This content explores the theoretical possibility and profound paradoxes of time travel, drawing heavily on physics concepts like relativity and quantum mechanics, and questioning the nature of free will and causality.
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foreign
it's a cold moonless night with a Mist
as he turned across the road fog
swallows both the sound and headlights
of the oncoming car you see it too late
embrace yourself for impact
then out of nowhere an elderly stranger
shoves you out of Harm's Way is hit by
the car and ends up in a heap on the tarmac
tarmac
their life ends as yours is saved later
a police officer hands you an envelope
found in the pocket of the deceased it
is addressed to you you open this
envelope and find a note with two words
scribbled on it
simply reads
study Physics whoever wrote this note
saved your life at the expense of theirs
so it only seems fair you follow their advice
advice
decades later you become a much
celebrated physics professor your
specialist subject Albert Einstein's
theories of Relativity you've always had
a hunch that time travel is possible and
one day finally you crack it at the
world's very first time machine is set
buzzing before you only then does the
penny drop how would you never realize
before you now have the power to go
anywhere and any when but you know
exactly what you have to do you take the
letter you've kept all these years out
of its frame on the wall place it back
inside its original envelope and put the
envelope in your pocket stepping into
your time machine you set the controls
to take you back to a cold moonless
Misty night arriving just in time to
push your younger self out of the path
of the on-rushing car
seems simple enough
but the real Joy of time travel stories
like this one comes once you begin to
scratch beneath the surface and some
incredibly profound questions begin to emerge
emerge
first of all who wrote the letter
the older version of you simply takes
the letter they'd already been given as
a teenager back in time the letter is a
seemingly creatorless entity stuck in a
Time Loop this is the bootstrap paradox
secondly do these time Loops keep on
repeating you save your teenage self so
they grow up to travel back in time to
save their teenage self who grows up to
travel back in time and so on you are
stuck in a Relentless cycle of living
and dying but the letter isn't
does it just keep getting older and more
worn and tattered
this is the restoration paradox
and whose idea was it to study Physics
anyway the notion seems to appear out of
nowhere a thought with no origin this is
the ex-niello Paradox and if events like
this can actually happen then it also
appears you have no free will no choices
in life you are always going to invent
the time machine and had absolutely no
option to do otherwise it
to happen had always happened if you
don't invent a time machine then you
can't save your teenage self and you
yourself would therefore die as a
teenager there is something in your
future that's firmly fixed this is the
predestination paradox
and finally what if you change your mind
unfortunately for your older self this
is not an option you can't get cold feet
when you arrive and that the younger you
get hit by the car how could you be
there to have second thoughts in the
first place
this is the auto infanticide Paradox it
is impossible to kill a younger version
of you even through the act of not doing something
something
and so not quite such a simple story
welcome to the weird and wonderful world
but of course traveling freely through
time is impossible you might say you
can't simply visit yesterday or Rush
head long through millennia
and yet this too is not quite so
straightforward despite its Myriad
impracticalities there are slivers of
possibility lurking at the edge of
physics temporal loopholes lying just
Out Of Reach
and so the question arises if time
travel were to be possible just how
could we do it [Music]
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the Sun beats down as the distinctive
whooping barks of howler monkeys Echo
around the humid swamps the current in
the muddy winding river is so strong
that you don't even have to paddle your
canoe you just sit back and take in the
sights and sounds as you're carried along
Jack Finney arguably one of the finest
proponents of time travel fiction
compared our experience of time to a
river like this one it seems like we're
relentlessly ferried Downstream from the
past to the future we have no need to
propel ourselves but slowing down
speeding up or turning around and
heading back Upstream doesn't seem
possible either but what happens to the
parts of the river that we've left
behind or in other words what is
yesterday go
it certainly seems as if it just
disappears yet making the same argument
about an actual River would seem
ludicrous you're unlikely to accept that
the riverbed is torn up and mysteriously
vanishes just because you've disappeared
around the next bend the Upstream parts
of the river are still there even if you
aren't in the same way it seems as if
tomorrow isn't real until it is that
somehow it magically appears once the
curtain falls on today but that would be
like new water springing up to make the
river of time longer only once you
approach where would it come from
and so over the years scientists have
revealed that time really is like that
River with its length already laid out
in its entirety from its source high in
the foothills of the Big Bang to its
Estuary at the universe's eventual
demise the past present and future
existing alongside one another your
great great grandchildren every bit as
real as you are now your great great
grandparents still alive and well
frolicking in the swells Upstream in the
vast River of time
philosophers know this idea as
eternalism it's the opposite of
presentism which argues that only now
exists and that the past is gone and the
future yet to emerge debates between
eternalist and presentist philosophers
have raged since the days of ancient
Greece indeed the 4th Century
philosopher Saint Augustine compared the
present to a knife's edge straddling the
perceived past and an imagined future
to a physicist
eternalism is known as the block
universe theory and is based on Albert
Einstein's much-lauded general
relativity one of the most successful
theories in the history of science [Music]
in 1905 Einstein publishes the special
theory of relativity it does away with
Isaac Newton's notion of absolute time
there is no universal clock that
everyone can use to agree on when
something happens instead as the name
suggests time is relative depending on
how they move through space one Observer
could see event a happen before event B
while another sees event B occur before
event a both observers are equally
correct an event in your past could be
in someone else's future this is yet
another reason why all our yesterdays
cannot simply vanish how can they when
the events they contain might form part
of somebody else's tomorrows as well as
this special relativity cemented the
fact that space and time are not
completely distinct from one another in
1908 Hermann minkowski develops
Einstein's work and properly unites
space and time once and for all
says that moving through space affects
how you measure time because the three
dimensions of space and one of time are
actually woven together into a single
fabric that he called space-time but how
do we know that space and time are
actually bundled up together into the
space time that the block universe is
made of proof would eventually come in
tests of Einstein's general theory of
relativity which he presented to the
back in the 1840s astronomers had
harnessed Newtonian gravity to discover
an entirely new planet using Newton's
equations to predict the future path of
Uranus they had found that it appeared
to deviate from that path the French
astronomer Urban leverier pulled over
the equations for months searching for
an answer was Newtonian gravity wrong
after all not quite the verrier
suggested that an unseen more distant
planet pulls on Uranus and affects its
orbit if correct Newton's laws of
gravity would be Vindicated and in 1846
Neptune was found
despite this success and although he
didn't know it yet the verrier was about
to hammer the first nail into Newtonian
gravity's coffin planets don't have
circular orbits but slightly changing
elliptical ones with the sun offset from
the center Newtonian physics claimed
that these slightly changing orbits was
due to the gravitational pull of the
other planets and Newton's equations
were correct in their predictions for
all of them except one Mercury
the verrier realized this in 1859 and
the search began for another new planet
but no such world was ever found instead
the answer came from Einstein on the
18th of November 1915 he wrote a letter
to the German mathematician David
Hilbert today I am presenting a paper in
which I derive out of general relativity
the perihelion motion of Mercury
discovered by leverier no gravitation
[Music]
imagine the fabric of space-time as the
surface of a trampoline
if you place something heavy in the
center it will sag in the middle
likewise the presence of a massive
object like the sun creates a dip in the
fabric of space-time physicists call
this a gravity well
Einstein was arguing that there is no
Newtonian gravitational pull at all it
is merely a mirage as the innermost
planet Mercury moves more deeply into
the Sun's gravity well than its
neighbors the deeper it goes the more
the local space-time becomes curved and
it's this increased curvature that
causes Mercury's orbit to change for all
the other planets the curvature is
subtle enough that Newtonian gravity
gives the same answer only in the
extreme gravitational environment that
mercury inhabits does Newtonian gravity
show itself for what it really is a good approximation
approximation
gravity is not a pull it's the result of
curved space-time
since then astronomers have tested this
idea in Myriad ways from solar eclipses
to swirling black holes and Rippling
gravitational waves and in every
experiment space-time passes with flying colors
colors
our entire Cosmos of past present and
future merely a four-dimensional block
of malleable space-time that just sits
there a supernatural being that could
somehow look upon it from the outside
world would clearly see yesterday today
and tomorrow happily coexisting
but where did this all come from as with
everything else scientists believed that
the Big Bang birthed all of space-time
in other words all of space and all of
time the regions of time we refer to as
the past the present and the future are
all there together in their entirety and
so it is easy for two people to exist
then and now as they do here and there
they can be separated by centuries of
time just as they can be separated by
kilometers of distance [Music]
[Music]
any object within this block Universe
can be given for coordinates
three spatial to say exactly where it is
and one temporal to say exactly when it
is imagine that an object's location in
the block is marked by a DOT even the
dot of a physically stationary object
moves because its time coordinate is
constantly changing a second ticks after
second and today relentlessly becomes
tomorrow join all the dots belonging to
one particle and you have a line that
Maps out its path through the block
Universe a world line you are nothing
but a collection of particle World lines
woven together and not in space-time
shedding and absorbing New World lines
as you move towards the day the knot
that is you and does itself or is undone
all the actions you will ever take from
what you'll have for breakfast every day
to if and who you'll marry and the
houses and jobs you'll flip between are
all entirely mapped out you can't
deviate from your journey any more than
a train can deviate from its tracks
in this way the block universe is like a
book the beginning the middle and the
end are already written and unchangeable
every choice you think you're making is
actually pre-written and so one that you
are always going to make a character
caught in a love triangle on page 150
may be unaware that it gets resolved 50
pages later but resolved it will be and
in the way it was always going to be
and this is why our time traveler from
the opening story had to invent the time
machine an older version of them had
already indelibly appeared on an earlier
page their world lines eventually looped
background to intersect with themselves
but can this really happen can you
travel to the seemingly long distant
past or indeed the far-flung future
if over then is no different to over
there it seems that the door to time travel
travel
in a distant corner of the universe a
gargantuan supermassive black hole is
holding sway over an entire galaxy
its intense gravity is pulling
stretching and ripping up whole solar
systems hot gas and dust whirl around it
as its huge magnetic field spins up and
spits out shrapnel into the void
among this debris is a tiny subatomic
bullet a proton which races across the
cosmos as as close to the speed of light
as it's practically possible to get it
has 100 quintillion times more energy
than an ordinary particle of light and
after traveling over a billion light
years it slams into Earth's atmosphere
with 21 million times the energy of
collisions inside the famed Large Hadron
Collider at CERN
it is so energetic that to this day
astronomers know it as the oh my God particle
an example of an ultra high energy
cosmic ray it is a reminder that the
Earth sits in a Celestial shooting
gallery and when Rays like this one
strike our atmosphere the impact is so
violent that it bursts into an invisible
fireworks display of subatomic particles
pions muons and neutrinos erupt and
Cascade down to the ground in fact
cosmic rays are so plentiful that they
give you an annual dose of radiation
equivalent to three chest x-rays and for
one Italian physicist cosmic rays became
an obsession his groundbreaking work on
the muons created by cosmic rays
producing some of the first evidence
that time travel to the future is not
only possible but commonplace
and his name
Rossi is behind the wheel of an old bus
snaking along the highest road in the
world where pine trees compete with
snow-capped mountains to reach the sky
eventually he arrives at the place he's
driven over a thousand miles from
Chicago to reach
Echo Lake
nestled high up in the Mount Evans
Wilderness of Colorado part of the world
famous Rocky Mountains it sits almost 2
up in the mountains Rossi is measuring
the muons created by cosmic rays new
ones are unstable particles and so they
quickly break down or Decay physicists
call the time it takes for half of a
group of muons to make this change their
Half-Life it is just 1.56 millionths of
a second but Rossi's measurements reveal
something curious far more muons are
reaching the ground than there should be
even though the muons are traveling at
more than 98 of the speed of light over
15 half-lives should still elapse by the
time they make it down to Echo Lake with
each successive Half-Life removing 50 of
the remaining muon population just 0.1
percent should reach Rossi and so how
come he is able to detect significantly
more than that
the remarkable answer was that the muons
were time traveling into our future
Einstein's special theory of relativity
says that you can think of anything
moving through space-time as having a
budget equal to the speed of light the
more of that budget you spend on moving
fast through space the less you spend on
time as the muons are traveling at close
to the speed of light considerably less
time passes for them than for Rossi from
their perspective fewer half-lives have
elapsed and so there simply hasn't been
enough time for 99.9 of them to Decay
this effect is called relativistic time
dilation and when Rossi factored it in
exactly the right number of muons reach
the ground [Music]
[Music]
but you may ask does this really count
as time travel
imagine instead that you are the muon
but instead of traveling from the upper
atmosphere to Echo Lake you travel on a
big loop around the universe at the same
98 of the speed of light and return home
25 years older those of us left on Earth
would have been spending less of our
space-time Budget on speed than you and
so it will cost us more in time but to
us you have spent your budget
differently for going time to travel
through space 25 years may have passed
for you while you were gone but
125 have elapsed on Earth
you have skipped forward a whole century
indeed such a journey could mess with
the normal generational order of family
relationships as is poignantly explored
in the short story memories of my mother
by Ken Liu
and so time travel to the future is more
than possible we just have to accelerate
humans to close to light speed that is
far from straightforward however because
the faster you travel the more energy
you have Einstein's famous equation E
equals m c squared tells us that energy
and mass are interchangeable so if you
gain energy you also gain mass in other
words the faster you travel the heavier
you get to an outside Observer a person
traveling at 99.99 of the speed of light
would weigh almost two tons or about the
same as a rhino and accelerating them to
that speed would require as much energy
as everyone on Earth currently uses in
three months [Music]
[Music]
understandably we are very far from
pulling that off but even today time
dilation leads to some bizarre consequences
consequences
take Mark and Scott Kelly both former
NASA astronauts who also happen to be twins
twins
Mark was born six minutes before his
brother however Scott has spent about 10
times longer in space 520 days to Mark's
54 days during these extra days Scott
was whizzing through space-time faster
than his brother spending more of his
Budget on speed and less on time in
other words more time has passed for
Mark than for Scott Mark may have
started out life six minutes older than
his twin but he is now 6 minutes and 5
milliseconds older had Scott been
traveling as fast as a muon instead
their age Gap would have widened to
nearly six and a half years despite
initially being born on the same day and
as if that weren't enough there is
another entirely separate form of
experimentally verified time dilation a
second way to time travel into the future
future
picture a clock except this Clock Works
differently to any you've seen hanging
on a wall it is simply made of two
horizontal mirrors a beam of light
bounces between the two mirrors and
every time the light hits a mirror the
clock ticks let's imagine you have two
identical copies of this clock you set
them running before taking one of them
deep inside a gravity well like the ones
explored in the last section space time
is more curved inside a gravity well and
so the space time between the clock's
mirrors will also be curved the beam of
light will have no option but to follow
this longer curved path in other words
the clock will now tick less frequently
than the clock you've left behind far
from the gravity well bring them back
together and more time will have elapsed
on the clock that did not encounter the
gravity well
but how can this be used to time travel
into the future well go and enter a deep
gravity well and then return to Earth
more time will have passed on Earth than
for you you could hop forward a century
just as you did by traveling at close to
the speed of light however to achieve a
genuinely significant amount of time
traveled using this method you'll need a
very deep gravity well indeed one far
bigger than the sun's a supermassive
black hole should do the trick as long
as you're careful not to get sucked in
and just such a scenario was used as a
key plot point in the film into Stella
suggested by the famous physicist Kip Thorne
Thorne
but Earth also creates its own gravity
well in space-time when Rossi drove his
bus up the winding road to Mount Evans
he was moving further out of this well
in other words he moved from a region
where space-time was more curved to a
region where it was less curved he aged
at a fractionally faster rate up the
mountain than he did down the mountain
this difference is so tiny that a human
is never going to notice but atomic
clocks do the best are such good time
Keepers that they are accurate to within
a 15 billionth of a second every year if
you sink up to atomic clocks on the same
shelf of a laboratory and then place
them on shelves of different heights
they will noticeably drift out of time
indeed in 2022 physicists measured this
gravitational time dilation using Ultra
precise atomic clocks placed just a
and finally you're likely to rely
heavily on time dilation probably
without even realizing it
GPS has become an indispensable part of
modern life for many of us our phones
and cars can tell where they are on
earth thanks to an armada of satellites
swarming around the planet these
satellites send out time-stout signals
and the quicker you receive one the
nearer you must be to that satellite in
order to achieve this the satellites
carry atomic clocks which are subject to
both forms of time dilation they are
both moving through space-time faster
than us and sit further outside Earth's
gravity well the time on the clocks has
to be artificially altered in order to
account for the fact that they are
moving through time at a different rate
to us
these experiments leave physicists in
little doubt that time travel to the
future is possible it is established
scientific fact but what about heading
in the other direction
is it possible to head upstream and
it is 2011. Barack Obama is President
Prince William and Kate Middleton in
newlyweds and NASA's iconic space
shuttle lands for the last time
in September physicists make a shock
announcement One That Rocks the
scientific establishment they seem to
have observed particles called neutrinos
traveling faster than the speed of light
true it would shatter one of the holiest
doctrines in physics and opened the door
to backwards time travel the Bold claim
comes from the Opera experiment buried
deep under Italy's Grand Sasso Mountain
designed to detect neutrinos via 730
kilometers through the Earth from CERN
near Geneva and Switzerland making that
Journey at the speed of light would take
a mere 2.4 milliseconds yet some of the
neutrinos appear to be taking 16
nanoseconds less than that in other
words they seem to have gotten there
traveling faster than the speed of light
Einstein's special theory of relativity
says that the speed of light is an
impenetrable barrier the faster you
travel the heavier you get the heavier
you get the more energy it takes to
increase your speed further eventually
there comes a point when it requires an
infinite amount of energy to increase
your speed so you can't and that cutoff
point is the speed of light
while a massless particle of light can
travel at light speed anything with mass
cannot reach light speed let alone
exceed it and so inevitably within a
year the neutrino mystery was solved the
experimental equipment was faulty the
neutrinos weren't superluminal after all [Music]
[Music]
the initial finding caused such a stir
because traveling faster than light
would allow you to travel backwards in
time and the story of why starts with a
young Dutchman by the name of Willem
Jacob van Stockholm
in 1937 van stockham publishes a paper
with a seemingly dull title of the
gravitational field of a distribution of
particles rotating about an axis of
symmetry one mundane in name it was
revolutionary in nature for it showed
that Einstein's equations allowed closed
time-like Curves in space-time
our world lines usually stretch from the
past to the future but a closed
time-like curve is a world line that
doubles back on itself closing a loop as
it intersects the line at a previous
point a pensioner following such World
lines could revisit the moment of their
birth or even save their teenage self
from a car accident
Von stockham had demonstrated that at
least on paper there is nothing in the
general theory of relativity to forbid
backwards time travel except that at
first glance it seems bending your
future world line into a closed
time-like curve requires you to travel
faster than the speed of light to travel
outside of what is known as your liked
cone if your world line is the singular
path that you actually take through the
blocky Universe then your light cone
represents all the paths that it would
have been at least possible for you to take
take
imagine that you are stationary in space
your world line moving forwards in a
straight line through time only what are
your options for where you can go in the
future well assuming that we stick with
special relativity you were confined by
the fact that nothing can travel faster
than the speed of light which is
approximately 300 000 kilometers per
second one second from now you cannot be
more than 300 000 kilometers away from
where you are currently yet you could be
up to 600 000 kilometers away in two
seconds and nine hundred thousand
kilometers in three the further ahead in
time you go the bigger the region of
space-time that your future world line
is allowed to be in becomes that's why
it's called a light cone the edges of
the cone represent the furthest a beam
of light can be from where it starts at
any given point in space-time and so
unless you can travel faster than a beam
of light you are trapped within the same
light cone meaning that in order to Loop
your future world line around so that it
into sects with itself at an earlier
point you would have to leave your light
cone in other words time travel to the
Past appears impossible unless you can
move faster than the speed of light foreign
foreign
but there is another way to do it if you
constantly tilt your light cone so that
your closed timeline curve to yesterday
is always within it and to do this you
would need to encounter space-time with
an incredibly high amount of curvature
in 1974 the American physicist Frank
tippler explored this very situation
specifically what would happen to a
cylinder containing at least 10 times
the mass of the Sun and rotating along
its longest axis he showed that
following a precise spiraling trajectory
around this cylinder would warp your
local space-time in such a way that your
light cone would gradually tilt
backwards and as your world line must be
somewhere within the cone you would then
be ferried from the present to the Past
visiting a tip the cylinder and then
journeying back to Earth would see you
arrive before you left in the first place
place
however in order to tackle the
fiendishly difficult mathematics tippler
had to assume that the cylinder is
infinitely long he couldn't conclusively
prove that a shorter one would do the
trick it would also have to rotate
billions of times a minute to warp
space-time enough hardly features of a
practical time machine then in the 1990s
Stephen Hawking added another layer of
complexity he argued that getting a
tipless cylinder that isn't infinitely
long to work would also require huge
amounts of negative energy and it is
here that quantum mechanics enters our
story one of the pillars of modern
physics alongside Einstein's general
theory of relativity it deals with the
subatomic world according to quantum
physics even empty space isn't truly
empty pairs of so-called virtual
particles are popping in and out of
existence all the time energy is
borrowed from the vacuum to create them
taking the local energy just below zero
in the same way that your bank balance
drops into negative territory when you
go into the overdraft and borrow money
from the bank just like the bank however
the universe eventually wants its money
back so negative energy is out there but
it's limited to Tiny regions of space
for fleetingly short amounts of time as
a result closed time-like curves may
exist in the subatomic realm but
creating a Time Loop big enough for a
human to travel along to revisit the
past seems an entirely different proposition
but there may be another way to achieve
the same thing a way to cheat the
sacrosanct rule of the speed of light as
the cosmic speed limit
a wormhole
in 1985 the celebrated astronomer author
and TV presenter Carl Sagan published a
novel called Contact in it an advanced
alien civilization reaches out to
humanity with the design of a device
capable of sending a single person
across the vast Chasm of space to the
star Vega taking into account that
nothing can go faster than light this 25
light-year Journey should take an
extremely long time so instead Sagan
wanted a way to get his heroine there
far faster but without invoking faster
than light travel even though the book
is finished Sagan phones his friend Kip
Thorne for advice Sagan overnights the
manuscript of Thorne who reads it in the
car the next day on his way to his
daughter's graduation
few quick calculations follow and Thorne
has the answer
what Sega needed in his story is a wormhole
wormhole
imagine space-time as a sheet of paper
with the Earth at one end and Vega at
the other now instead of traveling the
length of the paper you fold the paper
in half after all we know that space
time can be curved and bent Earth and
Vega are now right on top of each other
hop the gap between them and you get
there in no time at all from the outside
it would look like you've got there by
traveling faster than the speed of light
but at no point did you actually break
through that sacred barrier and so it
follows that a wormhole can not only
transport you to a different part of
space but it can also send you back in
time let's say the wormholt of Vega is
already set up first you need to attach
the Earth mouth of the Wormhole to a
rocket and drag it round on a loop
Through the Universe At Close to the
speed of light as the Wormhole has been
spending more of its space-time Budget
on speed less time will have elapsed
from its perspective this means that if
you jump into the Wormhole upon your
return to Earth you will emerge at the
other end and in the past crucially
though you will also be in the Vegas
star system so it is backwards time
travel but not heading back into Earth's history
history
but there is still a way to pull that
off too set the Wormhole up in the right
way and the time difference between its
mouths will be greater than the time it
will take you to return to Earth via the
conventional route you will arrive on
Earth before you jumped into the
wormhole in the first place at least
that's the theory in practice it gets a
little more complicated Thorne realized
that the gravitational pull of anything
traveling Through the Wormhole will act
to pull it shut you would need something
to help prop it open this again could be
negative energy but physicists have also
explored the possibility of using
everything from quantum entanglement to
in 1985 Jay Richard Gott and William
hiscox solved Einstein's equations for
cosmic strings six years later God
realized that they could also be used as
a time machine just like spiraling
around a tipless cylinder take the right
path along cosmic strings and you could
end up tilting your light comb backwards
quantum entanglement might be another
option physicists know how to link two
particles in such a way that changing
one instantly changes the other even if
you separate them by huge distances it's
as if the information about the change
you've made has traveled between the
particles faster than the speed of light
but it has turned out to be impossible
to actually transfer data super
luminally using this method so it seems
there are multiple ways you might be
able to travel backwards in time without
breaking the light speed barrier but
were it to be possible what could you do
about the paradoxes
well quantum theory might offer a
solution there two
two [Music]
handcuffs dig into your wrists and sweat
streams down your face as you stare at
the table in front of you on that table
rests a glass vial of lethal gas next to
a hammer on the end of a robotic arm a
robotic arm connected to a machine that
is constantly measuring subatomic
particles these particles can be in one
of two states A or B and the chance of
measuring each outcome is 50 50. it's
effectively a convoluted coin toss if
the outcome of the measurement is a
nothing happens but if it is B the
hammer Falls the vial is shattered and
you die
the machine will make 100 measurements
before the people watching from the
other room will allow you to leave your
odds of survival are around one in a
million trillion trillion you can feel
your heart pounding in your chest as the
machine makes its first measurement
a you relax slightly only to tensor
began ahead of the next measurement a
few seconds later
a again after the tenth a in a row you
begin to feel hopeful surely you can't
survive this after 99 consecutive A's
you're practically giddy it all comes
down to one final measurement the
machine clicks a
a
your captors are true to their word and
although you don't know it yet your
unlikely survival against seemingly
impossible odds just provided proof of
the many worlds interpretation of
quantum theory with potentially
important consequences for resolving
this is a human version of the famous
Schrodinger's cat thought experiment the
original imagined victim was a feline in
a box with the same vial of poison and
Hammer quantum physics says that instead
of being in state a or state B the
particle is initially in both state a
and state B at the same time something
physicists call a superposition of
States except that means that the vile
is also simultaneously unbroken and
broken and that the cat and you were
both alive and dead until the point the
next measurement is made is there
something about actually making the
measurement that forces the universe to
finally choose one way or another not
according to the many world's
interpretation of quantum theory
brainchild the physicist Hugh Everett
III it says that every time there's an
event in the quantum world that has more
than one outcome all outcomes happen
at that point the universe fractures
into two copies of itself outcome a
happens in one outcome B in the other
reality is therefore constantly
branching off to Fashion a tree of
possibilities the Great River of time
bifurcates into infinitely many separate
streams there are some in which you were
never born and others where you are
president of the Earth
if true making 100 measurements with the
poison and Hammer machine would have
created 100 different branches of
reality you died in 99 of them and
survived in one as the sole surviving
copy of yourself you Marvel at your own
luck the 99 copies of yourself in the
other branches though aren't so fortuitous
fortuitous
covering a sliding scale from those who
died after one measurement to those who
went through the whole process only to
this is known as the quantum immortality
experiment there will always be a
version of you that stubbornly refuses
to die no matter how many times the
machine makes a measurement in a sense
it's a game of Russian Roulette that you
can never lose while physicists do not
know whether this interpretation of
quantum physics is the right one the
many worlds Theory could help us resolve
many of the paradoxes that are often
associated with time travel to the Past
take for example the most famous Paradox
of all the grandfather paradox
you use your time machine to go back and
murder your grandfather as a child
however that means he never reaches
adulthood never meets your grandmother
and so your own mother is never born if
your own mother was never born then you
weren't either so how did you go back in
time to murder your grandfather in the
first place writers philosophers and
scientists have come up with various
ways to circumvent this issue Stephen
Hawking introduced what he called the
chronology protection conjecture it
states that backwards time travel is
simply impossible the universe would act
to rob you of any opportunity to go back
in time cause and effect sacrosanct and
tightly policed Igor novakov mused on
something similar here's eponymous
self-consistency principle says that you
can go back in time but that you can't
change what's already happened if you
revisit an earlier page in the book of
The Block Universe you cannot rewrite
what is already there attempt to murder
your younger self and you will fail
maybe the gun jams perhaps they sneeze
and you miss tries you might you could
never succeed this of course raises
other problems however whether events in
your future are predetermined or even
whether Free Will exists nor would
nobikov's self-consistency principle
work in all cases as Barack shashani
Jared wogan and Jacob Hauser show in a
series of papers published between 2020
imagine you have a working time machine
you also have a set of balls that are
either black or white crucially when two
balls meet the encounter changes their
color so a black ball becomes white and
a white ball becomes black let's say a
black ball travels through the time
machine where it whacks into its past
self the encounter changes both balls to
White the Collision also knocks the ball
that's already time traveled onto a path
that eventually sends it through the
time machine a second time in this
scenario the exact same ball exits the
exact same time machine at the exact
same moment in the past and yet it is
seemingly two different colors when it
does so this peculiar Paradox cannot be
explained away using nobikov's
self-consistency principle but according
to shashani wogan and hausa it can be
explained using parallel time streams
the many worlds of quantum mechanics
they argue that time travel to the past
is only possible if you emerge in a
different stream from the one you left
so the black ball would start in time
stream a but would emerge from the time
machine in time stream B in this
alternate time stream it's the other
ball that would end up going through the
time machine after the Collision instead
not the one that's been through once
already the ball that emerges from The
Time Machine the first time is a
different color from the one that
emerges the second time but that's now
okay because it is no longer the same ball
ball
this kind of extreme mental gymnastics
takes some time to wrap your head around
but it does offer a potential solution
to the grandfather Paradox if you enter
a separate time stream when you go back
in time then you can kill the version of
your grandfather there safe and the
knowledge that the version of your
grandfather that's crucial to your
logical existence remains alive and well
in the time stream you left behind your
free will is restored giving you the
choice to do whatever you want
it is possible to use many worlds to
circumvent the bootstrap Paradox as well
in our tale of the time traveler saving
their teenage self the origin of the
letter wasn't clear no one seemed to
write it but perhaps we were missing
characters from our story the version of
you in time stream a could write the
letter travel back in time to time
stream B and give it to their younger
self that teenager then chooses physics
grows up to invent the time machine and
travels back in time to time stream C to
give their teenage self an apparently
authorless letter so we can also explain
the bootstrap Paradox away but we need
three versions of you existing in three
if that seems like using a sledgehammer
to crack a nut you're not alone it could
be a shining example of just how humans
can tie themselves into intellectual
knots the number of splitting branches
could be infinite
as the physicist Bryce DeWitt said every
Quantum transition taking place in every
Star in every Galaxy in every remote
corner of the universe is splitting our
local World on earth into Myriad copies
of itself
but John wither the very man who coined
the phrase wormhole in the first place
fell out of love with the idea I have
reluctantly had to give up my support of
that point of view in the end because
I'm afraid it carries too great a load
of metaphysical baggage
and yet over the years the main world's
interpretation has gained traction with
physicists and now has a growing and
loyal following
an endless array of views each a little
different from the next playing out
every possible outcome of every possible
event in constantly fracturing streams
along the Great River
of time
and so as we travel along our world
lines at varying speeds within the great
block Universe around us time travel to
the Future isn't just possible but has
already been done it is being done every
day of course traveling backwards is
less clear but it seems plausible at
least on paper with a little helping
hand from the counter-intuitive world of
quantum mechanics for now we'll just
have to make do with the memories of
yesteryear safe in the knowledge that
you've been watching the entire history
of the universe don't forget to like And
subscribe and leave a comment to tell us
what you think thanks for watching and
we'll see you
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