When Worlds Collide

[Blood Legend]

If two galaxies, equal in mass were to pass through one another, what's the likelihood stars and planets would smash into each other?

Short story shorter, I got an eldritch horror making threats and I want to know how credible his boasts are.

[David Johnston2]

Originally Posted by Blood Legend

If two galaxies, equal in mass were to pass through one another, what's the likelihood stars and planets would smash into each other?

Short story shorter, I got an eldritch horror making threats and I want to know how credible his boasts are.

Any collisions, even in the sense of misses close enough to disrupt planetary systems would be unlikely freak events I think.

[whswhs]

Originally Posted by Blood Legend

If two galaxies, equal in mass were to pass through one another, what's the likelihood stars and planets would smash into each other?

Short story shorter, I got an eldritch horror making threats and I want to know how credible his boasts are.

What shape are they? Elliptical, spiral, irregular? If spiral, is the passage flat to flat or edge to edge? What's the mean interstellar separation?

[ericthered]

If he can get high enough speeds on the collision, interstellar medium from the other galaxy can turn into nasty radiation.

The most likely collisions would be from objects in the colliding galaxy's oort clouds - likely small objects, but fast enough to be nasty.

If he can aim well enough to have the colliding core pass through the general area of earth space, that's a whole different matter.

[Murrkon5]

Everything I've read in passing on NASA and astronomy sites say the amount of empty space between stars and such means a galactic collision is a whole lot of nothing.

[Johnny1A.2]

Originally Posted by Murrkon5

Everything I've read in passing on NASA and astronomy sites say the amount of empty space between stars and such means a galactic collision is a whole lot of nothing.

Yes, but with a caveat.

Stars do apparently very occasionally collide, especially in close quarters (by interstellar terms) like globular clusters. If the two galaxies are passing through each other, a few collisions might just happen, given hundreds of billions of chances.

But the big thing is not collision, it's disruption. The gravity of each galaxy will throw the other into disarray for a while, increasing the number of near-passage events between stars (among other things). A near passage is still unlikely but far more likely than a true collision, and a near passage can be quite catastrophic enough for the inhabited planets of the stars involved.

A star passing too close to Sol, for ex, depending on size, mass, how close the graze is, and relative velocities, could do such things as disrupt planetary orbits, raise the rate of impactor hits, etc.

But the thing to keep in mind is that galaxies passing through each other happens slowly on a mortal scale. Very, very slowly. We're talking about millions of years for the entire event to unfold.

[Johnny1A.2]

Originally Posted by Blood Legend

If two galaxies, equal in mass were to pass through one another, what's the likelihood stars and planets would smash into each other?

Short story shorter, I got an eldritch horror making threats and I want to know how credible his boasts are.

Credible on what time scale? A galactic passage at normal galactic velocities takes megayears to unfold. Do your characters care if the EH's threats will take 837,184 years before their world is affected?

If the EH can move the galaxies fast enough to have events unfold on a human time scale...yeah, he's badass. In multiple ways.

[whswhs]

Originally Posted by Johnny1A.2

Credible on what time scale? A galactic passage at normal galactic velocities takes megayears to unfold. Do your characters care if the EH's threats will take 837,184 years before their world is affected?

Charles Stross's "Palimpsest" has an utterly brilliant treatment of this, coming closer to Stapledonian sf than anything else I've seen.

[johndallman]

Originally Posted by Blood Legend

If two galaxies, equal in mass were to pass through one another, what's the likelihood stars and planets would smash into each other?

Galaxies have dimension of thousands to hundreds of thousands of light years. As several people have pointed out, a collision will be going on for kilo- to mega-years, and isn't really a pressing problem on a human timescale. Provided, that is, it's happening at the usual sort of speeds for galactic encounters.

However, if he can threaten to start a collision, can he move a galaxy into position for a collision, say, next year? If so, he can move entire galaxies at speeds much greater than light, which is a fairly potent ability. If he can do this, he can presumably drop Earth into a star, or shift a few pre-supernovae to nearby locations. He has a problem in that there's a certain lack of evidence for his attacks: it will take many thousands of years for light to travel to let Earth see that a galactic collision is underway, and weeks or months to let us see the nearby giant unstable stars.

Something small-scale might work better. Move Earth out into intergalactic space (just Earth, not the Sun), leave it there to start cooling down for a couple of days, put it back into the Solar System, and then start issuing demands. That should get people's attention.

[ericthered]

Speed is key in this case. If he can move the two galaxies into each other on a time scale relevant to the heroes, even if its a mere '50' years, Its a lot more threatening. If the entity can maintain that speed through the collision, the small collisions can actually do damage.

Note that if you accelerate the milkyway (rather or along with the thing hitting it), the speed of light isn't a barrier -- the lorentz transformations will shorten the distance to your target. This is a LOT more power than causing collisions in Mega years.

I assume an entity making this sort of threat either doesn't have the precision to attack more precisely, doesn't comprehend that the folks he threatening won't care about the time scales involved/don't mind exactly what stars are in the sky, is making a general statement about the might of his powers (I've benchpressed battleships, you will be no problem), or he's saying what he'll do out of spite after he's taken more practical measures.