Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 10/08/2019 16:58:22

Title: What is a Spin
Post by: yor_on on 10/08/2019 16:58:22
We all know that if you would translate the spin of a electron to a 'spinning top' then it would have to spin faster than light, which is a limit for anythings speed. Then we come to this  " In the not-so-recent past we delved into some of the nitty-gritty of vector bosons such as the force particles of the Standard Model. We saw that relativity forces us to describe these particles with four-component mathematical objects. But alas, such objects are redundant because they encode more polarization states than are physically present. For example, a photon canít spin in the direction of motion (longitudinal polarization) since this would mean part of the field is traveling faster than the speed of light. "  https://motls.blogspot.com/2011/10/who-ate-higgs.html

Isn't that argument flawed? That  ' a photon canít spin in the direction of motion (longitudinal polarization) since this would mean part of the field is traveling faster than the speed of light. '

I agree to that a 'spin' can't be ftl, but I find it harder to agree to that a quantum mechanical spin can't take any 'direction/polarization' it want. As this argument seems to state. A quantum mechanical spin has no classical counterpart, as far as I know?

Actually I've been wondering about that before too.
Title: Re: Spin
Post by: yor_on on 15/08/2019 12:15:42
Hmm, not sure how you think? As in a superposition falling out in two states?. If you do then you're pretty close to this https://physics.stackexchange.com/questions/154468/difference-between-spin-and-polarization-of-a-photon

It seems that you use wave theory to define the polarization, and that by doing so you reach those conclusions. But if you look at the first comment made, then " Basically if the electric field always points in plus or minus the same direction, then that's linear polarization, and it could in theory be in any direction by adjusting the relative magnitude of an x polarized one and a y polarized one (that are in phase with each other)." by Timaeus

Anna V usually have some good stuff too. What one can notice there is that " Please note that the individual photons have spin either along or against their direction of motion, while the electric fields are perpendicular. These are built up non-trivially, it is the handedness of the electric field vector ( which defines polarization classically) as it progresses in space and time that connects the electric fields to the spin direction. "
Title: Re: Spin
Post by: yor_on on 15/08/2019 12:56:07
But superpositions is a very deep question in itself which this pdf illustrate pretty good I think. You don't need to know what it means by calling electrons 'soft' or 'hard', or giving them a color to see the logic. " Superposition & the paradoxes of quantum mechanics "

https://www3.nd.edu/~jspeaks/courses/2007-8/20229/_HANDOUTS/quantum-mechanics.pdf
Title: Re: Spin
Post by: chris on 15/08/2019 14:15:12
Can you edit the title of this post to make it a question please.

Thanks
Title: Re: What is a Spin
Post by: yor_on on 15/08/2019 15:32:15
There should be a possibility of just changing the header Chris? But I didn't find it.
Title: Re: What is a Spin
Post by: evan_au on 15/08/2019 23:33:20
Quote from: yor_on
There should be a possibility of just changing the header Chris? But I didn't find it.
Just edit your original post, and the title is editable to change the title of the thread.

Quote from: esquire
fermion matter adopts a 1/2 spin as a measure instrument. it is a tool to gauge mass traveling at the speed of light.
Electrons are spin=1/2 fermions, but they can never be accelerated up to the speed of light, as that would require infinite energy.
Scientists know this, because the LEP (https://en.wikipedia.org/wiki/Large_Electron%E2%80%93Positron_Collider) (a predecessor to the LHC, in the same tunnel) managed to accelerate electrons and positrons very close to the speed of light, but could not reach c.

Quote
gravitational force has a spin factor of 2, it is also not subject to velocity.  its spin 2 motion factor exceeds that of a spin 1 motion.
How do you account for the detection of a neutron star merger, where the gamma rays and gravitational waves arrives almost simultaneously (<2s apart) over a travel time of around 130 million years.

This shows that photons and (hypothetical) gravitons travel at (almost) exactly the same speed.

Note that the alignment here is between the "ringdown" phase of the gravitational waves and the start of the gamma-ray burst.
- The fact that gravitational waves were detected for 100 seconds prior to the merger reflects the fact that these two neutron stars had been circling closer and closer for millions of years.
- It is only in the last 100s before merger that the gravitational wave signal was intense enough for us to detect with current equipment
- It is only in the first 2 seconds after the merger that the gamma ray signal was intense enough for us to detect with current equipment

See: https://en.wikipedia.org/wiki/GW170817
Title: Re: What is a Spin
Post by: yor_on on 16/08/2019 14:15:21
Esquire, I have difficulties following your thoughts. They seem more fitting to 'New Theories', in where you first would need to define how this 'linear friction' created by a mass in a uniform motion (in a vacuum) creates a classical spin. What I'm discussing is 'spin' as described from a main stream perspective. The only use I have for a 'classical spin' here is to lift it up and then recognize that a quantum mechanical spin has no classical counterpart.
Title: Re: What is a Spin
Post by: alancalverd on 17/08/2019 11:39:15
As with all things quantum mechanical, you can't assume macroscopic properties from the name. We know an electron behaves in a magnetic field "as if" it were indeed spinning, but the fact that its "spin" is quantised tells us that it isn't really like a classical spinning top.

Beware of "charm" and "strangeness"!
Title: Re: What is a Spin
Post by: Colin2B on 17/08/2019 18:25:12
There should be a possibility of just changing the header Chris? But I didn't find it.
Sorry @yor Iíve been away and just catching up. Open your first post and go to drop down on top right, select modify and you can change the subject. When you save every post will have its subject changed

Esquire, I have difficulties following your thoughts. They seem more fitting to 'New Theories', in where you first would need to define how this 'linear friction' created by a mass in a uniform motion (in a vacuum) creates a classical spin. What I'm discussing is 'spin' as described from a main stream perspective. The only use I have for a 'classical spin' here is to lift it up and then recognize that a quantum mechanical spin has no classical counterpart.
Agreed. The posts are misleading and should be in new theories, when I get a moment Iíll move them.

The point Alan makes is important:
As with all things quantum mechanical, you can't assume macroscopic properties from the name.
Quantum physicists are very fond of analogies, and labels which donít mean the same same as the classical world, and which certainly shouldnít be taken literally.


Title: Re: What is a Spin
Post by: jeffreyH on 17/08/2019 23:17:09
"For example, a photon canít spin in the direction of motion (longitudinal polarization) since this would mean part of the field is traveling faster than the speed of light."

If we look at this simplistically then the path of any oscillation would describe a velocity faster than light in any orientation, for the photon. This shows that the quantum mechanics of the photon is not a classical phenomena. You cannot ascribe a velocity to a quantised angular momentum.

Forget classical mechanics.
Title: Re: What is a Spin
Post by: yor_on on 20/08/2019 14:47:24
Np Collin :)

Just want to recommend a book if one is interested in the logic's behind Quantum mechanics.
"Quantum Mechanics and Experience" by David Z Albert

It's a puzzle in which he describes it using a awful lot of logic, and some mathematics to support the building :)

" This book was written both as an elementary text and as an attempt to add to what we presently understand, at the most
advanced level, about what seems to me to be the central difficulty at the foundations of quantum mechanics, which is the difficulty about measurement. The first four chapters are a more or less straightforward introduction to that difficulty: Chapter 1 is about the idea of superposition, which is what most importantly distinguishes the quantum mechanical picture of the world from the classical one, and which is where everything that's puzzling about quantum mechanics comes from. Chapter 2 sets up (in a way that presumes nothing at all, insofar as I understand how to do that, about the mathematical preparation of the reader) the standard quantum-mechanical formalism and outlines the conventional wisdom about how one ought to think about that formalism. Chapter 3 is about the Einstein-Podolsky-Rosen argument and how that argument was stunningly undercut by Bell (and it is urged there, by the way, that what Bell's discovery actually amounts to is very frequently misunderstood; it is urged that Bell discovered something not merely about hidden-variable theories but also about quantum mechanics, and also about the world). Finally, Chapter 4 explicitly sets up the measurement problem. "
Title: Re: What is a Spin
Post by: yor_on on 20/08/2019 15:05:11
And yes Jeffrey, that sentence was what made me write. The guy writing it is very good, and usually have a clear way of expressing and presenting his thoughts, but that one just didn't make sense to me.
Title: Re: What is a Spin
Post by: Colin2B on 22/08/2019 09:03:49
Just want to recommend a book if one is interested in the logic's behind Quantum mechanics.
Yes, Iíve read it.
Starts off ok, but later muddies the water by introducing a degree of unnecessary mysticism (as if there wasnít enough in QM - unnecessary mysticism that is) and his own Ďmany mindsí interpretation into measurements.
I also wish he had kept to describing properties as actual properties rather than inventing artificial properties, just confuses the beginner moving forward.
Iím not aware that his many minds is taken seriously except by philosophers.
Title: Re: What is a Spin
Post by: yor_on on 22/08/2019 17:40:35
Ahh, haven't got to that yet. I liked the beginning though, and 'many worlds' are to me just as mystical. What's worse with 'many worlds' is that if you believe in that one it doesn't really matter what happens to you, it will be okay anyway, somewhere else. It's like 'spreading your genes' but now in a ever more bifurcating universe(s). Never heard of many minds so it will be interesting.
=

You could think of it as physics answer to a all-seeing 'God'. Personally It hurts my head thinking of bifurcations bifurcating, ad infinitum, Clifford.

Had to clean up the syntax a little too :)
Title: Re: What is a Spin
Post by: CPT ArkAngel on 29/08/2019 20:06:30
A possible solution is that the spin is directly the angular moment (or rotational moment) and the polarization is its interactions with the vacuum field. Here, the vacuum field would represent the electric field of the surrounding charges in relation to the (relative) wavelength of the photon. A kind of spin networks. This would explain a lot...

A linearly polarized beam of light in the Y-axis is totally stopped by a polarizer in the X-axis, but if you introduce a polarizer at 45 degrees in-between, some photons pass through as the square of the cosine of the angle between the polarizers.
Title: Re: What is a Spin
Post by: yor_on on 29/08/2019 22:33:19
No CPT, if that was the answer a electron would be spinning faster than light. It's not a angular momentum.
Title: Re: What is a Spin
Post by: CPT ArkAngel on 30/08/2019 00:20:13
Why are you so sure of this? The charge could appear as a point or a dot and the mass-energy could be rotating within a Compton wavelength. What is measured is the charge. There is no experiment proving the contrary. Where does motion come from? Add a little of Relativity and you can go quite far... The official size of the electron doesn't mean anything, if you consider what is measured and how it is measured. In the case of the proton, you have 3 charges that is why you can get a size. If you have only one, how can you do it if you can't keep it at rest? You can't really touch it. It's all indirect measurements...
Title: Re: What is a Spin
Post by: yor_on on 30/08/2019 01:46:01
I first saw it presented a long time ago, and I'm sure I wrote about it, somewhere?

" The physical interpretation of Pauli's "degree of freedom" was initially unknown. Ralph Kronig, one of Landť's assistants, suggested in early 1925 that it was produced by the self-rotation of the electron. When Pauli heard about the idea, he criticized it severely, noting that the electron's hypothetical surface would have to be moving faster than the speed of light in order for it to rotate quickly enough to produce the necessary angular momentum. This would violate the theory of relativity."

https://physics.stackexchange.com/questions/170345/why-does-the-electron-spin-with-a-particular-tilt

Notice "  That's because if you "measure" the component in any direction whatsoever you always get Īℏ/2, so clearly you aren't measuring a pre-existing component of some preexisting vector. If there were a preexisting vector what would happen if you measured the component orthogonal to that vector? You can't get zero since you always get Īℏ/2. What you are really doing is polarizing the spin so that later it will give the same result on future measurements as what you just got."
=

I'll add this for the history of it. https://medium.com/nakshatra/how-the-works-of-schrodinger-pauli-and-dirac-brought-about-the-existence-of-antimatter-258f1adfd334
Title: Re: What is a Spin
Post by: yor_on on 30/08/2019 02:24:59
Well, you can 'touch' it. That's what they did at Lunds university where they 'photographed it'. Which is another mystery actually as it's supposed to be 'spread out', unless you measure it. https://phys.org/news/2008-02-electron.html
Title: Re: What is a Spin
Post by: CPT ArkAngel on 30/08/2019 05:36:29
Yes I remember, we already discussed about that a while ago. I can't find any other info on the image of the electron. Even the video on youtube is unavailable. It looks like a spinfoam. For the electron spin measurement, the problem is the measurement itself. The electron needs to be entangled with the detector to be measured... The spins correspond to the angular momentum of the electron mass rotating at the speed of light with a Compton wavelength. The electron has at least 2 perpendicular components (as all elementary particles), so it might be a part of the problem. 
Title: Re: What is a Spin
Post by: Jarek Duda on 30/08/2019 08:38:12
Spin of electron and of photon are very different - only the former has accompanied magnetic dipole moment, which leads e.g. to Larmor precession, or much more sophisticated acrobatics like spin echo: for nucleus in NMR, for electron in pulsed EPR: https://en.wikipedia.org/wiki/Electron_paramagnetic_resonance#Pulsed_electron_paramagnetic_resonance

(https://upload.wikimedia.org/wikipedia/commons/9/97/GWM_HahnEchoDecay.gif)
Title: Re: What is a Spin
Post by: yor_on on 30/08/2019 12:34:36
I think the short history is pretty enlightening, especially the equation of Paul Dirac needing four degrees of freedom, although a electron spin only presents two of them, the other two later found to belong to its anti particle (positron.)
=

You can find the movie here:  ftp://ftp.aip.org/epaps/phys_rev_lett/E-PRLTAO-100-004808/Mauritsson.mov
Title: Re: What is a Spin
Post by: Colin2B on 30/08/2019 22:20:47
I can't find any other info on the image of the electron. .......It looks like a spinfoam.
Please donít introduce new theories into this section. Thanks.

For the electron spin measurement, the problem is the measurement itself. The electron needs to be entangled with the detector to be measured...
If you mean entanglement as in Ďquantum entanglementí, then no because neither electron nor detector share a common history. If, on the other hand you mean that the measurement depends on the interaction of electron with detector, then I agree.

The spins correspond to the angular momentum of the electron mass rotating at the speed of light with a Compton wavelength.
As has been pointed out before, nothing is spinning. Physicists love analogy and illustrative ideas, but taking them literally is full of pitfalls eg wave/particle duality.
In the case of electron spin the measurement is very simple as it is associated with a magnetic moment; it requires no entanglement, just pass them through a magnetic field and see which way they curve.
Title: Re: What is a Spin
Post by: Jarek Duda on 31/08/2019 10:13:49
As has been pointed out before, nothing is spinning.
Indeed, seeing spin as spinning charge is unsuccessful, for example it cannot be stopped in particles.
However, imagining it as "internal twist of configuration" - its topological charge, seems to make more sense.
Here are some topological charges of vector field:
(https://i.imgur.com/NhnUyCa.png)
Quantum rotation operator says that rotating spins 's' particle by theta angle, the phase rotates by 'theta * s' - exactly as for above topological charges.
They are realized e.g. in fluxons/Abrikosov vortex ( https://en.wikipedia.org/wiki/Fluxon ) in superconductor - corresponding to quants of magnetic field ... spin of particle comes with magnetic dipole.
Title: Re: What is a Spin
Post by: CPT ArkAngel on 31/08/2019 19:44:31
QM is probabilistic. The measurement is not integrated in it in anyway. Without the correct interpretation, you are stuck. The problem is you make the same mistakes most physicists do. Your interpretation is magical. There is no randomness in the universe. What QM says is that all particles are related to all other particles. Then you understand that randomness is just due to a lack of knowledge and imagination... It does not belong to new theories, it is what any good theorist does, looking for better solutions... Entanglement is everywhere. It is apparent in ultra cold physics. QCD proves that QM needs a more realistic interpretation, simply because it goes beyond QM.
Title: Re: What is a Spin
Post by: CPT ArkAngel on 01/09/2019 21:14:47
Just to correct myself.

The measurement is so well integrated in QM that you have no idea what is its role in the wave equation. Like others said, QM is not really a theory but rather an operating system contained in a black box, more precisely a dark grey box. You merely perceive the surface...
Title: Re: What is a Spin
Post by: Colin2B on 01/09/2019 22:46:26
It does not belong to new theories,
Spinfoam belongs in New Theories, not in this section of the forum. Please keep it there.
See https://www.thenakedscientists.com/forum/index.php?topic=66954.0