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An Introduction to Quantum Field Theory

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Re: An Introduction to Quantum Field Theory

#51

Post by danbruc »

jpmattia wrote: post #49
danbruc wrote: post #3

Are quantum fields actual elements of reality or just a convenient mathematical tool to deal with many particle systems? One of the questions I am struggling to find a satisfying answer for for quite some time. Depends on whom you ask? We can't tell because both ways of thinking are completely equivalent? Fields are real! No, they are just a tool! Are there issues with real fields forming a preferred reference frame?…

> Are quantum fields actual elements of reality Yes. Just as Schroedinger/Heisenberg quantum theory describes the behavior ("reality") for particles on a small scale, quantum-field theory describes the behavior of fields on a small scale.

In the meantime I looked for a lecture I watched some time ago and where I remembered that it suggested that fields are not real.

This lecture leaves no doubt that fields are not real but just a mathematical tool. The relevant part are the first 30 minutes. I am still not absolutely confident about that but just because it is so often suggested, including the people in this thread, that fields are real. But I am now at least almost convinced that particles are real and fields are not. Really worth watching.

https://m.youtube.com/watch?v=tnA7bh7dTqY

Re: An Introduction to Quantum Field Theory

#52

Post by Rusky »

danbruc wrote: post #51
jpmattia wrote: post #49

Earlier quoted context omitted.

> Are quantum fields actual elements of reality Yes. Just as Schroedinger/Heisenberg quantum theory describes the behavior ("reality") for particles on a small scale, quantum-field theory describes the behavior of fields on a small scale.

In the meantime I looked for a lecture I watched some time ago and where I remembered that it suggested that fields are not real. This lecture leaves no doubt that fields are not real but just a mathematical tool. The relevant part are the first 30 minutes. I am still not absolutely confident about that but just because it is so often suggested, including the people in this thread, that fields are real. But I am now…

What exactly makes something "not real" and only a tool? Is relativity "just a mathematical tool" because planets don't have differential equation solvers in them?

Re: An Introduction to Quantum Field Theory

#53

Post by danbruc »

Rusky wrote: post #52
danbruc wrote: post #51

Earlier quoted context omitted.

In the meantime I looked for a lecture I watched some time ago and where I remembered that it suggested that fields are not real. This lecture leaves no doubt that fields are not real but just a mathematical tool. The relevant part are the first 30 minutes. I am still not absolutely confident about that but just because it is so often suggested, including the people in this thread, that fields are real. But I am now…

What exactly makes something "not real" and only a tool? Is relativity "just a mathematical tool" because planets don't have differential equation solvers in them?

Real is probably a bad word and I can't really give a sharp definition but I can provide an analogy.

Take a soccer field with two teams and a ball on it. I would prefer to describe that as 23 particles of a few different kinds (ball, field player, goal keeper) and different properties (team membership, mass, fitness, whatever).

You could certainly invent a ball field and a field for both teams that are zero everywhere except where the ball or a player is located but you certainly wouldn't suggest that there are ball and soccer player fields in the universe even if they perfectly describe what happens on the playing field. On the other hand we could certainly agree that there are really players and a ball located somewhere on the field with specific masses, velocities and so on.

Maybe you can look at it from a point of redundancy. If you describe ball and players as particles with locations and momenta you get a description that naturally matches what is going on. If you use a field description you have to place a lot of constraints on what is allowed and what not, players disappearing, a second ball appearing and so on. The field description has just a lot more degrees of freedom than the particle description and you have to impose a lot of constraints to suppress field configuration that are not physically possible. So it also a kind of Occam's razor argument to pick a simple and minimal description.

Re: An Introduction to Quantum Field Theory

#54

Post by Rusky »

danbruc wrote: post #53
Rusky wrote: post #52

Earlier quoted context omitted.

What exactly makes something "not real" and only a tool? Is relativity "just a mathematical tool" because planets don't have differential equation solvers in them?

Real is probably a bad word and I can't really give a sharp definition but I can provide an analogy. Take a soccer field with two teams and a ball on it. I would prefer to describe that as 23 particles of a few different kinds (ball, field player, goal keeper) and different properties (team membership, mass, fitness, whatever). You could certainly invent a ball field and a field for both teams that are zero everywher…

Quantum physics is not about a game of soccer. It's about phenomena that do things that actually do look strange when just looking at particles.

I don't see how a "natural" description is any different from the word "real" here- fields describe things particles can't, especially when trying to use your "natural" intuition.

Re: An Introduction to Quantum Field Theory

#55

Post by danbruc »

Rusky wrote: post #54
danbruc wrote: post #53

Earlier quoted context omitted.

Real is probably a bad word and I can't really give a sharp definition but I can provide an analogy. Take a soccer field with two teams and a ball on it. I would prefer to describe that as 23 particles of a few different kinds (ball, field player, goal keeper) and different properties (team membership, mass, fitness, whatever). You could certainly invent a ball field and a field for both teams that are zero everywher…

Quantum physics is not about a game of soccer. It's about phenomena that do things that actually do look strange when just looking at particles. I don't see how a "natural" description is any different from the word "real" here- fields describe things particles can't, especially when trying to use your "natural" intuition.

If you haven't yet watch the first half hour of the lecture I linked to - this makes the point way more clear than I ever could in a few comments. And it essentially settles the question. Besides the fact that there are so many people claiming that fields are real but at least for the moment I will assume that they are just wrong or until I hear a convincing argument that this point of view is wrong. There are some hints in some of the comments here that fields are really necessary in some circumstances but they are not specific and I am not knowledgeable enough to judge them. Will have to do some research on that.

And by the way I am in no way suggesting any classical particle model, even without fields you still have the entire quantum mechanical machinery and there are certainly non-classical things going on. The question is whether you really need fields to describe some aspects of nature or if good old quantum mechanics is good enough and quantum field theory just makes things mathematical more accessible.

Re: An Introduction to Quantum Field Theory

#56

Post by T-A »

danbruc wrote: post #43
danbruc wrote: post #27

Earlier quoted context omitted.

Now I have to dig out a statement by Nima Arkani-Hamed where he says something like "[...] should bother you if you believed that fields are real." - I of course forgot the most important part but I still remember in which lecture series he said that. If my memories does not mislead me, he was really arguing that you should not think of fields as real things for very concrete reasons. Trying to find it.

This took way longer than I imagined. I won't repeat the argument here in detail but it essentially boils down to the argument that fields theories are just an effective description of long distance behavior ignoring what is really going on at short distances, that fields are just a convenient tool to make locality manifest and that field theories contain a lot of redundancies due to this. The statement I was referri…

http://arxiv.org/abs/1204.4616 :)

Re: An Introduction to Quantum Field Theory

#57

Post by danbruc »

T-A wrote: post #56
danbruc wrote: post #43

Earlier quoted context omitted.

This took way longer than I imagined. I won't repeat the argument here in detail but it essentially boils down to the argument that fields theories are just an effective description of long distance behavior ignoring what is really going on at short distances, that fields are just a convenient tool to make locality manifest and that field theories contain a lot of redundancies due to this. The statement I was referri…

http://arxiv.org/abs/1204.4616 :)

Ohhh no! Just when I thought I got the answer. Just skimmed it and I draw some hope from the fact that the author uses the unconfirmed Unruh effect and wave function collapse, which is as far as I know not a thing because it is not unitary, to explain the double slit experiment. So maybe it is the author who is confused and not the others he is complaining about and I won't have to change my mind once again but I will have to read it way more carefully before drawing any conclusions.

Re: An Introduction to Quantum Field Theory

#58

Post by Retra »

danbruc wrote: post #48
Retra wrote: post #46

Earlier quoted context omitted.

That's a non-issue. Everything you've ever experienced is mapped through your perceptions, and thus it's all illusory. Physics is about coming up with language that describes our experiences accurately, not distinguishing between meaningless philosophical inventions.

Everything we know about the world has at some point to go through our senses but it is still a reasonable assumption that there is a real world outside and independent of our minds. When we collide particles in an accelerator and then look at the results on a computer screen you can hardly argue that our vision disturbs what the experiment tells us about the world out there in any significant way. The whole goal is…

You're not asking what the world is really like, you're asking about whether or not a model of reality is a fiction or not. Physicists model reality. Every model is a fiction. Some are just more useful fictions than others.

Re: An Introduction to Quantum Field Theory

#59

Post by Rusky »

danbruc wrote: post #55
Rusky wrote: post #54

Earlier quoted context omitted.

Quantum physics is not about a game of soccer. It's about phenomena that do things that actually do look strange when just looking at particles. I don't see how a "natural" description is any different from the word "real" here- fields describe things particles can't, especially when trying to use your "natural" intuition.

If you haven't yet watch the first half hour of the lecture I linked to - this makes the point way more clear than I ever could in a few comments. And it essentially settles the question. Besides the fact that there are so many people claiming that fields are real but at least for the moment I will assume that they are just wrong or until I hear a convincing argument that this point of view is wrong. There are some h…

"Good old quantum mechanics" is quantum field theory.
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