Graduate Do bosons contradict basic probability laws?

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The discussion centers on the implications of probability theory when applied to bosons, particularly their indistinguishability. It highlights a discrepancy between expected probabilities for mutually exclusive events and the actual outcomes when measuring bosons in different compartments. The probability of finding two bosons in one compartment is argued to be influenced by the experimental setup, leading to different results based on whether one measures only the box or both the box and compartments. Participants emphasize that the probabilities depend on how the system is prepared and the assumptions made during measurement. Ultimately, the conversation raises questions about the nature of probability in quantum mechanics and the interpretation of experimental results.
  • #61
Philip Koeck said:
Summary:: The probabilities of mutually exclusive events are additive. For bosons this does not seem to be the case. How can we explain this?
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How can we understand this?
Does it mean that different distributions of bosons in compartments are not mutually exclusive?
That would mean that there is some sort of overlap between the event of 2 bosons being in the upper compartment and the event of 1 being in the upper and 1 in the lower, somehow both distributions can exist at the same time.
I won't go into the probability math of this, I rather look at it physically rather than mathematically. And the way I picture bosons is that they are not mutually exclusive, several such particles can occupy the same space at the same time as each other. For example, the most common example of a boson is the photon. Within a given space and time, you can fit a nearly infinite number of photons in at the same time, and they don't interfere with each other. But with a fermion, that's not the case, there only one particle of a certain type can occupy the same time and space, due to the Pauli exclusion principle. The Pauli exclusion principle doesn't apply to the bosons, only to the fermions.

However, the amount of bosons you can fit into a certain space and time is not infinite (just nearly so, a very high number). The limit seems to be the Planck density, that is if you pack in the Planck energy's worth of bosons into a Planck volume of space, then the boson capacity is reached and you can't go any further, and a black hole is produced instead.
 
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  • #62
bbbl67 said:
I won't go into the probability math of this, I rather look at it physically rather than mathematically. And the way I picture bosons is that they are not mutually exclusive, several such particles can occupy the same space at the same time as each other. For example, the most common example of a boson is the photon. Within a given space and time, you can fit a nearly infinite number of photons in at the same time, and they don't interfere with each other. But with a fermion, that's not the case, there only one particle of a certain type can occupy the same time and space, due to the Pauli exclusion principle. The Pauli exclusion principle doesn't apply to the bosons, only to the fermions.

However, the amount of bosons you can fit into a certain space and time is not infinite (just nearly so, a very high number). The limit seems to be the Planck density, that is if you pack in the Planck energy's worth of bosons into a Planck volume of space, then the boson capacity is reached and you can't go any further, and a black hole is produced instead.
I meant that the events are mutually exclusive. For example one event could be that there is 1 boson in state A and 1 boson in state B. Another event could be that there are 2 bosons in state A.
If these two events can't happen at the same time then they are mutually exclusive in the sense of probability theory, if I understand correctly.
I'm not completely convinced that this has to be true for bosons, but I don't want to make them weirder than they are.
 
  • #63
PeroK said:
Looking at what you've done, you got the same answer for distinguishable and indistinguishable fermions. That can't be correct, because identical fermions (or to be precise, fermions with a symmetric spin state, hence anti-symmetric spatial wave-function) have a repulsive exchange force, so would tend to be found further apart.
That's an interesting detail. I was also surprised.
In my derivation I don't use the concept of space. I just have two groups of states that I call "volumes".
Both distinguishable and indistinguishable fermions block the states they are in, so maybe one shouldn't expect any difference in their behaviour.
I'm not even sure whether distinguishable fermions correspond to anything real anyway.
 
  • #64
Philip Koeck said:
I'm not even sure whether distinguishable fermions correspond to anything real anyway.

Of course they do; consider, for example, an electron and a proton.
 
  • #65
Philip Koeck said:
Both distinguishable and indistinguishable fermions block the states they are in

If by "block" you are referring to the Pauli exclusion principle, that only applies to indistinguishable fermions. An electron and a proton would not "block" each other from being in the same state.
 
  • #66
PeterDonis said:
If by "block" you are referring to the Pauli exclusion principle, that only applies to indistinguishable fermions. An electron and a proton would not "block" each other from being in the same state.
In my derivations I always talk about identical particles even if they are distinguishable.
 
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  • #67
Philip Koeck said:
In my derivations I always talk about identical particles even if they are distinguishable.

Then I'm not clear about what you mean by "identical" particles.
 
  • #68
PeterDonis said:
Then I'm not clear about what you mean by "identical" particles.
Identical simply means the same in all properties. Two large objects (maybe atom clusters) could be identical, but it's still possible to keep track of them and see which particle is where or in which state. So they are distinguishable.
Indistinguishable means that it's principally impossible to tell which of two particles is in state A and which is in state B. There is no measurement that would give you that information.
 
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  • #69
Philip Koeck said:
Identical simply means the same in all properties.

It can't literally be "all properties" unless the state of the particle is not one of its properties. But perhaps you mean just all invariant properties like charge or magnitude of spin.
 
  • #70
Philip Koeck said:
Identical simply means the same in all properties. Two large objects (maybe atom clusters) could be identical, but it's still possible to keep track of them and see which particle is where or in which state. So they are distinguishable.

No, that's not true. The wavefunction is such that you can't tell which is which. Whatever this derivation of yours is, it's not quantum mechanics.

It's evident you don't know QM. That's fine. None of us know everything about everything. But maybe before coming here and telling su it's wrong it would be a good idea to learn what it says? It is probably also not a good idea to call this an A-level thread if you don't have a graduate-level background in QM.
 
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  • #71
PeterDonis said:
It can't literally be "all properties" unless the state of the particle is not one of its properties. But perhaps you mean just all invariant properties like charge or magnitude of spin.
Yes, that's what I meant. One could think of identical atom-clusters having different kinetic energies, for example.
 
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  • #72
Vanadium 50 said:
No, that's not true. The wavefunction is such that you can't tell which is which. Whatever this derivation of yours is, it's not quantum mechanics.

It's evident you don't know QM. That's fine. None of us know everything about everything. But maybe before coming here and telling su it's wrong it would be a good idea to learn what it says? It is probably also not a good idea to call this an A-level thread if you don't have a graduate-level background in QM.
I didn't actually intend my question to be about QM. I was thinking in terms of statistical physics, both classical and quantum.
I'm also not saying that anybody is wrong. On the contrary, I'm interested in finding a possible contradiction between the concept of distinguishable particles and QM. Does QM actually say that identical, distinguishable particles can't exist?
The ideas I discuss are not my own either. Many authors have discussed the statistical physics of distinguishable particles. A starting point could be Robert Swendsen. Here's one of his papers: https://www.researchgate.net/publication/325700825_Probability_Entropy_and_Gibbs_Paradoxes.

More papers:
J R Ray 1984 Eur. J. Phys. 5 219

https://www.researchgate.net/profile/Robert_Swendsen?_sg%5B0%5D=x7WFLGQBnj_yLtNDHRaXQQYKCOqu316_LjBDrGl0PYITlXC8S3W-sbNGMA3CMW29v465TG0.6B40YFlyYiUotq2XMAlsuT2t5O1YXy4ASAboH31H85_EEmGgRalec7vc7Z6XbpHqg91V9SgX9J6jZ04oJQZO0g&_sg%5B1%5D=GcrHY0XV7OCP7C2n8xnBhPit4dE6SX4S7Kp6drZPaKSxojJJyxa8Ykc0mPz6OEfphiG2shk.pKdWZxrCY6Yp_lBNbUiG465NexdKEmT7Uotg8mKNW8Buw6FSAA6liFd3R7iLYckyxafPW4x-GHIF4jjRLKV3Ng
The ambiguity of "Distinguishability" in statistical mechanics
June 2015, American Journal of Physics 83(6):545-554
DOI: https://www.researchgate.net/deref/http%3A%2F%2Fdx.doi.org%2F10.1119%2F1.4906793?_sg%5B0%5D=kun7bmb5aWZ3M59C3kpOVpn7ATEF9fIia4kxGevGSN3SonF0SSd-2uI2kOSwRwc7IBCG7AriQK4CQtIsKRbA-XnZYw.OFTUyFQabl3fbuend8y9vkstQk_teOAk6MTuUHWw3hCEEQWAjhz-IAn_zFqKkafoG2OPXNqpQE_A5Hf1De65aw
 
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  • #73
Sure, you can imagine a world with classical indistinguishable particles, as turn-of-the-last-physicists did when wrestling with the Gibbs paradox. But that's not the world in which we live. The world in which we live is fundamentally quantum mechanical.
 
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  • #74
Vanadium 50 said:
I have no idea. You're the one who says this isn't QM but should be in the QM section.
I never said the original question had nothing to do with QM. I said it was intended as a question about statistical physics (both classical and quantum) rather than QM. I'm very open towards whatever QM has to say about it. For example if QM can show that identical, distinguishable particles can't exist then a lot of the discussion around the statistical physics of distinguishable particles might be completely meaningless at least within physics.
 
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  • #75
Philip Koeck said:
Then I'm not clear about what you mean by "identical" particles.
One shouldn't say "identical particles" when one means "indistinguishable particles"...
 
  • #76
vanhees71 said:
One shouldn't say "identical particles" when one means "indistinguishable particles"...
I agree. I would say identical particles are not necessarily indistinguishable. If they can be tracked they are distinguishable.
 
  • #77
Well they can't be tracked in QM so...
 
  • #78
Philip Koeck said:
I would say identical particles are not necessarily indistinguishable. If they can be tracked they are distinguishable.
As far as the math of quantum mechanics (quantum field theory if you want to do it properly) is concerned, if they can be tracked they aren't identical.
 
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  • #79
Philip Koeck said:
I agree. I would say identical particles are not necessarily indistinguishable. If they can be tracked they are distinguishable.
"Identical particles" is just a often used misnomer. The expression is used synonymously with "indistinguishable particle", and indistinguishable particles are particles with all intrinsic properties (i.e., the quantum numbers needed to label an asymptotic free one-particle state for vanishing momentum, i.e., the particle at rest). These are within the Standard Model: spin, electric charge, color charge, flavor, and weak hypercharge.
 
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  • #80
vanhees71 said:
One shouldn't say "identical particles" when one means "indistinguishable particles"...
vanhees71 said:
"Identical particles" is just a often used misnomer. The expression is used synonymously with "indistinguishable particle", and indistinguishable particles are particles with all intrinsic properties (i.e., the quantum numbers needed to label an asymptotic free one-particle state for vanishing momentum, i.e., the particle at rest). These are within the Standard Model: spin, electric charge, color charge, flavor, and weak hypercharge.
Now you got me confused tbh. Are you arguing that one should just never use the term “identical particles”? Or do you actually have some situation in mind where “identical particles” would not be used synonymously with “indistinguishable particles”, and want to reserve “identical” for this?
 
  • #81
As I said, I never use the expression "identical particles". I only wanted to say that you find this expression in many textbooks and papers meaning "indistinguishable particles".

In quantum mechanics the particles are really indistinguishable. It's a feature you cannot intuitively understand, because it's something we are not used to in our experience with macroscopic objects which obey to a very good approximation classical laws. Macroscopic objects can be individually followed. You just mark, e.g., a ball somehow and then you can distinguish it from other similar balls by this mark. Formally you can follow its trajectory from its initial position at some time ##t_0## and identify this individual object at any later time ##t##.

This you cannot in general anymore for an individual particle in a many-body system. The many-body quantum state of indistinguishable quantities must be either symmetric or antisymmetric under exchange of two particles, describing either bosons or fermions (where the bosons have necessarily integer and fermions necessarily half-integer spin).

Take two indistinguishable particles in non-relativistic quantum mechanics. Then you can describe a pure quantum state with a two-particle wave function ##\Psi(t,\vec{x}_1,\sigma_1, \vec{x}_2,\sigma_2)##, where ##(\vec{x}_j,\sigma_j)## are positions and spin-z components (##\sigma_j \in \{\pm s,\pm (s-1),\ldots \}##). The physical meaning is, according to Born's rule, given by the two-body probability distribution
$$w(t,\vec{x}_1,\sigma_1,\vec{x}_2,\sigma_2)=|\Psi(t,\vec{x}_1,\sigma_1;\vec{x}_2,\sigma_2)|^2,$$
where
$$\mathrm{d}^3 x_1 \mathrm{d}^3 x_2 w (t,\vec{x}_1,\sigma_1,\vec{x}_2,\sigma_2)$$
is the probability to find one particle with spin component ##\sigma_1## within a volume elment ##\mathrm{d}^3 x_1## around the position ##\vec{x}_1## and one particle with spin component ##\sigma_2## within a volume element ##\mathrm{d}^3 x_2## around the position ##\vec{x}_2##.
You can only say that much about indistinguishable particles: It doesn't make sense to say you find a specific particle around ##\vec{x}_1## and another specfic particle at ##\vec{x}_2##.

Indeed from ##\Psi(t,\vec{x}_2,\sigma_2;\vec{x}_1,\sigma_1)=\pm \Psi(t,\vec{x}_1,\sigma_1;\vec{x}_2,\sigma_2)## (upper sign bosons, lower sign fermions) you get
$$w(t,\vec{x}_2,\sigma_2;\vec{x}_1,\sigma_1)=w(t,\vec{x}_1,\sigma_1;\vec{x}_2,\sigma_2),$$
i.e., it's not observable which individual particle is which.
 
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  • #82
vanhees71 said:
As I said, I never use the expression "identical particles". I only wanted to say that you find this expression in many textbooks and papers meaning "indistinguishable particles".

You'll be pleased to learn that Chapter 5 of Griffiths' QM Book is entitled "Identical Particles"!
 
  • #83
Well, I'm not in favor of this textbook anyway,... As I said, it's unfortunately common jargon in the physics literature. You cannot help it. One only has to carefully explain the meaning to the students.
 
  • #84
PeroK said:
You'll be pleased to learn that Chapter 5 of Griffiths' QM Book is entitled "Identical Particles"!
So does Sakurai.

I think that "indistinguishable" comes from classical statistical physics, and is independent from identical particles. It just turns out that QM showed us that identical particles are fundamentally indistinguishable!
 
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  • #85
Yes, I know. Even Weinberg uses "identical particles". I don't say that any textbook that uses this phrase is bad. Then there'd be almost no textbook left, I guess.
 
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  • #86
vanhees71 said:
In quantum mechanics the particles are really indistinguishable. It's a feature you cannot intuitively understand, because it's something we are not used to in our experience with macroscopic objects which obey to a very good approximation classical laws. Macroscopic objects can be individually followed. You just mark, e.g., a ball somehow and then you can distinguish it from other similar balls by this mark. Formally you can follow its trajectory from its initial position at some time ##t_0## and identify this individual object at any later time ##t##.
This marking, does it have to be an actual change to the objects or could it just be in an image of the objects? For example if one could follow the trajectories of large molecules or clusters (for example in a solution) using some high speed, high resolution, imaging technique and then just mark the objects in the computer and track them, would that make normally indistinguishable particles distinguishable?
 
  • #87
In Walter Greiner's "QUANTUM MECHANICS An Introduction" (Fourth Edition) one reads:

"One characteristic of quantum mechanics is the indistinguishability of identical particles in the subatomic region. We designate as identical particles those particles that have the same mass, charge, spin etc. and behave in the same manner under equal physical conditions. Therefore, in contrast with macroscopic objects, it is not possible to distinguish between particles like electrons (protons, pions, α particles) on the basis of their characteristics or their trajectory. The spreading of the wave packets that describe the particles leads to an overlapping of the probability densities in time (Fig. 15.1); thus we will not be able to establish later on whether particle no. 1 or no. 2 or another particle can be found at the point in space r. Because of the possible interaction (momentum exchange etc.), dynamical properties cannot be used to distinguish between them, either." [Italics in original, LJ]
 
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  • #88
Imo Shankar explained it pretty well. At least I feel like I understood the difference.
 
  • #89
AndreasC said:
Imo Shankar explained it pretty well. At least I feel like I understood the difference.
What difference?
 
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  • #90
Philip Koeck said:
This marking, does it have to be an actual change to the objects or could it just be in an image of the objects? For example if one could follow the trajectories of large molecules or clusters (for example in a solution) using some high speed, high resolution, imaging technique and then just mark the objects in the computer and track them, would that make normally indistinguishable particles distinguishable?
You can of course follow individual particles and make a distinction between identical particles in different locations. For instance, when researchers trap an electron in a Penning trap and keep it there for days on end, they know it is always the same electron until it escapes the trap.

Likewise, if I follow a single carbon dioxide molecule that I expire, I can differentiate it from one you just breathed in.

It is only when these identical particles are part of the same system or can somehow interact that the indistinguishability plays a role.

I recommend reading Feynman on the subject: https://www.feynmanlectures.caltech.edu/III_04.html
 
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