Physics of Light

This is the same in QFT... and QFT deals quite happily with scalar fields. Electromagnetism is classical, as is Relativity for that matter. The difference between scalar and vector fields is irrelevant at this level of discussion. Both occur in classical field theory and qunatum field theory. I would describe the conclusion of MM as that the aether was unobservable for some reason. Lorentz proposed his transformations to explain why it was unobservable. True, but not what I was saying. Yes it is, it is very similar... although obeying quite different rules.

Yes, it is. Yes we do. They are called QFT and GR.

No, it's not inert. Under the Standard Model, existence is a combination of various fields: the classical metric field of GR, the U(1)xSU(2)xSU(3) bosonic fields of the Electroweak and Strong forces (photon, W and Z, and gluon), and the associated fermionic "matter" fields: the 6 leptons (electron, muon, tauon, and their respective neutrinos) and the 6 quarks (up, down, strange, charm, top, bottom).The "vacuum" is just the ground-state of these fields, constantly bubbling and seething, interacting with each other. There's a lot going on. If you include quantum gravity, then not only do you have virtual particles popping into existence and disappearing, but also virtual black holes doing the same! One of our ideas is that of the "quantum foam" where the very topology of space-time bubbles away. This is where we get the Sci-Fi idea of extracting a wormhole from the foam and blowing it up to usable size.As ever, this decription of theoretical physics makes it sound as if it is pure qualitative speculation. Don't be fooled... this is all tightly constrained exceptionally precise mathematics.One major goal of theoretical physics is to unify these separate fields into one master field which would constitute everything. Grand Unified Theory (GUT) is the attempt to get the Electroweak and Strong unified. Supersymmetry (SUSY) is the attempt to get bosonic and fermionic fields ("force" and "matter") unified. Quantum Gravity (QG) is the attempt to get a description of gravity on a similar footing to the other fields (or vice-versa) so enable possible unification. The combination of all of these is the start of the Theory of Everything (TOE). Superstring Theory (and its successor, M-theory) is one attempt at such large scale unification.

What is the experimental evidence for all this. I mean, it is great to have a set of equations that we think describe the sitution, but what is the hard data of experiments that predict things.I know that some aspects of GR are only recenlty been able to be tested (the one where rotation of the world creates small 'time eddies'). We only had the instrumentation to confirm that the last few years.

I think we are talking past each other, and not in a way that is helpful to anwering the OP questions. That QFT and classical field theory both deal with scalar and vector fields is true, but not relevant to the original question:Is there a physical "bulk medium" required for EM waves to propagate through, like a solid for acoustic waves?IMO it is misleading to qualitatively think of the photon gauge field in this way, as it is not a macroscopic bulk property of matter. And of course, macroscopic properties of matter are ultimately related to elecromagnetic fields as this is what holds matter together chemically, but the wave properties in a macroscopic medium are different level of physics than the properties of gage fields which are , as you point out, only possible to understand in QFT.As you've pointed out, however, qualitative pictures can be misleading in general, and physics is better described mathematically.

The notation Cavediver used that looks like U(1)xSU(2)xSU(3) etc. is group theory. One of the experimentally testable consequences of this is that certain group symmetries of fields lead to predictions that a certain number of elementary particles should exist with certain properties. This has largely been verified with particle accelerator experiments (at least with the up, down, strange,and charm quarks).The top and bottom quarks were discovered not too long ago and the process of filling out the table of particles with these may still be ongoing - maybe Cavediver knows more about the current status.

You have given me food for thought. thank you.
Quatum mechanics and theory would be a great playground if my life were different.This message has been edited by 2ice_baked_taters, 02-18-2006 11:01 AM

Which bit?Quantum Field Theory? Well Quantum Electrodynamics is the 2nd best tested theory ever devised, with a prediction of the gyromagnetic ratio accurate to the level of measurability at some 11-12 places of accuracy. This also provides the best test of Special Relativity, as QED (and all of QFT) is utterly dependent upon SR.General Relativity? The most accurately tested theory ever, with the prediction of binary neutron star spin-up accurate to the level of measurability at some 12-13 places of accuracy. This is just one of the bestter experimental tests of GR.The rest of the Standard Model physics? Well, CERN, Fermilab and DESY don't seem to be having any problems working their stuff.Don't worry... fundemental physics has a more firm bedrock of evidence than probably any other area of science. It's just not appreciated.

You are more than welcome. And speaking of welcome, welcome to EvC So true. I don't regret leaving academia, but it still makes me sad from time to time to be on the outside...

Yes, absolutely. Physical... the Casimir Effect certainly demonstrates that. Bulk... it encompasses space-time, so yes. Medium... well, that's what we are discussing. That shouldn't be a problem. In acoustics, the solid forms the background medium. In QED, the photon and electron fields form the background medium. And the mathematics is almost identical (modulo quantum stuff like spin and statistics). If you are not already familiar, check out the mathematics of phonons.Sound is just propegation of disturbance through a solid. Light is just propegation of disturbance through a quantum field. A different level, absolutely. We are at a much more fundemental level. But the principles are so very similar as demonstrated by the similarity of the mathematics. Sound in solids and light in the photon field are both just cases of bunches of connected harmonic oscillators...

So the medium is there. Just not in the typical physical sense. The medium manifests itself in a different state of exitation?
So then light is indeed like waves through water in that it is the evidence of energy passing through a medium, though not describable in the typical physical sense. Then I would conclude that the particle properties of light are simply further evidence of the energy traveling through the "virtual medium" I believe the particle properties of light to be misleading.

That's basically it. What you have to remember is that we also are just excitations of this medium! Well, yes and no. The very quantum nature of the field means that there is a minimum level of excitation. This minimal level propegates across the field, and this is what we call a photon - a "particle". But the real photons that we capture in detectors are a bit more complicated than this...The thing we call an electron is more obviosuly un-particle like when you get close to it. It is a seething mess of interactions between the photon and electron field.

Thank you, thank you.So that which we think of and expierence as physical matter is a mode of excitation that we can percieve. That would make sense, since the the nature of much of the tools we use to understand this are of that same level of excitation.
Amazing, to me anyway, how through the discusssion of the nature of light I have arived at the conclusion that the physical world is a mode or expression of energy or force.
Along this same line of thought I will make this assertion:
Though it cannot be described mathimatically, "yet" I would put forth that the force of will should be entertained by the science comunity.
But that is for another forum

I still don't find this analogy compelling. With phonons, you still have the crystal lattice of the solid as a propagation medium. Unless you think of gauge fields as forming a kind of lattice, I don't see how the analogy is that helpful. I've heard of lattice gauge theories but am not familiar with them, is this what you are getting at ? Is the "lattice" something structural, or a computational artifice? My field is applied physics . I had one course on QFT but haven't had occasion to use it.

We model the field as an infinitely dense lattice of harmonic oscillators. "Lattice" QCD, for example, essentially uses a finite sized lattice of oscillators simply for computational tractability.I guess you may have learnt QFT entirely from the perspective of Path Integrals, missing out canonical quantisation, in which case you will have by-passed much of this.