There is no shortage of musings about what it would be like to travel through time. In this article, I invite you to a thought experiment that need not imagine a magical craft that vanishes and reappears at different points in a timeline.
In fact, let’s get this “machine” business out of the way. I’ll tell you exactly how to travel forwards and backwards in time. Note that forward motion is already possible but is limited by our patience and preservation of the observer’s organism. So the first half is solved. Backwards movement is the only meaningful topic to discuss here.
What is the difference between now and fifty years ago? The difference is that much of the particles and energy in the universe were in different locations and at different states.
So if you’d like to go backwards 50 years, simply rearrange the universe to its original layout and you’d objectively find yourself having traveled through time. Simple!
Shuffled Deck Universe
Now we’ll take this a step further. Imagine a fictional universe that consists only of a wine glass and a hammer. Everything else is nothingness. No stars or galaxies in this universe, only the glass and the hammer.
Back in our universe, we write a computer program that can simulate every single possible combination of every particle’s position and every potential state of energy. It creates a random order catalogue of every possible combination for us and we can view any combination as a little playing card.
There would be a lot of cards in this deck, and we know these cards would be in it.
Looking at these, it’s tempting to say this documents a timeline in this universe from left to right. But we know these are just three conveniently picked cards. Most of the cards in the deck look like unrecognizable clouds of particles. In some cards, the hammer is a rabbit. In some cards, the hammer has a wine glass for a stem. We know this because we have every possible combination. It is a very very big deck.
Similarly if we could observe a snapshot of a universe and saw the card on the right, we’re tempted to say the first two cards preceded it. But we don’t know the starting state of the particles. There could never have been the middle card and we just happened to check in when the particles randomly came together to form a hammer and some shattered glass. It’s tempting to stick to a narrative but it’s truly just as likely for particles to bump into each other forming a whole wine glass as it is a shattered one. The universe doesn’t care about what we expect a glass to look like.
Now an observer is found in the universe. He tells us he saw the hammer hit and break what was once a whole glass. Now it’s looking pretty good that we can say this is a moment in time and the three cards are in order on a timeline of events.
But what if I told you we still aren’t looking at the universe? We are still looking at the computer’s vast library of combinations? This observer, his blood vessels, his brain, and even his thoughts and memories are manifested by the particular position of particles and states of energy at this snapshot in time.
Again, we’ve only picked a card from the deck that is convenient for our human minds. There are also cards where the observer is thinking of a banana. There are cards where the observer’s brain is a banana. There are cards where the observer is a banana.
Moreover, if this were real, the observer in this snapshot would believe in time itself. He would think one event preceded the other because the particles were aligned so. He doesn’t actually have cause or effect. He is like the card as he is truly a set of particles frozen in place. It is the particles of his moment of thought that allow him to believe another state preceded this one, whatever preceded means.
Just like the computer program, there is no correct sorting of the cards. The program was designed only to generate every possible combination. Because we see similar looking combinations that imply cause and effect does not mean they belong together in order. In fact we’d never be able to reconcile alternate futures in these timelines anyway.
Is this very moment actually an eternal fixed or ethereally coincidental rearrangement of matter and energy? Does time even exist?
Maybe if you wait a second you’ll see we advanced just a second into the future. Or maybe we didn’t. Maybe this second was the true state all along and our memory of the previous second is part of that. We are actually a single frame of a movie: A coincidental convergence between two chaotic clouds of particles in the frame before and after.
Maybe the entirety of time actually is the output of the program. All of time is the full set of all combinations of all matter and energy. There is no correct sort order. There is nothing connecting one configuration to another. They are all independently possible configurations.
The time traveler
Imagine one studious scientist actually does build a device that rearranges the universe so that he may “travel” “backwards” in “time.”
He turns on the machine and it works perfectly. Unfortunately, the particles from 200 years ago include the particles that make up his being. That long ago, some were in the Earth, some were in stars, some were in flowers. The particles making up him and his machine and everything in the universe are meticulously put back to their original location 200 years ago. It is “now” 200 years ago.
Effectively there was no traveling to observe. It is like we moved from one arbitrary card to the other in our computer program. Nothing links these two configurations to say we even moved from one to the other. The universe is either configured as today minus 200 years or it is configured as just today.
The Hollywood version of time travel lets us make extra particles. We can keep the observer preserved. But this is a paradox. 200 years ago, the observer wasn’t there. So strictly speaking, if he stays preserved and rearranged the universe, there will be missing pieces. He did not travel to the correct configuration. He would need to have created extra matter and energy to begin with, something that has never happened in the universe before.