Stern-Gerlach experiment used to probe the clash of quantum theory and gravity | Hacker News Reader

Stern-Gerlach experiment used to probe the clash of quantum theory and gravity

quantamagazine.org

69 pointsnsoonhui33 comments

This experiment is not really "lesser known" as the article claims. It is known by everyone in physics, because it is so fundamental. And fascinatingly simple in the concept of the setup to prove a quantum effect.

To the public, perhaps it is "lesser known" but so is all that isn't CERNs LHC or Einsteins theory of relativity.

Yeah, the article says, "It was indeed one of the most important experiments in physics of all time". Presumably the "lesser known" applies to the lay public, who know schrödinger's Cat (a thought experiment, not a lab one) and maybe the two-slit experiment. The concept of quantum spin and its demonstration is less well known to non-experts. I'm certainly no expert, but the concepts of spin, probability amplitudes, and bosonic vs fermionic behavior fascinate me.

Tough call there, I'd probably rank Michelson-Morley ahead of the double slit, but could see room for disagreement.

> the lay public, who know schrödinger's Cat

Even then, the vast majority only knows the _name_ of it and something about the cat being both living and dead. I doubt many people could even tell you it is an experiment, thought or otherwise.

I learned about it in school, I think in chemistry class. So it is not only known by physicists.

It used to probe the clash of quantum theory and gravity, but it still does too.

Very true, nothing obscure about S-G. Every time you ask "What time is it?", the question is ultimately answered by a cesium beam in an S-G apparatus.

At least for now, before the optical clocks take over...

Jerrold Zacharias produced a great video about the Stern-Gerlach experiment in 1967: https://www.youtube.com/watch?v=AcTqcyv-V1I

thank you, very interesting!

Love the idea of millions of diamond atoms traveling multiple paths through the quantum realm.

I wonder what the maximum number of atoms will be 1000 years from now...

The quantum/classical limit is a function of temperature and not size, though it's generally harder to bring a large system sufficiently close to ground state to avoid decoherence.

It is not yet known what the limit actually is. Theoretically, a very hot mass of atoms that doesn't interact with its surrounding environment could still be in a superposition and display quantum effects - though it's very hard to perform such an experiment. Decoherence happens because of interactions with the environment, it doesn't happen for an isolated system as far as the theory goes. In fact, proponents of MWI believe that the whole universe, which is an isolated system, is itself a quantum system.

The separation of experiment and environment is an artificial distinction. Decoherence is caused by random phase shifts, which may appear due to interaction with the environment, but by the entropic definition of temperature, a sufficiently large and warm object will be disorderly enough to have this effect as well. You can not be hot and coherent at the same time, as the first implies a large degree of disorder and the other implies a large degree of order.

> In fact, proponents of MWI believe that the whole universe, which is an isolated system, is itself a quantum system.

MWI doesn't make any experimentally verifiable predictions as far as I'm aware, so is not particularly in this discussion.

There are no diamond atoms, just carbon. Diamond is an arrangement of carbon atoms in a particular crystal lattice.

I don't understand the experiment: even if there are only two orientation for the spin, the atom, itself, can be in any orientation?

It's a quantum degree of freedom, not one that is spatial.

You are confusing classical physics (the orientation of the atom) with quantum mechanics (the spin of the atom). The spin of the atom can only be +1 or -1, regardless of the coordinate system (i.e. orientation) you choose to measure it in.

It is comparable to the polarization of light. You can filter it in a certain direction, but it, too, is a quantum property. While light cannot pass through two 90deg rotated polarization filters. Ir can pass if you put a 45deg polarization filter between them. That can not be explained classically.

But the two lines they get on the pin-head-sized region is physical and has a particular physical orientation? How can the spin not be a physical orientation and yet the experiment result is all about the physical location of the lines?

I'm sure the issues are with the details of how the experiment is explained, but I still don't understand.

Quantum angular momentum is a different thing from the ordinary spin of a spinning top. It's not about mass revolving around a physical center. It's just there, almost as if you painted "The angular momentum is 10^-34 Joule-seconds, pointing to the left wall of the laboratory" on it.

Despite that, a magnet acts on it exactly the same as if it were a spinning piece of charged metal. So it doesn't start as a spatial difference, but it becomes one once you pass it through the field.

And one of the ways you can tell it's not the same as a spinning piece of metal is that the amount of spin is always exactly the same, regardless of how you orient the field. It's always that number I gave you above, called h-bar. The only question is whether it's positive or negative; it's going to be exactly one or the other.

That's not what would happen to a regular object. For a regular object, you'd sometimes get 100% of h-bar, and sometimes 50%, and sometimes 0%, and sometimes -100%, depending on the angle between the spin and your apparatus. Just like if you were trying to measure the width of a piece of wood with a ruler: it depends on how you angle the ruler. Somehow, for quantum things, it's always exactly 100% or -100%.

100% things go one direction; -100% things go the other direction. You get exactly two lines, separated physically in space, even though there was no such separation in the original charged particle.

It turns out that you can only measure one component of the spin vector at a time. So you choose some direction in space, call that direction the “z” direction and orient your Stern-Gerlach apparatus in that direction. Important point: SG only separates two beams along this one direction so is only sensitive to differences in the z component of spin vector. You find two final beams. Effectively the measurement has “snapped” the z component of spin onto one of two discrete values. The system is rotationally symmetric in the sense that you could have arbitrarily chosen any other direction in space to call z and the measurement results would be similar. Indeed it’s very interesting to consider the results of orienting various SG apparati in different directions and then chaining together by feeding output of one as input beam of another (as is imagined in some textbooks).

Since all theories trying to unite and reconcile quantum theory with general relativity kind of failed, I wonder if one of the two theories doesn't have some structural errors.

That far is known already: entropy and time have to be kind of bolted on to quantum theories. And there is a veritable zoo is "interpretations" of quantum theory. Meanwhile, general relativity struggles with singularities, dark energy, and dark matter.

General relativity has no problem with dark energy or dark matter. Singularities, yes. And nonrenormalizability is an issue (with trying to make it quantum).

That's right. The problem is more that it doesn't say anything useful about dark energy and dark matter. Apart from allowing us to infer their existence.

General relativity doesn't say anything useful about electromagnetism or baryonic matter either; I'm not sure you can lay an absence of an explanation of dark matter at its feet. For dark energy of course is much harder to say...

It feels natural to expect it from General relativity as the dominant force at cosmic scales is gravity. The various MOND proposals are admittedly niche, and we might eventually be able to fully explain Dark matter without such modifications.

I thought the broad assumption is that both do, but they also bith kake a wide array of predictions which are useful; unification is about subsuming the demonstrated useful predictions of each within a common explanatory framework (ideally one which also adds additional predictive power, though parsimony is a win without that).

M Theory ??https://youtu.be/plU31-qtZ78?si=G5nAmw5zp8kbaouL

And I wonder why we try to unit and reconcile them. Do we try to unite and reconcile different branch or mathematics? I get that we like to have metanarratives, but in my opinion, this might be a dead end.

The framing of "uniting" or "reconciling" QM and GR is actually bad.

The real problem is this: both QM and GR have been proven by experiments to be wrong. GR predicts entirely wrong results for the motion of electrons and other small particles. QM predicts wrong results at galactic scales.

However, they both work very very well at certain scales. So we don't want to get rid of them. Instead, we want to find some rule that tells us when to use the math of GR, and when to use the math of QM, so that we don't need to guess and just pretend that they are right.

I agree with that, but I wasn't clear I think.

You can find euclidian geometric proof of some standard arithmetic theorems, but geometry isn't the right tool to prove really complex arithmetic theorems.

Try to make euclidian geometry complete enough to resolve all arithmetic, and you will probably find yourself in front of contradictions (I stopped doing math after my bachelor, so I clearly might be wrong about that).

Why do we need to expand th9se theory? First correct the issues left within the defined context. Then find more contexts and see if you can define a new theory or use an old one?

Langlands Program.

To oversimplify, the fundamental lemma of the project posits a direct connection between the generalized fundamental representation of a finite field with its group extension to the automorphic forms under which it is invariant. This is accomplished through abstraction to higher dimensional integration, by an equivalence to a certain analytical group as an absolute extension of its algebra. Consequently, this allows an analytical functional construction of powerful invariance transformations for a number field to its own algebraic structure.

https://en.wikipedia.org/wiki/Langlands_program

This is not true, but I can see how one can lead to think this is the case given the poor journalistic reporting on the subject.

At the time being we have no experimental access to the regime where quantum gravitational effects are relevant. So no theory of quantum gravity has been falsified experimentally. That is, none have failed because they didn't match the experiments. The experiments have not been carried out yet because we don't have the technological ability to do them or we haven't had the ingenuity to infer how to retrieve experimental data from available sourced.

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