These are the questions that keep Mark Zuckerberg up at night

As CEO of a social network that has more than 1.4 billion monthly active users, Mark Zuckberg is certainly familiar with thinking big.

We can assume that his mind often swirls with questions about beaming Internet to people all over the world, finding new uses for artificial intelligence, and turning virtual reality into an every-day part of people's lives.

But today we found out what other kinds of big questions keep Zuckberg up at night.

Zuckerberg held a live Q&A session on Tuesday where he answered a bunch of questions that people left on his Facebook wall.

"I would like to know a unified theory of gravity and the other forces," famed physicist Stephen Hawking posted. "Which of the big questions in science would you like to know the answer to and why?"

Via his response, here are the big questions that Zuckerberg ponders:

I'm most interested in questions about people. What will enable us to live forever? How do we cure all diseases? How does the brain work? How does learning work and how we can empower humans to learn a million times more? I'm also curious about whether there is a fundamental mathematical law underlying human social relationships that governs the balance of who and what we all care about. I bet there is.

These questions aren't all together surprising from Zuckerberg: Before he dropped out of Harvard, he was a psychology major.

During the same Q&A, Zuck also spouted his thoughts on how people will use Facebook technologies in the future:

"One day, I believe we'll be able to send full rich thoughts to each other directly using technology," he wrote. "You'll just be able to think of something and your friends will immediately be able to experience it too if you'd like. This would be the ultimate communication technology."