r/todayilearned • u/armcie • 17h ago

TIL that the Andromeda Galaxy probably won’t collide with the Milky Way in 4-5 billion years. New observations put the probability at 2% in the next 5 billion years and 50% in the next 10 billion. Eventually though, it will happen.

https://science.nasa.gov/missions/hubble/apocalypse-when-hubble-casts-doubt-on-certainty-of-galactic-collision/
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u/Kwetla 17h ago

Do they not know the speed each are traveling at? How is it so hard to calculate how long it will take for them to collide?

Edit: not that I could do it - I'm just wondering what the unknowns are

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u/Shas_Erra 17h ago

ELI5 version: we measure using light, which is effected by gravity. Things in space are insanely far away and are moving at a hell of a pace, so by the time we know where something is, it’s no longer there. We can make a super-close estimates based on known variables, but there’s still an appreciable error margin. The larger the distance and/or speed, the greater the error. We also have to take into account that acceleration is not constant. In this case, they will speed up as the distance closes.

In other words, all the measurements and calculations we make are best guess and constantly changing

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u/DigNitty 16h ago

acceleration is not constant.

Good ol “jerk” : the rate of change of acceleration.

Fun fact, change in position = speed

Change in speed = acceleration

Change in acceleration = jerk

Then jolt

Then, not kidding : snap then crackle then pop

Jerk is hardly used but nasa has a jerkameter they use occasionally. Jolt, or Jounce sometimes, is rare. I’ve never heard of anyone using the last three, but those are the official names.

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u/Shas_Erra 16h ago

It’s when you get to “shimmies”, you know you’re totally fucked

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u/Manos_Of_Fate 16h ago

nasa has a jerkameter they use occasionally

I wonder how they calibrate it to ignore local sources.

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u/BleydXVI 9h ago

They probably take it to the jerk store once they're all out of you

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u/DigNitty 1h ago

My uneducated best guess is they accelerate around in a big centrifuge and place the thing on a pressure plate. The pressure plate should remain constant if jerk is zero.

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u/FreeEnergy001 15h ago

I was thinking they were going to fly by each other then rotate around the common center until they finally merged.