PassGAMSAT Science EClass 14

(1)

1 | P a g e © P a s s G A M S A T

(2)

Learning Objectives

This lesson will expose the students to the latest theories in physics. The interchangeability of matter and energy involves several theories which attempt to explain the creation of the universe and where we are headed.

1. More on Matter plasma exotic states of matter

2. Fundamental Forces gravity electroweak strong 3. Nuclear Physics 4. Particle Physics force-carrying particles matter particles

particle summary tables

5. Cosmology: The Nature of the Universe big bang vs. steady state

finite vs. infinite strings vs. branes

(3)

3 | P a g e © P a s s G A M S A T MORE ON MATTER

Plasma

You have seen plasma, even if you weren't aware of it. It's the glow inside a neon light or a fluorescent light bulb, the heat of an arc welder, and the aurora borealis (or aurora australis). Plasma is a gas that is superheated until its atoms dissociate and become ionized. The electrons separate from their nuclei and the entire mass shows a high degree of energy. Plasma TVs work with plasma between two thick

glass plates. The interior of our sun and of all stars is plasma. Some physicists estimate that 95% of the matter in the universe is in the plasma state.

Plasma behaves as a dynamic fluid, and exhibits periodic fluctuations of electron density called Langmuir waves (after the physicist who discovered them). Much research into nuclear fusion reactions has focused on how to contain plasma to produce usable, controllable power. The core of a star (such as our sun) is essentially a nuclear fusion furnace. Plasma can also conduct electric or magnetic energy. Plasmas can be very hot, as in a star's core, or cool (a relative term, since "cool" plasmas are still several thousand degrees Celsius!) as in

technological applications such as lighting and TVs.

Plasmas created in laboratories for study are contained within strong magnetic fields; however, current technology is inadequate to maintain the containment for very long intervals.

Exotic States of Matter

In 1994 physicists at the CERN supercollider in Switzerland demonstrated the existence of a state of matter which had been predicted in the mid-1970s soon after the discovery of quarks, as the standard model theory of particle physics was being developed. It is similar to plasma

(4)

but cooler and much more dense, and is composed of the subatomic particles quarks and gluons. Their movements are much more organized than those within plasma, and "beam" matter exhibits chiral symmetry.

The existence of the sixth state of matter was predicted by Albert Einstein in 1925. If plasma and beam matter are superheated gas, Bose-Einstein condensation is supercooled gas. Einstein theorized that if a gas could be cooled almost to absolute zero it would condense into a unique state in which all of the atoms have exactly the same location and energy level. In 1995 a team at the University of Colorado was able to create and photograph a Bose-Einstein condensate. They were awarded the Nobel Prize for Physics in 2001.

THE FUNDAMENTAL FORCES Gravity

Gravity was first quantified by Galileo and defined by Isaac Newton. Newton's theory of gravity was that all bodies are attracted to one another in proportion to the mass of each; the bigger the body or object, the greater its gravitational attraction. Gravity is the weakest of the fundamental forces, but it's the one we notice the most because it works over large distances.

The currently accepted theory of gravitation that is most consistent with experimental observation data is Einstein's theory of general relativity. He started from Newton's observations and his own theory of special relativity (which explained the equivalence of mass and energy), and combined them to form a geometric theory of gravitation. General relativity describes gravity as a geometric property of a curved, four-dimensional spacetime, which is related to the mass-energy and momentum of any radiation or matter. It primarily deals with wave phenomena. The mathematical predictions of general relativity have been demonstrated experimentally, and no circumstances have yet been observed that contradict it. There are circumstances general relativity cannot explain, but it has stood the test of time.

(5)

Quantum theorists have been working toward a quantum theory of gravity; these ideas call for the existence of a particle called a graviton which carries the force. Various proposed

theories, such as string theory, superstring theory, and quantum loop gravity, fill some of the gaps of general

relativity, but all have limitations. A single theory which unifies gravity with the other fundamental forces may be possible, but it is still in the distant future.

Electroweak Force

The electroweak force is the unification of the electromagnetic force and the weak nuclear force. They were unified in the late 1960s by Glashow, Salam, and Weinberg, for which they won the 1979 Nobel Prize for Physics.

The electromagnetic force is also a unification, of much longer duration. Prior the late 1800s electricity and magnetism were thought to be separate. James Clerk Maxwell demonstrated in 1873 that they were both manifestations of the same force. This force governs interactions of charged particles and explains how electricity works.

The weak nuclear force (or weak interaction) is responsible for beta radioactive decay. In beta decay, a neutron is converted to a proton by emitting an electron. The atomic number of the element increases by one and the mass is unchanged. The neutron becomes a proton by changing the flavor of one of its quarks from down to up; it emits a W- boson which then splits into a high-energy electron and an electron antineutrino. The electron is given off as beta radiation. The weak force governs the stability of elementary particles.

(6)

similar. The photon of electromagnetism is similar to the W+, W-, and Z bosons of the weak force, and at that energy level they all behave the same way.

Strong Nuclear Force

The strong force is the strongest of the fundamental forces, and is mediated (carried) by bosons called gluons, which react to the "color" of quarks.. It is what binds quarks together as protons and neutrons, and what overcomes the repulsion of the positively charged protons to hold them together in an atom's nucleus. The strong force is strongest at the quark level; it diminishes at the nucleon (proton & neutron) level, and gets progressively weaker as atomic numbers increase. This may explain why all elements with atomic numbers higher than 82 are radioactive; they simply don't have enough force to hold their nuclei together.

NUCLEAR REACTIONS

Atomic and nuclear physics have scary connotations in vernacular usage. Mention the word "atomic", and the automatic word-association is "bomb"--think Nagasaki and Hiroshima. For the word "nuclear" the association is "power plant accident"--think Chernobyl, Three Mile Island and the latest at Fukushima, Japan. Nuclear power and atomic weapons are applications of a branch of physics which, at the most fundamental level, addresses the structure of all matter and the interactions that formed the universe.

As noted above, the sun's core is a plasma generated by nuclear fusion. Aside from the potential practical applications of nuclear fusion for power generation, understanding of the reactions involved sheds light (pun intended!) on the fundamental forces and the structure and composition of matter itself.

Since plasma is an ionized gas (electrons are separated from their nuclei), some

cosmologists believe that for the first few nanoseconds after the Big Bang the entire universe was a plasma and the fundamental forces were united. As it cooled and expanded, the

(7)

gravitational force split off first, and led to the formation of the elementary particles--quarks and leptons--which eventually further coalesced to become protons and neutrons and finally atoms.

Researchers in high-energy plasma and nuclear physics attempt to re-create the conditions of the early universe with the goal of understanding how matter formed and how the

fundamental forces that govern it are related. Current technology is not yet able to produce high enough levels of energy to simulate the theoretical conditions of the Big Bang.

PARTICLE PHYSICS

Particle physics grew out of nuclear physics, as researchers gradually discovered the existence of the various subatomic particles and how they interact. At the most fundamental level everything is composed of the same elementary particles, hence is related.

One of the postulates of Einstein's theory of special relativity is that matter and energy are equivalent and interchangeable. If matter can be energized sufficiently, it will break down.

(8)

That is the premise of particle accelerators (supercolliders). The basic concept is to smash atoms or smaller particles into one another and see what comes out. The particles are accelerated by huge, powerful superconducting electromagnets around the perimeter of a vast (27 km long) circular underground tunnel. The results of the collision are captured by very sensitive detectors. The particle collisions appear in the detectors as "jets", narrow cones of particles which are fragmented and recombined (hadronized).

Force-Carrying Particles

The equivalence of matter and energy also helps to explain the fundamental forces, which are carried by a group of particles called bosons or vector bosons. They include the W+_{, W}-_{, and Z}

bosons of the electroweak force, the photon, and the gluons of the strong force. The W+_{, W}-_,

Z, photon, and the hypothetical Higgs boson are elementary (thought to be indivisible); most other bosons are composites. Bosons are named after the Indian physicist Satyendra Nath Bose. They are not required to follow the Pauli exclusion principle; i.e. more than one boson can occupy the same quantum state (energy level). This gives them a "flexibility" (as opposed to "stiffness" or "rigidity" of fermions), and allows them to carry force and form the

Bose-Einstein condensate at very low temperatures.

All particles have a quantum condition called "spin". It was initially thought that particle spin was similar to that of the Earth on its axis, but a particle doesn't have an axis. Spin actually

(9)

describes the geometry of the particle. A particle of spin zero appears the same when viewed from any direction; a spin 1 particle has only one "right-side-up", and must be rotated 360º to have the same appearance again. A particle with a spin of 2 appears the same when rotated only 180º. All of the bosons have whole-number spin.

(10)

10 | P a g e © P a s s G A M S A T Matter Particles

The matter particles (fermions and hadrons) have spin of 1/2. This is difficult to visualize, but it basically means that the particle must be rotated 720º before its appearance is the same. There are two main classes of elementary (currently believed to be indivisible) matter particles; the entire group of elementary particles is called fermions, named for Italian- American physicist Enrico Fermi. Leptons (from a Greek word meaning light [weight, not illumination]) include the electron, the muon, the tau lepton, and a neutrino for each of the first three; only the electron and its neutrino occur in nature. Quarks come in six "flavors" and three "colors", and are the building blocks of the larger composite particles called hadrons which include protons and neutrons. For every particle there is an antiparticle, which is designated by an overline above the particle symbol. For example, the symbol for a proton is p; for an antiproton, p.

The hadrons are divided into two categories: baryons (from a Greek word meaning heavy) and mesons, whose mass falls between that of a lepton and that of a baryon. Hadrons are all composed of smaller particles, mostly quarks and neutrinos. The strong force carried by gluons binds them together. Of the six types of quarks, only the up and down quarks occur in nature. All other particles have been observed only in accelerators.

Each baryon is composed of three quarks, and each meson is is composed of one quark and one anti-quark. Because fermions must follow the Pauli exclusion principle, the three quarks in each baryon must be of different colors. "Color" in this context does not refer to the visual

(11)

phenomenon of light wavelengths; rather, it refers to a quantum condition of quarks required by the Pauli exclusion principle. It is also called the color charge, but isn't the same as electrical charge. As previously noted, quarks are held together by the strong interaction, mediated by gluons, which also carry a color charge. The theoretical framework for the interaction is called quantum chromodynamics (QCD), and is analogous to (though more complicated than) quantum electrodynamics (QED) for the electroweak interaction. Together QCD and QED are the pillars of the Standard Model of particle physics.

QCD was developed to explain why individual quarks have never been observed. Their

existence had been predicted, and the hadrons behaved as if they had point-like constituents, but no particle collision ever produced a free quark. One of the postulates of QCD is that particles which have a color charge (quarks and gluons) must be bound to at least one other particle; this concept is called color confinement. Thus when atomic nuclei collide in a particle accelerator, the three-quark protons and neutrons fragment and smaller hadrons such as glueballs and quark-anti-quark mesons appear; a process called hadronization. The new particles leave the impact site at various angles, producing the jets that are seen in the particle detectors.

(12)

The immediate result of the collision is a quark-gluon plasma (QGP), from which the new hadrons emerge. Since quarks are paired, only two jets would be expected; the third is the gluon which joined the quarks. The red lines in the center diagram represent quark-gluon strings, which result from the strong interaction between quarks, between gluons, and

between quarks and gluons. In one theoretical variation called jet-fluid string QCD, the quark- gluon plasma is a mass of partons (generic, unidentified elementary particles--the word parton was coined in the late 1960s by particle theorists who didn't accept Gell-Mann's quark idea) which coalesce into quarks and gluons, then mesons and glueballs. A jet parton is joined to a fluid parton by a jet-fluid string. Where things get really interesting and complicated is that some versions of cosmological string theory hold that the universe consisted of a quark-gluon plasma for the first few microseconds of its existence, and the quark-gluon strings of QCD may be analogous to cosmic superstrings.

Particle Summary Tables

Force-carrying Particles: Bosons

Name Symbol Force Particles

acted upon

Mass (GeV/c2₎

Charge Spin Notes

photon γ (lowercase gamma) electroweak electrically charged 0 0 1 mostly electromagnetic W- boson

W- _electroweak _{quarks, leptons 80.39} _-1 ₁

weak force W+

boson

W+ _electroweak _{quarks, leptons 80.39} ₁ ₁

Z boson Z0 _electroweak _{quarks, leptons 91.19} ₀ ₁

gluon g strong quarks, gluons 0 0 1

Higgs boson

H0 _mass _all _{<1.4 TeV} ₀ _{hypothetical--}

existence not yet demonstrated by experiment

(13)

13 | P a g e © P a s s G A M S A T Elementary Matter Particles: Fermions

Category Name Symbol Force involved: Acted upon by: Mass (GeV/c2

) Charge Spin leptons electron e electroweak γ, W-,_W+_,Z0 _0.000511 _-1

½

electron neutrino ve electroweak γ, W-,W+,Z0 (0-0.13)x10-9 0

½

muon μ electroweak γ, W-, W+ ,Z0 0.106 -1

_½

muon neutrino vμ electroweak γ, W-,W+,Z0 (0.009-0.13)x10-9 0

½

tau τ electroweak γ, W-,_W+_,Z0 _1.777 _-1

½

tau neutrino vτ electroweak γ, W-,W+,Z0 (0.04-0.14)x10-9 0

_½

quarks

up u electroweak, strong γ, W-,_W+_,Z0 _{gluons 0.002} _2/3

½

down d electroweak, strong γ, W-,_W+_,Z0 _{gluons 0.005} _-1/3

½

charm c electroweak, strong γ, W-,_W+_,Z0 _{gluons 1.3} _2/3

½

strange s electroweak, strong γ, W-,_W+_,Z0 _{gluons 0.1} _-1/3

½

top (or truth)

t electroweak, strong γ, W-,_W+_,Z0 _{gluons 173} _2/3

½

bottom (or beauty)

b electroweak, strong γ, W-,_W+_,Z0 _{gluons 4.2} _-1/3

½

Composite Matter Particles: Hadrons

Category Name Symbol Quark content Mass (GeV/c2_{) Charge Spin} _Notes

mesons pion π+ _ud _0.140 ₁ ₀ bosonic hadrons kaon K- _su _0.494 _-1 ₀ rho ρ+ _ud _0.776 ₁ ₁ B-zero B0 _db _5.279 ₀ ₀ eta-c ηc cc 2.980 0 0 baryons and anti- baryons proton p uud 0.938 +1 1/2 fermionic hadrons anti-proton p uud 0.938 -1 1/2 neutron n udd 0.940 0 1/2 anti-neutron n udd 0.940 0 1/2 lambda λ uds 1.116 0 1/2 omega Ω- _sss _1.672 _-1 _3/2

There are actually and hypothetically many more types of particles. These tables list only the more common and important ones.

(14)

COSMOLOGY: The Nature of the Universe

Cosmology is more than mere astronomy. Cosmology attempts to discern the very nature of the universe from its origin to the present, and even speculates on its ultimate fate. It attempts to explain how the objects we observe came to be what and where they are, and delves into religious and philosophical questions of why.

Most major religions believe in a supreme being or beings who created the universe, the earth, and humanity. A few reject all scientific inquiry into origins as blasphemous, but most are willing to coexist with it. Likewise some scientists reject religious belief in a Creator, but most acknowledge that a supreme Creator can explain things that science cannot. We won't debate that here, merely

summarize the scientific views.

Big Bang vs. Steady State

Most scientists now agree that our current universe began as an infinitely dense quantum singularity which exploded in a massive burst of heat and energy almost fifteen billion years ago. Prior to the mid-1960s, however, the Big Bang was a matter of debate. Many theorists believed that the universe was steady-state--had always been the size it is now. Evidence that galaxies are moving away from each other was noticed as

early as 1912; an astronomer at the Lowell Observatory in Flagstaff, Arizona, Vesto Slipher, measured the spectral shifts in the light of spiral nebulae. He observed that 36 of the 41 galaxies he studied had redshifts, indicating that they were moving apart. The idea that the entire universe was actually expanding was introduced in 1917 by Dutch astronomer Willem deSitter. He proposed a universe that was static (in accordance with the conventional ideas of the time), and

contained no matter at all. His idea had an intriguing mathematical oddity: if particles were added to it, they behaved as if they were moving away from each other. The same year, Albert Einstein also proposed a model of the universe. His

(15)

model was static as well, neither expanding nor contracting. In order to account for the observed redshifts and apparent movement, he introduced the mathematical fiction of a "cosmological constant" into his theory of general relativity. Einstein himself later called it his "greatest blunder". Einstein's model of the universe was referred to as "matter without motion"; deSitter's, "motion without matter."

After the Big Bang, the infinitely dense, infinitely hot maelstrom began to expand. Each time the size doubled, the temperature fell by half. The density decreased as a function of age; the average density of the universe is inversely proportional to the square

of its age.

When the new universe was one second old, the temperature had dropped to ten thousand million degrees--a thousand times hotter than the interior of our sun. Matter would have consisted only of leptons and bosons, although a few baryons would start to form.

(16)

Direct observational evidence of the Big Bang was discovered in 1965 by Arno Penzias and Robert Wilson. They were working at the Bell Telephone Laboratory, trying to identify and remove a persistent background noise in satellite

communication. After painstakingly removing as much interference as possible, they found an irreducible level of cosmic radiation which had a temperature of about 3º Kelvin. After reading about theoretical work which predicted low-temperature background radiation as a result of the Big Bang, they concluded that their bothersome interference was, in fact, that background radiation. Other theorists reviewing their results agreed; Penzias and Wilson were awarded the Nobel Prize for physics in 1978 for their discovery.

Finite vs. Infinite

Now that we know the universe is expanding, the next question is, will it continue expanding forever, or will it eventually reach a finite limit? If there is a finite limit, what will happen when it is reached? This question was addressed by Alexander Friedmann, a professor at the University of Leningrad, in the early 1920s.

(17)

Friedmann (correctly) believed that Einstein was wrong in his assumption that the universe was static. He proposed the idea of universes whose curvature varied; the curvature k can be positive, negative, or zero. All of Friedmann's universes begin with a bang.

If k is zero, the universe is flat, continuously expanding, and infinite. Its end, if there is one, will be a "whimper". If k equals -1, space is hyperbolic, but expands infinitely and if it ever ends it will also be a whimper. If k equals +1, space is spherical, unbounded, expanding, but finite. It will eventually reach a maximum size and start to contract; the contraction will then continue to an infinitely dense singularity--the "big crunch"--and another bang. Each universe is finite, but the number of universes in the cycle of bang, expansion, contraction, and new bang is un-knowable. Such a model is called an oscillating universe. Oscillating universe models could eventually reach a stage called "heat death": each new cycle has increasing entropy and the heat of matter becomes more evenly distributed across space, rather than condensed into galaxies, stars, and planets. The entropy would eventually reach a point where no more work could occur.

Until the mid-1990s, most cosmologists assumed that the expansion of the universe was decelerating as the energy from the Big Bang dissipated and the force of gravity opposed the expansion. However, startling and unexpected results from a 1994 study of redshifted supernovae showed that the rate of expansion is actually increasing. It was initially thought to be a fluke or a measurement error, but other teams found similar results in later studies. Why is the expansion occurring? Theorists have proposed a repulsive force which they call dark matter or dark energy, which may be up to 75% of the total matter in the universe. So far they haven't been able to identify exactly what dark matter is, but some think it may be an entirely new form of matter, not made of the same particles as visible matter.

The discovery of accelerating expansion required major revision of cosmological theories; Einstein's cosmological constant was resurrected to account for dark energy--maybe it wasn't such a great blunder after all. In an accelerating universe there obviously would never be a contraction phase or "big crunch"; no oscillation or "heat death", either. The question of finite vs. infinite still isn't settled, nor is the

(18)

ultimate fate of the universe. One researcher proposed a new possible ending: a "big rip". If the rate of expansion continues to accelerate, it could eventually reach a point where it would overcome the gravitational attraction between galaxies, stars, planets, and even subatomic particles. The universe would then be torn apart and matter would disintegrate.

Strings vs. Branes

So just exactly how is the universe structured? How are the fundamental forces related, and can the gravity of general relativity be reconciled with quantum mechanics and the Standard Model of the strong and electroweak forces?

String theory originated in the late 1960s as an early attempt to explain the strong nuclear interaction. It was later discarded for that purpose, supplanted by QCD. Cosmologists then took it on. In the last thirty years it has gone through a number of modifications and additions, and is now thought to be the leading candidate for a quantum theory of gravity which can unify all of the fundamental forces.

In classical quantum mechanics, a particle is a mathematical point which has no dimension; in string theories it is a one-dimensional loop. The vibration of a string represents the movement of a particle. String theories allow for a string to have a spin of 2, which would be a graviton, giving a quantum condition for gravity. By the early 1990s there were five distinct, competing versions of string theory in

circulation. In 1995 string theorists discovered "dualities" that related all five versions into a single superstring theory; each version described one of many quantum vacua (spaces). Superstring theory allows for the existence of p-branes (p is the number of dimensions), which are endpoints for strings; the word "brane" is a shortening form from "membrane".

Brane-world theories hold that our observable four-dimensional universe is

contained on a cosmic membrane (shortened to brane) inside a higher-dimensional bulk. Some versions include ten or eleven dimensions, but since all but four of them are wrapped in the bulk we don't need to worry about them.

(19)

String theories and brane theories have been merged into M-theory, a candidate for a unified "theory of everything". M-theory proposes an 11-dimensional

universe, of which seven are microscopically small. The remaining four are the three spatial and one time dimensions of general relativity. The diagrams above show how a particle becomes a string, a string becomes a membrane, and a membrane becomes a volume.

Another way of visualizing multiple dimensions is to imagine our four-dimensional universe as a membrane floating in the 11-dimensional spacetime. Then, imagine the space-time as a cylinder, and wrap the membrane around it. Then make the diameter of the cylinder progressively smaller until it becomes a string.

(20)

Exercises

1. List several examples of plasma.

2. How is beam matter similar to plasma? How is it different?

3. List the three fundamental forces and the processes they govern. 4. Briefly describe how a particle accelerator works.

5. How are bosons thought to carry force? 6. Describe the two categories of fermions. 7. Describe the two categories of hadrons.

8. Why have single (unbound) quarks or gluons never been observed? 9. Describe the observational evidence of the Big Bang.

10. Describe Friedmann's three models of the universe. Which one is now known to be correct?