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**Physics, Astronomy & Cosmology / Re: Are all photons mediated in the same way through the quantum vacuum**

« **on:**18/07/2019 08:51:32 »

I sorry, but we don’t have the time to run a detailed course on QED/QFT but I’ll try to answer as much as time allows and then suggest that you spend time trying to understand the theory via a course in Quantum Systems.

Feynman originally developed the diagrams as a calculation aid for students on his course on quantum theory. Just as Einstein had a ‘thing’ about quantum theory, Feynman had a ‘thing’ about field theory and wanted to express the interactions as particles - there are indications that he later regretted this terminology. The virtual particles are internal processes and are only metaphors for multivariate integrals. The term is often used between researchers as jargon/shorthand rather than meaning particles as we think of them eg electron. The real problem is that in physics a lot of prior understanding is taken for granted, so a QED/QFT text book doesn’t expect you to leap in without studying a few years of physics and quantum theory and so doesn’t bother to explain the background.

The virtual particles are metaphors for multivariate integrals, there are no 'time processes of virtual particles. Nobody ever has written down an equation for the time evolution of virtual particles. While a case can be made that virtual particles exist at least as lines on paper, no such case can be made for their time evolution.

You keep mentioning space when referring to vacuum.

3D space (which can have a vacuum) is not the same as quantum vacuum. A quantum vacuum is the lowest energy state of a quantum system, any quantum system eg molecular bonds, it is not the same as spacial vacuum. If we take a simple Newtonian system eg a weight hanging from a string it has zero energy as it hangs there (disregarding the energy in the mass of the weight and the potential energy which would be released if you cut the string); if you set this system oscillating it will have its maximum ke at the bottom and its minimum at the top of the arc - where it spends most of its time.

The quantum version of this oscillator has its minimum energy at the bottom - where it spends most of its time - but this energy is not zero. This none zero minimum energy is a feature of all quantum systems and is due to the fact that there is uncertainty in the position and momentum of the particles. It’s not that they are flitting about, it’s a way of handling the uncertainty of a probabilistic system.

You don’t really borrow energy from the quantum vacuum, the correct way to look at this is with time/energy uncertainty ie Δt ≥ h/2ΔE . What this is telling you is that if you want to detect the presence or absence of a particle whose rest mass energy (E = mc

He was criticised by a number of physicists for this article as being misleading. Remember also the difference between physics and the maths we use to describe it - "existence" for physics means "measurable", for mathematics "possible to be included in a self consistent theory”, quite often there is confusion in the use of these terms.

I’ve already given you links on the problems of virtual particles being considered real (in the physics sense) and the problem of considering them as cause of Hawking radiation, but here is another link explaining, try to read and understand beyond where it says “this visualization is not for real particles, but virtual ones. They are calculational tools only, not physically observable entities”. https://medium.com/starts-with-a-bang/ask-ethan-how-do-hawking-radiation-and-relativistic-jets-escape-from-a-black-hole-b7a4ef7d9bdf

Just to recap on virtual particles and vacuum fluctuation, important to read and understand before diving into your QED book:

https://www.physicsforums.com/insights/misconceptions-virtual-particles/

https://www.physicsforums.com/insights/physics-virtual-particles/

https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

Also some comments by Prof Baez http://physicsfaq.co.uk/Relativity/BlackHoles/hawking.html

A photon does not need to be redefined to have em characteristics, they are embedded in its Maxwell equation source where a photon is just a quantisation of the em gauge field (in other words it

Feynmans virtual photons are just mathematical simplifications of field interactions, for peasants, which I think I realized already.you obviously have a very poor opinion of students of quantum theory

Feynman originally developed the diagrams as a calculation aid for students on his course on quantum theory. Just as Einstein had a ‘thing’ about quantum theory, Feynman had a ‘thing’ about field theory and wanted to express the interactions as particles - there are indications that he later regretted this terminology. The virtual particles are internal processes and are only metaphors for multivariate integrals. The term is often used between researchers as jargon/shorthand rather than meaning particles as we think of them eg electron. The real problem is that in physics a lot of prior understanding is taken for granted, so a QED/QFT text book doesn’t expect you to leap in without studying a few years of physics and quantum theory and so doesn’t bother to explain the background.

I am correct in concluding that Feynmans virtual particles do not exist in reality,Correct.

The virtual particles are metaphors for multivariate integrals, there are no 'time processes of virtual particles. Nobody ever has written down an equation for the time evolution of virtual particles. While a case can be made that virtual particles exist at least as lines on paper, no such case can be made for their time evolution.

and are not the same virtual particles as those predicted to exist in space by Casimir via calculations based on the uncertainty principle, supported experimentally. Giving rise to various quantum foam/gravity theories.They are the very same, but their ‘existence’ is not predicted or supported experimentally. Experiments can confirm that results of the calculations represented by the shorthand/diagrams are correct (real).

Further more virtual particle pairs borrowing there energy from the quantum vacuum of space momentarily, are nothing to do with QFT, other than they both use the term virtual particles.They are the same.

You keep mentioning space when referring to vacuum.

3D space (which can have a vacuum) is not the same as quantum vacuum. A quantum vacuum is the lowest energy state of a quantum system, any quantum system eg molecular bonds, it is not the same as spacial vacuum. If we take a simple Newtonian system eg a weight hanging from a string it has zero energy as it hangs there (disregarding the energy in the mass of the weight and the potential energy which would be released if you cut the string); if you set this system oscillating it will have its maximum ke at the bottom and its minimum at the top of the arc - where it spends most of its time.

The quantum version of this oscillator has its minimum energy at the bottom - where it spends most of its time - but this energy is not zero. This none zero minimum energy is a feature of all quantum systems and is due to the fact that there is uncertainty in the position and momentum of the particles. It’s not that they are flitting about, it’s a way of handling the uncertainty of a probabilistic system.

You don’t really borrow energy from the quantum vacuum, the correct way to look at this is with time/energy uncertainty ie Δt ≥ h/2ΔE . What this is telling you is that if you want to detect the presence or absence of a particle whose rest mass energy (E = mc

^{2}) is equal to ΔE, you need to look for at least a time Δt. You can look for longer if you like, but this is the minimum observation time needed to ensure that the uncertainty in your measurement is less than the energy of the particle. If you look for less time, your energy uncertainty will be bigger than the mass energy, and you can't be sure whether the particle was really there or not and in QM the probability is nonzero, but if you look for a longer time they aren’t there - you didn’t see them. Again this is a way of handling uncertainty in the equations.Adding further to my confusion I googled for a link on evidence for the Casimir effect experiment which I know exists, and stumbled across this link https://www.scientificamerican.com/article/are-virtual-particles-rea/ Which leads me to think perhaps the terminology used by different Quantum theories leads to further confusion.

He was criticised by a number of physicists for this article as being misleading. Remember also the difference between physics and the maths we use to describe it - "existence" for physics means "measurable", for mathematics "possible to be included in a self consistent theory”, quite often there is confusion in the use of these terms.

I’ve already given you links on the problems of virtual particles being considered real (in the physics sense) and the problem of considering them as cause of Hawking radiation, but here is another link explaining, try to read and understand beyond where it says “this visualization is not for real particles, but virtual ones. They are calculational tools only, not physically observable entities”. https://medium.com/starts-with-a-bang/ask-ethan-how-do-hawking-radiation-and-relativistic-jets-escape-from-a-black-hole-b7a4ef7d9bdf

Just to recap on virtual particles and vacuum fluctuation, important to read and understand before diving into your QED book:

https://www.physicsforums.com/insights/misconceptions-virtual-particles/

https://www.physicsforums.com/insights/physics-virtual-particles/

https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

Also some comments by Prof Baez http://physicsfaq.co.uk/Relativity/BlackHoles/hawking.html

I think you must be misreading, it would be unusual for a book on QED to mention antenna and radio waves specifically, all em fields are treated in the same way no matter what their energy. Are you sure your not looking at the evanescent field close to the antenna?No. They are real photons.

This statement disagrees with my text book, section on QED. I will re read the section very slowly, I do not think I have misunderstood what I read.

Are you possibly redefining what a photon is, and giving it EM characteristics?

A photon does not need to be redefined to have em characteristics, they are embedded in its Maxwell equation source where a photon is just a quantisation of the em gauge field (in other words it

**the field measured with a value of one quanta when detected). See first few paragraphs of: https://www.imperial.ac.uk/media/imperial-college/research-centres-and-groups/theoretical-physics/msc/current/qed/Photons.pdf***is*The following users thanked this post: flummoxed