe fitting the conclusion that there is no gravity there's just
space-time its curvature and a senate as John Wheeler famously put it
space-time grips mass telling it how to move mass grips space-time telling it
how to curve relative to the earth we don't move very fast even jet airplanes
move negligibly close to the speed of light so relative to earth we move almost
exclusively through time as such we are more affected by the way time is curved
by mass than how space is curved this has led many to claim that for the most
part you feel as though you're being pushed into the ground not because of a
force called gravity but because time is moving faster for your head than for your
feet down is relative and always changing but it exists because of and is always
in the direction of slower time Bertrand Russell called this the law of cosmic
laziness everything is naturally steered towards where time is slowest we call
this falling going so you don't have to keep anything on the down-low time will
take care of that for you and as always thanks for watching [Music] remember
that you can support Vsauce and Alzheimer's research by subscribing to the
Vsauce curiosity box the current one comes with a code to get a free copy of universe
sandbox 2 which is amazing and a whole host of other science toys and tools
picked by myself Jake and Kevin I love it all so I hope to see you at brain
candy live we are coming to many many cities very soon hopefully one near you
by going to the show you can see Adam and I doing things that will you may not
have seen us do before we also explore the science and common misconceptions
behind all things err maybe have said too much maybe not I hope to see you
there and as always thanks for watching [Music]