Does this explain what generates gravity?

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#5,581
system7 said:
Neutrino Mathematician Terence Tao
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Tao asks, is it a rabbit or a duck?

1741395673811.png


Cue my neutrino joke:

A neutrino walks into a bar, and the barman says, "What can I get you?".

"Nothing, I'm just passing through." :rofl:
 
#5,583
I have decided that there is no easy way to explain the mysteries of the Neutral Kaon and Neutrino Oscillations.

Though it is a very "Fire Hose" of research in Modern Physics. And something that keeps me up at night.

But as Doctor Taylor Swift recently said: "Never be ashamed of Trying."


Here AD and TRACKING COOKIE free for the benefit of the secretive @Galu.

Surely ADs can suggest things to improve your life? And I wonder what he is trying to hide from? 🤔

Therefore, if there is popular demand, I shall TRY. 🙂
 
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#5,585
I have a feeling Schrodinger's Equation is not going to hack it here. 🙁

I have skimmed a Warwick University teaching paper on all this. 62 pages!

Wiki is almost as difficult:

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

Seems a neutrino flavour is a superposition of 3 mass states, and actually it is not known in what order they are arranged. So a Tau and Mu could be lighter than an Electron type.

It is also not known if Neutrinos are Dirac particles or Majorana ones, in which case the antineutrino would be the same as a neutrino.

This is the current state of knowledge:

Neutrinos 1 Current Knowledge.png


And Table 2, which is mixing angles, which I have seen before in Quark studies:

Neutrinos 2 Table 2 appendix.png


I am not too sure how this works, but neutrinos seem to be readily scattered by electrons, and the Z can be thought of as a heavy Photon, though the v Tau is unexplained here:

Neutrinos 3 The Feynman Diagrams.png


The Sterile Neutrino (A hypothetical Dark Matter candidate) is supposed to be very massive, around 45.6 GeV or more, which is more than a half of the W and Z Bosons, so is energetically unlikely to be easily detected.

Mixed Neutral Kaons do something similarly "strange" when passing through matter. Left to their own devices, one of them preferentially decays, but in matter the original equal creation probability is restored.

Neutral Kaon Peculiarities.png


https://en.wikipedia.org/wiki/Kaon#Oscillation

Best left to Professionals, one feels. But interesting to see how it is done.

🙂
 
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#5,586
system7 said:
It is also not known if Neutrinos are Dirac particles or Majorana ones, in which case the antineutrino would be the same as a neutrino.
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Ettore Majorana (left) and Paul Dirac (right) proposed two different answers to the question of whether neutrinos and antineutrinos are different particles or a single particle masquerading as two.

1741468461540.png


If neutrinos are Dirac particles, their matter and antimatter versions are very different.

If they are instead Majorana particles, the matter and antimatter components are the same thing.

Apparently, if neutrinos are Majorana particles, this opens up all sorts of new kinds of physics.

The Sterile Neutrino
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I was skimming through this earlier: https://www.symmetrymagazine.org/article/the-search-for-the-sterile-neutrino

From the article I've extracted and edited a brief history of the neutrino:

In 1930 Pauli wrote of his idea that the emission of a particle with neither charge nor mass explained the problem of the missing energy in beta decay.

A few years later, Fermi figured out a complete theory of nuclear beta decay incorporating Pauli's particle, which he christened the "neutrino".

In the 1950s, a detector placed next to a nuclear reactor confirmed the existence of the neutrino.

In the 1960s, astrophysicists, to their surprise, detected only a third as many neutrinos coming from the Sun as they expected.

In the 1990s, researchers discovered that the neutrino could "oscillate" between three different "flavours". Since oscillation implies mass, the discovery also let them know that neutrinos were not massless as they had thought.

Soon, strange results from neutrino oscillation experiments indicated that there may be a fourth flavour of neutrino that would interact even less strongly with matter than the known flavours. Physicists called the hypothetical missing neutrino flavour a "sterile" neutrino.
 
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#5,587
Yes, I think you have summed it up pretty well there. The fierce maths of it all is here at Warwick University, but not for less than postgrad Physics level, I'd say:

https://warwick.ac.uk/fac/sci/physi...d/stuff/neutrinolectures/lec_oscillations.pdf

It's all a nightmare of 4 component equations, but broadly can be followed as to how this mixing stuff is calculated.


I have been spending time at MetaFilter, which is just an ace site for discussion of nearly everything by sensible people.

"What would the World look like if the Kremlin had installed its top agent in the White House?" .... Yes, Welcome Home. 🤣

I have found the answer as to what Planck's Constant actually is!

This in a discussion of some dubious YouTube videos about Planck Units.

https://www.metafilter.com/206890/Planck-units

I have no idea where he's getting "sizes" for the quarks and neutrinos. I've written on Metafilter before that atoms aren't mostly empty:
If I were to make one mark on the world as a science communicator, it would be to put the misconception that “atoms are mostly empty space” into the same useful-but-wrong dustbin as the Bohr model. [...] An atom is full of electrons, which get bigger when they are cold.
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The number in the video for "the length scale of a quark," about one-thousandth the radius of a proton, is about the same as the "weak scale" that I refer to in that old post. But the neutrino also interacts at the weak scale. Maybe he's taking the square roots of cross sections? I really don't know. In any case, below the weak scale, everything is so intensely quantum-mechanical that it doesn't really make sense to talk about length at all anymore. Instead, we talk about energy.

Most people know that there is a connection, which has something to do with Einstein, between energy and mass; that sentence is generally enough to make them jump up and say "E = mc2." This is why physicists will talk about the masses of subatomic particles in energy units. Nobody uses the electron mass of ten-to-the-minus-twenty-mumble kilograms to do practical computations, any more than we measure atomic radii in furlongs. The electron mass is about 0.5 MeV/c2, where the mega-electron-volt is an energy unit, and where we frequently don't bother saying or writing the "c2" part.

When you start talking about angular momentum in quantum mechanics, you introduce Planck's constant ℏ, and that lets you make a similar connection between energy and length:
ℏc ≈ 200 MeV•fm
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This relationship between energy and length turns out to be super-useful when you're thinking of forces that are mediated by massive fields. (Other folks might say "mediated by massive particles," but the murky boundary between a continuous field and its particle-like excitations is the whole reason this video about particle sizes is a little wobbly.) It turns out that if the mediating field has a mass m, then the associated interaction has a length scale like (ℏc)/(mc2).

For example: protons and neutrons are attracted to each other by pion exchange; the pion mass 140 MeV is related to the maximum nuclear diameter of a few femtometers. But protons and neutrons repel each other by exchanging rho and omega mesons, which have masses closer to 800 MeV; this is related to the observation that nucleons act like they have a smaller hard core. The "weak length scale" is about a five hundred times shorter than the "strong length scale" because the weak interaction fields, associated with the particles W± and Z, have masses which are about five hundred times more than the pion mass.

But in practice, we don't actually talk in more than a handwaving way about things happening "inside of a proton." The inside of a proton is very complicated, but it's what we call a "stationary state": there's only one way to be a proton, and that's what protons are doing, all of them, all of the time. We talk instead about the energies of interactions where these other fields play an important role. If you want to see weak interactions, you can (a) wait a long time, (b) measure very carefully, or (c) hit stuff together with energies of 100 GeV or more, so that the W and Z fields have plenty of energy to make their particle-like excitations.

Notice that there is an awful lot of "about" and "or so" and "goes like" in these descriptions. You've maybe had the experience of doing a page of algebra, and at the end everything cancels out and you're left with an answer like "three" or "two-fifths." That happens a lot; we call those results "fractions of order one." You can actually accomplish quite a bit of physics without doing any of that page of algebra, in a technique called "dimensional analysis": you figure out what the relevant constants are in your problem, multiply and divide them so they have the units you want, and work with that unit combination. You know that any number you come up with will probably turn out to be wrong by a factor of two or one-third or something. But that's not really any different from knowing that you're going to the grocery store tomorrow or the next day, and that you'll spend fifty or a hundred dollars, even though you haven't yet made your list.

That's what the Planck scale is: the result of dimensional analysis. When we end up talking about quantum mechanics and gravity at the same time, we're going to have c, ℏ, and G available as constants. There's one way to combine them to make a length, so any intrinsic quantum-gravity phenomena are going to have that length scale. Or, if you don't believe in lengths after this little essay, you use ℏc to convert that length to an energy. If things collide with that energy, quantum-gravity effects are going to be important.
posted by fantabulous timewaster at 10:19 PM on December 25, 2024 [26 favorites]
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See, it crops up in angular momentum as well as Photon energy. But it's really about length scales for different energies. Preferably in GeV in Particle Physics.

There is a nice discussion of whether Atoms are mostly empty too.

If I were to make one mark on the world as a science communicator, it would be to put the misconception that “atoms are mostly empty space” into the same useful-but-wrong dustbin as the Bohr model (where we pretend electrons travel around the nucleus like little planets).

The misconception that “atoms are mostly empty” comes from illustrations in textbooks that show protons and neutrons as colored balls that are about the same size. A caption nearby explains that protons and neutrons have about the same mass, while the electron is 2000 times less massive, so the electron is included in the illustration as a smaller ball. Then there is some (correct) text about how the protons and neutrons are packed close in the nucleus, but the volume of the nucleus is only 10-15 = 0.0000 00000 00001 the volume of the atom. There may even be a (correct) statement that, while we know the size of the protons and neutrons, the electrons are “point particles” whose size is “too small to measure.” It’s either stated explicitly, or the reader is left to believe, that the list of things which could fill the atom has been exhausted, and that the atom must therefore be nearly all vacuum. But that conclusion is wrong. The atom is full of electrons, which get bigger when they are cold.

The colored-ball model, where protons and neutrons are one size and electrons are smaller, makes intuitive sense because all the materials normal humans interact with on a daily basis all have approximately the same density: the same density as water, within a factor of five or so. (You’ve probably handled styrofoam but never aerogel; your experience wearing gold jewelry does not prepare you for the experience of handling a gold brick.) The major exception is air, a thousand times less dense than water, which many educated adults sometimes forget is a material that carries mass at all. If protons, neutrons, and electrons were all made of “particle-stuff” with comparable density, then the big-ball/small-ball model would be a good one. But: they’re not, and it’s not.

We talk about the “intrinsic size” or a proton or neutron because there are short-range interactions that you can only detect below a certain length scale. Some of those short-range nuclear interactions are responsible for “hard-core repulsion” among nucleons, so nuclear matter has the same density whether the nucleus is small or large. A hopper full of tennis balls has constant density, for the same reason. Also, if you force two protons closer together than this “hard core,” these short-range interactions excite internal degrees of freedom and you get something that’s not-a-proton-anymore. Intrinsic size makes sense for protons and neutrons.

Electrons have a “natural size” from electromagnetism, which is slightly bigger than a proton or a neutron. But when you interact with electrons, there is absolutely nothing that changes at this length scale. Electrons do have an “extra interaction” that “turns on” at a particular length scale, which is about 1000 times shorter than the radius of a nucleon. But that’s not special to the electron: every particle that we know about participates in the weak interaction. And no matter how hard we look, the electron doesn’t seem to have any sub-structure that we can excite. Everything interesting that happens when electrons interact at short distances seems to be something interesting about the vacuum, not anything particular to the electron. It’s less that “the electron has zero size” and more that “the electron doesn’t have any size”: questions about intrinsic size, applied to the electron, give nonsensical or contradictory answers.

But there is an extrinsic size parameter associated with an electron, because there’s no such thing as an electron at rest. The de Broglie wavelength of any particle goes like the inverse of that particle’s momentum. And if you figure out how fast an electron trapped in an atom is moving, and turn that into a wavelength, what you get is ... the size of the atom. If there are lots of electrons trapped at the same atom, they take on increasingly baroque shapes that occupy the same volume with “zero overlap.” A low-math analogy might be to imagine a conch shell, which takes up a lot of volume but also has a long empty spiral where the snail used to live, and to imagine a design where you could get two conch shells to nest, one inside of the other.

(This extrinsic size isn’t special to electrons either: people use cold neutrons to study the structure of crystals, because the low-momentum neutrons are bigger than the distance between atoms and therefore interact with many atoms at once.)

I think that “cold electrons are huge” is just as pithy as “atoms are mostly empty space,” while also being more correct, being more interesting, and having more predictive power.
posted by fantabulous timewaster at 9:41 AM on March 28, 2021 [10 favorites]
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See, the electric force is VERY powerful. Didn't we get 3 Million Tons for two grains of sand 30 metres apart, or something?
 
#5,588
system7 said:
Planck Units
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What I know about Planck length:

If you measured the diameter of an atom in Planck lengths, and you counted off one Planck length each second, it would take you 10,000,000 times the current age of the universe!

system7 said:
whether Atoms are mostly empty
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What I know about electrons in the atom:

Physicists no longer consider that electrons move in set orbits. That idea has been superseded by the 'quantum model' of the atom where electrons are 'clouds of probability' and their position is uncertain.

ℏc ≈ 200 MeV•fm
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This basically means that to measure a nuclear length of ∼1 fm, you need a large energy of ∼200 MeV.

Here's a mathematical exercise for you, Steve.

Given that

h (Planck's constant) = 6.626068 x 10⁻³⁴ J·s
c (speed of light) ≈ 3 x 10⁸ m/s
ħ = h/(2π)

Calculate ħc in MeV·fm.
 
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#5,590
Galu said:
What I know about electrons in the atom:

Physicists no longer consider that electrons move in set orbits. That idea has been superseded by the 'quantum model' of the atom where electrons are 'clouds of probability' and their position is uncertain.
Click to expand...

I have decided to correct your misapprehensions one post at a time. Thus, given infinite time, we may make a Modern Physicist of you.

Sure, but what does it mean to say that an electron “is somewhere within” an orbital? It means that electron somehow has a position uncertainty Δx which is much smaller than the characteristic size of the orbital. If your mental picture of how nucleons and electrons look is a big-ball/small-ball model, you might say an electron has been “found” when its position uncertainty Δx is like the size of nucleon. But an electron which is localized so precisely has an enormous momentum uncertainty Δp; nearly the entire phase space for such an interaction has the electron ejected from the atom afterwards. You may have determined where the electron was, but you’ve done so destructively. Make an electron hot, and it gets small enough for you to decide you have found it.

The “probability cloud” interpretation comes from the era before relativistic field theories, and seems to rely on a semiclassical notion of “a particle.” People who use field theories to solve problems tend to be unbothered when I say something like “cold electrons are big.”
posted by fantabulous timewaster at 7:03 PM on March 28, 2021 [1 favorite]
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https://www.metafilter.com/190906/Travel-the-Universe-with-a-sheet-of-A4#8084301

It seems impossible to perform a similar miracle on "Professor" Brian Cox who still drones on in that unfetching Northern accent:


I was depressed by the notion of falling into "The Vast Infinite Depths of a Black Hole" and being "Crushed by Gravity", but was cheered by a all too brief cameo appearance by "The Dinosaurs", who I always respected for their feral single-mindedness.
 
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#5,591
system7 said:
we may make a Modern Physicist of you
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So, my education in modern physics lies in the hands of some unknown guy who calls himself 'fantabulous timewaster' and talks of hot and cold electrons. I wonder if his work has been peer reviewed? :scratch2:

However, the talk of cold electrons leads me to consider just how cold electrons can actually be.

Apparently, when electrons are cooled down, their energy reduces and they form a stable quantum structure known as 'electron ice'.

Scanning tunneling microscopy (STM) has revealed images of 'electron ice' some 90 years after its existence was predicted by Eugene Wigner:

1741537892598.png

STM images of electrons forming the Wigner crystal

https://interestingengineering.com/science/cold-electron-ice-image-captured
 
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#5,594
I just see a green dot. 🤔

Paul Dirac in Tie.jpg


I have also discovered that if I "ignore" @Galu, the whole thread is much improved, with just a few incomprehensible one liners from all and sundry, and simple (but not too simple) Physics from yours truly!

Sabrina Hassenpfeffer continues to amaze me. "How Do Gravitons Escape a Black Hole?" It is something that has often puzzled me.


Just when I think she is going to expalin it, unless I blinked and missed it, she switched to a blatant ad for "Brilliant" to learn more. 🙁
 
#5,597
I wondered where I had seen the "Galu Girl" before. It is on my bookshelf!

If you stare at it long enough, the blonde becomes brunette, and the brunette becomes blonde,

But the really weird thing is the shoes turn RED. 🤔
 

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#5,598
A Ship carrying Jet Fuel has Collided with a Ship carrying Sodium Cyanide in the North Sea.

https://www.bbc.co.uk/news/live/cgq1pwjlqq2t

This is only considered seriously dangerous if the Sodium Cyanide is ignited whereupon it forms a deadly Cyanide gas cloud if I remember my Chemistry.:

Ship Collision 2.jpg


Where is this?

Ship Collision.jpg


Should I be concerned in Portsmouth, near Southampton if you don't know? Surely we are usually dominated by the warm South-West wind?

Ship Collision 3 NW Wind.jpg


Oh NO! Looks like "Game Over" for young system in Portsmouth, UK. What is that smell of almonds? GOODBYE MY FRIENDS! 😢
 
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#5,600
handanim.gif


I seem to have survived a terrible night. I was reading my pulp fiction books late, and playing my favourite Frank Sinatra record (And now the END is NEAR...)

Interestingly, if you stare at the cover of my detective novel, the wallpaper and staircarpet and improvised blanket turns from Pink to Green.

I awoke, seemingly in heaven when I had a cup of tea and THREE Malted Milk biscuits. I'm alive, another day, thankyou God! I was touched by TNT's gracious message, and thank him.


I really didn't fancy Liverpool's chances against PSG, given their flukey win in the first leg so switched the wireless to the last bastion of Reithian values of BBC Radio 4 in a Tik-Tok world gone crazy.

Saturn versus Jupiter Brian Cox.jpg


https://www.bbc.co.uk/sounds/play/m0028d2f

When addressing a Home Counties audience Sir Brian Cox loses the northern vowels and constant repetition of the words HUGE, AMAZING, VAST and whatnot.

I will not tell you which planet won. I was as surprised as anyone.

MARS, because it is chocolatey!
 

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