Gold Member
Hello;

I remember being taught long ago that radioactive decay is random, but, no one ever explained to me why. Surely there has to be a reason for it? Or is it simply the case of it not being random? (particles in gases don't move randomly, it is dependent on various factors)

Thanks.

Matterwave
Gold Member
It is random just like (almost) all other Quantum Mechanical phenomena.

Yes, radioactive decay is truly random. There is no explanation for that. It is just a fact of life. Quantum mechanics can calculate the probability of decay, but it cannot tell when a given atom will decay.

Eugene.

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The decay rate of some radionuclides is known to vary, depending on environment. Recently, several isotopes which are normally stable were found to be radioactive when the bare nucleii (I forget which ones) were stored in a particle accelerator ring. This is due to the Coulomb field of the electron cloud of the atom inhibiting radioactive decay. Beginning about 50 years ago, experimenters have been looking for variations in the decay rate of beryllium-7, an isotope that decays only by electron capture (absorbs a K-shell electron). The conjecture is that the atomic environment (electron density) might affect the decay rate (~53 days). The randomicity of radioactive decay refers to the variations in instantaneous decay rate (e.g., decays in a short time interval) relative to the average decay rate. The number of radioactive nuclei divided by the decay rate is a constant equal to the mean lifetime. For all measured radioactive isotopes, no unexpected deviation has been seen outside the usual time-interval or Poisson statistics of variations from the average decay rate.

Bob S

Even if these observations are correct, they do not change the conclusion about the randomness of decay. These articles talk about the influence of external conditions on the decay rate. The point of quantum randomness is that with a given decay rate it is not possible to predict when a given atom/nucleus will decay.

Eugene.

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Instead saying the decay as random, it is better to see it as a statistical phenomenon.??
what you say?
branching ratios are just statistics (i mean per 100 decay and so on)

Gold Member
I saw an illustration to demonstrate how to think of radioactive decay some time ago and I'll give my version of it here:

Imagine a small steel BB shot inside of a ping pong ball. There is a small hole in the ping pong ball just a little larger than the BB. Now shake the ping pong ball. At some point the BB will exit the ball (radioactive decay), but when that actually happens is totally up to chance. The larger the hole, the shorter the "half life".

vanesch
Staff Emeritus
Gold Member
Instead saying the decay as random, it is better to see it as a statistical phenomenon.??

What's the difference ?

Nothing big difference..i am comfortable to think random as statistical one..
Random: it is random..
Statistical: values are same on statistical base..always same branching ratio for every 100 decays..

I saw an illustration to demonstrate how to think of radioactive decay some time ago and I'll give my version of it here:

Imagine a small steel BB shot inside of a ping pong ball. There is a small hole in the ping pong ball just a little larger than the BB. Now shake the ping pong ball. At some point the BB will exit the ball (radioactive decay), but when that actually happens is totally up to chance. The larger the hole, the shorter the "half life".

The ping-pong ball model would seem to argue more towards a deterministic, rather than random, prediction of the event. There is great complexity in the factors determining how one shakes the ping-pong ball, how the BB rattles around, and at what time it's position coincides with the hole, but is that truly random? Likewise, can quantum mechanics truly make the claim that it is fully able to account for the interactions of all the forces and particles acting upon a radioactive nucleus? Is it not possible that there are forces, particles, and interactions as yet unaccounted for that ultimately determine when a particle decays?

The uranium-238 nucleus, for instance, contains 146 neutrons and 92 protons which are not rigidly glued together, but in constant flux. There are an incredible number of positional changes that this nucleus constantly undergoes in terms of the relative position of each of these particles. Doubtlessly, some of these positions are more stable than others. When a sufficiently unstable position is reached, then the nucleus decays. The stability of the nucleus at any given time would be "highly sensitive to initial conditions," and thus could be determined by a non-random chaos model.

Furthermore, our ability to create the atomic bomb demonstrates quite clearly that when a high-energy particle comes in contact with a radioactive nucleus, it can be induced to decay. Is there any proof that natural radioactive decay does not occur from the "random" contact of nuclei with a high-velocity particle such as a neutrino?

This is not to say that at present (or possibly ever), that it is within human ability to predict when a particular nucleus will "naturally" decay, but unpredictable does not equal random.

mgb_phys
Homework Helper
The ping-pong ball model would seem to argue more towards a deterministic, rather than random, prediction of the event.
The same decay maths applies to anything where you half a large number of components that are independant. If hard drives fail randomly with an average life then the rate of failure of a large number will follow the same behaviour as radioactive decay.

Likewise, can quantum mechanics truly make the claim that it is fully able to account for the interactions of all the forces and particles acting upon a radioactive nucleus? Is it not possible that there are forces, particles, and interactions as yet unaccounted for that ultimately determine when a particle decays?
It's possible that there is a 'hidden variable' but there is no evidence for it and no reason to suppose one except believing that "God doesn't play dice"

Furthermore, our ability to create the atomic bomb demonstrates quite clearly that when a high-energy particle comes in contact with a radioactive nucleus, it can be induced to decay.
Fission isn't the same sort of decay.

The same decay maths applies to anything where you half a large number of components that are independant. If hard drives fail randomly with an average life then the rate of failure of a large number will follow the same behaviour as radioactive decay.

It's possible that there is a 'hidden variable' but there is no evidence for it and no reason to suppose one except believing that "God doesn't play dice"

Fission isn't the same sort of decay.

My point regarding fission is that the nucleus is susceptible to external stimuli. Perhaps it is an oversimplification, but my understanding of alpha decay is that the nucleus (and particularly the soon-to-be alpha particle) gets sufficiently excited that the electromagnetic repulsion becomes greater than the strong nuclear force and thus the alpha particle "breaks off." Any number of essentially unpredictable conditions (such as collision with a high-velocity neutrally charged particle such a neutrino at a time when the nucleus is at a higher-energy state than usual) could cause the nucleus to enter a sufficiently unstable state to decay. This unpredictability could create the illusion of randomness in a non-random situation.

My oversimplified understanding of the quantum mechanics model is that the nucleons are occupying positions and energy levels at random and not necessarily as the result of any interaction with another particle or photon.

In my opinion, an inability to predict is not the same as proof of randomness. I do not insist that "God does not play dice." Perhaps he does. Or perhaps the random behavior of particles is God's continuous involvement in shaping the course of events. It is interesting that as science has progressed, it has gone full circle back to the notion that natural events are ultimately unpredictable if one takes a sufficiently microscopic view. In a random model, events occurring "because God willed them to" is as good an explanation as any.

The uranium-238 nucleus, for instance, contains 146 neutrons and 92 protons which are not rigidly glued together, but in constant flux. There are an incredible number of positional changes that this nucleus constantly undergoes in terms of the relative position of each of these particles. Doubtlessly, some of these positions are more stable than others.

No, the nucleus arranges itself in discrete energy states, just like the electrons do. It's more complex since the attraction is mutual rather than being simplified to a point, but it is the same idea. Nothing will change unless it changes by a discrete quantum of energy, which is in the gamma-ray range at the very least.

Decay is caused by the genuine random nature of measuring the "collapse" of the wave function. A nucleon has a small probability of being farther away from the center, and once it finds itself there, it is no longer stuck via the strong force.

vanesch
Staff Emeritus
Gold Member
In my opinion, an inability to predict is not the same as proof of randomness.

There is no possible "proof of randomness" of anything. It is fundamentally impossible to prove that something is "fundamentally random". This is something that comes up regularly. The best proof against the existence of a proof of randomness is the fact that one can always make the hypothesis of the existence of a "Book of Events" somewhere hidden in an unaccessible corner of the universe by one or other deity or whatever, where all (past and future) events are written down.

I do not insist that "God does not play dice." Perhaps he does. Or perhaps the random behavior of particles is God's continuous involvement in shaping the course of events. It is interesting that as science has progressed, it has gone full circle back to the notion that natural events are ultimately unpredictable if one takes a sufficiently microscopic view. In a random model, events occurring "because God willed them to" is as good an explanation as any.

Maybe God is just playing the tape of the universe on his recorder. That tape is then the "Book of Events" and all future events are on the tape, even though it "didn't happen yet".
That's what I mean: that hypothesis is a logical, philosophical possibility (in other words, that the "laws of nature" are nothing else but just "reading" page by page, the "events in the universe" ; and there happen to be certain correlations within these events, which we think we can call "laws of nature"). So these events are then not "random" (because they are written in a book and it "couldn't happen otherwise"), but they are unknowable, and the best we can do is to base ourselves upon the statistical correlations we discovered as "physical laws", and to imagine that they correspond to some causal but partly random "dynamics". This also means that from tomorrow on, what we think are the "laws of nature" could change entirely, and all regularity we derived up to now was just superficial, simply because this evening, a page in the Book of Events will be turned that changes chapters, and the correlations after are totally different than those before.

As such a deterministic ontological view (the view of the Book of Events) can literally comply with ANYTHING that happens, it means two things:
- at least one deterministic view is compatible with just ANY laws of physics or observations or whatever
- this hypothesis is totally useless and unscientific in Popper's sense, in that it is of course utterly unfalsifiable.

From this follows that one can never, in a scientific way, PROVE that anything is "truly random". The only way to prove that something is NOT random, is by indicating that we know deterministically what will happen in each individual event. But the opposite is impossible.

In my opinion, we cannot predict when a given atom will decay does not mean radioactive decay is truly random. Apparently it's very hard (and maybe impossible) to calculate because there are plenty of factors involved but atoms still follow physics laws. There are laws that we have not yet discovered and may never will but i don't think atoms will decay whenever they desire.

When people say something is random, it think they mean it's random to them.

Pengwuino
Gold Member
Apparently it's very hard (and maybe impossible) to calculate because there are plenty of factors involved but atoms still follow physics laws. There are laws that we have not yet discovered and may never will but i don't think atoms will decay whenever they desire.

Do you have any backing for this?

Do you have any backing for this?

I truly think your question is the best catalyst for science.

In my opinion, we cannot predict when a given atom will decay does not mean radioactive decay is truly random. Apparently it's very hard (and maybe impossible) to calculate because there are plenty of factors involved but atoms still follow physics laws. There are laws that we have not yet discovered and may never will but i don't think atoms will decay whenever they desire.

When people say something is random, it think they mean it's random to them.

You are mistaken. This is the line of thought that led to Einstein's "Doesn't play dice" quote. But since then we have learned that there is genuine randomness. It is unpredictable and unknowable even in principle, and "hidden variables" have been ruled out.

You are mistaken. This is the line of thought that led to Einstein's "Doesn't play dice" quote. But since then we have learned that there is genuine randomness. It is unpredictable and unknowable even in principle, and "hidden variables" have been ruled out.

Can you expand on this post please?

Many years ago I imagined that the apparent randomness of radioactive decay was simply the result of the random reconfigurations of nucleons. Eventually I learned that this model does not explain the fact that sometimes alpha or beta particles are ejected from the nucleus with much less energy than the potential energy barrier that such particles would have to overcome to get out of the nucleus. The explanation in first year college physics texts is that the particles do not "climb" over the barrier but rather "tunnel" under the barrier.

This kind of explanation is another way of saying "we don't know what's really happening in radioactive decay". I recently heard of several researchers who have detected a variation in the rate of decay (sorry, I don't know what isotope) that corresponds to solar flares. Furthermore they think they have seen a variation in decay rate that corresponds to Earth's distance from the sun. If these results are correct, I would speculate that perhaps neutrinos may play a role in the rate of radioactive decay.

I don't believe in randomness as it is used to describe radioactive decay. Probability is a convenient mathematical tool, but provides no insight to the underlying process of decay. Heisenberg gave us a valuable tool, but I believe Einstein was right when he insisted that there must be a deeper truth beyond quantum physics.

Gold Member
I don't believe in randomness as it is used to describe radioactive decay. Probability is a convenient mathematical tool, but provides no insight to the underlying process of decay. Heisenberg gave us a valuable tool, but I believe Einstein was right when he insisted that there must be a deeper truth beyond quantum physics.

It's hard to disagree with this. Probability is indeed a tool and not an explaination. It's one thing to say a train will arrive at a station at 5:00pm with a 90% reliability but this is not an explaination as to why it will arrive at that time. There has to be an underlying cause to everything even if you must dive into deeply disturbing and speculative territory. I simply don't see how anything can be truly random. Gus Grissum tried to explain the explosive bolt on his capsule "just blew" but it was clearly pointed out that explosive bolts "don't just blow". And so it is with anything, things don't just happen, they happen for a reason. This is not to say that the reason for radioactive decay is easily explained, and I suspect it will require a new paradigm. Anyone that suggests random events truely happen is simply playing the "god of the gaps" game and is not dealing in science.

It's hard to disagree with this. Probability is indeed a tool and not an explaination. It's one thing to say a train will arrive at a station at 5:00pm with a 90% reliability but this is not an explaination as to why it will arrive at that time. There has to be an underlying cause to everything even if you must dive into deeply disturbing and speculative territory. I simply don't see how anything can be truly random. Gus Grissum tried to explain the explosive bolt on his capsule "just blew" but it was clearly pointed out that explosive bolts "don't just blow". And so it is with anything, things don't just happen, they happen for a reason. This is not to say that the reason for radioactive decay is easily explained, and I suspect it will require a new paradigm. Anyone that suggests random events truely happen is simply playing the "god of the gaps" game and is not dealing in science.

That is an a priori assumption on your part. There is no good reason why the universe couldn't exhibit truly random behavior.

Gold Member
That is an a priori assumption on your part. There is no good reason why the universe couldn't exhibit truly random behavior.

Yes there is. For one thing look at your sentence, you said "the universe" (that which creates an action) "exhibit" (is causing), "random behavior" (a random behavior). There is no avoiding it, if something happens, something or someone is doing it. One can't even write a sentence about random behavior without implying something causing it. And if something causes a random behavior, then the random behavior isn't random.

Yes there is. For one thing look at your sentence, you said "the universe" (that which creates an action) "exhibit" (is causing), "random behavior" (a random behavior). There is no avoiding it, if something happens, something or someone is doing it. One can't even write a sentence about random behavior without implying something causing it. And if something causes a random behavior, then the random behavior isn't random.

Not at all; you are changing the meaning of words. "Exhibits" has nothing to do with concepts of "causing" a behavior. It only talks about what we observe.

You are assuming that, if we dig down deep enough, at bottom we'll find that nature is completely deterministic. We'll find there was some property of nature that made this nucleus decay at time $t$, and not at any other time. That's fine for you to assume that, and I wouldn't be surprised if it were true. All I insist is that you recognize it for what it is: an assumption.

And if you take that assumption to be unassailably true a priori, then it is ironic for you to tell others that they are "not dealing in science" when they question it.