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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
may all have the
same root

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
-
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


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)

Main Injector Ring

D0

CDF

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
-
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
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.