What hold galaxies together?

Which is why no-one bothers.
In a non-inertial FoR the classical laws of mechanics don't apply!

 

Sarcasm?

 

Actually is done all the time, for example in machinery design.

The formulas taught in high school don't apply, but the classical laws of mechanics still apply but with more complicated formulas.

 

Newton's Laws do not apply in a non-inertial FoR. Remember "at rest or in steady motion"?

 

It is evident that if you're the valve caps of a running bicycle, motions and force directions won't follow the Newton's Law, as stated by Newton. Nevertheless, as it is made every day in solid mechanics, nothing stops you from writing the Newton's Law in other referentials. New terms appears in the equations, for instance the Coriolis force.

 

I like this short reframing of words by BARRY. There is a lot of shortcuts and abuse of the word Gravity. The etymology of this word is not universaly acknowledged, but in Sanskrit, the word "Gurutva" represent the power to attract. When one of the oldest scriptures in the world spoke of gravity https://www.artofliving.org/in-en/culture/amazing-india/oldest-scriptures-spoke-of-gravity. This concept could have been passed to Persians, to the Greeks, before being re-used by Isaac Newton. Cautiously can we say that gravity represent attraction. It's not a force. It's not an acceleration, It's a word used to formulate what is observed, a concept that is re-used everytime new theories, or laws, are established, in order to explain the nature or the properties of this attraction.

( The authors of the article given in the link above may be very talented in history and culture, but they seem to be poor physicists. Please do not throw the baby out with the bathwater when you will read "We know today from modern science that astronauts have difficulty as gravity is very low or nil in space". The gravity in the ISS is 90% of the earth gravity. Hard not to be fooled by misunderstanding of gravity and inertia, especially when reading articles for the general public)

 

NASA disagrees with you. A bunch of scientists and engineers there.
What Is Gravity? | NASA Space Place – NASA Science for Kids https://spaceplace.nasa.gov/what-is-gravity/en/

 

May I suggest that NASA has dumbed it down for kids? The title "NASA Science for Kids" kind of gives it away.
I can assure you that in classical mechanics, gravity is an acceleration. As I said, on the Earth's surface, its value is 9.8 metres per second per second. That's an acceleration.
NASA's approximation from that reference is misleading at best, downright wrong at worst:
"Gravity is the force by which a planet or other body draws objects toward its center. The force of gravity keeps all of the planets in orbit around the sun."
As you know, gravity keeps planets accelerating towards the sun and their tangential velocity keep them from hitting it.

 

Gravity is not an acceleration. Acceleration is a concept based on what was called the "Fluxion theory". To my opinion, this is where lies the true genius of Isaac Newton, that made him able to formulate the causality between action and reaction. Before the establishement of the theorems and lemma that constittutes the mathematical concept of Derivative, he postulate that the rate of speed change was proportionnal to the inertia. And he named the associated cause : Force. When he applied his theory to the Gravitation, he postulates that the associated force that causes attraction could be proportionnal to the product of the masses. The gravitationnal acceleration is found by equating this force with the expression of the rate of speed change, with the units that you gives : m.s-2. Mathematicaly correct, confirmed by the observations, being able to draw prediction, expressed with simplicity, the newtonian approach had every required qualities to be a "good law", according to Physics's principle. However, as time went by, scientists, becoming aware of the phenomenon of aberration of light, pushed the newtonian theory to its bound and tried to explain how a masslesss particle, the photon, could be deviated in the presence of massive objects. On the Deflection of a Light Ray from its Rectilinear Motion - Wikisource, the free online library https://en.wikisource.org/wiki/Translation:On_the_Deflection_of_a_Light_Ray_from_its_Rectilinear_Motion. But how can we define an acceleration, with her newtonian meaning, when it comes with massless objects ? This flaw has stimulated the interest of Einstein. Finally, he used a different mathematical approach to explain the Gravity, replacing the gravitationnal acceleration by a curvature tensor. From this time, the gravitationnal acceleration is still a handy concept, but you won't be able to study high energy phenomenon with it. To explain the aberration of light, you have to admit that, in modern physics, Gravitation is to be seen as space time curvature.

So, yesterday, Gravity accelerated. Today Gravity curves. And tomorrow ?

 

The concept that the simplest explanation has to be the best one, has no base in reality. It is only a common sense approach to make life easier. However, it eventually became Dogma. For example, there is no evidence that any frame of reference is the actual 0,0,0 for the Universe. However, we pick one that is convenient and treat it as such.

 

Nice summation.
There remains a difference between knowing something and understanding it.

Occam's Razor = the Law of Parsimony is simply a good place to start your investigations.

Learning about gravity, for example, from a textbook or a university professor is a good way to know something, but I do not understand it at all. When I took classical mechanics, Newton's laws of motion and the Law of the Conservation of Energy are easy enough to understand, but my understanding of those principals interfere with any understanding of the concept of gravity as something other than a force. The force, applied between bodies creates the acceleration of gravity. To then describe gravity as curved space without a force also removes the concept of acceleration. We are no longer attracted towards bodies of mass, but rather are immersed in a space and time that has us, in one moment, at a distance 'a' from said mass and in the next moment, at a distance a+/-tb from that mass. There is an element of magic in such a concept, to me.
I'm still back there with Newton, and Joule, and Mayer. I believe the math, but I don't believe it tells us what is really going on.

 

Normally I ignore your ramblings, but with this one I came close, but finally decided against it.

 

As long as you are not an astrophysicist, it will cause no harm, but in that case, your writing about dark matter should indeed called ramblings.

 

I just found this nice little explanation from Veritasium
Summary:
In Newtonian mechanics, gravity is an acceleration.
In General Relativity, gravity does not exist. Mass curves spacetime.
The video did not cover gravity in particle physics.

 

Nope. Doesn't do it for me. Most especially the laser beam experiment in the accelerating rocket ship and the canceling out of the gravitational acceleration while keeping the Normal force that is, by definition, a direct consequence of that acceleration. Mathematical hocus pocus by a consummate mathemagician.

If light is constant at speed C, then it would curve downward in an accelerating frame of reference because the rate of movement of the target wall would be moving faster than the source wall was moving by the time the light reached the target wall. Light can't experience acceleration because it is already going as fast as it can go.

That means, between the time the light was fired from a laser at what, to light, was a constant speed, the target accelerated away from that beam in a perpendicular direction. If this experiment were done with a bullet, the bullet would actually behave very similarly.

Fire the bullet at a target moving perpendicular to the aim of the bullet, and, no matter how fast you were going, the bullet would get to its target, but fire that same bullet under an accelerating frame of reference, the bullet would appear to be deflected in the opposite direction to acceleration.

The real difference I see between the beam of light and the bullet is, if the beam of light were fired at a target moving at high speed, it would deflect even if there was no acceleration, because it can't travel any faster than it does. The new location of the target, between firing the laser and hitting its target, would be farther away than the distance it had been when the target was first aimed at, and at a different trajectory.

I want to know how the scientists conducting the laser experiment were able to observe the effects predicted by Einstein for an outside, non-accelerating frame of reference? Who saw that beam traveling straight across the accelerating firing range?