Download Time From the Perspective of a Particle Physicist

Time From the
Perspective of a Particle
Physicist
David Hedin
Department of Physics
August 8, 2001
Outline
• Time in our everyday lives
• Looking back in time examples from Astronomy
• Looking back in time examples from high energy
accelerators
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
same root
may all have the
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
The Year
Two Indicators
• Due to the Earth’s tilt the
Length of the Day
and
Sun’s 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
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 Earth’s daily spin and yearly
orbit about the Sun
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
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-monthyear “bothered” philosophers and
theologians. Understanding this “random”
motion (and the planets were even worse)
by Copernicus, Kepler, Galileo, Newton
gave us modern science
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
1700,1800,1900,2100,2200 (just those
divisible by 400)
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
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
Galaxies and Hubble’s Law
• It has been observed (first by Hubble in the
1920’s) that galaxies are moving away
from us and that the further away they are
the faster they are moving (v=Hd)
• 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
More on Hubble’s 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
Fire and Ice
Robert Frost - 1923
Some say the world will end in fire,
Some say in ice.
From what I’ve 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.
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
Fermilab
• World’s 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
Fermilab Accelerator - 2 TeV Energy
Tevatron Ring(1 km radius)
CDF
D0
Main Injector Ring
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????
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”
Why is Gravity so Weak?
• The Weak, Strong, and Electromagnetic
Forces all have about the same strength at
Fermilab’s 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
Hawking’s primary work. Will be
necessary to understand the very, very
early universe when all 4 forces had the
same intrinsic strength
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
gravity’s weakness (though not
completely)
• SUSY particles could explain “missing
mass” observed in astronomy
Weakness of Gravity III
Extra Dimensions
• Let’s 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
CONCLUSIONS
• The understanding of the motion of the
Sun, moon, and planets - which are day-today 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
Universe’s 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 Nature’s
underlying particles and forces. There is
not yet a Theory of Everything but we are
working on it.