Which Way Is Down?

published on July 12, 2020

Hey Vsauce Michael here down here butwhich way is down and how much does downway well down raise about a hundredth ofa gram per cubic centimeter it is lightand airy which makes it a great sourceof insulation and buoyancy for waterbirds but if you let go of down it falls

Downso that's which way down is it's thedirection gravity is pulling everythingin now for someone on the other side ofthe earth my down is their up but whereare falling things going why do things

Fallare they being pushed or pulled or is itbecause of time travel first thingsfirst let's turn the Sun into a blackhole we can do that using universesandbox 2 this simulator will blow your

Mind I love it in fact I love it so muchI put a code to keep the game for freein the current curiosity box if you'renot subscribed to the box yet you aremissing out okay look for the purposesof this video we want the solar system

And here it is notice that everything'smoving pretty quickly around the Sunthat's because we currently have thegame set so that every second thatpasses for us is 14 days almost in thegame if I change this to one second

We're looking at the solar system inreal time you'll notice that it almostlooks like it's frozen even though theearth is traveling around the Sun atabout thirty kilometers per second itbarely appears to be moving that is how

Vast space is anyway let's go back to 14days I like that motion now look at theSun it is not currently a black hole butwe can change that what we need to do iscompress the Sun so let's lock its massso that it doesn't change while we make

Its radius smallerlet's make its radius as small as we canand oh where to go what's still thereit's just become a black hole prettyspooky but now let's look at the rest ofthe solar system all right

Zooming out and huh nothing's changed Imean something's changed it's colder anddarker but nothing's flying off intospace or getting sucked in you see byshrinking the Sun we didn't change thedirection of down for the planets

They're always being pulled by gravitytowards its middle and making it smallerdidn't move where the middle was butalso the strength of that force pullingthem to the middle of the Sun stayed thesame that gives us a clue as to what

Down is the clue is the other thing wedidn't change mass mass is a measure ofhow hard it is to accelerate somethingto change its motion now right now thesetwo balls have zero motion relative tome slapping around this hollow plastic

Ball is pretty easy but doing the sameto this solid steel ball there's a lotharder now gravity and weight havenothing to do with thisgravity acts downward not against myhorizontal slapping of course gravity

Does contribute to friction but frictionworks against me when I start moving theball but works with me when I stop theball and the steel ball is harder tostop than the plastic ball thedifference is mass the steel ball is

More massive it's more resistant tohaving its motion changed mass is anintrinsic property it does not depend onwhat's around or change from place toplace it can sometimes be thought of asthe amount of matter

Something has your mass is the sameregardless of where you are on the moonon earth in the middle of intergalacticspace floating around but all of thissaid mass does seem to care about what'saround mass loves company

Things with mass and/or energy areattracted together by a force that wecall gravity the feeling of gravity isjust you and the earth being attractedto one another now every portion of anobject with mass attracts other portions

Towards it the average of all thispulling is an attraction between centersof mass giant things like Earth exert anobvious pull but everything does even abaseball these two baseballs areattracted together by their own gravity

Z' except their masses are so small theforces minuscule and it can't overcomefriction or push air out of the waythey're never gonna come together but ifyou put two baseball's one meter apartin the middle of empty space where no

Other forces could act on them theywould literally fall together andcollide it would take three days tohappen but it would Isaac Newton foundthat the strength of the force bringingtwo things together is equal to the

Product of their masses divided by thedistance between their centers of masssquared times big G the gravitationalconstant if you make one of two objectsmore massive or move them closertogether the force will be stronger and

This force of attraction is what we callweight so mass is intrinsic whereasweight depends on what's aroundnow a weird thing happens when you weighyourself on most scales weight is aforce but scales display pounds or

Kilograms which are units of mass what'sgoing on is that a scale is activated bya force any force it doesn't have to becaused by gravity the scale thendisplays what amount of mass near thesurface of the earth would be attracted

To the earth with the force it'sdetecting now since scales tend to beused on the surface of the earth bypeople on which the only force acting isgravity they tend to not be very far offbut they can be easily tricked and they

Further lead to the confusion betweenmass and weightnotice that weight is mutual you arepulled down by earth with the same forcethat you pull up on earth according to ascale I weigh 180 pounds on earth

And the earth weighs 180 pounds on mebut because the Earth's mass is so muchgreater than my ownand because the more massive somethingis the more it resists being moved ourequal and opposite weight forces

Accelerate me a lot more than the earthif you drop a pencil from a height of 6feet the pencil doesn't just fall to theearth more precisely they both cometogether they're attracted to each otherby equal forces but the same force moves

The pencil a lot more than the earthwhen you let go of the pencil the earthis literally pulled up to the pencil bythe pencils own gravity a distance equalto about 9 trillion the width of aproton that same force moves the pencil

The remaining distance which is stillpretty much 6 feet at the height of theInternational Space Station's orbit youand Earth are attracted about 10% lessthan when you're on the surface about88 times your mass but not zero for

This reason weightless astronauts inzero-gravity are neither weightless norin zero gravity their weight force failsto bring them and earth together becausethey move horizontally so quickly thatthey fall just as fast as Earth's

Surface curves away from them and eventhough they're experiencing 90% of thegravity you and I are feeling right nowthat's why they don't just fly awaythere are no forces called g-forces toresist their weight since everything

Around them is falling to its resistanceto your weight force stress deformationthat is needed for you to feel weightwhat astronauts in orbit actually lackis apparent weight likewise a heliumballoon has weight I mean it's made out

Of matter it clearly has mass so it'sattracted to theearth let's try to measure its weightforce that's a-ok it has negativeapparent weight that's because it'sattraction to the earth is weaker than

The buoyant forces from the air aroundit that push it up now while it moves upit pushes air molecules down but theytransfer that force widely not justdirectly down onto the scalebuoyant forces are caused by the fact

That whenever you are immersed in afluid like water or air moleculeslowered down are at greater pressurethat are being pressed by the weight ofall the molecules above them and arecloser to earth so they're pulled to it

With a stronger force now having greaterpressure means they pack a bigger punchwhen they collide with things sohorizontally those collisions cancel outbut vertically the stronger collisionsfrom below went out providing lift a

Buoyant forcenow this even happens on your own bodyacross its surface area air lifts youwith a force of about one Newton whichis equal to the weight force of an appleso if you weighed yourself in a vacuum

You would weigh about this much more butthat's not all Earth's spin causes it tobulge at the equator so the closer youare to it the further you are fromEarth's center of mass and the less youractual gravitational weight will be down

Is always changing I mean where isEarth's center of mass it would alwaysbe the same as Earth's geometric middleif Earth's composition was uniform butearth contains pockets of massive rockat different depths water mountains it's

Got moving changing insides and air andseasonal ice and though they're far awaygravity extends forever from everythingso the moon the Sun the planets all ofthem pull on you negligibly but trulyyou weigh about a millionth of your

Weight less when the moon is directlyabove you this chunky shifting balanceof material on earth and everwhere else in the universe means thatdown is always changing on top of thatEarth's spin skews what you consider the

Direction of down away from its centerof mass because the push you get fromEarth's spin seems to slightly lift youreducing your apparent weight andbending down towards the equator the netresult is an apparent weight reduction

At the equator of about half of apercent if a scale guesses your massmust be 200 pounds at the polesit'll guess that you're 199 at theequator the 98 multiplier used so oftenin physics is calculated based on how

These factors affect someone at 45degrees latitude all of these influenceson the direction of down result in atotal vertical deflection that's onlyever at most a few arc seconds anywhereon earth that's not enough to be felt

But changes in direction and strengthcan be used to study the shape of theseafloor determine what's under you oreven help you discover ancient buriedrooms point is all of our downs aren't abunch of radially symmetric lines down

Is an uncombed mess now since solidsdon't flow they can have shapes thatdon't pay much mind to this but watercan flow so ignoring influences likewind and tides the surface of oceans andlakes and puddles is always

Perpendicular to down if water couldpass through land or if earth weresubmerged in water gravity would be thesame everywhere along its bumpy surfacesuch a surface is called a geoid and canbe drawn at any altitude if you wanted

To build a table that completelyenclosed the earth it would have to haverolling undulation z' nearly 100 metersat some points in order to be level sothat a ball placed anywhere on itwouldn't roll here is Earth's goe

Exaggerated a thousand times you'd weighabout a hundredth of a percent less afew grams here than you would say herewhere gravity is a bit stronger point isthe strength and directiondown is variable by location and changes

Over time so down is a fluctuatingvector easy enough except why shouldmatter attract matter in the first placeIsaac Newton was able to describeattraction but not explain ithumanity got closer however when Albert

Einstein introduced his general theoryof relativity Einstein thought a lotabout the fact that everything falls tothe ground at the same rate no matterhow massive something is when dropped itwill accelerate towards the earth down

Gaining about 98 meters per second forevery second that it falls that meansthat a hammer that's quite massive and anot so massive feather when dropped fromthe same height will hit the ground atthe same time okay what just happened

Was in error in order to fall throughair a thing has to push air out of theway but if it has a large surface areaand a low weight force it will have alot of air to move but will be able tomove that air very quickly in a vacuum

Things do fall at the same rateregardless of mass this was famouslydemonstrated by Apollo 15 commanderDavid Scott on the moon that's weirdright I mean if a more massive object ispulled with a greater force shouldn't it

Fall faster well Newton's explanationwas simple larger masses are attractedwith greater forces but will alsorequire more force to be accelerated thesame as a less massive thing something ahundred times more massive might require

A hundred times the force but it will bepulled by gravity 100 times more soeverything falls to earth at the samerate what a fun coincidence right maybenotEinstein realized that there's another

Way for things to appear to falltogetherof their masses imagine a feather and ahammer floating in space in a room ifthe room is suddenly accelerated up at98 m/s^2

The feather and the hammer will hit thefloor at the same time furthermorewhether it's the room coming up to meetthem or gravity being suddenly switchedonneither object will feel any force

Pushing them there's no way to tellwhich of these happened this isEinstein's famous equivalence principlehe once admitted that his greatestthought ever was that of a man fallingoff of a roof while falling the man

Would not feel any forces on him eventhough he's speeding up freefall isindistinguishable from floating alone inspace from having no forces on you fromnot being moved what if gravity isn't aforce at all what if things fall not

Because they're being pushed or pulledbut because they're not being pushed orpulled to see how this could be we needto talk about straight lines what I havehere is a retractable ID badge holderthis is a great way to test for straight

Paths because the string is always kepttaut the card I have behind has twolines drawn on it and if while I pullthe string out it always stays betweenthose two lines I will know that I neverturned while I pulled it because any

Turn will translate into a differentangle between the lines on the card andthe string now if I put two of these ona flat table and pull them out alwaysensuring that they go straight aheadthey will never meet they will be

Forever parallel but now let's put themon a sphere a curved surface again Ipull both strings forward making surethat they always are pulled out straightno turning wait they came togetherwell they didn't turn look maybe there's

Some kind of weird force that pulled myhands together and just like gravity Ididn't feel it but it happened no whathappened was not the result of a forceit was just a natural result ofcurvature you might be thinking wait a

Second are those really straight lines Imean they don't look that straight to meOh also what if they've just moved alonglatitude lines then they'd never cometogether and those look pretty darnstraight but they're not a straight line

Never turns and although latitude lineslook straight at first glance followingone requires turning to find straightline paths on surfaces whether they'reflat like this or curvedI love the ribbon test now you can use

An actual ribbon but I have found that astrip of paper works even better let'stake a look at this path right here it'sstraight at first but then it curves nowif two people are traveling along thiscurve and they want to stay together the

Person on the inside will have to covera shorter distance than the person onthe outside since both sides of thisstrip of paper cannot change theirlengths they'll help us find a straightpath

If the strip of paper can lay flat we'llknow that we have found a straight lineand as you can see the strip can layflat and follow the straight part ofthis path but when it comes to the curvein order to follow the path now the

Strip well it has too much material onthe inside and that material bunches upand leaves the plane therefore we knowthat this part of the path is notstraightlet's use the ribbon test to find

Straight lines on the surface of a conewell from the looks of it aligneddirectly from the base to the tip seemslike it would be straight and sureenough yeahthe ribbon lays flat on that path but

What about a ring around the cone nopedoesn't work shorter distances aroundnearer the tip of the cone mean thatthere's too much ribbon up at the top soit doesn't lay flat let's see what elseis there though besides this well if I

Start here and just allow the ribbon tolay flat huh I get a little curvylooking shape like this I say curvylooking because while to someone say atthe base this path might seem to go upslow down change direction and then fall

Down faster and faster since a ribbon onsuch a path is flat it's actually forinhabitants on the cones surfaceperfectly straight if we trace theribbons path on to the cone we can seethis clearly because a cone can be

Flattened a straight line on a curvedsurface is called a geodesic here is ageodesic on a sphere the Equator is onehere's another a line of latitude is nota geodesic it's not a straight line tosee why let's try to follow it with the

Ribbon you know what I have to keep kindof lifting it yeah see distances aroundthe sphere becomes shorter as we go upso there's too much material on theribbon up here and it leaves the surfacethis path contains turns and in order to

Turn a force has to act on you if noforces did this is the path you wouldtake notice that the ribbon beginsmoving due eastbut then falls southFalls Einstein realized that curvature

Could cause things to be seeminglyattracted to one another without needingto invent the existence of forces likegravity but attraction only happens ifthings move along the surface if theystay still they well they don't come

Together so for something at resthow does falling begin I mean the thinghas to move in this direction but it'sat rest rightwell yes but it's only at rest in spaceand that's not the whole story

Up down forward backward and left-rightare all you need to describe where anevent occur but a complete descriptionwill also need to describe when togetherthese four dimensions formed a settingin which everything in our universe

Happens space-time since we can talkabout a falling pencil using just onespatial dimension up and down we can usea piece of paper to model space-time forit okay so we've got up and down but wehave to add another direction the pencil

Moves in time now if no forces act onthe pencil it won't move through spaceit will only get older and as you cansee if all it does is get older it won'tfall if space-time was flat when I letgo of the pencil it wouldn't go anywhere

But now let's allow the earth which ismassive to manipulate space-time intosay a cone now with no forces acting onit every part of the pencil follows astraight line but on a cone as we sawearlier such a path will look like this

It will fall this is because distancesaround the cone are shorter higher uptime runs faster further from a massiveobject but to go straight not turn everypart of the pencil must cover an equaldistance in space-time like this

Only when the pencil hits the earth doesthe repulsion of their mutual electronsprovide a force pushing the pencil off ageodesic for the earth time is a seriesof slicesfrom this evolution the pencils

Force-free geodesic is why it falls nota push or pull just the pencils naturaltendency to follow a straight line untilsomething acts on it now we only usedone dimension of space and one of timebecause visualizing our two universes

Three of space and one of time wouldtake us beyond the limits of what couldbe shown on paper or screens but mathcan take us there general relativityallows us to calculate how much mass andenergy curved space-time and has been

Used to explain things that Newton'solder theory of falling as the result offorces couldn't like anomalies in theorbit of mercury which orbits nearestthe Sun and is therefore most affectedby the sun's grip on space-time many

Other experiments have confirmed generalrelativity's picture of the universefitting the conclusion that there is nogravitythere's just space-time its curvatureand a senate as John Wheeler famously

Put it space-time grips mass telling ithow to move mass grips space-timetelling it how to curve relative to theearth we don't move very fast even jetairplanes move negligibly close to thespeed of light so relative to earth we

Move almost exclusively through time assuch we are more affected by the waytime is curved by mass than how space iscurved this has led many to claim thatfor the most part you feel as thoughyou're being pushed into the ground not

Because of a force called gravity butbecause time is moving faster for yourhead than for your feet down is relativeand always changing but it existsbecause of and is always in thedirection of slower time Bertrand

Russell called this the law of cosmiclaziness everything is naturally steeredtowards where time is slowest we callthis falling goingso you don't have to keep anything onthe down-low

Time will take care of that for you andas always thanks for watchingremember that you can support Vsauce andAlzheimer's research by subscribing tothe Vsauce curiosity box the current onecomes with a code to get a free copy of

Universe sandbox 2 which is amazing anda whole host of other science toys andtools picked by myself Jake and Kevin Ilove it all so I hope to see you atbrain candy live we are coming to manymany cities very soon

Hopefully one near you by going to theshow you can see Adam and I doing thingsthat will you may not have seen us dobefore we also explore the science andcommon misconceptions behind all thingserr maybe I've said too much maybe not I

Hope to see you there and as alwaysthanks for watching

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