Time travel may be more possible now than ever imagined, and today, cutting-edge technology and the latest research into black holes and wormholes is helping experts get much closer to discovering how to travel through time.
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LearningTranscript
00:00the time machines of science fiction offer infinite possibilities but could time travel
00:12ever be science fact I don't want to blow your minds here but time travel is not even remotely
00:18science fiction it is absolute science reality time itself may be something you can bend and
00:25stretch them so in some respects time travel may be every bit as real and every bit as strange as
00:31our wildest science fiction fantasies by investigating time travel we're unraveling
00:38the deepest mysteries of the cosmos thinking about time travel can teach us a lot about the nature of
00:46our universe it forces us to take on some of the toughest unanswered questions in all of physics
00:52the more we learn about how the universe works the stranger it gets
00:56Cambridge England 2009 world-renowned physicist Stephen Hawking threw a party
01:19canapes were prepared champagne bored but friends and family weren't on the guest list the only
01:32people invited were time travelers here's somebody who worked on the physics of black holes worked on
01:39the physics of time and he thought to himself if time travelers exist they might all come together
01:46at one specific point in space and time for a party the invites gave a place date and time but they
01:55weren't sent out until after the party happened he only invited people from the future who could travel back into
02:06the past professor hawking waited and waited and waited
02:15unfortunately no one showed up is this proof that time travel doesn't exist well no maybe maybe he's just
02:21known in the future as having thrown really crappy parties a party without guests isn't much of a party
02:29good time travelers jump back in time and liven things up we're all moving into the future that is in essence time travel you're traveling into the future at 60 seconds per minute
02:43it's kind of a cop-out though when you talk about time travel you want to talk about leapfrogging into the future or going into the past if we want to go to Stephen Hawking's party which is now in the past how do we do that
02:57one way would be to change our passage through time according to Albert Einstein that's possible
03:11a hundred years ago Einstein started a scientific revolution which requires us to let go of our common
03:19sense ideas about what space and time are so instead of thinking of our universe is a three-dimensional place
03:25that just changes over time we should think of reality is this four-dimensional place called space-time
03:30if you stop and think about it all of your observations of time are directly coupled to watching something move in space right
03:41what is a day really but the rising and the setting of the Sun or an hour but the motion of a hand on a clock
03:48the three dimensions of space are linked with one dimension of time making a four-dimensional space-time continuum
03:59for wannabe time travelers that's good news it means motion through space is connected to motion through time
04:10we move through space time not space or time and the way this works is that if I'm standing still and I'm not moving through space very quickly then I move through time as fast as is possible
04:25this DeLorean doesn't look like it's moving but it is it's moving through time the car its driver and the road it's parked on are all moving through time at the same rate second by second
04:43but when the driver hits the gas
04:50some of that movement through time is converted into movement through space
04:57as soon as I have motion through space some of my intrinsic movement through space time is now taken up by that motion as I move faster through space I move slower through time
05:10scientists call this time dilation
05:17it turns fast moving humans into time travelers
05:25March 27 2015
05:28astronaut Scott Kelly traveled to the International Space Station
05:38his year-long mission was to study the effects of space flight on the human body
05:45Scott was the perfect candidate because back on Earth he had an identical twin
05:52Mark
05:53they did this for a variety of reasons to explore the effects of space travel and weightlessness on the human body using as controlled an experiment as possible
06:07Lack of gravity wasn't the only difference between the twins
06:14Scott was orbiting Earth at 17,000 miles an hour
06:19So compared to his Earth-bound twin Scott moved forwards through time
06:26this time travel into the future isn't just an abstract physics concept
06:31Scott the orbiting twin literally jumped into the future by a fraction of a second
06:37when Scott finally returned back to Earth
06:41because of his rapid speed he aged just a little bit slower than his brother and he was actually younger by a tiny fraction of a second
06:5117,000 miles an hour is fast but to jump more than a fraction of a second into the future
07:00Scott needed to go way faster
07:03What if Scott Kelly had wanted to let the Earth age a thousand years while he was in orbit for one year
07:10how fast would he have had to orbit the Earth to do that and it turns out he'd have to orbit at almost the speed of light
07:17to put it in perspective just how fast that is the fastest human piloted vehicle in history was Apollo 10
07:24that went at 25,000 miles per hour
07:27you would need to go more than 25,000 times faster than that
07:32that's pretty fast
07:34in the future we might try to build a spaceship with advanced propulsion capable of light speed
07:45but the laws of physics won't make it easy
07:49it would take an infinite amount of energy to accelerate something
07:53a car a marble a galaxy whatever to the speed of light
07:57and so for that reason we think that the speed of light is itself a truly unbreakable speed
08:03if you want to take a human sized spacecraft
08:06and accelerate it to 10% of the speed of light
08:09let alone 90% or 99% of the speed of light
08:13it requires more energy than humanity has ever used in its entire existence
08:19and probably will ever use in its entire existence
08:22jumping forward in time isn't simple
08:26but the physics of the universe make it possible
08:30going close to the speed of light slingshots you into the future faster
08:36but it does not take you to the past in any way
08:38it's not a way to go backwards in time and visit anyone's party
08:42a super fast time traveling spaceship
08:46can't take us back to Hawking's party
08:49but what about a time machine that exists out in the cosmos?
09:00in 2009 Stephen Hawking held a party for time travelers
09:15no one showed up
09:16could that situation ever change?
09:19so here we are in the future
09:22and we'd really love to go to that party
09:25I heard there's great snacks
09:26how do we get back there?
09:29we know extreme speeds can send us into the future
09:33but the universe has another force that messes with time
09:38gravity
09:41remember that there is only something called space time
09:44not separate space and time
09:46and what gravity really is is a bending of space time itself
09:51think of space time like a rubber sheet
09:56massive objects like planets and stars stretch it
10:01bending space and passages of time
10:04as you get closer to something with a lot of gravity
10:08time and space are stretched
10:10and that really does mean that time goes more slowly
10:14it even happens on earth
10:18here time runs more slowly close to the ground
10:23so what this means is if you live high up in an apartment building
10:26your clock is ticking by slightly more quickly
10:30than people living at the bottom of the apartment building
10:32you feel the earth's gravity slightly differently than they do
10:35if you live in the top floor of a luxury high rise in a penthouse
10:41you're actually aging more quickly than someone who lives in the basement
10:47these time differences are just tiny fractions of a second
10:52but there is a place in the universe where powerful gravitational forces slow time dramatically
11:00a black hole
11:03a black hole is a region of space where the space is so curved that not even light can escape
11:10a black hole in many ways is a natural time machine
11:14the closer you get to a black hole the more into that gravity the slower time goes
11:19at the center of the Milky Way sits Sagittarius A-star
11:28a supermassive black hole with the mass of 4 million suns
11:34to use this natural time machine we would have to send a spacecraft
11:40once that spacecraft gets near the black hole strange things will begin to occur
11:45the mission control would see the astronauts say
11:49hello
11:52and the astronauts would hear them answer
11:54oh my god I'm worried about you is everything okay
11:56apparently speaking too fast
11:58and then the astronauts would respond
12:00oh I'm fine
12:04they would seem to be moving in slow motion
12:06the crew steer the craft into orbit around the supermassive black hole
12:16mission control might see the craft orbit every 16 hours
12:21but for the crew the orbit is far shorter
12:25the immense gravity of Sagittarius A-star
12:28slows the craft's time relative to mission control
12:32if you enter a strong gravitational field like near a black hole
12:39and then you come back
12:41you will have experienced less time than someone who just stayed behind here on Earth
12:45but it never feels strange to you
12:48you always look at your wristwatch and the clock is ticking at exactly the same rate as you would expect
12:53you don't even notice that you're in a gravitational field until you come back and compare your clocks to the people who left behind
12:59in this way traveling close to a black hole and then coming back allows you to accelerate your passage through time compared to people who stayed behind
13:08so you're jumping in time you really are time traveling in that way
13:12if the gravity outside a black hole accelerates a spacecraft through time
13:19what does the inside do?
13:21to find out the crew send a manned probe towards the black hole's event horizon
13:29if you could maintain communication with them
13:32one of the things you would observe is that everything would get reddened
13:35that the light is actually losing energy as it comes out of that gravity of the black hole
13:39it'll get dimmer and dimmer and eventually as it falls right onto that event horizon it just fades out and freezes
13:46at the event horizon the probe appears to freeze in time and fade away
13:54but on board the probe time doesn't change a bit
14:01the crew plunge into the black hole
14:05inside immense gravitational forces might stretch the probe like spaghetti
14:11if the craft survives the crew push on towards the central singularity
14:16a place where the laws of physics do not apply
14:21a singularity is a true discontinuity
14:25a causal break in the fabric of space-time itself
14:28that's a fancy way of saying that we have no idea what happens beneath it
14:32if singularity is a break in space-time
14:37could it let us jump through time?
14:41what happens on the inside of a supermassive black hole
14:46is all very much in the realm of very advanced theoretical physics
14:52in fact the singularity at the center of a supermassive black hole
14:56it may be possible to even go through it
14:58there's many interpretations of what it could potentially mean
15:04parallel universes or time travel
15:08it could be that space and time gets far more chaotic
15:17different points in space and time connect to each other in every direction
15:20so at the very heart of a black hole you indeed may be able to access any point in space or time in the universe
15:30we can't know for sure if the singularity is a portal through time
15:36what we do know is crossing a black hole's event horizon is a one-way trip
15:41that's the thing about black holes you ain't coming out
15:47to return to the present after visiting professor hawkins party
15:52we'll need a different kind of time machine
15:55one that lets us come back
15:58they might exist
16:00but they might also crush anything that enters
16:04you
16:11in the movies time travel is as easy as hitting 88 miles an hour
16:23or diving into a black hole
16:26we've seen the concept of time travel into the past
16:30very often in movies and in tv
16:33did they get it right? did they get it wrong?
16:35it's hard to tell
16:37so the way that we currently understand time travel
16:41in a real sense is through either traveling very quickly
16:45or through a gravitational field
16:48all of these things will bring you into the future
16:51but not into the past
16:53could physics offer a different route to the past?
17:00stars and planets curve space-time
17:03black holes bend it infinitely
17:08but strange theoretical objects called wormholes
17:14could punch right through space-time
17:17connecting two different points in time
17:21with a tunnel
17:23so if you think about the fabric of space-time
17:26is this giant sheet
17:28and you want to get from one point to the other
17:30what a wormhole will do
17:31what a wormhole will do
17:33is that it will provide a bridge
17:35between the two points
17:36making them next to each other
17:40travelers would enter one end of the wormhole
17:45and exit in a different time
17:50allowing direct access to far away places
17:59and since wormholes connect points in space and time
18:06they could unlock real-life time travel
18:10there are some solutions to general relativity
18:14that allow for a concept of wormholes
18:16where if you entered it
18:19and could somehow survive traveling through it
18:21you would exit the wormhole at a time before you actually entered it
18:24right? so this would quite literally be time travel
18:27travelers would need to ensure the wormhole's entry point
18:33is anchored in the present
18:35while the exit is locked in the past
18:37turns out there's a way to do that
18:40you take two ends of a single wormhole
18:45a tunnel through space-time between them
18:47now you take one of those and you speed it up
18:49to near the speed of light
18:51it will freeze in time
18:53by time dilation
18:55on the other hand
18:57this end of the wormhole will continue to travel through time
19:00let's say in the far future
19:02you want to travel back to the point where those wormholes are created
19:05you just enter this end of the wormhole
19:08the one that's been ticking forward in time
19:10and you'll emerge from the frozen wormhole
19:13back where you started from
19:15but the furthest back you could travel is limited
19:20you wouldn't be able to go back before the moment you created it
19:25right? so you could create this time machine here and now
19:28and then people in the future could come back to the moment you created it
19:32a wormhole time machine won't let us go back to Hawking's party
19:38unless it was created before the party took place
19:42and there's a bigger problem using wormholes for time travel
19:48if we found a wormhole and tried to use it to travel backward in time
19:53really the gravitational field would be so strong
19:56that it would all just collapse into a black hole
19:58of course you need to survive passage through a wormhole
20:02and to do that you need to essentially hold open the throat of the wormhole
20:08there's only one way to do that
20:11to keep the wormhole open requires negative energies
20:16that sounds bad and it should sound bad
20:19we don't know whether you can make these kinds of negative energies
20:21people talk about exotic forms of energy that could push apart these wormholes
20:29but we don't know of anything of that sort
20:32the closest we know of is the dark energy that is supposedly accelerating the expansion of the universe
20:38dark energy pushes the universe apart
20:41but isn't exotic enough to hold open a wormhole
20:47it doesn't have negative energy
20:51but some scientists hope we'll find something that does
20:55so first people said weird stuff like that just totally can't exist
21:01but then another kind of weird stuff that we were told couldn't exist
21:05dark energy turned out to actually exist
21:08so now we're not so quick and fast and loose anymore
21:14to just say oh are we sure that can't exist
21:16someday we may discover a substance with negative energy
21:22opening up the possibility of wormholes
21:25and of traveling backwards through time
21:28but there may be another way to travel to the past
21:32by controlling time itself
21:35time itself may be something you can bend and stretch
21:39there may even be different versions of time
21:40so in some respects time travel may be every bit as real and every bit as strange
21:46as our wildest science fiction fantasies
22:05time travel inspires incredible journeys of science fiction
22:08and traveling to the past would be the ultimate vacation
22:14if I could time travel into the past
22:17I would love to experience ancient Rome at the height of the Roman Empire
22:23I would travel 13 billion years in the past
22:27and I would watch our galaxy form
22:29well I can tell you if I were a time traveler
22:31I would love to show up for Stephen Hawking's party
22:34but is this actually possible?
22:36could we ever travel back into the past?
22:40if we could travel back in time
22:42the possibilities would be endless
22:45but backwards time travel
22:47also causes mystifying temporal paradoxes
22:51even in science fiction time travel is all about paradoxes
22:56is it possible that you can influence your own past
22:59and the most simple way of putting this is the grandfather paradox
23:01what if you could go backwards in time and kill your grandfather?
23:07in that case how could your parents have been born?
23:10how could you have ever been born?
23:12but if you were never born and you didn't exist
23:15how did you kill your grandfather?
23:17you just run in circles
23:19it doesn't make any sense
23:20it's logically impossible
23:24it seems like the laws of the universe will not allow you to travel back in time
23:28but maybe there's a loophole
23:30there could be a way to travel back in time without creating a paradox
23:39thanks to the way that space and time are linked
23:43once you believe in four-dimensional space-time
23:48you begin to conceptualize reality as the whole four-dimensional thing
23:53which you then call the block universe
23:55it's like a four-dimensional block of stuff
23:57the different slices are different moments of time
24:04in the block universe the past, present, and future coexist
24:09if you could step outside of this entire framework and see this block universe
24:17you would see the entire history of the universe from time zero to time infinity
24:22sitting in front of you
24:24from dinosaurs roaming the earth 150 million years ago
24:30to humans colonizing the solar system hundreds of years in the future
24:35and Hawking's party for time travelers back in 2009
24:41in the block universe all of history exists simultaneously
24:48astrophysicist Paul Sutter explains
24:53you can think of the block universe as a film reel
24:57where the past and future already exist
25:00they're just frames on the same film
25:02all the frames already exist
25:06they're just right there
25:08but we experience them in a particular order
25:11and in a particular direction
25:13based on, you know, a particular turn of the handle
25:17just like a handle turning a film reel
25:21time flows from past to future
25:24but since every moment in time exists
25:27as a frame somewhere on this reel
25:31then surely we can visit them
25:34if the idea of the block universe is really true
25:37that makes time travel more understandable and more possible
25:41we just need to find a way to get to different parts of this reel
25:44to do that
25:46we have to find a way to travel through time
25:48we know planets and black holes curve space-time
25:58but Einstein's equations reveal
26:01that really massive objects moving around each other
26:04can drag space-time into a loop
26:08the regions of our universe most likely to harbor
26:11the greatest possibility for something crazy like time travel
26:13is in the most extreme regions of space-time curvature
26:18you can imagine a very complicated situation
26:21where you had enough mass
26:23and it was moving in such a way
26:25that you could twist space upon itself
26:27theoretical objects called naked line singularities
26:32could do just that
26:34like the hearts of two black holes
26:36but stretched out infinitely
26:39two naked singularities moving close to each other
26:43could create a looped path through space-time
26:47called a closed time-like curve
26:50a closed time-like curve
26:53is a very special kind of path through space-time
26:56where you have some starting point
26:58and you start moving through space-time
27:01just like you advance in frames in this piece of film
27:04and it just so happens in a closed time-like curve
27:08that your ending frame is exactly the same as your beginning frame
27:14so as you move through space you start moving into your future
27:19but you also move into your own past
27:22and you end up at exactly the same point where you started
27:26both in space and in time and you've closed the loop
27:30with closed time-like curves you may be able to visit your own past
27:36by looping space-time
27:39but traveling in the block universe has a big drawback
27:43you can never alter the past
27:47if this block universe idea is correct
27:52this movie real universe that all of time exists all at once
27:55that solves the grandfather paradox
27:58you can't go back in time to kill your grandfather
28:01because you haven't
28:03you never will
28:05you never will have done it
28:07you can't do it
28:09because it didn't happen
28:11time travelers in a block universe can't change history
28:15so since we know that no one attended Stephen Hawking's party
28:20no one ever will
28:22by investigating time travel
28:27scientists are unraveling mysteries of our universe
28:31but one question remains unanswered
28:34why does time seem to only run in one direction?
28:40how is it then that we remember the past but we don't know the future?
28:45this seemingly obvious question turns out to have its explanation in the origin of our universe
28:50shockingly
28:51the passage of time isn't set in stone
29:01time can be bent
29:03slowed
29:04even frozen
29:19but our experience of time seems fixed
29:23time only flows in one direction
29:26there just is a direction to time in a way there's not a direction to space
29:31there's no difference between up down left right forward backward
29:35but there's still a difference between yesterday and tomorrow
29:38why does time seem to run forwards
29:42and not backwards?
29:44so many things in our everyday life only make sense in one direction of time
29:49you break an egg it doesn't suddenly become an egg again
29:52you scramble an egg it doesn't become whole
29:54you know that there's sort of directions of things
29:57this arrow of time seems to be linked to the chaos and disorder we see in our day-to-day lives
30:06best explained perhaps over a coffee
30:12if I have a mug of coffee
30:14there's only one way for all the little bits and pieces of the mug and the liquid in the coffee
30:18to be in this shape and it's right here in front of me
30:22the mug is in what's called a highly ordered state
30:26but if I shove it off the table
30:29and it smashes into a million pieces
30:33we'll never see all those pieces and the bits of liquid reassemble into the shape of the mug again
30:40we know the shattered mug won't reassemble itself
30:46in scientific terms
30:48the disorder
30:50or entropy of the coffee mug increases
30:53but never decreases
30:55and across the universe
30:58entropy always increases
31:00just like across the universe
31:02time flows from past to future
31:04everything in the universe is gradually becoming more and more disordered
31:12but why?
31:14we never really think about broken eggs reassembling themselves
31:18and that actually may go all the way back to what the conditions of the big bang were like
31:2213.8 billion years ago
31:24space-time rapidly expanded from a tiny point
31:36in the blink of an eye
31:38the universe was born
31:40this marked the first moment of time
31:43our current understanding of the universe is that there was a time zero
31:48there was a moment that the universe came into being
31:52and that is the big bang
31:54the big bang seems to have been an incredibly low entropy state
31:58everything was very ordered
32:00very dense and very hot
32:02so there was really nowhere for entropy to go but to increase from that state
32:07at time zero the universe expanded from a highly ordered dense speck of energy
32:15380,000 years later
32:21the first atoms formed
32:23gradually gas began to clump together
32:26something like 200 million years later
32:29that the first stars formed
32:31and then those formed into galaxies sometime after that
32:34as the universe ages and expands
32:37it becomes more and more disordered
32:41galaxies move further and further apart
32:45in trillions of years
32:47disorder will rule
32:49star building gas will run out
32:52no new stars will form
32:55when the last stars die
32:57the universe will become cold and dark
33:03the accelerated and continual and forever expansion of our universe
33:08might make for a frankly depressing end
33:10there will come one day when the very last star in the universe just fizzles out
33:14and that is it
33:15the big bang may explain why time seems to flow in one direction
33:24from the past through the present and to the future
33:29right down to the last detail
33:33the rise of entropy in the universe explains why you can scramble an egg from a whole egg
33:40but it's a little harder to make a whole egg from a scrambled one
33:46and entropy could be a big problem for wannabe time travelers
33:51the arrow of time means that things get more chaotic over time
33:55so if you were to go back in time it breaks that law of entropy
33:59and is entropy in fact the reason why we cannot travel into the past
34:04that that is getting back to a part of the universe where the energy itself was different
34:09the level of disorder was different
34:10maybe this law of entropy requires us to keep moving into the future
34:15the arrow of time seems to be another nail in the coffin for traveling to the past
34:23but some scientists think there could be a workaround
34:28time travelers might travel to the past in the quantum realm
34:38though in our macroscopic world we don't experience time travel in both directions
34:44it could be that the quantum realm may allow that to be possible
34:47and quantum time travel could change everything we know about reality
34:54we experience the flow of time in one direction
35:12in one direction
35:14forwards
35:16clock hands never reverse
35:19broken eggs stay broken
35:22and people only attend a party if they're invited before it takes place
35:27but there is a place in the universe where this arrow of time
35:34might run both ways
35:35the subatomic realm ruled by quantum mechanics
35:39in quantum mechanics we do know that the sub sub sub sub atomic world is a very strange place
35:50microscopic particles build everything we see in the universe
35:56quarks leptons and bosons tiny building blocks that play by their own rules
36:03the laws of quantum mechanics
36:07in the quantum world subatomic particles can travel through walls
36:13or pop in and out of existence
36:17but the laws of quantum mechanics have an even stranger property
36:22they appear to be reversible
36:25in quantum mechanics there's no difference between moving to the future and moving to the past
36:29as far as we currently know in the laws of physics
36:33in the quantum realm the arrow of time may break down
36:38in March 2019 Russian scientists put this to the test
36:44using a quantum computer
36:47they simulated an electron traveling a fraction of a second backwards in time
36:50the team calculated that this backward motion can spontaneously happen in the real world
37:06though perhaps only once in the thirteen point eight billion year history of the universe
37:12on the microscopic level the laws of physics are time reversal and variant
37:19so this idea of time travel actually appears in the quantum realm
37:24at least in the mathematical calculations
37:26if the quantum realm's arrow of time runs forward and backwards in the real world
37:34quantum particles could offer a new route to Stephen Hawking's party
37:41but it might not be a comfortable ride
37:44if this idea of quantum time travel is true then you could go to Stephen Hawking's time travel party
37:49but you'd have to do it one subatomic particle at a time
37:53there are more particles in the human body than grains of sand on the earth
38:00so safely deconstructing someone into subatomic particles and then rebuilding them
38:08probably isn't going to happen
38:10but could quantum particles pave the way to a different kind of time travel?
38:15we send information using quantum particles every day
38:23electrons carry signals inside your computer
38:27and photons carry cell phone signals into space and back
38:33could we encode information onto a set of particles and send them back in time?
38:40perhaps to our younger selves?
38:44if you can just send information back in time
38:48that could already make you very very rich
38:51just go to next week
38:53send back the stock market prices and let me know
38:56we have some stuff to talk about
38:58sending information to the past
39:02to alter the present is a tantalizing idea
39:05perhaps we could send invites for Professor Hawking's party
39:11to scientists back in 2009
39:14but even if that's possible
39:17we may never know if they even got the message
39:20quantum mechanics throws a monkey wrench into this
39:24and suggests that maybe the past can branch into many different futures
39:28if you have an interaction between two subatomic particles
39:34and there's a probability it'll go one way
39:37and a probability it'll go another way
39:39to us observing it, it only seems to go one way
39:42but there's this interpretation of quantum mechanics that says
39:46they both happen
39:48you've now created two universes
39:50the timeline is split
39:52in the quantum world
39:55sending a particle
39:57invitations to a party
39:59or even a DeLorean back in time
40:01could create
40:03a new timeline
40:05in the new timeline
40:07Hawking's party
40:09might have been packed with party goers
40:11but we aren't part of that timeline
40:14and neither is our Stephen Hawking
40:17you're not time traveling back into your own universe
40:19and changing things
40:21you're traveling to another universe at that point of time
40:24and changing things from there on forward
40:26and it doesn't matter if you change things then
40:28because in that universe
40:30you don't get born later to go back in time to change things
40:32that happened in another universe
40:34I know this stuff is hard to understand
40:36it's hard to explain too
40:38maybe if there are an infinite number of universes
40:40there's an alternate version of me that understands it better
40:43I hope he has more hair
40:44for now time travel is still science fiction
40:52so in my personal view nothing is going to go backward in time
40:58particles information anything like that
41:00sometimes you hear reports of something going backward in time
41:03or being undone or whatever
41:05it's really nothing more than a fancy version of playing a movie backward
41:09but perhaps someday scientists will discover a source of exotic matter
41:14to prop open a wormhole
41:16or find a way to bend space-time back on itself
41:20you know never say never because what we consider science now
41:25would have been considered science fiction or the lunatics of a madman a century ago
41:31but I'm holding out a little bit of hope
41:34because very smart people have tried to prove that it's actually impossible and failed
41:41you should never say never
41:43and along the way maybe we'll learn a bit more about how the universe works
41:49time travel is definitely more science fiction than science fact
41:53but thinking about time travel and trying to understand why it might not be possible
41:58is really interesting and can teach us a lot about the nature of our universe
42:04it's also really fascinating to think about this because it forces us to take on some of the toughest
42:10unanswered questions in all of physics
42:13and will ultimately probably lead to a deeper understanding of the very nature of reality
42:18to take the analogy of Alice in Wonderland
42:21the universe really does keep leading us farther and farther down the rabbit hole
42:24further down the rabbit hole