Are tracks in collision experiments proof of particles?

[A. Neumaier]

I'd like to discuss the question in the title, following up on my remark quoted below.

In fact, strictly speaking, the measurement results are not even properties of the individual system but properties of the detector in contact with the particle field determined by the preparation. One can completely avoid mentioning the individual microscopic systems. Indeed, what one measures in a collision experiment are ionization tracks and tracks of deposited energy - properties of the detection fluid or wires. Quantum mechanics predicts how the statistics of the tracks in the detector is related to the state of the source, both macroscopically determined stuff.

The particles themselves remain invisible and their properties may even be regarded as completely hypothetical.

Note that I don't want to repeat the discussion in
https://www.physicsforums.com/threads/tracks-in-particle-detectors-and-quantum-paths.758778
so maybe reread that one first!

The traditional analysis is given in the paper
N.F. Mott, The Wave Mechanics of $\alpha$-Ray Tracks, Proc. Royal Soc. London A 126 (1929), 79-84.
see also http://arxiv.org/abs/1209.2665 and https://en.wikipedia.org/wiki/Mott_problem

 

[A. Neumaier]

The reason I ask the question is that, as discussed e.g. in the context of this post of mine, there is a semiclassical treatment of the photodetection process in which a photodetector responds to a classical electromagnetic field (where the notion of a photon doesn't make sense) in the typical way that is considered as heralding photons appearing in the detector. But this is obviously not the case.

Thus the question arises where similar discrete detection events that are usually considered as showing the detection of particles can also be interpreted as responses of a quantum detector to a classical external field.

The most interesting class of such detection events (apart from photon counters) are tracks in a bubble chamber (or their modern analogues, wire detectors).

 

[vanhees71]

In the case of massive particles like an electron, I'd say you can measure the charge over mass ratio by applying an external magnetic field or you measure, e.g., the energy loss in the detector material, which is characteristic for this ratio. Perhaps you find something more concrete when googling for particle ID.

In case of photons it's not very easy to make sure to detect only one photon. The oldfashioned treatment a la Einstein's paper of 1905 is very misleading, because indeed it's fully explanable with semiclassical modern quantum mechanics, semiclassical meaning here that the electromagnetic field is treated as a classical field and the bound electrons in the material quantized and then using first-order time-dependent perturbation theory, as detailed in my Insights article

https://www.physicsforums.com/insights/?s=sins+in+physics+didactics

To be sure to have only precisely one photon one way is to use parametric downconversion to create a polarization-entangled photon pair and detect one of the photons as a "trigger". Then you know that you have one and only one other photon.

 

[A. Neumaier]

In the case of massive particles like an electron, I'd say you can measure the charge over mass ratio

But this is a property of the electron field, not of a single electron. Thus one possibly has the same kind of ambiguity as in the case of photons.

 

[fresh_42]

But this is a property of the electron field, not of a single electron. Thus one possibly has the same kind of ambiguity as in the case of photons.

Do they interpret, e.g. the traces in a bubble chamber after calculating expectations from the SM, or the other way round? I'm asking because I wonder how spins are 'detected'.

 

[jimgraber]

I think all those thousands of people at CERN think that they work at a particle collider, and talk about particles all the time. Nevertheless, it is still possible that the wave picture or QFT could be more accurate than the particle picture (QM, or whatever you call it.) I think other posters have made the point that the particle picture is at least pretty good FAPP. Therefore I think it should be acknowledged that the claim that particles do not exist can only be true (if it is) in a highly technical manner of speaking and not in the ordinary meaning of the terms. This should not be interpreted to depreciate a very technical wave based explanation, particularly if it is in some way more accurate or more precise than the particle based explanation. However, there should be some proof that it is a true rival theory, not just a rival terminology.

Just my less than $.02 worth.

 

[strangerep]

Thus the question arises where similar discrete detection events that are usually considered as showing the detection of particles can also be interpreted as responses of a quantum detector to a classical external field.

The most interesting class of such detection events (apart from photon counters) are tracks in a bubble chamber (or their modern analogues, wire detectors).

Now you've got me wondering whether the analysis in Mandel & Wolf for the flat 2D detector case could be extended to a 2nd order analysis for a 3D detector.

After all, ionization chambers can detect both gamma rays and alpha/beta rays, so why should the latter be fundamentally different in terms of particle-vs-wave-vs-field?

 

[Jano L.]

But this is a property of the electron field, not of a single electron. Thus one possibly has the same kind of ambiguity as in the case of photons.

It can be regarded as property of both.

There are many reasons electrons are considered particles rather than field. Going back to Millikan's measurements, oil drop was found to have only electric charge that is multiple of elementary charge $e$. If electron was a field, one would expect the electric charge of the oil drop to be distributed continuously, not in multiples of $e$.

 

[Greg Bernhardt]

@Vanadium 50 @mfb thoughts?

 

[A. Neumaier]

there should be some proof that it is a true rival theory, not just a rival terminology.

There are two rival theories: Interacting quantum field theory, where electrons are fields and particles exist only asymptotically (since Fock space is essentially an asymptotic concept), and quantum mechanics, where electrons are particles with ghostlike properties. They are considered to be compatible, but the relation between the two (via the S-matrix) is only very thinly discussed in the literature.

In quantum field theory it is impossible to speak of a sequence of single electrons moving from a source to a detector, while in quantum mechanics this is the standard picture. Thus there is something to be reconciled.

My question is whether there is actual proof that electrons (and other particles) in quantum mechanics really exist, or whether - similar to nonexistent photons detected by a photodetector coupled to an external classical electromagnetic field - they are just ghosts manifesting themselves only through the discrete responses of macroscopic quantum detectors to an electron fields.

 

[A. Neumaier]

I think all those thousands of people at CERN think that they work at a particle collider, and talk about particles all the time.

People also talk about photons all the time, although this is a very fleeting (and - as the semiclassical treatment of the photoeffect shows - much more questionable) concept.

Having good terminology that captures what ''really'' happens is important, I think, though not as important as having it right in the formal treatment that decides upon what can be predicted and how well.

 

[vanhees71]

But this is a property of the electron field, not of a single electron. Thus one possibly has the same kind of ambiguity as in the case of photons.

Well, that's also an interpretation as is the particle picture. Of course, by definition within relativistic QFT a particle is an asymptotic-free Fock state of definite occupation number 1, and as you write yourself in the first postings of this thread the appearance of tracks in a medium is well-understood since the early days of modern quantum theory (see the there cited paper by Mott).

If you are very precise you can argue that in an detector like a cloud or wire chamber you don't observe electrons but in-medium quasi-particles ;-)).

 

[A. Neumaier]

by definition within relativistic QFT a particle is an asymptotic-free Fock state of definite occupation number 1

Yes, but I had asked for a sequence of electrons (many, well-separated in time). There is no asymptotic picture for these, only for a single electron!

So the sequence of electrons only makes sense if you take the S-matrix from QFT and interpret the sequence of electrons in QM! Which is of course the conventional procedure but nevertheless very strange, if one thinks that QFT should be able to describe the source, the particles and the detector by a single (complicated) state of the quantum fields involved.

 

[vanhees71]

Well, perhaps there's some way to understand the tracks of an electron in a cloud chamber using quantum electrodynamics (in the medium). What we really see are of course droplets condensing due to ionization. So one would have to calculate the condensation probability density given a single electron in the chamber.

 

[A. Neumaier]

one would have to calculate the condensation probability density given a single electron in the chamber.

For a single electron, this can probably be made to work similar to Mott's analysis.

But again the problem is how to model a train of electrons in a single beam on the QFT level, which (given a single state) describes the dynamics of fields everywhere in space-time - rather than on the QM level, which (given a single state) describes what happens under temporal repetition (''identical preparation'') of the same situation.

 

[vanhees71]

That's also an interesting question. As far as I know from talks of accelerator physics, they treat the particles in the accelerators as classical particles. This works obviously very well. I guess, in a first approximation you can just use magnetohydrodynamics or the Vlasov equation to describe the beams in an accelerator on a continuum level. Then the argument would be that you can approximate the Kadanoff-Baym equation with a Boltzmann-Vlasov equation very well.

 

[zonde]

Are tracks in collision experiments proof of particles?
"Proof" is math term. Answer obviously is no. No observation can prove some model.

 

[mfb]

In quantum field theory it is impossible to speak of a sequence of single electrons moving from a source to a detector, while in quantum mechanics this is the standard picture. Thus there is something to be reconciled.

You can model such a sequence with suitable wave packets. If the sequence is finite (but as long as you want), the usual approach of non-interacting initial and final states with interaction in between works nicely.

I don't get the point of the discussion. In principle, it is possible to work with quantum field theory everywhere. It is also possible to use general relativity for an inclined slope problem. It is just needlessly complicated.

In particle accelerators, particles are treated as classical objects. You need some input from quantum mechanics, e.g. the power and spectrum of synchrotron radiation, but once you have those inputs you can use classical trajectories of the accelerated particles. Classical thermodynamics with time- and space-dependent external fields.

In the collision process itself, QFT is unavoidable.

After the collision, the description is (nearly) classical again: you have particles flying in different directions. Decoherence happens so quickly with every interaction that quantum effects are not relevant here. If particles decay, the actual decay process needs QFT again, but only to determine the lifetime, branching fractions, angular distributions and so on, not for the propagation of the initial or final particle. Mixing is a bit special, because you need some quantum mechanics in flight, but again you can cover that as effect based on the classical flight time.

 

[Vanadium 50]

@Vanadium 50 @mfb thoughts?

I was going to leave this thread alone, but to me it sounds like angels and pinheads. Of course particles have tracks and of course they exist, at least in the sense that they can be counted. On the theoretical side, anything I can care about can be calculated and compared with theory. So if this isn't completely mathematically rigorous, I don't much care. It's not the first time in my life I have done a calculation that wasn't perfectly rigorous, and I don't expect it to be the last.

 

[A. Neumaier]

they treat the particles in the accelerators as classical particles. This works obviously very well. I guess, in a first approximation you can just use magnetohydrodynamics or the Vlasov equation to describe the beams in an accelerator on a continuum level. Then the argument would be that you can approximate the Kadanoff-Baym equation with a Boltzmann-Vlasov equation very well.

Often one can indeed do the latter. But both the Kadanoff-Baym equations and the Boltzmann-Vlasov equations are field theories in phase space, not particle theories.

Instead of particles one has only phase space densities. Thus talking about particles seems to be simply a left-over from the 19th century when Boltzmann derived his equation from a classical particle picture.

 

[A. Neumaier]

So if this isn't completely mathematically rigorous, I don't much care. It's not the first time in my life I have done a calculation that wasn't perfectly rigorous, and I don't expect it to be the last.

Are tracks in collision experiments proof of particles?
"Proof" is math term. Answer obviously is no. No observation can prove some model.

I am not interested here in mathematical rigor. ''proof'' has a far more general use than only in math. You can call it instead ''conclusive evidence'' or ''confirmation'', as in the recent LIGO announcement

11 February 2016 -- For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the Earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein's 1915 general theory of relativity and opens an unprecedented new window onto the cosmos.

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

 

[mfb]

Instead of particles one has only phase space densities. [/B]Thus talking about particles seems to be simply a left-over from the 19th century when Boltzmann derived his equation from a classical particle picture.

The concept of particles is incredibly useful. Why would you stop talking about particles? You would replace it with a lengthy description saying the same thing all the time, so what do you gain?

 

[Dr. Courtney]

As in most areas of science, I think "proof" is too strong a word.

Tracks are _evidence_ for particles and may support or refute various related hypotheses and theories.

 

[A. Neumaier]

The concept of particles is incredibly useful. Why would you stop talking about particles? You would replace it with a lengthy description saying the same thing all the time, so what do you gain?

On the formal side I want to understand how one level of description arises form the more fundamental level below. This is a very legitimate question. Nobody denies that thermodynamics or hydromechanics are incredibly useful, but people still want to understand how they are derived from a more fundamental level and consider this important physics. By doing so one gains important insights and even calculational tools for making predictions. Thus, precisely because the concept of particles is incredibly useful, I want to see clearly how it arises from the underlying quantum field picture beyond the (apparently almost nonexistent) discussion in books and articles. Perhaps the discussion exists and I am only unaware of it, but there is definitely something to be understood.

On the informal side there are many puzzles of quantum mechanics caused (in my opinion) by uncritically using a particle picture far beyond its range of validity. Thus it is important to delineate the range of validity of the particle picture. This can be done only if one understands in some detail how it derives from the underlying quantum field picture, and which approximations are made.
Then one can assess the errors in this approximations and find out in which range the particle picture is appropriate and where it breaks down.

My guess is that the particle picture is appropriate under similar conditions as where the geometric optics approximation is appropriate to model electromagnetic waves, and is inappropriate when these conditions are violated.

The question in the title of the thread is just one particularly concrete example where one can try to investigate the issue since there is prior work on it.

 

[A. Neumaier]

After the collision, the description is (nearly) classical again: you have particles flying in different directions. Decoherence happens so quickly with every interaction that quantum effects are not relevant here.

This would be worth a more detailed discussion. Mott's analysis suggests that after the collision the scattered part forms a spherical wave (and not particles flying in different directions) until the wave reaches the detector.

Thus, where does the decoherence happen? During the flight to the detectors, or due to the contact with the detection elements? How can one see that decoherence happens very quickly in the present case? Why is the result of the decoherence a collection of particles flying in different direction? rather than something nonlocal? I'd appreciate references that discuss this in the context of collision experiments.

 

[A. Neumaier]

You can model such a sequence with suitable wave packets. If the sequence is finite (but as long as you want), the usual approach of non-interacting initial and final states with interaction in between works nicely.

Would it be a 1-particle state whose wave function consists of N pulsed wavepackets? But if it contains N electrons wouldn't it have to be an N-electron state? Then its wave function would have to be in N-particle configuration space. What is ''the usual approach'' for this? Is there a book or survey article explaining it?

The only approach I know (which I therefore am inclined to consider as ''the usual approach'') is to treat this purely on the quantum mechanical level as a repeated preparation of a 1-particle system, in which the temporal aspect is completely ignored.

In principle, it is possible to work with quantum field theory everywhere. [...] It is just needlessly complicated.

I have no idea how this should be implemented inside a quantum field picture. If it is possible, as you claim, please give me enough hints that outline how you convinced yourself of this possibility.

 

[mfb]

If you incoming particles are described as suitable wave packets, the outgoing particles will be suitable wave packets as well.

This would be worth a more detailed discussion. Mott's analysis suggests that after the collision the scattered part forms a spherical wave (and not particles flying in different directions) until the wave reaches the detector.

Yes, that's what quantum mechanics predicts. You do not need to consider that to predict the measurement results of detectors, however, because you know in advance that decoherence will happen, based on the detector design (solid matter + particle that interacts via electromagnetism or the strong interaction => decoherence in the position of impact). You can choose an easier model - flying particles.
For final states with neutrinos only you probably don't get decoherence in most collisions, but those events you don't see anyway.

Would it be a 1-particle state whose wave function consists of N pulsed wavepackets?

Or N particles. Whatever you like. I don't know books discussing this. It looks like an unnecessary complication compared to the analysis of each collision individually.

With "usual approach" I mean the scattering calculation: start with a non-interacting state, then let it scatter, then end with a non-interacting state.

 

[Robert100]

On the formal side I want to understand how one level of description arises form the more fundamental level below. This is a very legitimate question. Nobody denies that thermodynamics or hydromechanics are incredibly useful, but people still want to understand how they are derived from a more fundamental level and consider this important physics. By doing so one gains important insights and even calculational tools for making predictions. Thus, precisely because the concept of particles is incredibly useful, I want to see clearly how it arises from the underlying quantum field picture beyond the (apparently almost nonexistent) discussion in books and articles. ...

A. Neumaier, it appears to me that the problem with this discussion is that :
(a) some people are replacing your question with a different (easier, but different) question, and then answering that, or
(b) for some reason can't understand your question (even though I got it right away - and it is a meaningful, clear and important question!)
(c) are telling you "Your question isn't important, because I can ignore it with easier practical observations and calculations"

The people in group (c) really upset me, because they are not physicists. They are engineers, or video game programmers, or people who never cared about philosophy, and the actual history of 16th to 20th century physics.

The entire program of 16th to 20th century physics has been about this: What is reality? How does some particular description of reality arises form an even more fundamental level below? That is precisely how all progress in physics occurs, and it upsets me greatly that people are effectively disparaging your question, saying "Well, there are easier was to calculate things." Well of course there are! But that response is not grasping the point or your question at all.

I want to encourage you to keep hammering away at the hand-waving responses, with your verbally precise, physically important questions.

Robert

 

[jerromyjon]

Are particles proof of particles? Maybe virtual particles are the only "real" particles and all "real" particles aren't.

 

[A. Neumaier]

because you know in advance that decoherence will happen, based on the detector design (solid matter + particle that interacts via electromagnetism or the strong interaction => decoherence in the position of impact). You can choose an easier model - flying particles.

This step is not so obvious. Why is this known in advance? Why does decoherence imply that one can replace the spherical wave by flying particles? Wouldn't this mean decoherence in a preferred momentum basis, not decoherence in position? I would like to see papers that actually support this with proper formulas and derivations, not just uncheckable allusions to collective knowledge. That it works in practice is good for the practitioner but not a sufficient explanation for the theorist.

With "usual approach" I mean the scattering calculation: start with a non-interacting state, then let it scatter, then end with a non-interacting state.

That's precisely the step that lacks a detailed quantum field description when applied to multiple particles. Quantum field scattering theory just defines a scattering matrix for a single particle colliding with a target, prepared at time $-\infty$, detected at time $+\infty$, and leaves the interpretation of the S-matrix to ordinary QM and its ensemble interpretation. This is consistent with experiments, and most people may be content with that.

But I want to understand why! QFT should be a fundamental theory, hence should allow in principle to model a train of electrons as a process happening at finite times inside its own framework, and within this model one should be able to derive the interpretation of the S-matrix dynamically instead of having to postulate it in addition to its formalism! What I am interested in is how quantum field theory can achieve that. Saying that one can simplify things to get the correct predictions doesn't answer this more fundamental quest!

 

[mfb]

Why is this known in advance?

We know the detector, and the detector will still be there once the spherical wave function arrives.

Why does decoherence imply that one can replace the spherical wave by flying particles? Wouldn't this mean decoherence in a preferred momentum basis, not decoherence in position?

In a basis of detector positions (to a good approximation), which is neither a pure momentum nor a pure position basis as seen from the initial collision.
I don't have links to papers.

That's precisely the step that lacks a detailed quantum field description when applied to multiple particles. Quantum field scattering theory just defines a scattering matrix for a single particle colliding with a target, prepared at time $-\infty$, detected at time $+\infty$, and leaves the interpretation of the S-matrix to ordinary QM and its ensemble interpretation.

The target is a second particle already. I am not aware of limits for the number of participating particles - the number of outgoing particles is certainly not limited, why should the number of incoming particles be? The whole process is time-symmetric (in principle).

 

[Orodruin]

The entire program of 16th to 20th century physics has been about this: What is reality? How does some particular description of reality arises form an even more fundamental level below? That is precisely how all progress in physics occurs, and it upsets me greatly that people are effectively disparaging your question, saying "Well, there are easier was to calculate

This statement is fundamentally flawed and misguided in terms of what physics is. Physics and many other natural sciences are about finding a description which describes reality, not a quest for what it actually is, which is the domain of philosophy. Physics is an empirical science, driven by the will to do experiments to figure out how Nature behaves, create theories about it, and then test the new predictions arising from this. There is no underlying quest for reality, just the question of how it will behave when we change the input parameters and developed based on flaws in current theories, be they actual empirical flaws or more constructed theoretical ones.

If there is an easier way to describe how something works, then this is exactly the path we should take based on Occam's razor. Otherwise you are inviting wild speculation without experimental verification, which leads to a slippery slope towards crackpot land.

 

[Jilang]

It can be regarded as property of both.

There are many reasons electrons are considered particles rather than field. Going back to Millikan's measurements, oil drop was found to have only electric charge that is multiple of elementary charge $e$. If electron was a field, one would expect the electric charge of the oil drop to be distributed continuously, not in multiples of $e$.

I have been thinking about this on the drive home from work strangley enough, before I saw this post. The electrons are in oil are quantised as they are confined in atoms/molecules and so are "measured". Perhaps, just as spin is only quantised when electrons are subjected to a magnetic field, the other properties are similar?

 

[Bruno81]

Are chairs proof of particles?

 

[A. Neumaier]

If electron was a field, one would expect the electric charge of the oil drop to be distributed continuously, not in multiples of e.

This would hold for a classical field but not for a quantum field. In quantum mechanics, discreteness is not rigidly associated with decomposability into pieces.

Orbital angular momentum is also quantized, but nobody deduces from the http://espace.library.uq.edu.au/view/UQ:161172/UQ161172.pdf the existence of angular momentum particles.