The Paradox of Time Travel

On June 28th, 2009 Stephen Hawking hosted a cocktail party for time travelers. He sent out an invitation to all time travelers to attend to the party in his Documentary series Into the Universe With Stephen Hawking. No time travelers took Hawking up on his invitation. That poses an interesting question, does that mean time travel will never be invented or just that Stephen Hawking threw lousy parties?

In this post, we'll conduct a few thought experiments to discover for ourselves that not only does time travel appear to be a technology far beyond our reach, But its invention is its own undoing. You don't need any math or physics to prove this to yourself, just a little logic.

Everyone Wants Time Travel

I'll keep this section brief to avoid stating the obvious. It takes very little imagination to understand how any individual could benefit from time travel. Simply pick up a sports almanac and never miss a bet for the rest of your life. That works for you but inherently every dollar you earn is a dollar lost for some non-time traveler.

The other obvious value add to society would be predicting the weather and other natural events. Knowing how bad a flood will be, how cold the winter will get and when a earthquake will happen would save huge amount of lost value to people around the world.

It's fair to say if there's ever a tiny chance we could invent time travel, whole governments will stop at nothing in order to invent it as quickly as possible.

What exactly is Time Travel?

Story writers have come up with countless sets of rules for time travel. Let's establish a minimum baseline for what must be true of a time machine.

A Time Machine Cannot Alter What Has Happened

You can prove this to yourself with a simple thought experiment. Imagine you accidentally dropped a beloved vase; you have a friend with a time machine so you ask them to travel back in time and save the vase. You watch them depart in the machine and then... what?

If they fail to save the vase:

It is unfortunate, but it makes sense. Maybe they went back a day too far, maybe there was just no way to prevent it, maybe they are the reason it was smashed in the first place.

If they save the vase

After they depart and rescue the vase, you're standing there holding a bag of broken vase. They can't return to you holding the recovered vase because you hold the pieces, where else could you have gotten them from?

The answer is they never did rescue your vase. What they really have is not a time machine so much as a way to travel between multiverses. From their perspective they travel back in time to rescue the vase and then travel back to the present to return it. But in reality there's one universe where the "time traveler" just vanished without a trace and another where they showed up unprompted and stole and returned a vase.

While the ability to travel between multiverses is awesome for the traveler; it is useless at best to all others who are left behind.

Paradoxes

A paradox is when an action from the past and the future contradict. For example; our time traveling friend would cause a paradox by saving our vase since the vase being broken is why we asked them to travel back in the first place. It would seem like we could never resolve if we would ask them to go back in time at all. But there's one key detail that prevents this sort of loop; Every present has a single past and every past has a single future.

This makes sense, you can remember your past so any present where you took different actions to get there would still be different from one another. Imagine the timeline is a highway and you are a car driving along it. In this example Time travel would putting the car in reverse. If you have a loop/paradox that would be a road with no on or off ramps. You can't get onto that road because to have an on ramp would mean there are multiple pasts for a single future.

What can you do with time travel?

Temporal Computation

Let's consider the simplest possible use of a time machine. We wont send people or things back in time. We'll just send a text message to the past. You could actually solve very complicated problems efficiently. Imagine I want to find what page of a book has a quote I like in it. I could use this simple procedure to find it nearly instantly:

1. Check my phone for a text from the future with a page number
2. Check that page in the book
   1. If I don't see the quote: Text my past self the next page number
   2. If I do see the quote: Text my past self the correct page number

While in many parallel loops you looked at all the other pages, you will only experience the one loop where you jump directly to the correct page. This is because we know time travel cannot alter the past, so the only reality where this situation resolves itself correctly is one where you do not alter it. Where you send the page number back to yourself that you remember receiving.

We're taking advantage of what I call instability to find our answer. We're causing a paradox who's only resolution is our answer. It is easy to imagine ways to use this for dramatically more complicated problems. There are just a few things you have to be sure of in order to use this:

• A solution must exist: Some problems have no real solution. For example if you try to find what number you can square to get -1 it doesn't matter how high or low you count you'll never find a solution. It is not clear what would happen if you tried.
• The solution space must be countable: If your answer is for example an irrational number you will never find a solution that's perfectly correct, and you need to be able to know what solution to attempt after a failed attempt.

Any problem that fits into these criteria could be solved near instantly with relatively small effort.

Why Can't We Time Travel?

Brian Greene has a Wonderful TED talk that among other things talks about Johannes Kepler trying to figure out why the earth is 91 million miles from the sun. Kepler thought there should be some astronomical reason behind it. Brian Green says Kepler's question was wrong, the question should have been "Why do we Humans find ourselves on a planet 91 million miles from the sun?". That answer is easy, because much closer and we'd cook, much further and we'd freeze. We could have ended up on another planet if it had been the right distance from our star.

We can relate this idea to our question of why isn't there time travel; "Why do we humans find ourselves in a world where time travelers don't show up to parties?". I believe the answer is clear, if we did invent time travel the chaos it would cause in its own past would be so great, that we would destroy any chance of inventing time travel in the first place.

What Chaos In the Past?

Let's imagine you go back in time to visit your great grandparents in the past. You hardly talk to them, you don't tell them who you are, you're interacting as little as possible. It is safe because you aren't causing enough disturbance in the past to interfere with the eventual invention of time travel...right?

This is where I think many people would point to the butterfly effect; but something much more sinister could happen. Consider that you carry countless viruses and bacteria with you that you have become immune to over time. What happens if one of your viruses manages to stay in the past, and infect someone maybe not seriously but say it manages to survive through various hosts all the way back to you. Now, when you go back in time not only are you carrying the original virus, but you carry the virus after a few generations of evolution. You can imagine how that virus would nearly instantly evolve into something no human immune system could handle.

Now it is easy to dismiss this and say "well I'll just be really careful, I wont get close to anyone etc." But consider that if you can time travel others can and will as well. Someone will make this mistake. What that means is if we can time travel we will put ourselves in the classic "I killed my ancestor" paradox.

Stability is the Spice of Time

We can apply some of what we learned with temporal computation to this loop of a terrible virus evolving instantly. We know that the loop will continue until it finds a stable state, one where what gets sent back in time is the same as what we originally saw come back.

In this case what's the "stable state"? I think it has to be one where time travel is never invented; because we are here.

The Simple Truth Behind Time Travel

After diving into the world of time travel, we've uncovered a simple truth: it's more than just a technological hurdle; it's a conceptual one. Our explorations show that time travel, as fascinating as it is, seems to conflict with the very fabric of reality. It turns out, the absence of time travelers in our world isn't just coincidence; it's a hint that time travel, for all its allure, may prevent its own invention.