Name: We Traveled Back in Time. Now Physicists Are Angry.
Uploaded: 2026-01-15T16:56:29.000Z
Duration: 11 min 20 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Congratulations, you're a time traveller.

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 space time.

What you experience as time passing and your life happening is actually you moving through the time dimension.

While you can go forward, backward, left and right, up and down in the space dimensions, in time, you can only go forward.

You have far more control over how you move through time.

To explain how this actually works, without making a maths video, we have to make a lot of physicists grumpy.

So please keep in mind that we're still simplifying and lying a bit.

Everything in our universe moves at the speed of light through four dimensional space time.

Your speed through space time is the sum of your separate speeds through time and space.

Even if you're not moving through space dimensions, you're 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 space time.

If you could somehow truly stand still in space, you'd still move through time at the speed of light.

And the other way round, 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 zero.

In the time dimension, they're 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're 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.

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.

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's usually worded, the bus 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 had another person moving way faster or slower than you, both end up traveling different distances in the time dimension.

The story that's 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're no longer the same age.

And the younger space twin is ready to start.

Years of therapy for being abducted by theory.

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're 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.

This way you can witness the end of the universe or look at the final thing that will ever happen.

We should mention that there's another way to jump forward in time.

Massive objects bend space time, causing time to go slower near them.

The gravity of our planet makes time pass 0.000000007% slower than in deep space, far from any gravitational fields.

If you move into a really strong gravitational field, like around a black 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 faraway journey will always mean leaving your home as you knew it forever.

But now that we know all of this, there's actually a way we could use the rules of our universe to travel backwards through time.

So you always move through space time 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.

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.

They could exist and would have wild properties like getting f 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.

It seems there are things in our universe that are impossible to mix.

Unfortunately, the theory of relativ 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.

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We Traveled Back in Time. Now Physicists Are Angry.

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