What force holds an oxygen atom to another in O2?

A] gravity

B] electromagnetic force

C] strong nuclear force

D] weak nuclear force

The strong nuclear force occurs because of the exchange of massive “mesons” between protons/neutrons.

We will discuss the constituents of mesons & protons & neutrons shortly.

Through careful experimentation, you discover a force of attraction between two people.

It only acts over a short range! There is no force if the separation is > 1 m.

In keeping with physical nomenclature, you call this force “smell”. There are apparently two smells, “nice” and “nasty”. Like attracts like.

What is the mass of the odoron, the quantum of the smell field?

A] zeroB] smaller than an electronC] about as big as a strongly flavored quarkD] the same as a ton of bricks

The mass of the exchange particle is only one possible limitation on range (through the Heisenberg uncertainty.)

The force will have a short range if the energy required for “long range action” exceeds the Heisenberg limit.

This is the case with quark-quark interactions.

Because the gluon carries color “charge”, a gluon field emits more gluons. The energy in all these gluons must be considered in the Heisenberg limit… in other words, there is an “effective mass” that you get when you take into account all the interaction energy.

This is why a) the color force is finite range and b) color cannot be isolated.

All observable hadrons are colorless. (Pulling a red quark out of a proton would require so much energy that you

would make an anti-red and a red, ending up with the proton and a meson.)

Murray Gell-MannProfessor of Physics(now) University of New Mexicoproposed Quarks in 1964

Quarks(Baryons) Leptons Antiparticles

Top & Bottom rows are different “flavors”Proton decay is u d + e+ + e

This conserves lepton number & baryon numberAnd also keeps the “flavor difference”* constant in the universe(adding one net particle of each flavor.)

* Electrons, muons, taus are also said to have different flavor. This is not conserved.

Antiparticles

What’s the deal with antiparticles?Dirac wrote down a relativistic version of the Schroedinger Eqn.He found it had both positive and negative energy solutions for e-. (Not surprising, since it had to be second order in time!)

Rather than discard the negative energy solutions as spurious,(as any sane or modest person would do)he hypothesized that the negative energy states were filled with unobservable electrons. Moving an e- up out of this sea would leave a hole of + charge, that acts just like an antielectron.

Feynman showed that antiparticles are just regular particles going backward in time. This eliminates the need to postulate an “unobservable” e- sea… perhaps at the psychic expense of worrying about “backward time”.

The Big Bang & The Expanding Universe

Distant galaxies are receding (Doppler shift of spectral lines)

The Big Bang & The Expanding Universe

There is (probably) no center for expansion.To have a center, you need a boundary.

The Big Bang & The Expanding UniverseIf the universe is expanding, it must have been denser and hotter in the past. We can sample physics at higher temperatures (higher energies) in accelerator experiments, and thus we can extrapolate the current universe back to very

nearly its beginning.

<380,000 years after the bang, the average energy per particle is > electronic binding energy. There are no atoms, only ions, which scatter light. The universe is

opaque.

A hundred seconds after the bang, the average kinetic energy per particle is greater than nuclear binding energy: there are no nucleii

A microsecond after the bang, there is quark soup… no neutrons, or protons.

Prof. Fields makes quark soup on Long Island.

What happened before the universe was quark soup is mostly speculation. We don’t have experiments to test different ideas. We think the forces should be

unified.

You are here

We think we understandphysics until here (or so)

thought

Gravitational Lensing & Dark Matter

The bending of light by massive galaxies, And the rotation rates of galaxies Both indicate A LOT more mass than can be accounted for by stars & dust.

Accelerating Expansion - “Dark Energy”

When we look carefully at the Doppler redshifts of distant galaxies, they are smaller than Hubble predicts.

The expansion of the universe used to be slower!

Gravitational attraction should slow the expansion down!

Maybe the universe has stuff in it pushing the expansion - “dark energy”

Maybe general relativity is wrong.

Maybe there is large-scale structure in the universe we are unaware of.

A few things we don’t understand

• We haven’t been able to write a sensible & complete theory combining General Relativity and QM

• Although the quantization of fields seems to work well (and makes verified predictions), it also predicts an infinite energy density (recall that even the ground state of a quantum system has some energy!)

• Although electrons appear to be pointlike particles, that would give them infinite “self energy” in their Coulomb field, and so infinite mass.

• We don’t understand why charge is quantized, and mass isn’t.

• We don’t understand WHY there are three generations of matter.

• We don’t understand the accelerating expansion of the universe

• We don’t know what dark matter is.

What is a perfect theory of everything?

In physics, a perfect theory (IMO) shows how all physical laws and behaviors arise from the smallest set of postulates.

In a perfect theory, we look at the small set of postulates and say, “given that these are true, the universe could not be other than it is.”

(As an example, given that the spacetime has its geometry, + Coulomb’s law, all of electromagnetism must follow!)

There is a regression to smaller and smaller sets of postulates, and more basic (quasi-philosophical) questions. For example, why does spacetime have Minkowski geometry? We don’t know.

BUT WE CAN KNOW!“The most incomprehensible thing about the universe is that it is comprehensible!” - A Einstein