Sunday, April 24, 2016

Peter Ward Tries It On

Peter Ward asks

1.  What physically is a photon? The standard answer is a photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation. This is more a concept than a description of what a photon is physically. Is it simply a massless oscillation in space?
Eli goes to the Google:  A photon is the vector boson which carries the electromagnetic force.  It is a massless particle of spin one and zero charge.  Single photons are labelled by energy, momentum and polarization where energy, E = hν and momentum k = 2π/λ

Eli might ask what is an electron?  A sufficient answer is that it is a particle of spin 1/2, unit charge and a mass of 9.10938291 x 10-31 kg.

Quantum electrodynamics provides rules for calculating the probability of photons interacting in various ways with charged particles including electrons.  As with any quantum anything, QED provides an instruction sheet on what to do, but the epistemology oft is lacking.  As John Bell once wrote there is both speakable and unspeakable in quantum mechanics and venturing into the latter is brain threatening.  Ce la vie.

It is worth mentioning that in QED charged particles do not interact directly with each other but do so by exchange of photons.  Using QED one can calculate at least in principle the probability of an electron scattering off an electron, changing the motion (or quantum state) of the electron or charged nucleus in space and time, etc.

Quantum behavior is difficult, but tractable.  Playing the game of why in the quantum realm is not recommended unless you shrink down a bit and acquire some practice.   Assembling the machinery on the blackboard scale takes some time, and anybunny who wants to see the bottom line first might go to minute 37 in the video below where Feynman calculates the interaction of two electrons and then go back and view the entire lecture.

So no, photons do not beat their kids, and asking when they stopped is not going to lead to a fruitful discussion.  They simply will not allow themselves to be forced into your theory of knowledge of choice, but humans can figure out how they will and do behave.
4. Do the photons interact with each other in space? If not, why not? If yes, how?
See the lecture.  Since photons can decay into electron-positron pairs (or other beast pairs at super high energies) and the other photons can interact with the charged particles before they recombine, yes in principle, in practice not damn much in labs with budgets under the price of unicorns.  Without virtual pair production uncharged massless particles like photons do not interact.  No gravitational attraction either, they are massless.

What about interference?  Well from the QED point of view this is a function of the interaction of photons with the charged particles at the detector, that is the interference does not exist until it is mediated by the interaction at the detector.  That also answers the question of where the photon is, it is where the detector detects it.  Some, not Eli to be sure, may not like that but that's the engineering level report.
7. We talk of an electromagnetic field that can be mapped out in three dimensions and time with a suitable sensor. What is the physical relationship of such a field to photons?
In the interest of getting to bed and the comforts of Ms. Rabett, Eli will hand this one off to Lubos and return to the other questions tomorrow.


Dan Riley said...

In GR, all energy gravitates, including photons. In classical terms, the EM field contributes to the stress-energy tensor; that implies that any unified theory that includes quantum gravity is expected to have interaction terms between photons and gravitons. You can see this by thinking about the EM binding energy contribution to mass, or what happens to the mass of a black hole that absorbs a photon, or just think about the implications of photons following geodesics (which is experimentally confirmed) without any reciprocal effect.

EliRabett said...

Missed that

Aaron said...

I think of "Mother Nature" as an embodiment of physics, chemistry, and math. I do not try to understand everything that a very senior and sophisticated person might do. Some things just are.

Hank Roberts said...

> Some things just are
until they aren't

Bryson said...

Interesting to see Ward turn to the metaphysics of quantum theory here. But what matters is that the math has been found reliable when applied to light, including emission (spectra and BB) and absorption, and to heat and its transfer. The reliability of the math is what he needs to address if he's going to avoid the standard results.

John said...

Peter Ward apparently has been baffled by the particle-wave duality. Light behaves like a wave in some ways, and like a particle in other ways. This was on the scientific frontier about a century ago, but have long since been figured out. By now it's in the textbooks.

Peter Ward should read a good introductory physics textbook with coverage of an introduction to modern physics. For example, Knight, Physics for scientists and engineers.

The physics profession knows enough about these topics to address global climate change. In other words, progress climate change research does not require any further advances on the topics that Peter Ward is concerned about: e..g., QED or the nature of photons. Again, these were once hot topics. It wasn't just Einstein. For example, Arthur H. Compton won the Nobel Prize in 1927 for his discovery and explanation of the Compton effect, which involved the concept that light had both particle-like and wave-ike properties.

John Hughes said...

ITYM C'est la vie.

Unable to comment on the QM bits.

Greg said...

>No gravitational attraction either, they are massless.

Wait up there, not correct. For example, photons confined within a volume (for the sake of argument, the inside of a sealed box with totally reflective surfaces) will produce the same gravitational effect as an amount of matter having a mass equivalent to the energy of the photons, in the same volume, at least when measured a good distance from the box. The actual gravitational effect of photons is complicated by their pressure and the whole thing has be discussed in terms of a stress-energy tensor, but the original statement is definitely wrong.

luminous beauty said...

The most parsimonious theory is; photons at rest have zero mass. Photons are not at rest.

Hank Roberts said...

>> they are massless.
> Wait up there, not correct.

Gravitational lensing, right?

Well, the further I look, the stranger it gets, e.g.

"... Neurons, brains, space … these are just symbols we use, they’re not real. It’s not that there’s a classical brain that does some quantum magic. It’s that there’s no brain! Quantum mechanics says that classical objects — including brains — don’t exist...."

8c7793aa-15b2-11e5-898a-67ca934bd1df said...

Quanta Magazine has definitely run off the rails lately. And that creepy guy that writes for Forbes has been having some nice meltdowns as well. Then there is the endless Woit Motl feud. This has been going on for a while now, people (scientists) just can't stand ambiguity.

Hank Roberts said...

So it's actually pronounced "strength eerie"?
Works for me.