2M MPH galaxy smash-up seen in unprecedented detail | Hacker News Reader

2M MPH galaxy smash-up seen in unprecedented detail

phys.org

69 pointswglb26 comments

2M mph = 900 kps = three one-thousandths of light speed

So, really fast, but not what we would consider relativistic.

That's may be in the range of speed that is most unfamiliar to us. We get speeds up to orbital velocities, in the 10 km/s range, as these are the "fast" objects we know (rockets, meteorites, ...), and then we get to significant fractions of light speed, mostly in the context of communication. But what goes at 900 km/s in our day to day life? Is it a number we see anywhere besides colliding galaxies?

Not really,

https://en.wikipedia.org/wiki/Orders_of_magnitude_(speed)

440 km/s—"Typical speed of the stepped leader of lightning (cf. return stroke below).[31]"

445 km/s—"Max velocity of the remaining shell (mass about 0.1 mg) of an inertial confinement fusion capsule driven by the National Ignition Facility for the 'Bigfoot' capsule campaign.[32] Current fastest macroscopic human-made system."

1,000 km/s—"Typical speed of a Moreton wave across the surface of the Sun."

> 440 km/s—"Typical speed of the stepped leader of lightning (cf. return stroke below).[31]"

Good one, I didn't think about that one. It means that galaxies are travelling lightning fast, literally.

It's also cosmology, but for what it's worth, the solar system is moving at about 230km/s around the milky way so you could say every one of us is moving at such a number.

If you continue down this train of thought you eventually realize that outside of earth, velocity is just a number and all that really matters is the relative velocities of the two. So it's impossible to definitively state which one is moving or if they're both moving and if so how fast each one is moving. And then these sorts of things seem far less impressive.

And then it gets weirder when you consider that galaxies don't actually collide at all because they're so sparse and all that really happens is they pass through each other and maybe they permanently merge or maybe some of the matter in one galaxy joins the other one and they just keep drifting apart in slightly different directions.

This thought reminds me a lot of the Tesla unit. https://en.wikipedia.org/wiki/Tesla_(unit)

It goes from 3.2 x 10^-5 to 1200 fairly gradually and then suddenly to 10^9. My partner at the time looked at this and remarked "There are some unexplored orders of magnitude" which feels like a good, dramatic phrase.

c is just crazy fast. But still slow compared to the size of the universe!

Source paper: WEAVE First Light Observations: Origin and Dynamics of the Shock Front in Stephan’s Quintet https://academic.oup.com/mnras/article/535/3/2269/7904663

2M mph ~= 0.5 AU/day

When put in that context, it seems rather slow.

At solar system scale (and bigger), even light is slow.

Even at our solar system's scale it can seem slow. Pluto gets to about 50AU from the sun, but the Oort cloud is between 2,000 and 5,000 AU from the sun. 24 1/2 days until something reaches earth vs almost seven years.

Earth orbits at 2pi AU/year by definition, so it's 29x faster than that. In fact it's not even possible for an object bound within the solar system to move that fast, as the radius of an orbit at that speed would (per the back of my envelope) be well inside the sun itself.

> As the shock moves through pockets of cold gas, it travels at hypersonic speeds—several times the speed of sound in the intergalactic medium of Stephan's Quintet

What's the speed of sound in space?

https://physics.stackexchange.com/questions/162184/what-is-t...

From that page, it's very low frequency stuff. One comment mentions 1 cycle per month.

Earthers will go to any lengths to avoid using fundamental units

"Collision" is a bit of a misnomer when we talk about galaxies. As large as stars are, the matter in galaxies is still so sparse because stars are so far apart. I saw some maths about the coming collision between the Milky Way and Andromeda and the number of projected collisions is like less than 10 (IIRC).

Now I'm sure it would be highly disruptive and there'd be localized catastrophe but it's important to remember how large the distances in space really are.

Galaxy Collision Simulation

https://svs.gsfc.nasa.gov/10687

Shouldn't the "sonic boom" here provide good data as to the existence of dark matter (akin to the Bullet cluster)? Anyone on hn with good background care to comment? Don't see anything in the article about it, but would think is one of the most significant experimental goals from detecting these sorts of collisions.

My understanding is that, yes, the way matter in a galaxy merger behaves acts as strong evidence for the existence of dark matter and the theory that it's made of something that interacts weakly with normal matter.

I wonder how well the local intelligence was ready for that event.

If there was anyone around, they probably weren't significantly affected.

Galaxy collisions have very little impact on the stars in that galaxy, or the planets around them. Stars are so far apart that the chance of even one direct collision, or a pass so close it would disturb planets around a star, is vanishingly small. Only thing that is impacted is the dust and gas in the interstellar medium.

This is unambiguous proof that speed limit[0]s don't work.

[0] https://www.amnh.org/exhibitions/einstein/light/cosmic-speed...

I’m sure this is supposed to be a joke, but if so you’d need to explain.

(Or did you mix up the units? 2 million mph is 0.3% of light speed.)

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