How should I respond to someone's research "elevator pitch"? I am starting my PhD in cosmology in October of this year. I have already met some of the older PhD students and postdocs in the department where I will be studying, and I am going to be attending two summer schools before I start the PhD where I will meet more new people, from other institutions.

It seems a natural conversation opener to ask people what their research is about or what they are working on at the moment, and in response I generally get their 1-2 minute elevator pitch summarising their work.

My question is: what questions are best to ask them to keep the conversation going after they have given this spiel when I am a novice in the field?

I don't want to squander what could lead to a valuable discussion, but at the moment I can do very little except make some banal comment such as "that sounds interesting..." and let the discussion drift away from the nitty-gritty details of the field, because most of the time the things they tell me about are completely new to me (but presumably old hat to them). 

I don't feel that I know my own topic well enough yet to explain it properly in return- so what could I say instead?
what questions are best to ask them to keep the conversation going after they have given this spiel when I am a novice in the field?


You're overthinking this. Pretend it's a conversation between human beings: do you have a question about what they said? Ask it. Don't you have any question or you find their research topic utterly boring? Change topic of conversation. Exactly as you would do with anyone else.

Summer schools frequently have students with different backgrounds and from different countries; you will be all in the same situation, and for most of you it will be a new place: there will certainly be things around you that can be subject of conversation.

As for your last question:


  I don't feel that I know my own topic well enough yet to explain it properly in return- so what could I say instead?


Tell the truth: That you're going to start your PhD on topic X, that you like it for reason Y, but you still don't know much about it because you just started and that you will be super-happy if you can meet someone there working on the same topic.