Exact cause of radioactive decay?

[Darwin]

TL;DR

The EXACT cause of a radioactive decay event

I recognize that radioactive decay is a spontaneous process in which an unstable atomic nucleus breaks into smaller, more stable fragments, but exactly what is it that causes an atom to decay at a particular time rather than at some other time?

 

[Vanadium 50]

It's probabilistic.

 

[Darwin]

It's probabilistic.

Yes I know, but its probability doesn't address its cause.

What I'm looking for is the "why," not the likelihood of its appearance. Why does a decay event happen when it does and not at some other time?

 

[symbolipoint]

Yes I know, but its probability doesn't address its cause.

What I'm looking for is the "why," not the likelihood of its appearance. Why does a decay event happen when it does and not at some other time?

The way you are asking still gives just 'probability'. What you really want is, just the first part of what you ask: WHY DECAY?

But maybe what Vanadium 50 means, is that nothing more is known other than certain atoms will decay at a certain rate, and other kinds of atoms will not.

I am curious about this, too. What makes the atom or isotope of any kind of atom decay? What is there about the atom which makes it need to decay? What is about the atom on its inside that forces it to eventually decay, if it could?

 

[etotheipi]

There is a very nice outline of the theory here as well as a calculation for the transition rate (excited state to ground state with emission of gamma) which makes use of Fermi's golden rule starting at the end of page 96, if you want to take a stroll into the weeds.

There's also this set of notes which I have only skimmed but they also seem to give a good overview.

 

[Vanadium 50]

You're asking for something that doesn't exist. It is truly random and not deterministic.

 

[PeterDonis]

exactly what is it that causes an atom to decay at a particular time rather than at some other time?

Nothing does. At least, nothing that can be described by our best current models in quantum mechanics or detected by our best current experiments. As far as we can tell, there is no reason beyond random chance why one particular atom decays at a particular time.

 

[vanhees71]

Summary:: The EXACT cause of a radioactive decay event

I recognize that radioactive decay is a spontaneous process in which an unstable atomic nucleus breaks into smaller, more stable fragments, but exactly what is it that causes an atom to decay at a particular time rather than at some other time?

The cause of radioactive decays depend on the specific decay. E.g., $\alpha$ decay is due to the tunneling effect (Gamow-theory of $\alpha$ decay), $\beta$ decay is due to the weak interaction, and $\gamma$ decay is due to the electromagnetic one.

The lifetime of any specific nucleus is in all these cases random due to quantum mechanics. So there is no cause that a specific nucleus decays at the specific time you observe it to do so, i.e., there's some probability that the nucleus decays within a given time (measured from the time you estabished it to be there). It's given approximately by $P(t)=1-\exp(-\lambda t)$.

As you see, only for a stable nucleus, where $\lambda=0$ you are certain that it never decays. That's the case if the nucleus is an exact energy eigenstate of the complete Hamiltonian.

 

[Darwin]

You're asking for something that doesn't exist. It is truly random and not deterministic.

And that's what I'm trying to ferret out. If what you say is true then radioactive decay is truly an event that has absolutely no cause. It could just as well never happen as happen. An unstable atomic nucleus may never lose energy by radiation. In fact, trillions upon trillions of unstable atomic nuclei may be in the process of existing forever. I know this probably comes across as an exaggeration, but if you're correct then it would seem this remains as a distinct possibility.

What bothers me is that decay events have a temporal component to them which expresses itself as at time "X" rather than "y" or "Z." and because no cause for this has been found the default conclusion is that "There is none," rather than "We haven't been able to find one yet." So I guess my difficulty is science's leap to an extremely farfetched reason, uncaused randomness, which has never shown up in any other aspect of existence.

 

[PeroK]

So I guess my difficulty is science's leap to an extremely farfetched reason, uncaused randomness, which has never shown up in any other aspect of existence.

It's the clockwork determinism of classical physics that I find far fetched! Apart from a few simple cases, where was there ever any evidence for that?

 

[Vanadium 50]

I can only tell you what the answer is. I can't make you like it.

 

[Darwin]

The cause of radioactive decays depend on the specific decay. E.g., $\alpha$ decay is due to the tunneling effect (Gamow-theory of $\alpha$ decay), $\beta$ decay is due to the weak interaction, and $\gamma$ decay is due to the electromagnetic one.

The lifetime of any specific nucleus is in all these cases random due to quantum mechanics. So there is no cause that a specific nucleus decays at the specific time you observe it to do so, i.e., there's some probability that the nucleus decays within a given time (measured from the time you estabished it to be there). It's given approximately by $P(t)=1-\exp(-\lambda t)$.

As you see, only for a stable nucleus, where $\lambda=0$ you are certain that it never decays. That's the case if the nucleus is an exact energy eigenstate of the complete Hamiltonian.

While you say the cause of an atomic nucleus emitting an alpha particle is the tunneling effect, it's been my understanding that tunneling can only take place after the decay event. Am I wrong? If I am, then there must be some component within the tunneling effect that determines the "when" of the decay event.

 

[Darwin]

It's the clockwork determinism of classical physics that I find far fetched! Apart from a few simple cases, where was there ever any evidence for that?

Aside from supposed uncaused atomic decay do you know of any other event in the universe that is uncaused (undetermined)? If so, please share.

 

[Darwin]

I can only tell you what the answer is. I can't make you like it.

Thank you.

 

[PeroK]

Aside from supposed uncaused atomic decay do you know of any other event in the universe that is uncaused? If so, please share.

Randomness occurs on at least two levels:

All QM phenomena, not just decay, are random. Scattering, atomic photon emission etc.

Leaving aside QM, we have classical chaos theory. Although theoretically deterministic, classical systems of sufficiently complexity (i.e. all but the simplest, stable systems) are so depending on initial conditions that their behaviour is random and unpredictable. For example: turbulence, stock market behaviour, spread of diseases, sporting events etc. In many cases, to predict a system's state beyond a short time would require initial conditions to a precision that take you into the realm of QM.

 

[PeterDonis]

it's been my understanding that tunneling can only take place after the decay event. Am I wrong?

Yes. The tunneling is the decay event. "Tunneling" and "decay event" aren't two separate events; they are two different ways of describing the same event.

 

[PeterDonis]

Aside from supposed uncaused atomic decay do you know of any other event in the universe that is uncaused (undetermined)?

All quantum events are "uncaused" in the same sense that atomic decay is. QM only predicts probabilities of different possible outcomes; it does not tell you that anyone outcome is determined. (The only exceptions are the edge cases in which you are making a measurement on a system that is already in an eigenstate of the measurement observable--for example, if you measure a qubit to have z-spin up and then make another z-spin measurement right afterwards.)

 

[Darwin]

Randomness occurs on at least two levels:

All QM phenomena, not just decay, are random. Scattering, atomic photon emission etc.

Leaving aside QM, we have classical chaos theory. Although theoretically deterministic, classical systems of sufficiently complexity (i.e. all but the simplest, stable systems) are so depending on initial conditions that their behaviour is random and unpredictable. For example: turbulence, stock market behaviour, spread of diseases, sporting events etc. In many cases, to predict a system's state beyond a short time would require initial conditions to a precision that take you into the realm of QM.

Dynamical systems that are expressed by chaos theory are more than theoretically deterministic, but actually deterministic, even those we commonly call random. And unpredictability has no bearing on the deterministic nature of an event. All unpredictability does is announce our inability to assess the factors that bear on its creation.

Predictability and unpredictability are a non-factors in causation/determinism.

 

[PeroK]

Dynamical systems that are expressed by chaos theory are more than theoretically deterministic, but actually deterministic, even those we commonly call random. And unpredictability has no bearing on the deterministic nature of an event. All unpredictability does is announce our inability to access the factors that bear on its creation.

Predictability and unpredictability are a non-factors in causation/determinism.

The theory is deterministic but then is it a good model for the system? If a system can be described poorly by a deterministic theory and better by a statistical theory, why ultimately prefer the deterministic one? Unless you have an a priori belief that nature must be deterministic.

Anyone, none of this helps with your difficuties over QM.

 

[PeroK]

@Darwin PS I think my point is this: I genuinely never saw QM probabilities as the "bolt from the blue" that you do. I've always seen probabilities everywhere.

 

[Darwin]

The theory is deterministic but then is it a good model for the system? If a system can be described poorly by a deterministic theory and better by a statistical theory, why ultimately prefer the deterministic one? Unless you have an a priori belief that nature must be deterministic.

Not sure what you have in mind by "The theory is deterministic," but the thing is, statistical theories, nice as they may be, don't tackle the functional "whys" of an event. And, other than the possibility of uncaused subatomic events, everything else in the universe is determined (has a cause). However, as I asked before, if you know of some other event in the universe that is uncaused (undetermined), please share.

 

[Darwin]

Yes. The tunneling is the decay event. "Tunneling" and "decay event" aren't two separate events; they are two different ways of describing the same event.

Thank you for clearing this up.

 

[Darwin]

@Darwin PS I think my point is this: I genuinely never saw QM probabilities as the "bolt from the blue" that you do. I've always seen probabilities everywhere.

And I think probabilities are just about everywhere, if one cares to put events into that format. What is the probability that I'll be in bed before 10 PM tonight? From past practices I put it at about 0.01%

 

[PeterDonis]

Not sure what you have in mind by "The theory is deterministic,"

He means that the theory is a model, and a model is not the same as the thing being modeled. The fact that classical mechanics is a deterministic theory does not mean the real world that classical mechanics provides a model of must be deterministic.

statistical theories, nice as they may be, don't tackle the functional "whys" of an event.

You're assuming that there has to be some "functional why". What if there isn't one?

You might not like such a possibility, but it is a possibility.

as I asked before, if you know of some other event in the universe that is uncaused (undetermined), please share.

Both @PeroK and I have already answered this, in posts #15 and #17.

 

[nrqed]

Summary:: The EXACT cause of a radioactive decay event

I recognize that radioactive decay is a spontaneous process in which an unstable atomic nucleus breaks into smaller, more stable fragments, but exactly what is it that causes an atom to decay at a particular time rather than at some other time?

The reason that, as others pointed out, one can not predict the particular time a given nucleus will decay is that the world is quantum mechanical. In a deterministic world, given enough precise data about the state of a nucleus at a given time, one could predict when it would decay (maybe not in practice, but in theory). It is because of quantum physics that we cannot.

 

[Darwin]

He means that the theory is a model, and a model is not the same as the thing being modeled. The fact that classical mechanics is a deterministic theory does not mean the real world that classical mechanics provides a model of must be deterministic.

You've evidently dropped a significant word here or added an unnecessary one because at it stands your sentence isn't making sense.

You're assuming that there has to be some "functional why". What if there isn't one?

If there isn't one then how does an event come into being?

You might not like such a possibility, but it is a possibility.

No problem with possibilities if they're meaningful to the issue, which isn't the case with establishing the reason an event (other than a quantum event) arises.

Both @PeroK and I have already answered this, in posts #15 and #17.

Sorry, I was referring to events other than those of a quantum nature. This shift in subject came about when PeroK said "Leaving aside QM, we have classical chaos theory. . . ." in post 15 and started talking about "deterministic, classical systems."

 

[Darwin]

The reason that, as others pointed out, one can not predict the particular time a given nucleus will decay is that the world is quantum mechanical. In a deterministic world, given enough precise data about the state of a nucleus at a given time, one could predict when it would decay (maybe not in practice, but in theory). It is because of quantum physics that we cannot.

Thank you for your input.

 

[vanhees71]

While you say the cause of an atomic nucleus emitting an alpha particle is the tunneling effect, it's been my understanding that tunneling can only take place after the decay event. Am I wrong? If I am, then there must be some component within the tunneling effect that determines the "when" of the decay event.

The understanding of the $\alpha$ decay in terms of Gamow's theory is that in a nucleus $\alpha$-particles are preformed due to strong correlations in the nuclear many-body system and that these $\alpha$ particles tunnel through the barrier. Given the quite simplistic approach the corresponding prediction for the lifetime is quantitatively not too bad.

 

[PeterDonis]

You've evidently dropped a significant word here or added an unnecessary one because at it stands your sentence isn't making sense.

Do you understand the difference between a model and what is being modeled? Do you understand that, just because a model has some property, that does not mean what is being modeled must have the same property?

The fact that one particular model of the universe (classical physics) is deterministic does not mean the actual, real universe must be deterministic.

If there isn't one then how does an event come into being?

If randomness is fundamental, it just does.

I was referring to events other than those of a quantum nature.

According to QM, all events have "a quantum nature". The quantum nature just isn't practically detectable in all cases. But it's always there.

 

[Darwin]

The fact that one particular model of the universe (classical physics) is deterministic does not mean the actual, real universe must be deterministic.

If the universe isn't deterministic on the classical mechanic scale then what is it? What governs its day-to-day operation?

If randomness is fundamental, it just does.

"It just does" is hardly persuasive.

The randomness would only be an expression of our inability to lock down the causes and figure out the resultant event. It's like the roll of a pair of dice. Although we call the outcome random because we're unable to pin down the relevant initial conditions and the specific forces that determine the outcome, we're well aware that they do exist to produce an inevitable result. "The dice had to come up a 3 and a 4 because . . . ." And this is the only sense in which "random" has any meaning because the only other sense of random is, "utterly and completely without cause." which we know, or should realize, is not how our world functions.

According to QM, all events have "a quantum nature". The quantum nature just isn't practically detectable in all cases. But it's always there.

But the quantum nature of the event would never rise to the level where it would affect causation in the classical mechanic world. As Mark Tegmark, physicist, cosmologist, and professor at the Massachusetts Institute of Technology said in regard to quantum events affecting neural functions of the brain,

"The main argument against the quantum mind proposition is that quantum states in the brain would decohere before they reached a spatial or temporal scale at which they could be useful for neural processing. This argument was elaborated by the physicist, Max Tegmark. Based on his calculations, Tegmark concluded that quantum systems in the brain decohere quickly and cannot control brain function."
source

And likewise, at the quantum level a state may decohere, but it has no affect on other processes at the level of the classical mechanic world. Where we function there is always a "because . . . . " even if we can't pin it down.