B Is there a definition of randomness?

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The discussion on the definition of randomness reveals that there is no formal definition applicable to philosophical inquiries about randomness, despite established concepts in probability theory. A random process is characterized as unpredictable but can be described probabilistically, and its predictability may vary depending on the theoretical framework used. The conversation touches on the idea that correlation between variables can influence perceptions of randomness, though this does not necessarily diminish the inherent randomness of each variable. The concept of randomness is further complicated by quantum mechanics, where certain processes are fundamentally unpredictable, challenging deterministic interpretations. Ultimately, the nature of randomness remains a complex interplay between predictability, theory, and philosophical considerations.
  • #91
Zafa Pi said:
Physics is loaded with undefined terms as opposed to math. You mention theory, and there is measurement and random. The OP was concerned about random, so sticking with that, it seems (@PeroK, @andrewkirk , and me) that random is contextual.

I suggest that a random sequence is generated by certain specified physical processes. For example coin flipping. More general, lab measurements that correspond to QM measurements that are random variables, e.g. measuring electron spin at 90° from spin up electrons at 0°.
Both of the above example have been found to satisfy randomness checks for two valued uniform sequences, but they are not generated by any algorithm.

How about?
Random := The value(s) produce by an objective* physical* process that when repeated yields a sequence that passes randomness tests.
* objective means repeatable by others. * physical means non-algorithmic, like coin flips.

Now note this not a math definition. Is it any more vague than defining length?
I understand, but it seems self-referential: defining randomness in terms of passing random tests. Maybe you can say that the outcome can only be determined probabilistic , over the long run. No method exists * which can predict with 100% accuracy the outcome of the experiment.

Maybe can exist
 
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  • #92
Curiose said:
It's like the difference between having a set of statistics about something, and literally being an omnipotent observer of that system in real time, having much more detailed statistics about factors which may not seem to affect the outcome but actually do. This is why I say that it is not randomness that exists, but rather a lack of observational knowledge.

But, what if there is a physical, theorectical limit to observation knowledge? This could come from a) there being a limit on how accurate a measurement of position could possibly be; and/or b) a limit to the knowledge of two observables - the HUP (Heisenberg Uncertainty Principle) essentially says that the more accurately you know the position of a particle, the less accurately you know its momentum - in any case, you can't know both to an arbitray precision; and/or c) a piece of data that is intrinsically random, such as the spin on an electron, which may be theorectically unknowable.

This ties into my post above. There is no certainty that Quantum Mechanics will one day be replaced by a "deterministic" theory, where the HUP and the instrinsic probabilities disappear. And, unless that happens, then there are clear limits to observational knowledge.
 
  • #93
PeroK said:
There is no way to be certain that the probabilities apparently observed in the Stern-Gerlach experiment and predicted by QM will ever go away.

"Apparently observed" is a wise choice of words. Technically, we don't observe specific probabilities. We observe actual frequencies of occurence. Can we observe the distinction between "there is some probability involved" versus "the result is always the same" for a given situation? - i.e. distinguish between certainty versus a probability very near 1?

I find it interesting to consider whether the concept of (physical) probability must always involve a notion of time - at least "time" in the sense of something that has a "before" and "after" aspect. For example, we think of a probability that a fair coin "will" land heads in a scenario where the coin does land. So there is a "collapse" of the probability to a definite outcome even in this non-quantum-mechanical model.

One can try to circumvent the time aspect by thinking about "ensembles". For example, we can think about an "ensemble" of fair coin tosses that were done in the past. However, to extract a probability model for a coin toss of experiment from that point of view, we have to introduce the idea that an experiment is "chosen at random" from the ensemble. So implicitly we have the the idea of the probability of heads before the experiment is chosen then a definite result after the experiment is chosen.
 
  • #94
WWGD said:
I understand, but it seems self-referential: defining randomness in terms of passing random tests.
I didn't do that. The randomness tests were included to ensure the physical process wasn't do something ridiculous like turning out the same digit over and over.
For example, if one looks at the displayed temperature (F) at Times Square at noon on June 1st, I would say the leading digit is not uniform (over the 10 digits), but I would not be surprised if the 2nd digit was (with respect to randomness tests).
 
  • #95
PeroK said:
The answer of "definitely", which is assumed by some as an a priori property of a fundamentally deterministic universe is not correct. There is no way to be certain that the universe is fundamentally deterministic. There is no way to be certain that the probabilities apparently observed in the Stern-Gerlach experiment and predicted by QM will ever go away.
You have said this before and I agreed. However, you didn't respond to my statement
Zafa Pi said:
Also the evolution of the electron state through the S/G is governed by a deterministic unitary process.
 
  • #96
Curiose said:
What I'm saying is that if I knew the state of every air particle, the exact pressure/position of the thumb, how many atoms the thumb and the coin had in them, the exact height of the point of release, the exact distance to the ground, the exact layout of the ground or surface of which the coin was falling, maybe down to the micron, and the environmental conditions directly surrounding and interacting with the coin as it flips through the air, and probably a number of other physical factors which it is currently impossible to measure, I could predict which side the coin would end up on when it finally came to rest.
In spite of many physicist saying this, I find it nonsense. It is untestable, not even wrong, philosophical fluff. I find it on par with: If I knew enough I could predict anything.
PeroK's statement is germane,
PeroK said:
But, what if there is a physical, theorectical limit to observation knowledge?
You then go on to say,
Curiose said:
But pretty much, since I don't know those things and have no interest in developing some sort of analysis device which could, for me, it is impossible to know, thus I would say it is "random".
I like the last 6 words. See post #85.
 
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  • #97
Zafa Pi said:
I didn't do that. The randomness tests were included to ensure the physical process wasn't do something ridiculous like turning out the same digit over and over.
For example, if one looks at the displayed temperature (F) at Times Square at noon on June 1st, I would say the leading digit is not uniform (over the 10 digits), but I would not be surprised if the 2nd digit was (with respect to randomness tests).
I see, so you first define the randomness test and then use that. Ok, sorry I misread you.
 
  • #98
A truly random phenomena must have some probability of failing a randomness test.
 
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  • #99
Stephen Tashi said:
A truly random phenomena must have some probability of failing a randomness test.
Although uncertain, the possibility exists; Mr Pi can appreciate simplicity to a greater degree, using his contextualy more constrained version of randomness...

Defined as... Deterministic Unridiculous Randomness (DUR) .

Zafa Pi said:
The randomness tests were included to ensure the physical process wasn't do[sic] [recte to] something ridiculous like turning out the same digit over and over.
 
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  • #100
Stephen Tashi said:
A truly random phenomena must have some probability of failing a randomness test.
I would use possibility rather than probability.
The definition I gave in post #85 has that built in. It's the 1st time I've tried to give a definition, what deficiencies do you find with it?
 
  • #101
OCR said:
Defined as... Deterministic Unridiculous Randomness (DUR)
Deterministic? Where did that come from?
 
  • #102
Zafa Pi said:
The definition I gave in post #85 has that built in. It's the 1st time I've tried to give a definition, what deficiencies do you find with it?

I suggest that a random sequence is generated by certain specified physical processes. For example coin flipping. More general, lab measurements that correspond to QM measurements that are random variables, e.g. measuring electron spin at 90° from spin up electrons at 0°.

As you said, you haven't given a mathematical definition. Until you explain how to determine which physical processes are among the "certain specified physical processes", we don't have a specific physical definition.

A definition of "randomness" that only classifies a process as "random" or "not random" isn't very useful. It would lump tossing a fair die in the same category as tossing a loaded die.
 
  • #103
Zafa Pi said:
Deterministic? Where did that come from?
It just " locks in " ... Unridiculous .
So you can have randomness, with absolutely no foolishness.

Is that speculation ?
 
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  • #104
  • #105
Zafa Pi said:
Random := The value(s) produce by an objective* physical* process that when repeated yields a sequence that passes randomness tests. * objective means repeatable by others. * physical means non-algorithmic, like coin flips.

it seems to me that @andrewkirk gave the most scientifically relevant answer. A process is random or not relative to a theory for predicting it.

A randomness test can at best check whether something is predictable by simple algorithmic means.

Of course, what this means is that some aspects of a sequence might be random, while other aspects are not. For example, there might be a sequence

12121212124121212121212121251212121212121212...

which is mostly an alternation between 1 and 2. So a "randomness test" will fail, saying that it's not random. But the departure of the sequence from predictability may be random.

The other way around would apply, as well. There could be a sequence that looks completely random, but is actually completely predictable. The digits of pi is an example.

If you're interested in how hard it is to predict a sequence, I think computational complexity theory is more appropriate. Given a sequence of digits you can characterize how random it is by how much it can be compressed algorithmically.
 
  • #106
PeroK said:
But, what if there is a physical, theorectical limit to observation knowledge? This could come from a) there being a limit on how accurate a measurement of position could possibly be; and/or b) a limit to the knowledge of two observables - the HUP (Heisenberg Uncertainty Principle) essentially says that the more accurately you know the position of a particle, the less accurately you know its momentum - in any case, you can't know both to an arbitray precision; and/or c) a piece of data that is intrinsically random, such as the spin on an electron, which may be theorectically unknowable.

This ties into my post above. There is no certainty that Quantum Mechanics will one day be replaced by a "deterministic" theory, where the HUP and the instrinsic probabilities disappear. And, unless that happens, then there are clear limits to observational knowledge.

To be more specific, I'm not necessarily saying that randomness exists, but I'm also not saying that it doesn't exist. I definitely don't have privileges that break quantum observation, lol.

And yes, the limits are clear. Assuming that human beings and human logic are the highest point on some hierarchy we imagine exists, thus creating our measurement error in any possible measurement device that we can currently imagine. To be more precise, we will never have a finalized and perfect definition of the physical world, thus we will never have a fully agreed upon definition of what is random.

Zafa Pi said:
Random := The value(s) produce by an objective* physical* process that when repeated yields a sequence that passes randomness tests.
* objective means repeatable by others. * physical means non-algorithmic, like coin flips.

I like this definition of randomness as it is useful for most purposes and clearly delineates physical phenomena from math.

It's theoretical, but I don't think it's fluff. Un-testable by current standards? yes. There is still a humongous debate about whether or not reality is deterministic. I am liking what's going on with Quantum Gravity theory with their quasicrystals and information theoretic notion of reality which takes neither side of the debate. We can only measure phenomena which we can sense (with our senses or some device), and even then, the measurement of that phenomena changes the outcome, plus is very prone to error. Are there dimensions in physical systems which we are not taking into account and could be measured to approximate an outcome, but we simply cannot measure them yet?

I'm not saying that we are in a simulation. That is, I think, too far off topic. But I will give a simulation test example of how randomness can be explained. The problem being that you don't know if physics is the observed result of some higher order algorithm which sits behind the true laws governing the physical system in which the coin is being tossed. You have to take into account "the observer" which is also theoretical, but I mean, if we can't prove randomness exists, then randomness is also theoretical.

Let's take the idea and inverse it.

If you assume that the observer exists, technically any device which senses a physical phenomenon is an observer of that phenomenon whether or not it comprehends what it is sensing. Let's say that I make a program which takes input from a camera, performs a couple of filter operations on the incoming data and creates a 2D space which is scattered with colored points that represent the color edges being sensed in the current frame. So my space has the following dimensions: (x, y), (r, g, b) and the x,y sub-space is filled with points by the edge filter using the r,g,b data from the original video frame. Now let's say that I also populate this space with "observers" which follow some rules that govern their behavior.

What my little simulated observers are sensing are the edge point positions and color that are the result of multiple filters working constantly on some input data. The filter input data is coming from "my reality" or what you refer to as the physical world, whereas the filtered output data is a simplified projection of that raw data. To further complicate things, let's assume that the simulated observers can recall and track edge shapes from the points they observe. They will always ever be observing a filtered 2D projection of a 3D space. Let's also assume that they have some ability to remember patterns and thus predict near-future outcomes like a shape they will see, or where and in what orientation that shape will turn up.

They will never be able to fully predict where and when in their reality, the points that represent my hand will show up, because they don't know my hand exists as a hand. The idea of what a hand is supersedes them, since they don't have hands or bodies. A hand is only represented in their space as a recognizable set of points with specific relationships in 2D+Color+Edge space, which occur in some positions at some rotations, etc. The physics of how a CCD camera works, how the edge filter algorithm works, and how my own decision making process works are all impossible to know for these observers because their observable reality is a product of these perception filters used to create their reality space. Their reality is a projection of my reality that is further filtered before they can even perceive it. These are the observational limits of their perception as set forth by filtering (projection) process.

It is to say, I can accurately predict where the points that represent my hand will show up in their space and in roughly which configuration those points will be, because I am an outside observer of their reality and I can perceive the extra dimensions which affect that reality, but they cannot (imposed measurement limit). No matter how intelligent these lower dimensional observers are, the best they can ever do is create a statistical observation of where my hand might show up with some certainty quantifier. They might even make up equations like the Heisenberg Uncertainty Principle to formally define this and try to explain the phenomenon as random because it saves them the time of trying to imagine something that eludes them about their own perception of what they believe to be reality.

I'm not saying that this microcosmic example is true for our perception of reality, I'm saying that we never stand a chance of knowing if randomness truly exists or not. Perhaps it's just an artifact of the laws that govern the physical space we exist in, which could be generated by a higher-dimensional space in which other laws apply.

It is proven that each perception of reality is not the same as other's perceptions. The Pauli exclusion principle perhaps is a clue that either we are not seeing the same data, or that we are seeing the same input data, but through the chained filters of our "perceptive ability" and the distinct point of view through which we perceive the space. No two measuring devices can exist in the same physio-temporal position, thus no two perceptions of the same data will ever result in the same final input which is then processed by the perceiver. This is yet another filter, the entire process taking time to actually happen. In a very real way, the perceptive filter of our reality is represented physically by a sense organ or device. What we see is what we get. Are we all inside of a higher dimensional reality which, upon perception, collapses into a lower dimensional representation of that reality?

That brings us to the idea of fractals. A fractal is a projection of a higher dimensional space onto a lower dimensional plane. Point by point at any scale or n-dimensional rotation, can be calculated up to the currently working infinity limit, which is rendered in the projection as negative space. When projection happens, one or more dimensions must be collapsed into another dimension on the plane of interception/observation. When your eye perceives light, it generates signals which are rendered in your brain as a 2D projection. If you have two eyes, you have a higher sense of depth. This is a perception of a dimension of physical space which we call depth. This third perceived dimension helps us further define and comprehend the data we are perceiving inside of the space in which we exist.

Is it hard to imagine that the factors operating in a coin toss elude our limits of perception given the limited physical representation of reality at which we interact and observe? No... and that's why we have statistics. Statistics smooth for "randomness" by counting, summing, and the like, in order to account for the error in our ability to recognize data as an ordered pattern due to those perceptive limits.

This is really a great question! Thanks for the inspiration. I'm loving this forum, I feel like I've been missing out on talks like this IRL.
 
  • #107
I wonder if it would be meaningful to throw-in the idea of some Godel-like results, if we view a theory as a collection of axioms together with rules of inferences. Then there will always be some non-random events ( not provable within the system).
 
  • #108
stevendaryl said:
it seems to me that @andrewkirk gave the most scientifically relevant answer. A process is random or not relative to a theory for predicting it.
PeroK said:
Randomness in QM is different, because you have perfect information. You have an ensemble of electrons that are spin-up in the z-direction; you measure their spin in the x-direction and you get spin-up 50% and spin-down 50%.

The theory of QM predicts this and suggests that there is no further information that could possibly be available to you (hidden variables) that would allow you to predict when an electron will be spin-up and spin-down.

Tossing a coin is random because you have inexact information about the experiment.
QM states that measurements are random variables, and it also states that the evolution of the state of an electron (and S/G device) obey a deterministic law.
What ever theory one proposes it is a subjective decision (with consensus) which physical instruments and processes are modeled by the theory.

Here is my simple theory: A coin flip is a two valued uniform r.v.
Most would agree that a "fair" coin flipped from the Eiffel Tower or in a wind tunnel is a physical device that is modeled by the theory. I now define that processes/result as random. And most would agree, though some would say random due to ignorance. I find this silly since saying there is information (but inexact) is math or philosophy not physical (no way to test). Same goes for determinism in general.
Any other r.v. can be generated by the coin flip. Flip it a 100 times and you have have a number from [0,1] chosen uniformly. Take functions of that to get any other continuous distribution. With an average of two flips of the fair coin you can get any other two valued r.v., e.g. 1 with probability 1/π, 0 with probability 1 - 1/π.

Now this may too slow in practice, but so what it's a definition of random. What the OP asked for.
Now, what if someone asks if the results of a S/G apparatus (above) are random. Well what does the consensus say, is it distinguishable from coin flipping? If not then it's random, if it is how so.
 
  • #109
PeroK said:
The theory of QM predicts this and suggests that there is no further information that could possibly be available to you (hidden variables) that would allow you to predict when an electron will be spin-up and spin-down.
Unless you have/apply ALL the information in the universe?
 
  • #110
entropy1 said:
Unless you have/apply ALL the information in the universe?
One problem is that "all the information in the universe" may not be well defined. Especially if the universe is infinite.
 
  • #111
PeroK said:
One problem is that "all the information in the universe" may not be well defined. Especially if the universe is infinite.
Ok. Suppose we know ALL there is to know about the universe, EXCEPT the outcome of the spin-measurement of our electron. Would that imply that we THEN could calculate what the outcome will be? (Almost the same question, I realize)
 
  • #112
entropy1 said:
Ok. Suppose we know ALL there is to know about the universe, EXCEPT the outcome of the spin-measurement of our electron. Would that imply that we THEN could calculate what the outcome will be? (Almost the same question, I realize)
I don't believe that is a well defined question. But, to take it at face value, I don't know. There is nothing in physics that demands that we would know the outcome in advance. And QM suggests that we couldn't necessarily know the outcome.

Just to be clear: it's the assumption that ultimately all uncertainty can be, theoretically at least, swept away that I believe is wrong.
 
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  • #113
I was wondering, if variable A is random, it is said we cannot predict its outcome. However, can't we really? We can't for a single instant, however we can or should very well for the distribution of a set of generated outcomes; that is the principle of probability theory, right? Maybe this is contrary to situations where we can predict a single outcome, but not a distribution, like when I drive through town and I can predict when I will go right or left, but not what the ratio between right or left will be. Anyway, in case of our variable A, the distribution of outcomes should be pretty predictable, right?
 
  • #114
entropy1 said:
Ok. Suppose we know ALL there is to know about the universe, EXCEPT the outcome of the spin-measurement of our electron. Would that imply that we THEN could calculate what the outcome will be? (Almost the same question, I realize)

According to quantum mechanics, no. There is nothing in the far reaches of the universe or in the details of subatomic particles that would allow you to predict the result of a measurement, in general. (There are certainly cases where the result is predictable, but in many cases, it is not.)
 
  • #115
stevendaryl said:
According to quantum mechanics, no. There is nothing in the far reaches of the universe or in the details of subatomic particles that would allow you to predict the result of a measurement, in general. (There are certainly cases where the result is predictable, but in many cases, it is not.)
So 'who/what' is 'deciding the outcome' then?
 
  • #116
entropy1 said:
So 'who/what' is 'deciding the outcome' then?

Quantum mechanics doesn't say anything about what or who decides the outcome. So if you need an answer, then you have to have some theory that goes beyond quantum mechanics.

To say it's random is the same thing as saying that nothing decides the outcome.
 
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  • #117
stevendaryl said:
To say it's random is the same thing as saying that nothing decides the outcome.
So, if there is no decision, maybe both outcomes are real, right? Depending on the interpretation? And maybe both are real given a certain probability? (for my understanding)
 
  • #118
If randomness could be defined then it's not random.
Sort of a non sequitur
 
  • #119
rootone said:
If randomness could be defined then it's not random.
Sort of a non sequitur
I defined random in post #108. How does that definition make it not random?
 
  • #120
Zafa Pi said:
I defined random in post #108. How does that definition make it not random?
Isn't a coin flip from the Eiffel Tower random because the effect (the yieling of the result) and the cause (the flipping of the coin) have such a complicated relationship (chaoticly), that the relationship can't be described, not conceived and not even traced back that FAPP there IS no relationship between cause and effect?
 
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