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.

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

meopemuk
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.

otto9K9otto
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Bob S
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

meopemuk

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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Rajini
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".

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

What's the difference ?

Rajini
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.

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.

JDługosz
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.

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.

nemoover
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.

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?

nemoover
Do you have any backing for this?

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

JDługosz
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.

Rear Naked
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?

TheCaptain
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.

chogg
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.

chogg
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.

Gold Member
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.

The reason I reject it only being an assumption and accept it as a fact and the reason I consider it an abandonment of scence is because science is the study of the reasons things are the way they are in nature. If it ever comes to be that there is indeed a dead end to all logic, i.e. a truly random event, then this becomes the end of science for there is nothing left once we reach the point of something just happening for absolutely no reason at all. A truly random event in my mind is exactly the same sort of concept as saying "God has always just existed and that's why we are here". It is a meaningless thing to say and cannot be considered science.

chogg
The reason I reject it only being an assumption and accept it as a fact and the reason I consider it an abandonment of scence is because science is the study of the reasons things are the way they are in nature. If it ever comes to be that there is indeed a dead end to all logic, i.e. a truly random event, then this becomes the end of science for there is nothing left once we reach the point of something just happening for absolutely no reason at all. A truly random event in my mind is exactly the same sort of concept as saying "God has always just existed and that's why we are here". It is a meaningless thing to say and cannot be considered science.

I see the study of randomness not as stopping science, but expanding its scope. What I would need to consider something science is not hard underlying determinism, but simply "discoverable regularities". Probabilistic laws are still laws, after all. If it turned out that such laws actually exist, I'd prefer science to be able to say something about them.

Similarly, a truly random event is not a "dead end to all logic"; it merely requires an extension to logic. A logical argument requires each of its premises has a definite truth state. When we relax this restriction, we obtain the probability calculus, which can describe truly-random affairs just as well as the pseudo-random behavior we usually encounter. By no means would we be unable to reason about a truly random event.

Incidentally, the probability calculus much better reflects how science is actually done; logic is rarely if ever used in science. After all, science progresses by induction, not deduction. Under logic, induction is a fallacy, but probability calculus handles induction quite nicely.

I do not want to exclude the possibility of true randomness a priori, because it is too limiting for the scope of science. I think we do better to draw our line in the sand at "discoverable regularities".

Neandethal00
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.

I know it's not science, but my logically illogical conclusion is nothing in the universe is random. It's just a matter of time when probablity in QM will be removed. But at the present time probability in science satisfies our demands. Then why not use it instead of saying 'God does it' or 'a miracle happens'.

Gold Member
I see the study of randomness not as stopping science, but expanding its scope. What I would need to consider something science is not hard underlying determinism, but simply "discoverable regularities". Probabilistic laws are still laws, after all. If it turned out that such laws actually exist, I'd prefer science to be able to say something about them.

Similarly, a truly random event is not a "dead end to all logic"; it merely requires an extension to logic. A logical argument requires each of its premises has a definite truth state. When we relax this restriction, we obtain the probability calculus, which can describe truly-random affairs just as well as the pseudo-random behavior we usually encounter. By no means would we be unable to reason about a truly random event.

Incidentally, the probability calculus much better reflects how science is actually done; logic is rarely if ever used in science. After all, science progresses by induction, not deduction. Under logic, induction is a fallacy, but probability calculus handles induction quite nicely.

I do not want to exclude the possibility of true randomness a priori, because it is too limiting for the scope of science. I think we do better to draw our line in the sand at "discoverable regularities".

I don't have a problem with "discoverable regularities" but I think it would be wise not to confuse this with cause and effect as a reason why something happens. Probablilities and discoverable regularities are both ways to characterize an event, but they cannot be used to explain the event. I should emphasize that I do not limit myself to having things explained only through bosonic events, I don't dismiss "action at a distance", or "fields" that have no material form, or any other extreme action as possible explainations for events that occur in nature including seemingly random events.

The laws of probability are not laws of nature, they are laws of probability. Laws of probability are the rules you can depend on to watch a probabilistic outcome occur as you would expect it to. The laws of nature are the only true laws. I will however submit that I can see how one can argue that a law (such as a random event) can be stated only using characterizations such as "following this or that law of probability" and leaving that as the true explaination for the event. For example one can say that there is a 100% probability that if an antimatter particle and a matter particle colide, they will transform into pure energy. This could be called a fundamental law of nature that requires no further explaination. But is this good enough? Is no further explaination required because this is territory that science cannot or will not explore for whatever reason so it is the end of the discussion?

I believe one cannot just say something happens because it happens (it's a law) and take that for the reason that it happens. If science starts to do this then there is no end to how far up the ladder this could go. Before you know it the flattened rotational curves of spiral galaxies, because they are observed 100% of the time, can simply be explained away as being a fundamental law. There would then be no need to speculate about dark matter as a cause for such an event.

I still submit everything has to happen for a reason and that is a fundamental law that can be taken a priori.

Mentor
The laws of probability are not laws of nature, they are laws of probability. Laws of probability are the rules you can depend on to watch a probabilistic outcome occur as you would expect it to. The laws of nature are the only true laws.
Do you think that we will someday develop the ability to calculate ahead of time where a photon will land after passing through a double-slit?

Probability certainly is a mathematical, logical construct, but so is the rest of math. Why must the universe follow the rules of addition and subtraction? It just so happens (no, I don't actually think it is a coincidence) that the universe behaves in a logical way, therefore the logic of math applies to it.

Your position that eventually we will be able to predict the behavior of probabilistic systems is not a popular one in mainstream science. Pretty much everything we know about probabilistic systems implies that they are not just not knowable now, but are inherently unknowable.
I believe one cannot just say something happens because it happens (it's a law) and take that for the reason that it happens. If science starts to do this then there is no end to how far up the ladder this could go. Before you know it the flattened rotational curves of spiral galaxies, because they are observed 100% of the time, can simply be explained away as being a fundamental law. There would then be no need to speculate about dark matter as a cause for such an event.
Scientists don't take it lightly - they're not going to just start throwing up their hands and stopping investigating in favor of accepting 'it just is'.
I still submit everything has to happen for a reason and that is a fundamental law that can be taken a priori.
Unless you're God, you're not entitled to write such laws.

Gold Member
Do you think that we will someday develop the ability to calculate ahead of time where a photon will land after passing through a double-slit?

No I don't. For one thing the rules laid out in the Heisenberg uncertainty principle forbid it. However, submitting to this does not mean I accept that the double slit phonomenon does not have an underlying cause. It's simply a matter of logistics as to why we will probably never get to the bottom of it.

Probability certainly is a mathematical, logical construct, but so is the rest of math. Why must the universe follow the rules of addition and subtraction? It just so happens (no, I don't actually think it is a coincidence) that the universe behaves in a logical way, therefore the logic of math applies to it.

Indeed, it does seem to and I have no argument with that.

Your position that eventually we will be able to predict the behavior of probabilistic systems is not a popular one in mainstream science. Pretty much everything we know about probabilistic systems implies that they are not just not knowable now, but are inherently unknowable.

This is not my position. I agree that ever predicting such events is most likely, highly unlikely. My only position is that everything happens because something makes it happen. Probability is popular because it is an excellent tool for predicting the outcome of events, but again, it does not and will never explain the reason for these events. You may wish to take the position that because the outcome of an event precisely matches the predicted outcome of a particular law of probability, that this makes the law of probability the underlying cause of the event simply because you (and I) agree that the universe follows the logic of math. This is a leap of faith. There is no direct connection between the two, only a correlation. To say this another way, I believe the laws of probability follow random events and can be used as a tool to follow random events, but this does not mean that random events follow the laws of probability because again, the laws of probability are not true laws of nature, they simply describe it.

Unless you're God, you're not entitled to write such laws.

Oh come now, physicists write laws like this all the time. "The speed of light is a constant" is one of a bazillion examples.

Mentor
No I don't. For one thing the rules laid out in the Heisenberg uncertainty principle forbid it. However, submitting to this does not mean I accept that the double slit phonomenon does not have an underlying cause. It's simply a matter of logistics as to why we will probably never get to the bottom of it.
So....you accept that the HUP applies to some things, but not others that it is currently applied to?
This is not my position. I agree that ever predicting such events is most likely, highly unlikely. My only position is that everything happens because something makes it happen. Probability is popular because it is an excellent tool for predicting the outcome of events, but again, it does not and will never explain the reason for these events. You may wish to take the position that because the outcome of an event precisely matches the predicted outcome of a particular law of probability, that this makes the law of probability the underlying cause of the event simply because you (and I) agree that the universe follows the logic of math. This is a leap of faith.
An experimental result is most certainly not a leap of faith except insofar as all experiments depend on the same "leap" that we're not just extraordinarily lucky to see such logic/consistency.
There is no direct connection between the two, only a correlation. To say this another way, I believe the laws of probability follow random events and can be used as a tool to follow random events, but this does not mean that random events follow the laws of probability because again, the laws of probability are not true laws of nature, they simply describe it.
Well lets go a step further: if there is more to it than probability, if we may eventually be able to predict outcomes that are currently modeled as probabilistic, then there must be a pattern that we aren't seeing. If there is a pattern that we aren't seeing, then what we think is random really isn't random -- it means there are errors in our predictions that are so small that we haven't seen them yet. Since we haven't found patterns in the randomness (which would, of course, contradict the concept of "randomness"), evidence supports/strengthens the conclusion that these things really do have a random element. And the more experiments done, the stronger the conclusion that these things really do have a random element and the smaller and smaller the dark corner that a pattern may yet lie in.
Oh come now, physicists write laws like this all the time. "The speed of light is a constant" is one of a bazillion examples.
Most certainly not. That's a postulate and a theory, which has corroborating evidence. You've elevated your idea above that. You believe, despite contradictory evidence, that the true functioning of the universe is a certain way. That's beyond science - the only way to know such a thing would be to be the one who actually wrote those laws into the programming of the universe.

Delta Kilo
Interesting question. I can see where Buckethead is coming from. I too, tend to draw a distinction between the inability to measure something precisely on one hand and a total randomness (as in lack of cause) on the other.

For example, classical mechanics is considered fully deterministic. In theory if you know the positions and velocities of all molecules in 1 litre volume of gas with sufficient accuracy, you should be able to predict their positions at some future time, say 1 second ahead. In practice, something like moving 1kg of mass by 10cm somewhere in the vicinity of Sirius is sufficient to throw spanner into the works here on Earth and make the positions completely unpredictable (Penrose gave this argument in one of his books). Then there are pesky questions about knowing the position of all particles in the universe, solving gazillion-body problem, not to mention photons coming from the very fringes of the observable universe which you can't predict because you haven't seen them yet.

Nevertheless we still call it deterministic. Why is that? I think this is because, when a (classical) particle hits the screen, we don't just say "it's random", we have an explanation ready, we say, well it hit here and not there because the sum total of all forces must have been such as to produce this kind of trajectory. If only we knew the forces beforehand we could've surely predicted where it was going to hit.

Now let's look at HUP. We know what it says but why exactly does it say it, where does it come from? Well, from non-commuting projection operators. And where do these come from? From the observables, measurement, wavefunction collapse, Born rule etc. And these? At this point we are supposed to shut up and calculate.

But but but. Just like with the gas pressure, to get more and more accurate results we will have to look at individual molecules, so with quantum measurement we will have to treat the entire measurement apparatus quantum-mechanically. Obviously we can't just replace a hugely complicated system with lots of interacting degrees of freedom (measurement apparatus) with a simple operator and expect to get exactly the same results? Surely this must be some kind of idealization, simplification or generalization just like the gas pressure is the generalization of the forces of individual molecules hitting the wall?

What I'm driving at is, the process of measurement, wavefunction collapse, observables, their projection operators, and therefore HUP are all likely to be emergent phenomena. In other words, HUP is valid for a ideal measurement which is only an approximation for the real measurement, arising from our ignorance of quantum-mechanical nature of the measurement apparatus and its environment.

So, when (this time quantum) particle hits the screen we could say, well it hit here and not there because the relative phases of the wavefunctions of everything the particle had ever interacted with (the atoms of the screen, the source, the two slits, the CMBR photon that was passing by and everything that was entangled with it since the beginning of time), yeah all these phases just happened to be aligned so. Yeah, if only we knew all these phases beforehand we surely could have predicted where it was going to hit. Honestly

Gold Member
So....you accept that the HUP applies to some things, but not others that it is currently applied to?

No, I didn't say that nor is that my perspective. My understanding is that HUP can be applied to any subatomic random event. I'm not sure what this has to do with our discussion however. The HUP is a principle about what we can expect when trying to analyze a random subatomic event, it is not a theory of the random event itself.

An experimental result is most certainly not a leap of faith except insofar as all experiments depend on the same "leap" that we're not just extraordinarily lucky to see such logic/consistency.

Again, you seem to be confusing my stance that probability calculations and random events are not the same. If you are using probability to make observations about any number of unrelated random events and you find that your calculations match, all you can say about it is that the random events seem to be random, you cannot say that random events are truely random.

Well lets go a step further: if there is more to it than probability, if we may eventually be able to predict outcomes that are currently modeled as probabilistic, then there must be a pattern that we aren't seeing. If there is a pattern that we aren't seeing, then what we think is random really isn't random -- it means there are errors in our predictions that are so small that we haven't seen them yet. Since we haven't found patterns in the randomness (which would, of course, contradict the concept of "randomness"), evidence supports/strengthens the conclusion that these things really do have a random element. And the more experiments done, the stronger the conclusion that these things really do have a random element and the smaller and smaller the dark corner that a pattern may yet lie in.

This is a very good point and I appreciate what you are saying here and it does make me wonder about the "algorithm" used by nature to create a seemingly random event. Perhaps this is really gets to the heart of the matter. If our computer generated statistical outputs almost perfectly match those generated by nature, then one could say that nature uses an algorithm as well that is 100% repeatable given the same input. Our computers are not able to generate a truely random event, only a simulation, and perhaps nature is up against the same wall.

Mannix99
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.
Actually, recents observation tends to show that radioactive decay is NOT random. The issue is that so far, noone is able to say why.