We Traveled Back in Time. Now Physicists Are Angry.
You’re going forward through time one second every second. Congratulations, you’re a time traveler! A bit lame, but let's start here to get to the fun, real time travel to ride on dinosaurs and high-five Einstein.
Time isn’t really a thing that passes but a dimension, a direction you can move in. We think our universe is four-dimensional, with three spatial and one-time dimensions smooshed together into a fancy thing we call spacetime. What you experience as time passing and your life happening is actually you moving through the time dimension. And this dimension is a bummer. While you can go forward-backward, left and right, up and down in the space dimensions, in time you can only go: Forward.
You only get older, never younger. You can’t make detours, but this is an illusion. You have far more control over how you move through time. To explain how this actually works without making a math video, we have to make a lot of physicists grumpy, so please keep in mind that we are simplifying and lying a bit. Ok. So there is a very strange rule: Everything in our universe moves at the speed of light through four-dimensional spacetime.
Your speed through spacetime is the sum of your separate speeds through time and space. It's impossible for you to stay still. Even if you are not moving through space dimensions, you are moving through the time dimension, blasting face-first into the future. You can slow down in the time dimension by moving faster through the space dimensions, but in total, you will always move at the speed of light through spacetime. If you could somehow truly stand still in space, you'd still move through time at the speed of light. And the other way around.
Photons, light particles, move at the speed of light through space. They don’t experience any time passing because their speed in that time dimension is 0. In the time dimension, they are frozen in place. If you see light on Earth, from the photon’s perspective, it was just on the surface of the sun and then suddenly crashed into your eye with nothing happening in between. Remember we are telling a science story here; in real life, things are more complicated – for one, it's impossible to truly stand still relative to space itself, you can only be standing still relative to other things IN space. And you can’t really define a reference frame for photons as we just did.
For details, check out our sources, but now back to our story. Ok, so everything in the universe moves at the same breathtaking speed, but this speed is split between space and time. Which leads to all sorts of wacky things and where we can see time travel happen.
Real and Easy Time Travel. When a bus drives past you, it's moving a little bit faster through space than you are. So it moves slower through time than you. Or how it is usually worded: The car is moving slower through time relative to you. While this makes things feel like weird physics magic, it's very simple. Move faster through space, go slower in time. Move slower through space, go faster in time.
And if you add another person moving way faster or slower than you, both end up traveling different distances in the time dimension. The story that is usually told to demonstrate this is to get twins, force one of them into a rocket against their will, shoot them through space super fast while the other twin chills on Earth. And because one twin moves very fast through space while the other moves slowly on Earth, they move at different speeds through time. When the rocket finally gets home, they are no longer the same age, and the younger space twin is ready to start years of therapy for being abducted by theoretical physicists. This is the twin paradox, and it makes perfect sense within the rules of the universe.
If you want to learn more about the paradoxes of time, we made a video about it. But in a nutshell, you are using time dilation, the fact that your position in time is subjective and only true for yourself. With sufficiently fast rockets, you could get as far as you want into the future. Technically, this way, you could witness the end of the universe or look at the final thing that will ever happen! We also made videos about that!
We should mention that there's another way to jump forward in time. Massive objects bend spacetime, causing time to go slower near them. On Earth, the effect is tiny—the gravity of our planet makes time pass 0.00000007% slower than in deep space, far from any gravitational fields. Hardly useful for time travel, but it’s a start. If you move into a really strong gravitational field, like around a black hole, the closer you get to the event horizon, the slower time passes for you.
If we kidnap our twin again, they won’t notice it at all—their watch ticks as fast as it always does. But from the outside, they seem to almost stand still. Only when the twin leaves the gravitational field and comes back to Earth do they realize they time traveled. Ok, so you can time travel to the future by going fast or near a black hole. But this is pretty lame and has literally only downsides except for people who want to sort of teleport into the future.
And it makes building a gigantic human space empire super hard—because even if we develop the technology to go really super fast between stars, going on a far-away journey will always mean leaving your home as you knew it forever. But now that we know all of this, there is actually a way we could use the rules of our universe to travel BACKWARDS through time.
The Past is a Faraway Land – Let’s go there. So you always move through spacetime at the speed of light. If you go fast through the space dimension, you slow down in the time dimension. And here we find an actual opening for backwards time travel: The theory of relativity, which may be the most solid scientific theory we know, does NOT forbid faster-than-light motion through space. IF you move through the space dimension faster than the speed of light—superluminal—you need to move through the time dimension with negative speed.
That means you go BACKWARDS through time relative to the rest of the universe—into the past. Actually, scientists have been looking for superluminal particles and given them a name: “tachyons”. They could exist and would have wild properties like getting faster when they lose energy and, of course, traveling backwards in time. Finally REAL time travel! Just like with photons, it isn't really possible to clearly define how time passes for tachyons themselves—it depends on how you move through spacetime relative to them. But now, for the first time, some observers could actually see tachyons literally traveling backwards in time.
Which means they could be used to directly interact with the past. In theory, you could use them to send messages to your past self and change all sorts of things that have already happened. Maybe with some tachyon-powered sci-fi future tech, you could see a star die before it was born. Meet your great-grandparents. Prevent your past self from making that one dumb mistake, win the lottery, or get eaten by dinosaurs. And create juicy time paradoxes that also kind of break the universe...
Oh, what's that? (Short Pause) Right. Sigh... It seems there are things in our universe that are impossible to mix. Unfortunately, the theory of relativity specifically forbids anything that starts out traveling slower than the speed of light, to reach the speed of light. Because moving faster through space requires energy, and the energy required to reach the speed of light is literally infinite. Even if you could gather all the energy of the universe, you couldn’t accelerate a grain of sand to the speed of light. You can get as close as you like, but you can never reach it.
It is possible to move at the speed of light, as photons do. But they ALWAYS move at the speed of light. If you move superluminally, you will always move superluminally. And sadly, so far, we don’t have the tiniest bit of evidence that tachyons exist. While they work on paper, most scientists think they are not real. So for now, it seems that time travel into the past is strictly impossible, no matter how hard we try. The past is a faraway land, and while we can look at it in pictures, movies, and our memories, it's locked away. Forever outside of our reach.
While this is sad, it leaves you with the most important time there is: Right now, the moment where you actually exist—and a future that's yours to forge. If you want, you could even time travel to it. Luckily, shaping the future is much easier than trying to change the past. With the right knowledge and problem-solving skills, you’ll be ready for whatever obstacles or opportunities lie ahead. Which is where our friends at Brilliant come in.
Brilliant will make you a better thinker and problem solver, with thousands of interactive, bite-sized lessons on just about anything you may be curious about—from physics and maths to technology, data analysis, and beyond. You can build real knowledge in just minutes a day—a terrific use of your precious, present moments.
On Brilliant, you’ll learn through discovery by trying things yourself. Their course on Scientific Thinking, for example, gets you hands-on with fascinating questions like the ones in this video. And since each course is like an immersive workshop, you’ll gain knowledge of key concepts while building your intuition—helping you learn to think like a scientist.
kurzgesagt has also partnered with Brilliant to create a series of lessons to take your scientific knowledge to the next level. Each one is like a one-on-one version of a kurzgesagt video, letting you further explore the topics of popular videos like rabies, mammalian metabolism, or climate science and supernovae. And you can get started whenever, wherever—right from whatever device you’d like. To get hands-on with kurzgesagt lessons and explore everything Brilliant has to offer—from AI and programming to math, science, and beyond—start your free 30-day trial by signing up at Brilliant.org/nutshell. There’s even an extra perk for kurzgesagt viewers: anyone signing up through our link will get 20% off an annual membership once their trial ends.
We found a way to evade the laws of the universe and go on a truly breathtaking time travel adventure. Are you ready? Well, too bad, here we gooooo! Look, it’s the human Era! 12,000 years of human innovation brought to life. Absolutely incredible how our ancestors laid the foundation for the life we lead today. Let’s go back even further and take a look at the timeline of life – wow humans are only a tiny blip at this scale. Mammals haven’t been around for long either.
Here is the age of the dinosaurs—and going further back, we can see where it all began. You can now go on your very own time travel adventure with our Timeline of Human History and Timeline of Complex Life Posters. They are expert-approved and designed with great love and care. Get them both to get the full time travel effect and to support everything we do on this channel.