I guess the analogy still holds with gravity instead of charge? So while the "real" mass of each object is compressed so "small" that normally colliding will be improbable, their gravitational influence is much much bigger and pretty much fill the entire empty space. And this is without even considering the milky way center that influences the whole galaxy.

This is a really insightful comment.

Interesting that galaxies can collide without a star hitting another star.. but would it also be that the distances are so vast that the stars dont even (or mostly dont even) affect each other?

I don't know about the percentage of affected stars in a typical galaxy collision but note that you don't need a collision for two stars to affect each other. We're not speaking of interaction like on a pool table. The gravity will significantly affect star systems that pass significantly close from each other.

> I don't know about the percentage of affected stars in a typical galaxy collision but note that you don't need a collision for two stars to affect each other.

True, but the way in which they affect each other is that they try their best to collide.

From that perspective, it's kind of weird if collisions don't happen.

Is it? Think of a single star during a galactic collision. It wants to collide with every star in the other galaxy. It gets pulled this way and that, so, in sum, it decides to collide with the oncoming galaxy as a whole. Unfortunately, despite having billions or trillions of stars, that other galaxy is mostly empty space. The poor original star that we were tracking was unable to collide with any star at all.

Well, why do we keep attempting to collide with the Sun instead of striking out for the center of the Milky Way? Everything you just wrote applies just as much to the Earth as it does to a star heading through a foreign galaxy.

And yet the galaxies usually get ripped apart.

Why would stars hit each other during galactic collisions when stars inside of a single galaxy already almost never hit each other? Apparently reading it here of all the stars in the galaxy, a stellar collision only happens once every 10 000 years.

Interestingly enough, when galaxies collide, the supermassive black holes at their centres do typically merge as their gravity is strong enough to attract each other.

It's by the way not particularly unlikely for stars to pass through other solar systems. Apparently about 70 000 years ago a binary star system flew through the Oort Cloud.

Thinking of the scales involved, even at the speed of light probably two masses couldn’t pull each other fast enough to collide (if you are colliding the two galaxies at the fastest manifestible speed which is the speed of the light).

That’s hand wavy because if you’re imagining like grabbing the two galaxies and smashing them together… that’s like giga levels of magnitude faster dynamics than the speed of light, ha!

You’d have to like put the galaxies on top of each other and leave them there, probably for years, for gravity to get everything moving quickly enough (and then for the speed of light to even bridge the distance!) before really anything would happen. I think.

Engineer but not a physicist.

An other aspect is that orbital mechanics are weird/chaotic. Specially when you get masses that are similar scale. Like why hasn't moon hit earth yet?

So with two galaxies colliding I would not expect the stars to hit each other as result of collision. But as result of interactions when speeds increase.

This is not at all what happened. \Rutherford, Geiger, and Marsden ran an experiment. The data was totally incompatible with a solid blob of atomic mass, but instead suggested that the positive charge of the atom was concentrated into a tiny space.

Yes but why is positive all we care about when it comes to the definition of space? If we fire neutrinos, basically 100% of the atom is 'empty space'. If we fire protons, 99% is empty. If we fire electrons, none is empty.

Why do we say it is mostly empty just because of 2/3 of these things don't interact? Disproving the plum-pudding model was significant but saying the atom is 99% empty space is misleading. We know there are billions of virtual particles appearing and disappearing all the time within the atom and space, they just cancel out.

The word “empty” carries no semantic content under this construction of the term. Terms like “empty” are always used contextually, hence, why only a fool would correct their partner for mentioning the empty refrigerator by remarking on the presence of air.

Unless you think Sagan would have been surprised by the presence of an electric field (mediated by virtual particles…) between electrons and protons in an atom it’s quite likely you’re choosing an obtuse understanding of the term.

This is kind of my point, Sagan was a great science communicator but really missed the mark here.

A solid wood table is not mostly empty by any common contextual definition. Photons do not pass through it freely, your hand won't pass through it freely, and the electron clouds of the carbon atoms in the wood are physical in almost every common sense of the term. They very much push back on any other electron cloud that comes near enough and are generally the only part of the atom that does the interacting.

While it might be true that almost all the mass is concentrated in the center of the atom, that's not what people mean by empty. Houses aren't considered empty just because most of the mass is in the foundation.

I agree it's more complicated and the technical term is https://en.wikipedia.org/wiki/Cross_section_(physics) For each particle, it's different acording to what is hitting it.

Anyway, for atoms we mostly care about colisions with charged particles. I'd say that they are almost empty.