The Nature of the Universe, Time Travel and More...

[Will The Serious]

After reading a little bit more about galaxy formation, shape and evolution theories, most of the answers I read just seem to say some version of, because that is the shape they take.

However, I liked this answer best, from Astronomy (https://astronomy.stackexchange.com/questions/32947/what-decides-the-shape-of-a-galaxy#:~:text=As stars form in one,this makes spiral arms appear.)
These are just hypotheses.

First mechanism: The arms of a galaxy are density waves.
Picture a traffic jam: all the cars are slowed down on a portion of the highway, but the cars that are in the jam aren't always the same ones. Each car reaches the traffic jam, slows down, gets to the end of the jam and speeds back up. This is what the stars are doing in the spiral arms: a star reaches the arm, slows down because of the higher density, gets to the end of the arm, resumes its normal speed. As a result, the arms spin slower than the individual stars.

Second mechanism: The arms appear as star formation propagates through the galaxy.
The leading edge of the arm is where star formation occurs. The trailing edge is where stars die (so are no longer visible). In this case, there is not more matter in the arms, it simply is more visible.

As stars form in one region, the largest ones cause shock waves to spread through the interstellar medium, igniting the formation of new stars. This causes the star-forming region to propagate through the galaxy. Combined with the differential rotation of the galaxy, this makes spiral arms appear.

a star reaches the arm, slows down because of the higher density, gets to the end of the arm, resumes its normal speed. As a result, the arms spin slower than the individual stars.

I'm beginning to get what publiusr meant by traffic jam. Not the shape, but the dynamics. But, why would the star "resumes its normal speed"? That doesn't sound like conservation of energy. I suppose of there is such a thing as the "interstellar medium", then something like bernoulli's principle could explain the acceleration out of the spiral arm. What I wonder, from all the attempts to explain that I've read, why a spiral? If it is the result of some example of Cosmic ripples, gravity wave, waves in the interstellar medium, etc. Why not ripples like on a pond surface? Why a disk shape, flat and polar-like symmetry?

If material isn't being spewed out from the center from some equatorial spin, what is the reason some galaxies (most as I understand it) form flattened disks and spiral?

-Will

 

[Asbo Zaprudder]

galaxy - Why are galaxies disk shaped? - Astronomy Stack Exchange
The same goes for the solar system, by the way, at least as far as the Kuiper Belt.
Here's a paper explaining the dependence of galaxy formation on angular momentum:
ANGULAR MOMENTUM AND GALAXY FORMATION REVISITED - IOPscience
The Milky Way is not a perfectly flat disc in any case.
Milky Way galaxy is warped and twisted, not flat - BBC News

 

[publiusr]

It isn't that the traffic patterns are spirals...but how stars can bunch up.

On materials
https://www.secretprojects.co.uk/threads/what-new-materials-are-there.18181/page-22#post-667570

 

[Asbo Zaprudder]

The speed thing likely depends on the inhomogeneity of the gravity field. Although a star passing through a region of increased stellar density should accelerate, when it gets farther in toward the centre of mass, the nett local gravitational attraction should tend to zero. Leaving the region causes deceleration. However, as it's an N-body problem, where N >> 2, it's hard to be certain what happens without numeric simulation and even so, certain scenarios are likely chaotically dependent on initial conditions.

I have only skimmed this, but here are the notes for a complete university course on galaxies from the University of Groningen in the Netherlands from 2007. This is probably a good starting point for the knowledge required to discuss such topics seriously.

Galaxies course Homepage (rug.nl)

It looks similar to the structure of a course I took about 50 years ago at university, although the science has moved on a lot, of course.

 

[KamenRiderBlade]

Latest Webb Space Telescope data confirms Hubble’s value for expansion of Universe

 

[Unicron]

 

[Asbo Zaprudder]

Lagrangians and Noether's first theorem are 2nd year undergraduate-level magic.

What about statistical thermodynamics?
Although the second law is somewhat mysterious regarding the arrow of time, Stephen Wolfram believes he can explain it (and quantum mechanics) with branchial theory. I find it is very easy to get tangled in tautological knots and self-referential loops. Our cognition is correlated with increasing entropy and the expanding universe which we measure using clocks and standard candles.

I have no explanation for the Hubble Tension, but I would have bet (and lost) on it being down to systematic errors in distance measurement as happened in the early 20th century with Cepheid variables.

 

[Will The Serious]

Latest Webb Space Telescope data confirms Hubble’s value for expansion of Universe

"We've measured it using information in the cosmic microwave background and gotten one value," Ars Science Editor John Timmer wrote. "And we've measured it using the apparent distance to objects in the present-day Universe and gotten a value that differs by about 10 percent. As far as anyone can tell, there's nothing wrong with either measurement, and there's no obvious way to get them to agree."
Considering all the various attempted ways the Hubble Constant has been measured, one might conclude that the variations in outcomes could be explained by unequal expansion of local space. To compare gravitational echoes or lensing from a few super nova to the CMB, seems very likely to give different answers. It would be like measuring the energy output of an explosion based on one piece of shrapnel landing somewhere within the total blast radius.

-Will

 

[Asbo Zaprudder]

Yes, our assumptions of homogeneity and isotropy might well be incorrect - especially as some recently discovered galaxy associations appear to be larger than the maximum size considered possible.

Cosmic megastructures: The largest objects in the Universe - BBC Future

 

[Asbo Zaprudder]

Johnathan Oppenheim claims his theory of postquantum gravity explains the observations attributed to dark matter and dark energy, but, as Sabine Hossenfelder points out, the theory doesn't have the right scaling properties to agree with observation.

 

[KamenRiderBlade]

"We've measured it using information in the cosmic microwave background and gotten one value," Ars Science Editor John Timmer wrote. "And we've measured it using the apparent distance to objects in the present-day Universe and gotten a value that differs by about 10 percent. As far as anyone can tell, there's nothing wrong with either measurement, and there's no obvious way to get them to agree."
Considering all the various attempted ways the Hubble Constant has been measured, o one might conclude that the variations in outcomes could be explained by unequal expansion of local space. To compare gravitational echoes or lensing from a few super nova to the CMB, seems be likely to give different answers. It would be like measuring the energy output of an explosion based on one piece of shrapnel landing somewhere within the total blast radius.

-Will

Or the Hubble Constant might not be a "Constant" and more of a variable with a range of values depending on which part of space you're sampling from.

Ergo, different parts of space have different rates of expansion, ergo non-uniform expansion over time & space.

 

[Asbo Zaprudder]

In other words, anisotropic and inhomogeneous.

 

[Will The Serious]

Yup. Exactly.

-Will

 

[publiusr]

Could “they” have found Gravitons?
https://phys.org/news/2024-03-experimental-evidence-graviton-particle-quantum.html

Maybe?

Somebody tell me that this can yield a new form of propulsion…

Sabine, you’re my only hope…

 

[Will The Serious]

Perhapse, the obstacle scientists are struggling with to unify Quantum Mechanics and General Relativity lies in the quest for some fundamental particle. As each particle is discovered the next question has to be asked, what makes up that particle.

"Paths of gravitons in photon structure, color changes and magnetic color have spin and curvature speed"
https://www.researchgate.net/figure...d-magnetic-color-have-spin-and_fig1_308890409

What I would want to know is, how are these curved paths, these spins around a point without an obvious force to create the angular change, created? Does it not require a force to cause an acceleration in every case?

Somebody tell me that this can yield a new form of propulsion…

Probably not a new form of propulsion, but perhapse a new way of harnessing an existing form of propulsion. Maybe to use the propagation of gravitons or graviton-like particle, we would need to figure out how to change our own matter state to something else more quantum-like. If all matter has graviton interaction in the same way, to unbalance that condition and effect movements beyond a normal state of free fall, a would probably need to change our relationship with the matter/gravity space-time environment.

-Will

 

[Asbo Zaprudder]

Could “they” have found Gravitons?
https://phys.org/news/2024-03-experimental-evidence-graviton-particle-quantum.html

The excitations described are analogues of spin-2 particles, a property shared with gravitons (theoretically), but they are not gravitons. They could be useful for investigating the behaviour of such particles, however. According to Weinberg’s low energy Theorem,any massless spin-2 field must couple to the stress–energy tensor in the same way that the gravitational field does and so give rise to a force indistinguishable from gravitation. However, I do not know that these spin-2 analogues are massless. They might have a gravitational effect, but perhaps over a shorter range if they have effective mass. Such a conjecture should be readily testable, experimentally, at least to put limits on any pseudogravitic effect.

Perhaps, the obstacle scientists are struggling with to unify Quantum Mechanics and General Relativity lies in the quest for some fundamental particle. As each particle is discovered the next question has to be asked, what makes up that particle.

"Paths of gravitons in photon structure, color changes and magnetic color have spin and curvature speed"
https://www.researchgate.net/figure...d-magnetic-color-have-spin-and_fig1_308890409

What I would want to know is, how are these curved paths, these spins around a point without an obvious force to create the angular change, created? Does it not require a force to cause an acceleration in every case?

I found the original paper and a few others by the same researchers, but I'm not inclined to read them - firstly, as the poor English makes them hard to follow and, secondly, as the papers do not appear to have been subject to peer-review and published in a reputable journal. The list of references they quote also makes it hard for me to take the work seriously. There are lots of pet theories out there and no-one has time to consider them all.

ETA: Talking of pet theories, I'm intrigued by the notion of the amplituhedron - unfortunately, my mathematical ability isn't up to understanding it to any great depth. I like the fact that it has the potential to eliminate the need for using Feynman diagrams and invoking virtual particles, which physicists such as Dyson and Dirac thought a hack and which have been described as the modern-day version of epicycles.

In mathematics and theoretical physics (especially twistor string theory), an amplituhedron is a geometric structure introduced in 2013 by Nima Arkani-Hamed and Jaroslav Trnka. It enables simplified calculation of particle interactions in some quantum field theories.
...
The twistor approach simplifies calculations of particle interactions. In a conventional perturbative approach to quantum field theory, such interactions may require the calculation of thousands of Feynman diagrams, most describing off-shell "virtual" particles which have no directly observable existence. In contrast, twistor theory provides an approach in which scattering amplitudes can be computed in a way that yields much simpler expressions. Amplituhedron theory calculates scattering amplitudes without referring to such virtual particles. This undermines the case for even a transient, unobservable existence for such virtual particles.

The geometric nature of the theory suggests in turn that the nature of the universe, in both classical relativistic spacetime and quantum mechanics, may be described with geometry.

Calculations can be done without assuming the quantum mechanical properties of locality and unitarity. In amplituhedron theory, locality and unitarity arise as a direct consequence of positivity. They are encoded in the positive geometry of the amplituhedron, via the singularity structure of the integrand for scattering amplitudes. Arkani-Hamed suggests this is why amplituhedron theory simplifies scattering-amplitude calculations: in the Feynman-diagrams approach, locality is manifest, whereas in the amplituhedron approach, it is implicit.

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

All Nima Arkani-Hamed's talks on YouTube seem to be illuminating and engaging. I wish I understood his ideas better. If spinors are conceptually the square roots of vectors, whatever that means, I struggle to wrap my head around twistors at all, even though they supposedly make spinors more inteliglible. Sometimes, one must realise one's limits and let the smart people get on with it. If it were easy, it would have been worked out decades if not centuries ago and be taught in secondary schools as calculus is.

 

[publiusr]

It seems when you work with thin films…you can get a bleed-over between forces:

I am hoping that layers of metamaterials can achieve what no one-bulk substance can…
https://phys.org/news/2024-03-topological-metamaterial-amplifies-exponentially.html

 

[Asbo Zaprudder]

I found the following video on spinors helpful. It's the introductory video in a series.

As for twistors, let's ask Roger Penrose, who was the inventor:

I don't think my brain is up to grasping it beyond a superficial level (true of so many things).

Here's an interesting take on spinors and twistors and why they are useful in fundamental physics.

 

[Will The Serious]

Just a short primer.
Prime numbers are those numbers that are only divisible by themselves and the number '1'. The set of all primes starts with the number '2', and because all even numbers are multiples of 2, every prime number after that, is odd. Except for the number '2', no new prime number can be a multiple of two.

The next prime number is '3'. '2' and '3' are the only two prime numbers that follow in immediate sequence in the set of Integers or Natural numbers. After that, there are lots of primes that appear as "pairs", in that they are only two numbers apart, like '3' & '5', '5' & '7', '11' & '13'. In fact, it is generally accepted that there are an infinite number of prime pairs.

'3', like '2', is a prime number, thus eliminating all other numbers that are divisible by '3' as a next possible prime number. So, no subsequent prime numbers will land in any third place on the number line, such as: 6 (3x2), 9 (3x3), 12 (3x4 and 6x2), or 15 (you get the picture). The thing is, every time a prime number appears, the following sequence of number will never include another prime that appears in that prime's number of places from the parent prime. I mean, for the nth prime number, no prime will subsequently appear in the positions of n+n, 3n, 4n, 5n, ... and so on. Every prime number, by the nature of primes, becomes the least common denominator of its multiples. (There are no primes that have another prime as one of its factors. By definition, all prime numbers have only itself and '1' as factors).

Since there is only one prime for its multiples, and an infinite number of multiples for each prime factor, the further along the Integer numberline we go, the less likely we are to find the next prime number. Yet, they go on into infinity, just like their multiples.

Every prime number, thus behaves exactly like every other prime number with only the exceptions of '2', being even, and '3' being right next to '2'. Yet, according to the abstract I just read in which some mathematicians claim to have found a way to predict the next prime (in other words, they can determine the nth prime number), All they need to do that with are, the four primes: '2', '3', '5', and '7'.
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4742238
From these, they have put together a Periodic Table of Primes, or PTP.
"We identify 48 integers out of a period of 2×3×5×7=210 to be the roots of all primes and composites without factors of 2, 3, 5, and 7, each of which is an offspring of the 48 integers uniquely allocated on the PTP."

I wish my math skills were better, because... How does that work?

-Will

 

[Asbo Zaprudder]

^Fasinating.

Goldbach's Conjecture is that all even numbers greated than 2 are the sum of precisely two primes. It has been demonstrated for all even numbers up to 4x10^18, but remains unproven. I suspect that there is a form of number theory that has yet to be uncovered/invented - depending on whether or not you are a Platonist - a new layer of abstraction that will shed light on some, but perhaps not all, outstanding questions about primes.