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Will the gravity of two non-ionic tungsten atoms at rest a hundred light years apart (many relevant details omitted) eventually draw them together?

Physics Asked by owlsupport on February 16, 2021

The original question was evidently far too long to fit into the title, so I’ll post it here in the body:

"If two non-ionic tungsten atoms spontaneously popped into existence at rest a hundred light years apart in the approximate center of a hypothetical non-expanding universe that is a billion light years across but utterly devoid of all other ordinary matter and energy, including "dark matter," would the gravity of the widely separated atoms eventually bring them together into a close encounter or even an outright collision? If so, how closely will each atom approach the speed of light just before that encounter?"

I honestly have wondered about this on occasion for the longest time (in truth, decades) and hope that my obvious ignorance of physics hasn’t rendered the question as phrased trivial or otherwise uninteresting. I’ve tried to phrase it for maximum conceptual simplicity. Nor is it quite clear how to properly formulate this question for a straightforward Google query — my attempts thus far have attracted only a flood of irrelevant results that dance around the core question. The major search engines don’t seem able to cope with so many common physics keywords. -_-

P.S. No, this is not a homework question! I’m just an uneducated dude with odd questions buzzing around in his head and some experience in writing clearly for commercial clients. o_o

One Answer

The force of gravity has no limits, so both atoms will feel a tug from the other one, even if it's 100 light years away. The magnitude of the force will be very minor, around $10^{-96}$ Newtons. The total gravitational potential energy around $10^{-81}$ J, which will result in a collision speed of about $10^{-28}$ m/s. Even if the atoms closed the distance at their top speed the entire time, it would take $10^{36}$ years for them to collide, which exceeds the age of the universe by almost 30 orders of magnitude!

As another way to think about it (in reverse), particles that are very far apart and get drawn in by gravitation alone will collide at roughly escape velocity. Since a tungsten atom is very tiny, its escape velocity is miniscule - you need very little velocity to launch yourself as far away as you'd like, never to return.

NOTE: I need to double check some of these figures, I think I might have messed something up with the fact that there are two atoms moving toward a central point. Regardless, the forces and speeds involved will be very, very tiny.

Answered by Nuclear Hoagie on February 16, 2021

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