# How fast does fire travel?

Discussion in 'Physics & Math' started by Stryder, Sep 26, 2002.

1. ### chrootCrackpot killerRegistered Senior Member

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By the way, this is hilarious:

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- Warren

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3. ### James RJust this guy, you know?Staff Member

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Frencheneesz,

Your idea that a particle might "really" be in one of three different states and vary between them very rapidly, rather than being in a superposition of the three states (as is the quantum picture), is a type of <b>hidden-variable</b> theory.

Similarly, a person in favour of hidden variables might explain the quantum 2-slit effect by saying "the particle really goes through one or the other slit. It has a hidden variable which tells it which slit to go through. We can't measure that hidden variable, but the process is deterministic nonetheless."

Sounds good, doesn't it? The trouble is that hidden variable theories have been ruled out by experiments. Quantum mechanics cannot act through hidden variables. There's a large body of theory dealing with the problem - try looking up the "Bell inequalities".

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5. ### FrencheneeszAmazing MemberRegistered Senior Member

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"can yo' ass tell da difference betwizzeen random, 'n hella hard to predict?"

LOL! Thats hella funney.

"Otherwise, you cannot have randomness "

Right, but I'm saying that it isn't random, just hard to predict.

"Observe that they each decay (typically) into a muon neutrino and an electron. Observe that the time when each decays is random, and obeys a very precise Poisson distribution."

I doubt that you can OBSERVE that something is random. You can OBSERVE that something is unpredicatable, and that can be for many reasons. Also, as you have said, the muon is composed of more fundemental particles (the muon nutrino and electron), and I'm pretty sure that those two particles would not be traveling at the same relative speed, but would be orbiting around eachother probably at somewhat high speeds. The unknown of the relative speeds of the muon nutrino and the electron INSIDE the muons boundaries is in itself enough to cause the system to be unpredictable and look random.

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7. ### FrencheneeszAmazing MemberRegistered Senior Member

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James R:

"Sounds good, doesn't it? The trouble is that hidden variable theories have been ruled out by experiments. "

How has that theory been ruled out by experiment? I took a quick look at "bell inequalities" and was reminded of a quantum idea that when you observe a particle it "collapses" into a state with only one state. That type of thinking seems absured to me. After all, the universe does not know when we are looking at it.

If you scoff at my "belief" that the universe is deterministic, then I don't see any reason why you would embrace superpositions that disappear as soon as we look at it.

8. ### chrootCrackpot killerRegistered Senior Member

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Well, more specifically, hidden variables have been ruled out by the mathematics of QM, which is regarded as being the most successful scientific theory ever discovered. Bell's work simply illucidated the issue. Mathematically, there cannot be any hidden variables.
One view has the support of experimental evidence, the other does not.

- Warren

9. ### FrencheneeszAmazing MemberRegistered Senior Member

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"there cannot be any hidden variables. "

I think a hidden variable might be something a bit different than what I was describing. My description had nothing to do with hidden thing... I think. Could you describe what a hidden variable was?

"One view has the support of experimental evidence, the other does not. "

By definition, superposition "disappears" or "collapses" as soon as it is observed, therefore, it cannot have ANY experimental evidence.

10. ### On Radioactive Waveslost in the continuumRegistered Senior Member

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sounds like a paradox

11. ### James RJust this guy, you know?Staff Member

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<b>Frencheneesz:</b>

<i>...the muon is composed of more fundemental particles (the muon nutrino and electron), and I'm pretty sure that those two particles would not be traveling at the same relative speed, but would be orbiting around eachother probably at somewhat high speeds.</i>

Actually, a muon is a kind of heavy electron. It is not made up of anything more fundamental. As far as we can tell, it is a point particle, just like an electron. It does not in any way contain a neutrino or an electron.

<i>How has that theory been ruled out by experiment?</i>

There was a very clever experiment involving entangled photons, performed in France by Alain Aspect and collaborators. (Search for "Aspect" along with "Bell inequalities" for more information.)

<i>I took a quick look at "bell inequalities" and was reminded of a quantum idea that when you observe a particle it "collapses" into a state with only one state. That type of thinking seems absured to me. After all, the universe does not know when we are looking at it.</i>

It depends what you call an observation. Few people seriously think a human observer is required to collapse a quantum state. Observation has a wider meaning than that.

<i>I think a hidden variable might be something a bit different than what I was describing. My description had nothing to do with hidden thing... I think. Could you describe what a hidden variable was?</i>

A hidden variable is a particle's way of knowing what it should do when measured. For example, suppose two electrons are entangled so that if one has spin "up", the other has spin "down". We separate the electrons by several light years, then look at one of them. Suppose it is spin up. Then somebody observing the other one will instantly find that the other one is spin down, even though no signal has had time to travel from one to the other at the speed of light.

A hidden variable explanation of this would be that before we separate the electrons, one of them gets the hidden variable label "spin up", and the other gets the "spin down" label. We don't know which one we'll get until we look, but it is set from the beginning. The quantum explanation says something different. It says that neither of the electrons actually has a definite spin direction until one of them is measured; when that happens, the quantum state collapses so that both are opposites. However, the outcome is totally random as to which is up and which is down.

As I said, the hidden variable picture is ruled out by experimental results. The quantum picture, though much weirder, is correct.

<b>chroot:</b>

<i>Well, more specifically, hidden variables have been ruled out by the mathematics of QM, which is regarded as being the most successful scientific theory ever discovered. Bell's work simply illucidated the issue. Mathematically, there cannot be any hidden variables.</i>

Not quite right. Bell's mathematics predicts the results of certain experiments both with and without hidden variables. It provides a way to see if reality involves hidden variables or not. The experiments have been done, and they match Bell's predictions for the case of no hidden variables. So, mathematically, there <b>could</b> be hidden variables, but we know from experiment that quantum mechanics doesn't work that way in reality.

12. ### thedIT GopherRegistered Senior Member

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Age is irrelevant but it does help understand the level of knowledge and background some one has.

I remember first hearing about QM and not accepting it at the same age. Only after I studied it formally some years later did I see the real power of it.

QM may be non-deterministic at its heart but it is very predictive.

They are, they are. The leading edge of theoreticians are trying to Unify them. String theory was an early attempt.

Brian Green was an early worker on String Theory, almost a founder. His work was to show that vibrational 'strings' could produce particle like properties.

There's the difference, you don't believ it. I strongly suspect you have not studied it in detail only relying on pop-sci write up's. Try,

http://scienceworld.wolfram.com/physics/topics/EarlyQuantumMechanics.html

For a very good introduction. You'll note that this site mentions that the results of QM provide philosphically intersting results that are still debated strongly. Like this thread.

I disagree. If you have a particle of known mass in a known gravitational field and you know it's velocity/acceleration vectors you can predict accurately where the particle will be at any time. OK, in real complex systems there may be complicating factors. But again, it is possible to predict where things will be. I am ignoring inherently chaotic systems at present, that's another story.

In QM this is inherently impossible.

Read the link I gave above. That is one possible philosophical interpretation.

What you descibe is the explanation for how electrons escape atoms to become electrical conductors. Tunneling is different. The particle simply has a probability of existing outside the well.

Again, Physics understands this and you have to quote the limiting errors in an experiment. All possible sources of error have to be considered. If a result is not as expected the first place you look is systematic errrors in equipment.

Physics means precise +/- errors of equipement.

Uncertainty is not like that. In classical physics you can measure postition and momentum to an arbitrary precision. Uncertainty says you can not. If you knew position to a high level of precision (within equipment error) know hardly anything (or nothing) about momentum.

Give me a better definition of non-random? If it's not predictable surely it is random or, at best, chaotic.

What you are describing is equipment resolution. Or that a sufficiently fast oscilation of states appears continuous. Again, not the same thing as random and it cannot explain things like discrete spectra.

I'll let JamesR and Chroot tackle Hidden Variables as I'm not as clued up on that. All I know is a friend with a greater grasp of this than me tried to formulate a hidden variables theory 13 years ago. He should be getting his certificate of sanity any day now

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13. ### thedIT GopherRegistered Senior Member

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Not even close I'm afraid. Try reading http://particleadventure.org for a description of how fundamental particles behave and the terminology.

Just because a high energy particle decays to other particles doe snot mean it is composed of those particles. For exmaple a neutron decays to neutrino, electron and proton. Yet an electron and proton combines to a neutron without the neutrino.

Similar, a photon can decay to a electron/positron pair and vice versa. Yet electrons are leptons obeying fermi stats and photons are neither leptons or hadrons and are bosons. They are radically different particles.

In short, fundamental particles may have no internal structure and yet still decay to other fundamental particles.

14. ### thedIT GopherRegistered Senior Member

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Two things, the universe is deterministic at the Macro level we live in. All we are saying is that our common sense does not apply to the microscopic world. Secondly is a deep philosophical point,

If a man is talking in a forest where woman can not here him, is he still wrong?

Of course the Universe does not know we are looking at it.

15. ### chrootCrackpot killerRegistered Senior Member

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Mm... kinda. Beta decay is: neutron -> proton + e<sup>-</sup> + antineutrino.

Inverse beta decay is: e<sup>-</sup> + proton -> neutron + neutrino.

The emission of a neutrino is essentially the same as the capture of an antineutrino.

- Warren

edited for back'ards use of [ ] brackets

Last edited: Nov 5, 2002
16. ### chrootCrackpot killerRegistered Senior Member

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Good catch. One proves via experiment, not by math. I've grown too accustomed to the phrase "Bell's inequalities prohibit hidden variable theories."

- Warren

17. ### FrencheneeszAmazing MemberRegistered Senior Member

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"As far as we can tell, it is a point particle, just like an electron. It does not in any way contain a neutrino or an electron."

If it decays into smaller particles, how is it possible not to be composed of them?

"Few people seriously think a human observer is required to collapse a quantum state. Observation has a wider meaning than that."

What would that wider meaning be?

"It says that neither of the electrons actually has a definite spin direction until one of them is measured"

Why the heck would anything change when it is measured? I'm pretty convinced that the "measuring" has nothing to do with it, but it is for some other reason that measuring might induce, if it exists at all that is.

"OK, in real complex systems there may be complicating factors."

An atom is quite complex enough, right? We seem to have no concept of the electrons speed, we say it doesn't exist, so I think that would change our predictions by a large factor if we just ignored the electrons.

"Just because a high energy particle decays to other particles doe snot mean it is composed of those particles. "

I think that might be in violation of the laws of conservation. Especially the following statement: "For exmaple a neutron decays to neutrino, electron and proton. Yet an electron and proton combines to a neutron without the neutrino."

"Two things, the universe is deterministic at the Macro level we live in. "

That cannot be true if the universe is random.

18. ### thedIT GopherRegistered Senior Member

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Note to self, always double check facts before posting with 3 screaming kids in room. Simple errors are stupid.

19. ### chrootCrackpot killerRegistered Senior Member

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A pi-zero can spontaneously decay into two gamma-ray photons. Each of those photons can go on to spontaneously produce an electron-positron pair. Does the pi-zero contain gamma-ray photons? Do the gamma-ray photons contain electrons and positrons?

It seems on the subatomic scale, particles can literally be transmuted, sometimes spontaneously, sometimes via interaction with other particles. Despite the common notion that light and matter are different, to a particle physicist, it's all the same stuff. It just happens that photons have zero rest mass. They might not be the only such particles.

In any case, there is no experimental justification for the belief that a particle somehow "contains" its decay products.
"Observation of a system" is any interaction which causes some change in the external world. A happenstance photon that hits one member of an entangled pair is enough to "collapse" any superposition of the pair.
You're welcome to convince yourself of anything you'd like, but you might benefit from a bit more thought before you seal your own coffin. I'm a local realist myself, at least on macroscopic scales. Perhaps I've just been cultured to thinking that's the way things should be. In any event, the mathematical model of a particle existing in a superposition of states until measured is a successful one. Whether or not the philosophical implications reach far enough to really describe Schroedinger's cat as being both alive an dead in the box is an issue of active debate.
Yes, thed was missing the lowly neutrino. In any event, the laws of conservation are not really laws in QM -- everything obeys the Heisenberg uncertainty principle, and systems can, for example, violate energy conservation for a predictably brief amount of time.
On macroscopic scales, the deviations of QM predictions from classical predictions are neglible. For example, a baseball thrown into the air can, for all intents and purposes, be regarded as having a definite momentum and position at all times. Quantum mechanics says we actually can't know both precisely, only to a very very high (but finite) precision. The allowed precision is so enormous that we literally can't detect the Heisenberg uncertainty principle operating at macroscopic scales.

I know you don't like those kinds of concepts, but you're already used to at least one: the incredibly small size of atoms. In your everyday life, you don't interact with anything that would depend upon the properties of individual atoms. Your body, the food you eat, the baseball you throw -- they all have so many tiny atoms that you only experience a continuum.

- Warren

20. ### thedIT GopherRegistered Senior Member

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The point I was trying to make is that life in the world of particle physics is vastly different to what we see in everyday reality.

When neutron decay was first measured they found that the masses of the electron and proton combined, where less than the mass of the neutron. This apparently defies conservation of energy. To redress the situation it was postulated that a new particle was created. One that interacted with matter very weakly and had low mass. The Neutrino.

But there is more going here than meets the eye. A neutron is a hadron made of 3 quarks, or a so called baryon. Internally to the neutron it has 2 down quarks and 1 up quark held together by 3 gluons. No sign of either the electron or anti-neutrino. When the neutron decays one down quark converts to an up quark which releases energy from the system. This is in the form of an electron and anti-neutrino and the neutron turns into a proton. <a href="http://particleadventure.org/particleadventure/frameless/npe.html">Full details here</a> as I'm not about to go into a full discussion of quantum chromodynamics/quantum electrodynamics and weak currents.

Common sense says that these fundamental particles are like solid balls that 'stick together' and then come apart. Reality at this level is very, very different to what you expect.

Why not? Once you get interactions of more than a few particles the randomness appears to smooth out into determinstic behaviour. Understanding why is one goal of modern physics.

21. ### FrencheneeszAmazing MemberRegistered Senior Member

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"Despite the common notion that light and matter are different, to a particle physicist, it's all the same stuff. It just happens that photons have zero rest mass. "

I can see this type of concept in terms of string theory, but not in terms of QM. With QM, you say that particles just change randomly, where as in string theory, the waving of the strings would follow quite different rules and allow "transmutations" for certain string interactions.

""Observation of a system" is any interaction which causes some change in the external world. A happenstance photon that hits one member of an entangled pair is enough to "collapse" any superposition of the pair."

Ok, that makes much more sense. In that case, what phisically happens to a particle when it "collapses" and why would it do this if it is "hit" by a particle?

"You're welcome to convince yourself of anything you'd like, but you might benefit from a bit more thought before you seal your own coffin."

I think in the above quote you made, you explained to me quite clearly that my statement is justified. Not by observation does a superposition "collapse" but through an interaction with a particle which may be induced by "observation".

"violate energy conservation for a predictably brief amount of time."

How so? Why only for a bit? I seem to be edging toward doubting this statement.

"Quantum mechanics says we actually can't know both precisely, only to a very very high (but finite) precision."

But we can know both to a very high precision? Really? Well... that sounds quite like what I was arguing. It STILL seems to me like the uncertainty principle has everything to do with precision and very little to do with randomness. EVERY time someone talks about this principle, they always mention precision. If the uncertainty principle actually worked the way you argue, there should be a definition excluding references to precision.

"The allowed precision is so enormous that we literally can't detect the Heisenberg uncertainty principle operating at macroscopic scales."

The differences between macroscopic and microscopic precision is very small. Macroscopic objects have no "point" where we can say it is because it covers such a large area. The only advantage macroscopic objects have in measurement is that it doesn't get knocked around as much when hit by something used to measure.

22. ### FrencheneeszAmazing MemberRegistered Senior Member

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"Once you get interactions of more than a few particles the randomness appears to smooth out into determinstic behaviour. "

Ok, I see your point. It would work that way sinse the many particles "randomness" would average out to be deterministic.

I can accept that particles COULD have randomness, maybe. BUT I cannot accept that "things" in general are random. I could accept that the fundemental particles, however many there are, would have certain probabilities of doing this or that. But what I cannot accept is that there are no fundemental particles with fundemental randomness and everything has its own completely independent probabilities.

To me, you guys are saying that this cup (composed of millions of atoms) has a 1% probability of spontaniously cracking. What I THINK makes more sense is saying that the cup has 1% chance of spontaniously breaking BECAUSE the atoms inside have a 1% chance of doing something which is caused by individual random basic particles. My idea is that all particles act independantly, therefore, the reason radioactive decay is "random" should depend on the probabilities of the particles composing the atom to be at certain places in certain states at the same time, not just arbitrary randomness of an entire atom.

23. ### chrootCrackpot killerRegistered Senior Member

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What makes you think QM prohibits it? Eh?
The unobserved particle is in a superposition of (perhaps many) quantum states. When the particle is observed, it chooses one of the possible quantum states, at random. If you observe a large number of identical systems each undergoing the same observation, you will see a probability distribution emerge that is related to the probability amplitudes associated with each possible change of state. We cannot say WHY the particle chooses one state or the other -- given the exact same starting conditions, a particle simply chooses either one or the other. QM does not tell you WHY the particle does what it does -- QM predicts the outcome of experiments. There is no experiment that can tell you WHY a particle does what it does.
Ok, we just differ in our definitions of "to measure."
Energy and time are non-commuting observables (the phrase "non-commuting" refers to a mathematical relationship between the two operators that correspond to the observables 'energy' and 'time'). You cannot know both precisely. If you know energy precisely, you cannot know precisely the time period during which the particle has that energy. As a result, a system can violate energy conservation for a period of time such that

delta-t * delta-E > h-bar / 2 * pi

I don't understand why you'd "edge toward" disbelief. After all, quantum mechanical tunnelling (which depends upon the energy-time uncertainty) is utilized to great effect in everything from tunnel diodes to lasers to Josephson junctions. In fact, our understanding of the rate of the proton-proton chain (by which the Sun liberates the majority of its energy) is wholly dependent on the concept of quantum mechanical tunnelling. The fact that we understand tunnelling so thoroughly that we can build technological devices to use it is an indication, that, well, it really happens.
Well, it's rather difficult to define uncertainty in non-mathematical terms without using the word 'precision.' In any event, uncertainty has nothing to do with the inadequacies of human-built measurement equipment. Even a theoretically perfect measuring device will not be capable of giving you a precise momentum and position of an electron at the same time. Quantum mechanics dictates a fundamental maximal precision, and man-made machines impose additional error. The two are not the same! I've explained this like three times now. Quit acting like you don't understand it!
No, precision is precision. The statement "you cannot know the position of [insert object] any better than within two angstroms" is a statement independent of scale. It has radically different consequences at different scales, however. If you can't know the position of your baseball any more specifically than within two angstroms, you won't likely even notice. You won't likely even be able to build a machine to notice.

On the other hand, if you can't know the position of your electron any more specifically than within two angstroms, well, you don't know a hell of a lot about it at all.

- Warren