This is one item on the list that I came up with, and I meant it as “favorite time period that they have traveled to” instead of, say, a certain year of the show. Sorry for any ambiguity there.

The Doctor certainly seems to spend a lot of time on Earth in whatever the current year is (strange, that) but we do see Earth in the future and the past as well. So many great episodes have taken place at various times in Earth’s history. They’ve traveled to Ancient Rome, Victorian England, World War II, and Pre-Revolutionary France, to name a few. And of course they have traveled to many different time periods elsewhere in the universe. The time period that I picked for today is

The End

I’m lumping together the end of Earth and the end of the universe because they have a similar emotional impact, though the science is different.

The Doctor travels to some version of ‘The End’ in several episodes: “The End of the World,” “Utopia,” “Listen,” and “Hell Bent.”

This may seem like a depressing topic, but to me it isn’t. The end of the universe (or even the earth) is far enough away that it doesn’t really seem sad. It’s sort of abstract. You can ponder what will eventually happen to the human race, or to space itself.

“The End of the World” gets the premise correct – in about 5 billion years, the sun will use up all of its available hydrogen fuel. This point is considered to be the beginning of the end of a star’s life. The core will start to collapse and heat up, causing the outer layers of the sun to expand. It will grow into a type of star called a red giant, possibly engulfing the earth. However, our planet becomes inhospitable long before this, about 1 billion years from now. This is due to the sun gradually increasing in brightness over time.

The end of the universe is another story. There are several different scenarios detailing how the universe might end. The one that episodes like “Hell Bent” and “Listen” seem to be hinting at is the ‘big freeze’ aka the heat death of the universe. Heat always flows from hot to cold, so given enough time, the entire universe will come to a uniform temperature slightly above absolute zero. At this point there will be no stars left, and no energy available to do anything.

Another scenario is the ‘big crunch,’ which envisions the universe sort of like a rubber band. Right now the universe is expanding, but at some point it may stop expanding and start to collapse in on itself.

The opposite of this is the ‘big rip,’ where the expansion accelerates, spurred on by dark energy, until the entire universe is ripped apart.

There are many more ideas out there regarding the fate of the universe, but the scientific consensus seems to favor the ‘big freeze,’ at least right now.

I always end the semester in my physics classes by talking about the end of the universe. It gives everyone something nice and optimistic to think about over summer/winter break!

What’s your favorite time period? Is there one you would like to see them visit next?