All language subtitles for [English (auto-generated)] Speed—Video [DownSub.com]

since speed is the fundamental core of

what drives the special theory of

relativity let's start at the beginning

and ask the most basic question of all

which is what is speed well we all know

the answer to that but let's get all on

the same page so if that car say has a

speed of a hundred kilometres per hour

we all know what that means if we look

at how far it's gone divided by how long

it takes it to get there so let's say

this is one hour journey if it's going

100 kilometres an hour we know that it

will have traveled 100 kilometers in

that hour that's what speed is so in

essence speed is nothing but distance

that an object travels divided by

duration now Express in that language

speed might seem like I don't know a

kind of boring concept a pedestrian

concept why concern yourself with speed

the answer to that is clear if we

recognize that distance is a measure

that has to do with space duration is a

measure that has to do with time so if

we find as we are going to find going

forward that speed has unusual features

when the speeds involved get very big

near the speed of light what we really

will therefore be learning is that space

and time have weird strange features

that is what we are after

ok good that's where we're headed let's

start by first thinking about the non

strange features of speed the features

that we all hold in our intuition so

what are the basic features of speed

well first off speed is a concept that

even before Einstein was known is a

concept that is relative what do I mean

by that well imagine we look at this car

say it's going 100 kilometers per hour

what we need to say is 100 kilometers

per hour relative to the road why if

that Road is itself moving let's say

it's on a boat the boat is moving then

the cars speed relative to the water is

not a hundred kilometers per hour and if

we zoom out and look at that car on the

surface of the earth and we realize that

the earth itself is spinning around the

earth itself is going in orbit around

the Sun with respect to the frame of

reference our perspective right now that

car is executing a pretty complicated

motion not just 100 kilometers per hour

so it's always vital when you're talking

about speed to recognize that you can

only ever frame the idea of speed for

ordinary objects that we encounter as

the object has this and that speed

relative to this or that object you need

to specify the reference in order that

the speed that you are specifying has

any meaning at all okay so that's a very

basic feature of speed that it is

relative another basic feature speed is

that it is additive and subtractive so

what do I mean by that well let's

imagine that we have two characters who

are playing a game of catch with say

football George and Gracie two

characters that we are going to

encounter often in our discussions and

imagine that they're throwing that

football back and forth at say five

meters per second now

that's all well and good completely

understood but let's imagine that they

go out to play a game of catch on

another day and Gracie is surprised to

see that George has a hand grenade now

she doesn't like hand grenade

so she runs away when he throws it

because she knows that by running away

from the hand grenade she can change the

speed at which it approaches her she can

subtract her own speed from the speed of

the oncoming grenade and that way have

it approach her not say at the initial

speed at which it was tossed which for

instance might be just like the football

five meters per second instead if she

runs away at three meters per second she

knows that now the grenade will approach

her more slowly at two meters per second

and that is a good thing when it comes

to hand grenades similarly it's the case

that if an observer like Gracie is to

not run away but say run toward an

object that is being thrown at her the

speed with which he approaches the

object will be added to the rate at

which it approaches her so if it was

thrown at five and she's running toward

it at three well we all know that that

means it will approach her at eight

meters per second speed is additive and

subtractive you can change the speed

with which an object is coming toward

you by either running toward it or away

from it now let me quickly mention just

as a small footnote these basic

calculations that we've done here we

surprising are going to find that they

are only approximate when you take into

account some of the strange features of

relativity but that's something that we

will encounter later especially if

you're doing the math version of this

course but in terms of the basic idea

that comes from our intuition you

certainly would anticipate that if you

run toward an object at speed will

approach you more quickly if you run

away from an object it'll approach you

more slowly third basic feature of speed

that again we are all familiar with is

this when you are executing a very

special kind of motion what we call

stint velocity motion motion that has a

fixed speed fixed magnitude in a fixed

direction then you can't feel that

motion right you can't feel that motion

for a very good reason if you are

executing constant velocity motion you

are completely justified in claiming to

be at rest and the rest of the world

moving by you in that sense constant

velocity motion is very special because

it is motion that is completely

subjective there is no absolute notion

of being in motion when the velocity is

constant so let me give you a quick

example of that so let's imagine that we

have one and the same physical situation

described from two different

perspectives so what I'm going to

imagine is having George and Gracie

floating in space okay floating in space

now here's George's view of the events

he looks out and he sees a character

Gracie coming toward him and she waves

as she passes as does he his perspective

is that he is stationary and she is

rushing by him good now I'm going to

show you exactly the same situation but

from Gracie's perspective so what does

Gracie say from her perspective she says

that she is stationary out there in

space she looks out into the distance

and she sees George rushing by he waves

she does too and he goes on his merry

way is one perspective right and the

other wrong absolutely not you are

completely justified if you're not

accelerating if you aren't changing your

speed or changing the direction of

motion to say that you are stationary

now

the reason why I am emphasizing constant

velocity motion is something that you

have all experienced right if you are in

a car and you take a show

our turn you feel your body being pushed

this direction you know that you are

moving if you are in an airplane and

it's taking off as it accelerates as it

speeds up you feel yourself pushed back

into the seat you know that you are in

motion but if you're not accelerating

you don't feel the motion and in fact

there is no way for you to detect the

motion at all so for instance if you

imagine that George and Gracie we're in

two floating laboratories out there in

space and they do experiments to work

out the laws of physics they will work

out exactly the same laws because there

will be absolutely no remnant no

experimental implication of their

relative motion if that motion is a

constant speed in a fixed direction no

way to determine your state of motion

because you are justified in saying that

you are at rest now that idea that idea

that you can claim to be arrest that

there's no implication of constant

velocity motion does not start with

special relativity it does not start

with Albert Einstein this is an idea

actually that goes way way back it goes

all the way back to Galileo and Galileo

wrote a wonderfully poetic description

of this idea let me show you a little

visual representation of what he said

and I'll just read his words too while

this plays so he said shut yourself up

on a large ship and there procured gnats

flies and other small winged creatures

he said let a bottle be hung up which

drop by drop lets forth its water into

another narrow neck bottle placed

underneath then with a ship lying still

observe how the winged animals fly with

like velocity told all parts of the room

how the distilling drops all fall into

the bottle place underneath then he says

having observed all these particulars

may

the ship move with whatever velocity you

please so here it is the ship is going

into motion and he says that so long as

the motion is uniform by which he means

constant velocity you shall not be able

to discern the least alteration in all

the four named effects nor can you

gather by any of them whether the ship

is moving or standing still that is the

very same idea that constant velocity

motion cannot be detected it has no

impact on your observations old idea

back with Galileo so where then does

Einstein come into the story

Einstein's new contribution is to say

that among the four named effects that

Galileo was talking about he was just

talking about the gnats and flies and

the water dropping into the bottle place

underneath that those things would not

change if you go into motion so long as

it's uniform Einstein added something to

the list

Einstein added to the list of things

that would not change he added the speed

of light this is the surprising new

insight of Einstein let's see what it

means