We Traveled Back in Time. Now Physicists Are Angry.

Summary

This script delves into the concept of time travel, explaining that while moving forward in time is inevitable, the rate at which we experience it can be altered by speed and gravity. It introduces the twin paradox and time dilation, showing how traveling at high speeds or near massive objects can affect the passage of time. The script also explores the theoretical possibility of backward time travel through superluminal motion, but ultimately concludes that, according to our current understanding of physics, traveling to the past remains impossible. It encourages focusing on the present and the future, with a call to action to enhance one's problem-solving skills through educational platforms like Brilliant.

Takeaways

• 🕰 Time Travel Concept: The script introduces the idea of time travel, suggesting that time is not a thing that passes but a dimension we can move through, part of a four-dimensional spacetime.
• 🌌 Universe Dimensions: It explains that our universe is considered four-dimensional, with three spatial dimensions and one time dimension, all intertwined into spacetime.
• 🚀 Forward Movement: The human experience of time is likened to moving forward through the time dimension, with no ability to move backward, unlike the spatial dimensions.
• 💡 Speed of Light Rule: The script mentions a fundamental rule that everything in the universe moves at the speed of light through spacetime, a combination of speed in time and space.
• 🌐 Time Dilation: It discusses the phenomenon of time dilation, where moving faster through space results in moving slower through time, as illustrated by the twin paradox.
• 🌌 Gravitational Time Slowing: The effect of gravity on time is highlighted, with time moving slower near massive objects, such as a black hole.
• 🔮 Future Time Travel: The script suggests that traveling to the future is theoretically possible through high-speed travel or being near a black hole, due to the effects of time dilation and gravitational time slowing.
• 🚫 Past Time Travel: It explains that, according to the theory of relativity, traveling back in time is not possible because no object with mass can reach or exceed the speed of light.
• 🌌 Superluminal Motion: The concept of superluminal motion (faster than light) is introduced, which theoretically could allow for backward time travel if particles like tachyons exist.
• 🚀 Energy Limitation: The script points out the impossibility of accelerating any object with mass to the speed of light due to the infinite energy required, thus making superluminal travel and time travel to the past unlikely.
• 🌟 Embracing the Present: The narrative concludes by emphasizing the importance of focusing on the present and shaping the future, rather than attempting to change the past.

Q & A

What is the concept of time travel presented in the script?

The script presents time travel as moving through the time dimension in a four-dimensional universe, where time is one of the dimensions of spacetime.

Why can't we move backward in time according to the script?

In the script, it's mentioned that while we can move in any direction in space, in the time dimension, we can only move forward, which is considered an illusion but a rule of our universe.

What is the speed of light rule in the context of moving through spacetime?

The script explains that everything in the universe moves at the speed of light through four-dimensional spacetime, which is the sum of separate speeds through time and space.

How do photons experience time according to the script?

Photons, being light particles, move at the speed of light through space and do not experience any time passing because their speed in the time dimension is 0, making them frozen in time.

What is the twin paradox mentioned in the script?

The twin paradox is a thought experiment where one twin travels through space at high speeds while the other remains on Earth, resulting in the traveling twin aging slower and being younger upon return.

How does the script explain time dilation?

Time dilation is explained as the effect where moving faster through space results in moving slower through time, and vice versa, affecting the aging process of individuals differently.

What is the effect of gravity on the passage of time as described in the script?

The script states that massive objects bend spacetime, causing time to pass slower near them. This effect is minimal on Earth but becomes significant near a black hole.

What is the concept of superluminal motion and its relation to time travel?

Superluminal motion refers to moving faster than the speed of light through space, which according to the theory of relativity, would require moving backward through time.

What are tachyons and their proposed properties in the script?

Tachyons are hypothetical superluminal particles that, if they exist, would travel faster than light and could be used for backward time travel, with properties like getting faster when they lose energy.

Why is backward time travel considered impossible according to the script?

Backward time travel is considered impossible because the theory of relativity forbids anything from accelerating to the speed of light from below it, as the energy required is infinite.

What alternative does the script suggest for 'time traveling'?

The script suggests that instead of trying to change the past, we should focus on the present and the future, which we can shape with knowledge and problem-solving skills.

Outlines

00:00

🕰️ The Fundamentals of Time Travel and Spacetime

This paragraph introduces the concept of time travel in a literal sense, emphasizing that time is not something that merely passes but a dimension we move through, known as spacetime. It explains the universe as four-dimensional, with three spatial dimensions and one time dimension. The key point is that while we can move freely in space, our movement in time is unidirectional, always forward. The script then delves into the idea that everything in the universe travels at the speed of light through spacetime, with our speed through time being inversely related to our speed through space. It introduces the notion of time dilation, exemplified by the twin paradox, where one twin traveling at high speeds in space ages slower than the one remaining on Earth. The summary also touches on the effects of gravity on time, as demonstrated near massive objects like black holes, where time passes more slowly the closer one gets to them.

05:01

🌌 The Pursuit of Backwards Time Travel and Its Limitations

The second paragraph explores the possibility of traveling back in time, a concept that is theoretically allowed by the theory of relativity if one could move faster than the speed of light, a phenomenon referred to as superluminal motion. It introduces the hypothetical particle 'tachyon,' which could move faster than light and thus travel backward in time. The script discusses the implications of such particles, including the potential to interact with and change the past, creating time paradoxes. However, it ultimately concludes that according to the current understanding of physics, nothing with mass can accelerate to the speed of light, making superluminal motion and the existence of tachyons highly unlikely. The paragraph ends on a philosophical note, encouraging the audience to focus on the present and the future, which are within our reach and can be shaped by our actions.

10:01

🗺️ Time Travel Adventures with Historical Timelines

The final paragraph shifts from theoretical physics to a more tangible form of time travel—the exploration of history. It invites the audience to embark on a journey through time by using 'Timeline of Human History' and 'Timeline of Complex Life' posters. These visual tools are designed to provide an educational and engaging experience of history, allowing individuals to witness the progression of human civilization and the broader timeline of life on Earth. The paragraph serves as a call to action, encouraging the audience to support the channel and gain a deeper understanding of our past by acquiring these educational resources.

Keywords

💡Time Travel

Time Travel is the concept of moving through time, analogous to moving through space. In the video, it's discussed as a dimension where one can move forward but not backward, highlighting the limitations of our perception of time. The script explores the idea of time travel not as a fantasy but through the lens of theoretical physics, suggesting that moving at the speed of light or experiencing strong gravitational fields could affect one's experience of time.

💡Spacetime

Spacetime is a four-dimensional continuum that combines the three dimensions of space and the one dimension of time into a single fabric. The video describes it as 'smooshed together,' which is a simplified way to convey the complex idea that our universe is experienced as a unity of space and time. The concept is central to understanding how moving through space can affect the passage of time.

💡Twin Paradox

The Twin Paradox is a thought experiment in special relativity where one twin travels through space at high speeds while the other remains on Earth. Upon the traveling twin's return, they find they have aged less than the twin who stayed behind, illustrating the concept of time dilation. The video uses this paradox to explain how relative motion affects the experience of time, emphasizing the subjective nature of time passage.

💡Time Dilation

Time Dilation is a difference in the elapsed time measured by two observers due to the relative difference in their velocities or gravitational fields. The script mentions time dilation to explain how moving at different speeds or being in different gravitational fields can cause individuals to age at different rates, which is a direct consequence of moving through spacetime at different rates.

💡Speed of Light

The Speed of Light is a universal constant in a vacuum, approximately 299,792 kilometers per second, and is the maximum speed at which all energy, matter, and information in the universe can travel. The video script states that everything in the universe moves at this speed through spacetime, emphasizing that this speed is constant regardless of an object's motion through space.

💡Photons

Photons are elementary particles that are the basic units of light. They always travel at the speed of light and, from their perspective, time does not pass. The script uses photons to illustrate the concept that at the speed of light, time effectively stops, which is a key aspect of understanding time dilation and the relativity of time.

💡Gravitational Field

A Gravitational Field is a region around a massive object where it exerts a force on other objects due to gravity. The video explains that strong gravitational fields, such as those near a black hole, can slow down time for an observer within that field. This is an example of how gravity can warp spacetime, affecting the flow of time.

💡Black Hole

A Black Hole is a region of spacetime with a gravitational force so strong that nothing, not even light, can escape it. The script mentions black holes as an extreme example of gravitational fields affecting time, where time can appear to almost stand still for an observer near the event horizon.

💡Superluminal

Superluminal refers to speeds that exceed the speed of light. The video script discusses the theoretical possibility of superluminal motion, which would require an object to move backward through time. This concept is central to the hypothetical idea of backward time travel, although it is currently considered impossible.

💡Tachyons

Tachyons are hypothetical particles that always travel faster than light. The script introduces tachyons as a means to explore the possibility of backward time travel, suggesting that if such particles existed, they could be used to interact with the past. However, it also notes that there is no evidence for their existence and most scientists consider them theoretical constructs.

💡Relativity

Relativity, specifically the theory of relativity developed by Albert Einstein, is a fundamental principle of physics that describes the behavior of objects in motion and the effects of gravity on spacetime. The video script uses the theory of relativity to explain why moving faster than light is considered impossible, as it would require an infinite amount of energy.

Highlights

Time is not a thing that passes but a dimension, part of a four-dimensional construct called spacetime.

We only move forward in time, unable to go backward, unlike spatial dimensions.

Everything in the universe travels at the speed of light through spacetime, a combination of speed through time and space.

Photons, as light particles, move at the speed of light in space and experience no time passage.

The twin paradox demonstrates time dilation, where one twin traveling at high speeds ages slower than the one on Earth.

Massive objects like black holes bend spacetime, causing time to pass slower near them.

Time travel to the future is theoretically possible by moving fast or being near a massive object.

The theory of relativity does not forbid motion faster than light, which could enable time travel to the past.

Superluminal particles, or 'tachyons', could travel backwards in time but their existence is unproven.

The energy required to reach the speed of light is infinite, making it impossible for objects to accelerate to light speed.

Time travel into the past seems impossible based on current understanding of physics.

The importance of living in the present and shaping the future is emphasized over attempting to change the past.

Brilliant.org offers interactive lessons to improve problem-solving skills and scientific thinking.

Kurzgesagt has partnered with Brilliant for a series of lessons to deepen scientific knowledge.

Time travel adventures can be experienced through the Timeline of Human History and Timeline of Complex Life posters.

The posters are expert-approved and support the channel's educational content.

Transcripts

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 space time, 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 Far Away 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  

and 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.