# How Random Is Random ?

cyrusabdollahi said:
The fact that you cant take into account the infinite parameters affecting the consideration is exactly why it is random.
I thought you'd say exactly that as I was typing that sentence. (which also means that the human mind is not as random as it seems:surprised :rofl: :rofl: )

But the fact still remains that at least theoretically nothing is random(even with the near infinite conditions).
The diffraction phenomenon is equally the same as any other such phenomenon that requires the infinite parameters.

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BobG
Homework Helper
arunbg said:
I have always thought that the phenomena of diffraction and interference of EM waves is explained by the classical wave theory of Maxwell and Huygens constructions.Correct me if I am wrong.

Although I haven't yet studied this phenomena with an electron beam, considering what you said about the impossibility of determining where the electron eventually lands , it seems like this is only due to lack of sufficient knowledge or information regarding the nature and interactions involved in the phenomenon.
Otherwise please clarify why it is impossible to determine the coordinates
(measuring errors apart).

A word on probability distributions.
Of the very few probability distributions that I have studied (like velocities of molecules of a gas) the theories come close to predicting the actual distribution and of course it is possible to obtain the exact distributions once we take the infinite or so parameters affecting into consideration.
So there is nothing "random" about it (again measurements apart).
Even with a random dice roll, you have a pattern of distribution. There's more ways for two dice to add up to 6 or 7 than there are to add up to 12. That doesn't mean you can accurately predict that one die will be a 2 and the second a 5.

That's what cyrus is talking about with the double slit experiment (the single slit experiment is probably a better example, though). You can predict the pattern created by many photons, but you can't predict which path any given photon will take.

Even with a random dice roll, you have a pattern of distribution. There's more ways for two dice to add up to 6 or 7 than there are to add up to 12. That doesn't mean you can accurately predict that one die will be a 2 and the second a 5.
Why do we always keep returning to sqare 1 ?
Let me repeat my question a second time (3 overall).
Is anything in the universe completely random at all levels of observation ?

The outcomes of the roll of a die may appear completely "random" to a casual observer but to an expert (and hardworking) physicist, if he takes all conditions involved, the outcomes are anything but random.They can be explained by a handful of fundamental laws of physics.
The same is the case of the electron beam.

My question is not regarding the relevance of probability, rather the limit to which probability is relavent.

Astronuc
Staff Emeritus
pi-r8 said:
You seem to be saying that EVERYTHING in nature is random. Not just dice rolls, but even such apparently simple phenomenom as pendulum and planets orbiting the sun. I'd like to know why you consider those to be random.
Well, in one sense everything is deterministic, however we just can't process all the information.

On the other hand, on a die roll, things like air pressure, air resistance, the temperature of the die are variable, hence random.

Cyrus makes a good point about 'uncertainty'. It is inherent in Nature.

I think the problem one is facing is that there has to be an outcome, but is the outcome necessarily random? Statistically, one must consider a population of outcomes, and from that determine if something is random.

No, I don't think there is a complete random, when there are too many variables to consider, we conveniently put it as random or chaos.

If you look at human history and the development of science and technology, certain random and chaos in the ancient times became predictable and solvable with today's computing power as we can handle more variables. Something random at this moment in time might become predictable. So, is there a true randomness? I don't think so.

Just as the definition of "infinite", is there truly an "infinite" or is that the number is too big to comprehend, we again conveniently put it as infinite.

What about a decaying nucleus? Two nuclei that are completely identical in every single respect. One of them decays, and one of them doesn't. This doesn't follow cause and effect. And its completely random, no way of ever predicting which will decay and which won't, no matter how good your measuring instruments.

And its also my understanding in the double slit experiment that the point of detection of any individual electron/photon is completely random. All we can ever know about the particle is its wave function, and all that gives us is probabilities. But where it actually hits is completely unpredictable.

arunbg said:
Ok , so you can't measure the position and momentum of a body with absolute certainty.
But is it not quite certain that the particle indeed exists somewhere in some state which cant't be predicted through measurement or do we also have to take its dual nature into account ?
Nope, this is what heisenberg was trying to point out. Its not the case that the particle has a definate position and we just can't measure it. The particle actually doesn't have a clear position and momentum. When you take into acount the dual nature, and you represent the particle as a wave packet, in order to localize that wave packet over an increasingly small area, you must necessarily increase the uncertainty in its momentum.

Firstly, i'm totally new here so what I say may be rubbish.

Kazza_765 said:
What about a decaying nucleus? Two nuclei that are completely identical in every single respect. One of them decays, and one of them doesn't. This doesn't follow cause and effect. And its completely random, no way of ever predicting which will decay and which won't, no matter how good your measuring instruments.
I think that the notion of nucleus decay being random is not due to the decay occuring at that exact moment by chance, but rather that we don't have sufficient knowledge to determine what the trigger for nuclear decay really is.

Take the example of a random number generator from a computer. Although the numbers that come from it are seemingly random to an observer, once the algorhythm for generating the numbers is revealed, any observer with an identical computer can "predict" the next number by punching the same input conditions into an identical machine. In the same way, if we knew the causes of nuclear decay and were able to measure them, it would be possible for a complex algorhythm for predicting the nuclear decay to be set up based on the input conditions (the state of the atom).

An even more interesting point that stems from this idea is the notion of predestination. If everything in the universe followed a fixed set of rules governing their behaviour, it would mean an infinite number of universes created from an identical big bang would follow the exact same path of development.

The implications of this would be that everything in this current universe is following a path of reactions that cannot be deviated from. I am aware, however, that there is a notion of human free will that would technically create "randomness" and destroy such a predestination. However, once you look at humans as simply a complex interaction of energy and mass this notion disappears; why should the reactions between the particles in the human brain be any less predictable than the reactions of the particles inside a cooking loaf of bread?

It seems that everything is predestined?

I know that this argument is probably hugely flawed somewhere.. or everywhere! I'd love for you guys to come and tear it apart :)

Cheers,
Hanny

Go back to Schroedinger's Cat. The original experiment was designed to make sure it was random whether or not the particle decayed.

the outcome of a roll of a die is also random no computer, no matter how fast and accurate can predict what it will be before it is rolled.

rcgldr
Homework Helper
Any algorithm that calculates the "nth digits" of pi or similar constants generates random sequences, and there are programs to do this, so computers can be used to generate random sequences. Go to the link below, click on algorithms, then "nth digit ...":

constants.html

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so a chaotic pendulum has no physical value which has a measurable chance to happen?

A small contribution for the thread.

Since this is still in general discussion, there was something interesting I recall on how a mathematican considers the random nature of pi; that is it not random, but displays every pattern possible.

I feel this thread needs to be where it should.