Light, Waves, Particles etc

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Re: Light, Waves, Particles etc

#101  Postby zaybu » Oct 07, 2011 1:54 pm

twistor59 wrote:
zaybu wrote:


I'm not asking how it is represented mathematically, but what is its physical nature.


I'm not sure if we can say that it has a "physical nature", at least not in general, and not directly, it's a component in a model. Using this component you can construct observables, with which you can make (probabilistic) predictions about measurable quantities.


And those measurable quantities are mass, charge, spin, parity. Never mind that the initial assumption is that we have particles in the model.

This reminds me of Boltzman who postulated that a gas was made of atoms and proceeded to change the whole field of thermodynamics. He never saw atoms, and atoms were only confirmed years after his death. Yet, during his lifetime, he was ridiculed for believing that atoms were real. I guess you would have been one of these people laughing at him.
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Re: Light, Waves, Particles etc

#102  Postby Teuton » Oct 07, 2011 2:52 pm

hackenslash wrote:
Teuton wrote:
This means that operators aren't physical quantities (properties) themselves but mathematical representations thereof.

Still getting all your definitions from the dictionary of what colour is my lint.
http://hyperphysics.phy-astr.gsu.edu/hb ... moper.html
http://en.wikipedia.org/wiki/Operator_% ... _Operators


"…The other sorts of mathematical objects which we shall need to know something about are operators.
Operators are mechanisms for making new vectors out of old ones. An operator on a vector space, more particularly, is some definite prescription for taking every vector in that space into some other vector; it is a mapping (for those readers who know the mathematical meaning of that word) of a vector space into itself."


(Albert, David Z. Quantum Mechanics and Experience. Cambridge, MA: Harvard University Press, 1992. p. 25)

Mappings are functions, functions are abstract mathematical objects, and as such they aren't concrete physical objects.
(See: http://mathworld.wolfram.com/Map.html)
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Re: Light, Waves, Particles etc

#103  Postby Teuton » Oct 07, 2011 3:08 pm

twistor59 wrote:
Teuton wrote:But physical reality cannot be reduced to mere possibilities or probabilities. The real properties of spacetime points or regions must be actual determinate properties.

Emphasis mine. But that is the very heart and soul of quantum theory. A system might or might not possess a definite value of such a quantity until a measurement is made. In the cases were it does not possess a definite value, quantum theory is not making definite predictions about individual systems, but rather about ensembles of systems.


Physical reality is physical actuality and not a "semireal" limbo or twilight zone of mere possibilities, potentialities, or probabilities. Physical objects or systems must have some physical properties, since nothing can exist propertylessly; and I don't know what it means to say that they don't have definite, determinate properties unless physicists have measured them, i.e. that unmeasured physical reality swims in a sea of indeterminacy or vagueness. I doubt that there are and can be indeterminacies and vaguenesses in the world that are ontological rather than semantic. That is, I doubt that vague predicates or concepts represent real properties that are vague themselves.
(For example, is it even coherent to say that an object has a mass but none of the possible determinate properties belonging to that determinable property, e.g. a mass of exactly 3kg?)

twistor59 wrote:It depends on the point of view you take - maybe the spacetime point itself is an abstraction ....


Actually, I do think that spacetime points and point particles are mathematical abstractions or idealizations. (But I believe in the physical reality of non-zero-dimensional spacetime regions and particles.)
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Re: Light, Waves, Particles etc

#104  Postby twistor59 » Oct 07, 2011 3:11 pm

zaybu wrote:
twistor59 wrote:
zaybu wrote:

I'm not asking how it is represented mathematically, but what is its physical nature.


I'm not sure if we can say that it has a "physical nature", at least not in general, and not directly, it's a component in a model. Using this component you can construct observables, with which you can make (probabilistic) predictions about measurable quantities.


And those measurable quantities are mass, charge, spin, parity. Never mind that the initial assumption is that we have particles in the model.

This reminds me of Boltzman who postulated that a gas was made of atoms and proceeded to change the whole field of thermodynamics. He never saw atoms, and atoms were only confirmed years after his death. Yet, during his lifetime, he was ridiculed for believing that atoms were real. I guess you would have been one of these people laughing at him.


I really don't understand where you get this impression that I don't believe in particles. Could you quote a post from me which states or implies this ...
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Re: Light, Waves, Particles etc

#105  Postby Teuton » Oct 07, 2011 3:15 pm

cavarka9 wrote:
That depends on how small can one consider a particle to be and how its intrinsic properties are. Perhaps particles can only come in groups, that too could be the case. Hence I used the word structure. Which could in other cases be used to consider say strings or other mathematical objects, they too are structures.


If particles are 0-dimensional and strings are 1-dimensional, then they lack an interior, i.e. then they are (internally) structureless.
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Re: Light, Waves, Particles etc

#106  Postby twistor59 » Oct 07, 2011 3:17 pm

Teuton wrote:
twistor59 wrote:
Teuton wrote:But physical reality cannot be reduced to mere possibilities or probabilities. The real properties of spacetime points or regions must be actual determinate properties.

Emphasis mine. But that is the very heart and soul of quantum theory. A system might or might not possess a definite value of such a quantity until a measurement is made. In the cases were it does not possess a definite value, quantum theory is not making definite predictions about individual systems, but rather about ensembles of systems.


Physical reality is physical actuality and not a "semireal" limbo or twilight zone of mere possibilities, potentialities, or probabilities. Physical objects or systems must have some physical properties, since nothing can exist propertylessly; and I don't know what it means to say that they don't have definite, determinate properties unless physicists have measured them, i.e. that unmeasured physical reality swims in a sea of indeterminacy or vagueness.


It means that certain pairs observables are such that a system can't possess definite values of both observables simultaneously. If my system has a definite value of A, then B is indeterminate. The best I can do is say that if I have an ensemble of identically prepared systems, all with the same fixed value of A, then those systems will have a distribution of values of B. Quantum theory places some limits on how narrow that distribution can be.
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Re: Light, Waves, Particles etc

#107  Postby Teuton » Oct 07, 2011 3:21 pm

"Was the world wave function waiting for millions of years until a single-celled creature appeared? Or did it have to wait a little longer for some more highly qualified measurer—with a Ph.D.?"

(Bell, J. S. "Quantum Mechanics for Cosmologists." In: J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, 2nd ed., 117-138. Cambridge: Cambridge University Press, 2004. p. 117)

8-)
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Re: Light, Waves, Particles etc

#108  Postby twistor59 » Oct 07, 2011 3:32 pm

Teuton wrote:"Was the world wave function waiting for millions of years until a single-celled creature appeared? Or did it have to wait a little longer for some more highly qualified measurer—with a Ph.D.?"

(Bell, J. S. "Quantum Mechanics for Cosmologists." In: J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, 2nd ed., 117-138. Cambridge: Cambridge University Press, 2004. p. 117)

8-)


That quote is talking about the measurement process, sometimes misleadingly called "wavefunction collapse". The generally (but not universally) accepted modern view on this is that it takes place via the process of decoherence, and thus does not require an "observer". It just needs the system to be a bit leaky.
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Re: Light, Waves, Particles etc

#109  Postby zaybu » Oct 07, 2011 4:52 pm

twistor59 wrote:
zaybu wrote:
twistor59 wrote:

I'm not sure if we can say that it has a "physical nature", at least not in general, and not directly, it's a component in a model. Using this component you can construct observables, with which you can make (probabilistic) predictions about measurable quantities.


And those measurable quantities are mass, charge, spin, parity. Never mind that the initial assumption is that we have particles in the model.

This reminds me of Boltzman who postulated that a gas was made of atoms and proceeded to change the whole field of thermodynamics. He never saw atoms, and atoms were only confirmed years after his death. Yet, during his lifetime, he was ridiculed for believing that atoms were real. I guess you would have been one of these people laughing at him.


I really don't understand where you get this impression that I don't believe in particles. Could you quote a post from me which states or implies this ...


Well, you started this thread in a debate that started on another thread, in which you wrote: "I would prefer to refer to photons as "field quanta", and reserve the term "particle" for entities that had position operators." After which I pointed out in QFT, the position operators are fields, and from there we were arguing about fields being "particle exchanges", which you disagree that we only see in and out states, which I questioned why we should even pursue the Higg boson, a so-called virtual particles not to be found in any in or out states, but then you claimed we should do our outmost to see a free Higgs boson, which would be an out state. So I really don't know where you stand. You've been skirting around the issue.

For me, the Standard Model is very clear: there are two types of "particles": 1) fermions with half-integral spins, which make up what we consider as bulk matter; 2) bosons, with integral spin that mediate the known four basic forces of nature (fields). My claim from the very beginning has been that when we explore nature at smaller and smaller scales, what we see is particles. You disagree, what exactly, I really don't know.
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Re: Light, Waves, Particles etc

#110  Postby campermon » Oct 07, 2011 5:07 pm

Has anyone mentioned 'wavicles' yet?

:ask:
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Re: Light, Waves, Particles etc

#111  Postby Teuton » Oct 07, 2011 5:58 pm

campermon wrote:Has anyone mentioned 'wavicles' yet?
:ask:


How about "dunnos"? ;)
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Re: Light, Waves, Particles etc

#112  Postby cavarka9 » Oct 08, 2011 10:41 am

campermon wrote:Has anyone mentioned 'wavicles' yet?

:ask:


some one mentioned strings.
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Re: Light, Waves, Particles etc

#113  Postby cavarka9 » Oct 08, 2011 10:43 am

Teuton wrote:
cavarka9 wrote:
That depends on how small can one consider a particle to be and how its intrinsic properties are. Perhaps particles can only come in groups, that too could be the case. Hence I used the word structure. Which could in other cases be used to consider say strings or other mathematical objects, they too are structures.


If particles are 0-dimensional and strings are 1-dimensional, then they lack an interior, i.e. then they are (internally) structureless.


only one of those could be the ultimate units, am very busy right now for few days. so will miss the threads
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Re: Light, Waves, Particles etc

#114  Postby twistor59 » Oct 08, 2011 11:44 am

zaybu wrote:
twistor59 wrote:
zaybu wrote:

And those measurable quantities are mass, charge, spin, parity. Never mind that the initial assumption is that we have particles in the model.

This reminds me of Boltzman who postulated that a gas was made of atoms and proceeded to change the whole field of thermodynamics. He never saw atoms, and atoms were only confirmed years after his death. Yet, during his lifetime, he was ridiculed for believing that atoms were real. I guess you would have been one of these people laughing at him.


I really don't understand where you get this impression that I don't believe in particles. Could you quote a post from me which states or implies this ...


Well, you started this thread in a debate that started on another thread, in which you wrote: "I would prefer to refer to photons as "field quanta", and reserve the term "particle" for entities that had position operators." After which I pointed out in QFT, the position operators are fields,


But fields and position operators are two completely different things.

zaybu wrote:
and from there we were arguing about fields being "particle exchanges", which you disagree that we only see in and out states, which I questioned why we should even pursue the Higg boson, a so-called virtual particles not to be found in any in or out states, but then you claimed we should do our outmost to see a free Higgs boson, which would be an out state. So I really don't know where you stand. You've been skirting around the issue.

For me, the Standard Model is very clear: there are two types of "particles": 1) fermions with half-integral spins, which make up what we consider as bulk matter; 2) bosons, with integral spin that mediate the known four basic forces of nature (fields). My claim from the very beginning has been that when we explore nature at smaller and smaller scales, what we see is particles. You disagree, what exactly, I really don't know.



OK, if I haven't made myself clear, I'll try again:

1) This started with a discussion of photons. In the sentence you quote, I was trying to make the point that photons often display behaviour which is not particle-like at all. You can discuss localization of an electron, by which I mean that you can define a position operator, and talk about position eigenstates (Newton Wigner). You cannot do the same for photons. "Particle" is not an ideal term for either really, but it's less ideal for massless spin 1 bosons than for the other cases.

2) You objected that discussion of single photons in terms of wavefunctions has been superseded by the treatment of photons in QED. I counter objected that the usual QED treatments do not tell the whole story - they are fine for computing scattering cross sections in particle physics, but not so useful for quantum optics - in these scenarios it still helps to think in terms of wavefunctions.

3) At some point, to tro...err, to liven up the debate a bit, I threw in the fact that I didn't believe that VIRTUAL photons necessarily represented elements of reality.

So, this is a summary of my viewpoint:

* "Particle" is a bit of a crappy term because it suggests little bullets. "Quantum" is better because it sounds mysterious, and these critters are mysterious. However, we're stuck with "particle". So be it.
* Photons, electrons, W, Z, quarks etc are real entities. Their properties can be measured.
* Photons aren't as easy to localize as electrons.
* VIRTUAL (meaning internal lines in Feynman diagrams) photons, electrons, W, Z, quarks etc are not "elements of reality". At least I think they're not - they're only present because we do perturbation theory, which is just an approximation technique.
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Re: Light, Waves, Particles etc

#115  Postby Teuton » Oct 08, 2011 1:06 pm

cavarka9 wrote:
Teuton wrote:
If particles are 0-dimensional and strings are 1-dimensional, then they lack an interior, i.e. then they are (internally) structureless.

only one of those could be the ultimate units, am very busy right now for few days. so will miss the threads


0D objects and 1D objects exist only in the abstract realm of mathematics. Concrete physical objects are (at least) 3D objects.
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Re: Light, Waves, Particles etc

#116  Postby twistor59 » Oct 08, 2011 1:41 pm

Teuton wrote:
cavarka9 wrote:
Teuton wrote:
If particles are 0-dimensional and strings are 1-dimensional, then they lack an interior, i.e. then they are (internally) structureless.

only one of those could be the ultimate units, am very busy right now for few days. so will miss the threads


0D objects and 1D objects exist only in the abstract realm of mathematics. Concrete physical objects are (at least) 3D objects.


How do you know this ?
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Re: Light, Waves, Particles etc

#117  Postby Teuton » Oct 08, 2011 2:41 pm

twistor59 wrote:
Teuton wrote:
0D objects and 1D objects exist only in the abstract realm of mathematics. Concrete physical objects are (at least) 3D objects.
How do you know this ?


That's a conceptual truth. 0D, 1D, and 2D objects, i.e. points, lines, and surfaces, are geometrically ideal boundaries of physical objects but not physical objects (substances) themselves. They are too "thin", too insubstantial to be physical objects (substances); they aren't bodies (in the most general sense of the word, including nonsolid bodies).

"Elementary particles in the ordinary view of things are point particles. A point can’t have many, many properties. A point is too simple to have properties. However, we know that elementary particles have a lot of properties. They have spin, they have electric charge, they have something called isotopic spin, they have a quantum number called color - it’s not got anything to do with ordinary color - they have generations that they belong to, there are whole catalogs of different kinds of quantum numbers, of different kinds of properties that quarks, electrons, netrinos, or photons have. It sounds unreasonable for a point to have that structure. So the feeling most of us have is that, at some level, if you look deeply enough into things, you‘ll discover that particles aren’t points. That they must have all kinds of internal machinery that gives them these properties."

(Leonard Susskind, interview by George Zarkadakis, April 27, 2009. Feline Quanta Blog. http://felinequanta.blogspot.com/2009/04/interview-with-leonard-susskind.html.)
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Re: Light, Waves, Particles etc

#118  Postby Teuton » Oct 08, 2011 3:10 pm

Teuton wrote:
0D objects and 1D objects exist only in the abstract realm of mathematics. Concrete physical objects are (at least) 3D objects.


This means that if there are atoms in the literal sense of the term, i.e. truly fundamental particles, then they must be simple, partless (componentless/constituentless) 3D atoms. Of course, many ask how it is possible for a 3D object to be spatially partless when any (nonzero) volume of space can be partitioned into smaller (nonzero) subvolumes. This is a good question and the only good answer might come from the theory called loop quantum gravity, according to which there are indivisible atoms of space that cannot be partitioned into any smaller subvolumes.

"According to loop quantum gravity, space is made of discrete atoms each of which carries a very tiny unit of volume. … One consequence of this is that there is a smallest possible volume. This minimum volume is miniscule – about 1099 of them would fit into a thimble. If you tried to halve a region of this volume, the result would not be two regions each with half that volume. Instead, the process would create two new regions which together would have more volume than you started with. We describe this by saying that the attempt to measure a unit of volume smaller than the minimal size alters the geometry of the space in a way that allows more volume to be created."

(Smolin, Lee. Three Roads to Quantum Gravity. New York: Basic Books, 2001. p. 106)

So, if a fundamental 3D particle occupies exactly one space-atom, then one can consistently and intelligibly say that it lacks spatial parts.
(It might be the case that the particle-atoms are ontologically reducible to the space-atoms so that the latter would be the fundamental physical objects.)

If there are no 3/4-dimensional particle-atoms or space/spacetime-atoms, then matter or space/spacetime is "gunky", i.e. then it is infinitely divisible and every part of it has proper parts ad infinitum. (This means that "gunky" stuff isn't ultimately composed of simple point-sized objects, which lack proper parts.)
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Re: Light, Waves, Particles etc

#119  Postby Teuton » Oct 08, 2011 3:37 pm

"Quantum mechanics is, at least at first glance and at least in part, a mathematical machine for predicting the behaviors of microscopic particles — or, at least, of the measuring instruments we use to explore those behaviors — and in that capacity, it is spectacularly successful: in terms of power and precision, head and shoulders above any theory we have ever had. Mathematically, the theory is well understood; we know what its parts are, how they are put together, and why, in the mechanical sense (i.e., in a sense that can be answered by describing the internal grinding of gear against gear), the whole thing performs the way it does, how the information that gets fed in at one end is converted into what comes out the other. The question of what kind of a world it describes, however, is controversial; there is very little agreement, among physicists and among philosophers, about what the world is like according to quantum mechanics. [my emph.] Minimally interpreted, the theory describes a set of facts about the way the microscopic world impinges on the macroscopic one, how it affects our measuring instruments, described in everyday language or the language of classical mechanics. Disagreement centers on the question of what a microscopic world, which affects our apparatuses in the prescribed manner, is, or even could be, like intrinsically; or how those apparatuses could themselves be built out of microscopic parts of the sort the theory describes."

(http://plato.stanford.edu/entries/qm/)

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Re: Light, Waves, Particles etc

#120  Postby twistor59 » Oct 09, 2011 8:19 am

Teuton wrote:
twistor59 wrote:
Teuton wrote:
0D objects and 1D objects exist only in the abstract realm of mathematics. Concrete physical objects are (at least) 3D objects.
How do you know this ?


That's a conceptual truth. 0D, 1D, and 2D objects, i.e. points, lines, and surfaces, are geometrically ideal boundaries of physical objects but not physical objects (substances) themselves. They are too "thin", too insubstantial to be physical objects (substances); they aren't bodies (in the most general sense of the word, including nonsolid bodies).

"Elementary particles in the ordinary view of things are point particles. A point can’t have many, many properties. A point is too simple to have properties. However, we know that elementary particles have a lot of properties. They have spin, they have electric charge, they have something called isotopic spin, they have a quantum number called color - it’s not got anything to do with ordinary color - they have generations that they belong to, there are whole catalogs of different kinds of quantum numbers, of different kinds of properties that quarks, electrons, netrinos, or photons have. It sounds unreasonable for a point to have that structure. So the feeling most of us have is that, at some level, if you look deeply enough into things, you‘ll discover that particles aren’t points. That they must have all kinds of internal machinery that gives them these properties."

(Leonard Susskind, interview by George Zarkadakis, April 27, 2009. Feline Quanta Blog. http://felinequanta.blogspot.com/2009/04/interview-with-leonard-susskind.html.)


That sounds like philosophy speak. Even the Susskind quote. Statements like "a point is too simple", "It sounds unreasonable".. point to speculation, not science.

We started with "concrete physical objects are (at least) 3D objects". The problem is with the word "are". I don't know how philosophers interpret it, but "are" for me in this context could be replaced by "is modelled by". So I would mean that a physical object can be put into a correspondence with a 3D mathematical object. The model would be a good one, deserving of the term "are" if the mathematical object could then be used to predict accurately the behaviour of the physical object.

With this definition it is not 100% clear that physical objects have to "be" at least 3D.
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