Understanding QFT and the Role of Particles in Quantum Fields

[Varon]

It is said that in QFT, "particles" are just momentum and energy of the field. Is this just a conjecture or modelling to solve certain problems like photons coming out from electrons as it lower energy states or create a solution to other problems, or is it 100% proven that there are really no particles but just quantum fields and waves?

If the latter is so. Then one shouldn't think in terms of particles in the double slit and the measurement problem solved? Note Bohmian mechanics, Many worlds, even Copenhagen assumes particles are like solid balls and for nearly a century, countless debates ensued what happens in between emission and detection. The picture may be simplier if there is no particles but just quantum fields and wave at the double slit.

How come Bohm and Everett didn't think in terms of QFT but focused on the particles picture where QFT says quantum field is the primary, particles just momentum and energy of the quantum field?

What do you think?

 

[The_Duck]

The measurement problem has nothing to do with particles in particular. The measurement problem is how we get from a superposition of states to one single observed reality. QFT has superposition in exactly the same way as nonrelativistic quantum mechanics, only now it is superpositions of different possible field states instead of different possible particle positions or whatever.

 

[Varon]

The measurement problem has nothing to do with particles in particular. The measurement problem is how we get from a superposition of states to one single observed reality. QFT has superposition in exactly the same way as nonrelativistic quantum mechanics, only now it is superpositions of different possible field states instead of different possible particle positions or whatever.

Ok. Let's focus on electrons being momentum and energy of the electron field. Is this categorical? Or is just a modelling view that doesn't necessarily mean it is so? How come in the following article they are figuring out the shape of electron treating it like particles. Unless one can say they are figuring out the shape of the momentum and energy of the electron field? Pls. elaborate on this (others too who can share some valuable insight). Thanks.

http://www.npr.org/2011/05/25/136656087/what-shape-are-electrons-scientists-try-to-find-out

 

[TriTertButoxy]

It is said that in QFT, "particles" are just momentum and energy of the field. Is this just a conjecture or modelling to solve certain problems like photons coming out from electrons as it lower energy states or create a solution to other problems, or is it 100% proven that there are really no particles but just quantum fields and waves?

Together from experimental results from atomic physics (quantum optics) and particle physics, QFT has pretty much been verified -- everything is fields.

If the latter is so. Then one shouldn't think in terms of particles in the double slit and the measurement problem solved? Note Bohmian mechanics, Many worlds, even Copenhagen assumes particles are like solid balls and for nearly a century, countless debates ensued what happens in between emission and detection. The picture may be simplier if there is no particles but just quantum fields and wave at the double slit.

In quantum field theory the field does not replace the wave-function. Wave-functions are still there, and they still collapse. Let me illustrate:

In elementary quantum mechanics, the dynamical quantity is position. Here, the quantum mechanical uncertainties are captured by the wave-functions which are functions of position. Its square magnitude has the interpretation of the probability of finding the particle at a certain position.

Similarly, in quantum field theory, the dynamical quantity is the value of the field at every spatial point, called the field configuration. The field configuration may be a plane-wave, or something static like the electric field in a capacitor. Here, the quantum mechanical uncertainties are captured by wave-functions which are functions of field configurations. Its square magnitude has the interpretation of the probability of finding the field with a certain field configuration. Note that here, we are potentially talking about waves of waves.

Wave-functions are still there in quantum field theory. And they collapse when you make a measurement. The measurement problem is not solved.

 

[TriTertButoxy]

Ok. Let's focus on electrons being momentum and energy of the electron field. Is this categorical? Or is just a modelling view that doesn't necessarily mean it is so? How come in the following article they are figuring out the shape of electron treating it like particles. Unless one can say they are figuring out the shape of the momentum and energy of the electron field? Pls. elaborate on this (others too who can share some valuable insight). Thanks.

http://www.npr.org/2011/05/25/136656087/what-shape-are-electrons-scientists-try-to-find-out

The particles that emerge from quantum field theory are an interpretation of the discretization of the energy levels as a result of quantizing the field.

The reason, in the article you posted, they (the scientists) are figuring out the 'shape of electron' because the public still thinks in terms of particles. What the scientists are really finding is that the electron is not a complicated (relatively speaking), correlated excitation of many fields like the proton. Or alternatively, the electron can be described by a local field. (The proton can't)

 

[Varon]

The particles that emerge from quantum field theory are an interpretation of the discretization of the energy levels as a result of quantizing the field.

The reason, in the article you posted, they (the scientists) are figuring out the 'shape of electron' because the public still thinks in terms of particles. What the scientists are really finding is that the electron is not a complicated (relatively speaking), correlated excitation of many fields like the proton. Or alternatively, the electron can be described by a local field. (The proton can't)

So when thinking about double slit experiments. We mustn't think in terms of particles asking if it passes thru the left or right slit.. and the truth being that the electron field passes thru both slits as quantum field or wave?

 

[TriTertButoxy]

So when thinking about double slit experiments. We mustn't think in terms of particles asking if it passes thru the left or right slit.. and the truth being that the electron field passes thru both slits as quantum field or wave?

This is essentially correct.

 

[Varon]

This is essentially correct.

So most popular science books like those written by Brian Greene and John Gribbin and hundreds of others who mentions about particle either taking the left or right slits are flat lying to us or dumb down it so much that the writers believe it themselves??

But if the electron quantum wave passes thru both slits and they hit the detector at all regions simultaneously. What made one electron appear at the detector. Do you believe that one of the million existing electrons which comprise the detector just got triggered randomly just like what Neumaier described? If so. What is the measurement problem which you said still exist?

 

[TriTertButoxy]

So most popular science books like those written by Brian Greene and John Gribbin and hundreds of others who mentions about particle either taking the left or right slits are flat lying to us or dumb down it so much that the writers believe it themselves??

Yes, it's dumbed down. But I wouldn't say the writers believe it themselves. They know exactly what's going on.

But if the electron quantum wave passes thru both slits and they hit the detector at all regions simultaneously. What made one electron appear at the detector.

This is where the wave-function collapses. If you're using quantum mechanics to describe the system, then it is exactly as you know it to be -- the particle is found to be at a particular point. If you're using the field theory description, then the wave-function of the field configuration collapses in such a way so as to make it look like a quantum of excitation is localized near where the electron appeared at the detector. Either way, the wave-function has collapsed.

 

[Varon]

Yes, it's dumbed down. But I wouldn't say the writers believe it themselves. They know exactly what's going on.


This is where the wave-function collapses. If you're using quantum mechanics to describe the system, then it is exactly as you know it to be -- the particle is found to be at a particular point. If you're using the field theory description, then the wave-function of the field configuration collapses in such a way so as to make it look like a quantum of excitation is localized near where the electron appeared at the detector. Either way, the wave-function has collapsed.

Ok. What do you think of the following argument that the wave function never collapse. What happens in the detector is thus (according to Neumaier "no collapse" interpretation):

"It arrives at the various places of detector with different intensities, and these intensities stimulate all the electrons. But because of conservation of energy, only one can fire since the first one that fires uses up all the energy available for ionization (resp. jumping to the conduction band), and none is left for the others"

The above assumes the detector has millions of existing electrons already as these are used for detection so the quantum field never actually collapses but just trigger one of them as described. Can you refute it or offer some experiment setup where this theory is falsified?

 

[mattt]

That "alternative explanation" you quote from Neumaier seems to me quite interesting, but I have to think about it more carefully, because I have never heard of it before.

I want to see that argument with all the details, because at first sight I do not see any difference at the experimental level.

 

[Varon]

That "alternative explanation" you quote from Neumaier seems to me quite interesting, but I have to think about it more carefully, because I have never heard of it before.

I want to see that argument with all the details, because at first sight I do not see any difference at the experimental level.

Pls. read this thread beginning message #15 https://www.physicsforums.com/showthread.php?t=490492

Then if you see the fatal flaw.. hit it.

 

[Varon]

also pls. try to answer the following thread as it's related to analysis of it, thanks.

https://www.physicsforums.com/showthread.php?t=502538

 

[kith]

So when thinking about double slit experiments. We mustn't think in terms of particles asking if it passes thru the left or right slit.. and the truth being that the electron field passes thru both slits as quantum field or wave?

No. It seems to me, that you have some fundamental misconceptions about physics.

Physics is not about "truth". It is about developing theoretical models, which are able to predict the behaviour of nature. So if two models predict the same, both are valid and in general, the more simple model is used. Since quantum field theoretical calculations are very complicated, the theory is only used when it is the only one available (mostly relativistic situations).

Naturally, it can be instructive to consider simple, single particle problems in the more complicated language of quantum field theory. But to do that, you need substancial knowledge of both theories. From a quantum field theoretical viewpoint, problems with localized field configurations are sophisticated.

 

[A. Neumaier]

Naturally, it can be instructive to consider simple, single particle problems in the more complicated language of quantum field theory. But to do that, you need substancial knowledge of both theories. From a quantum field theoretical viewpoint, problems with localized field configurations are sophisticated.

Not really. In quantum optics they are used routinely as coherent states. Indeed, they are the most easily produced forms of states.

 

[Varon]

Anyone remember how it first occurred to physicists that particles are really momentum and energy of the field?

In the double slit then, since there are really no particles emitted but buckyball fields, then the buckyball field simply literally split at the slits and interfere at the detector and what happens was one of the millions of existing electrons in the detector simply get triggered making us think wrongly that it is due to the original buckyball hitting and triggering the electron? What happens to the buckyball field is it is simply smeared all over the screen. But if this were true, how come the molecules in our body don't get smeared out all over our organs making us jell-o or jelly like creatures?

 

[Oudeis Eimi]

Anyone remember how it first occurred to physicists that particles are really momentum and energy of the field?

Somewhere in the 1930's.

In the double slit then, since there are really no particles emitted but buckyball fields, then the buckyball field simply literally split at the slits and interfere at the detector and what happens was one of the millions of existing electrons in the detector simply get triggered making us think wrongly that it is due to the original buckyball hitting and triggering the electron? What happens to the buckyball field is it is simply smeared all over the screen. But if this were true, how come the molecules in our body don't get smeared out all over our organs making us jell-o or jelly like creatures?

Or more simply: why is there a localised detector to begin with? The standard answer
is environmentally induced decoherence. As a matter of fact, I suspect in many cases
of interest, the (1-particle) buckyball field would become well localised on the detector,
which would act as a strongly interacting environment for it.

In general, macro- and mesoscopic systems interact too strongly with their (ever-present,
even in a 'vacuum'), environment, and this interaction tends to make the purely quantum
behaviour extremely fragile.

 

[stefanbanev]

So most popular science books like those written by Brian Greene and John Gribbin and hundreds of others who mentions about particle either taking the left or right slits are flat lying to us or dumb down it so much that the writers believe it themselves??

But if the electron quantum wave passes thru both slits and they hit the detector at all regions simultaneously. What made one electron appear at the detector. Do you believe that one of the million existing electrons which comprise the detector just got triggered randomly just like what Neumaier described? If so. What is the measurement problem which you said still exist?

Electron remains in superposition state, it is "observer" who actually is splitting and each branch witnesses its own decoherence (I would say a virtual decoherence). The MWI picture is getting elegant and natural once we recognize that it is not universe splits apart (it always remains in superposition state) but rather "observer" is constantly branching. Our memory perceives a single path from root to current node. In fact the "Time" from this point of view is not one dimensional – it seems one-dimensional only thanks to our perception. Such picture is kind of trivial follow up of MWI, it is rather some psychological reason people are reluctant to give up one dimensional time; they try to preserve "self-unity" and with splitting universe it is simpler to do ;o)

Stefan

 

[Varon]

Electron remains in superposition state, it is "observer" who actually is splitting and each branch witnesses its own decoherence (I would say a virtual decoherence). The MWI picture is getting elegant and natural once we recognize that it is not universe splits apart (it always remains in superposition state) but rather "observer" is constantly branching. Our memory perceives a single path from root to current node. In fact the "Time" from this point of view is not one dimensional – it seems one-dimensional only thanks to our perception. Such picture is kind of trivial follow up of MWI, it is rather some psychological reason people are reluctant to give up one dimensional time; they try to preserve "self-unity" and with splitting universe it is simpler to do ;o)

Stefan

Don't you understand what Neumaier (the new von Neumann) is saying. There was no particle in the first place so it is a wrong question to ask whether there is particle that goes thru the left or right slit. Also since there is no particles, one musn't find any particles at the detector. It is meaningless. What happens is one of the million of existing electrons at the detector (which used electrons for detection) is simply triggered by the pure wave that hit the detector. Neumaier said Einstein photoelectric effect explanation was false because it was really the wave the hit the electrons and I think we must take back Einstein Nobel.

But Neumaier, if wave indeed hits the electrons in photoelectric effect. How come intensity of light is not able to dislodge the electron but it should be the frequency? It was for this reason that Einstein proposed photons are particles in the first place. Where did he do wrong?

 

[stefanbanev]

Don't you understand what Neumaier (the new von Neumann) is saying. There was no particle in the first place so it is a wrong question to ask whether there is particle that goes thru the left or right slit.

Well, get back and reread what I said. The "particle" as such is an emergent notion the result of decoherence. What I said that decoherence is a relative to observer and "electron" remains in superposition state described by wave function.

 

[Fyzix]

Don't even waste your time listening to Stefan, he couldn't answer to the major technical problems with MWI.
So when he continues to advocate it, he is just spreading misinformation.

 

[stefanbanev]

Don't even waste your time listening to Stefan, he couldn't answer to the major technical problems with MWI.
So when he continues to advocate it, he is just spreading misinformation.

>he is just spreading misinformation

What "misinformation"? Please be more specific. I spread no information at all; I just communicate the interpretation I like; if you do not like it just ignore no need to follow me unless you have a specific objections to what I said.