Chat gravity refers to the degree of familiarity with another person, which, if you chance to encounter them on the street, will determine if you merely greet them in passing or stop to talk.

聊天重力指你跟另一個人的熟悉程度決定了你們在街頭偶遇的時候是只打個招呼就過去，還是停下來聊一會兒。

For example:

We saw each other on the sidewalk and said ’hello’ to each other. But she was just a friend of a friend I’d only met once or twice before, and there wasn’t enough chat gravity to merit stopping and chatting.

我們在路邊相遇，互相打了個招呼。不過，她只是我一個朋友的朋友，我們也只見過一兩次面，所以也就沒有足夠的聊天重力讓我停下來跟她攀談了。