Today
:What does expansion mean?
Hubble proved that the universe is expanding. The cosmic microwave background is the remnant of the embryonic universe that was produced from the big bang. Consequently, we have the notion that the universe started from a dot and expanded outward, graduating from a radiation dominated period to the matter dominated phase in which we now live. The question arises into what did the universe expand? That is, if everything is the universe, how can it expand into anything. There is really no good non-mathematical answer to this question. But there are two approaches that can be taken. First, we have learned from relativity that laws of physics do not change if we alter our vantage point. Hence, we could easily have taken the view that the universe is not expanding but rather that everything in it is contracting. As a consequence, it appears that everything is receding. In acutality we cannot know that this is not the correct view. But we do have the intuitive notion that everything really is not the size of a dot. Another approach is to assume that the universe (vacuum) stretched off to infinity at the beginning. The nice thing about infinity is that if we add something to infinity we still have infinity. Hence, if a tiny dot on vacuum started to expand, the question of what it was expanding into does not arise if we assume the vacuum already stretched off to infinity. We really cannot know that the universe is not infinite. Hence, this seems to be a reasonable alternative to the expansion dilemma.
Unification of forces
There are four known forces, gravity, strong nuclear (QCD), electromagnetism (QED), and weak nuclear. Other than gravity, ALL the forces involved in elementary physics problems (" sliding friction, static friction, normal, spring, string, etc) are manifestations of electromagnetism. The strong nuclear force binds the nucleons together; the weak nuclear force produces radioactive decay.
E M
You've seen one unification already: electricity and magnetism.
Notice that these seem like entirely separate phenomena at first glance (say from 500 BC to 1700 AD- a long glance!)
Links between them were found:
Moving electric charges exerted forces on magnets.
Moving magnets exerted forces on electrical charges.
The UNIFIED theory (Maxwell's equations) does more.
It predicts the existence of a new type of wave: E-M waves. The properties of that wave (speed, polarization, energy density, momentum) all are DERIVED from the theory unifying the forces. In this case, the predicted new wave wasn't so new- it was just light. In other cases, the predicted new waves/particles from unifications are usually new enough that one must do new experiments to find them.
The unified theory is more than the sum of the parts. Rember what happened when we put quantum mechanics and special relativity together. We obtained both spin and antimatter. Neither theory by itself has either of these properties. Spin and antimatter are emergent properties resulting from the unification of QM and SR. There are many examples of emergent properties in nature.
In atoms, electrical forces are much stronger than magnetic forces. At very high temperature (high enough so that relative velocities on the order of c are common) magnetic and electrical effects are of about the same size. In a world of charged particles flying around near speed c, it would not make sense to have separate treatments of "magnetic" and "electric" forces, since these would obviously be manifestations of the same effects. Remember that these fields transform into each other in Lorentz transformations.
Magnetic and electric forces become separable only if you pick a particular reference frame in which most things aren't moving fast. I.e. if the local environment strongly breaks the Lorentz symmetry of relativity, the forces look different. In the full symmetry environment, they obviously belong to a unified theory.
These features- prediction of new particles, connection between symmetry loss and apparent disunity of forces- will be found in the other unifications, including the one fully achieved (electro-weak), the one with some serious proposals and experiments (grand unification), and perhaps the one in embryo ("theory of everything").
All forces are the result of particle exchange
I claim
(without proof) that one can regard the EM force to be the result of the exchange of photons. For example, when one electron bounces off another one, one can draw this picture:
There are two key properties of the EM interaction: 1) the incoming and outgoing particles are the same and 2) the interaction between the particles is mediated by a massless particle, namely the photon. Because the photon is massless, it can travel a large distance. Consequently, the forces it can mediate are long-range. This intuitive notion will always be true in physics, namely the less massive the mediator of the particle interaction is the more long range is the interaction.
The electroweak interaction
The EW interaction is mediated by two particles that are massive, the W and Z particles. The mass of these particles is 80GeV/c^2 and 91 GeV/c^2, respectively. All leptons experience the weak interaction. Leptons come in pairs where one member of the pair is the neutral counterpart of the other: (e,n_e), (m,n_m),(t,n_t) where m is a muon and n_m is the muon neutrino, t is the tau particle and n_t the tau neutrino, e is the electron and n_e the electron neutrino. Aside from being mediated by a massive particle, the weak interaction can also change the incoming and outgoing particles. For example an electron can be changed into its neutrino. For example a typical interaction mediated by the weak interaction is (e,m) interacting with the W particle and giving rise to (n_e,n_m). Photons cannot mediate this reaction. In fact it was as a result of reactions of this sort that it was deduced that the weak interaction had to exist. In all instances, the weak interaction does not always result in a changing of the particle type. The Z particle can mediate an interaction involving neutrinos only. Here is a summary of the similarities and differences between the EW and EM forces.
Nevertheless, there are similarities. In particular, within its short range, the weak interaction is actually the same strength as EM. It has been known since 1935 (Yukawa) that the short-range forces are due to the exchange of massive particles. Therefore, in the late 1950s it was proposed that the weak interactions are mediated by heavy
intermediate vector bosons.
Suppose the vacuum is full of Higgs particles
The synthesis of EM and EW was provided in the 1960s by Glashow, Weinberg and Salam
(and others). Its main features are the following:• The photon doesn’t interact with h, so it remains massless.
• The W and Z particles become massive, and the weak force becomes short-range. This explains the observed differences between weak and EM interactions.
• The masses of all particles (quarks, electrons, etc.) result from the interaction with the vacuum.
Mass is no longer an intrinsic property of isolated objects. If we could change the properties of the vacuum or of the interactions with it, we could change the masses. Indirect evidence exists.
The theory makes many confirmed predictions,
e.g. the existence of "neutral currents"- a previously unexpected type of particle interaction. These neutral current interactions were actually observed before the theory was developed, but were assumed to be a part of the miscellaneous experimental background events. The one attempt to calibrate the background indicated that it did not account for the events, but no one was sufficiently interested to follow up that calculation to see if it was reliable- until the theory was developed.
This story is well told in A. Pickering's book "Constructing Quarks". If you read it, just remember that the first and last chapters have no rational connection with the remainder of the book or with any other aspect of human experience.
Two important unconfirmed predictions of the theory:
• The Higgs particle must exist. It may be discovered in the next 10 years.
• As with the magnetized iron, at high temperatures the symmetry is restored. The vacuum density of h becomes zero, and particles become massless. This is a difficult experiment. It should have happened in the early universe.
GUT
The next unification would be that of the electroweak force and the strong nuclear force, which is described by QCD (quantum chromodynamics). The strong interaction is fundamentally different from all other forces: Its strength increases as the distance increases. Hence, the ground state of particles experiencing the strong interaction is a tightly bound state. Quarks are the only particles that experience the strong interaction. It is for this reason that there are not free quarks found in nature. There are a variety of different proposals for this unification. Each involves distinct experimental predictions
for new particles, how the strengths of the interactions depends on the length scale, etc. Other than the search for the Higgs particle, high-energy accelerator physics is primarily concerned with sorting out these effects, and finding the proper form of the GUT. One of the earliest proposals was due to Georgi and Glashow. However, this theory predicted that the proton was unstable. This turned out not to be true. However, it seems that super symmetry has saved the theory of Georgi and Glashow.
What about gravity?
The first of the fundamental forces to be found is the hardest to integrate into the unified framework.
Finding some deeper theory than GR is NOT just an optional whim on the part of people who like unified theories. The present form of GR and QM are NOT CONSISTENT, so there must be some deeper form which applies in the realm where quantum effects become important (very short times/distances, high energies).
So far the only proposals that look like they have a chance to give GR in the usual regime without making contradictions in the high-energy regime are proposals involving more space-time dimensions: string theory and its relatives.
The hypothetical new paradigm
Assuming that the effort to unify QM and GR is successful, our view of the universe will have become one in which all forces (fields) and particles are the same kind of entity. This includes the geometry of the universe, which GR has made into a dynamical entity. Gravity (at long wavelengths, low energies) is thought to be mediated by a massless particle called the graviton.
The structure of the universe, and of spacetime itself, is determined by the interactions between the various particles. This may lead to an issue of uniqueness and self-consistency. For a given general form of theory, is there only one set of particles and interactions that might be found, or are their possible ranges of coupling constants, etc.? If the form of the theory doesn't specify all those numbers, what does?