﻿TWENTIETH 
  CENTURY 
  PHYSICS 
  MILLIKAN 
  195 
  

  

  It 
  is 
  well 
  known 
  with 
  what 
  joy 
  the 
  astronomers 
  have 
  seized 
  upon 
  

   this 
  fact 
  to 
  enable 
  theiii- 
  to 
  escape 
  their 
  otherwise 
  insuperable 
  dif- 
  

   ficulties 
  encountered 
  because 
  the 
  sun, 
  for 
  example, 
  can 
  not 
  possibly 
  

   have 
  been 
  pouring 
  out 
  heat 
  as 
  long 
  as 
  it 
  is 
  now 
  known 
  to 
  have 
  been 
  

   doing, 
  if 
  it 
  is 
  merely 
  a 
  hot 
  body 
  cooling 
  off. 
  If, 
  however, 
  it 
  has 
  the 
  

   capacity 
  at 
  the 
  enormous 
  temperatures 
  existing 
  in 
  its 
  interior, 
  say 
  

   40,000,000° 
  C, 
  of 
  transforming 
  its 
  very 
  mass 
  into 
  radiant 
  energy, 
  

   then 
  these 
  particular 
  difficulties 
  disappear. 
  But 
  what 
  a 
  shock 
  it 
  

   would 
  be 
  to 
  Lord 
  Kelvin 
  if 
  he 
  could 
  hear 
  the 
  modern 
  astronomers 
  

   talking 
  about 
  the 
  stars 
  radiating 
  away 
  their 
  masses 
  through 
  the 
  

   mere 
  act 
  of 
  giving 
  off 
  light 
  and 
  heat, 
  and 
  this 
  is 
  now 
  orthodox 
  

   astronomy. 
  

  

  And 
  again, 
  if 
  they 
  do 
  so 
  in 
  accordance 
  with 
  the 
  Einstein 
  equation, 
  

   then 
  is 
  it 
  not 
  more 
  than 
  probable 
  that 
  the 
  process 
  is 
  also 
  going 
  on 
  

   somew^here 
  in 
  the 
  opposite 
  sense 
  and 
  that 
  radiant 
  energy 
  is 
  con- 
  

   densing 
  back 
  into 
  mass, 
  that 
  new 
  worlds 
  are 
  thus 
  continually 
  form- 
  

   ing 
  as 
  old 
  ones 
  are 
  disappearing? 
  These 
  are 
  merely 
  the 
  current 
  

   speculations 
  of 
  modern 
  physics, 
  based, 
  however, 
  upon 
  the 
  now 
  fairly 
  

   definite 
  discovery 
  that 
  conservation 
  of 
  matter 
  in 
  its 
  nineteenth 
  

   century 
  sense 
  is 
  invalid. 
  

  

  Some 
  time 
  ago 
  I 
  was 
  one 
  of 
  the 
  speakers 
  at 
  a 
  forum, 
  and 
  in 
  the 
  

   course 
  of 
  my 
  address 
  I 
  used 
  the 
  word 
  " 
  spirit 
  " 
  a 
  number 
  of 
  times. 
  

   When 
  questions 
  were 
  afterward 
  called 
  for, 
  a 
  man 
  arose 
  in 
  the 
  rear 
  

   of 
  the 
  room 
  and 
  with 
  a 
  somewhat 
  hostile 
  air 
  asked 
  if 
  the 
  speaker 
  

   would 
  define 
  what 
  he 
  meant 
  by 
  the 
  word 
  " 
  spirit." 
  I 
  replied 
  that 
  

   if 
  the 
  interrogator 
  would 
  be 
  good 
  enough 
  to 
  define 
  for 
  me 
  the 
  word 
  

   " 
  matter 
  " 
  I 
  would 
  attempt 
  to 
  define 
  for 
  him 
  the 
  word 
  " 
  spirit." 
  

   The 
  attempt 
  was 
  not 
  called 
  for. 
  And, 
  in 
  fact, 
  in 
  view 
  of 
  the 
  

   growth 
  of 
  twentieth 
  century 
  physics 
  and 
  the 
  changes 
  in 
  our 
  con- 
  

   ception 
  of 
  matter 
  that 
  it 
  has 
  brought, 
  it 
  is 
  to-day 
  quite 
  as 
  difficult 
  

   to 
  find 
  a 
  satisfactory 
  definition 
  of 
  " 
  matter 
  " 
  as 
  of 
  " 
  spirit." 
  

  

  Fifth. 
  But 
  what 
  do 
  we 
  now 
  know 
  about 
  the 
  nature 
  of 
  this 
  phenom- 
  

   enon 
  which 
  we 
  have 
  called 
  radiant 
  energy, 
  with 
  the 
  aid 
  of 
  which 
  the 
  

   masses 
  of 
  the 
  stars 
  are 
  being 
  dissipated 
  into 
  space? 
  In 
  a 
  word, 
  

   where 
  is 
  now 
  the 
  nineteeth 
  century 
  jDhysics 
  of 
  the 
  ether? 
  

  

  The 
  physics 
  of 
  the 
  ether 
  meant 
  in 
  1890 
  the 
  physics 
  of 
  electro- 
  

   magnetic 
  waves, 
  and 
  it 
  means 
  precisely 
  that 
  now. 
  Electromagnetic 
  

   waves 
  are 
  sharply 
  and 
  definitely 
  recognizable 
  by 
  certain 
  observed 
  

   properties. 
  Thus, 
  in 
  the 
  first 
  place, 
  electromagnetic 
  waves 
  travel 
  

   through 
  space 
  with 
  an 
  exactly 
  measurable 
  speed, 
  namely, 
  the 
  speed 
  

   of 
  light, 
  i. 
  e., 
  186,000 
  miles 
  per 
  second. 
  Second, 
  they 
  all 
  exhibit 
  a 
  

   definite 
  measurable 
  periodicity, 
  or 
  frequency, 
  which 
  divided 
  into 
  

   the 
  velocity 
  of 
  light, 
  gives 
  the 
  wave 
  length. 
  Third, 
  they 
  all 
  exhibit 
  

   another 
  measurable 
  property 
  described 
  by 
  the 
  words 
  " 
  state 
  of 
  

   polarization," 
  the 
  precise 
  definition 
  of 
  which 
  need 
  not 
  here 
  be 
  given. 
  

  

  