What is spin up and spin down?

  • #51
Are you familiar with the Bohmian interpretation of QM?
If not, I believe you might like it.
For a somewhat wider context see also
http://arxiv.org/abs/quant-ph/0609163
Thanks for the link and it basically says it all. I'm not nearly as frustrated with QM as Zapper thinks I am. The real fact is that I'm tired of everyone making all sorts of claims as fact that just can't be known as such. People talk about entangled particles communicating faster than light, yet its not proven. Honestly I'm not even sure what experiment you'd concoct that would prove such a claim. How would you measure the communication rate to be faster than light when we can't possibly measure something faster than light.

I've never said QM wasn't very cool, only that we try to make it a philosophical debate instead of accepting that we just don't know.

Thanks for the link though, at least my opinion seems to be shared by one person... :-)

glenn
 
  • #52
ZapperZ
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Ok, I see your point. And just for the record, its not that I'm picking on QM. Its more that when trying to ask what I would think are simple questions I get 20 different answers, and most sound more like double talk than an actual answer. I actually like learning QM's, I just get frustrated sometimes with the lack of understandings and yet the non-ending flow of philosophical ideas...
That's nothing! Try asking if light has mass. You'll get varying interpretation also. This occurs in many places in physics and not just restricted to QM. Yet, it doesn't bother physicists very often because we know the "physics", rather than the human-language-interpretation of physics. You seem to be confusing between the two. People can disagree with what such-and-such means, but they don't disagree about that such-and-such. They are not the same thing!

The problem here is that you are trying to learn about physics, not physics itself. When you do that, of course you will run into varying approaches and interpretation. Compound that to the fact that you are not seeing the actual physics - all you get are handwaving descriptions. That's like seeing only the shadow of a cow during different times of the day. You're getting confused that it looks differnt each time you come back to take a look at it, when in actuality, you should have been paying attention to the cow itself!

If you want to understand physics, then learn physics like the rest of us. There's no shortcut. But don't fool yourself into thinking you are learning physics when you are actually learning ABOUT physics.

Zz.

Zz.
 
  • #53
If you want to understand physics, then learn physics like the rest of us. There's no shortcut. But don't fool yourself into thinking you are learning physics when you are actually learning ABOUT physics.

Zz.

Zz.
Boy, I thought I was learning... lol. I guess asking questions or not completely understanding a subject at its start was by passed in your study. I wish I could be so lucky but it takes time for me. I have to read, ask questions, get answers and repeat. Sometimes when answers don't make sense I have to ask them again. I just hope you're not a teacher... :-)

I will say, I've asked a couple different questions on this board and you seem to be the only one that hunts me down and gets right to the "You don't know anything" remarks. I guess we can't all be a smart as you, but its obvious your intelligence doesn't help any in communication skills.

As I've repeated to you multiple times, I'm not attacking QM's. I'm just trying to figure out things such as spin, entanglement and so forth. When the answers don't make sense I have to ask again. This thread itself is a perfect example of the different types of answers you get to a simple question by the OP.

Please stop attacking and just answer questions. If I'm off base its ok to explain to me why, you don't have to get personal and start telling me to go to school. I'm in school, I'm just not finished yet and didn't realize I needed to wait until that day before I could ask questions or share my opinion... right or wrong.

glenn
 
  • #54
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The problem here is that you are trying to learn about physics, not physics itself.
But if you learn something ABOUT physics, as a byproduct you may also learn a part of physics itself, perhaps even a pat that otherwise you would never learn by directly learning only physics itself. In fact, the creators of science called physics probably started with thinking ABOUT physics.
 
  • #55
ZapperZ
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Boy, I thought I was learning... lol. I guess asking questions or not completely understanding a subject at its start was by passed in your study. I wish I could be so lucky but it takes time for me. I have to read, ask questions, get answers and repeat. Sometimes when answers don't make sense I have to ask them again. I just hope you're not a teacher... :-)

I will say, I've asked a couple different questions on this board and you seem to be the only one that hunts me down and gets right to the "You don't know anything" remarks. I guess we can't all be a smart as you, but its obvious your intelligence doesn't help any in communication skills.

As I've repeated to you multiple times, I'm not attacking QM's. I'm just trying to figure out things such as spin, entanglement and so forth. When the answers don't make sense I have to ask again. This thread itself is a perfect example of the different types of answers you get to a simple question by the OP.

Please stop attacking and just answer questions. If I'm off base its ok to explain to me why, you don't have to get personal and start telling me to go to school. I'm in school, I'm just not finished yet and didn't realize I needed to wait until that day before I could ask questions or share my opinion... right or wrong.

glenn
And you don't see what I was 'attacking'? You didn't just ASK a question. If I were to attack people who just ask, then I'd be all over the place. You did more than just that. You liberally sprinkled your posts with definitive statements of what you think of it is based on faulty knowledge. You even showed no hesitation to make blatant characterization of people who have responded to you as either not understand of what they're explaining, or wanting to HIDE the info. THIS is what I asked you to stop.

If you do not understand something, ask. If the answer still isn't fully clear, ASK some more. But to then proclaim that no one understands light is absurd. That would qualify as an overly-speculative opinion, the same way you are drawing up your conclusion about me based on the flimsy "evidence" that you gathered on here.

I would also suggest that you study it NOT by starting from the philosophical interpretation. It is my strong opinion that THAT is the wrong starting point, because you will start it with already a point of view in mind. If this is how you wish study science, then I guess that rather explains what has transpired here.

Zz.
 
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  • #56
ZapperZ
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But if you learn something ABOUT physics, as a byproduct you may also learn a part of physics itself, perhaps even a pat that otherwise you would never learn by directly learning only physics itself. In fact, the creators of science called physics probably started with thinking ABOUT physics.
I'm not so sure about that, since I can't tell what the "creators of science called physics" were thinking way back then. And I certainly do not doubt that one can get some superficial knowledge of physics by learning about physics. My point is that they are not the same thing. You learn more about physics by reading pop-science books than learning physics. If they are the same thing, we could simply dump Griffith's QM text and get the student to just read Gribbin's "In Search of Schrodinger's Cat" book. It would certainly be a lot more entertaining.

The problem here is not about these sources of information. The problem is the perception that these "about physics" knowledge is sufficient and equivalent to "physics" knowledge. I'm sure you have seen many instances on the internet where people who have read only books like Hawking's "Brief History of Time" somehow now proclaim to have ample knowledge to talk about General Relativity and Cosmology. While I have often encourage people to read such books, I have never encouraged those people to have the delusions that such books have actually taught them physics. You do not teach someone by popping out things out of nowhere with no logical series of steps. And certainly, you cannot teach physics via handwaving arguments exclusively.

The OP's question of what is spin up and spin down is perfectly valid. I also think that you and others on here have sufficiently answered that question. However, this thread has deteriorated into a question of defending QM and such philosophical issues.

Zz.
 
  • #57
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Mind you that spin comes into play in QM not necessarily in connection with the Dirac equation.
The necessity to introduce the spin came out in order to explain the experimental results (Zeeman effect). It is known as the Pauli theory of spin. It is usually mentioned in the connection with the non-relativistic limit of the Dirac theory of electron. The only detailed textbook discussion of it known to me is given by V.A. Fock, “Foundations of quantum mechanics”, 2nd edition, 1976. This is last book written by V.A. Fock and it seems that there is no English translation. Pauli obtain the correct magnetic moment of the electron (g=2). The electron is described in terms of two component spinors only (no negative energy solutions, no sea and no Zitterbewegung present). Pauli invention of the electron spin breaks the algebraic structure of the 2-dim complex Hilbert space, the mathematical framework of the standard QM (Pauli introduced the 4-dim C2 Clifford algebra). The Dirac theory of the electron uses the 16-dim C4 Clifford algebra. It is worth to mention that P.A.M. Dirac was never satisfied with that.

Regards, Dany.
 
  • #58
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ZapperZ, I completely agree with what you say above. Still, I have to emphasize that a boundary between "physics" and "about physics" is not strict. For example, a reader who read the physics textbook of Mesiah or Schiff could say that the Griffith's textbook you mention above is not completely physics but something between physics and about physics.
 
  • #59
reilly
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Originally Posted by lightarrow View Post
To me, it's not so sad the fact to study QM at graduate level, it's said the fact it wasn't taught so well to make me understand what is physics and what is mathematics (maybe it just depends on the teachers I had at university).
Judging from what some other people says, I would say I'm not the only one with this problem.

A typical example is this: a very low energy photon is emitted from a light source and hits a screen and we detect a bright pointy flash on it.
Question: how can you prove *experimentally* that the photon has travelled from the source to the screen, if you destroy it in such a measure? So, how can you prove to me that the photon *physically* and not mathematically, really exist from source to screen?

I still haven't received a satisfying answer to this question from the many physicists I have asked it. They answer that the photon exist the same; I wonder how then they worry about QM paradoxes, if they talk about "gosts" and not about physical objects.

Some says that physics is not simply what we can measure. Ok, so, what is physics exactly? What does "to exist" mean in physics?
Why they don't write this on books? How can people hope to understand completely QM if they "seem" not to understand what that exactly mean?


Excellent objections! :approve:
I believe that physicists avoid talking about these things for two reasons:
1. They do not know the answers.
2. If they dare to talk about it, they will be accused for being too philosophical rather than scientific.

First, most of us have had lousy teachers -- so, you have to study harder and read everything about the subject.

About the photon path: How do you know that a pulsed radar signal actually travels back and forth from transmitter to, say, airplane, back to the receiver? The problem you state is not restricted to QM.

If you look at the history of physics, you will see that the assumption of light/photon propagation is the best game in town, for both classical and quantum physics.(Recall Hughens.) If you don't like that assumption, then put it to the test. Move the screen around, say directly toward the source:you will find experimental evidence that the photon does indeed propagate as advertised.


Demystifier: Your comment about "being philosophical" simply is not true, unless you can cite an example or two. And your comment about lack of understanding is also not true, which I believe I've demonstrated. Also, consider lasers; you can see the propagation of photons with your very own eyes.

Regards,
Reilly Atkinson
 
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  • #60
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I'm sure it's fairly well known that to some extent, particles do not follow paths (in the QM formalism, particularly the Feynman formalism), instead they have probabilities that they would follow (and therefore have followed) a certain path; and in a very real (or complex.. haw haw haw) sense, the probability has a contribution from every possible path between those points.

In that sense, it doesn't make sense to say a photon took this path between two points. The formalism tells us every possible path somehow contributed to it. Whether or not the mathematical formalism is to be taken seriously in this context, however, is up to philosophers, is it not?
 
  • #61
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I'm sure it's fairly well known that to some extent, particles do not follow paths (in the QM formalism, particularly the Feynman formalism), instead they have probabilities that they would follow (and therefore have followed) a certain path; and in a very real (or complex.. haw haw haw) sense, the probability has a contribution from every possible path between those points.
Wrong. Consider the coherent wave packet (minimum uncertainty state). It will move exactly as described by Reilly. If you refuse to “see the propagation of photons with your very own eyes” it is your personal problem.

In that sense, it doesn't make sense to say a photon took this path between two points. The formalism tells us every possible path somehow contributed to it. Whether or not the mathematical formalism is to be taken seriously in this context, however, is up to philosophers, is it not?
Not. In that sense, it does make sense for those that know and understand the mathematical formalism (it was discussed and demonstrated in details by E. Schrödinger and W. Heisenberg).

Regards, Dany.
 
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  • #62
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Consider the coherent wave packet (minimum uncertainty state). It will move exactly as described by Reilly. If you refuse to “see the propagation of photons with your very own eyes” it is your personal problem.
And what if we consider non coherent wave packets? How will they move? Is that also my personal problem?
 
  • #63
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First, most of us have had lousy teachers -- so, you have to study harder and read everything about the subject.
Probably you have answered this a million times: which QM books do you consider up to date and very good to begin?
About the photon path: How do you know that a pulsed radar signal actually travels back and forth from transmitter to, say, airplane, back to the receiver? The problem you state is not restricted to QM.
A pulsed radar signal is made of many photons, so it can be revealed along its path putting small detectors in space, without affecting it considerably.

As I said in my previous post, my problem is not about photons in general and not even with a single photon, if it has a considerable energy; in this case it "becomes a particle" in the sense I said: its path can be traced without affecting it considerably.
My problem is with single photons of such a low energy that you are forced to destroy them in the detection.
Maybe you could argue that it's not sensible to state the existence/non existence of photons on their energy only, however this is exactly my doubt.
I would say more: maybe the lower the energy, the more those properties we are measuring don't belong to that particle alone but to the entire system particle/measuring apparatus? And the greater the energy, the more that particle has an intrinsic existence?
Your comment about "being philosophical" simply is not true, unless you can cite an example or two. And your comment about lack of understanding is also not true, which I believe I've demonstrated. Also, consider lasers; you can see the propagation of photons with your very own eyes.
Of course you weren't answering me here because it wasn't me to make those comments.
Regards.
 
  • #64
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And what if we consider non coherent wave packets? How will they move? Is that also my personal problem?
Yes. In the physical problem that we discuss now (“a very low energy photon is emitted from a light source and hits a screen and we detect a bright pointy flash on it”, see above), I have no idea why you want to consider non coherent wave packets.

Regards, Dany.
 
  • #65
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Perhaps; what about a particle propagating in general?

In reilly's description, I thought he was talking about the general case of "what happens between measurements" and saying how it should not be relegated to philosophy. In that sense, I took his proposition in the most general case possible: a particle propagating from A to B.

This is why I want to consider non-coherent wave packets: because I enjoy discussing general physics; not that limited to very special cases.
 
  • #66
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This is why I want to consider non-coherent wave packets: because I enjoy discussing general physics; not that limited to very special cases.
Non-coherent wave packets are fine, but what wrong with very special cases?

Suppose you leave your low energy department and enter high energy school. Everybody is non-coherent. Consider for simplicity a friend with a beautiful name: charm. He is point-like, certainly not described as a coherent state. He hates free propagation and likes to be voluntarily confined. He enthusiastically participates in all fundamental interactions. In order that you will not be boring, he is surrounded by crowd family of spin ½ brothers and spin 1 sisters and all together play for you such a concert that you will immediately forget everything about philosophy and general physics just trying to catch what happens around you. I am sure that you will enjoy that company too.

Regards, Dany.
 
  • #67
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You see, Dany, I was responding to reilly. You conveniently didn't consider the part of my post discussing how I was responding to the problem of "what occurs between measurements" by taking the Feynman formalism seriously.

Exactly what problem you have with that, or why you are even telling me I'm wrong, or even that you have any problem with that at all, I don't know.

Like, what exactly are we discussing here? That Gaussian wave packets maintain their shape as they propagate? Or that the Feynman formalism is not to be taken as providing a serious description of nature? Or are we discussing nothing at all?
 
  • #68
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Like, what exactly are we discussing here? That Gaussian wave packets maintain their shape as they propagate? Or that the Feynman formalism is not to be taken as providing a serious description of nature? Or are we discussing nothing at all?
We discuss here a question:” What is spin up and spin down?”. In addition, we discuss Reilly’s statement related to that:” Move the screen around, say directly toward the source: you will find experimental evidence that the photon does indeed propagate as advertised… you can see the propagation of photons with your very own eyes.” I add that Gaussian wave packets maintain their shape and trajectory according to QM as well as to CM.

The statement that the Feynman formalism is not to be taken as providing a serious description of nature I see first time in my life. After checking second time your post #60 I came to conclusion that you discussing nothing at all.
 
  • #69
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The statement that the Feynman formalism is not to be taken as providing a serious description of nature I see first time in my life.
OK, so you do take that formalism seriously. Then when I said

masudr said:
In that sense, it doesn't make sense to say a photon took this path between two points. The formalism tells us every possible path somehow contributed to it.
why did you disagree? From what I know, this is precisely what the Feynman formalism indicates. Perhaps I am wrong on that. Am I?
 
  • #70
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You conveniently didn't consider the part of my post discussing how I was responding to the problem of "what occurs between measurements" by taking the Feynman formalism seriously.

Exactly what problem you have with that, or why you are even telling me I'm wrong, or even that you have any problem with that at all, I don't know.

Why did you disagree? From what I know, this is precisely what the Feynman formalism indicates. Perhaps I am wrong on that. Am I?
After second thought, perhaps, you are right and my response was not adequate. Indeed, it is your question and definitely not mine. It is legitimate, but I am not qualified enough to discuss it (however, I doubt that this is precisely what the Feynman formalism indicates). Each one of us brings his/her interests and individuality into discussion.

I never was interested in questions related to the formal equivalence of the different mathematical descriptions of the same physical phenomena. For me, it is like different human languages, it is obvious that the numerous mathematical frameworks exist that describe the same thing. I even think that something wrong with Dirac demonstration of the non-equivalence of the Heisenberg and Schrödinger pictures in the relativistic QM.

Perhaps, my reaction was connected with the psychological trauma of the “childhood”. Sorry. During my PhD studies my supervisor whom I deeply respect (he was former PhD student of J. Schwinger with extensive knowledge and virtuosity in the functional analysis) was convinced that the content of my investigation is pointless since it was “proven” that it is equivalent to the standard formulation (E.P.Wigner never accept that). He required doing something much more complicated (E6 GUT enclosure into C7 Clifford algebra framework) with the physical motivation completely obscured for me. I remember that we even quarreled with him: in order to drop him from my back, I said that I am a primitive mind and not able to consider anything more complicated than the next numerical system (after that accident he call me Simplicio).

I did not even identify your question, but equally well you did not identify mine. I am relatively foreigner in the low energy region; my “natural” environment is hep-th: elementary particles physics. I came here to PF several months ago with the only purpose: to understand better the role of the coherent states.

My last paper concerns not with QM but with the wave mechanical reformulation of the classical mechanics using J. von Neumann conjecture that it is dispersion free physical theory. The astonishing and the totally unexpected result was that the basis of the corresponding Hilbert space is unique (rigid). So, what it is? It is clear that they should be Gaussians which allow the discontinuous transition from delta x*delta p > h/2 to delta x*delta p = 0. Intuitively, the most suitable candidates are the coherent states. Therefore, I am looking into QM to understand from where they come into the game. I do not think that this is trivial; in addition, that what I see eventually in the single photon/electron experiments of A.Aspect and A.Tonomura.

Regards, Dany.
 
  • #71
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[...]
If you look at the history of physics, you will see that the assumption of light/photon propagation is the best game in town, for both classical and quantum physics.(Recall Hughens.) If you don't like that assumption, then put it to the test. Move the screen around, say directly toward the source:you will find experimental evidence that the photon does indeed propagate as advertised.
Regards,
Reilly Atkinson
Ok. To perform this experiment I have to send a new photon and have a, let's say, closer to the source, screen position. But in this way the situation is different because:
1. it's not the same photon anylonger.
2. the new photon is in a different environment (different screen position).
Furthermore, the photon hits the screen in a point but we don't know where exactly; we only know the probability to find it in an area dA around point P is proportional to [tex] |\Psi|^2{dA}[/tex]
 
  • #72
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Excellent objections! :approve:
I believe that physicists avoid talking about these things for two reasons:
1. They do not know the answers.
2. If they dare to talk about it, they will be accused for being too philosophical rather than scientific.
:rofl:

But it can get worse, some physicists like to pretend they do know the answers, so obviously discussing these "philosophical' matters make them feel uncomfortable to say the least. :smile:
 
  • #73
reilly
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Let's suppose I wanted to check bullet trajectories, rather than photon ones. In either case I must repeat the experiment with new bullets of new photons; in both cases , with a wide enough screen, I'll get a hit at any distance from the source. Check out the details of statistical sampling theory; will set your mind at ease.

Coherent or not, photons propagate, but "spread differently' according to the construction of the photon packet.
********************************************88
Originally Posted by Demystifier View Post
Excellent objections!
I believe that physicists avoid talking about these things for two reasons:
1. They do not know the answers.
2. If they dare to talk about it, they will be accused for being too philosophical rather than scientific.


But it can get worse, some physicists like to pretend they do know the answers, so obviously discussing these "philosophical' matters make them feel uncomfortable to say the least.




Are there no honest physicists? What's the problem? Examples?
Regards,
Reilly Atkinson
 
  • #74
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Coherent or not, photons propagate, but "spread differently' according to the construction of the photon packet.
But the coherent states are spreaded but do not spread. That makes them very special cases. I do not know another example with that property. Do you?

Regards, Dany.
 
  • #75
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Let's suppose I wanted to check bullet trajectories, rather than photon ones. In either case I must repeat the experiment with new bullets of new photons; in both cases , with a wide enough screen, I'll get a hit at any distance from the source. Check out the details of statistical sampling theory; will set your mind at ease.
I don't understand what you mean. With bullets is completely different: I detect a bullet's position in two different points of space while it is "in fly" and then I can find its velocity, compute its trajectory and so find the exact point on the screen it will hit; and it will hit exactly that point. You can't do it with (low energy) photons.
 

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