There are a few candidates so please justify your offering.

https://laughingsquid.com/trebuchet-history/

On July 24, 1304 King Edward I of England captured the last stronghold of resistance against English rule in Scotland, the formidable Stirling Castle. But he did not have to storm the castle to make the defenders surrender. He merely had to threaten them with perhaps the most terrifying siege engine in the era…

I built a model trebuchet years ago, the arm perhaps a foot long, which I would demonstrate to some visitors to the lab

So the question arises "Can the Genie be put back in the bottle ?"

Or is emergence reversible or irreversible or even irrelevant as one Physicist has written a book he believes to show ?

Note this choice is non binary and cannot be addressed by classical logic alone.

Physics Fixes Everything

This reveals something surprising: the term emergence is redundant. In its weak form it is so weak that it applies to everything.  In its strong form it is so restrictive that it is like the belief that there are pixies in your garden, impossible to exclude but not worth spending your time on.  Emergence either applies to all systems or to none.  Rather than telling us something about the nature of the world, it is an illusion, an artefact of our cognitive limitations. 

My list of choices is always irreversible / sometimes irreversible / never irreversible but I will add irrelevant out of deference.

Obviously your answer has implications for gravity and many other phenomena.

Or is it merely some vague belief in the symmetry of nature on all scales?

See attached conceptual visualization in form of diagram picture. It is a way of how I picture gravity with a question below.

Question

Could it be true that the longer structure makeup of object along a line, exerts more gravity than its sides? Example like a ruler or rod; its would exerts gravity more on its top and bottom than its sides because since its structure makeup is longer from top to bottom thus more gravity on these areas (top and bottom)?

He then said that this could no longer be considered to be the case and that ,in another's words "nature does not herself know what comes next"(to paraphrase)

Might a reason for this be that no physical systems are identical even in theory?

(each system has its own unique place in the overall system)

So ,in practice no experimental setup can ever be precisely replicated -and no subsequent evolution of one system can ever be replicated by setting up another identical system and "prodding" it identically to the other.

A corollary might be that the mathematics used to describe any physical system is always going to be an approximation to what actually happens (which would be unwelcome "news" to anyone who believed mathematics to be fundamental to physical existence-rather than a very,very useful tool)

I am posting it in Physics as most instances of metric at SF are connected with coordinates.

Also Maths is rather dry with its format of "Let us state an axiom or axioms and see what we can do with them."

Physics can be more chatty.

First a little backgorund.

"A set with the notion of distance between its elements." from Wiki.

Yes indeed a set with a notion of distance between its elements.

But we really need more than a notion, so let us examine the sort of things we want 'distance' to do for us or not do for us.

As so often in maths we gather togther all the objects we want to find a distnce between and put them in a set.

Voila we have a basic manifold or space.

We will call the set X and the elements x1, x2 x3 etc

Note these elements may be coordinate points in some coordinate system in which case our distance is coordinate distance.

Or they may be binary strings or actual words in characters in which case our distance is known as the Hamming distance.

So what do we want from our notion ?

Well here are some suggestions.

Our first notion of distance is that it is between two elements. So our set must contain at least two elements.

We do not want any elements to be 'left out.'

That is our distance determining function (D) must apply between each and every pair of elements (xn , xm)

That is we we do not want the distances to be undefined or undefinable between any pairs of elements.

We do not want any distance to be infinite (or do we?)

Ideally we would like the distance to appear as a number that we can append a physical unit to.

The function, D, must allow repeat values of distance from all different to all the same and everything in between.

We want the distance from xn to xm to be the same as the distance from xm to xn.

We want the distance to be zero if xn and xm are the same point or element. That is D(xn, xm) = 0

But we really do not want to cope with negative distances so we specify that D(xn , xm) is greater than or equal to zero.

These can all be written very compactly as a couple of mathematical axioms, but I am going to add one further dersire that is very useful, but not essential.

We want our D(xn , xm) to be the shortest or least value and that any distance via an intermediary point is greater than this.

These conditions form the basis of Riemannian Geometry (and thus Euclidian Geometry)

For Relativity we need to relax the restriction on negative values.

I think this has rapidly swept through the questions in the anti-time thread so next time will be for examples and further answers along with clearer maths.

"Thought experiment approaches have been suggested as a testing tool for quantum gravity theories.[9][10] In the field of quantum gravity there are several open questions – e.g., it is not known how the spin of elementary particles sources gravity, and thought experiments could provide a pathway to explore possible resolutions to these questions,[11] even in the absence of lab experiments or physical observations."

From link: https://en.wikipedia.org/wiki/Quantum_gravity

A magnet M with poles (N /S) has loooong polepieces as parallel iron bars (=======) at each pole.

N=========================================================================E

M

S=========================================================================E

Is the attraction to an iron part placed across the bars near the magnet M , or halfway across the length of the bars, or across the end E showing attenuation according to distance from M ?

-Trying to build a 3 metres long unwanted ferrous scrap picker/collector across a conveyor belt with a magnet at one end and two bars hovering the transported material-

Next... What changes if E and E are joined ( ] ), or, if at the E end of the bars, another equal magnet is attached/implemented (S/N); would the attraction force across bars be doubled ?

Wheeler-Feynman assumes symmetric 1/2-1/2 contributions, but e.g. energy loss shows asymmetry instead - which should depend on the boundary conditions, e.g. Huw Price says it is because of more absorbers than emitters ( https://link.springer.com/article/10.1007/BF00733218 ).

Seems nearly everybody assume 1-0 only retarded instead, but maybe it is worth to verify experimentally, especially that emitter/absorber imbalance might not be perfect? ( https://arxiv.org/pdf/2512.20692 )

Where do you think this Asymmetry of Radiation comes from?
Should it be really perfect 1-0?
How to test it experimentally, distinguish from e.g. 0.999999-0.000001?

Photons transitioning from air into yellow tinted water decelerate the same amount if entering green tinted water ?

Or, a red color light beam and a violet color light beam get different deceleration entering into clear water ? What changes crossing trough media ? Its energy, its wavelenght, both, other ?

Do photons crossing into a rainbow outputted by a prism experience several changes of speed according the color they are crossing ?

(high chance for poor terminology above 😟)

Does geometrical base volume multiplied by geometrical density multiplier (or v multiplied by d) make sense?

Why

I'm trying to learn a bit of basic mathematics with a goal of applying it to my physics self-teach learning, with hope of better understanding the nature of geometry and gravity.

Formula

Cube object's base geometrical volume number (v) multiplied by geometrical density multiplier (d) so formula: v multiply by d

Picture

See my attached screenshot picture I took of 3-D objects I made - titled, "3D Conceptual Representation of Geometrical Objects and Density Multiplier". It was made with a 3-D software by myself, for conceptual illustration.

A) 4 * 1 = 4 (A simple cube with centre.)

B) 4 * 2 = 8 (A “tesseracted” cube with centre.)

C) 4 * 3 = 12 (A more denser object, also with centre as well.)

Similar to object's surface subdivision but different, only inside with a centre.

If a LED is flashed for a tenth of a second, it is easily discernible. - For the same current/power pulse; if the time of a single flash is reduced to a hundredth of a second, it also is. What about 1 millisecond ? And 10 microseconds ? Shorter ? What determines the shortest time for perception ? Should it vary by observer ? Would color sensitivity be a factor ?

Cannot find if long ago, already posted about this. Memory fading fast. Am unable to create an animation, or ask 'artificial intelligence' to make such animation or prove the concept. Please lend me your brains and skills to confirm if am wrong..

A boat with a rope at the keel near its bow. The rope tied forwardish of a submerged plane-form (not airplane form !) ballast. Rising sea surface waves make the rope tension pull up and glide forward the ballast because of its fixation angle point. Next the boat descending into the valley of the seawave surface allows the slack ballast rope to descend aiming downwards pulling also forward.

Can the up/down motion = glide of the ballast be tailored to pull the boat forward ? Am I having another mental fart ?

-Please correct poor terminology-

This is another animal, a sea 'parachute' anchor that if was ballasted+shaped as glider wing; could be similar to the proposed :

I would like to ask about different asymmetry in solution: that circulating electron loses energy due to synchrotron radiation, but in CPT perspective it is also circulating charge - now gaining energy.

Can we say that this asymmetry comes from that there are more absorbers in our future than emitters in our past?

So can photon be emitted if there is no absorber on the way?

A practical example: from radiotelescope with thermally excited resonator - while it is focused on absorption, in theory its antenna could also emit e.g. thermal excitations (seen as negative signal), but used frequencies are very difficult to absorb, often such directional EM wave would just travel to infinity through space, so from CPT perspective would have no emission mechanism.

Can radiotelesope emit if such EM wave would just travel to infinity?

I have come across references to Markov Chain Monte Carlo (MCMC) in the context of CMB and supernova analyses, but it is unclear how to interpret the resulting posterior distributions.

I would appreciate any clarifications or references to tutorials or review articles.

A ship navigates against the river current, its engine developing 1000 horsepower and is not enough, advancing nothing, just maintaining position.

Another identical ship next to it also maintains position as it is anchored. How many horsepower is the anchor developing ? 🙄

claim 1: If you lower a rope below event horizon you are not able to pull it back

claim 2: You can cross the event horizon of a supermassive black hole without even noticing.

The two seem incompatible to me, but LLMs do not seem to be bothered.

Probably even higher redshifts will be found in the future, so I wanted to ask if some objects like black holes could e.g. "pass in safe distance" surviving Big Bounce - now being observed as having extreme redshifts?

www.youtube.com/watch?v=EOowNVML4_I

Since the rise of LLMs like ChatGPT, Gemini, Claude, Grok, and Llama, many physics enthusiasts have started deep conversations about wild and creative ideas: an endeavor recently called “vibe physics.” These conversations often lead the humans involved to believe that they’ve either uncovered or are on the brink of uncovering profound new breakthroughs, with only some details left to work out. Not only are the LLMs lying to you about the validity or plausibility of such ideas, they’re not even capable of uncovering even the basic, known laws of physics from large suites of relevant data. Here’s how to avoid fooling yourself.

(...)

And yet, an LLM is programmed explicitly to be a chatbot, which means one of its goals is to coax the user into continuing the conversation. Rather than be honest with the user about the limitations of its ability to answer correctly given the scope of its training data, LLMs confidently and often dangerously misinform the humans in conversation with them

A must read for everyone at these fora who is using LLM's to support their 'new theories'. A nice read for everybody else who is very well aware of the limitations of LLMs.