Life in the Fast Lane
Musings about rapid galaxy encounters
Author’s note: this is a very rough draft of the third section of a piece, or possibly book chapter, about galaxy interactions. You can find the first two sections here.
Many galaxy interactions are short lived: a brief encounter after which each galaxy goes its own separate way, never to see the other again. This happens when galaxies pass too far from each other, or at too high of speeds. But while the interaction is fairly short, these so called high speed fly byes can still leave their mark. Here I’d like to pause and wonder about these short lived interactions, representative of so many of the daily encounters in my life. What do these interactions look like, how are the galaxies changed as a result? How is it that two galaxies that never touch can still make such a difference in each other’s lives?
Flybys, shaped at high speeds
So what do these distant interactions look like? The pictures we have of interactions in progress are really beautiful. As galaxies approach they pull more strongly on the stars and gas on the near sides of the galaxies, often forming a long bridge between them, and streams of stars on the far sides, trailing behind in long narrow tails, like mice holding hands. As they pass stars are flung out in great curving arcs - streaming behind, and also ahead, of their original hosts. Some stars might actually get exchanged in the process. Some form of cosmic highway robbery I suppose, or maybe just a gift for a passing friend, like cookies given in exchange for a favor.
As they pass each other their properties start to change. The tidal stresses propagating through the galaxy can prompt density waves that enhance spiral structure, or create bursts of newly formed stars. Even a short pass leaves evidence of the passage. It’s an interesting feature of galaxies that they often shine brighter and more beautifully under duress. The shocks and pressures of interactions squeeze their gas, causing it to collapse into new shining stars, manifesting in new glorious blue light.
Of course, for these brief encounters the galaxies eventually go their own way. The great tails of stars and gas linger a little, but slowly disperse into space, fading beyond notice, like the scent of new bread after the oven is off. For many galaxies it’s impossible to see that there was any connection, each again flying solo through the vastness of space.
How is it that life is so often like that? Those friendships which you feel in the moment are so strong they’ll never end. Like when we all signed each others middle school year books in 8th grad and promised reunions over bonds we were certain would never fade. And yet, we drift apart. New people enter our lives, time dulls our memories. The stars we shared fade into the background constellation of our memories. Maybe we remember who they when we scroll through our facebook friends, and maybe not. And yet, that interaction, though faded, is still part of me. Or is it?
Tides, the Difference Makers
What is it that creates such change, even when galaxies never touch? Gravity acts over long distances, even galactic ones. So it makes sense that a galaxy passing another might be redirected by its pull, like a skater by the outstretched hand of her dancing partner. But it turns out there’s something even more important than the gravitational pull on the whole galaxy: is the difference in gravitational pulls on different parts of a galaxy that makes the biggest difference.
This difference in pulls goes by a more familiar name - tides. The same mechanism that causes water on the sides of earth to rise — pulled up by the gravitational lift of the moon — can cause great shifts in passing galaxies. The pull of gravity depends on the distance to an object. Because galaxies are so large, stars on one side are pulled harder than stars and gas in the middle of the galaxy, and those in the middle of the galaxy are pulled harder than stuff on the far side. These differences in forces can be so strong they pull a galaxy apart, such that stars and gas nearby the passing companion can get completely pulled off the galaxy, and more, the bulk of the galaxy can actually get pulled away from the stars and gas on the far side.
I wonder if it isn’t the difference in attractive forces, more than the forces themselves, that really shape my life. Differences that determine which tugging team wins, which direction I go. And those things that pull on my attention but leave my heart behind, or that pull on my heart, and leave my soul in empty space. Sociology textbooks write about cultural deviance. “What deviant actions or attitudes have in common is some element of difference that causes us to think of another person as an ‘outsider.’” But it’s not just people who have moved outside of cultural norms, it also can be the bulk pulling away from people, leaving them out - deviant. Like tides creating tension across a galaxy, societal strain encourages deviance, which in turn encourages social change. Tidal tensions drive the changes in galaxies as they interact, tidal tension transforms trajectories, and changes appearance in big beautiful ways. Invisible tides are the drivers of change, the forces that underly the and promote the continued life of a galaxy.
Great job with applying galactic interactions to personal interactions Luke! I never knew you were a writer.