﻿PHYSICS 
  OF 
  THE 
  UNIVERSE 
  JEANS 
  169 
  

  

  lives 
  its 
  appointed 
  life 
  serenely 
  undisturbed 
  by 
  external 
  accidents 
  of 
  

   heat 
  or 
  pressure; 
  when 
  its 
  hour 
  strikes 
  it 
  will 
  cease 
  to 
  exist 
  as 
  

   uranium 
  and 
  will 
  proceed 
  to 
  disintegrate 
  into 
  lead, 
  helium, 
  and 
  

   radiation. 
  Its 
  nucleus 
  slips 
  back 
  to 
  a 
  state 
  of 
  lower 
  energy, 
  the 
  lost 
  

   energy 
  being 
  put 
  in 
  evidence 
  as 
  emitted 
  radiation. 
  On 
  the 
  other 
  hand 
  

   the 
  change 
  produced 
  in 
  ordinary 
  atoms 
  by 
  electronic 
  rearrangement 
  

   is 
  extremely 
  susceptible 
  to 
  external 
  physical 
  conditions. 
  Every 
  

   spectroscopist 
  knows 
  how 
  to 
  chip 
  off 
  one, 
  two, 
  or 
  even 
  three 
  electrons 
  

   from 
  the 
  atom 
  at 
  will. 
  Nevertheless, 
  as 
  was 
  first 
  made 
  clear 
  in 
  a 
  

   remarkable 
  paper 
  which 
  Einstein 
  published 
  in 
  1917,^ 
  the 
  difference 
  

   is 
  merely 
  one 
  of 
  degree 
  and 
  not 
  of 
  kind. 
  

  

  The 
  electrons 
  in 
  an 
  atom 
  are 
  free 
  to 
  move 
  from 
  one 
  orbit 
  to 
  another, 
  

   and 
  as 
  the 
  various 
  possible 
  orbits 
  have 
  difi'erent 
  energies, 
  the 
  atom 
  

   constitutes, 
  to 
  some 
  extent, 
  a 
  reservoir 
  of 
  energy. 
  For 
  example, 
  

   the 
  hydrogen 
  atom 
  consists 
  of 
  a 
  single 
  proton 
  as 
  central 
  nucleus, 
  

   and 
  a 
  single 
  electron 
  revolving 
  round 
  it. 
  According 
  to 
  Bohr's 
  

   theory, 
  the 
  electron 
  can 
  revolve 
  in 
  orbits 
  whose 
  diameters 
  (or 
  major 
  

   axes) 
  are 
  proportional 
  to 
  the 
  squares 
  of 
  the 
  natural 
  numbers, 
  1, 
  4, 
  

   9, 
  16, 
  25, 
  ... 
  The 
  differences 
  of 
  energy 
  between 
  the 
  various 
  

   orbits 
  are 
  easily 
  calculated; 
  for 
  example, 
  the 
  smallest 
  two 
  orbits 
  

   differ 
  in 
  energy 
  by 
  16 
  X 
  10~'^ 
  erg. 
  If 
  we 
  add 
  16 
  X 
  10"^^ 
  erg 
  of 
  energy 
  

   to 
  an 
  atom 
  in 
  which 
  the 
  electron 
  is 
  describing 
  the 
  smallest 
  orbit 
  of 
  

   all, 
  it 
  crosses 
  over 
  to 
  the 
  next 
  orbit, 
  absorbing 
  the 
  16 
  X 
  10"'^ 
  erg 
  in 
  the 
  

   process 
  and 
  so 
  becoming 
  temporarily 
  a 
  reservoir 
  of 
  energy 
  holding 
  

   16X10"^^ 
  erg. 
  If 
  the 
  atom 
  is 
  disturbed, 
  it 
  may 
  of 
  course 
  discharge 
  

   the 
  energy 
  at 
  any 
  time, 
  or 
  it 
  may 
  absorb 
  still 
  more 
  energy 
  and 
  so 
  

   increase 
  its 
  store. 
  But 
  if 
  it 
  is 
  left 
  entirely 
  undisturbed, 
  the 
  electron 
  

   must, 
  after 
  a 
  certain 
  time, 
  lapse 
  back 
  spontaneously 
  to 
  its 
  original 
  

   smaller 
  orbit. 
  If 
  it 
  were 
  not 
  so, 
  Planck's 
  well-established 
  law 
  of 
  

   black-body 
  radiation 
  could 
  not 
  be 
  true. 
  In 
  this 
  process 
  the 
  atom 
  

   ejects 
  16X10~^^ 
  erg 
  of 
  energy 
  in 
  the 
  form 
  of 
  radiation 
  and, 
  as 
  a 
  

   consequence, 
  experiences 
  a 
  diminution 
  of 
  mass 
  to 
  the 
  extent 
  of 
  

   1.8X10"^^ 
  gm. 
  Thus 
  a 
  collection 
  of 
  hydrogen 
  atoms 
  in 
  which 
  the 
  

   electrons 
  describe 
  orbits 
  larger 
  than 
  the 
  smallest 
  possible 
  is 
  similar 
  

   to 
  a 
  collection 
  of 
  uranium 
  atoms 
  in 
  that 
  the 
  atoms 
  spontaneously 
  

   lapse 
  back 
  to 
  their 
  states 
  of 
  lower 
  energy 
  as 
  a 
  result 
  merely 
  of 
  the 
  

   passage 
  of 
  time, 
  losing 
  mass 
  and 
  emitting 
  radiation 
  in 
  the 
  process. 
  

  

  We 
  have 
  spoken 
  of 
  adding 
  16X10"'^ 
  erg 
  of 
  energy 
  to 
  a 
  hydrogen 
  

   atom 
  in 
  its 
  state 
  of 
  lowest 
  energy. 
  We 
  can 
  not 
  of 
  course 
  do 
  this 
  

   simply 
  by 
  pouring 
  miscellaneous 
  energy 
  on 
  the 
  atom, 
  and 
  expecting 
  

   it 
  to 
  drink 
  it 
  up. 
  The 
  hydrogen 
  atom 
  only 
  accepts 
  energy 
  which 
  is 
  

   offered 
  it 
  in 
  the 
  form 
  of 
  radiation 
  of 
  precisely 
  the 
  right 
  wave 
  length; 
  

   it 
  treats 
  all 
  other 
  radiation 
  with 
  complete 
  indifference. 
  Every 
  atom 
  

  

  » 
  Phys. 
  Zeitsch., 
  vol. 
  48, 
  p. 
  122, 
  1917. 
  

  

  