what propels light?

[Peter Ward]

Originally Posted by sjastro

So a nuts and bolts explanation of a mechanism is required otherwise the mechanism is not real.


The physics explains why photons exist.

Regards

Steven

Ah..there was some irony/humor there Steve....

That said, yes, I understand the energy has to go somewhere, and "an electron emits a photon"...we can even determine its energy / wavelength....but to labour the point *how* does an electron emit a photon?? i.e does the Photon "creation" process have a deeper mechanism other than popping into existence?

[Octane (Humayun)]

I just wanted to say that this has been a very interesting thread to read, thus far. Keep it going, people.

H

[Regulus (Trevor)]

Well done chris. A good answer to a good question.

[sjastro]

Originally Posted by Peter Ward

Ah..there was some irony/humor there Steve....

That said, yes, I understand the energy has to go somewhere, and "an electron emits a photon"...we can even determine its energy / wavelength....but to labour the point *how* does an electron emit a photon?? i.e does the Photon "creation" process have a deeper mechanism other than popping into existence?

Peter,

First of all one needs to define what QFT tells us about photons.

QFT tells us why photons exist and why they have certain physical properties such as zero mass.
It tells us the conditions under which photons are created without providing an explanation as to how they are created.

A single photon is defined as a vector, it carries an intrinsic angular momentum as "clockwise" or "anticlockwise" spin. Observers in space would therefore see the photon differently according to their position in space. If the photon appeared the same to all the observers it would have a spherical symmetry.
While this appears to be trivial, the non spherical symmetry of a photon tells us the conditions under which they are created.
In the hydrogen atom the ground state exists in a spherically symmetrical state. If the atom is excited and the electron moved to a higher energy state, the symmetry can no longer be spherical. When the electron returns to it's ground state a non spherically symmetrical photon is emitted. This cannot happen if both the ground and excited states of the atom are spherically symmetrical.
It forms part of the selection rules for electron transistions on which quantum mechanics is built on.
There is a subtle relationship between symmetry and angular momentum.

QFT extends the notion of symmetry and allows to us explain why the photon has certain properties. I use the term explain rather than describe as QFT has also made predictions for other particles and bosons according to their symmetry that were unknown at the time, but have been subsequently discovered.

Calculations involving quantum field theory are universal throughout high-energy particle physics. Here we will briefly summarize a few more notable successes.
As stated above, one the greatest achievements of QED is the very labor intensive, but extremely precise, calculation of the angular magnetic moment of the electron. In classical quantum mechanics, the magnetic moment of the electron, called g, should be exactly 2.0. The very small deviation from 2 is called the anomalous magnetic moment, and can be experimentally measured to extremely high precision. The very labor-intensive theoretical predictions from QED match the experimental measurement to one part in a billion, a precision unparalleled in all of science. Also, the magnetic moment of the muon can be predicted to one part in a billion.
Also, as stated above, the Electroweak Theory, a unification of QED and weak force, predicted the existence and approximate mass of the Z boson before its observation.
QCD has successfully described jet events at particle colliders. In these events, a particle and antiparticle are smashed together in a collider; the resulting energy then turns into a quark and anti-quark pair. The quark and anti-quark each then split into vast, complex showers of other high-energy particles, called "jets", which are seen as showers of particles in opposing directions in the particle detector. If the quark and anti-quark produce showers immediately, it is called a two-jet event. In about 10% of all cases, a quark or anti-quark will emit a gluon, which then splits into still more particles, thus displaying a three-jet event. QCD successfully models the probability and momentum distributions of the jets.
Non-perturbative methods have recently been very successfully used in QCD. Lattice gauge QCD calculations, requiring vast computer power, have computed the masses of the proton, neutron, by computing the energy of the interaction E of the quarks and gluons on a lattice, and then employing Einstein's equation m = E/c2. These calculations compute the masses of several important particles entirely a priori, with no tuneable free parameters input to the model, except the strong coupling constant.

Regards

Steven

[snas (Stuart)]

Steven

Just wanted to say that I am glad to see you posting again. While my biologist's eye view of astronomy is coming from a much lower level than yours, I have always enjoyed the discussions created by both yourself and Craig and have even occasionally learnt something! Like I have learnt on this thread where photons come from. Would like to see Craig come back too because he was prolific starter of interesting threads!

Stuart

[cwjohn (Chris)]

Steven

A physicist states that a photon is created so that the Lagrangian for the electromagnetic field is invariant under a U(1) symmetry operation. While this may sound like goobledygook to you, Bert and 99.99% of the population, when simplified it means a photon is created to preserve the conservation laws. It makes perfect sense.

This does not sound like goobledygook to me, and it did not sound so when I was first taught it decades ago, and I also suspect Bert and Peter have also been educated to this extent also.

I think however, you have to consider the import of your language here.

First of all "A physicist states" implies that you speak for all physicists. This is a somewhat arrogant statement. By virtue of an honours degree in physics I suppose I am theoretically a physicist albeit not a practicing one. I would not state that a photon is "created ..." but rather that a photon is "described by the Lagrangian....".

So you say a photon is created to preserve the conservation laws. Why? Because Noether said so in 1920.

This makes perfect sense???? Why?

Nobody argues that QFT and the math of symmetry describe the Standard Particle Theory par excellence. Many emminent physicist (for example Lee Smolin) do not accept this as a reasonable end point for our deliberations. And indeed if this were to be the case there are many very smart physicists wasting their time investigating string theory and quantum loop gravity.

Perhaps you are most correct when you say

The use of the word real smacks too much of a metaphysical argument.

I think this is definitely true and it is an argument that goes back to Einstein and Bohr and continues to this very day. You are content to accept mathematical descriptions as reality, and indeed you have quoted the leading proponents of this proposition (a metaphysical proposition to boot). There are others (and I fall into this camp) who do not accept this as "reality" but rather seek deeper explainations. However, to lump all physicists into your way of thinking is not reasonable.

[sjastro]

Chris,

Originally Posted by cwjohn

Steven

First of all "A physicist states" implies that you speak for all physicists. This is a somewhat arrogant statement. By virtue of an honours degree in physics I suppose I am theoretically a physicist albeit not a practicing one. I would not state that a photon is "created ..." but rather that a photon is "described by the Lagrangian....".

That is your assumption which is incorrect.
I’m stating the mainstream view which is supported by a vast majority of physicists who partake in research.

So you say a photon is created to preserve the conservation laws. Why? Because Noether said so in 1920.

This makes perfect sense???? Why?

It makes perfect sense particularly when looked at from a historical perspective.

Noether’s theorem is the evolution of an idea starting from around the 1830s when Newtons second law F=ma was derived by defining the gravitational field as a dynamical system using a Lagrangian.
The use of a Lagrangian did not require Newtons second law to be known beforehand.
Similarly the Lagrangian of an electromagnetic field defined as a dynamic system doesn’t require photons to be known beforehand either. A photon is a prediction of the local invariance of the Lagrangian under a U(1) or rotation transformation.
If you believe that QFT is simply describing a phenomena than how do you explain the other gauge bosons which are predicted by QFT but were not observed at the time.

Nobody argues that QFT and the math of symmetry describe the Standard Particle Theory par excellence. Many emminent physicist (for example Lee Smolin) do not accept this as a reasonable end point for our deliberations. And indeed if this were to be the case there are many very smart physicists wasting their time investigating string theory and quantum loop gravity.

Both string theory and quantum loop gravity draw heavily on QFT. Neither theory departs from the symmetry theme of QFT and both are examples of gauge theories.
The very smart physicists still work on QFT such as Ed Witten who has made significant contributions.
Why raise string and quantum loop theories in the first place?
Neither provides the "realistic" approach you are seeking.

I think this is definitely true and it is an argument that goes back to Einstein and Bohr and continues to this very day. You are content to accept mathematical descriptions as reality, and indeed you have quoted the leading proponents of this proposition (a metaphysical proposition to boot). There are others (and I fall into this camp) who do not accept this as "reality" but rather seek deeper explainations. However, to lump all physicists into your way of thinking is not reasonable.

I have to ask you the obvious question, is there any theory in physics that satisfies your criteria of providing a realistic approach?

Regards

Steven

[cwjohn (Chris)]

Steven

I’m stating the mainstream view which is supported by a vast majority of physicists who partake in research

Well we have moved from "all physicists" to the "vast majority". I suspect when pressed most of this "vast majority" would conclude that QFT describes rather than explains. I suspect they would not accept as a suitable description of a photon as:

Similarly the Lagrangian of an electromagnetic field defined as a dynamic system doesn’t require photons to be known beforehand either. A photon is a prediction of the local invariance of the Lagrangian under a U(1) or rotation transformation.

Here you state that a photon is in fact a "prediction" as defined by a Lagrangian etc which is a mathematical construct. You also describe it as a "vector". You also state it carries "intrinsic angular momentum" but at no stage do you state what "it" is. You also state QFT "tells us the conditions under which photons are created without providing an explanation as to how they are created."

So this "prediction" which is a "vector" is defined by a theory which says nothing about the creation of said "prediction" or "vector". Does this really make sense to you on reflection.

Furthermore you state that because QFT has predicted other particles (a fact that is beyond doubt) that ipso facto it explains "the ontological and epistomological basis of these particles". This is a logical fallacy which cannot be sustantiated on any evidential basis.

You already state you dont know how a photon is created (or QFT doesnt, I guess you may). If you believe you know what a photon is then state it simply, but not as a "prediction", or a "vector".

I thought religion was confusing???

Just kidding. I reached for one of my texts on QFT (Zee QFT in a nutshell 2003). In the intro he says

It struck me as limiting that even after some 75 years, the whole subject of QFT remains rooted in this harmonic paradigm (Schrodingers equations and extensions)... We have not been able to get away from the basic notions of oscillations and wave packets. Indeed, string theory, the heir to QFT is still firmly founded on this harmonic paradigm. Surely, a brilliant young physicist will one day take us beyond."

As to your question

I have to ask you the obvious question, is there any theory in physics that satisfies your criteria of providing a realistic approach?

As I have stated before all theories of physics that do not deal at the Plank level. Its not that I don't accept theories of physics at the quantum level. I certainly do, but I maintain that they are descriptive and predictive. They explain a real result, but they do not explain what is really happening. As Zee says when some brilliant person takes us beyond that barrier then we will see the next breakthrough in physics.

Just my opinion.

[Kal (Andrew)]

Originally Posted by sjastro

That is your assumption which is incorrect.
I’m stating the mainstream view which is supported by a vast majority of physicists who partake in research.

This may be what you intended to say but it is not what you initialy communicated. You would have been better off by starting with "The mainstream view of a physicist is .. "

[sjastro]

Well we have moved from "all physicists" to the "vast majority". I suspect when pressed most of this "vast majority" would conclude that QFT describes rather than explains. I suspect they would not accept as a suitable description of a photon as:

I’m stating the mainstream view which is supported by a vast majority of physicists who partake in research

What has the mainstream view got to do with whether physicists consider QFT to being a phenomenological theory or not?
What I stated was that QFT is a mainstream theory supported by the vast majority of physicists (and that includes the string and quantum loop gravity theorists) otherwise it would not be mainstream. It has nothing to do with a physicists personal take on whether QFT explains or describes things.

You have completely taken my statement out of context.

Here you state that a photon is in fact a "prediction" as defined by a Lagrangian etc which is a mathematical construct. You also describe it as a "vector". You also state it carries "intrinsic angular momentum" but at no stage do you state what "it" is. You also state QFT "tells us the conditions under which photons are created without providing an explanation as to how they are created."
So this "prediction" which is a "vector" is defined by a theory which says nothing about the creation of said "prediction" or "vector". Does this really make sense to you on reflection.

Furthermore you state that because QFT has predicted other particles (a fact that is beyond doubt) that ipso facto it explains "the ontological and epistomological basis of these particles". This is a logical fallacy which cannot be sustantiated on any evidential basis.

You already state you dont know how a photon is created (or QFT doesnt, I guess you may). If you believe you know what a photon is then state it simply, but not as a "prediction", or a "vector".

Once again your propensity of distorting of my statements to create a strawman argument is duly noted.

What is obvious is that you have a problem comprehending the objectives of QFT.

The simplest explanation I can give is a piece of history which clearly shows the differences between a theory that describes (a phenomenological theory) and a theory that explains (a non phenomenological theory).

In the 1960s the physicist Murray Gell-Mann came up with the EightFold Way theory which described the grouping of particles into octets according to their quantum numbers.
There was a missing particle but its properties were predicted according to its position in the group. The particle was discovered and its experimental properties closely matched the predictions.

The Eightfold Way theory is a rare example of a phenomenological theory in particle physics that has led to a prediction of a particle.
The Eightfold Way theory cannot explain however why the particles are arranged in octets or why each particle possesses the properties it has.

The Eightfold Way theory was a stepping stone to the development of Quantum Chromodynamics (QCD) which explains the Eightfold Way theory, all without having to invoke stories about creation which you seem to be fixated on.

Your argument is fallacious because it attempts to pidgeon hole QFT into a "creationist" theory which it clearly isn't. Creation itself isn't theorized in QFT. The "conditions" for creation however are through symmetry preserving (photons) or symmetry breaking (W and Z bosons).

As I have stated before all theories of physics that do not deal at the Plank level. Its not that I don't accept theories of physics at the quantum level. I certainly do, but I maintain that they are descriptive and predictive. They explain a real result, but they do not explain what is really happening. As Zee says when some brilliant person takes us beyond that barrier then we will see the next breakthrough in physics.

Just my opinion.

"Macro" physics is much more phenomenological than quantum mechanics. Why is it that scientists require a quantum mechanical explanation for dark energy and dark matter given by your definition quantum mechanics is purely descriptive?

Regards

Steven

[cwjohn (Chris)]

Steven

The discussion seems to have drifted somewhat. I do not think it is constructive to bore everyone about whether a theory is phenomological or non-phenomological.

We are not poles apart. Our differences are more philosophical than scientific.

The bottom line is that you are satisfied that you clearly understand what a particle is, and you have expressed it here. I find your definition unsatisfactory as do many others, physicists and non physicists alike, and I have expressed that here.

I am sure people will come to their own conclusions on this matter.

It was an interesting discussion nontheless.