(March 6, 2016 at 10:23 am)Alex K Wrote:

(March 6, 2016 at 10:15 am)little_monkey Wrote: No, you didn't understand my post. Seeing means detected by not only your eyes, but also by any detector that will indicated it has been hit by those particles, either by a flash or a jump in voltage. Virtual particles are exchanged between two other particles, so those particles are NOT going to hit your eyes or any other detector. It's that simple.

What's the conceptual difference between particles in a detector (which is made from particles) and "two other particles"?

The difference is not in the concept in terms of what "is" a particle, but that are involved differently. The particles that can be observed are those registering on your eyes ( or any detectors you wish to use). So you can say they interact with you (or an observer/detector). The virtual particles are those involved in the interaction between other particles. Therefore, they are not interact with you (or any other observer/detector). The word "virtual" is to distinguish these two cases, but a photon is a photon whether it interacts with you, hence its a "REAL" particle, or it interacts between two electrons, hence "VIRTUAL".

I don't need to deny the existence of god. I don't care if there's a god. I don't know why people keep on assuming that atheists are going to join the nearest death cult just because there's a god.

I don't care, either way. It just so happens that it's an ill-defined concept, with no explanatory power. Notice how every religion does the same trashing of science, right up to the point where they arbitrarily declare their own story book to just happen to contain everything you need to know. Because someone living in the past knew the name of a town they lived near or whatever.

(March 6, 2016 at 10:35 am)little_monkey Wrote:

(March 6, 2016 at 10:23 am)Alex K Wrote: What's the conceptual difference between particles in a detector (which is made from particles) and "two other particles"?

The difference is not in the concept in terms of what "is" a particle, but that are involved differently. The particles that can be observed are those registering on your eyes ( or any detectors you wish to use). So you can say they interact with you (or an observer/detector). The virtual particles are those involved in the interaction between other particles. Therefore, they are not interact with you (or any other observer/detector). The word "virtual" is to distinguish these two cases, but a photon is a photon whether it interacts with you, hence its a "REAL" particle, or it interacts between two electrons, hence "VIRTUAL".

You again make an arbitrary distinction between "other particles" and "you or observers".
Why do you say that virtual particles don't interact with you or other observers? That's just not true. The electrons in your eye are affected by virtual particle effects just as any other electrons. The electrons in your eye for example have an anomalous magnetic moment , ergo they interact with virtual particles.

(March 6, 2016 at 10:00 am)Alex K Wrote: Jesus Harris, did you write that novel yourself or paste it from somewhere? "people who are not trained in pQFT", who talks like that!

Maybe I can address this wall of text later. For now suffice it to say that indeed, what you call the first law (let's call it conservation of energy) is not so straightforward in curved spacetime. No, energy of particles in a curved spacetime is not preserved, that's an observational fact. Space can carry negative energy to compensate for the positive energy of its contents.

I was avoiding this discussion but you have fetched my legs into it.

(March 6, 2016 at 9:06 am)Harris Wrote: Can quantum mechanics create Universes from nothing? No!

Of course not, and neither can anything else.

(March 6, 2016 at 9:06 am)Harris Wrote: In the Quantum Field Theory view, actual particles are viewed as being detectable excitations of underlying Quantum Fields. Virtual Particles are also viewed as excitations of the underlying fields, but appear only as forces, not as detectable particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles. Thus, in mathematical terms, they never appear as indices to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modelled.

I would like to come back to this statement, in which you seem to trace back the reason why particles and virtual particles have a different "ontological status" (one really exists, the other is an artefact of calculational method) to the fact that one appears as scattering matrix-elements, whereas the others don't. Since you are eager to discuss S-matrix theory, you must be aware that this distinction is problematic because

1. S-Matrix elements are infra-red divergent due to soft emission, and only lead to physically meaningful results if they are combined with virtual corrections in scattering cross-sections. From this it appears to me that real emission of particles alone isn't even defined, and that the distinction between the two classes of particles based on arguments from S-Matrix theory is problematic. What's your perspective?

2. The S-Matrix is a somewhat artificial construct obtained by assuming that ingoing and outgoing particles stop interacting, and then taking the time to +/- infinity and using the resulting asymptotic free states as "external particles". This works great when describing scattering processes in which isolated, massive and stable particles meet and part again, but if you depart from this idealized scenario, things like 1.) happen. Also, if you just look at the unitary time evolution operator without taking it to asymptotic times, the distinction between what's an S-Matrix index and what isn't surely isn't as clear-cut any more.

Do you disagree? Or do you subscribe to an axiomatic S-Matrix approach to particle physics in which there is no local Lagrangian and no time evolution? Because that worked out so well?

(March 6, 2016 at 12:03 pm)Alex K Wrote:

(March 6, 2016 at 10:35 am)little_monkey Wrote: The difference is not in the concept in terms of what "is" a particle, but that are involved differently. The particles that can be observed are those registering on your eyes ( or any detectors you wish to use). So you can say they interact with you (or an observer/detector). The virtual particles are those involved in the interaction between other particles. Therefore, they are not interact with you (or any other observer/detector). The word "virtual" is to distinguish these two cases, but a photon is a photon whether it interacts with you, hence its a "REAL" particle, or it interacts between two electrons, hence "VIRTUAL".

You again make an arbitrary distinction between "other particles" and "you or observers".
Why do you say that virtual particles don't interact with you or other observers? That's just not true. The electrons in your eye are affected by virtual particle effects just as any other electrons. The electrons in your eye for example have an anomalous magnetic moment , ergo they interact with virtual particles.

You're really having a hard problem. It's a matter of definition. If you can't get that, I'm wasting my time with you.

(March 6, 2016 at 3:39 pm)little_monkey Wrote:

(March 6, 2016 at 12:03 pm)Alex K Wrote: You again make an arbitrary distinction between "other particles" and "you or observers".
Why do you say that virtual particles don't interact with you or other observers? That's just not true. The electrons in your eye are affected by virtual particle effects just as any other electrons. The electrons in your eye for example have an anomalous magnetic moment , ergo they interact with virtual particles.

You're really having a hard problem. It's a matter of definition. If you can't get that, I'm wasting my time with you.

Maybe I misunderstand you. Can you briefly clarify what you mean is a matter of definition, and how does that relate to the question whether virtual particles really exist?

(March 6, 2016 at 4:25 pm)Alex K Wrote:

(March 6, 2016 at 3:39 pm)little_monkey Wrote: You're really having a hard problem. It's a matter of definition. If you can't get that, I'm wasting my time with you.

Maybe I misunderstand you. Can you briefly clarify what you mean is a matter of definition, and how does that relate to the question whether virtual particles really exist?

double posting, sorry

(March 6, 2016 at 5:15 pm)little_monkey Wrote:

(March 6, 2016 at 4:25 pm)Alex K Wrote: Maybe I misunderstand you. Can you briefly clarify what you mean is a matter of definition, and how does that relate to the question whether virtual particles really exist?

Basic definition: two electrons interact, they exchange a photon. That photon is "virtual". It's not going to be a photon that any of my detectors  - eyes, voltmeters, or whatever apparatus I have at hand - will register. The photon goes from one electron to the other. Period. It's virtual.

Now, should a photon be emitted by an electron and not absorbed - for whatever reason, then that photon can be detected (by my eyes or any other detector). That photon is observable, so it's not a virtual photon. The reason you can see is that there are gazillions of those photons floating around, bouncing everywhere and then.

Now if you understand that basic definition, then we can talk about interactions, what are they, how are they calculated, what role does the Heisenberg Principle play, what are Feynman diagrams, what's their use, what's their limitations, what's renormalization, what's gauge theory, what role does symmetry play in QFT, what's the advantages of going through the Hamiltonian as opposed to the Lagrangian formulation, what about the Feynman path integral, what's the core idea behind QFT, and why do we need QFT instead of QM, and a dozen other topics. But if you have trouble of understanding basic definition, we're not going to get anywhere.