"To be fair, this is a matter of belief," he said, "But it's a belief that's informed by the laws of physics that we know." In other words, these guys aren't blindly guessing, but they're out far enough on the frontier of knowledge that respectable physicists can agree to disagree.

If there's only one universe and you can't change the present from the past, then how can we change the future in the present? This raises the contentious possibility that we can't. Goldberg says this may be the way it is. Others see loopholes that would allow us to have the kind of free will we seem to be experiencing. "The right way to think about the future is that it's changeable," said Carroll.