FROM THE VERY SMALL
TO THE VERY LARGE
Theoretical High Energy Physics in
the 21st Century
Michael Dine
June, 2007
New York Times
Reports a debate among cosmologists about
the Big Bang.
lll1.html
Dr. Tyson, who introduced himself as the Frederick P. Rose
director of the Hayden Planetarium, had invited five
"distinguished" cosmologists into his lair for a roasting
disguised as a debate about the Big Bang. It was part of
series in honor of the late and prolific author Isaac
Asimov (540 books written or edited). What turned out to be
at issue was less the Big Bang than cosmologists'
pretensions that they now know something about the
universe, a subject about which "the public feels some
sense of ownership," Dr. Tyson said.
"Imagine you're in a living room," he told the audience.
"You're eavesdropping on scientists as they argue about
things for which there is very little data."
Dr. James Peebles, recently retired from Princeton, whom he called
"the godfather"; Dr. Alan Guth from the Massachusetts
Institute of Technology, author of the leading theory of
the Big Bang, known as inflation, which posits a spurt of a
kind of anti-gravity at the beginning of time; and Dr. Paul
Steinhardt, also of Princeton, who has recently been
pushing an alternative genesis involving colliding
universes.
Rounding out the field were Dr. Lee Smolin, a gravitational
theorist at the Perimeter Institute for Theoretical Physics
in Waterloo, Ontario, whom Dr. Tyson described as "always
good for an idea completely out of left field - he's here
to stir the pot"; and Dr. David Spergel, a Princeton
astrophysicist.
But Dr. Smolin said the 20th-century revolution was not
complete. His work involves trying to reconcile Einstein's
general relativity, which explains gravity as the
"curvature" of space-time, with quantum mechanics, the
strange laws that describe the behavior of atoms.
"Quantum mechanics and gravity don't talk to each other,"
he said, and until they do in a theory of so-called quantum
gravity, science lacks a fundamental theory of the world.
The modern analog of Newton's Principia, which codified the
previous view of physics in 1687, "is still ahead of us,
not behind us," he said.
Although he is not a cosmologist, it was fitting for him to
be there, he said, because "all the problems those guys
don't solve wind up with us."
Today, you are listening to someone seemingly more out in left field
-- a particle physicist.
Particle physics: seeks to determine the laws of nature at a
``microscopic” – really submicroscopic, level.
What does this have to do with the Big Bang?
EVERYTHING!
With due respect to the New York Times, articles like
this give a very misleading impression.
We know:
•There was a Big Bang
•This even occurred about 13 Billion Years Ago
•We can describe the history of the universe,
starting at t=3minutes
•There is now a huge amount of data and a picture
with great detail.
There are lots of things we don’t know. With due
respect to Lee Smolin, the correct address for these
questions is Particle Physics.
•What is the dark matter?
•Why does the universe contain matter at all?
•What is the dark energy?
•What is responsible for ``inflation”?
•What happened at t=0?
We can’t answer any of these questions
without resolving mysteries of particle
physics. We need to know that laws of nature
which operate at the smallest distances we can
presently imagine.
Physical Law – What are we after?
Newton: F=ma
FG= M1M1/R2
Probably the most famous physical
laws. UNIVERSAL
Newton could use his laws to
explain the motion of the
planets, the moon. Haley –
comets.
Electricity and Magnetism:
Maxwell
Wrote down the laws of electricity
and magnetism; ``Maxwell’s equations.”
Light, radio waves (Maxwell predicted),
and other radiation all part of the same
set of phenomena.
So two sets of laws. These describe most of the phenomena of our
day to day experience: gravity, light, electricity, magnetism…
With these, scientists of the late 19th century understood the motion
of the planets in great detail, and made great technical progress.
They started, as well (somewhat inadvertently) to explore the world
of atoms.
The end of the 19th century saw the discovery of the first
elementary particle, by Thompson – the electron.
EINSTEIN
Excited by Maxwell’s equations and also puzzled. There
seemed to be a maximal speed at which light could travel.
Puzzled, also by the problem of the photoelectric effect – the
emission of electrons by light.
Also wondered about the existence of atoms. Were they real, or
just a trick to understand the periodic table?
Einstein’s Extraordinary year:
1905
•Photoelectric effect – the idea that lights come in
packets of energy – the beginnings of the photon
concept
•Explanation of the Brownian motion – basic to
physics, chemistry, biology – clinched the idea that
atoms were real.
•Special relativity – time and space are relative
concepts; depend on the observer. But the speed of
light is absolute: all observers agree about it.
General Relativity
Now, a deeper understanding of the laws of
electricity and magnetism. But Einstein didn’t
know how to reconcile Newton’s laws with the rules
of relativity. E.g. in Newton’s laws, action at a
distance. Didn’t make sense; electricity and
magnetism don’t work this way.
Einstein’s clue: the equality of gravitational and
inertial mass. Inertia – something to do with space
and time. So gravity?
F=ma
FG= mM/R2
Inertia
Gravitation
The equality of gravitational and inertial mass
was first tested in experiments by the Hungarian
scientist Eotvos in the late 1800’s:
Einstein and the General Theory
of Relativity
After almost eleven years of struggle, Einstein announced
his general theory of relativity in 1916. A theory in which
gravity arises as the distortion of space and time by energy.
Proposed three experimental tests:
•Bending of light by the sun
•Perihelion of Mercury
•Red Shift
General Relativity and the
Universe
Gravity is unique among the forces in that it is
always attractive. So it acts on things at the
surface of the earth, on the planets, on stars,
and on the universe as a whole. So Einstein
and others tried to apply his theory to the
universe.
But the universe is complicated, varied. How
to proceed?
Einstein + Copernicus
Assume the universe is homogeneous and isotropic –
no special place or direction.
Einstein’s equations have no Static solutions.
The universe expands!
Einstein was very troubled – remember that at that time (c. 1920)
Astronomers didn’t know about galaxies!
HUBBLE (1921)
Galaxies move away from us at a speed proportional to
their distance
The Cosmic Microwave Background
In the past, the universe must have been much
hotter: Big Bang. Gamow, Peebles: if true, there
should be a ``glow” left over from this huge explosion
(but of microwave radiation, not light).
Objects give off a characteristic spectrum of
electromagnetic radiation depending on their
temperature; ``blackbody.” The temperature then
was 10,000 degrees; today it would be about 3
degrees
Discovered by Penzias and Wilson
(1969).
Today: thanks to COBE satellite, the best
measured black body spectrum in nature.
Artist’s Rendering of COBE
COBE measured the temperature of the universe:
More detailed study of the
CMBR:
From satellites and earth based (balloon)
experiments. Most recently the WMAP
satellite.
Detailed information about the
universe:
Aside: we understand this as a quantum phenomenon;
the “measurement” was done a long time ago.
COMPOSITION OF THE
UNIVERSE
From studies of CMBR, of distant Supernova
explosions, and from Hubble and GroundBased observations we know:
• 5% Baryons (protons, neutrons)
• 35% Dark Matter [???] (zero pressure)
• 65% Dark Energy [????] (negative pressure)
A Confusing Picture: Where Do
We Stand?
We have a good understanding of the history of the
universe, both from observations and well
understood physical theory, from t=180 seconds.
BUT:
• We don’t know why there are baryons at all!
• We don’t know what constitutes 95% of the
energy of the universe.
• We know that the universe underwent a period of
violent expansion (inflation) at about 10-30 seconds
after the big bang. What caused this?
But: we’ve gotten ahead of our
story.
We started out talking about laws of nature.
We had Newton, and with him an
understanding of the planets; then Maxwell,
and an understanding of the electromagnetic
spectrum, and now Einstein, and we have
started to think about the universe as a
whole. But a lot happened between 1905
and these discoveries.
New particles, new laws
•
•
•
•
1895 – discovery of the electron
1911 - discovery of the atomic nucleus
1920’s – quantum mechanics
1930’s – the neutron, and understanding of
the atomic nucleus.
• 1930’s – discovery of antimatter.
LOOKING STILL DEEPER
By the 1940’s, much progress, but much not well
understood:
• Photons -- the quantum mechanics of
electrodynamics (QED)
• The precise laws underlying the nuclear forces
To go further: theoretical developments
Experiments probing distances smaller
than the size of nuclei
Quantum Electrodynamics
Feynman, Schwinger, Tomanaga:
detailed understanding of how quantum
mechanics and electricity and magnetism
work together. Predictions with awesome
precision. E.g. the magnetism of the
electron explained in terms of the
electron’s charge and mass to one part in
1012.
Looking Deeper
The late 1940’s launched the era of large
particle accelerators. Over the next 50
years, numerous elementary particles,
understanding of the basic constituents of
matter and the forces between them – the
Standard Model. Critical interplay between
theory and experiment.
Success of the Standard
Model
Standard Model
is extremely successful
Experimental discovery of
all of its matter constituents
and force carriers
Simple common approach
to describe all (relevant)
forces: gauge principle
Self-consistent at the
level of quantum
corrections
Detailed Comparison with
Experiment
One missing piece: the Higgs
particle
•In SM, responsible for the masses of the quarks,
leptons, W§, Z0
•Mass not predicted by the model, but know
something from experiment.
Peter Higgs
•Should be discovered at the LHC.
•Much work on this by H. Haber
The Standard Model Higgs Boson
time [year]
Last missing particle in SM
(EW symmetry breaking – mass)
Light SM Higgs preferred
MH = 126 +73 -48 GeV
< 280 GeV (95% CL)
Higgs Search at LEP:
mass limits:
obs. mh >
exp. m >
h
114.4 GeV
115.3 GeV
Fundamental open questions
Is There a Higgs particle? The hierarchy problem: why is
Higgs mass so small? (dimensional analysis: mH » 1019
mp )
from LEP
114.4 GeV/c2 < mH< 1 TeV/c2
Lots of parameters – masses, couplings of
quarks, leptons. Where do they come
from?
Can’t just add general relativity to the SM;
not clear consistent with quantum
mechanics.
SM describes only a tiny fraction of our
universe – what is the dark matter ? What
about the dark energy
from theory
One possible new phenomenon:
Supersymmetry
• A new symmetry among the elementary
particles. ``Fermions ! bosons; bosons !
fermions.
Not only a new symmetry of nature, but if
this idea is right, then it explains what the
dark matter is! [Banks, Dine, Haber]
An attractive Extension: Supersymmetry
Symmetry between
Fermions ↔ Bosons
(matter)
(force carrier)
... doubled particle spectrum ... ☹
Consequences of Supersymmetry
• Automatically provides an explanation of
the dark matter
• Predicts one of the fundamental constants of
nature.
• Provides an understanding of why there is
more matter than antimatter in the universe
• Predicts lots of phenomena observable at
the LHC.
Interaction Strength in Supersymmetry
without SUSY
... BUT some Standard Model
Problems solved ...
... extension in string theory is
candidate for Grand Unified Theory ...
... lightest SUSY particle stable
⇨candidate for dark matter ...
... unification of forces ...
with SUSY
1 TeV
Interaction energy in GeV
l
c
l
c
l
c
q
q
g
q
l
l
c
l
q
Production and decay of superparticles at the LHC. Here, jets,
Leptons, missing energy.
In a broad class of supersymmetric models, the
lightest new particle is stable (R-parity); typically
the partner of the Higgs or Z boson or photon.
Produced in early universe.
N
W+
N
W-
Range of supersymmetry parameters consistent with dark matter density;
here partner of photon is essentially the dark matter.
I am a fan of the supersymmetry hypothesis; I'm not
alone. About 12,000 papers in the SPIRES data base. If
true, quite exciting: a new symmetry of physics, closely
tied to the very nature of space and time. Dramatic
experimental signatures. A whole new phenomenology,
new questions. But neither the limited evidence nor these
sorts of arguments make it true; there is good
experimental as well as theoretical reason for skepticism.
This is not the only explanation offered for the hierarchy,
and all predict dramatic phenomena in this energy range.
• Large extra dimensions
• Warped extra dimensions
• Technicolor
• It’s just that way (anthropic?)
Hypothetical answers to a set of fundamental questions:
• Too many parameters
• Charge quantization
• Quantum general relativity
• Dark Matter
• Dark energy
• Baryogenesis
STRING THEORY
String theory has pretensions to attack the remaining
problems on this list:
• A consistent theory of quantum gravity
• Incorporates gauge interactions, quarks and leptons,
and other features of the Standard Model.
• Parameters of the model can be calculated, in principle.
• Low energy supersymmetry emerges naturally – all of
this proliferation, which seemed artificial, almost
automatic.
Has string theory delivered?
• String theory is hard. We don’t have a wellunderstood set of principles. Some problems of
quantum gravity are resolved, but many of the
challenges remain.
• String theory seems able to describe a vast number
of possible universes, only a small fraction of
which are like ours.
• Until recently, no progress on one of the most
difficult challenges to particle physics: the dark
energy, but this has changed.
We are at the dawn of a very exciting era. We
may resolve some of our fundamental questions.