I Think Faster Than Light Travel is Possible. Here's Why.

Summary

The video script explores the possibility of faster-than-light (FTL) travel and communication, challenging the common belief that it's impossible due to the speed of light being a universal limit. It discusses Einstein's theory of Special Relativity, the energy required to accelerate objects, and the misconceptions surrounding mass and energy. The script also addresses potential paradoxes related to time travel and argues that current theories, including General Relativity, may not fully capture the complexities of space-time, suggesting that a quantum gravity theory could offer new insights. The presenter encourages physicists to reconsider the potential for FTL travel and ponders on the implications for interstellar communication and the perception of Earth's dullness to extraterrestrial life.

Takeaways

• 🌌 The speaker posits that intelligent life on other planets might find Earth too uninteresting to contact us, and suggests that our inability to send information faster than light is a sign of our technological limitations.
• 🚀 The video aims to challenge the notion that information cannot travel faster than light, a concept rooted in Einstein's theory of Special Relativity.
• 🔬 It explains that the speed of light is constant for all observers and does not change with the observer's motion, as demonstrated by the Michelson-Morley experiment.
• ⚡ The energy required to accelerate an object with mass to the speed of light is infinite, suggesting that only massless particles, like photons, can achieve this speed.
• 🤔 The script questions the validity of the argument that infinite energy is needed to reach the speed of light, pointing out that most of an object's mass comes from binding energy, not rest mass.
• 🧠 It discusses the Higgs field as the source of mass for fundamental particles, and how particles were massless in the early universe before the Higgs field condensed.
• 🕰 The video disputes the idea that faster-than-light travel would lead to time travel paradoxes, arguing that the direction of time on a spaceship could be different from an observer's perception.
• 🔮 The script suggests that our current understanding of space-time through General Relativity is incomplete and may not accurately represent the possibilities of faster-than-light travel.
• 🚫 It argues against the formal reasons often cited against faster-than-light travel, stating that they are based on theories that may not hold up in the context of quantum gravity.
• 💡 The speaker encourages physicists to reconsider the possibility of faster-than-light travel and to challenge conventional thinking in the field.
• 📚 The video concludes by promoting an interactive learning platform, Brilliant.org, as a tool for engaging with complex scientific concepts like those discussed in the video.

Q & A

What is the speaker's opinion on the existence of intelligent life on other planets?

The speaker believes that there is intelligent life on other planets, but they think we haven't been contacted because we are too boring for them to bother with.

What is the central topic of the video the speaker is discussing?

The central topic of the video is the possibility of breaking the speed of light limit and the implications it has for faster-than-light travel or communication.

Why does the speaker believe that the common understanding of the speed of light as a limit might be wrong?

The speaker believes it might be wrong because the argument that it takes an infinite amount of energy to reach the speed of light has some issues, including the fact that it doesn't consider the origin of mass and the conditions in the early universe.

What is the theory of Special Relativity, and how does it relate to the speed of light?

Special Relativity is a theory by Albert Einstein that states the speed of light in a vacuum is the same for all observers, regardless of their relative motion. It implies that the speed of light is a universal constant and a fundamental limit.

Why does the speaker think the argument for the impossibility of faster-than-light travel based on energy requirements is flawed?

The speaker thinks it's flawed because it doesn't account for the fact that most of the mass of objects is actually binding energy, and in the early universe, particles were massless and moved at the speed of light, contradicting the infinite energy requirement.

What is the Higgs field, and how does it relate to the mass of particles?

The Higgs field is a field that permeates the universe, and its condensate gives particles their mass. When particles interact with the Higgs field, they acquire mass, which is why most of an object's mass comes from this interaction rather than the mass of the particles themselves.

What is the electroweak symmetry breaking, and how does it relate to the mass of particles?

Electroweak symmetry breaking is a phase transition that occurred about 10^-11 seconds after the Big Bang, where the universe cooled down and the Higgs field condensed. It is when particles acquired mass for the first time.

What is the concept of a time-like closed loop, and why is it problematic for the theory of faster-than-light travel?

A time-like closed loop is a theoretical construct in which an object could travel in a loop through time, potentially allowing for time travel and causality paradoxes. It is problematic because it suggests that faster-than-light travel could lead to logical inconsistencies.

Why does the speaker argue that the possibility of faster-than-light travel does not necessarily imply time-travel paradoxes?

The speaker argues that the direction of time on a faster-than-light spaceship could be different from what an external observer perceives, and that general relativity, which accounts for gravity, does not necessarily support time-travel paradoxes as special relativity does.

What is the current state of our understanding of space-time, and why does the speaker believe it's incomplete?

Our current understanding of space-time is based on General Relativity, which does not integrate well with quantum theory. The speaker believes it's incomplete because we need a theory of quantum gravity, which we do not yet have, and this new theory might change our understanding of causality and locality.

What is the role of the sponsor Brilliant in the video, and how can viewers benefit from it?

Brilliant is a sponsor that offers interactive courses on various subjects, including physics and mathematics. Viewers can benefit from it by actively engaging with the material to better understand concepts like Einstein's theories and quantum mechanics.

Outlines

00:00

🌌 Speculations on Extraterrestrial Intelligence and Light Speed Limit

The speaker begins by expressing a belief in extraterrestrial life and humorously suggests that the reason for the lack of contact is Earth's lack of excitement. They challenge the common belief that information cannot travel faster than light, citing physics as a reason they disagree with. The video's main topic is the possibility of breaking the speed of light limit. The speaker references a previous video on faster-than-light travel that was not well understood, promising a clearer explanation this time. They also touch on recent discussions of unexplained aerial phenomena, humorously dismissing the idea that these could be extraterrestrial in origin due to Earth's 'boredom,' but acknowledging the possibility. The video then delves into Albert Einstein's theory of Special Relativity, which posits that the speed of light is constant across all observers, leading to the concept that light speed is a universal limit. The speaker uses the example of water hoses and laser pointers on a moving train to illustrate the constancy of the speed of light. They conclude the paragraph by discussing the implications of this theory, hinting at the unexpected consequences that challenge our understanding of physics.

05:01

🚀 The Myth of Infinite Energy for Light Speed Travel

The speaker explores the concept that accelerating an object with mass to the speed of light requires an infinite amount of energy, a conclusion drawn from Einstein's equations. They argue that this idea is flawed, pointing out that the theory does not forbid faster-than-light travel, but rather suggests it's impossible to accelerate from below to above the speed of light. The speaker also challenges the acceptance of infinity in this context, noting that physicists typically dismiss infinities as signs of flawed mathematics. They introduce the concept that most of an object's mass is actually binding energy, particularly within atomic nuclei, and that true mass comes from the Higgs field. The Higgs field condensate, they explain, gives particles mass, but this was not always the case in the early universe. The speaker uses the analogy of water vapor condensing to explain how the Higgs field condensed as the universe cooled, giving particles mass. They argue that the energy released during this phase transition was finite, contradicting the theory that reaching the speed of light requires infinite energy. The paragraph concludes by suggesting that the barriers to understanding faster-than-light travel may not be as solid as previously thought.

10:05

🕰 Time Travel Paradoxes and the Misconceptions of Light Speed Travel

The speaker addresses the common argument that faster-than-light travel could lead to time travel paradoxes, using a thought experiment involving Alice and Bob observing a spaceship moving at different speeds. They explain how, according to special relativity, an object moving faster than light could appear to move backward in time from Bob's perspective. However, the speaker challenges this notion, arguing that just because Bob perceives the sequence of events differently, it doesn't mean that the spaceship's internal direction of time is reversed. They point out that the assumption that all observers must be treated the same in special relativity does not necessarily hold when considering gravity, which is not included in the theory. The speaker suggests that general relativity, which does account for gravity, might provide a different perspective on the issue. They conclude by emphasizing that the argument against faster-than-light travel based on time travel paradoxes is not as solid as it seems and that our current understanding of space-time, which is incomplete, may change with the development of a theory of quantum gravity.

15:08

🔗 The Co-Moving Frame and Its Implications for Time Travel

The speaker delves deeper into the implications of general relativity and the concept of a co-moving frame, which is a reference frame that moves along with the average motion of matter in the universe. They explain that if faster-than-light travel were only allowed forward in time within this frame, it would not be possible to create time loops, thus avoiding time travel paradoxes. The speaker uses this example to argue that the possibility of faster-than-light travel does not inherently lead to paradoxes. They also suggest that the motion of matter in the universe might not be directly related to the possibility of faster-than-light travel, but it serves as a useful illustration. The paragraph concludes by questioning the validity of the argument against faster-than-light travel based on time travel, hinting that our understanding of space-time and the development of a quantum gravity theory could further challenge these assumptions.

20:12

🌐 The Need for a Quantum Gravity Theory and Its Impact on Light Speed Travel

In the final paragraph, the speaker argues against the belief that faster-than-light travel is impossible, stating that our current theories, including General Relativity, are incomplete because they do not reconcile with quantum theory. They suggest that the development of a quantum gravity theory could significantly alter our understanding of causality and locality, potentially changing the arguments against faster-than-light travel. The speaker expresses optimism that the formal reasons against such travel are not as robust as they seem and encourages physicists to reconsider the possibility. They conclude by sharing a personal anecdote about Einstein's theories and promoting an interactive learning platform, Brilliant.org, which offers courses on various scientific subjects, including special relativity and quantum mechanics, to help deepen understanding of these complex topics. The speaker also mentions a special offer for viewers interested in using Brilliant's services.

Keywords

💡Intelligent life

Intelligent life refers to beings with cognitive abilities comparable to or exceeding human intelligence. In the video's context, it is used to speculate about extraterrestrial life and their potential reasons for not contacting Earth, suggesting that humans might be 'too boring' due to our lack of technological advancement, such as the inability to send information faster than light.

💡Faster-than-light (FTL)

Faster-than-light (FTL) is a term used to describe the hypothetical speed that exceeds the speed of light in a vacuum. The video discusses the possibility of FTL communication and travel, challenging the common belief that it is impossible according to the theory of Special Relativity. The script explores the implications of FTL on the potential for alien contact and the advancement of human technology.

💡Special Relativity

Special Relativity is a theory proposed by Albert Einstein that deals with the physics of objects moving at constant speeds, particularly those close to the speed of light. The video explains that according to this theory, the speed of light is a universal constant and that it has been traditionally believed to be an unbreakable barrier for information or travel. The script questions this notion and explores its implications.

💡Speed of light

The speed of light is a universal physical constant, approximately 299,792 kilometers per second in a vacuum, and it is considered the maximum speed at which information or matter can travel according to the theory of Special Relativity. The video discusses this speed as a central concept, arguing that the common understanding of it as an insurmountable limit may be flawed.

💡Mass-energy equivalence

Mass-energy equivalence, represented by the famous equation E=mc^2, is a concept from Einstein's theory of relativity that equates mass to energy. The video uses this principle to explain why accelerating an object with mass to the speed of light would require an infinite amount of energy, thus challenging the idea that the speed of light is an absolute limit.

💡Higgs field

The Higgs field is a theoretical field that exists throughout the universe, postulated by the Standard Model of particle physics. The video explains that particles acquire mass through their interaction with the Higgs field. The script uses the Higgs field to argue against the idea that it takes an infinite amount of energy to reach the speed of light, as particles were massless before the Higgs field condensed after the Big Bang.

💡Electroweak symmetry breaking

Electroweak symmetry breaking is a phase transition that occurred in the early universe, leading to the Higgs field condensing and particles acquiring mass. The video script uses this concept to argue that the energy released during this phase transition was finite, contrary to the implication that reaching the speed of light would require infinite energy.

💡Time-travel paradoxes

Time-travel paradoxes refer to logical inconsistencies or contradictions that arise when considering time travel, particularly when it involves events affecting their own cause. The video discusses the argument that faster-than-light travel could lead to such paradoxes, but then refutes this by explaining that the perception of time can differ between observers and that general relativity might not support such paradoxes.

💡General Relativity

General Relativity is Einstein's theory of gravity, which describes the curvature of spacetime due to mass and energy. The video contrasts it with Special Relativity, explaining that while Special Relativity suggests time-travel paradoxes could occur with FTL travel, General Relativity, which accounts for gravity and the large-scale structure of the universe, might not support this conclusion.

💡Quantum gravity

Quantum gravity is a hypothetical theory that attempts to reconcile the principles of quantum mechanics with those of general relativity. The video suggests that our current understanding of space-time and the prohibitions against FTL travel may be upended by the future discovery of a viable quantum gravity theory, hinting at the possibility of a more complex and less restrictive universe than currently understood.

💡Brilliant.org

Brilliant.org is an online platform offering interactive courses in science, technology, engineering, and mathematics (STEM). The video script mentions it as a resource for viewers to actively engage with and deepen their understanding of complex topics like Einstein's theories, which are central to the video's theme of exploring the possibility of faster-than-light travel.

Highlights

The speaker believes in the existence of intelligent life on other planets but suggests that they haven't contacted us because we're too boring.

The video aims to discuss the possibility of breaking the speed of light limit, a topic the speaker previously covered but admits was not well understood.

Unexplained aerial phenomena, previously known as UFOs, are mentioned, with the speaker expressing skepticism about their extraterrestrial origin.

The concept that light speed is a limit comes from Einstein's Special Relativity, where light speed is constant for all observers.

The Michaelson-Morley experiment confirmed that the speed of light in a vacuum is always constant, regardless of the observer's motion.

The energy required to accelerate an object to the speed of light is infinite, according to Einstein's equations, implying that only massless objects like light can reach this speed.

The speaker argues that the idea of light speed as a limit is not technically correct due to issues with the argument and the nature of mass in physics.

Most of an atom's mass comes from binding energy, not the mass of its constituent particles, challenging the traditional view of mass in Einstein's equations.

The Higgs field is introduced as the source of mass for fundamental particles, contrasting with the massless photons that do not interact with it.

In the early universe, particles were massless and moved at the speed of light before the Higgs field condensed, a process that released finite energy.

The speaker points out that the argument for light speed being a barrier is flawed because it doesn't account for the early universe conditions.

Time-travel paradoxes are often cited as a reason why faster-than-light travel is impossible, but the speaker challenges this notion.

The co-moving frame in general relativity is used as an example to show that faster-than-light travel doesn't necessarily lead to time-travel paradoxes.

The speaker suggests that our current theories of space-time may be incomplete and that a future theory of quantum gravity could change our understanding of faster-than-light travel.

The video concludes by encouraging physicists to reconsider the possibility of faster-than-light travel and questioning the validity of common arguments against it.

The speaker promotes active engagement with scientific material and mentions Brilliant.org as a resource for learning more about physics and mathematics.

Transcripts

I believe there’s intelligent life on  other planets. And the most plausible  

reason why they haven’t contacted  us is that we’re too boring. I mean,  

we haven’t even figured out how to send  information faster than light. Pathetic.

But wait, let me guess. You’ve  heard that it’s impossible to  

send information faster than the speed  of light because, er, physics. Yes,  

I’ve heard that too. But I think it’s wrong.  And in this video, I want to explain why.  

Is it possible to break the speed of light  limit? That’s what we’ll talk about today.

If you’ve been following this channel for a  really long time, first of all, thank you,  

I know it isn’t always easy. Second,  you may remember that I made a video  

about faster than light travel already a few  years ago. But I think no one understood it. 

In fact, when I watched it again  recently, I didn’t understand it either.  

So please give me a second chance. Because I think  it’s becoming increasingly relevant to get this  

right. And this time, I’ll try to do it better.  I’ll even let you leave the toilet seat up.

In the past year or so, there’s been a lot  of talk about unexplained aerial phenomena,  

formerly known as UFOs. I don’t actually  believe any of those are of extra-terrestrial  

origin because, as I said, we’re just too  boring for aliens to bother visiting us.

Then again, what do I know? Maybe some of those  aerial phenomena really are space probes from  

alien species. And if we want to properly  evaluate how likely that is, we need to talk  

about the possibility of travelling faster  than light, or at least sending information  

faster than light. Because if it’s possible  at all, then that’s what the aliens are doing.

The idea that the speed of light is a limit  comes from Albert Einstein’s theory of Special  

Relativity. Yes, this guy again. The speed  of light plays a special role in his theory  

because it’s the only speed that’s the same  for all observers. And just to make sure,  

I mean the speed of light in vacuum. The  speed of light in a medium, any medium,  

is slower than the speed of light in vacuum,  

and depends on how you move relative to the  medium. But the speed of light *in vacuum  

does *not depend on how fast you move because  there’s nothing for you to move relative to.

I know this sounds about as exciting as flossing  teeth, but it has some unexpected consequences,  

and I don’t mean that your crowns pop off. Suppose you and your friend, let’s call him Bob,  

both have a water hose, and it spits out  water at, say 10 kilometres per hour. Bob  

gets on a train which moves at 200 kilometres  per hour. If you live in the United States,  

make that 20, then he turns on his  water hose again. The water moves  

with 10 kilometres per hour relative to  him. But how fast does it move relative  

to you? You’d expect it to be the speed of  the water plus the speed of the train, right?

Now imagine you don’t have water hoses but  laser pointers. They send out light with,  

well, the speed of light. Your friend  Bob gets on a train again. In vacuum,  

of course. Because this is theoretical  physics, where people don’t breathe,  

cows are spheres, and 3 is either equal to  pi or infinity, depending on whom you ask.  

How fast do you see the light of Bob’s laser?  You’d expect this to be faster than the light  

that comes out of your laser pointer by the  speed of the train, but not so. It moves with  

the exact same speed as yours. Because  the speed of light is always the same.

This is what was confirmed with the  famous Michaelson-Morley experiment,  

and it has a very odd consequence: You can’t  catch up with light. It doesn’t matter how  

fast the train is, light will still move  away from it with the speed of light.  

If that didn’t make your crowns pop off  you’ve probably heard it so many times  

before that you’ve forgotten how remarkable  it is. That, or you have a very good dentist.

We can quantify the difficulty of catching  up with light by asking how much energy it  

takes to accelerate an object. Let’s  suppose the object has a mass m. This  

mass corresponds to an energy which is  given by the most famous equation ever,  

E equals m c square, where E is the  energy and c is the speed of light.

But now we accelerate this massive object  from zero velocity to some other velocity,  

v. The energy you need for this acceleration  is the total energy of the object at the new  

velocity, minus the energy it previously had.  In Einstein’s theory, the total energy of an  

object that moves relative to you with  velocity v is given by this expression. 

Now if you want to know the kinetic energy,  

you take this and subtract the  same expression for zero velocity.  

So you get this somewhat messy expression, but  don’t despair, it isn't as bad as it looks.

For one thing, when the velocity, v, is  much smaller than the speed of light,  

then the ratio v over c is much smaller than  one. In this case, the complicated thing with  

the square root is approximately one plus one half  v over c square, the one cancels out and the c’s  

cancel out and you get one half m v square, which  you might remember is just the kinetic energy.

But we’re more interested in the case where  the velocity gets close to the speed of light,  

so v over c gets close to 1. Then  this factor gets close to zero,  

and the entire energy gets close to  one over zero, which is infinity. 

This means if you want to accelerate an  object until it reaches the speed of light,  

you need an infinite amount of energy. Another  way to put this is that the only way you can move  

at the speed of light is when your mass is zero.  Even a keto diet isn’t going to do that for you.

This is where the idea comes from that the  speed of light is a limit that you can’t cross.  

But… this argument has some issues. The first issue is that it doesn’t mean faster  

than light travel is forbidden in Einstein’s  theory. Indeed, his theory is entirely compatible  

with faster-than-light travel. The problem  seems to be instead that you can’t accelerate  

from below the speed of light to above the speed  of light. It’s more like a barrier than a limit.

The second issue is more a peculiarity. It’s  that on all other occasions when physicists  

see some quantity go to infinity, they’ll tell  you that infinity is unphysical and a sign that  

the maths doesn’t properly work. Big bang, black  holes, non-renormalizable effective field theory,  

whatever. If there’s a singularity,  they’ll say it’s a mathematical artefact  

and not real. They don’t say that in  this case, and I think they should.

The third issue is that we have a counterexample  to the claim that one needs an infinite amount  

of energy to reach the speed of light,  which makes the argument extremely suspect.  

But to see why I say this, I first need  to tell you where mass comes from. No,  

it’s not too much cheese, it’s simpler than that.

Most of the mass of objects around you isn’t  really mass, it’s binding energy. You see,  

almost the entire mass of atoms is in the nucleus.  The nucleus is made of neutrons and protons,  

and the neutrons and protons are each  made of three quarks. For the neutron  

that’s two down and one up, and for the  proton it’s two up and one down. Quarks,  

not thumbs, I mean. The quarks do have  masses, but if you add them together,  

the sum is far less than the mass  of either the neutron or proton.

Instead, most of the mass of neutrons and protons  is the binding energy from the strong nuclear  

force that holds them together. We *interpret  it as mass because E equals m c square. But  

this means it’s really odd to put the mass of an  object into this equation in Einstein’s formula.  

Because really if you look at the object  microscopically, most of it isn’t mass. And,  

yes, that means most of you isn’t mass either.  You’re almost entirely made of pure energy.  

Though when I see how much time you spend  watching YouTube I find that hard to believe.

What’s with the remaining mass, the part that  isn’t binding energy? Electrons and quarks  

do have masses, albeit very small ones.  These masses come from the Higgs-field,  

not to be confused with the Higgs-boson. To be  more precise, the masses come from the condensed  

Higgs field. This Higgs-field condensate  fills the entire universe and drags on  

particles. It’s kind of like the 19th century  aether, but with two important differences.

First, the aether was believed to be necessary  for light to travel. But for the Higgs-field  

it’s the opposite. The particles of light,  the photons, are massless, which means they  

don’t feel the Higgs field at all. But other  particles do feel it. When the field condenses,  

it sticks to the particles. That slows them  down and it looks to us like they have a mass.

Another difference between the condensed  Higgs-field and the aether is that the  

Higgs-condensate looks the same for everyone,  regardless of how fast they move. It’s just a  

number at each point in space-time and everyone  agrees on what this number is. It’s like the  

number of socks in your washing machine.  Doesn’t matter how fast the spin cycle is,  

the number of socks doesn’t change. Or if  it does, I guess it’s time for new socks.

The aether on the other hand was believed to  be basically like a fluid. Some people would  

drift with the flow, and some people would move  against it, and they’d see different things.  

This is *not the case for the Higgs-field and  its condensate. If you like technical terms,  

and I just know you do, it’s a Lorentz-scalar  and invariant under Poincare transformations.

Ok, so the masses of fundamental particles  come from the Higgs-field. But. This is  

only the case when the field is condensed and  that wasn’t the case in the early universe.

Think of an early morning in spring. No, not the  coffee, I mean the dew on the grass. Where does it  

come from? Well, air contains water vapour, which  means that individual water molecules float around  

in the air. But warm air can hold more water  vapour than cold air. If the air temperature  

drops during the night, the water molecules  collect to form drops which are too heavy  

to keep floating, and they fall to the ground. The Higgs field has done a very similar thing,  

not at night, but in the early universe. In  the early universe it was really hot. There  

was a Higgs-field but it wasn’t condensed,  kind of like the water vapour in the air.  

But then the temperature dropped, and the Higgs  field condensed. This condensate now fills the  

entire universe. But it was only when the Higgs  field condensed that particles acquired masses. 

It’s a phase transition called “electroweak  symmetry breaking” and it’s believed to have  

happened about 10 to the minus  11 seconds after the Big Bang at  

a temperature of 10 to the 15 Kelvin, that’s  much hotter than even the centre of the sun.

What all this means is that in the early  universe none of the particles had masses.  

They were all massless, and they were all moving  with the speed of light. Later they were not. And  

here’s the important bit: The energy that was  released in this phase transition was finite.  

If it hadn’t been, we wouldn’t be here, and  someone would have written a paper about that,  

I’m sure. But the equation that we looked at  earlier said that the difference in energy  

should have been infinite. What gives? Mathematically it’s pretty obvious what  

goes wrong with the earlier argument. If you  look at this equation again, you see that if  

this factor goes to zero, but the mass *also goes  to zero, then the ratio can well remain finite.

This doesn’t help us at all to travel at  the speed of light. Because we can’t just  

uncondense the Higgs field. Even if we could, it’d  basically evaporate the traveller and, I mean,  

I’m not a doctor, but that’s probably not healthy.  So, this isn’t going to let us build a warp drive.  

But it shows that the argument that the speed of  light is a barrier isn’t even technically correct. 

There is another reason that physicists  often bring up for why you can’t travel  

faster than the speed of light, which is that  it can allegedly cause time-travel paradoxes.

The argument goes like this. Suppose  Alice observes a spaceship which goes  

by faster than the speed of light.  Zoom there it goes. Her friend Bob  

can’t afford the new super-duper spaceship  and lamely zooms by in last year’s model,  

at merely 90 percent the speed of light. Then  Bob would see the space-ship going back in time.

Let’s draw this into a space-time diagram to see  why. The horizontal axis depicts one direction  

of space, so left and right, for example. And  the vertical axis is time. A spaceship which  

doesn’t move, according to this axis, just makes a  vertical line. A spaceship at constant velocity is  

a line which moves at some angle. By convention  a 45-degree angle is the speed of light.

Alice just sits there and moves on  this straight line. And everything  

that happens on a perfectly horizontal line  happens simultaneously, according to Alice.

The faster-than-light space-ship goes by like  this. And Bob moves on this line. The question  

is now what Bob sees. For this, let’s look at  two particular events. And let’s make sure those  

events have a clear arrow of time from entropy  increase, let’s say someone drops a raw egg. The  

guy in the spaceship stumbles here, and the  egg smashes to the ground here. This means,  

importantly, that time on the space-ship passes in  this direction, and *not in the other direction.

Since Bob is moving relative to Alice, he  sees different events happen simultaneously.  

I explained this previously in my video  on why the past still exists. So, well,  

either take my word for it  or watch the other video. 

For Bob, events that happen at equal times are on  these straight lines, not on horizontal lines. You  

can then see that for Bob the order of events is  that the egg first smashes to the ground and then  

gets dropped. It seems that for Bob the time order  of the faster than light ship is reversed, crazy!

The first reaction you may have to this is:  Who cares what Bob sees? I mean you can watch  

this video in reverse and that doesn’t mean  I actually spoke in reverse. Fair enough.

The second reaction is to point out that this  isn’t what either Alice or Bob see anyway. You  

can’t see a faster than light ship coming for  the same reason you can’t hear a supersonic  

plane coming. What do you want to see it with?  Instead, both Alice and Bob will only see the  

spaceship after it’s gone by and then they’ll  see it moving away in both directions. And again,  

you can say, so what? I mean gravitational  lensing distorts galaxies into rings, alright,  

but that doesn’t mean the galaxy is a ring.  It’s just some weird trick on our perception.

And that’s entirely correct… But, you know,  physicists have noticed that too. Thing is,  

this wasn’t the entire argument. There’s  a piece missing which goes like this. 

Imagine you are Bob, and there’s really a  spaceship that can go faster than light and  

according to you that goes back in time. Let’s not  ask what this means but what you can do with it.  

If the time on the spaceship really  goes forward this way, then you can  

give a message to the guys as they come by.  They take your message to Andromeda, hand it  

over to another faster-than light spaceship,  and the second ship brings the message back to  

you. It would then arrive before you sent it. This means you could send messages to yourself  

back in time, and *that causes a lot of  trouble. Imagine that this video greatly  

disturbs you and you send a message  to your younger self to not watch it,  

then you’d never have sent the message in the  first place, so did you, or didn’t you watch it?  

This type of construction is also called a  time-like closed loop, it’s a loop in time. 

The argument then concludes that if  faster-than-light travel was possible,  

that would lead to causality  paradoxes, so it must be impossible.

But this argument is also wrong. The reason  is that just because according to Bob there’s  

a spaceship going that way with a time  that goes forward on the space-ship in  

a direction that Bob calls backwards in time,  that doesn’t mean if a space-ship goes that way  

then its internal forward-in time direction  would be that way. If the time-direction on  

the ship goes that way, they can’t deliver  a message to your younger self. Instead,  

your younger self can send a message  there, and nothing’s weird about that.

Physicists do have a reason to assume that  time on the space-ship could go this way,  

but it’s not a good reason. It’s because in  special relativity all observers must be treated  

the same. In Special Relativity, if you think that  this is possible, then this must also be possible.

But Special Relativity is special because  it doesn’t contain gravity and this means  

it doesn’t actually describe reality. For this, we  need general relativity. And while the time-travel  

argument is correct in special relativity,  it is not correct in general relativity.

I know this video is some tough going so  let’s stop for a moment to appreciate where  

we are. I summarised the usual argument  for why faster than light travel leads to  

time-travel paradoxes. I’m about to explain why  this argument doesn’t apply in the real universe.

The usual argument uses special relativity  according to which only relative velocities  

are physically relevant. In special relativity,  you can’t be at a velocity of absolute zero,  

that just makes no sense. But the  real universe contains stuff, as  

you’ve probably noticed. You can take all this  stuff, calculate the average velocity that it  

moves with. And then you can define absolute  rest to be motion that has no relative velocity  

to the average of all that stuff. Since you  like technical terms so much, it’s called the  

“co-moving frame”. It’s the reference frame  that moves along with matter in the universe.

We are currently not at rest relative to the  average of stuff in the universe because the  

earth goes around the sun and the sun goes  around the centre of the milky way and the  

milky way is rushing towards something  called the big attractor that no one  

really knows what it is. If you wanted to  be at rest with the universe you’d have  

to run at 300 kilometres per second into  this direction. No, wait. This. Or, this?

Alright, so there’s matter in the universe that  moves one way and not another. But what does this  

have to do with the time-travel story? Suppose  you are Alice again but now you are Alice in a  

universe with general relativity and you  are moving with the stuff, you are in the  

co-moving frame. And now assume that faster  than light travel is only allowed forward in  

time in this particular frame. In this case you  can’t make loops in time, regardless of what  

Bob thinks he sees. The co-moving frame defines  one direction as forward in time. The only thing  

Bob can do is send two signals to Andromeda,  and there’s nothing Paradoxical about that.

You may wonder now what the motion of matter  should have to do with the possibility of  

faster-than light travel? This is a very good  question to which the answer is: Quite possibly  

nothing. I just used this as an example. It’s  an example to show that faster-than-light travel  

does not necessarily imply time-travel paradoxes.  The latter just doesn’t follow from the former.

To add one final reason why you shouldn’t trust  the argument that faster-than-light travel is  

impossible is that we know our current  theory of space-time, General Relativity,  

can’t be correct because it doesn’t work together  with quantum theory. This is why we need a theory  

of quantum gravity, and we still don’t have one.  We know however that causality and locality become  

really screwed up in quantum mechanics, and the  same is probably the case in quantum gravity. 

This is why I think it’s extremely  implausible that any argument about  

faster-than-light travel would survive in  the to-be-found theory of quantum gravity.

Of course you already know that no one’s figured  out how to travel faster than the speed of light.  

But I hope I have managed to convince at least  some of you that the formal reasons you may have  

heard against it are on shaky grounds. This is  why I believe physicists should think a little  

harder about faster-than-light travel. At the very  least, then maybe humans wouldn’t be so boring. 

​When I was in middle school my physics teacher  told me that very few people understand Einstein’s  

theories. Maybe that was once correct, but I can  very confidently tell you that it’s no longer  

the case today. I believe that everyone  can understand Einstein's theories today,  

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Thanks for watching, see you next week.