Particles vs Fields: What's More Fundamental?

[Demystifier]

In your opinion, what are more fundamental objects: particles or fields?
In other words, is QFT just a convenient mathematical way to calculate the properties of particles,
or
are particles just specific states of quantum fields?

 

[Demystifier]

I voted particles, although I am far from being completely convinced.
For a pedagogic review of arguments for both possibilities see
http://arxiv.org/abs/quant-ph/0609163

 

[CarlB]

I refuse to vote. I think that there exists a "now", and that when an event from the past is examined, one will find that a particle description fits it well. When one examines an event from the future, I think that this event also exists in spacetime, but must be described by a field.

The field is a sort of stress of spacetime, and as the moving hand of time passes through the event, the stresses become extreme, and eventually spacetime breaks down. Particles are the points at which spacetime has dislocated. The probability interpretation simply says that the dislocations are random, but are more likely in places where the stress is higher.

This is like Bohmian mechanics, but with the wave and particle not being operative, at the same time, for any single snapshot of the event.

 

[arivero]

pleres kai stereon to on, to de kenon to me on...

 

[Jheriko]

I prefer fields.

In a very simple sense they are more general. For example, it could be said that (infintely dense point) particles are a subset of fields, i.e. a field with a value defined for just one position and zero elsewhere. For quantum particles we have a probability distribution which is also a specific type of field... etc..

This imo makes them more fundamental, since you can model a particle as a field without any tricks involving infinity, but the reverse is not true.

 

[arivero]

It seems that force carriers, bosons, are more properly treated as fields, while fermions, in some sense matter fields, are more properly to be thought as particles. There is a subtle point here, that there is no such thing as a classical fermionic field: this is seen by explicitly putting the Planck constant in the Lagrangian, instead of setting it to 1 as usual.

One could think that an object whose main properties are spatial is a field, and an object whose main properties are kinematical/positional is a particle. But everything gets mixed in the quantum world.

It also happened to Democritus, that after clearly dividing spatial properties (no-thing) from positional ones (thing), got to the problem of how forces were communicated, and need to create a new concept, eidola, to understand our actual bosonic carriers. So at the end one has four categories: spatial boson, positional boson, spatial fermion, positional fermion.

For the mathematically minded, it could be better to think on de Rham duality instead of the wave/corpuscle issue. What is more fundamental, the form or the cycle? The density or the volume where we integrate this density? The volume or the density to be integrated in this volume?

 

[vanesch]

I voted particles, because I think fields are more fundamental :tongue2:
(and if I would have voted fields, I would then be in a state where particles are more important).

Honestly, the two descriptions are entirely equivalent. So what...

 

[robphy]

"QFT in curved spacetime" suggests that the notion of particle is dependent upon some structures that are not available in a generally curved spacetime.

See, e.g., http://arxiv.org/abs/gr-qc/0608018 "The History and Present Status of Quantum Field Theory in Curved Spacetime" by Robert M. Wald

 

[selfAdjoint]

It seems that force carriers, bosons, are more properly treated as fields, while fermions, in some sense matter fields, are more properly to be thought as particles. There is a subtle point here, that there is no such thing as a classical fermionic field: this is seen by explicitly putting the Planck constant in the Lagrangian, instead of setting it to 1 as usual.

One could think that an object whose main properties are spatial is a field, and an object whose main properties are kinematical/positional is a particle. But everything gets mixed in the quantum world.

It also happened to Democritus, that after clearly dividing spatial properties (no-thing) from positional ones (thing), got to the problem of how forces were communicated, and need to create a new concept, eidola, to understand our actual bosonic carriers. So at the end one has four categories: spatial boson, positional boson, spatial fermion, positional fermion.

For the mathematically minded, it could be better to think on de Rham duality instead of the wave/corpuscle issue. What is more fundamental, the form or the cycle? The density or the volume where we integrate this density? The volume or the density to be integrated in this volume?

This expresses my notions too. I voted "fields" but I actually believe that whatever comes prior to spacetime is the source of all, and its geometrical/topological/combinatiorial properties determine what we experience at low energies as the quantum universe.

 

[turbo]

My view is that particles are condensations of fields, and if so, fields are more fundamental. Of course, this wanders into chicken or egg territory since it is difficult to conceive of one without the other, so I may have wandered down a path that is insupportable. Condensations allowing the existence of particles have spatial and temporal aspects - I'm not sure if these can constrain the existence of underlying fields.

 

[CarlB]

My view is that particles are condensations of fields, and if so, fields are more fundamental.

If by "condensations", you mean places where fields become infinite (while becoming zero everywhere else, then I agree.

 

[turbo]

If by "condensations", you mean places where fields become infinite (while becoming zero everywhere else, then I agree.

I think that I can agree to this concept. Nature appears to accord to quantum weirdness in this regard.

 

[CarlB]

I think that I can agree to this concept. Nature appears to accord to quantum weirdness in this regard.

I'm now voted for fields. A dislocation eliminates the stress in the field except at a point (particle), where it becomes infinite (as in a delta function), by instead of defining an infinitesimal change to the coordinates (the derivative of which is the field), it defines a step function in the coordinates, a place where the old spacetime is no longer continuous, but instead suddenly jumps. The derivative is then a delta function and defines a point particle position.

 

[selfAdjoint]

CarlB, it seems to me that by setting up local infinities as a criterion of reality, you have captured the true spirit of particle physics!

 

[Demystifier]

Until recently, I was also thinking that fields are more fundamental. However, one of the reasons I have changed my mind is the (old) cosmological-constant problem. The point is that it is the problem only if you take the vacuum energy seriously, i.e., if you assume that there is something even in the absence of particles. (This something is a vacuum expectation value of the field energy-momentum.) On the other hand, if you assume that only particles have physical reality, the old cosmological-constant problem simply does not appear. (For an attempt to exploit this idea in more details, see
http://arxiv.org/abs/gr-qc/0611037 )

 

[Demystifier]

For the mathematically minded, it could be better to think on de Rham duality instead of the wave/corpuscle issue. What is more fundamental, the form or the cycle? The density or the volume where we integrate this density? The volume or the density to be integrated in this volume?

Interesting view, but the field-or-particle dilemma is more subtle than this.
See e.g. the post of robphy above.

 

[hellfire]

I have voted for fields. Quantum field theory on curved backgrounds shows that the field is the fundamental entity. What are the reasons to think that it is wrong? Isn't the CMB power spectrum produced during inflation an empirical (indirect) proof that the theory is correct?

 

[Demystifier]

I do not see how CMB confirms the observer dependence of particles suggested by QFT in curved backgrounds. Perhaps you had something else in mind?

 

[turbo]

Until recently, I was also thinking that fields are more fundamental. However, one of the reasons I have changed my mind is the (old) cosmological-constant problem. The point is that it is the problem only if you take the vacuum energy seriously, i.e., if you assume that there is something even in the absence of particles. (This something is a vacuum expectation value of the field energy-momentum.) On the other hand, if you assume that only particles have physical reality, the old cosmological-constant problem simply does not appear. (For an attempt to exploit this idea in more details, see
http://arxiv.org/abs/gr-qc/0611037 )

Let us suppose that the vacuum actually is a sea of virtual-particle pairs popping into existence and annihilating in accordance with the HUP. Is there a fundamental quantum law that could prevent this vacuum field from self-gravitating into collapse? What if, like real particles, virtual particles obey the Pauli exclusion principle and resist being packed into proximity with similar same-spin particles. This effect would dynamically balance the the vacuum's tendency to collapse and result in the fine-tuned CC that we observe, regardless of the density and energy of the field at any location.

 

[Demystifier]

Let us suppose that the vacuum actually is a sea of virtual-particle pairs popping into existence and annihilating ...

There are no virtual particles. (They are nothing but a verbalization of certain Feynman diagrams.) But if you rephrase all this in terms of fields, then it certainly makes sense qualitatively. But the problem is to achieve this quantitatively.

 

[Demystifier]

Another reason why particles might be more fundamental than fields is string theory. There are indications that string field theory (i.e. second quantization of strings) may not be the correct way to treat strings.

See also what S. Weinberg thinks on fields:
http://arxiv.org/abs/hep-th/9702027

 

[hellfire]

I do not see how CMB confirms the observer dependence of particles suggested by QFT in curved backgrounds. Perhaps you had something else in mind?

If QFT in curved backgrounds is wrong, then the prediction of the power spectrum would be without base. However, theory and observations agree.

 

[vanesch]

If QFT in curved backgrounds is wrong, then the prediction of the power spectrum would be without base. However, theory and observations agree.

I don't think that the idea is that QFT is wrong. It is just that QFT predicts different numbers of observed particles for different observers, hence meaning that particles are an observer-dependent concept. Typical example: the Unruh-effect.

 

[Demystifier]

If QFT in curved backgrounds is wrong, then the prediction of the power spectrum would be without base. However, theory and observations agree.

I agree with vanesch. I am not saying that QFT in curved spacetime is completely wrong, I am only saying that a PART of it could be wrong: the part that claims that there is no observer-independent notion of particles. In particular, CMB particles seen by us could be equally physical for any other observer.

 

[hellfire]

But the part that claims that there is no observer independent notion of particles arises from the same principles than the part that claims that a de-Sitter background generates a very typical power spectrum of density fluctuations that agree with observations. How to deny the first without putting the whole in doubt?

 

[robphy]

As a follow-up, here is a video colloquium from Dartmouth:

Friday, November 3, 2006
Robert M. Wald, Enrico Fermi Institute and Department of Physics, University of Chicago
Topic: "Quantum Field Theory in Curved Spacetime"

http://www.dartmouth.edu/~physics/news/colloquium.archives/archives.html

 

[Demystifier]

But the part that claims that there is no observer independent notion of particles arises from the same principles than the part that claims that a de-Sitter background generates a very typical power spectrum of density fluctuations that agree with observations. How to deny the first without putting the whole in doubt?

This number of particles is calculated by choosing one specific "natural" time coordinate of the de-Sitter background. It is possible that, in some way, this particular time coordinate is the right one. What I suggest is that a consistent quantum theory requires a preferred time coordinate. In fact, some other foundational aspects of quantum theory also seem to suggest the existence of a preferred time coordinate (or preferred foliation). At first sight this contradicts the spirit of relativity, but this is not necessary so if this "preferred" time coordinate emerges in a dynamical way as a manifestation of a choice of an initial confition or something similar. In fact, some models of that type already exist.

 

[hellfire]

I see, I have read something about this in your papers you have referenced here. This would indeed set up a preferred class of reference systems and there would be a "preferred" notion of particle, but how would this change the fact that the notion of particles is an observer dependent one? What do you exactly mean with "the right one"?

 

[Demystifier]

Once you have a preferred (or the right) notion of a particle, then this particle perceived by another observer is given merely by a covariant coordinate transformation. In particular, the vacuum will remain the vacuum to any observer. In this sense, particles are no longer observer dependent. In this case the Unruh effect is correctly treated only by Minkowski quantization (not by Rindler quantization) and this effect is no longer interpreted as the existence of "particles". In fact, I have never seen a convincing argument that the so-called "particle detector" introduced in the literature to obtain the Unruh effect with Minkowski quantization really catchs the essential features of actual particle detectors.

Another, more formal, way to explain what I mean is as follows. The "right" particles correspond to the right representation of the field algebra among many unitarily inequivalent representations.

 

[karfiol]

My view is that particles are condensations of fields, and if so, fields are more fundamental.

That is my view too.

Maybe it can be said that field is potential and particle is physical manifestation of that potential.

 

[Demystifier]

For yet another reason why particles could be more physical than fields see also
http://lanl.arxiv.org/abs/hep-th/0101036

 

[petm1]

Particles, before you can have movement you must have something to move. With that said I think that all fundamental particles move at c relative to us at all times, and because of their being time contracted, we only "see" them as a field.

 

[Hans de Vries]

Particles, before you can have movement you must have something to move. With that said I think that all fundamental particles move at c relative to us at all times, and because of their being time contracted, we only "see" them as a field.

Only massless particles can move at c.

There is Lorentz contraction of length and there is time dilation.
Something like "time contraction" doesn't exist. Classical
particles combined wit SR still gives you 'point-like' objects
and no fields.Regards, Hans.

 

[petm1]

Imo you can think of every "point like" particle as being a field, dual nature of matter, even those "point like" objects derived from SR are a field in their own reference frame. I use the term time contracted, because I can not think of another term that means the opposite of time dilation, in so far as I am talking about objects, fields, with less motion relative to us and not more. Keeping in mind that E = mc^2 then all objects with less motion than I are still tied to the speed of light squared.

 

[taylaron]

think about string theory... the concept that everything is made of incredibly small strings of energy (in comparison, if our solar system was the size of a quark, than a string would be about the size of a tree...pretty dang small...far smaller than particles.)