Is Time Travel Impossible? PBS Space Time English English (auto-generated)

Is Time Travel Impossible?YouTube↗ IA↗(long shot)
Education 926.6K views 24.7K likes 21 Oct 2019 Show Contributors 2

PBS Space Time 3.5M subscribers United States YouTube↗

Contributed subtitles

Vietnamese Fire Alarm Prevention as Antoanvn

Albanian Roland Biti

Click to Play Video

If time travel turns out to be possible, I hereby invite future time travelers to

if time travel turns out to be possible i hereby invite all future time

travelers to join me on set i'll post

I’ll post the address one year from today.

the address one year from today okay

Okay. Right now.

Well that's a bummer.

well it's a bummer [Music]

Time travel stories are cool because both the past and future are somehow more interesting

time travel stories are cool because both the past and the future are somehow

than the present and because everyone secretly wants a do-over.

more interesting than the present and because everyone secretly wants a

But so far it appears we are doomed to live consumed by regret in the eternal, boring present.

do-over but so far it appears that we are doomed to live consumed by regret in the eternal boring present

Time marches on, inexorably and only forward.

time matches on inexorably and only

Or so we thought until Einstein came along.

forward or so we thought until einstein

His special and general theories of relativity changed the way we think about time forever,

came along his special and general theories of relativity change the way we think about time forever and believe it

and believe it or not, their raw equations permit time travel.

or not their raw equations permit time

They even tell us how to do it.

travel they even tell us how to do it so let's

So let’s review the possibilities, and decide how possible time travel really is.

review the possibilities and decide how possible time travel really is the first

The first approach to time travel uses only special relativity, which describes how intervals

approach to time travel uses only special relativity which describes how

of time and space are stretched or contracted depending on relative speeds.

intervals of time and space are stretched or contracted depending on

A fast-moving spaceship appears to experience a slower rate of time compared to someone

relative speeds a fast-moving spaceship appears to experience a slower rate of time

waiting back on Earth.

compared to someone waiting back on

Do a trip around the galaxy at close to the speed of light and very little time might

earth do a trip around the galaxy at close to the speed of light and very little time

pass from the perspective of the traveler.

might pass from the perspective of the

But they’ll find a minimum of hundreds of thousands of years have passed

traveler but they'll find a minimum of hundreds of thousands of years have

when they get back to Earth.

passed when they get back to earth

However, that’s a one-way trip in time, and is really just traveling in the same temporal direction

however that's a one-way trip in time and it's really just traveling in the

at different rates.

same temporal direction at different

So the original Planet of the Apes style time travel is possible.

rates so the original planet of the ape style time travel is possible but it'd be

But it’d be nicer to be able to go back in time.

nicer to be able to go back in time

Actually, the math does sort of allow that.

actually the math does sort of allow

The spaceship’s clock slows down as it speeds up, and it stops completely at the speed of light.

that the spaceship's clock slows down as it speeds up and it stops completely at the

And at faster speeds, time should actually tick backwards.

speed of light and at faster speeds time should actually tick backwards

So if you could travel faster than light you could navigate a path to a point in spacetime

so if you could travel faster than light you could navigate a path to a point in

before you departed.

space time before you departed

We saw how to Iconstruct such a path in a previous challenge episode.

we saw how to construct such a path in a previous challenge episode

Of course we know that the laws of physics forbid faster than light travel.

of course we know that the laws of physics forbid faster than light travel

In order for any object with regular mass to even reach light speed it would need infinite

in order for any object with regular mass to even reach light speed it would

energy – which can never be obtained.

need infinite energy which could never

But notice I said “regular mass.”

be obtained but notice i said regular

We can hack the equations of special relativity by allowing mass to take values in the weird

mass we can hack the equations of special relativity by allowing mass to take values in the weird realm of imaginary

realm of imaginary or complex numbers.

or complex numbers

An object with imaginary mass is now restricted to only traveling FASTER than light, never slower.

an object with imaginary mass is now restricted to only traveling faster than light never slower that means it can

That means it can only travel backwards in time, not forwards.

only travel backwards in time not

We call a particle with imaginary mass a tachyon.

forwards we call a particle with imaginary mass a

If we could control tachyons then perhaps at least we could send information back in time.

tachyon if we could control tachyons then perhaps at least we could send information back in time

But do they exist?

but do they exist

Does imaginary mass exist?

does imaginary mass exist

This is an example where the equations of a theory technically allow something to

this is an example where the equations of a theory technically allow something

be true, but there’s still no good reason to believe that it is.

to be true but there's still no good reason to believe that it is we've seen

We’ve seen no evidence of tachyons, and common sense tells us we probably

no evidence of tachyons and common sense

tells us we probably never will so

So special relativity isn’t much help.

special relativity isn't much help

Fortunately we still have the general theory of relativity, which incorporates special relativity,

fortunately we still have the general theory of relativity which incorporates

but also explains the force of gravity as a result of curvature in the fabric

special relativity but also explains the force of gravity as the result of

of spacetime due to the presence of mass and energy.

curvature in the fabric of space-time due to the presence of mass and energy

But GR describes a warping of space AND time.

the gr describes a warping of space and

So maybe we can warp them enough to take us back to our own past.

time so maybe we can warp them enough to take us back to our own past the best

The best-known approach is through something called a wormhole.

known approach is through something

A wormhole is a particularly bizarre hypothetical consequence of general relativity.

called a wormhole a wormhole is a particularly bizarre hypothetical consequence of general relativity now if

Now, if space can be warped, then perhaps it can be stretched in such a way as to create a

space can be warped then perhaps it can be stretched in such a way as to create

tunnel between two points – and one whose internal distances could be very short, even

a tunnel between two points and one whose internal distances could be very

if the mouths of the tunnel are far apart.

short even if the mouths of the tunnel are very far apart this has the obvious

This has the obvious benefit of allowing you to teleport between distant points in space,

benefit of allowing you to teleport between distant points in space

but also between distant points in time.

but also between distant points in time

This is how you do it: take one stable wormhole large enough to be traversed.

this is how you do it take one stable wormhole large enough to

Accelerate one end to close to the speed of light or drop it into a deep gravity well

be traversed accelerate one end too close to the speed of light or drop it

– its rate of time flow will slow relative to the other end of the hole.

into a deep gravity well its rate of time flow will slow relative to the other end of the hole

Now bring the two ends back together.

now bring the two ends back together

They will be offset in time: one portal permanently stuck in the past of the other by some set interval.

they will be offset in time one portal permanently stuck in the past of the other by some set interval

Travel through the “future” end and you’ll exit in the "past."

travel through the future end and you'll exit in the past so this all sounds

So this all sounds straightforward enough.

straightforward enough but can wormholes

But can wormholes even exist?

even exist there are a number of ways

There are a number of ways they might – from connections between universes in the interiors

they might from connections between

of black holes to miniscule wormholes appearing and vanishing on the tiniest scales of space and time.

universes in the interiors of black holes to minuscule wormholes appearing and vanishing on the tiniest scales of

Now, these deserve their own episode.

space and time now these deserve their

But for now, to build a useful time machine a wormhole has to be large enough to fit through

own episode but for now to build a useful time machine a wormhole has to be large enough to fit through and it has

and it has to be stable.

The equations of GR do permit large wormholes, but they are definitely not stable.

the equations of gr do permit large wormholes but they are definitely not

They collapses on themselves instantly, leaving inescapable black holes.

stable they collapse on themselves instantly leaving inescapable black holes in order

In order to keep our wormhole time machine operational, its throat needs to be kept open.

to keep our wormhole time machine operational its throat needs to be kept

We need to counter gravity, and to do that we need another probably-non-existent form

open we need to counter gravity and to do that we need another probably

of mass – negative mass – also referred to as exotic matter.

non-existent form of mass negative mass also referred to as exotic

As far as we know, mass can only take on positive, real values, so a requirement of

matter as far as we know mass can only take on positive real values so a

negative mass seems a non-starter. However there may be hope.

requirement of negative mass seems like a non-starter however there may be hope

Really what we need to open the wormhole is a negative energy density.

really what we need to open a wormhole is a negative energy density

Some have argued that we already produce this in the Casimir effect, in which the energy

some have argued that we've already produced this in the casimir effect in

of the vacuum is lowered between two nearby conducting plates.

which energy of the vacuum is lowered between two nearby conducting plates

However there’s no clear path to translating this to a large-scale negative energy distribution

however there's no clear path to translating this to a large scale

that could keep a wormhole open.

negative energy distribution that could keep a wormhole open

And in fact we'd need entire planets – perhaps entire stars converted to negative energy to do this.

and in fact we'd need entire planets perhaps entire stars converted to negative energy to do this

Some other time travel options also involve using negative energy densities - for example

some other time travel options also involve using negative energy densities

the Alcubierre warp drive, which we already covered.

for example the alcubierre warp drive which we already covered in short if you

In short – if you have exotic matter you can probably time travel.

have exotic matter you can probably time

But is negative mass-energy as much of a non-starter as imaginary mass?

travel but is negative mass energy as much of a non-starter as imaginary mass

While the actual equations of general relativity themselves don’t prohibit it, there are

while the actual equations of general relativity themselves don't prohibit it

a set of secondary rules in GR that do.

there are a set of secondary rules in gr

These are the so-called energy conditions.

that do these are the so-called energy

They’re a set of requirements that do things like prevent negative energies and enforce

conditions there are a set of requirements that do things like prevent negative energies and enforce energy

energy conservation.

conservation but the energy conditions

But the energy conditions don’t have a really fundamental basis, and they're seen to be violated

don't really have a fundamental basis and they seem to be violated in some

in some cases – like with the Casimir effect.

cases like with the casimir effect we

We can’t completely rule out wormhole or warp drive time machines based just on the energy conditions.

can't completely rule out wormhole or warp drive time machines based just on

And as it turns out, there may be other ways to build time machines

the energy conditions and as it turns out there may be other ways to build

without either negative or imaginary masses.

time machines without either negative or imaginary masses

One example is the Tipler cylinder, conceived by Frank Tipler based on a solution to the

one example is the tipless cylinder conceived by frank tipler based on a

Einstein equations by Willem van Stockum.

solution to the einstein equations by

It’s simple: just build an infinitely long cylinder of extreme density and set it rotating

willem van stockham it's simple just build an infinitely long cylinder of extreme density and set it rotating

insanely quickly about its main axis.

insanely quickly about its main axis

It will drag spacetime in its vicinity into a sort of vortex.

it'll drag space-time in its vicinity into a sort of vortex

This generates sub-lightspeed paths through spacetime that form closed loops, ending up

this generates sub-light speed paths through space time that form closed

back where they started in both space and time.

loops ending up back where they started in both space and time

We call such paths closed time-like curves.

we call such paths closed time-like

A spaceship traveling along one of these curves could return to a point in its own past.

curves a spaceship traveling along one of these curves could return to a point in its own past

If you don’t have the budget for an infinitely long cylinder, you could try building just

if you don't have the budget for an infinitely long cylinder you could try

a very, very long cylinder… and be horribly disappointed.

just building a very very long cylinder and be horribly disappointed stephen

Stephen Hawking showed that unless the cylinder is infinitely long this doesn’t work –

hawking showed that unless the cylinder is infinitely long this doesn't work

unless you also modify the spacetime with negative energy.

unless you also modify the space time

In which case you might as well just build a wormhole.

with negative energy in which case you might as well just build a wormhole

So it turns out that it’s not so hard to find solutions in general relativity with

so it turns out that it's not so hard to find solutions in general relativity

closed timelike curves.

with closed time-like curves

Kurt Goedel, famous for his incompleteness theorem, discovered one

kurt godel famous for his incompleteness

and he wasn’t even a physicist.

theorem discovered one and he wasn't even a physicist he's involved an entire

His involved an entire universe, rotating about a central axis and with matter and dark

universe rotating about a central axis

energy perfectly balancing it against collapse or expansion.

with mata and dark energy perfectly balancing against collapse or expansion

So to build this time machine we just need construct an entire universe – which allows

so to build this time machine we just need to construct an entire universe

us to travel back in time only within that universe.

which allows us to travel back in time only within that universe

Dragging the fabric of space in a circle can give us our time-loops in very special,

dragging the fabric of space in a circle can give us our time loops in very

and frankly useless cases.

special and frankly useless cases

Another one is the interior of a rotating black hole – a so-called Kerr black hole.

another one is the interior of a rotating black hole a so-called ker

The maelstrom of spinning spacetime may generate closed timelike curves deep down below the event horizon.

black hole the maelstrom of spinning space time may generate closed time-like curves deep down below the event horizon so that's

So that’s fun: you can travel back to your own past, but never to the time before you

fun you can travel back to your own past but never to the time before you fell

fell into the black hole, which is probably the only thing you really want to do at that point.

into the black hole which is probably the only thing you really want to do at

Unless it’s an Interstellar-style black hole . . . general relativity doesn’t directly

that point unless it's an interstellar black hole general relativity doesn't directly

refute black hole time machine libraries.

refute black old time machine libraries

So it seems we have lots of ways to send things back in time, but it all seem useless for actually making

so it seems we have lots of ways to send things back in time but it all seems

time machines for one reason or another.

useless for actually making time machines for one reason or another

But the weird thing is that we don’t know of one consistent, fundamental law in physics

but the weird thing is that we don't know of one consistent fundamental law

that prohibits true time travel.

in physics that prohibits true time

And yet most physicists still think it’s impossible because time travel threatens the

travel and yet most physicists still think it's impossible because time travel threatens the common

common-sense chain of cause and effect.

sense chain of cause and effect it

It threatens causality.

threatens causality

Break causality and you can create paradoxes– time-travel to kill your grandfather and you

break causality and you can create paradoxes time travel to kill your grandfather and

would ever be born to time travel in the first place.

you would never be born to time travel

But there are no true paradoxes – only seeming paradoxes that point to a gap in our understanding.

in the first place but there are no true paradoxes only seeming paradoxes that point to a gap in

Stephen Hawking put it nicely with his Chronology Protection Conjecture.

our understanding stephen hawking put it nicely with his chronology protection

It states that the laws of physics will always prevent time travel or allow it

conjecture it states that the laws of physics will always prevent time travel

only when doesn’t cause paradoxes.

or allow it only when it doesn't cause

In other words, the universe has to make sense, time-travel or no.

paradoxes in other words the universe has to make sense time travel or no one way for

One way for a closed timelike curve to exist without causing a paradox is expressed in

closed time-like curves to exist without causing a paradox is expressed in the

the Novikov Self-Consistency Principle.

novikov self-consistency principle

Igor Novikov suggested that closed timelike curves are fine as long as they’re self-consistent.

igor novokov suggested that closed time-like curves are fine as long as they're self-consistent as long as the

As long as the backwards time-traveling configuration of matter always leads to exactly to the same

backwards traveling configuration of matter always leads to exactly the same

forward-traveling configuration.

forward traveling configuration in other

In other words, the loop creates itself.

words the loop creates itself

So I don’t know – you try to kill your grandfather, only to become your own grandfather?

so i don't know you accidentally kill your grandfather only to become your own

Like Fry, let’s not think too hard about that.

grandfather like fry let's not think too hard about that an

An alternative lies in Hugh Everett’s many-worlds interpretation of quantum mechanics, in which

alternative lies in hugh everett's many worlds interpretation of quantum

every possible universe exists, splitting off in an infinite tree.

mechanics in which every possible universe exists splitting off in an infinite tree

So if you travel back in time and prevent yourself being born – no problem - your

so if you travel back in time and prevent yourself being born no problem

photo doesn’t slowly fade away because you were still born in that other timeline.

your photo doesn't slowly fade away because you were still born in that other timeline or time travel could be

Or time-travel could be genuinely impossible.

genuinely impossible kip thorne suggests

Kip Thorn suggests there should probably be one fundamental law of physics that prohibits it -

that there should probably be one fundamental law of physics that

for example, the quantum vacuum may be unstable in the infinitely iterating loops

prohibits it for example the quantum vacuum may be unstable in the infinitely

of a closed timelike curves.

iterating loops of closed time-like curves

On the other hand, Kip was the consultant in Interstellar, so who knows what to believe?

on the other hand kip was the consultant in interstellar so who knows what to

In actual fact we can’t know until we have a full theory of quantum gravity –

believe in actual fact we can't know until we have a full theory of quantum gravity

until then we’re working with the approximate theories general relativity and quantum theory.

until then we're working with approximate theories general relativity

Approximate theories can make bad predictions – like the possibility of time travel.

and quantum theory approximate theories can make bad predictions like the possibility of time travel one

One final argument that time travel is impossible is that we don’t see time travelers.

final argument that time travel is impossible is that we don't see time

Stephen Hawking put this to rigorous test when he organized a cocktail party for time

travelers stephen hawking put this to rigorous test when he organized a cocktail party

travelers, only advertising the event after it ended.

for time travelers only advertising the event after it ended

Tellingly, no one showed up.

tellingly no one showed up

Though I don’t know – maybe there was a slightly better party somewhere else in, like,

though i don't know maybe there was a slightly better party somewhere else in

all of history.

like all of history

For now we seem doomed to time travel only forwards, and very slowly at that.

for now we seem doomed to time travel only forwards and very slowly at that we

We remain firmly in the grip of that one dimension that we can never halt nor reverse it's pace: time.

remain firmly in the grip of that one dimension that we can never halt nor reverse its pace time

Similar videos:

How Does Gravity Warp the Flow of Time?
PBS Space Time
1.1M views 32.5K likes -1 dislikes 10 Feb 202176% match

No Dark Matter = Proof of Dark Matter?
PBS Space Time
708.1K views 17.4K likes -1 dislikes 24 Apr 201975% match

Why Time Travel to the Past is Impossible
RealLifeLore
2.5M views 75.6K likes 3.5K dislikes 13 Apr 201875% match

The Holographic Universe Explained
PBS Space Time
3M views 42.5K likes -1 dislikes 10 Apr 201975% match

Gravitational Wave Background Discovered?
PBS Space Time
910.5K views 28.3K likes -1 dislikes 17 Feb 202175% match

Does Consciousness Influence Quantum Mechanics?
PBS Space Time
1.9M views 46.9K likes -1 dislikes 18 Feb 202074% match

Navigating with Quantum Entanglement
PBS Space Time
620K views 21K likes -1 dislikes 22 Dec 202073% match

Could We Terraform Mars?
PBS Space Time
2.1M views 42.1K likes -1 dislikes 16 Sep 201973% match

Does Time Cause Gravity?
PBS Space Time
2.3M views 58.4K likes -1 dislikes 24 Feb 202173% match

Will Wormholes Allow Fast Interstellar Travel?
PBS Space Time
1.4M views 31.5K likes -1 dislikes 22 Apr 202070% match

Is Dark Energy Getting Stronger?
PBS Space Time
811.3K views 17.2K likes -1 dislikes 20 Mar 201970% match

Loop Quantum Gravity Explained
PBS Space Time
1.2M views 27.3K likes -1 dislikes 15 Oct 201969% match

Electroweak Theory and the Origin of the Fundamental Forces
PBS Space Time
736.8K views 22K likes -1 dislikes 04 Nov 202069% match

Is Earth's Magnetic Field Reversing?
PBS Space Time
1.3M views 31K likes -1 dislikes 03 Sep 201968% match

What Happened Before the Big Bang?
PBS Space Time
1.6M views 32.7K likes -1 dislikes 19 Aug 201968% match

Deciphering The Vast Scale of the Universe | STELLAR
PBS Space Time
982.5K views 24.4K likes -1 dislikes 25 Jul 201968% match

How Black Holes Kill Galaxies
PBS Space Time
1M views 19.9K likes -1 dislikes 17 Jun 201968% match

Did Time Start at the Big Bang?
PBS Space Time
2.3M views 49.4K likes -1 dislikes 18 Jul 201968% match

How Many Universes Are There?
PBS Space Time
1M views 21.5K likes -1 dislikes 30 Sep 201968% match

What Caused the Big Bang?
PBS Space Time
1.3M views 27.8K likes -1 dislikes 06 Aug 201968% match

Podróż w czasie | Animacje Polish Sausage
Polish Sausage
4.6M views 68.9K likes 3K dislikes 09 Feb 201667% match

¿Cómo viajar en el tiempo?
CdeCiencia
1.3M views 41.6K likes 1.5K dislikes 08 Feb 201567% match

Time travel for $10!
StikBot Central
16.3M views 35.9K likes 6.6K dislikes 19 Feb 201667% match

Time Travel 2
The Playinghand Show
1.8K views 86 likes 6 dislikes 30 Apr 202067% match

Venus May Have Life!
PBS Space Time
475.4K views 21.5K likes 449 dislikes 05 Oct 202067% match

time travel
Nathan Brand
144 views 6 likes 0 dislikes 29 Nov 201767% match

Can We Break the Universe?
PBS Space Time
938.4K views 30.5K likes -1 dislikes 19 Jan 202166% match

The Alchemy of Neutron Star Collisions
PBS Space Time
933.1K views 19.5K likes -1 dislikes 06 Jun 201966% match

The Edge of an Infinite Universe
PBS Space Time
3.6M views 51.4K likes -1 dislikes 03 Apr 201966% match

Is The Universe Finite?
PBS Space Time
1.6M views 31.8K likes -1 dislikes 02 Dec 201963% match

Settings Privacy About Contact Donate