We are all time travelers, journeying together into the future. But let us work together to make that future a place we want to visit.

So some might imagine that I am an expert on time, although of course these days an expert is not necessarily a good thing to be.

Quantum physics tells us that no matter how thorough our observation of the present, the (unobserved) past, like the future, is indefinite and exists only as a spectrum of possibilities. The universe, according to quantum physics, has no single past, or history. The fact that the past takes no definite form means that observations you make on a system in the present affect its past.

Disorder increases with time because we measure time in the direction in which disorder increases.

Even if it turns out that time travel is impossible, it is important that we understand why it is impossible.

Up until the 1920s, everyone thought the universe was essentially static and unchanging in time.

The laws of science do not distinguish between the past and the future.