Time From the Perspective of a Particle Physicist

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Time From the Perspective of a Particle Physicist

• David Hedin
• Department of Physics
• August 8, 2001

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Outline

• Time in our everyday lives
• Looking back in time - examples from Astronomy
• Looking back in time - examples from high energy
  accelerators

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Markers of Time

• DAY Sun at maximum height
• MONTH length of time it takes for the moon to
  make an orbit around the Earth (repeats phase
  every 29.5 days)
• Most early cultures use the day and month to mark
  time
• moon-month-measure-man may all have the same
  root

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Length of Day and Month are changing

• Friction between the Earth and the Moon
  (seen daily in tides)
• Day becomes .002 seconds longer each century
• Moon receeding from the Earth by 4 cm each year
• 500,000,000 years ago there were
• 22 hours in a day
• 400 days in a year
• Billions of years in the future there will be
• 1 day 47 present days
• 1 month 1 day
• Earth-Moon frozen into a pair with no additional
  spin for the Earth alone

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The Year

• Two Indicators
• Due to the Earths tilt the
• Length of the Day and
• Suns path through the sky
• vary. One year returns to the same spot
• More dramatic further north (Stonehenge)
• Which stars are overhead changes with seasons.
  Gives passage of year
• Passage of time at night also given by stars
  apparent motion
• Stars Calendar and Clock

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Star Wheel

• Stars move East to West over the course of one
  Night (in circle about the North Star)
• Stars move East to West by 2 hours per month
  and return to the same position after one Year
• just Earths daily spin and yearly orbit about
  the Sun

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What Year Is It?

• Almost all societies agree - 2001
• But where should time really start?
• Formation of Earth 4.5 billion years ago
• Formation of Sun 5.0 billion years ago
• Sun will continue as a star for another
• 5 billion years and then transform into
• a Red Giant and then a White Dwarf.
• The Sun orbits the center of the Milky
• Way galaxy every 250 million years
• So now in the 20th galactic year
• Formation of Universe 13 billion years ago

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365.242 days in a year

• Ancient calendars were Lunar
• Babylon - 12 months 6 with 29 days and 6 with
  30. Add 13th month occasionally (also used in
  India and similar in China)
• Egypt - 12 months each 30 days plus 5 extra
• Polynesia - 13 lunar months drop 1 occasionally
• Priests would determine when to add extra months
  and day
• Very tempting to have 360 days in a year and 12
  months of 30 days. nice numbers
• Lack of correlation between day-month-year
  bothered philosophers and theologians.
  Understanding this random motion (and the
  planets were even worse) by Copernicus, Kepler,
  Galileo, Newton gave us modern science

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365.242 days in year

• If normal year has 365 days need extra 24
  days/century and extra 2 days/millenium
• 46 BC Julius Ceaser (really Sogigula an Egyptian)
  - Julian calendar with leap day every 4 years.
  But 8 too many days every 1000 years so.
• Gregorian calendar adopted
• Spain and Catholic Europe 1582
• England
  1751
• Russia
  1918
• which immediately skipped 10 days (in 1582).
  No leap day on century years 1700,1800,1900,2100,2
  200 (just those divisible by 400)

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Time in Astronomy

• Lives of Stars
• stars are born in interstellar gas clouds
  and die once they have exhausted their nuclear
  fuel
• more massive stars burn their fuel faster
  and may only exist for 1 million years while
  stars like our Sun (or smaller) will exist for 10
  billion years or longer
• Clusters of Stars
• stars are formed in groups with some massive
  and some small. The size of the largest stars in
  the cluster tells us how old all the stars in
  that cluster are.
• The oldest clusters are formed from the
  primordial material of the Universe (before stars
  existed) and their atomic composition is a fossil
  record from the first few minutes after the
  Universe was created

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Galaxies

• Stars come in groups of 200 billion or more like
  our own Milky Way Galaxy
• as they are so bright galaxies can be seen if if
  extremely far away - 5 billion light years
• as light takes time to travel to us, looking at
  galaxies very far away means we are looking back
  in time. Soon the latest telescopes will have
  study more and more galaxies from the time when
  they (and the stars they contain) were first
  formed

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Galaxies and Hubbles Law

• It has been observed (first by Hubble in the
  1920s) that galaxies are moving away from us and
  that the further away they are the faster they
  are moving (vHd)
• Indication that the Universe is expanding, and it
  has been ever since it was created in the Big
  Bang about 13 billion years ago
• Understanding how the expansion rate changes with
  time tells us about the inherent mass and energy
  which makes up the Universe, a more precise
  values for its age, and what its fate will be
• Current data is perplexing most of the mass is
  missing and due to some unexplained new types
  of matter and there is vacuum energy which is
  acting like a kind of anti-gravity and
  accelerating the expansion

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More on Hubbles Law

• As the Universe expands it cools down. At its
  earlier times it was much, much hotter.
• If the Universe keeps on expanding forever
  everything will come to a cold end, the stars
  will all end there lives and no new ones will be
  formed
• if the expansion stops and a contraction begins
  the Universe will heat up as it returns to a
  state similar to when it was formed
• Not unambiguously known what fate will be

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Fire and IceRobert Frost - 1923

• Some say the world will end in fire,
• Some say in ice.
• From what Ive tasted of desire
• I hold with those who favor fire.
• But if I had to perish twice,
• I think I know enough of hate
• To say that for destruction ice
• Is also great
• And would suffice.

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Exploring Very Early Times

• Fossil evidence available to astronomy are
  remnants from the first few minutes after the Big
  Bang
• To explore back to earlier times we use our
  understanding of physics
• The earlier you go in time the hotter was the
  Universe. Particle accelerators can briefly
  reproduce those conditions. The highest energy
  machine is equivalent to about 1 picosecond
  (.000000000001) after the universe began
• Even earlier times can be understood by
  extrapolating symmetries in Nature but going back
  to the moment of Creation needs a complete
  knowledge of gravity and a more complete
  understanding of time itself

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Fermilab

• Worlds highest energy particle accelerator (2
  TeV 2 trillion electron volts
  2,000,000,000,000 eV total energy)
• collides protons with antiprotons
• located in Batavia - take I88 (or Rt 38) to Kirk
  Road (just east of toll booth then a few miles
  north on Kirk)
• VISITORS WELCOMED
• self-guided tour plus videos in main building
• Education center
• Nature trails and fishing
• Buffalo herd

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Fermilab Accelerator - 2 TeV Energy
Tevatron Ring(1 km radius)
CDF
D0
Main Injector Ring
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Matter-Antimatter Asymmetries

• We live in a world which is dominated by matter
  such as protons and electrons (but antimatter is
  readily made at accelerators and has some medical
  applications)
• But the very early universe had equal amounts of
  both matter and antimatter. After most
  annihilated with each other a very, very, very
  small excess of matter was left over to make
  everything including us
• WHY????

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Matter-Antimatter continued

• Matter-antimatter differences have also been
  observed in the decay of the strange quark (1964)
  and the bottom quark (2001)
• the underlying math which describes quarks shows
  that if there are at least 3 generations of
  quarks then you can have an asymmetry (which is
  why we are all very happy that the top and bottom
  quark exist!!)
• Still missing is a Theory of Everything which
  explains why there are three generations. (Carl
  Albright of NIU is working on it.) This ToE
  probably also explains mass differences
• Ongoing experimental efforts (including neutrino
  studies) gives constraints which theorists like
  Albright use as a guide in selecting models of
  the ToE

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Why is Gravity so Weak?

• The Weak, Strong, and Electromagnetic Forces all
  have about the same strength at Fermilabs energy
  (though not at room temperature) and are well
  described by complete relativistic quantum
  mechanical theories
• Gravity is much, much weaker. 37 orders of
  magnitude weaker than EM. Only dominates at
  larger distances as EM has both positive and
  negative charges and others are short-ranged
• No complete theory of gravity exists. Einstein
  tried for one and this is Stephen Hawkings
  primary work. Will be necessary to understand the
  very, very early universe when all 4 forces had
  the same intrinsic strength

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Weakness of Gravity II

• Need new theories, two of which are Supersymmetry
  and Extra Dimensions
• Supersymmetry (SUSY)
• postulates the existence of extra particles,
  which are partners to existing (selectrons
  partner to electrons, etc)
• All particles produced similarly in the very
  early universe. All forces also the same at that
  time
• As Universe cools during first picosecond SUSY
  particles freeze out as they are heavier. Their
  existence helps to explain gravitys weakness
  (though not completely)
• SUSY particles could explain missing mass
  observed in astronomy

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Weakness of Gravity III Extra Dimensions

• Lets assume (for fun) that we live in an
  11-dimensional world
• time
• normal 3-D position space
• 7 extra dimensions which are small
  (compactified is the physics term) less than 1
  mm
• Only gravity can communicate to the extra
  dimensions (all other particles and forces are
  confined to the normal 3D space)
• Fairly simple geometry explains why gravity
  appears weak - it has to spread its force fields
  over a larger space which thereby dilutes them in
  the normal 3D space itself

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CONCLUSIONS

• The understanding of the motion of the Sun, moon,
  and planets - which are day-to-day indicators of
  time - lead to the development of first astronomy
  and then physics
• By both studying the fossil record in stars
  and elsewhere and actually looking back in time
  by analyzing distant objects, astronomers have
  mapped out most of the Universes history from
  the Big Bang to today. Though the ultimate fate
  is still uncertain.
• To understand the first instances of time
  requires an understanding of Natures underlying
  particles and forces. There is not yet a Theory
  of Everything but we are working on it.