Is Causality in Quantum Mechanics as Clear-Cut as in Classical Mechanics?

  • Thread starter Thread starter Galactor
  • Start date Start date
  • Tags Tags
    Cause Qm
Click For Summary
Causality in quantum mechanics (QM) differs significantly from classical mechanics (CM), where cause and effect are clearly defined. In QM, while causality is accepted philosophically, it does not adhere to determinism, as events cannot be predicted with certainty but only with probabilities. Quantum decoherence suggests that interactions, rather than measurements, lead to the emergence of definite states from superpositions. The discussion highlights that single quantum events, like radioactive decay, exhibit randomness and lack clear causes, indicating potential incompleteness in QM. Ultimately, while QM can describe statistical behaviors, it struggles to provide explanations for individual events, suggesting a fundamental unpredictability in nature.
  • #31
Galactor said:
On a related matter, does QM allow for the coming into existence of a particle without there being a cause?

I think I can understand that there are causeless events but can we allow for the arisal of a particle without there having been a cause?

There is always a cause. As I mentioned before, radiation (alpha-decay or atom radiation) is a transfer to a lower state (tunnelling, for example) due to instability of the system. Any law in physics describes ensembles of experiments, not one event. We physicists must derive the laws from observations by treating data, processing data, reducing data. Taking only one point on the screen in the double-slit experiment is taking a photo with one-pixel camera. One photon will always arrive at the screen. But if it is what we study?
 
Physics news on Phys.org
  • #32
It seems very often that when people puzzle over the nondeterministic nature of QM they forget the fact that they as conscious beings are only analyzing their perspective of reality (I know I said conscious being in a physics forum but don’t worry, I’m not implying CCC). Take for instance this question:

meopemuk said:
Can any deterministic interpretation of QM predict when a single chosen radioactive atom will decay? I believe not. So, in my opinion, these interpretations do not add anything of practical significance to the basic formalism of QM.

What it is really asking is can any deterministic interpretation of QM predict when I WILL SEE a single chosen radioactive atom decay? I find it easiest to answer this in the context of MWI. Consider a path space containing this radioactive atom. You can think of this path space as a plane with each point representing an entire three dimensional space. The path space itself is completely deterministic. However, a conscious being by the very nature of its operation can only experience one four dimensional path in path space. So then your question boils down to what determines this beings path in path space? The answer of course is nothing.

Let me explain since this is a difficult concept to grasp. Consider first just a classical Newtonian space-time. There only exists one 4D path with a conscious being in it. So what determines where in time that conscious being is? The answer of course is nothing. That being feels as though there is some flow of time and that there is some idea of a present state. But in reality there are many instances of that being experiencing the “present” and “flow of time” at every point on the timeline. So any particular beings location in time is totally random. In a very similar way, any particular beings path in path space is also totally random.
 
  • #33
Bob_for_short said:
There is always a cause. As I mentioned before, radiation (alpha-decay or atom radiation) is a transfer to a lower state (tunnelling, for example) due to instability of the system. Any law in physics describes ensembles of experiments, not one event. We physicists must derive the laws from observations by treating data, processing data, reducing data. Taking only one point on the screen in the double-slit experiment is taking a photo with one-pixel camera. One photon will always arrive at the screen. But if it is what we study?

Bob, I don't think this is a fair representation of the OP's issue. There is no fundamental difference between a neutron which is on the verge of decay as compared to one which is not. That is the orthodox viewpoint. If there truly were a "cause", we would expect there to be a difference between these. Now, it is always "possible" that there is such a difference and we just don't know how to detect it. But according to both theory and experiment, there is NO such difference.
 
Last edited:
  • #34
Galactor said:
On a related matter, does QM allow for the coming into existence of a particle without there being a cause?

I think I can understand that there are causeless events but can we allow for the arisal of a particle without there having been a cause?

And again the answer is YES, particles can be created out of energy and there is nothing which "causes" this. An example would be a particle accelerator in which two high energy particles are brought together. You can get more particles "out" than the two you started with. (Of course the conservation laws still apply.) But the particle number can increase. Now, what causes that? Nothing. It is simply a branching ratio - a probability. You could get 4 particles out sometimes, 8 particles another, etc. But there is no known difference between the input particles that determines or causes the output particle count. Again, both theory and experiment agree on this.
 
  • #35
DrChinese;2436709/ said:
what causes that? Nothing. ... there is no known difference.

No. You can't say that an event is causeless only because it's ontologically impossible for anyone to know the causes. You simple can't.
 
  • #36
italiano vero said:
No. You can't say that an event is causeless only because it's ontologically impossible for anyone to know the causes. You simple can't.

You're right. But the measurement itself is the event. It doesn't make any sense to say the outcome of the measurement from your perspective is the event. So if you're not talking about an event then obviously there is no cause. The universe only appears indeterminate to us, but that doesn't mean that reality as a whole violates causality. Check out my post on page 2 for more detail...
 
  • #37
italiano vero said:
No. You can't say that an event is causeless only because it's ontologically impossible for anyone to know the causes. You simple can't.

I will say again: there is NO known cause. There is absolutely no HINT of the existence of a cause for any quantum event with a probability less than 100%. This matches theory perfectly.

Further: with radioactive decay, there has never been any way to segregate or otherwise identify nuclei on the verge of decay. If you were able to show me some of those, I would gladly rethink my position.

In the meantime, I will point out what I hope is obvious: the fact that fairies have not been documented is not proof that there are no fairies. Some people even believe in fairies. Ditto with causes for quantum events. Given the fact that theory supplies no causes for quantum events - and none have been discovered - you will understand why I say they do not exist. The idea that there are no causes for quantum events is as proven as it can get.
 
  • #38
Your problems with "lack of causes" origin from lack of comprehension to what extent the quantum things are strongly coupled and are many-variable systems. For example, an electron. It is permanently coupled to the quantized EMF. Together they represent a compound system with many degrees of freedom. You all forget it and seek "hidden" variables whereas they are here, explicitly given. It is impossible to control or measure or get information about the whole system with so many coupled variables.
 
Last edited:
  • #39
Bob_for_short said:
Your problems with "lack of causes" origin from lack of comprehension to what extent the quantum things are strongly coupled and are many-variable systems. For example, an electron. It is permanently coupled to the quantized EMF. Together they represent a compound system with many degrees of freedom. You all forget it and seek "hidden" variables whereas they are here, explicitly given. It is impossible to control or measure or get information about the whole system with so many coupled variables.

That assertion cannot be supported. There is not the slightest hint that any electron+field is fundamentally different than any other electron+field, which would be needed if there were causes to things like electron capture and similar. If all electrons are fundamentally identical, then there is no root cause for observed particle behavior. Which is completely in accordance with observation and theory.

If you disagree, perhaps you could cite a reference?
 
  • #40
So Bob, are you saying that any measurement is, in fact, a complicated interaction between the state and its environment and it must be treated within a many-body framework?

But if this is true, you are implying that a true many-body treatment of the system ( say, electron + quantized EMF) should be able to predict even a single decay, no?

Then why does the infamous measurement problem still exist today? Many-body physics has been with us for a long time.
 
  • #41
DrChinese said:
I will say again: there is NO known cause. There is absolutely no HINT of the existence of a cause for any quantum event with a probability less than 100%.

DrChinese,

I agree with you completely. We don't know the causes of apparently random quantum events. There is, of course, a chance that some day these causes will be discovered. But at this point nobody has any good idea what they might be. There are some proposals (like "hidden variables" or MWI) to go beyond standard quantum mechanics and "explain" the randomness. But no one of them was able to predict the timings of clicks in the Geiger counter, even approximately. So, presently these proposals are just speculations without any practical consequence. Our "knowledge" about the cause of quantum randomness has not improved a single bit since QM was invented more than 80 years ago. So, I will make a safe bet by saying that no such cause will be found ever. Nature has a certain degree of fundamental randomness. We should just accept it and move on.

Eugene.
 
  • #42
sokrates said:
So Bob, are you saying that any measurement is, in fact, a complicated interaction between the state and its environment and it must be treated within a many-body framework?
I said nothing about measurement but about causes of randomness.
But if this is true, you are implying that a true many-body treatment of the system ( say, electron + quantized EMF) should be able to predict even a single decay, no?
No, because it is impossible and I said it.
Then why does the infamous measurement problem still exist today? Many-body physics has been with us for a long time.
Many-body physics and physicist also speak of ensembles and probabilities, not of one elementary event.
 
Last edited:
  • #43
DrChinese said:
...If you disagree, perhaps you could cite a reference?
I can cite my own publications but this is forbidden here. Why not look in ...?
 
  • #44
Bob_for_short said:
I can cite my own publications but this is forbidden here.

My point exactly. It would be appropriate to label your opinions as speculative rather than as you have been (by implication), as established theory. You've been here long enough to know the standards, Bob! :smile:

[Note added in edit: Besides, I can see from some of your published work that you follow probabilistic concepts (by analyzing perturbative corrections, etc.) so it surprises me that you would be talking about cause and effect. I also see your "exact" (non-perturbative) solution for a scattering problem and will look at that a bit. But certainly you must understand that even such a solution hardly takes the probability out of QM.]
 
Last edited:
  • #45
DrChinese said:
My point exactly. It would be appropriate to label your opinions as speculative rather than as you have been (by implication), as established theory. You've been here long enough to know the standards, Bob! :smile:

OK, I will refer to textbooks where the electron is coupled to the quantized EMF. It is a fact. What is a consequence? The electron will stay still despite this coupling? Don't you feel the answer is here?
 
  • #46
Bob_for_short said:
I said nothing about measurement but about causes of randomness.
Causes of randomness? Hmm... To me measuring the x-component of a z-up spin and getting -x and +x with equal probability is randomness. Because it is unpredictable. But bringing in the many-body (environment+state) arguments, you just said that there's nothing mysterious in the randomness, as if there was an underlying cause for measuring -x and +x.
Bob_for_short said:
Don't you feel the answer is here?

No I don't feel it. I don't think you have convincing arguments either. IF you truly knew the causes of randomness you'd accomplish a great deed for quantum mechanics.
 
  • #47
DrChinese said:
...But certainly you must understand that even such a solution hardly takes the probability out of QM.]
My statement is not to take the probability out of QM, on the contrary. I say it is impossible to control the "whole world" so the probabilities are inevitable. OK, may be we speak of different things.
 
  • #48
Bob_for_short said:
Don't you feel the answer is here?

As sokrates says: no.

There have been all kinds of efforts to give us stochastic and other ideas about QM, usually in an attempt to restore some kind of causality to theory. Demystifier and Morgan are loosely working along these lines too (just to name a couple I happen to know about). But the devil is in the details, and clearly it is no simple matter to connect the dots. Regardless of how strongly you feel about the direction you are headed, I wouldn't come to any dramatic conclusions too early. Why do you think that Bell's Theorem and other no-go's are so important? It is precisely because initial results may lead you to think you are on to something, only to discover later that a complete and consistent theory is not possible.
 
  • #49
DrChinese said:
There have been all kinds of efforts to give us stochastic and other ideas about QM, usually in an attempt to restore some kind of causality to theory.
I have to blame myself. I cannot express myself clearly.

I did not mean to restore any causality in the classical sense. On the contrary, I wanted to furnish even more of quantum causes to support impossibility to predict one single event.
 
  • #50
Bob_for_short said:
I have to blame myself. I cannot express myself clearly.

I did not mean to restore any causality in the classical sense. On the contrary, I wanted to furnish even more of quantum causes to support impossibility to predict one single event.

Ah, I get it now. The field supplies the "causes", but then the field just adds that many MORE variables (which presumably are random anyway). So the electron itself does not contain the "hidden variable", it is external and many.

So you were objecting to the discussion of causes being internal to the particle (or something similar), if there were to be a cause.

Actually, there are multiple places the external "causes" could reside. In the Bohmian interpretation, it is a function of the positions of other particles of the system. In Relational Block World, you could say it is in the future. And you have it in the virtual ("dressed") field.

Regardless, there are still those nasty no-go theorems out there to consider. And entanglement! Good luck! I will look forword to more on this.
 
  • #51
DrChinese said:
Ah, I get it now. The field supplies the "causes", but then the field just adds that many MORE variables (which presumably are random anyway). So the electron itself does not contain the "hidden variable", it is external and many.
...And you have it in the virtual ("dressed") field.
If you mean me personally, then yes. Each charge comes with its own quantized EMF ("dressed" field). In this sense the QEMF represents the "internal" degrees of freedom of a compound system. I would not call them "external" like vacuum fluctuations in vacuum. Rather, they are internal degrees of freedom belonging to a real charge, capable of radiation wherever it is. So each electron (or charge) carries its own "dressing" with it. The charge in such a system is quantum mechanically smeared, not point-like. And as soon as there is no threshold for exciting soft modes, one cannot prepare the system in its ground state.

I do not think that in an empty space (classical vacuum) there are independent field fluctuations. They always come with charges and are a charge feature, in my humble opinion. Thinking so prevents me from paradoxes like how to harness/extract vacuum energy from empty space (with vacuum cleaner?).
 
  • #52
DrChinese said:
the fact that fairies have not been documented is not proof that there are no fairies.

yeah, but there is a foundamental difference between fairies and causes: at the macroscopic level there is causality. how can it be, if quantum world is random? The quantum causality must exist to explain the classical one. Also the brownian motion was thought at the beginning as random, only because no causes were observed. But this is in opposition with Classic causality: how can they coexist? They cannot. I know the difference between the brownian motion and the quantum one, but the example is pertinent.


If we cannot know if there are or not the causes, we can introduce them to explain Classic causality.

Earthquakes are unpredictable but caused. Who can predict a single quake? Is it random only for this?
 
Last edited:
  • #53
italiano vero said:
yeah, but there is a foundamental difference between fairies and causes: at the macroscopic level there is causality. how can it be, if quantum world is random? The quantum causality must exist to explain the classical one.

Sorry, that is circular reasoning! You are assuming classical causality (which may or may not exist - this is quite debatable, especially if you have free will) to justify micro causality.

Regardless, there is no physical requirement that macro causality require micro causality anyway. What do you have to compare it to? How can you prove this? Just asking "how can it be, if quantum world is random?" is not a convincing argument; how can ANY physical phenomena (entanglement, HUP, Pauli exclusion principle, general relativity, etc.) be?

My point was that it is NOT possible to disprove micro causality, but it MIGHT be possible to prove it. Of course, I doubt that, but I understand that others might not agree. But if you are asking me to DISPROVE causality, I can't and never will. But lack of evidence FOR causality does not constitute evidence FOR, it definitely tends to the AGAINST side given the investigation done so far.

If you take a photon polarized at 0 degrees and run it through a polarizing filter at 45 degrees, what causes the result? That is the specific question at hand. Theory and experiment agree: there is no known cause.
 
  • #54
DrChinese said:
If you take a photon polarized at 0 degrees and run it through a polarizing filter at 45 degrees, what causes the result? That is the specific question at hand. Theory and experiment agree: there is no known cause.

The result is always a radiated by the filter medium photon, not the incident one. So it is the medium who brings uncertainty in this particular case due to interaction with the incident photon.
 
  • #55
DrChinese said:
Sorry, that is circular reasoning!

oh, yeah, it is. For centuries thinkers debated on the causes of the events. Hume said that there is no connection between an event and another: causes doesn't exist, the motion of billiard balls is random and not causal. We will indeed not solve this problem here.

DrChinese said:
there is no physical requirement that macro causality require micro causality anyway. How can you prove this?

How can I prove that classical mechanics cannot be based on randomness? ... This is evident! Hume asks to the other philosophers to prove the existence of gravity. So they hit an object and this one fall down. The answer of Hume was "This is not a proof: how can you say that this is not random? How can you know that one day one object will not fall?" This is twisted. If the classical phisic laws are valid in the macroscopical context, regularity and causality must exist.

The existence of a single valid law proves the existence of causality: laws cannot exist in anarchy, in randomness, because randomness is lack of laws.

The causality exists and this is proved by the existence of any phisic law, and causality cannot be based on randomness. This is my position. Of course is criticizable, like any other position.

DrChinese said:
there is no known cause.

yes, no KNOWN cause.
 
  • #56
italiano vero said:
...The causality exists and this is proved by the existence of any phisic law, and causality cannot be based on randomness...

This is, as I said, circular. So is this conversation, as apparently you are not familar with the laws of quantum physics. You might want to learn more about these before you make statements like this. QM is probabilistic and yet has fairly rigid rules.
 
  • #57
DrChinese said:
If you take a photon polarized at 0 degrees and run it through a polarizing filter at 45 degrees, what causes the result? That is the specific question at hand. Theory and experiment agree: there is no known cause.

What makes you think, that the observer who've seen the photon any less real in comparison to the observer who have not?

-- Dmtr
 
  • #58
dmtr said:
What makes you think, that the observer who've seen the photon any less real in comparison to the observer who have not?

-- Dmtr

I don't understand your question. I didn't intend to draw the observer into the equation so much as say: a photon's polarization value at a specifc angle has no known cause that has as yet been demonstrated.
 
  • #59
DrChinese said:
I don't understand your question. I didn't intend to draw the observer into the equation so much as say: a photon's polarization value at a specifc angle has no known cause that has as yet been demonstrated.

I thought you did. You've said "if you take a photon polarized at 0 degrees", that assumes an observer correlated (entangled) with a photon at the certain state.

-- Dmtr
 
  • #60
dmtr said:
I thought you did. You've said "if you take a photon polarized at 0 degrees", that assumes an observer correlated (entangled) with a photon at the certain state.

-- Dmtr

OK, so how does that relate to what "causes" the subsequent polarization value?
 

Similar threads

Undergrad Lagrangian Mechanics, Continuous Particle Paths and QFT
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad How to derive orbital angular momentum of spins in quantum mechanics?
  • · Replies 2 ·
Replies
2
Views
794
Graduate Can Quantum Mechanics be postulated to exclude humans?
  • · Replies 44 ·
2
Replies
44
Views
5K
Undergrad One does not “prove” the basic principles of Quantum Mechanics
  • · Replies 24 ·
Replies
24
Views
918
High School Why is gravity a weak force at the atomic scale?
  • · Replies 8 ·
Replies
8
Views
1K
Undergrad Schrödinger equation and classical wave equation
  • · Replies 39 ·
2
Replies
39
Views
1K
Undergrad Is Causality Preserved in Quantum Mechanics?
  • · Replies 62 ·
3
Replies
62
Views
6K
Undergrad What Are the Key Questions and Challenges in Understanding Quantum Gravity?
  • · Replies 5 ·
Replies
5
Views
2K
High School Understanding Causality in Quantum Mechanics
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad Let's talk about the classical limit of QM
  • · Replies 13 ·
Replies
13
Views
2K