It's a very bright bar, but Sarah doesn't feel herself starting to burn, yet, even though this dress only goes just past her knees and elbows and isn't of sunblocking material.

She goes in, cautiously.

". . . Hello, there."

"Hello! First time in Milliways?"

"It is."

Anda grins. "It's an interdimensional restaurant! The counter is sapient and sells stuff from over 8^16 universes*! I loaded up on textbooks and have been waiting for an alien to come along and it's great to meet you!"

*Anda is using the word for "separate realities with different physical laws" not the word for "timelines/Everett branches". 

Oh, she's going to get to be the one who's saved (at least!) two worlds.

"Delightful.  What's your name, darling?"

"Anda, what's yours?" Oh hm possibly you shouldn't tell your real name to unfamiliar slightly purplish humanoids from other universes. Whatever; any given set of fictional tropes is probably totally wrong and if Anda's screwed in this interaction it'll be from something A's never heard of.

"That's pretty!  I'm Sarah.  Sarah Catalyst, if you need a distinguisher."

"I don't know any other Sarahs. Want to tell me about your universe or should I tell you about mine first?"

"You first, if you're offering."

"From what I've heard from Bar we have a relatively simple fundamental physics; minds are complex computations rather than ontologically fundamental. We have round planets orbiting giant nuclear-fusion-based stars and the language-using species on our planet are humans and parrots with humans being the only ones that develop new technology. We live in cities of a few million individuals with a mix of elected and appointed governments and have a planetwide computer network, small research outposts on our moon, travel to anywhere on the planet on a timescale of hours, and gradually increasing standards of living in most places most of the time. Humans are warm-blooded omnivores born helpless and raised by our parents and I'm phenotypically normal though my hair colour is artificial."

"My world has humans as well!  Though it's been quite some time since I've met one.  You're very good at thorough descriptions; I'm not sure I can match your detail."

"You look a lot more like a my kind of human than parrots do; do the species share a recent common ancestor? What's yours called? Also if you just say all the points where your universe is different from the description I gave that'll probably be detailed enough for a first pass."

"I'm myself an ex-human.  Our planets and stars sound similar but my people live less densely and more nomadically.  I'm terribly excited to learn more about your technology; I don't think we're as advanced.  And the rest I'm not very sure about."

"Ooooh, an ex-human? We don't have anything I'd describe as that yet; what'd you change? And I'm no engineer but I can help you pick topics to get books on from Bar."

"Well," she appears right next to Anda, and then back a few feet away, without any visible transition.  "I'm much faster, more durable, and happier.  To start with.  Annnnd," she singsongs, "I can make other people the same way.  - Other humans.  It doesn't work on parrots, sadly."

Anda jumps at the sudden proximity, then grins. "Oooooh, shiny--is the happiness a change to your underlying psychology or is it just that being fast and durable is fun? Also are you conceiving of this interaction as a trade interaction or a reciprocal altruism interaction without formal value tracking or a joint optimization scenario or other?"

"Being fast and durable is very fun.  I don't feel too different from when I was human but some people do.  But, ah, may I sidetrack and ask - are you already someone particularly special, in your world?"

"Special? Oh no, I'm incredibly boring. Except for having found this place, but whatever it uses to pick people it picked a boring one this time. Why, are you special?"

"As it happens I was the one at the start of the chain which led to almost everyone from my world becoming fast and durable and whatnot.  I'd be very happy for the chance to do it again."

"That's really cool! Is it, uh, contagious, in a way where 'make it available to anyone who wants it but let anyone who doesn't want it avoid it' is impossible or difficult? Or does 'start of the chain' just mean you invented it?"

"It's somewhat contagious but I think it would be possible to avoid it spreading to people who for whatever reason don't want it, now that I know enough to handle things with a little more finesse.  I certainly won't turn you without meaning to."

"Okay. I'll want more information on negative externalities I might cause before I decide whether to take you up on it. Was my question about how you're conceiving of this interaction lost in the conversationtree or did you not want to answer it?"

"I said I'd be very happy for the chance to do that again!  I'd also be very happy if you shared your technology with us but I'm not going to hold out for a trade while people are going around dying and living small lives!"

"Oh! Important thing about this place I forgot to say! While we're in here time is stopped in our home universes! Nothing is urgent and we can spend a week getting you lots of technology information and figuring out the logistics of distributing your thing."

"Exciting!  Is there a point in between a week and a few centuries where anything would change, or could I stay here truly as long as I might want, do you know."

"Uhhh, I only have another seventy or eighty years before I die of old age and am now even more curious about your transmissible transhumanity thing. Also food and a room costs money; I'm just planning to go home with enough resellable technology that it'll be okay if I spend a few months burning through my savings first. If your society hasn't invented tapcards or whatever we can figure something out."

"I won't let you die of old age.  And I don't need food, for the most part, and might not need a room.  But no, we've not invented tapcards."

'Won't let you' is kind of an unnerving way to phrase that but Anda is too happy about life extension to care. "How long are you going to live if nothing bad happens? And how long are your years? And what futuristic-for-you technologies are you most interested in picking up?"

"So far we've only died of bad things happening, and the oldest of us is around five or six hundred years of three hundred sixty-five and a quarter days each.  I would desperately like to go to the moon and the planetwide network is intriguing."

"That's so incredibly cool and I should probably not tell you to do it to me right now but I'm very tempted. I can get you books on rocketry and the internet--do you know the laws of motion that govern the movement of the planets? What are your society's main energy generation and storage technologies? What's your current state of the art in communications? So I know what prereqs to cover."

She does a little shoulder shimmy.  "I at least personally don't know those laws.  What sort of things do you classify as energy: electricity or something more futuristic?  We have non-planetwide networks for text and sound, in some places, and people sometimes serve as relays between those."

"Yeah, we run everything off electricity. Well, rockets use chemical fuels, but if you have abundant electricity you should be able to synthesize propane and lox and stuff. How do your current communications networks work? Do you have any kind of electricity-based calculating machine?"

"I don't know of one.  Some communications networks run wires everywhere in that network, and either send text by encoding it as sounds of various lengths or send sounds directly.  But radios are more common and they don't use wires.  - Does running around leaving notes at landmarks count as a communications network; we do an awful lot of that but it's not very technological."