Last week on the show we discussed time travel. I’ve always been rather fascinated with the concept of time travel. Where would you go? What would you do? And how would your time travel work? Would it be a car that’s been technologically enhanced? Perhaps an alien space ship that’s bigger on the inside than the outside? Or maybe you’d just fly a ship around the sun really fast?

“Were we able to navigate time as easily as we navigate space, our worldview would not just change, it would undergo the single most dramatic shift in the history of our species.” – Brian Green on Time Travel.

These are obviously questions that really can’t be answered. We can’t travel through time and its possible that time travel is simply against the laws of physics and can’t happen in our universe. So asking questions about time travel are sort of like debating what would happen if aliens existed. Not because aliens existing is a logical impossibility, but because since we only have a reference point of one (Earth) it’s really hard to theorize what aliens might be like.

There is no denying that the idea of time travel fascinates us humans. Modern science fiction is littered with stories of time travel. Everything from Back to the Future, the Terminator films, Doctor Who, Star Trek, and many others have used time travel to tell some pretty amazing stories.

And going further back in time…no pun intended, we have HG Well’s the Time Machine, Mark Twain’s A Connecticut Yankee in King Arthur’s Court, and even Charles Dickens’ A Christmas Carol.

And there are even references in ancient literature to characters traveling throughout time. This idea has clearly been with us for a long time.

But okay, that’s fiction. Is time travel possible? The answer seems to be “well….maybe….”

Stephen Hawking once suggested that the absence of tourists from the future is an argument against the existence of time travel.

According to General Relativity, time travel (of a sort) is possible. You just need to go at the speed of light and time from your perspective will remain the same, but time from some other perspective will progress. So if you went near the speed of light for a day, 1000 years will have passed on the earth. This isn’t quite the same as hopping in the Tardis and jumping ahead 1000 years. Going at or near the speed of light seems pretty much impractical for humans.

What about wormholes?

So imagine a tunnel in space-time that allows you connect two different points that are normally very far apart, but via the wormhole, only a short distance away. Could this in theory allow for time travel? Well not so fast. First, we’ve (and by ‘we’ I mean scientists) have never seen a wormhole, but general relativity doesn’t rule them out either. In order for time travel to occur, you would need the entrance of the wormhole to be progressing at one time while the exit is progressing at another speed. In actuality, it’s more like time-dilation.

Now time-dilation isn’t really what we mean by time travel. Most of us imagine that time travel essentially means the traveler steps “outside” of time and re-enters at a different space in time. For example I would step outside of 2012 and step back in at 1812. Time-dilation is where I would observe time passing at a certain speed (from my perspective) but someone at a different perspective would observe time passing at a different rate. So in theory I could experience time passing normally, but from someone else’s perspective time is passing much much slower.

Of course if the wormhole was actually connecting you to a different point in time or some other universe, that would change the rules slightly.

The problem with much of these ideas and theories is that; well, ideas and theories are one thing but practical testing is next to impossible. It seems that physics would say that travel to the future is conceivable. Travel to the past…well this is still a difficult question and most likely, will remain so until the singularity.

But like I said, when have we let physics get in the way of good conversation?

First, where would you go? – The past or the Future?

What would you do there? – Vacation? Sight see? or would you attempt to alter the cause and effect of time?

Would you say, go back in time and kill Hitler? Would you go to the future and see the winning lottery numbers and then come back to the present to win the lottery?

Would the risk of creating a universe ending paradox concern or limit your time travel? Consider for a moment you go back in time to see a famous historical event, return home only to discover you’ve accidently change history and now must keep going back in time to fix things only to change the timeline even more?

What about minor things? Would you go back in time and stop yourself from eating that shellfish that made you sick? or help yourself from failing that exam? Would you take the opportunity to right old wrongs or would you rather keep them, even if they are/were painful moments in your life?

What sort of time travel would you envision? We have many examples from fiction and a few ideas from science…

Given that my chosen field of study is history, it shouldn’t be a surprise that I would go to the past. I’m not sure exactly what I would do there. Probably take in some sights and answer some old questions. I would like to know how the Pyramids were built. Same with Stonehenge. I would also like to visit Shakespeare’s globe and take in a play or two.

I guess I would be like a time traveling tourist. I’d go visit the Hanging Gardens of Babylon, I’d like to try to figure out if the Clovis people were the first to colonize North America…

I don’t know if I would change history. I would certainly be tempted too. I guess it would depend on if I could undo my changes after the fact. As much as I’d like to right history’s wrongs, I think a certain argument can be made that learning from our past failures was and is an important step in our evolution as sentient beings.

The question of the dangers of time travel seem to be very convincing. Suppose I go back in time to see the Hanging Gardens and upon returning I discover I had inadvertently set off a chain reaction that resulted in the Nazis winning world war 2. I think at that point I’d just go back and kill Hitler… But nevertheless, time travel does suggest a certain amount of risk. Of course, so does space travel. Risk is part of the deal I suppose. And where would we be if we always turned away from something because of risk?

Now since time travel differs so much in our popular media, we should consider some of the ways time travel has been imagined.

1. There is a single fixed history, which is self-consistent and unchangeable. In this version, everything happens on a single timeline which does not contradict itself and cannot interact with anything potentially existing outside of it. Any actions taken by a time traveler were part of history all along, (so if you go back and kill Hitler, you’re not creating a paradox because, in essence, you were meant to kill Hitler.) Thus it’s impossible for the time traveler to “change” history in any way. The time traveler’s actions may be the cause of events in their own past though, which leads to the potential for circular causation and the predestination paradox; where the time traveler creates the event that caused them to travel in time…

There’s also the trend where history is fixed and depending on the creativity of the writer, something prevents the time traveler from interacting with the past…  a strange law of physics, or where the traveler is rendered a non-corporeal phantom unable to physically interact with the past

Or

2. History is flexible and is subject to change (Plastic Time)

Examples include Doctor Who and the Back to the Future trilogy. In some cases, any resulting paradoxes can be devastating, threatening the very existence of the universe. The extreme version of this (Chaotic Time) is that history is very sensitive to changes with even small changes having large impacts, i.e swatting a fly in the past causes the future to change drastically.

But there is also the version of this where History is change resistant in direct relationship to the importance of the event ie. small trivial events can be readily changed but large ones take great effort or simply cannot be changed or else they destroy the universe or something. Doctor Who seems to follow this line of thought.

In the Doctor Who episode The Waters of Mars, Captain Adelaide Brooke’s death on Mars is the most singular catalyst of human travel outside the solar system. At first, the Doctor realizes her death is a “fixed point in time” and does not intervene, but later defies this rule and transports her and her crew to Earth. Rather than allow human history to change, Captain Brooke commits suicide on Earth, leaving history mostly unchanged.

Or

3. Alternate timelines. In this version of time travel, there are multiple coexisting alternate histories, so that when the traveler goes back in time, he/she ends up in a new timeline where historical events can differ from the timeline he/she came from, but her original timeline does not cease to exist (this means the grandfather paradox can be avoided since even if the time traveler’s grandfather is killed at a young age in the new timeline, he still survived to have children in the original timeline, so there is still a causal explanation for the traveler’s existence). Time travel may actually create a new timeline that diverges from the original timeline at the moment the time traveler appears in the past.

One could think of this as if “of all the possibilities (say me having Jello or ice cream) instead of me choosing ice cream and that becoming the true “fixed historical event” in reality, both choices exist in seperate timelines. One where I take the ice cream and one where I take the Jello.

In the Star Trek The Next Generation episode “Parallels” had an example of what Data called “quantum realities.” His exact words on the matter were “But there is a theory in quantum physics that all possibilities that can happen do happen in alternate quantum realities,” suggesting the writers were thinking of the many-worlds interpretation of quantum mechanics.

I wish I had some concrete advice to end on, like if you ever travel back in time, don’t touch anything, even the slightest change could affect the future in ways you couldn’t possibly imagine. But, given the vast differences in opinion on what sort of time line we have, how time travel would work and also, how often do you get to travel in time?

Basically, use your best judgement. If you decide to change history use your critical thinking skills first and rationally weigh the consequences of your actions.