0:02 foreign
0:05 it's a cold moonless night with a Mist
0:11 as he turned across the road fog
0:13 swallows both the sound and headlights
0:15 of the oncoming car you see it too late
0:18 embrace yourself for impact
0:20 then out of nowhere an elderly stranger
0:23 shoves you out of Harm's Way is hit by
0:25 the car and ends up in a heap on the tarmac
0:26 tarmac
0:30 their life ends as yours is saved later
0:32 a police officer hands you an envelope
0:34 found in the pocket of the deceased it
0:38 is addressed to you you open this
0:40 envelope and find a note with two words
0:41 scribbled on it
0:43 simply reads
0:47 study Physics whoever wrote this note
0:49 saved your life at the expense of theirs
0:51 so it only seems fair you follow their advice
0:53 advice
0:54 decades later you become a much
0:56 celebrated physics professor your
0:59 specialist subject Albert Einstein's
1:01 theories of Relativity you've always had
1:03 a hunch that time travel is possible and
1:07 one day finally you crack it at the
1:09 world's very first time machine is set
1:12 buzzing before you only then does the
1:14 penny drop how would you never realize
1:17 before you now have the power to go
1:19 anywhere and any when but you know
1:22 exactly what you have to do you take the
1:24 letter you've kept all these years out
1:26 of its frame on the wall place it back
1:28 inside its original envelope and put the
1:30 envelope in your pocket stepping into
1:32 your time machine you set the controls
1:35 to take you back to a cold moonless
1:38 Misty night arriving just in time to
1:40 push your younger self out of the path
1:43 of the on-rushing car
1:50 seems simple enough
1:52 but the real Joy of time travel stories
1:54 like this one comes once you begin to
1:57 scratch beneath the surface and some
2:00 incredibly profound questions begin to emerge
2:01 emerge
2:05 first of all who wrote the letter
2:07 the older version of you simply takes
2:09 the letter they'd already been given as
2:12 a teenager back in time the letter is a
2:14 seemingly creatorless entity stuck in a
2:19 Time Loop this is the bootstrap paradox
2:21 secondly do these time Loops keep on
2:23 repeating you save your teenage self so
2:25 they grow up to travel back in time to
2:27 save their teenage self who grows up to
2:30 travel back in time and so on you are
2:32 stuck in a Relentless cycle of living
2:36 and dying but the letter isn't
2:38 does it just keep getting older and more
2:40 worn and tattered
2:43 this is the restoration paradox
2:45 and whose idea was it to study Physics
2:47 anyway the notion seems to appear out of
2:51 nowhere a thought with no origin this is
2:54 the ex-niello Paradox and if events like
2:56 this can actually happen then it also
3:00 appears you have no free will no choices
3:02 in life you are always going to invent
3:04 the time machine and had absolutely no
3:07 option to do otherwise it
3:10 to happen had always happened if you
3:11 don't invent a time machine then you
3:13 can't save your teenage self and you
3:15 yourself would therefore die as a
3:17 teenager there is something in your
3:21 future that's firmly fixed this is the
3:23 predestination paradox
3:27 and finally what if you change your mind
3:30 unfortunately for your older self this
3:33 is not an option you can't get cold feet
3:34 when you arrive and that the younger you
3:36 get hit by the car how could you be
3:38 there to have second thoughts in the
3:39 first place
3:43 this is the auto infanticide Paradox it
3:44 is impossible to kill a younger version
3:48 of you even through the act of not doing something
3:49 something
3:53 and so not quite such a simple story
3:56 welcome to the weird and wonderful world
4:03 but of course traveling freely through
4:06 time is impossible you might say you
4:08 can't simply visit yesterday or Rush
4:11 head long through millennia
4:13 and yet this too is not quite so
4:16 straightforward despite its Myriad
4:19 impracticalities there are slivers of
4:20 possibility lurking at the edge of
4:24 physics temporal loopholes lying just
4:25 Out Of Reach
4:29 and so the question arises if time
4:32 travel were to be possible just how
4:34 could we do it [Music]
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6:05 the Sun beats down as the distinctive
6:08 whooping barks of howler monkeys Echo
6:11 around the humid swamps the current in
6:13 the muddy winding river is so strong
6:15 that you don't even have to paddle your
6:18 canoe you just sit back and take in the
6:20 sights and sounds as you're carried along
6:25 Jack Finney arguably one of the finest
6:27 proponents of time travel fiction
6:30 compared our experience of time to a
6:32 river like this one it seems like we're
6:34 relentlessly ferried Downstream from the
6:37 past to the future we have no need to
6:39 propel ourselves but slowing down
6:41 speeding up or turning around and
6:43 heading back Upstream doesn't seem
6:45 possible either but what happens to the
6:47 parts of the river that we've left
6:50 behind or in other words what is
6:52 yesterday go
6:54 it certainly seems as if it just
6:57 disappears yet making the same argument
6:59 about an actual River would seem
7:01 ludicrous you're unlikely to accept that
7:04 the riverbed is torn up and mysteriously
7:06 vanishes just because you've disappeared
7:08 around the next bend the Upstream parts
7:11 of the river are still there even if you
7:13 aren't in the same way it seems as if
7:17 tomorrow isn't real until it is that
7:18 somehow it magically appears once the
7:21 curtain falls on today but that would be
7:23 like new water springing up to make the
7:26 river of time longer only once you
7:28 approach where would it come from
7:31 and so over the years scientists have
7:34 revealed that time really is like that
7:37 River with its length already laid out
7:40 in its entirety from its source high in
7:42 the foothills of the Big Bang to its
7:44 Estuary at the universe's eventual
7:47 demise the past present and future
7:49 existing alongside one another your
7:52 great great grandchildren every bit as
7:54 real as you are now your great great
7:57 grandparents still alive and well
8:00 frolicking in the swells Upstream in the
8:03 vast River of time
8:05 philosophers know this idea as
8:07 eternalism it's the opposite of
8:09 presentism which argues that only now
8:12 exists and that the past is gone and the
8:15 future yet to emerge debates between
8:17 eternalist and presentist philosophers
8:19 have raged since the days of ancient
8:21 Greece indeed the 4th Century
8:23 philosopher Saint Augustine compared the
8:26 present to a knife's edge straddling the
8:29 perceived past and an imagined future
8:31 to a physicist
8:34 eternalism is known as the block
8:37 universe theory and is based on Albert
8:39 Einstein's much-lauded general
8:41 relativity one of the most successful
8:45 theories in the history of science [Music]
8:51 in 1905 Einstein publishes the special
8:54 theory of relativity it does away with
8:57 Isaac Newton's notion of absolute time
8:59 there is no universal clock that
9:01 everyone can use to agree on when
9:03 something happens instead as the name
9:07 suggests time is relative depending on
9:09 how they move through space one Observer
9:12 could see event a happen before event B
9:15 while another sees event B occur before
9:18 event a both observers are equally
9:21 correct an event in your past could be
9:24 in someone else's future this is yet
9:26 another reason why all our yesterdays
9:29 cannot simply vanish how can they when
9:31 the events they contain might form part
9:34 of somebody else's tomorrows as well as
9:36 this special relativity cemented the
9:38 fact that space and time are not
9:41 completely distinct from one another in
9:43 1908 Hermann minkowski develops
9:45 Einstein's work and properly unites
9:48 space and time once and for all
9:50 says that moving through space affects
9:52 how you measure time because the three
9:54 dimensions of space and one of time are
9:56 actually woven together into a single
10:00 fabric that he called space-time but how
10:02 do we know that space and time are
10:04 actually bundled up together into the
10:07 space time that the block universe is
10:09 made of proof would eventually come in
10:12 tests of Einstein's general theory of
10:14 relativity which he presented to the
10:22 back in the 1840s astronomers had
10:24 harnessed Newtonian gravity to discover
10:28 an entirely new planet using Newton's
10:30 equations to predict the future path of
10:32 Uranus they had found that it appeared
10:35 to deviate from that path the French
10:37 astronomer Urban leverier pulled over
10:40 the equations for months searching for
10:42 an answer was Newtonian gravity wrong
10:45 after all not quite the verrier
10:48 suggested that an unseen more distant
10:51 planet pulls on Uranus and affects its
10:53 orbit if correct Newton's laws of
10:57 gravity would be Vindicated and in 1846
10:59 Neptune was found
11:01 despite this success and although he
11:04 didn't know it yet the verrier was about
11:06 to hammer the first nail into Newtonian
11:09 gravity's coffin planets don't have
11:12 circular orbits but slightly changing
11:15 elliptical ones with the sun offset from
11:17 the center Newtonian physics claimed
11:19 that these slightly changing orbits was
11:21 due to the gravitational pull of the
11:24 other planets and Newton's equations
11:25 were correct in their predictions for
11:29 all of them except one Mercury
11:32 the verrier realized this in 1859 and
11:34 the search began for another new planet
11:38 but no such world was ever found instead
11:41 the answer came from Einstein on the
11:44 18th of November 1915 he wrote a letter
11:45 to the German mathematician David
11:49 Hilbert today I am presenting a paper in
11:51 which I derive out of general relativity
11:53 the perihelion motion of Mercury
11:56 discovered by leverier no gravitation
12:04 [Music]
12:06 imagine the fabric of space-time as the
12:09 surface of a trampoline
12:10 if you place something heavy in the
12:12 center it will sag in the middle
12:14 likewise the presence of a massive
12:16 object like the sun creates a dip in the
12:19 fabric of space-time physicists call
12:21 this a gravity well
12:23 Einstein was arguing that there is no
12:27 Newtonian gravitational pull at all it
12:30 is merely a mirage as the innermost
12:32 planet Mercury moves more deeply into
12:34 the Sun's gravity well than its
12:36 neighbors the deeper it goes the more
12:38 the local space-time becomes curved and
12:40 it's this increased curvature that
12:44 causes Mercury's orbit to change for all
12:45 the other planets the curvature is
12:47 subtle enough that Newtonian gravity
12:50 gives the same answer only in the
12:52 extreme gravitational environment that
12:55 mercury inhabits does Newtonian gravity
12:58 show itself for what it really is a good approximation
13:00 approximation
13:03 gravity is not a pull it's the result of
13:05 curved space-time
13:07 since then astronomers have tested this
13:09 idea in Myriad ways from solar eclipses
13:12 to swirling black holes and Rippling
13:14 gravitational waves and in every
13:17 experiment space-time passes with flying colors
13:18 colors
13:21 our entire Cosmos of past present and
13:23 future merely a four-dimensional block
13:26 of malleable space-time that just sits
13:29 there a supernatural being that could
13:30 somehow look upon it from the outside
13:33 world would clearly see yesterday today
13:37 and tomorrow happily coexisting
13:41 but where did this all come from as with
13:42 everything else scientists believed that
13:45 the Big Bang birthed all of space-time
13:48 in other words all of space and all of
13:50 time the regions of time we refer to as
13:53 the past the present and the future are
13:55 all there together in their entirety and
13:57 so it is easy for two people to exist
14:01 then and now as they do here and there
14:03 they can be separated by centuries of
14:06 time just as they can be separated by
14:08 kilometers of distance [Music]
14:11 [Music]
14:13 any object within this block Universe
14:17 can be given for coordinates
14:20 three spatial to say exactly where it is
14:23 and one temporal to say exactly when it
14:25 is imagine that an object's location in
14:28 the block is marked by a DOT even the
14:30 dot of a physically stationary object
14:33 moves because its time coordinate is
14:35 constantly changing a second ticks after
14:38 second and today relentlessly becomes
14:40 tomorrow join all the dots belonging to
14:43 one particle and you have a line that
14:45 Maps out its path through the block
14:49 Universe a world line you are nothing
14:51 but a collection of particle World lines
14:54 woven together and not in space-time
14:56 shedding and absorbing New World lines
14:59 as you move towards the day the knot
15:03 that is you and does itself or is undone
15:06 all the actions you will ever take from
15:07 what you'll have for breakfast every day
15:09 to if and who you'll marry and the
15:12 houses and jobs you'll flip between are
15:15 all entirely mapped out you can't
15:17 deviate from your journey any more than
15:22 a train can deviate from its tracks
15:24 in this way the block universe is like a
15:27 book the beginning the middle and the
15:28 end are already written and unchangeable
15:31 every choice you think you're making is
15:33 actually pre-written and so one that you
15:36 are always going to make a character
15:38 caught in a love triangle on page 150
15:41 may be unaware that it gets resolved 50
15:44 pages later but resolved it will be and
15:47 in the way it was always going to be
15:49 and this is why our time traveler from
15:52 the opening story had to invent the time
15:54 machine an older version of them had
15:57 already indelibly appeared on an earlier
16:00 page their world lines eventually looped
16:02 background to intersect with themselves
16:10 but can this really happen can you
16:12 travel to the seemingly long distant
16:16 past or indeed the far-flung future
16:18 if over then is no different to over
16:21 there it seems that the door to time travel
16:21 travel
16:30 in a distant corner of the universe a
16:33 gargantuan supermassive black hole is
16:37 holding sway over an entire galaxy
16:39 its intense gravity is pulling
16:41 stretching and ripping up whole solar
16:44 systems hot gas and dust whirl around it
16:47 as its huge magnetic field spins up and
16:50 spits out shrapnel into the void
16:53 among this debris is a tiny subatomic
16:56 bullet a proton which races across the
16:59 cosmos as as close to the speed of light
17:02 as it's practically possible to get it
17:04 has 100 quintillion times more energy
17:07 than an ordinary particle of light and
17:09 after traveling over a billion light
17:11 years it slams into Earth's atmosphere
17:14 with 21 million times the energy of
17:17 collisions inside the famed Large Hadron
17:20 Collider at CERN
17:24 it is so energetic that to this day
17:27 astronomers know it as the oh my God particle
17:33 an example of an ultra high energy
17:35 cosmic ray it is a reminder that the
17:37 Earth sits in a Celestial shooting
17:39 gallery and when Rays like this one
17:42 strike our atmosphere the impact is so
17:44 violent that it bursts into an invisible
17:47 fireworks display of subatomic particles
17:50 pions muons and neutrinos erupt and
17:53 Cascade down to the ground in fact
17:55 cosmic rays are so plentiful that they
17:57 give you an annual dose of radiation
18:01 equivalent to three chest x-rays and for
18:04 one Italian physicist cosmic rays became
18:08 an obsession his groundbreaking work on
18:10 the muons created by cosmic rays
18:12 producing some of the first evidence
18:14 that time travel to the future is not
18:18 only possible but commonplace
18:19 and his name
18:27 Rossi is behind the wheel of an old bus
18:29 snaking along the highest road in the
18:31 world where pine trees compete with
18:33 snow-capped mountains to reach the sky
18:35 eventually he arrives at the place he's
18:37 driven over a thousand miles from
18:39 Chicago to reach
18:40 Echo Lake
18:42 nestled high up in the Mount Evans
18:44 Wilderness of Colorado part of the world
18:48 famous Rocky Mountains it sits almost 2
18:55 up in the mountains Rossi is measuring
18:58 the muons created by cosmic rays new
19:00 ones are unstable particles and so they
19:02 quickly break down or Decay physicists
19:04 call the time it takes for half of a
19:07 group of muons to make this change their
19:11 Half-Life it is just 1.56 millionths of
19:14 a second but Rossi's measurements reveal
19:17 something curious far more muons are
19:19 reaching the ground than there should be
19:21 even though the muons are traveling at
19:24 more than 98 of the speed of light over
19:27 15 half-lives should still elapse by the
19:29 time they make it down to Echo Lake with
19:31 each successive Half-Life removing 50 of
19:35 the remaining muon population just 0.1
19:37 percent should reach Rossi and so how
19:40 come he is able to detect significantly
19:41 more than that
19:44 the remarkable answer was that the muons
19:48 were time traveling into our future
19:51 Einstein's special theory of relativity
19:52 says that you can think of anything
19:55 moving through space-time as having a
19:57 budget equal to the speed of light the
19:59 more of that budget you spend on moving
20:02 fast through space the less you spend on
20:04 time as the muons are traveling at close
20:07 to the speed of light considerably less
20:10 time passes for them than for Rossi from
20:12 their perspective fewer half-lives have
20:14 elapsed and so there simply hasn't been
20:18 enough time for 99.9 of them to Decay
20:21 this effect is called relativistic time
20:23 dilation and when Rossi factored it in
20:26 exactly the right number of muons reach
20:28 the ground [Music]
20:30 [Music]
20:33 but you may ask does this really count
20:35 as time travel
20:38 imagine instead that you are the muon
20:40 but instead of traveling from the upper
20:42 atmosphere to Echo Lake you travel on a
20:44 big loop around the universe at the same
20:48 98 of the speed of light and return home
20:51 25 years older those of us left on Earth
20:53 would have been spending less of our
20:55 space-time Budget on speed than you and
20:58 so it will cost us more in time but to
21:00 us you have spent your budget
21:03 differently for going time to travel
21:06 through space 25 years may have passed
21:08 for you while you were gone but
21:12 125 have elapsed on Earth
21:15 you have skipped forward a whole century
21:17 indeed such a journey could mess with
21:19 the normal generational order of family
21:22 relationships as is poignantly explored
21:24 in the short story memories of my mother
21:26 by Ken Liu
21:29 and so time travel to the future is more
21:31 than possible we just have to accelerate
21:33 humans to close to light speed that is
21:35 far from straightforward however because
21:38 the faster you travel the more energy
21:41 you have Einstein's famous equation E
21:43 equals m c squared tells us that energy
21:46 and mass are interchangeable so if you
21:49 gain energy you also gain mass in other
21:51 words the faster you travel the heavier
21:54 you get to an outside Observer a person
21:57 traveling at 99.99 of the speed of light
22:00 would weigh almost two tons or about the
22:03 same as a rhino and accelerating them to
22:05 that speed would require as much energy
22:09 as everyone on Earth currently uses in
22:10 three months [Music]
22:12 [Music]
22:14 understandably we are very far from
22:17 pulling that off but even today time
22:19 dilation leads to some bizarre consequences
22:20 consequences
22:23 take Mark and Scott Kelly both former
22:26 NASA astronauts who also happen to be twins
22:28 twins
22:30 Mark was born six minutes before his
22:33 brother however Scott has spent about 10
22:37 times longer in space 520 days to Mark's
22:40 54 days during these extra days Scott
22:42 was whizzing through space-time faster
22:45 than his brother spending more of his
22:48 Budget on speed and less on time in
22:50 other words more time has passed for
22:53 Mark than for Scott Mark may have
22:55 started out life six minutes older than
22:58 his twin but he is now 6 minutes and 5
23:00 milliseconds older had Scott been
23:02 traveling as fast as a muon instead
23:05 their age Gap would have widened to
23:07 nearly six and a half years despite
23:11 initially being born on the same day and
23:13 as if that weren't enough there is
23:15 another entirely separate form of
23:19 experimentally verified time dilation a
23:21 second way to time travel into the future
23:22 future
23:26 picture a clock except this Clock Works
23:27 differently to any you've seen hanging
23:30 on a wall it is simply made of two
23:32 horizontal mirrors a beam of light
23:34 bounces between the two mirrors and
23:36 every time the light hits a mirror the
23:39 clock ticks let's imagine you have two
23:42 identical copies of this clock you set
23:44 them running before taking one of them
23:46 deep inside a gravity well like the ones
23:48 explored in the last section space time
23:51 is more curved inside a gravity well and
23:53 so the space time between the clock's
23:56 mirrors will also be curved the beam of
23:58 light will have no option but to follow
24:01 this longer curved path in other words
24:04 the clock will now tick less frequently
24:06 than the clock you've left behind far
24:08 from the gravity well bring them back
24:11 together and more time will have elapsed
24:13 on the clock that did not encounter the
24:15 gravity well
24:17 but how can this be used to time travel
24:20 into the future well go and enter a deep
24:22 gravity well and then return to Earth
24:25 more time will have passed on Earth than
24:27 for you you could hop forward a century
24:29 just as you did by traveling at close to
24:31 the speed of light however to achieve a
24:33 genuinely significant amount of time
24:36 traveled using this method you'll need a
24:39 very deep gravity well indeed one far
24:42 bigger than the sun's a supermassive
24:44 black hole should do the trick as long
24:46 as you're careful not to get sucked in
24:49 and just such a scenario was used as a
24:51 key plot point in the film into Stella
24:54 suggested by the famous physicist Kip Thorne
24:55 Thorne
24:58 but Earth also creates its own gravity
25:01 well in space-time when Rossi drove his
25:03 bus up the winding road to Mount Evans
25:05 he was moving further out of this well
25:07 in other words he moved from a region
25:10 where space-time was more curved to a
25:12 region where it was less curved he aged
25:15 at a fractionally faster rate up the
25:17 mountain than he did down the mountain
25:19 this difference is so tiny that a human
25:22 is never going to notice but atomic
25:24 clocks do the best are such good time
25:26 Keepers that they are accurate to within
25:29 a 15 billionth of a second every year if
25:31 you sink up to atomic clocks on the same
25:33 shelf of a laboratory and then place
25:35 them on shelves of different heights
25:37 they will noticeably drift out of time
25:41 indeed in 2022 physicists measured this
25:43 gravitational time dilation using Ultra
25:46 precise atomic clocks placed just a
25:53 and finally you're likely to rely
25:55 heavily on time dilation probably
25:58 without even realizing it
26:01 GPS has become an indispensable part of
26:04 modern life for many of us our phones
26:06 and cars can tell where they are on
26:08 earth thanks to an armada of satellites
26:10 swarming around the planet these
26:12 satellites send out time-stout signals
26:14 and the quicker you receive one the
26:16 nearer you must be to that satellite in
26:18 order to achieve this the satellites
26:21 carry atomic clocks which are subject to
26:24 both forms of time dilation they are
26:25 both moving through space-time faster
26:29 than us and sit further outside Earth's
26:31 gravity well the time on the clocks has
26:33 to be artificially altered in order to
26:35 account for the fact that they are
26:37 moving through time at a different rate
26:39 to us
26:42 these experiments leave physicists in
26:43 little doubt that time travel to the
26:46 future is possible it is established
26:49 scientific fact but what about heading
26:51 in the other direction
26:54 is it possible to head upstream and
27:04 it is 2011. Barack Obama is President
27:06 Prince William and Kate Middleton in
27:09 newlyweds and NASA's iconic space
27:12 shuttle lands for the last time
27:14 in September physicists make a shock
27:16 announcement One That Rocks the
27:18 scientific establishment they seem to
27:21 have observed particles called neutrinos
27:24 traveling faster than the speed of light
27:26 true it would shatter one of the holiest
27:29 doctrines in physics and opened the door
27:32 to backwards time travel the Bold claim
27:34 comes from the Opera experiment buried
27:36 deep under Italy's Grand Sasso Mountain
27:40 designed to detect neutrinos via 730
27:42 kilometers through the Earth from CERN
27:44 near Geneva and Switzerland making that
27:46 Journey at the speed of light would take
27:49 a mere 2.4 milliseconds yet some of the
27:51 neutrinos appear to be taking 16
27:54 nanoseconds less than that in other
27:56 words they seem to have gotten there
28:00 traveling faster than the speed of light
28:02 Einstein's special theory of relativity
28:04 says that the speed of light is an
28:06 impenetrable barrier the faster you
28:09 travel the heavier you get the heavier
28:11 you get the more energy it takes to
28:13 increase your speed further eventually
28:15 there comes a point when it requires an
28:17 infinite amount of energy to increase
28:20 your speed so you can't and that cutoff
28:24 point is the speed of light
28:26 while a massless particle of light can
28:28 travel at light speed anything with mass
28:30 cannot reach light speed let alone
28:33 exceed it and so inevitably within a
28:36 year the neutrino mystery was solved the
28:39 experimental equipment was faulty the
28:42 neutrinos weren't superluminal after all [Music]
28:45 [Music]
28:47 the initial finding caused such a stir
28:49 because traveling faster than light
28:51 would allow you to travel backwards in
28:54 time and the story of why starts with a
28:57 young Dutchman by the name of Willem
28:59 Jacob van Stockholm
29:02 in 1937 van stockham publishes a paper
29:04 with a seemingly dull title of the
29:07 gravitational field of a distribution of
29:09 particles rotating about an axis of
29:11 symmetry one mundane in name it was
29:13 revolutionary in nature for it showed
29:16 that Einstein's equations allowed closed
29:19 time-like Curves in space-time
29:22 our world lines usually stretch from the
29:24 past to the future but a closed
29:27 time-like curve is a world line that
29:30 doubles back on itself closing a loop as
29:31 it intersects the line at a previous
29:34 point a pensioner following such World
29:36 lines could revisit the moment of their
29:39 birth or even save their teenage self
29:40 from a car accident
29:43 Von stockham had demonstrated that at
29:45 least on paper there is nothing in the
29:47 general theory of relativity to forbid
29:50 backwards time travel except that at
29:52 first glance it seems bending your
29:54 future world line into a closed
29:57 time-like curve requires you to travel
29:59 faster than the speed of light to travel
30:03 outside of what is known as your liked
30:06 cone if your world line is the singular
30:07 path that you actually take through the
30:10 blocky Universe then your light cone
30:11 represents all the paths that it would
30:14 have been at least possible for you to take
30:14 take
30:17 imagine that you are stationary in space
30:19 your world line moving forwards in a
30:22 straight line through time only what are
30:24 your options for where you can go in the
30:26 future well assuming that we stick with
30:28 special relativity you were confined by
30:30 the fact that nothing can travel faster
30:32 than the speed of light which is
30:35 approximately 300 000 kilometers per
30:37 second one second from now you cannot be
30:40 more than 300 000 kilometers away from
30:42 where you are currently yet you could be
30:45 up to 600 000 kilometers away in two
30:47 seconds and nine hundred thousand
30:50 kilometers in three the further ahead in
30:52 time you go the bigger the region of
30:54 space-time that your future world line
30:57 is allowed to be in becomes that's why
30:59 it's called a light cone the edges of
31:01 the cone represent the furthest a beam
31:03 of light can be from where it starts at
31:05 any given point in space-time and so
31:07 unless you can travel faster than a beam
31:09 of light you are trapped within the same
31:12 light cone meaning that in order to Loop
31:14 your future world line around so that it
31:16 into sects with itself at an earlier
31:18 point you would have to leave your light
31:21 cone in other words time travel to the
31:23 Past appears impossible unless you can
31:27 move faster than the speed of light foreign
31:29 foreign
31:32 but there is another way to do it if you
31:34 constantly tilt your light cone so that
31:37 your closed timeline curve to yesterday
31:41 is always within it and to do this you
31:43 would need to encounter space-time with
31:47 an incredibly high amount of curvature
31:50 in 1974 the American physicist Frank
31:53 tippler explored this very situation
31:55 specifically what would happen to a
31:57 cylinder containing at least 10 times
31:59 the mass of the Sun and rotating along
32:02 its longest axis he showed that
32:04 following a precise spiraling trajectory
32:07 around this cylinder would warp your
32:09 local space-time in such a way that your
32:12 light cone would gradually tilt
32:15 backwards and as your world line must be
32:17 somewhere within the cone you would then
32:20 be ferried from the present to the Past
32:22 visiting a tip the cylinder and then
32:24 journeying back to Earth would see you
32:26 arrive before you left in the first place
32:27 place
32:29 however in order to tackle the
32:31 fiendishly difficult mathematics tippler
32:33 had to assume that the cylinder is
32:35 infinitely long he couldn't conclusively
32:37 prove that a shorter one would do the
32:39 trick it would also have to rotate
32:41 billions of times a minute to warp
32:44 space-time enough hardly features of a
32:47 practical time machine then in the 1990s
32:49 Stephen Hawking added another layer of
32:51 complexity he argued that getting a
32:53 tipless cylinder that isn't infinitely
32:56 long to work would also require huge
32:59 amounts of negative energy and it is
33:02 here that quantum mechanics enters our
33:04 story one of the pillars of modern
33:05 physics alongside Einstein's general
33:08 theory of relativity it deals with the
33:10 subatomic world according to quantum
33:12 physics even empty space isn't truly
33:14 empty pairs of so-called virtual
33:16 particles are popping in and out of
33:18 existence all the time energy is
33:20 borrowed from the vacuum to create them
33:22 taking the local energy just below zero
33:24 in the same way that your bank balance
33:26 drops into negative territory when you
33:28 go into the overdraft and borrow money
33:30 from the bank just like the bank however
33:33 the universe eventually wants its money
33:36 back so negative energy is out there but
33:38 it's limited to Tiny regions of space
33:41 for fleetingly short amounts of time as
33:43 a result closed time-like curves may
33:45 exist in the subatomic realm but
33:47 creating a Time Loop big enough for a
33:49 human to travel along to revisit the
33:51 past seems an entirely different proposition
33:57 but there may be another way to achieve
34:00 the same thing a way to cheat the
34:03 sacrosanct rule of the speed of light as
34:05 the cosmic speed limit
34:07 a wormhole
34:11 in 1985 the celebrated astronomer author
34:13 and TV presenter Carl Sagan published a
34:16 novel called Contact in it an advanced
34:19 alien civilization reaches out to
34:21 humanity with the design of a device
34:23 capable of sending a single person
34:26 across the vast Chasm of space to the
34:29 star Vega taking into account that
34:32 nothing can go faster than light this 25
34:34 light-year Journey should take an
34:37 extremely long time so instead Sagan
34:39 wanted a way to get his heroine there
34:42 far faster but without invoking faster
34:45 than light travel even though the book
34:47 is finished Sagan phones his friend Kip
34:49 Thorne for advice Sagan overnights the
34:51 manuscript of Thorne who reads it in the
34:53 car the next day on his way to his
34:54 daughter's graduation
34:57 few quick calculations follow and Thorne
34:59 has the answer
35:02 what Sega needed in his story is a wormhole
35:04 wormhole
35:06 imagine space-time as a sheet of paper
35:09 with the Earth at one end and Vega at
35:11 the other now instead of traveling the
35:13 length of the paper you fold the paper
35:15 in half after all we know that space
35:18 time can be curved and bent Earth and
35:20 Vega are now right on top of each other
35:22 hop the gap between them and you get
35:24 there in no time at all from the outside
35:26 it would look like you've got there by
35:28 traveling faster than the speed of light
35:30 but at no point did you actually break
35:32 through that sacred barrier and so it
35:34 follows that a wormhole can not only
35:36 transport you to a different part of
35:39 space but it can also send you back in
35:42 time let's say the wormholt of Vega is
35:45 already set up first you need to attach
35:46 the Earth mouth of the Wormhole to a
35:48 rocket and drag it round on a loop
35:50 Through the Universe At Close to the
35:53 speed of light as the Wormhole has been
35:54 spending more of its space-time Budget
35:57 on speed less time will have elapsed
35:59 from its perspective this means that if
36:01 you jump into the Wormhole upon your
36:03 return to Earth you will emerge at the
36:06 other end and in the past crucially
36:08 though you will also be in the Vegas
36:10 star system so it is backwards time
36:13 travel but not heading back into Earth's history
36:14 history
36:16 but there is still a way to pull that
36:18 off too set the Wormhole up in the right
36:20 way and the time difference between its
36:22 mouths will be greater than the time it
36:24 will take you to return to Earth via the
36:26 conventional route you will arrive on
36:28 Earth before you jumped into the
36:31 wormhole in the first place at least
36:34 that's the theory in practice it gets a
36:36 little more complicated Thorne realized
36:38 that the gravitational pull of anything
36:40 traveling Through the Wormhole will act
36:42 to pull it shut you would need something
36:44 to help prop it open this again could be
36:47 negative energy but physicists have also
36:49 explored the possibility of using
36:51 everything from quantum entanglement to
36:57 in 1985 Jay Richard Gott and William
37:00 hiscox solved Einstein's equations for
37:02 cosmic strings six years later God
37:04 realized that they could also be used as
37:06 a time machine just like spiraling
37:08 around a tipless cylinder take the right
37:10 path along cosmic strings and you could
37:12 end up tilting your light comb backwards
37:14 quantum entanglement might be another
37:17 option physicists know how to link two
37:19 particles in such a way that changing
37:21 one instantly changes the other even if
37:24 you separate them by huge distances it's
37:25 as if the information about the change
37:27 you've made has traveled between the
37:29 particles faster than the speed of light
37:32 but it has turned out to be impossible
37:34 to actually transfer data super
37:38 luminally using this method so it seems
37:40 there are multiple ways you might be
37:42 able to travel backwards in time without
37:45 breaking the light speed barrier but
37:47 were it to be possible what could you do
37:48 about the paradoxes
37:51 well quantum theory might offer a
37:53 solution there two
37:54 two [Music]
38:03 handcuffs dig into your wrists and sweat
38:06 streams down your face as you stare at
38:09 the table in front of you on that table
38:12 rests a glass vial of lethal gas next to
38:15 a hammer on the end of a robotic arm a
38:18 robotic arm connected to a machine that
38:20 is constantly measuring subatomic
38:22 particles these particles can be in one
38:26 of two states A or B and the chance of
38:29 measuring each outcome is 50 50. it's
38:32 effectively a convoluted coin toss if
38:34 the outcome of the measurement is a
38:37 nothing happens but if it is B the
38:40 hammer Falls the vial is shattered and
38:41 you die
38:44 the machine will make 100 measurements
38:45 before the people watching from the
38:48 other room will allow you to leave your
38:51 odds of survival are around one in a
38:54 million trillion trillion you can feel
38:56 your heart pounding in your chest as the
38:58 machine makes its first measurement
39:02 a you relax slightly only to tensor
39:03 began ahead of the next measurement a
39:05 few seconds later
39:09 a again after the tenth a in a row you
39:11 begin to feel hopeful surely you can't
39:14 survive this after 99 consecutive A's
39:16 you're practically giddy it all comes
39:19 down to one final measurement the
39:21 machine clicks a
39:22 a
39:24 your captors are true to their word and
39:30 although you don't know it yet your
39:32 unlikely survival against seemingly
39:35 impossible odds just provided proof of
39:37 the many worlds interpretation of
39:39 quantum theory with potentially
39:42 important consequences for resolving
39:50 this is a human version of the famous
39:53 Schrodinger's cat thought experiment the
39:55 original imagined victim was a feline in
39:58 a box with the same vial of poison and
40:00 Hammer quantum physics says that instead
40:03 of being in state a or state B the
40:05 particle is initially in both state a
40:07 and state B at the same time something
40:10 physicists call a superposition of
40:13 States except that means that the vile
40:16 is also simultaneously unbroken and
40:19 broken and that the cat and you were
40:22 both alive and dead until the point the
40:25 next measurement is made is there
40:27 something about actually making the
40:30 measurement that forces the universe to
40:34 finally choose one way or another not
40:35 according to the many world's
40:37 interpretation of quantum theory
40:39 brainchild the physicist Hugh Everett
40:42 III it says that every time there's an
40:44 event in the quantum world that has more
40:48 than one outcome all outcomes happen
40:50 at that point the universe fractures
40:53 into two copies of itself outcome a
40:56 happens in one outcome B in the other
40:58 reality is therefore constantly
41:01 branching off to Fashion a tree of
41:03 possibilities the Great River of time
41:06 bifurcates into infinitely many separate
41:09 streams there are some in which you were
41:11 never born and others where you are
41:14 president of the Earth
41:16 if true making 100 measurements with the
41:18 poison and Hammer machine would have
41:20 created 100 different branches of
41:23 reality you died in 99 of them and
41:27 survived in one as the sole surviving
41:29 copy of yourself you Marvel at your own
41:32 luck the 99 copies of yourself in the
41:34 other branches though aren't so fortuitous
41:35 fortuitous
41:38 covering a sliding scale from those who
41:40 died after one measurement to those who
41:43 went through the whole process only to
41:50 this is known as the quantum immortality
41:52 experiment there will always be a
41:54 version of you that stubbornly refuses
41:56 to die no matter how many times the
41:58 machine makes a measurement in a sense
42:01 it's a game of Russian Roulette that you
42:03 can never lose while physicists do not
42:05 know whether this interpretation of
42:07 quantum physics is the right one the
42:09 many worlds Theory could help us resolve
42:12 many of the paradoxes that are often
42:16 associated with time travel to the Past
42:19 take for example the most famous Paradox
42:22 of all the grandfather paradox
42:24 you use your time machine to go back and
42:26 murder your grandfather as a child
42:28 however that means he never reaches
42:30 adulthood never meets your grandmother
42:33 and so your own mother is never born if
42:34 your own mother was never born then you
42:37 weren't either so how did you go back in
42:39 time to murder your grandfather in the
42:42 first place writers philosophers and
42:44 scientists have come up with various
42:47 ways to circumvent this issue Stephen
42:49 Hawking introduced what he called the
42:52 chronology protection conjecture it
42:54 states that backwards time travel is
42:57 simply impossible the universe would act
42:59 to rob you of any opportunity to go back
43:03 in time cause and effect sacrosanct and
43:06 tightly policed Igor novakov mused on
43:08 something similar here's eponymous
43:10 self-consistency principle says that you
43:12 can go back in time but that you can't
43:15 change what's already happened if you
43:17 revisit an earlier page in the book of
43:19 The Block Universe you cannot rewrite
43:21 what is already there attempt to murder
43:23 your younger self and you will fail
43:26 maybe the gun jams perhaps they sneeze
43:28 and you miss tries you might you could
43:31 never succeed this of course raises
43:33 other problems however whether events in
43:36 your future are predetermined or even
43:39 whether Free Will exists nor would
43:40 nobikov's self-consistency principle
43:43 work in all cases as Barack shashani
43:45 Jared wogan and Jacob Hauser show in a
43:47 series of papers published between 2020
43:54 imagine you have a working time machine
43:56 you also have a set of balls that are
43:59 either black or white crucially when two
44:01 balls meet the encounter changes their
44:03 color so a black ball becomes white and
44:06 a white ball becomes black let's say a
44:07 black ball travels through the time
44:09 machine where it whacks into its past
44:12 self the encounter changes both balls to
44:14 White the Collision also knocks the ball
44:16 that's already time traveled onto a path
44:17 that eventually sends it through the
44:20 time machine a second time in this
44:23 scenario the exact same ball exits the
44:25 exact same time machine at the exact
44:28 same moment in the past and yet it is
44:30 seemingly two different colors when it
44:33 does so this peculiar Paradox cannot be
44:35 explained away using nobikov's
44:37 self-consistency principle but according
44:40 to shashani wogan and hausa it can be
44:44 explained using parallel time streams
44:48 the many worlds of quantum mechanics
44:50 they argue that time travel to the past
44:52 is only possible if you emerge in a
44:54 different stream from the one you left
44:57 so the black ball would start in time
44:59 stream a but would emerge from the time
45:02 machine in time stream B in this
45:04 alternate time stream it's the other
45:05 ball that would end up going through the
45:07 time machine after the Collision instead
45:10 not the one that's been through once
45:12 already the ball that emerges from The
45:14 Time Machine the first time is a
45:15 different color from the one that
45:18 emerges the second time but that's now
45:20 okay because it is no longer the same ball
45:22 ball
45:25 this kind of extreme mental gymnastics
45:27 takes some time to wrap your head around
45:29 but it does offer a potential solution
45:31 to the grandfather Paradox if you enter
45:33 a separate time stream when you go back
45:35 in time then you can kill the version of
45:37 your grandfather there safe and the
45:39 knowledge that the version of your
45:41 grandfather that's crucial to your
45:44 logical existence remains alive and well
45:47 in the time stream you left behind your
45:50 free will is restored giving you the
45:54 choice to do whatever you want
45:56 it is possible to use many worlds to
45:58 circumvent the bootstrap Paradox as well
46:01 in our tale of the time traveler saving
46:03 their teenage self the origin of the
46:05 letter wasn't clear no one seemed to
46:08 write it but perhaps we were missing
46:10 characters from our story the version of
46:12 you in time stream a could write the
46:14 letter travel back in time to time
46:16 stream B and give it to their younger
46:19 self that teenager then chooses physics
46:21 grows up to invent the time machine and
46:24 travels back in time to time stream C to
46:26 give their teenage self an apparently
46:28 authorless letter so we can also explain
46:31 the bootstrap Paradox away but we need
46:33 three versions of you existing in three
46:43 if that seems like using a sledgehammer
46:46 to crack a nut you're not alone it could
46:49 be a shining example of just how humans
46:51 can tie themselves into intellectual
46:53 knots the number of splitting branches
46:56 could be infinite
46:59 as the physicist Bryce DeWitt said every
47:01 Quantum transition taking place in every
47:04 Star in every Galaxy in every remote
47:07 corner of the universe is splitting our
47:10 local World on earth into Myriad copies
47:12 of itself
47:15 but John wither the very man who coined
47:16 the phrase wormhole in the first place
47:20 fell out of love with the idea I have
47:21 reluctantly had to give up my support of
47:23 that point of view in the end because
47:26 I'm afraid it carries too great a load
47:28 of metaphysical baggage
47:31 and yet over the years the main world's
47:33 interpretation has gained traction with
47:36 physicists and now has a growing and
47:37 loyal following
47:40 an endless array of views each a little
47:42 different from the next playing out
47:44 every possible outcome of every possible
47:47 event in constantly fracturing streams
47:49 along the Great River
47:50 of time
47:53 and so as we travel along our world
47:56 lines at varying speeds within the great
47:58 block Universe around us time travel to
48:01 the Future isn't just possible but has
48:05 already been done it is being done every
48:07 day of course traveling backwards is
48:10 less clear but it seems plausible at
48:12 least on paper with a little helping
48:14 hand from the counter-intuitive world of
48:17 quantum mechanics for now we'll just
48:19 have to make do with the memories of
48:22 yesteryear safe in the knowledge that
48:33 you've been watching the entire history
48:36 of the universe don't forget to like And
48:37 subscribe and leave a comment to tell us
48:40 what you think thanks for watching and
48:41 we'll see you
