# What is a particle?

1. Mar 12, 2010

### Naty1

(I don't have a specific question here, but observations/insights on the above paper or following excerpts are welcome....)

Marcus posted the above in another thread...And I do wish he would STOP that type of thing because everytime I read another paper I discover something else I had not thought about and therefore clearly do not understand!!!! (just joking,partly, but it never seems to end...peeling a layer simply reveals yet another layer...)

OMG, not only is space and time frame dependent, seems like particles are too...and in more ways than simply the virtual particle pairs of Unruh...

So there are some fundamental differences among theorists about what particles are....

So we are back to the old reliable, "I don't know exactly what it is, but at least I can measure whatever it is....thank heaven for "observables"...."

This sounds reminisescent not only of the Unruh effect but also things like "relativity of simultaneity" and the apparent affects of gravity (curved spacetime) on local versus distant time....is nothing constant in this universe except lightspeed????

2. Mar 12, 2010

### marcus

Heh heh Thanks for the backhand compliment, Naty.

I think besides the first all your other quotes are from the Rovelli Colosi 2004 paper.
I'm not sure there is anything very new or controversial. But Rovelli gave a nice talk with the same title at Abhay Ashtekar's birthday party conference, the 2008 "Abhayfest". I can get a link, if you want.
It sort of updates and sharpens the message of the paper, and there are Q&A with the audience afterwards.

I guess we all know that for example gravitons have no fundamental existence and gravity can be analyzed using gravitons (as a mathematical tool) only in restricted situations. Admittedly the particle idea is very useful as an analytical tool in situations where it applies.

As a general rule the world is not made of particles, it is more correct and less confusing to say that it is made of fields. Unless I'm mistaken all or most of us at the Forum realize this?

3. Mar 12, 2010

### meopemuk

I don't count myself in this group. As Naty1's quote said "particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier." This means that particles (not some mysterious fields) are the objects studied by real experimental physics. If "curved spacetime" does not agree with the particle concept, so bad for the "curved spacetime".

Eugene.

4. Mar 12, 2010

### rewebster

think of particles -> matter, is like dust -> dust bunnies

[that's just the Buddhist in me though]

5. Mar 12, 2010

### marcus

There we go! You are echoing what Carlo Rovelli, the LQG guy, says. This is what he says:
==quote==
...we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier...
==endquote==

You are agreeing with something taken out of context however. Maybe you should read the paper itself, that Naty was quoting.

http://arxiv.org/abs/gr-qc/0409054
What is a particle?
Daniele Colosi, Carlo Rovelli
(Submitted on 14 Sep 2004)
"Theoretical developments related to the gravitational interaction have questioned the notion of particle in quantum field theory (QFT). For instance, uniquely-defined particle states do not exist in general, in QFT on a curved spacetime. More in general, particle states are difficult to define in a background-independent quantum theory of gravity. These difficulties have lead some to suggest that in general QFT should not be interpreted in terms of particle states, but rather in terms of eigenstates of local operators. Still, it is not obvious how to reconcile this view with the empirically-observed ubiquitous particle-like behavior of quantum fields, apparent for instance in experimental high-energy physics, or "particle"-physics. Here we offer an element of clarification by observing that already in flat space there exist --strictly speaking-- two distinct notions of particles: globally defined n-particle Fock-states and *local particle states*. The last describe the physical objects detected by finite-size particle detectors and are eigenstates of local field operators. In the limit in which the particle detectors are appropriately large, global and local particle states converge in a weak topology (but not in norm). This observation has little relevance for flat-space theories --it amounts to a reminder that there are boundary effects in realistic detectors--; but is relevant for gravity. It reconciles the two points of view mentioned above. More importantly, it provides a definition of local particle state that remains well-defined even when the conventional global particle states are not defined. This definition plays an important role in quantum gravity."

As in any mathematical science there must be both operational definitions AND consistent mathematical models. Operationally, the particle is equated with the detection event. But can a complete mathematical picture be made exclusively of detection events?

Because context is often helpful to have, let me quote the entire third paragraph on page 2 of the Rovelli Colosi paper:

==quote, page 2==
To address these questions, we observe that if the mathematical deﬁnition of a particle appears somewhat problematic, its operational deﬁnition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier. Now, strictly speaking a particle detector is a measurement apparatus that cannot detect a n-particle Fock state, precisely because it is localized. A particle detector measures a local observable ﬁeld quantity (for instance the energy of the ﬁeld, or of a ﬁeld component, in some region). This observable quantity is represented by an operator that in general has discrete spectrum. The particles observed by the detector are the quanta of this local operator. Our key observation is that the eigenstates of this operator are states of the quantum ﬁeld that are similar, but not identical, to the Fock particle states deﬁned globally.
==endquote==

Last edited: Mar 12, 2010
6. Mar 12, 2010

### meopemuk

I like more the logic presented in Weinberg's "The quantum theory of fields" vol.1: The primary objects are particles described by irreducible unitary representations of the Poincare group. For realistic systems with varying numbers of particles we build the Fock space as a direct sum of products of irreducible representations spaces. Then the sole purpose of quantum fields (=certain linear combinations of particle creation and annihilation operators) is to provide "building blocks" for interacting generators of the Poincare group in the Fock space. In this logic quantum fields are no more than mathematical tools.

I think that gravity can fit into this logic pretty well if we reject the idea of "curved spacetime".

Eugene.

7. Mar 13, 2010

### tom.stoer

The method to use unitary representations of the Poincare group is stil OK in curved spacetime. You build a Fock space usingfield operators (creation and annihilation operators) and classify the locally measured excitations according to these representations. So locally the particle concept is well-defined.

The problem arises when you want to make statements which are globally valid, or when you change the reference frame as you do in the Unruh effect. Then you find that you haver to change Hilbert spaces. A zero-particle state (vacuum) is not mapped to another zero-particle state, so the two vacua are not one-to-one. The problem is that constructing the states uses a certain Minkowski vacuum which is not globally valid (as you can see in the Hawking effect). In the very end the Unruh and the Hawking effect have a rather similar origin (according to GR acceleration and gravitation are identical, the Unruh effect has something to do with a horizon as well)

But you should keep in mind that similar affects are knwon already from QFT in flat spacetime, e.g. neutrino oscillations: again you classify particles according to some representations of symmetry groups, but unfortunately the states in one irr.-rep. are mixed when you go to the other irr.-rep. The number of particles is not changed, but the particles states are mixed causing the oscillations.

So in the later case you can say that the transformation mixes states within one N-particle subspace, whereas in the Unruh-case the transformation mixes the different N-particle subspaces.

8. Mar 13, 2010

### meopemuk

I am against using words "Poincare group" and "spacetime" in the same sentence.

In the Weinberg's approach the Poincare group is NOT the group of isometries of the Minkowski spacetime. It is a group of transformations connecting different inertial observers. The fact that this group coincides with the group of isometries of the 4D pseudo-Euclidean spacetime is just a mathematical coincidence.

If we want to build a relativistic theory of an electromagnetic system, e.g., the hydrogen atom, we must build a unitary representation of the Poincare group in the Hilbert space of the two particles. This guarantees that system's descriptions by different inertial observers are correctly connected with each other.

Quite similarly, if we want to build a theory of gravitational interaction between the electron and the proton we need to build a (different) unitary representation of the Poincare group in the 2-particle Hilbert space. The fact that the electron and the proton interact gravitationally does not make the idea of inertial observer invalid. There is no need to introduce non-inertial observers and the curved spacetime.

The same is true in a less obvious example of a two-particle system - Sun+Earth. The ideas of inertial observers, the Poincare group, and its representations remains perfectly valid in this case as well. There is no fundamental difference between the Sun+Earth and proton+electron cases.

Eugene.

9. Mar 13, 2010

### Fra

I don't follow this conclusion, but I'll try to see if I can guess.

It seems your view of gravity here is an "external view", where the observer itself is not subject to gravitational interactions but he is observing a system which has "internally" gravitational interplay?

It seems then you constrain the "set of possible observers", to those where your scheme makes sense: ie. an external observer, making regular QM-style measurements on a small subsystem?

How does cosmological pictures fit into your view? Ie. the situation where a complex controlled laboratory is not available, simply because the observer itself is a small subsystem immersed in an unkonwn environment. Do you reject that scenario as outside science?

Not to change the topic but as I see it, we have the same core issue here as in the other thread.

It seems you stick to an extrinsic view, rather and intrinsic view. If we keep inflating the imaginary context of the extrinsic view, I see that we can keep a form of "simplicity" but this method seems to me to be in violation to the measurement ideal that you also seem to hold dear?

Ie. shouldn't the INFORMATION of the STATE OF transformations and symmetries in the set of all possible states of inforamtion about the system of study also be subject to the same measurement principle, rather than realism?

/Fredrik

10. Mar 13, 2010

### Dmitry67

Ha Ha. Unruh effect is cool.
It is a weapon against old school saying that "only real particles are real, virtual particles are just math"

11. Mar 13, 2010

### tom.stoer

It depends whether you talk about active or passive transformations.

All what I want to say is that you can construct the representations and this works locally, but not globally. Your problem with Minkowski spacetime and Poincare invariance fades away as soon as you introduce curvature, because the n Poincare invariance is no longer a global symmetry (nevertheless you can construct a gravitational gauge theory using Poincare invariance as local gauge group).

It is like asymptotic symmetries in particle physics (e.g. chiral symmetry is only an asymptotic symmetry in the naive quark model, nevertheless it is very usefull). You know that something breaks the symmetry, nevertheless you are using this symmetry to classify particles, put a structure on the Hilbert space etc. In a second step you calculate the effects of the symmetry breaking. In the case of curvature and acceleration its not only the "type" of the particle that changes, it's the "concept" of a particle.

But the particle concept based on asymptotic plane waves is a mathematical artefact. It is heavily used in perturbative calculations but is less usefull in non-perturbative regimes, e.g. in strong interactions. In strong interactions there is no known regime where it makes sense to talk about gluons as free (asymptotically observable) particles only. Because you have never seen a gluon, you are not bothered by that fact. In GR the surprise is due to the fact that there IS a regime where the particle concept makes sense, but that there are other regimes where it does NOT.

12. Mar 13, 2010

### Fra

To connect the different phrasings: local vs global is a different view of intrinsic vs extrinsic but where it's related as I see it.

The global connection of local observables is what I called "extrinsic".

The main distinction I make between local/global and instrinsic/extrinsic is that local/global somehow refers to position or distance on a spacetime or manifold. intrinsic/extrinsic as I think of of instead refers to what's decidable or at hand and what's not.

Extrinsic view then represents information that is not encoded by an observing system, it's just an imagined embedding. Similiarly there is an ambigousness in the global connections given only a local info.

(?) I think what Meopemuk suggested is that given a sufficiently complex embedding, the gravitatioanl interaciton inside the system can still be described relative to a massive reference. Because then a "constrained" semi-global descirption can still be encoded locally, if by local we mean beloning to the external context (the qualifying context). And this works fine for subsystems, but not for cosmological models or if you consider that the theory litteraly LIVES inside an horizon, embedded in large unkonwn environment.

/Fredrik

13. Mar 13, 2010

### Naty1

I almost did not post the original excerpts thinking nobody would be much interested...but it seemed interesting to me and I'm gald I did....and it reminded me again about 'local' versus 'global' reality...

I'm not sure I understand the first part entirely and did not know about the second... "restricted" in what sense??

I'd not readily agree that anybody knows so much about any of the fundamentals in physics...especially what "exists" or doesn't and whats "real" or not....If there is anything I've learned over the last six or eight years of catching up on physics (at least a little) it's that things at the most fundamental levels are NOT nearly so simple as they appear.... hence many theories, many mathematuical constructs, many interpretations.....which description "fits" usually seems to be associated with the theory you start with and the situation(s) you are describing....

14. Mar 13, 2010

### marcus

static typically flat geometry. The underlying geometry is restricted to some idealization.

I'll buy that! That's part of what I was trying to say. You can't just naively accept idealizations---they are all approximations with limited applicability. But I also think there are questions of degree. Maybe all concepts are incorrect and confusing but some conceptualizations are more incorrect and confusing than others.

15. Mar 13, 2010

### tom.stoer

Particles appear in rare situations, namely when they are registered.

The concept of (virtual) particles is useful in rare cases, namely when one wants to describe measurements, when one talks about plane wave states (asymptotically), effects in low order perturbation theory etc.

Even in standard QFT there are effects like instantons, condensates or vacuum expectation values, confinement etc. which cannot be described by particles but by fields (or field operators).

In QG there is another difficulty, namely that the methods of standard QFT do no longer apply; the Unruh effect is one prominent example, because the concept of a "particle" is shown not to be invariant. Two observers will in general not agree on the number of particles they observe.

16. Mar 13, 2010

### marcus

Both these comments spice up the discussion for me, I hope other people are entertained by the ideas as well. Dmitry, do you say here that Unruh effect blurs the distinction between real and virtual because (?) by accelerating the observer we can turn virtual ones gradually into real ones?

17. Mar 13, 2010

### marcus

I was going to say that sort of thing to Naty! The trouble with the particle concept is that one cannot attribute a permanent existence. It only exists at the moment it is detected.

The rest of the time there is a kind of spread out thing---a cloud---a wave---a field---something that is less "particular".

And as Rovelli Colosi show you cannot in general say how many particles are "there". This is semantically at the root of the particle idea---that one should be able to count them. If you can't even count them, forgetaboutit

18. Mar 13, 2010

### meopemuk

Yes, this is true. Observer is not a part of physical system. This separation system/observer is recorded in the formalism of quantum mechanics: physical system is represented by a vector in the Hilbert space, while observer (or measuring apparatus) is represented by a Hermitian operator.

Such separation is possible in the case of gravity as well. For example, if we are interested in the gravitational system Sun+Earth, then we should choose our inertial observers to be far from the Solar system, so that Sun's gravity has no effect on them. Then we can apply the Poincare group and relativistic QM formalism just as freely as in the proton+electron case.

There is a problem in applying quantum mechanics to such objects as Solar system or Universe. Quantum mechanics calculates probabilities. This implies preparation of multiple copies (ensemble) of identical systems. The Solar system and Universe are unique, so formally QM does not apply to them. But quantum effects at cosmological scale are negligible, so we can do well with classical mechanics only.

Eugene.

19. Mar 13, 2010

### meopemuk

But only these situations are relevant in physics! According to scientific method we are not allowed to speculate about things that cannot be registered/observed/verified. If we do use such unobservable things (e.g., wave functions, quantum fields, etc) in our formalism we should keep in mind that these are mathematical tools unrelated to the physical world.

Why this is an argument against particles? What's wrong with the fact that different observers see different number of particles? For example, I see one neutron. Fifteen minutes later (this can be regarded as an observer translated in time with respect to the observer from the preceding sentence) I see three decay products (proton + electron+ antineutrino). This does not undermine my trust in the particle picture.

Eugene.

20. Mar 13, 2010

### meopemuk

The number of particles is a QM observable similar to other observables like position, momentum, energy. I guess, you are not troubled by the fact that these other observables may have different values (and uncertainties) for different observers. Then you should not be troubled by the fact that different observers count different number of particles or that the particle count has usual quantum-mechanical uncertainty.

Eugene.

21. Mar 13, 2010

### tom.stoer

I am aware of this. But with the concept of particles alone you are not able to calculate anything beyond classical physics. No quantum mechanics, no atoms, no spectra, no nuclei, no nucleons, no quarks, ... they all rely on concepts like wave functions, field operators, Hilbert spaces, etc. You can't measure these mathematical entities, you can't register them, but have to life with them if you want to do physics. And in the very end the formalism (which you can't measure) produces results in nearly perfect agreement with measurements. So you can't avoid the conclusion that this unrealistic formalism describes nature.

This is not the problem I am talking about. I don't know if know what the Unruh effect means. Think about an unaccelerated observer sitting in completely empty space. There is nothing, no particle, no light - just void. So this is what he calls vacuum. No think about another observer moving with (constant) acceleration through the same region of space, quite close to the first observer. She sees not empty space but a kind of thermal heat bath filled with particles with temperature

$$T_\text{Unruh} = \frac{\hbar a}{2\pi k_Bc}$$

where a is the acceleration of the observer.

http://en.wikipedia.org/wiki/Unruh_effect

So the second observer sees something totally different than vacuum.

That means that both observers do not agree on the number of particles they observe.

The reason is that the definition of a "particle" relies on the definition of a vacuum state which has to be (at least asymptotically) flat; acceleration spoils this concept because acceleration is equivalent to gravity and therefore undermindes the basis of relativistic (which means special relativistic) quantum field theory. So both observers can construct quantum field theory, but both theories will be be inequivalent as they do not use the same notion of vacuum.

So switching on gravity means that (at least partially) the concept of particles becomes meaningless.

Last edited: Mar 13, 2010
22. Mar 13, 2010

### meopemuk

I have nothing against the mathematical formalisms of quantum mechanics or QFT, which involve wave functions, field operators, and Hilbert spaces. My point is that this formalism is simply a tool for describing/predicting results of measurements performed on particles. So, the primary physical objects are particles. Quantum fields are simply mathematical constructs which make writing of particle Hamiltonians somewhat easier (see Weinberg).

Yes, I know about the Unruh effect. I also know that this effect has not been observed yet. So, referring to this theoretical speculation does not prove anything.

Eugene.

23. Mar 13, 2010

### tom.stoer

I does! Maybe it does not prove anything regarding real particles (whatever this could be), but it proves that the theoretical concept of particles is cumbersome.

Of course you can talk about the registration of a particle in a measurement device, but can't calculate how that particle moved to the measurement device through a double-slit. As soon as you use the wave function the calculation works and it agrees with experiment. So perhaps the wave function is not real (whatever this should mean), but it is related to the physical world in the sense that it allowes you to predict physical reality. Unfortunately what you call reality (the particle) does not allow you to make this prediction.

24. Mar 13, 2010

### meopemuk

The fact that wave functions allow us to predict results of the double-slit experiment simply tells us that we should abandon the naive idea that particles are small balls moving along classical trajectories. Quantum mechanics tells us that it is wrong to ask how particles look when they are not observed (between the preparation and the measurement). This question simply does not have any satisfactory answer. The best we can do is to design a mathematical formalism (with wave functions, Hilbert spaces, quantum fields, etc.) which allows us to predict the results of measurements without giving us any clue about how the "unobserved particle" looks like.

I disagree that wave function tells us "how that particle moved to the measurement device through a double-slit". Wave function is just a (useful) mathematical abstraction. It should not be promoted to the rank of "physical reality".

It is OK to leave some questions (like "how things look like while they are not observed?") without answers as long as all questions related to actual observations are answered correctly.

Eugene.

25. Mar 13, 2010

### SimonA

I agree. But why then is it okay to invent a "many worlds" multiverse when we only observe one. Using "dark flow" as evidence would be like sending someone to jail just for doing something strange in the vicinity of a murder. The same goes for explaining youngs slit experiment.

But that leaves me unsatisfied. Is there a particle at all ? Is it a packet of energy in any dimension, or is it a wave that is only absorbed at a single point ? If the latter, then what is waving ? Space ? Time ? Consider a photon moving through a vacuum. Why does it move at constant velocity ? What does that say about the nature of space ?

Is it fair to say that we have a mathematical formalism that is realiable, but is it fair to say that we don't have even a basic idea about the nature of space, time, matter and particles ?