What is spin up and spin down?

[Anonym]

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.

 

[masudr]

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?

 

[lightarrow]

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.

 

[Anonym]

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.

 

[masudr]

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.

 

[Anonym]

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.

 

[masudr]

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?

 

[Anonym]

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.

 

[masudr]

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

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?

 

[Anonym]

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.

 

[lightarrow]

[...]
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 |\Psi|^2{dA}

 

[MeJennifer]

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.

 

[reilly]

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.
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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

 

[Anonym]

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.

 

[lightarrow]

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.