What is Time?

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11 Δεκ 2013 (πριν από 3 χρόνια και 8 μήνες)

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What is time?


Albert

Einstein

defined

time

as

“that

which

a

clock

measures”
.

T h e

d e f i n i t i o n

of

time

is

traditionally

considered

‘on

the

fence’

between

physics

and

philosophy
.

How

can

we

define

what

it

is?



Time

quantifies

or

measures

the

interval

between

events,

or

the

duration

of

events
.

Time

has

long

been

perceived

as

a

dimension

in

which

each

event

has

a

definite

(but

not

necessarily

unique)

position

in

a

linear

sequence,

but

as

differing

from

spatial

dimensions

in

that

"motion"

through

time

appears

restricted

to

having

only

a

forward

direction
.


For

everyday

purposes,

and

even

for

quite

accurate

measurements,

this

view

is

sufficient
.

However,

the

Classical Physics and Time Asymmetry


It

is

a

common

statement

that

Newtonian

physics

is

invariant

under

time

reversal
.

T h i s

s y m m e t r y

e s s e n t i a l l y

c o m e s

f r o m

t h e

f a c t

t h a t

t h e

f u n d a m e n t a l

N e w t o n i a n

e q u a t i o n,


F

=

d
p
/d t

r e m a i n s

u n c h a n g e d

w h e n

s u b j e c t e d

to

the

map




x

-
>

x


t

-
>

-
t


v

-
>

-
v


This

symmetry

would

seem

to

be

at

odds

with

thermodynamics,

which

violates

time
-
symmetry

via

the

total

increase

in

entropy

of

a

system

with

increasing

time
.

In

a

world

governed

entirely

by

time
-
symmetrical

laws,

could

the

second

law

of

thermodynamics

be

valid,

or

is

it

the

product

of

an

implicit

assumption

of

the

arrow

of

time?


Time, Relatively Speaking


By

the

end

of

the

19
th

century,

people

thought

that

time

flowed

uniformly,

independent

of

our

intrinsic

existence
.

In

1905
,

a

young

patent

clerk

named

Albert

Einstein

wrote

a

scientific

paper

that

changed

the

way

we

look

forever
.

He

hypothesized

that

the

passage

of

time

is

relative

to

the

person

measuring

it
.

He

stated

that

there

exists

a


time
-
dilation


when

one

is

moving

relative

to

another

which

is

quantitavely

given

by

the

Lorentz

transformation
:







where

t,t


are

the

times

measured

by

each

observer

in

their

respective

inertial

reference

frames,

x

the

position

of

the

unprimed

observer,

v

is

their

relative

velocity

and

c

is

the

speed

of

light
.

We

can

see

from

this

transformation

that

as

a

body

speeds

up,

the

time

measured

in

its

frame

dilates

in

comparison

to

that

of

the

other
.

In

1915
,

Einstein

published

another

ground

breaking

paper

in

which

he

incorporated

gravity

into

his

theory

of

relativity
.

He

showed

that

in

a

strong

gravitational

field,

time

also

slows

down

for

relative

observers
.

The

fabric

of

Einstein

s

universe

is

a

4

dimensional

space
-
time,

where

gravity

is

described

not

as

a

force,

but

as

a

curvature

of

this

space
-
time
.

A

body

in

gravitational

orbit

is

simply

following

a

geodesic

(the

path

of

shortest

distance)

in

4

dimensional

space
-
time
.


Arrow of Time


A

property

one

can

look

at

with

time

is

something

called

the


Arrow

Of

Time

.

When

real

time

is

discussed,

it

has

a

forward

and

backward

direction
.

The

laws

of

science

do

not

distinguish

between

the

past

and

the

future

but

you

can

see

the

sharp

contrast

in

everyday

life
.

Imagine

recording

a

glass

falling

off

a

table,

you

could

easily

tell,

while

watching

it,

when

the

tape

is

being

played

backward

or

when

been

played

forward
.

This

is

because

it

would

be

very

strange

to

see

a

glass

reforming

while

defying

gravity!


In

a

simulation

under

classical

laws,

it

is

found

that

by

reversing

the

velocities

of

all

of

the

particles

in

a

simple

gaseous

system

simultaneously,

a

net

decrease

in

entropy

did

indeed

result
.

However,

any

small

random

deviation

from

these

‘backward

path’

velocities

put

the

system

onto

an

upward

entropy

climb

once

more
.

It

is

postulated

that

random

influences

are

the

source

of

the

appearance

of

the

arrow

of

time

from

statistical

mechanics
.

Thi s

i dea

poses

some

i nt er est i ng

quest i ons

if

true
:

for

a

closed

system,

such

as

the

universe

as

a

whole

may

be,

would

classical

mechanics

then

produce

a

direction

of

increasing

entropy?

A n d,

if

randomness

is

the

key,

where

does

the

asymmetry

lie

that

causes

randomness

to

be

generated

in

one

direction

of

time,

and

not

the

other?


This

is

because

everything

must

obey

the

second

law

of

thermodynam
ics

which

states

that

in

a

closed

system,

disorder

{entropy}

must

always

increase

with

time
.

This

is

one

example

of

an

“Arrow

Of

Time”
.

Another

is

ones

own

personal

arrow

of

time
.

This

is

an

arrow

that

everyone

feels

time

pass
.

The

third

arrow

of

time

is

one

associated

with

the

expanding/contracting

of

our

universe
.

All

three

arrows

point

in

the

same

direction

defining

our

life

of

“remembering

the

past,

not

the

future”


Why

do

we

see

all

three

arrows

pointing

in

the

same

direction?

One

looks

to

the

anthropic

principle,

which

states

that


We

see

the

universe

the

way

it

is

because

if

it

were

different,

we

would

not

be

here

to

observe

it


This

answer

stems

from

various

arguments

producing

the

result

that

if

one

arrow

were

pointing

opposite

to

another,

life

would

not

be

possible

due

to

uncertain

results

of

non
-
standard

rules

of

thermodynamics

in

this

hypothetical

universe
.

So

whichever

arrow

of

time

y o u

f e e l

m o s t

i n t r i n s i c

to

you,

rest

assured

its

pointing

the

right

way
.


The universe tends


towards disorder

Time Reversible

Not exactly Time reversible

Ice



Low Entropy

Puddle


High Entropy

Time

The Beginning and End of Time


Most

physicists

agree

on

the


Big

Bang


theory

which

states

that

the

universe

exploded

out

of

a

single

point

or

singularity

sometime

between

13

and

15

billion

years

ago
.

It

was

initially

suggested

because

it

explains

why

distant

galaxies

are

receding

at

such

great

speeds
.


Time

,

as

it

is,

was

created

in

this

singularity

and

ends

in

a

singularity

cloaked

behind

the

event

horizon

of

a

black

hole
.

When

we

observe

very

distant

galaxies

we

are

in

fact

looking

back

in

time

at

the

early

universe
.

The

more

red

shifted

the

spectrum

of

their

light,

the

quicker

they

are

receding

from

us,

the

further

away

they

are

and

the

further

back

in

time

we

are

seeing
.

The

theory

also

predicts

the

cosmic

background

radiation,

the

afterglow

of

the

early

universe
.

In

the

very

earliest

fractions

of

a

second

after

time

t=
0
,

the

basic

constituents

of

matter

still

hadn't

come

into

existence,

and

the

fundamental

forces

of

nature

were

unified

into

one

force
.


And

the

future

of

the

universe?

Well

that

depends

on

the

gravitational

mass

of

the

matter

in

the

universe
.

If

it

is

great

enough

to

halt

the

universe's

expansion,

then

Universe

is

said

to

be

closed,

and

time

will

end

in

a

Big

Crunch

just

like

it

started

with

a

Big

Bang
.

Otherwise

the

Universe

is

said

to

be

open

and

galaxies

will

continue

receding

until

eventually

even

single

elementary

particles

will

be

separated

by

vast

distances
.

We

therefore

see

that

time

can

be

considered

as

a

quantity

which

can

be

changed,

manipulated

and

perhaps

reversed
.

The

t est s

of

Einstein

s

theories

have

been

thorough

and

very

successful

over

the

past

century

showing

that

time

is

not

just

a

concept

invented

by

human

civilization,

but

something

we

feel

by

virtue

of

our

existence

and

maybe

someday,

something

we

can

touch
.


References :




"Time Reversal Symmetry Violation and the H
-
Theorem“

-

A. Aharony, Physics

L
etters Vol 37A No1.



"The Enigma of Time“

-

P. T. Landsberg.



"Physics of Time Asymmetry“

-

PCW Davies.



"A Chronicle of Timekeeping“

-

W
. An
drews, Scientific American Vol.287 No.3



“Cosmology, a Very Short Introduction”


Peter Coles


scientific

understanding

of

time

underwent

a

revolution

in

the

early

part

of

the

twentieth

century

with

the

development

of

relativity

theory
.

Modern

physics

treats

time

as

a

feature

of

spacetime,

a

notion

which

challenges

intuitive

conceptions

of

simultaneity

and

the

flow

of

time

in

a

linear

fashion
.


Also,

from

ubiquitous

practical,

and

some

key

theoretical

evidence,

we

can

say

that

there

is

some

sort

of

orientation

of

time

that

qualitatively

distinguishes

‘future’

from

‘past’
.



So,

much

like

many

other

quantities,

we

may

try

to

define

time

by

giving
:

1
)

a

basis

for

an

orientation,

or

arrow

of

time,

and


2) an ability to measure the magnitude of a time interval.


John Tracey

Simon Hall

Michael Stewart

Stephen Hardiman

supervised by Dr. Stefan Hutzler



“The Road to Reality”


Roger Penrose



“Black Holes & Time Warps

-

Kip S. Thorne




“A Brief History of Time” Chapter 9, “Arrow Of Time”

-

Stephen W. Hawking




http://www.pbs.org/deepspace/timeline/

(for universe timeline timeline)



http://www.tycho.usno.navy.mil/leapsec.html



http://www.big
-
bang
-
theory.com

A

month

is

the

approximate

time

needed

for

the

moon

to

revolve

once

around

the

earth
.

The

lunar

month

actually

takes

29

days,

12

hours,

44

minutes

and

3

seconds
.

There

is

another

natural

time

divide

in

the

form

of

the

cyclic

change

in

weather
:

the

changing

seasons
.

The

cycle

lasts

for

one

year

-

now

defined

as

the

time

for

one

revolution

of

the

earth

around

the

sun

equal

to

365

days,

five

hours,

48

minutes

and

46

seconds

or

approximately

365
¼days
.

In

order

that

we

have

our

nice

365

days

in

one

calendar

year

we

introduce

an

extra

day

to

the

year

every

4

years

in

what

has

become

known

as

a

leap

year
.

To

account

for

the

minute

difference

of

11
minutes

and

14

seconds

less

than

365
¼days

we

make

a

century

year

a

leap

year

if

and

only

if

it

is

divisible

by

400
.

This

gives

us

an

extra

3
,
300

years

before

the

calendar

and

solar

year

again

differ

by

one

day
.


The

earth

is

undergoing

a

slight

deceleration

caused

by

breaking

action

of

tides

and

other

effects
.

Civil

time

must

be

adjusted

by

one

second

to

ensure

that

the

difference

between

a

uniform

time

scale

defined

by

atomic

clocks

does

not

differ

from

the

earth’s

rotational

time

by

more

than

0
.
9

seconds
.

This

has

become

known

as

a

leap

second
.




Measurement of Time


Before

the

dawn

of

civilisation

a

natural

time

divide

existed



Night

and

Day
.

Upon

the

arrival

of

organised

civilisation

and

government,

a

time

measurement

system

was

developed

unique

to

each

distinct

culture
.

The

Babylonian

and

Roman

systems

certainly

have

had

an

effect

on

the

system

used

almost

worldwide

today
.

The

Romans,

for

instance,

re
-
named

the

months

July

and

August

after

Julius

Caesar

and

Augustus

respectively
.



In

1967
,

at

the

Thirteenth

General

Conference

on

Weights

and

Measures,

it

was

agreed

to

define

the

second

within

the

SI

(Systeme

International

d’Unites)

System

as

the

duration

of

9
,
192
,
631
,
770

periods

of

the

radiation

corresponding

to

the

transition

between

two

hyperfine

levels

of

the

ground

state

of

the

cesium
-
133

atom
.

It

follows

that

there

are

then

60

seconds

in

a

minute,

60

minutes

in

an

hour

and

a

day

is

the

time

it

takes

for

the

earth

to

spin

on

it’s

axis,

or

24

hours
.

Seven

days

make

a

week
.

Keeping Time


In

the

light

of

Einstein's

theory

of

relativity,

it

is

a

truism

to

say

that

humanity

had

previously

seen

time

as

uniform,

regardless

of

position

or

velocity
.

Ho we v e r,

in

practical

terms

time

was

more

relative

five

hundred

years

ago

than

today
.

Al t hough

mec hani c al

c l oc ks

had

been

in

use

since

at

least

1283
1
,

primarily

for

the

purpose

of

religious

observance,

time

standards

were

still

dependent

on

the

location

at

which

they

were

measured
.

Towns

used

t he

posi t i on

of

the

sun

to

directly

determine

the

time
;

since

noon

differed

from

place

to

place,

so

did

the

time
.

It

was

not

until

the

development

of

railroad

that

there

grew

a

need

for

a

standardised

time,

the

first

such

standard

being

laid

down

in

Harvard

University

Observatory

in

1851
,

followed

closely

by

the

Royal

Observatory's

standard

time

for

Britain
.

T h i s

i n c r e a s i n g l y

a r b i t r a r y

i n t e r p r e t a t i o n

of

time

dissociated

it

from

the

sun

and

stars,

culminating

in

the

worldwide

standardisation

to

Greenwich

Mean

Time,

at

the

1884

International

Meridian

Conference
.

&

Oscillators


a basis for time


To

measure

the

length

of

a

physics

poster,

we

place

a

meter

stick

against

it

and

compare

its

endpoints

with

the

marks

of

the

meter

stick
.

To

measure

a

length

of

time,

we

need

a

corresponding

‘time

yardstick’
.

Ancient

humans

used

the

dawn

and

sunset

as

markers

of

time
;

later,

the

cycle

of

seasons

and

the

phases

of

the

moon
.

These,

of

course,

are

all

examples

of

oscillations,

and

oscillations

are

still

the

source

of

human

timekeeping,

in

every

form

from

quartz

crystals

to

cesium

clocks
.


The steady swing of
an oscillating
pendulum, provides
a basic yardstick for
measuring time

The large mass
of the planet
‘curves’ its local
space
-
time