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Post by Progenitor A on Mar 9, 2011 7:58:27 GMT 1
Of course the really interesting thing about beta-decay is that it was the continuous spectrum of the energies of the emitted beta-particle, that led to the idea that either energy wasn't being conserved, OR there was another particle that we couldn't see taking away the 'missing' energy. It had to be light, and electrically neutral, hence neutrino. But damn difficult to detect them which is why you need giant tanks of fluid deep underground............. Which then led to the solar neutrino problem, the fact that neutrinos didn't have zero mass, neutrino oscillations etc. Which tells us what? That , in common with everyone else on this board, you haven't a clue as to why the decay can be measured as a half-life, so you might just as well obfuscate by changing the subject! Perhaps you would like to explain simpler things, such as why in the equation v= Lambda x f f must be constant. ;D (Not vert nice, is it, this constant rubbishing?)
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Post by jean on Mar 9, 2011 10:09:08 GMT 1
You can see what's happening on this thread, can't you? Principled asks a 'Why?' question: If all atoms in an isotope are the same, why is it that some will decay quickly whilst others will not? Its quantum. There is no REASON why one nucleus decays now, whilst another takes another million years. It is a truly RANDOM event, and all that changes, for different elements, is the probability that any one atom will decay in the next second. STA answers, reasonably enough, that there is no answer to a 'Why'? question, which must be rephrased, and a different sort of question asked instead. But abacus naively persists: If you say there isn't a why then how can that be called science? Which enables nay to come in, finally, with what no-one has denied anyway: ...in common with everyone else on this board, you haven't a clue as to why the decay can be measured as a half-life... Even I can see that.
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Post by principled on Mar 9, 2011 11:37:42 GMT 1
STA I'd like to return to your post 21 in detail a little later. But, for now, I'm going back to basics. However, before I do let me say that I accept that QM can be counter intuitive and that for much at the sub-atomic level science has no answers, but what I am trying to do is draw the line that divides what we definitely know and from what is conjecture.
So. We have a bag of sweets that are identical except that half are green and half are red. Blindfolded, we take half the sweets and place them in a second bag. Now over "n" times probability says that we should find that each bag holds 50% green and 50% red sweets. BUT, we cannot guarantee that on any particular occasion the sweets in each bag will be 50/50. Let's replace the sweets with a bag of Beryllium isotope atoms (where green = those that decay during the first half life and red are those that remain). In this case the probability that each time we split the sample each bag will contain half the atoms that decay is 1. It is invariable. I find this most strange. We have replaced chance with certainty, so something must be going on.
I'd like to know more about the decay process. so let's take two isotopes of a similar material. Neon 19 has a half life of around 17 seconds and decays to Flourine 19. Neon 23 has a half life of 37 secs and decays to sodium 23. Could you use these two examples as a basis for a discussion on the process of decay? P
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Post by Progenitor A on Mar 9, 2011 13:14:53 GMT 1
It is indeed strange Principled No matter how many atoms there are in a sample (or your bag), the 1/2 life is still the same! Thus if we have a ton of an isotope and it takes 100 years to decay to 1/2 ton, then the 1/2 ton will also take 100 years to decay to 1/4 ton, and so on until, in the end we are left with a material that contains just 2 atoms of the isotope, and we know it will take 100 years for one of those atoms to decay (and we do not know which one). Also, a question is, how long will it take for the final atom to decay?
This process of decay occurs quite naturally in nature and is called exponential decay (indeed the exponential number e is often called the natural number) and can be expressed as
m=Me-kt where m= decayed mass M= original mass k is a constant for an isotope t is time
All manner of physical phenomena follow this law, including a clock-spring running down, the discharge of a capacitor, the melting of an ice cube, the cooling of a cup of tea. This law is sometimes expresed as the more you have the quicker you lose it. (The inverse is exponential growth which is the more you have the faster it grows!)
Someone might know why an isotope follows this law, I certainly do not. It could, I suggest, be related to temperature, and I suppose the acid test of that would be to cool an isotope to 0K and see if the decay continues!
Evidently there is a struggle going on between the strong nuclear foces that bind the nucleus together and the weak electrical forces trying to drive it apoart. In materials that do decay these forces must be about equally balanced and the nucleus must gain some extenal energy from somewhere that causes the repulsive electrical forces to escape, causing decay
But I am guessing
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Post by speakertoanimals on Mar 9, 2011 13:51:28 GMT 1
That's because it is wrong.
Its just the law of large numbers.
So suppose I have a nucleus which has a probability p that it will decay in the next second. That means there is a probability (1-p) that it will NOT decay.
If I take a sample Of N identical atoms, then I can compute the probability that a certain number (m say), and ONLY m will decay in the next second. The actual expression is a bit fiddly (how to select m examples from N comes in), but basically contains p (prob decay) to the power of m, and (1-p) to the power of (N-m).
Okay, so imagine we can do that. We can compute the probability that m atoms will decay in the next second. We can then draw a graph of probability, versus m.
Which is all very tedious! Except now the clever bit happens. As N becomes LARGE, we find that the probabilities become very strongly peaked about Np. No shit sherlock, because the definition of p as the probability that an atom will decay in the next second means that we measure it by taking the actual number from a sample size N that decay in the next second (our m), then say that p = m/N, AND take the limit as N increases without limit.
So, when it comes to lumps of matter, the point is the N is so large for even small chunks, that the law of large numbers can be applied, and for a sample size N, the number that decay in the next second will be Np.
SO for uranium, 1 mole weighs 238g, but that mole contains 10^23 atoms! So even a fraction of a gram contains a VAST number of uranium atoms.
It is also this law of large numbers that gives the exponential decay law. Basically, the proportion of atoms that will decay in the next second is fixed, just that since some have already decayed, the number that decay in the next second will be slightly less. When you put all the maths together, the exponential decay is what comes out.
Decay rates have NOTHING to do with temperature (or chemical composition) since none of that effects the energy levels of nucleons in tne nucleus, they just effect the outermost electrons in the atom, if anything.
WHY decay?
The point is that nucei hold together because being in a clump requires LESS energy than floating around free. How much different gives the binding energy per nucleon (proton or neutron). A different nucleus will have a different binding energy per nucleon. Hence if there is al allowed way for one nucleus to transform into one with a LOWER binding energy per nucleon, it will do that releasing energy in the process.
No more surprising than that atoms decay to states of lower eneregy if they can.
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Post by petergriffin on Mar 9, 2011 13:53:08 GMT 1
It is interesting to note that at least 2 Nobel laureates have been unable to give a simple explanation for the process of radioactive decay. Feynman and Gell-Man, both of which have been asked at lectures I have been to this very question. Both state it is impossible to give a simple explanation, but that is the nature of quantum mechanics, it is not deterministic by nature. It does seem like some form of black magic. But as Feynman said, just because you cannot come to terms with the way quantum mechanics works doesn’t mean it’s not so. If it doesn’t freak you out you haven’t understood the concept. If you take two atoms of neon 23 and watch them for a long time, the age of the known universe they will not decay, because you are watching them. WHY, no one knows, we do now the act of watching them causes this to happen. It’s just plan scary, but I am quite prepared to believe this will happen because its quantum mechanics it’s not some other dark force at work. (It’s mostly the uncertainty principle). Just because science cannot give me answer doesn’t make me believe there is no answer, just we haven’t found it yet. So there are some things to which there are no ways to explain a “why” sometimes we just have to take it as that’s the way it is. It’s not bollocks or just that science can’t answer the question it just is. STA and other have tried to give examples which are very good and scientifically correct, but they do give that magic simple two line explanation of how it happens, and I don’t think anyone could. If we could solve this on this board then we would all be off to Sweden to pick up our Nobel Prize.
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Post by speakertoanimals on Mar 9, 2011 14:09:39 GMT 1
Except you are still assuming there IS a why, that nature just isn't fundamentally non-deterministic -- and there is no reason, really, why it shouldn't be!
Quantum decay is rather odd. For everyday objects, we can imagine stable states (ball at the bottom of a valley), and metastable states (ball balanced in teeny dip on top of hill, but where it will decay if given a teeny nudge.
Whereas in quantum terms, we have a system like a nucleus in a dip as regards energy, yet WITHOUT any nudge, it can spontaneously, and with no sign of age or decrepitude beforehand, suddenly decide to transform into a different state.
I think this is just plain WRONG.
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Post by Progenitor A on Mar 9, 2011 14:17:12 GMT 1
It is interesting to note that at least 2 Nobel laureates have been unable to give a simple explanation for the process of radioactive decay. Feynman and Gell-Man, both of which have been asked at lectures I have been to this very question. Both state it is impossible to give a simple explanation, but that is the nature of quantum mechanics, it is not deterministic by nature. It does seem like some form of black magic. But as Feynman said, just because you cannot come to terms with the way quantum mechanics works doesn’t mean it’s not so. If it doesn’t freak you out you haven’t understood the concept. If you take two atoms of neon 23 and watch them for a long time, the age of the known universe they will not decay, because you are watching them. WHY, no one knows, we do now the act of watching them causes this to happen. It’s just plan scary, but I am quite prepared to believe this will happen because its quantum mechanics it’s not some other dark force at work. (It’s mostly the uncertainty principle). Just because science cannot give me answer doesn’t make me believe there is no answer, just we haven’t found it yet. So there are some things to which there are no ways to explain a “why” sometimes we just have to take it as that’s the way it is. It’s not bollocks or just that science can’t answer the question it just is. STA and other have tried to give examples which are very good and scientifically correct, but they do give that magic simple two line explanation of how it happens, and I don’t think anyone could. If we could solve this on this board then we would all be off to Sweden to pick up our Nobel Prize. Well done Peter! An excellent posting Because we do not know why does not mean that there is no why - that is the whole basis of the 'scientific method' Do you have corroborating evidence for the non-decay of an observed pair of neon 23 atoms? That is very strange.
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Post by abacus9900 on Mar 9, 2011 15:25:56 GMT 1
I have said this before many times and I don't care how much profanity is thrown at me, I shall state it again.
Unless you include the observer as a vital and intrinsic part of any experiment, which is simply another name for an observation, you will never understand why things often seem bizarre and unintuitive in QM. People tend to forget that any results obtained from experiments are the result of a chain of events culminating in conscious awareness. So, in the case of quantum decay we can never predict what is going to happen beforehand because something is occurring in the way the brain processes information, possibly at random, in the chain of events leading up to the actual observation. This means we should not regard quantum events as existing solely 'out there' in isolation but as part of a whole process in which we the observer are an essential part. With macro objects this effect is minimal because things tend to average out enough to makes big things behave predicably but in the microscopic world the role of the observer becomes all powerful, which is why it is pointless in trying to understand reality on the quantum level as acting separately from us.
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Post by speakertoanimals on Mar 9, 2011 15:40:25 GMT 1
Well, except that neon 23 has a measured half-life of 32s. Means that some neon 23 certainly does decay when someone is watching!
So as far as I can see, this claim that watched atoms don't decay , IF TRUE, we would have heard about it because it would overturn a centurys worth of quantum physics.......................
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Post by speakertoanimals on Mar 9, 2011 16:03:44 GMT 1
Neon 19 10 protons, 9 neutrons decays to flourine 19 (9 protons, 10 neutrons). Hence proton changes to a neutron, and positron emitted. Why? Because in general, although protons and neutrons in the nucleus both feel the strong nuclear force, which holds the nucleus together, there is a difference between neutrons and protons, in that protons are also positively charged, hence tend to repel each other. So sticking TOO MANY protons in tends to make the whole thing fall apart very quickly. WHY more than 1 proton at all? Because like electrons in atoms, nucleons in the nucleus form shells, seperate ones for neutrons and protons. Filled shells at 2, 8, 20 etc. Hence why don't just get all neutrons, because swapping a proton for a neutron when there are already lots of neutrons might meajn shifting to a higher shell, despite additional repulsive problem with protons as opposed to neutrons. So, neon 19 and fluorine, 10 protons 9 neutrons versus 9 protons, 10 neutrons. As we might have expected, fewer protons wins, hence goes from neon to fluorine. Neon 23 has 10 protons and 13 neutrons, and decays to sodium 23, with 11 protons and 12 neutrons. emits electron (negative beta decay) Why? Well, looks to me like 13 neutrons is a bit too many (remember the shells), and lower energy by switching to 11 and 12, despite extra positive charge of the new proton. If you look at this plot: en.wikipedia.org/wiki/File:Table_isotopes_en.svgyou'll see that stable nuclei are around the equal numbers of neutrons and protons at small atomic numbers, but the line drifts up towards MORE neutrons as we get heavier. Which sort of beta decay depends which side of the line we are on. So, neon 19 insights on one side, where neon 23 insights on the other. 17 versus 37 seconds -- all much of a muchness, as regards order of magnitude, trying to distinguish between 1.7 versus 3.7 is getting into fiddly details! Which is a damn lot of nuclear physics for a first go! Strange BUG here! The word I type as l i e s keeps coming out as insights!
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Post by abacus9900 on Mar 9, 2011 16:28:47 GMT 1
"The measurement problem in quantum mechanics is the unresolved problem of how (or if) wavefunction collapse occurs. The inability to observe this process directly has given rise to different interpretations of quantum mechanics, and poses a key set of questions that each interpretation must answer. The wavefunction in quantum mechanics evolves according to the Schrödinger equation into a linear superposition of different states, but actual measurements always find the physical system in a definite state. Any future evolution is based on the state the system was discovered to be in when the measurement was made, meaning that the measurement "did something" to the process under examination. Whatever that "something" may be does not appear to be explained by the basic theory." en.wikipedia.org/wiki/Measurement_problem
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Post by Progenitor A on Mar 9, 2011 18:05:45 GMT 1
Well, except that neon 23 has a measured half-life of 32s. Means that some neon 23 certainly does decay when someone is watching! So as far as I can see, this claim that watched atoms don't decay , IF TRUE, we would have heard about it because it would overturn a centurys worth of quantum physics....................... Read the statement properly! He said that if we observe 2 atoms of that isotope they never decay.
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Post by Progenitor A on Mar 9, 2011 18:10:46 GMT 1
I have said this before many times and I don't care how much profanity is thrown at me, I shall state it again. Unless you include the observer as a vital and intrinsic part of any experiment, which is simply another name for an observation, you will never understand why things often seem bizarre and unintuitive in QM. People tend to forget that any results obtained from experiments are the result of a chain of events culminating in conscious awareness. So, in the case of quantum decay we can never predict what is going to happen beforehand because something is occurring in the way the brain processes information, possibly at random, in the chain of events leading up to the actual observation. This means we should not regard quantum events as existing solely 'out there' in isolation but as part of a whole process in which we the observer are an essential part. With macro objects this effect is minimal because things tend to average out enough to makes big things behave predicably but in the microscopic world the role of the observer becomes all powerful, which is why it is pointless in trying to understand reality on the quantum level as acting separately from us. Keep on saying it! QM is a mystery that we do not understand as Feynman, Einstein, Tong, Penrose and others have said
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Post by speakertoanimals on Mar 9, 2011 18:18:37 GMT 1
I can read, it's still [snip]!
2 or 2 billion, it's still nonsense, claiming that a watched atom doesn't decay.
You're jolly unlikely to see ANY decays if you watch just two atoms of something with a long half-life, but that's just statistics, not some weird physical effect of watching stuff.
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