- 7 months ago
A discussion of whether or not time travel is possible and the mechanisms that may allow it or prevent it.
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00:01During the following program, look for NOVA's webmarkers which lead you to more information at our website.
00:11If we could travel backwards in time, it would be the ultimate thrill ride.
00:22All of history would be a fantastic tourist attraction.
00:30That adventure is commonplace in Hollywood, where time machines are fueled by imagination, rather than science.
00:45This is what makes time travel possible. The flux of capacity.
00:53But will traveling back through time ever be more than just fantasy?
01:01To find out, a scientist named Kip Thorne took physics to the limit.
01:09He was inspired by the science fiction story Contact,
01:13and discovered a way, however unlikely, that time travel might someday be possible.
01:19Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.
01:20Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.
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01:22Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.
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01:25Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.
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01:29Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.
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02:43If it ever becomes possible to travel back in time, it may happen because one man
02:55asked the right question.
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03:11scientific investigation of time travel.
03:14In contact, the heroine Eleanor Haraway was a radio astronomer
03:19engaged in the search for radio signals from extraterrestrial intelligence.
03:26Well, she received a signal.
03:29And the signal, after much decoding, turns out to be a machine.
03:34And the machine is a means of traveling great distances.
03:41Inside the machine, a space traveler would sit in what looked like an ordinary armchair.
03:48In the movie of Contact, the chair went high-tech, but the idea was the same.
03:54A mysterious machine designed by aliens that would transport a human across the universe.
04:03But in writing this story, Sagan ran into trouble.
04:06Our galaxy is so large that it would take his heroine thousands of years to reach her destination.
04:14That was my problem, to get her to a great distance away from the Earth in the Milky Way galaxy
04:21to meet the extraterrestrials and come back and do all that within the lifetime of the people she has left behind.
04:30What Sagan needed was a shortcut through the vast reaches of outer space.
04:37And this search would unravel the secrets of time travel.
04:41In the early 1980s, there was a common misunderstanding that you might be able to trap from one place to the other in the galaxy
04:51without covering the intervening distance by plunging into a black hole.
04:57But there was something about the whole idea that made me nervous.
05:01And it was for that reason that I contacted Kip Thorne.
05:06I was a little upset because he had the heroine in his novel traveling through a black hole.
05:19And I knew that you can't go into a black hole and come out somewhere else.
05:23The fundamental laws of physics forbid it.
05:25A black hole forms when a large star dies and collapses to a small, infinitely dense point with immense gravitational pull.
05:37This warps space so severely that everything nearby is sucked in and destroyed,
05:44making travel through a black hole impossible.
05:46If you go down through the horizon of a black hole,
05:51at the center you don't find a tunnel that leads you to some other place in the universe.
05:55What you find instead is a region where the material of which your body is made,
06:01the atoms, get stretched and squeezed beyond recognition.
06:04And then space and time themselves get stretched and squeezed beyond recognition and destroyed.
06:12But if you've gone inside a black hole trying to travel through it, that doesn't matter.
06:16You're dead.
06:17There was no way you could get through.
06:18I got back a long letter from Kip with about 50 lines of closely reasoned equations,
06:26which was a level of detail in response to my phone call that I had not anticipated.
06:34Rather quickly, I recognized that what he probably should do is replace the black hole
06:39as a means for rapid interstellar travel with a wormhole.
06:43At that time, wormholes were not something that were part of science fiction.
06:47They became part of science fiction as a result of this interaction between Carl and me.
06:53Wormholes are like tunnels in the fabric of space and time predicted by Einstein's equations.
07:00Thorne realized they might hold the answer to Sagan's dilemma.
07:04Our universe, it's three-dimensional, but we can pretend it's two-dimensional,
07:09so it's like this sheet of paper.
07:12And we live in Pasadena over here, and London is over there,
07:17and it's thousands of miles from Pasadena to London.
07:20This universe is curved up so that through hyperspace,
07:26the distance from Pasadena to London is only a few feet.
07:30And there is this pipe, this little wormhole,
07:33that will lead us from Pasadena to London across that very short distance.
07:38And it's like looking through a crystal ball.
07:40You see a distorted picture of what is going on at the other mouth of the wormhole,
07:46which may be in another galaxy, or it may be near the Star of Vega,
07:51or it may be in London.
07:55This shortcut was just what Sagan's heroin needed.
08:06I must have gone through a wormhole.
08:11Contact had started something new.
08:13It introduced the world to science fiction
08:18and also reintroduced to serious scientists
08:23the notion of a wormhole as something that was really worthy of thinking about.
08:30With wormholes on center stage,
08:33Thorne soon found they had other mysterious properties.
08:36They could be used for more than traveling great distances in space.
08:40They might also be used for time travel.
08:47If you have a wormhole,
08:48then you can turn them into time machines for going backward in time.
08:52We thought, how could we have been so stupid?
08:54We should have realized that.
08:55That's obvious.
09:00What became obvious to Thorne
09:02was that time could behave in strange ways inside a wormhole.
09:07Let's suppose that I have a wormhole
09:13with one mouth here
09:14and the other mouth over there.
09:16Now, there are three different possibilities
09:18for how time could be hooked up
09:20through the interior of that wormhole.
09:22The first is that
09:23when I stick my arm into this mouth,
09:26it came out over there simultaneously.
09:28The second possibility is that
09:31when I stick my arm into this mouth,
09:34it comes out over there only after some delay.
09:38The third possibility
09:39is that if I go into this wormhole mouth,
09:42then I come out over there
09:44before I ever even go in.
09:47Let's just see that.
09:48It was quite a surprise
09:54when I realized that
09:55with a single wormhole,
09:57you could have time hook up
09:58toward the future
10:00or toward the past
10:01and that you could actually manipulate the wormhole
10:03and change how time hooked up.
10:04That was a surprise,
10:06but a very satisfying surprise
10:07when I really understood how it worked.
10:13Suddenly,
10:14time travel seemed feasible.
10:15At least in theory.
10:19As a youngster,
10:20who was fascinated by the possibility
10:21of time travel in science fiction,
10:24to be in any way involved
10:26in the possible actualization of time travel
10:32is,
10:34it just brings goosebumps.
10:36A physicist working on the possibility
10:38of travel into the past
10:40has to be careful
10:42not to be labeled a crank
10:44or accused of wasting public money
10:46on science fiction fantasy.
10:51Don't say a word.
10:54Stand back in less than three minutes!
10:56I shall have escaped this age of madness!
11:03Nevertheless,
11:04that is an important question.
11:06Right now we are in one of those
11:10classic, wonderfully evocative moments
11:15in science
11:16when we don't know
11:17when there are those on both sides
11:20of the debate
11:21and when what is at stake
11:24is very mystifying,
11:28very profound.
11:29Time
11:39is something which
11:42at a fundamental level
11:43we don't understand.
11:45Newton, of course,
11:46thought time was like a river
11:47that flows.
11:49Time is
11:50the thing out there
11:52that flows
11:53and I go with the flow.
11:55Time
11:55is nature's way
11:57to keep everything
11:58from happening
11:59all at once.
12:01It is one of those concepts
12:03that is
12:04profoundly resistant
12:07to simple
12:08a definition.
12:15We continue with
12:17the sale
12:18of Albert Einstein's
12:20manuscript
12:20for the theory
12:21of relativity.
12:23Einstein's theories
12:25of relativity
12:26are invaluable
12:27because they lay
12:28the foundations
12:29of modern physics
12:30including the possibility
12:32of time travel.
12:33Number one
12:34of this catalog
12:34and I have a bid
12:37of two million dollars
12:38to start the bidding
12:39on this
12:39at two million dollars now.
12:40In his theory
12:41of special relativity
12:42Einstein declared
12:43that time
12:44can be altered
12:45by speed.
12:46The faster you move
12:47the slower time passes.
12:49Three million dollars
12:51at two million
12:52three hundred thousand dollars.
12:53In 1971
12:54Joe Haefeli
12:56and Richard Keating
12:57put the theory
12:58to the test.
13:00They flew four
13:01atomic clocks
13:01around the world
13:02to compare
13:03the passage
13:04of time
13:04on an airplane
13:05with time
13:06on the ground.
13:09I don't trust
13:10these professors
13:10who get up
13:11and scribble
13:12in front of blackboards
13:13claiming they
13:14understand it all
13:15because I've made
13:16too many measurements
13:17where they don't
13:18come up with
13:18the numbers they say.
13:19Richard had told me
13:21that in all
13:21the previous work
13:22he had done
13:22he had never seen
13:24the effect
13:24of time dilation.
13:25I had never seen it.
13:27I knew what was
13:28being predicted
13:29and that
13:30but it always seemed
13:31to me that
13:32the best proof
13:32is to measure it.
13:34To see how time
13:38slows down
13:39with speed
13:40they had to fly
13:41all the way
13:42around the world
13:42in the original experiment.
13:46Today's atomic clocks
13:47are more accurate
13:48so the warping
13:49of time
13:49should be apparent
13:50even on an ordinary
13:51transatlantic flight.
13:53What's the champagne
14:03for you, sir?
14:04Yes, one for me
14:05and one for my friend.
14:07Certainly.
14:10About five minutes
14:12past four local time
14:13which is five minutes
14:15past nine English time.
14:17Einstein said time
14:18is that which is indicated
14:19by clock.
14:20I think I know
14:22what a clock is
14:23therefore I think
14:24I know what time is.
14:25Seem to be following
14:26an unbelievably constant track.
14:28En route
14:29they collected data
14:30from the pilot
14:30then put it into
14:32Einstein's equations
14:33to predict the time change.
14:35We have accumulated
14:36so far
14:37a change of 34 nanoseconds
14:38and broadly speaking
14:40it seems to confirm
14:41the original prediction.
14:43The clock will have gained
14:44somewhere between
14:4537 and 40 nanoseconds
14:48when we land
14:48it in about half an hour's time.
14:54Suppose you would live
14:55for a hundred years
14:56and you would spend
14:58your entire life
14:59on one of these aircraft
15:00flying around the world.
15:03You could expect
15:04to be younger
15:05than people
15:06who did not do that
15:07by about
15:09one ten thousandth
15:11of a second.
15:12To measure exactly
15:21how much time changed
15:22relative to time on Earth
15:24the clock on the airplane
15:25had been synchronized
15:26with the international standard
15:28at the start of the trip.
15:29Then
15:30it was compared
15:30to the same standard
15:31when it landed
15:32in the United States.
15:33The clock on the plane
15:38disagreed
15:39with the ones
15:40on the ground
15:40by 40 billionths
15:42of a second
15:43just as Einstein predicted.
15:48At $2,700,000 now
15:51at $2,800,000
15:53at $2,900,000
15:55here at $2,900,000
15:56at $3,000,000
15:58$3,100,000
16:00$3,200,000
16:03$3,300,000
16:07$3,300,000
16:07is here.
16:08Don't wave back there.
16:10Fair warning now
16:11at $3,300,000
16:14But taken to an extreme
16:20Einstein's theory
16:22that time slows down
16:23at high speeds
16:24has even stranger consequences.
16:26This is sometimes
16:27described as the
16:28twin paradox
16:30two identical twins
16:31one of whom
16:33goes off on a voyage
16:34close to the speed of light
16:35the other of whom
16:35stays home
16:36when the
16:38space traveling
16:39twin returns home
16:40time has
16:42dilated
16:43for him
16:45or her
16:45that is
16:47he or she
16:48has aged
16:49only a little
16:49while the twin
16:50who has remained at home
16:51has aged
16:52at the regular pace
16:53and here
16:54you have two identical twins
16:55who may be decades
16:56apart in age
16:57Applied to a wormhole
17:00this curious phenomenon
17:01might make time travel possible
17:04There are several different ways
17:08to turn a wormhole
17:08into a time machine
17:09if you are a clever
17:11and infinitely advanced civilization
17:13by an infinitely advanced civilization
17:17I mean somebody
17:18who can do anything
17:20their heart desires
17:21except they can't violate
17:23the fundamental laws
17:24What they could do
17:28is take advantage
17:29of the twins paradox
17:30and send one mouth
17:32of a wormhole
17:32on a voyage
17:33into outer space
17:34As the wormhole mouth
17:42approaches the speed of light
17:43time slows down
17:45relative to the wormhole mouth
17:47that remains on earth
17:48At the end of its high speed voyage
17:55the traveling wormhole mouth
17:57returns to earth
17:58where it can be picked up
18:01by its owner
18:02Just like the twins paradox
18:10less time has passed
18:12for this mouth
18:13of the wormhole
18:13than for the other end
18:15that stayed behind
18:16on earth
18:16The wormhole
18:18is now a tunnel
18:20with each mouth
18:21located in a different time
18:23If I now go into
18:26this wormhole mouth
18:28today
18:29I will come out
18:30of that mouth
18:31yesterday
18:32So in theory
18:34an advanced civilization
18:35might turn a wormhole
18:37into a time machine
18:38But the practical details
18:40are something else again
18:41Where would they find
18:45a wormhole
18:46in the first place?
18:48The answer
18:49grew out of the work
18:51of Kip Thorne's
18:51former teacher
18:52I like to think
18:54of space and time
18:56as analogous
18:58to the ocean
18:59and changes in it
19:02is analogous
19:03to waves
19:04on the surface
19:05of the ocean
19:06But those waves
19:08of course
19:09don't show up
19:11when one's miles
19:12above the ocean
19:13it looks flat
19:16Then as one gets
19:17down closer
19:19to the surface
19:20one sees
19:21the waves
19:23breaking
19:23and
19:24the foam
19:26I see no way
19:33to escape
19:34the conclusion
19:35that similar
19:37foam-like structure
19:39is developing
19:41in space
19:43and time
19:45Wheeler thinks
19:51the space
19:51between atoms
19:52is filled with bubbles
19:53and this quantum foam
19:59as he calls it
20:00gives rise
20:01to incredibly
20:02tiny wormholes
20:03Let's suppose
20:06we begin
20:06at the size
20:07of Britain
20:07and we squeeze
20:09ourselves on down
20:10smaller and smaller
20:11past the size
20:12of a human being
20:13down to the size
20:14of an atom
20:14and on down
20:15to the size
20:15of an atomic nucleus
20:17we're then
20:18half way
20:18to the scale
20:19of quantum foam
20:20we have to then
20:21begin at that scale
20:22and go on down
20:23as much as we went
20:24from Britain
20:25to an atomic nucleus
20:25we have to keep
20:26on going down
20:27it's another
20:2720 powers of 10
20:29on down
20:30we get to
20:31a minuscule scale
20:32far smaller
20:32than anybody
20:33has ever been able
20:34to reach
20:35with experimental apparatus
20:36and there
20:37the fundamental laws
20:38seem to tell us
20:39there must be
20:40quantum foam
20:40space
20:42as we know it
20:43must cease
20:44to exist
20:45in this smooth
20:46sort of a way
20:46and must become
20:48something more like
20:48a froth of soap suds
20:50this quantum foam
20:55we can think of
20:56as having bubbles
20:57but we can also
20:57think of it
20:58as having little handles
20:59like the handle
21:00on a coffee cup
21:01that is a wormhole
21:03it's something
21:04that reaches
21:04from over here
21:06in the foam
21:06to over there
21:07Thorne speculates
21:09that if quantum foam
21:10exists
21:11an advanced civilization
21:13might be able
21:14to grab one
21:15of these tiny wormholes
21:16and enlarge it
21:17but there's another problem
21:19we then had to face
21:21the fact
21:22that the one kind
21:23of wormhole
21:23that we knew
21:24as a solution
21:25of Einstein's equations
21:26was a wormhole
21:27that lives
21:28for only a very short time
21:30it has a throat
21:31that expands
21:32opens
21:33and shuts
21:33in a flash
21:34so fast
21:36that anybody
21:36who tries to travel
21:37through it
21:38in that flash
21:39while it's open
21:40can't manage
21:41to get through
21:42they get crushed
21:43in the pinch off
21:43to travel
21:45through a wormhole
21:46a human being
21:48would have to find
21:48a way
21:49to hold it open
21:50so we then
22:01ask ourselves
22:01what is it
22:02that you need
22:03to thread through
22:04the wormhole
22:04to hold it open
22:05long enough
22:06for somebody
22:06to travel through it
22:07and Einstein's equations
22:09told us the answer
22:10we just had to do
22:11a page or two
22:11of calculation
22:12what you needed
22:13was something
22:14very exotic
22:15some material
22:16that has negative energy
22:17the bad news
22:21is that if you want
22:22a wormhole
22:23about one meter across
22:24which is a
22:25really minimal requirement
22:27for something
22:27to put a human through
22:28you need about
22:29minus one Jupiter's worth
22:31of this exotic matter
22:33so the repellent force
22:36of a planet-sized
22:37amount of exotic matter
22:39would be required
22:40to hold a wormhole open
22:41but it's not as far-fetched
22:45as it might sound
22:46scientists have found
22:48that tiny amounts
22:49of such negative energy
22:50can be made
22:51in the laboratory
22:52comes down here
22:54and supports this triangle
22:56all ordinary matter
22:57has positive energy
22:58but exotic matter
23:00can be produced
23:00by squeezing energy
23:02out of a vacuum
23:03created in a tiny gap
23:05between two metal plates
23:06now when you bring
23:07two plates close together
23:09photons of long wavelengths
23:12can't exist
23:13between the plates
23:14so they exclude
23:15some of this energy
23:16from the system
23:17when you do so
23:18the energy
23:19between the plates
23:20is lower than
23:21the energy outside
23:22and so there's a force
23:24between these two plates
23:25the force actually
23:28pulls the plates together
23:29and that's critical
23:31because
23:32that tells you
23:33that the energy density
23:34between the plates
23:35has to be negative
23:36and that is the key
23:38to it actually being
23:40exotic matter
23:41we just did it for fun
23:45and it's built with
23:47this junk we found
23:48around the lab here
23:50and in fact we can measure
23:51an extremely tiny force
23:53that experiment
24:02from our point of view
24:04is a proof
24:05in principle
24:06that at least
24:07small amounts
24:08of exotic matter
24:10effectively negative energy
24:12do exist
24:12in the real world
24:13if enough negative energy
24:16could be collected
24:17time tourists
24:18of the future
24:19might be able
24:20to hold a wormhole
24:21open long enough
24:22to make a safe journey
24:23into the past
24:24hoping that large amounts
24:30of negative energy
24:31might be possible
24:32and speculating
24:34that wormholes
24:35could be extracted
24:36and enlarged
24:37from the quantum foam
24:38Thorne took the plunge
24:40he published his ideas
24:46about time travel
24:47in one of the most
24:48prestigious physics journals
24:50my concern was
24:52the word time machine
24:53in the title
24:54and my worry was
24:56that the popular press
24:57would see this paper
24:58and would start
24:59to ballyhoo it
25:00in a manner
25:01that caused
25:02our serious
25:03scientific colleagues
25:04to pay no attention
25:06to it as being
25:06crackpot stuff
25:07very quickly
25:09the rest of the press
25:10grabbed hold of it
25:11here we are
25:12we've invented time travel
25:13there are other stories
25:14that basically had us
25:15building time machines
25:16in our own basements
25:17and I rather quickly
25:21pulled back
25:22and told the Caltech
25:23public relations office
25:24I do not give interviews
25:26on this subject
25:26I told my research group's
25:30administrative assistant
25:31I do not return
25:32telephone calls
25:33from the press
25:34on this subject
25:34I will talk about
25:35anything else
25:36but not time travel
25:37but Thorne's work
25:43brought other scientists
25:44out into the open
25:45when I realized
25:47that my friend
25:49and extremely
25:50great scientist
25:52Kip Thorne
25:52published it
25:53I immediately
25:53called him
25:54and told
25:55thank you so much
25:56for that
25:57now
25:57I will publish
25:59I will work
25:59on this
26:01subject also
26:03he was overjoyed
26:04on the other end
26:05of the phone
26:05he said
26:06oh Kip
26:06it's absolutely wonderful
26:08your paper's wonderful
26:09you've broken the barrier
26:11if you can do research
26:12about time travel
26:13then so can I
26:14Igor Novikov
26:16had been working
26:16on time travel
26:17in secret
26:18for years
26:19he was intrigued
26:20by the trouble
26:20that time travelers
26:22might cause
26:22if they went back
26:23and tried to change history
26:25once it appeared
26:27that time machines
26:28were a real possibility
26:29we then had to face
26:31the question of paradoxes
26:33of going back in time
26:34and changing history
26:35and thereby
26:36causing the foundations
26:38of physics
26:39to crumble beneath us
26:40you're in focus now Paul
26:44press the button Harvey
26:45press the button my friend
26:47send me back into time
26:49time travel would seem
26:52to lead to contradictions
26:53if one was able
26:55to go back
26:56and change the past
26:57the grandfather paradox
26:59is a very simple
27:01science fiction based
27:03apparent inconsistency
27:05at the very heart
27:08of the idea
27:09of backwards time travel
27:10if I have a time machine
27:12then there is the danger
27:13that I may go
27:14into the time machine
27:15go back in time
27:16kill my grandfather
27:18before my mother
27:19is conceived
27:20so that she can
27:21conceive me
27:22where does that
27:22then leave you
27:24how do I get here
27:26do you instantly
27:27pop out of existence
27:29because you were never made
27:31but then you couldn't
27:32have gone back
27:33and so on
27:34hurtling back
27:36through time
27:37creates paradoxes
27:38that have fueled
27:39dozens of Hollywood
27:40productions
27:40dudes
27:41you guys are gonna
27:42go back in time
27:43anything you do
27:44could have serious
27:44repercussions
27:45on future events
27:46do you understand
27:47this is gonna
27:59cause me
28:00a serious consideration
28:02of backwards time travel
28:04must confront
28:04these paradoxes
28:06billiard balls
28:07provided us a way
28:08to study paradoxes
28:10with time travel
28:11without getting into
28:13the nasty business
28:14of free will
28:14of human beings
28:15billiard balls
28:17behave in predictable
28:18ways according
28:19to the laws
28:19of physics
28:20cause always
28:21precedes effect
28:22but what happens
28:24if time travel
28:25is possible
28:26if I have a time
28:27machine
28:28the story is
28:29quite different
28:29imagine the pockets
28:31of this billiard table
28:32are the mouths
28:33of a wormhole
28:34time machine
28:35in this case
28:36I have only one
28:37billiard ball
28:38and I send that
28:39billiard ball
28:39into this mouth
28:40of the wormhole
28:42and it will then
28:43come out of that mouth
28:44before it entered
28:45this mouth
28:46hit itself
28:47and prevent itself
28:47from going into
28:48the first mouth
28:49voila
28:50a paradox
28:51it's the
28:52billiard ball version
28:53of going back
28:54in time
28:54and changing history
28:56of course
29:00this problem
29:01was discussed
29:02a lot
29:03in literature
29:06in movies
29:07in science fiction
29:09but I'm talking
29:10not about fantasies
29:12but real
29:13science
29:14having developed
29:17simple versions
29:18of the paradox
29:20we then set about
29:22trying to figure out
29:23how to solve these
29:24how will the laws
29:26of physics behave
29:27if time machines
29:27are permitted
29:28in searching
29:29for a resolution
29:30of the paradox
29:31we were led
29:32by a principle
29:33introduced by
29:34Igor Novikov
29:35which said that
29:36nature will only
29:38allow those behaviors
29:40that are absolutely
29:41self-consistent
29:42in order to be
29:45self-consistent
29:46a ball emerging
29:49in the past
29:50must knock itself
29:51into the time machine
29:52so it can come out again
29:55along a trajectory
29:56that will knock itself
29:58back into the pocket
29:59Novikov wrote
30:04mathematical equations
30:05for the possible outcomes
30:06of a ball hitting itself
30:08and found that
30:09a self-consistent solution
30:10always exists
30:12this implied
30:15that nature
30:16would not allow
30:17a paradox to arise
30:18this is the main principle
30:20all these events
30:22must be in self-consistency
30:24with each other
30:25it's so simple
30:27so obvious
30:28but more of that
30:30we gave the strict
30:32mathematical proof
30:33that this principle
30:35is the consequence
30:37of the basic ideas
30:40of the physics
30:41by the same token
30:44Novikov's findings
30:46implied
30:46that the laws
30:47of physics
30:48would prevent
30:49a time traveler
30:50from rewriting history
30:51what has happened
31:03has happened
31:04it cannot be changed
31:05it cannot be repeated twice
31:08in different ways
31:09human beings
31:11human beings
31:11aren't billiard balls
31:12and we might like to believe
31:14in free will
31:15so a sufficiently stubborn human
31:18would seem to be able
31:20to get around
31:20any sort of consistency condition
31:22by just demanding
31:24that he does something different
31:26I can have
31:32free will
31:33to walk along
31:35this wall
31:36without
31:38special equipment
31:39it's my free will
31:41can I do that?
31:42no
31:43I can't
31:44why?
31:45because of
31:46a law of physics
31:47because of
31:48the law of gravity
31:51it's forbidden
31:53laws of physics
31:56like gravity
31:57already restrain
31:58our free will
31:59Novikov thinks
32:06these same laws
32:07would restrain
32:07the free will
32:08of a time traveler
32:09bent on changing
32:11the past
32:11the dress code
32:18open up please
32:20immediately
32:21if time travelers
32:26can't change
32:26the past themselves
32:28could they alter history
32:29by sending a message
32:30into the past instead?
32:34Einstein's theories
32:34of relativity
32:35show that if something
32:36could travel faster
32:38than the speed of light
32:39it could be viewed
32:40as going backwards
32:41in time
32:42but relativity
32:43also says
32:44that's impossible
32:45yet this man
32:53may have taken
32:53a step in that direction
32:55because he claims
32:56to have sent
32:57information
32:57faster than light
32:59this signal
33:03is splitted
33:03into
33:04by an electronic
33:06mirror here
33:07into two parts
33:09so we can
33:10compare
33:10the signal
33:11one is moving
33:13through the air
33:14and the other one
33:14is moving
33:15through the barrier
33:16in this experiment
33:17Gunther Nims
33:18splits a microwave
33:19signal in two
33:20half goes through
33:22the air
33:23traveling at the speed
33:24of light
33:24and half is fired
33:25into a barrier
33:26to block the signal
33:28but that's not
33:29what happens
33:30this is an oscilloscope
33:32where you see
33:33the signal
33:34and then we can see
33:36which one
33:36is faster
33:38the two humps
33:39on the screen
33:40are not in the same place
33:41because the microwaves
33:42that went through
33:43the barrier
33:44got to the detector first
33:46apparently exceeding
33:48the speed of light
33:48only a very small part
33:52comes to the other side
33:53but it comes
33:55and this part
33:56comes at a velocity
33:58which is much faster
33:59than the velocity
34:00of light
34:01so how could the microwaves
34:03go faster than light
34:04and what was the role
34:06of the barrier
34:06Nims chalks it up
34:08to a strange phenomenon
34:09called quantum tunneling
34:11at the subatomic
34:13or quantum level
34:14the world is ruled
34:16by probability
34:16and chance
34:17and the seemingly
34:18impossible occurs
34:19all the time
34:20for example
34:22when a stream
34:23of particles
34:24like photons
34:25meets a barrier
34:25most bounce off
34:29but a few of them
34:31materialize
34:32on the far side
34:33of the barrier
34:33and continue
34:36on their way
34:37Nims detected
34:41the particles
34:42that appeared
34:42and measured
34:44how fast
34:45they got there
34:45I'm amused about this
34:51we did this
34:51for fun
34:52and when we figured out
34:54that it's faster
34:55than the velocity
34:55of light
34:56we did not think
34:58about
34:58its importance
35:00another expert
35:05in quantum tunneling
35:06is Raymond Chow
35:07he agrees
35:13with at least
35:13part of what
35:14Nims has found
35:14in our experiments
35:18we have measured
35:19that a single photon
35:21can tunnel
35:22across a tunnel barrier
35:23at 1.7 times
35:25the speed of light
35:26what bothers Chow
35:30is not that
35:30random photons
35:31seem to go
35:32beyond the speed
35:33of light
35:33but that Nims
35:36claims he can
35:37use tunneling
35:38to send information
35:39faster than light
35:40to have a genuine
35:53signal
35:53you really have
35:54to control
35:55the signal
35:55but in quantum
35:56mechanical tunneling
35:57it's a completely
35:59random process
36:00fundamentally
36:01we cannot
36:02we cannot send
36:04information
36:04with this
36:05tunneling particle
36:06yeah
36:09some colleagues
36:10were claiming
36:12that you cannot
36:12send information
36:13and then we started
36:14to transmit
36:16Mozart 40
36:17and this is
36:19for instance
36:19the original
36:20tape
36:21that's what
36:25we sent
36:25at a speed
36:26of 4.7
36:27times the velocity
36:29of light
36:30and the distance
36:32of about
36:3214 centimeter
36:33whether you can
36:35recognize
36:36Mozart 40
36:37or not
36:37despite the
36:47randomness
36:48and uncertainty
36:49of the tunneling
36:49process
36:50Mozart seems to
36:51have gone through
36:52the barrier
36:52the essential
36:55question is
36:56what is a signal
36:57or what constitutes
36:58information
36:59has he really
37:01sent a signal
37:03in the sense
37:04of information
37:05faster than the speed
37:06of light
37:06this is where
37:07professor Nims
37:08and I
37:09are company
37:09because we don't
37:10really have a
37:11rigorous definition
37:12of what is
37:14information
37:14at the quantum
37:15level
37:15maybe that
37:26this is
37:27not information
37:28for American
37:28colleague
37:30but
37:30for a German
37:31or a British
37:32colleague
37:32I think
37:33Mozart 40
37:34has some
37:35information
37:35in it
37:36transmitting
37:39Mozart
37:39is one
37:40thing
37:40convincing
37:45others
37:46that you've
37:46sent it
37:46faster than
37:47light
37:47is another
37:48and so
37:52the debate
37:53continues
37:53with neither
37:54side budging
37:55I
37:57consist
37:58no
37:58no
37:59I
37:59not consist
37:59I
38:00insist
38:01on it
38:01that we
38:02have
38:02and we
38:03can
38:03transmit
38:04signals
38:05faster than
38:06the velocity
38:06of light
38:07Nims
38:09has found
38:10little support
38:10for this
38:11claim
38:11and few
38:14think
38:14this will
38:14ever allow
38:15information
38:16to go
38:16backwards
38:17in time
38:17still
38:20the idea
38:21that time
38:22travel
38:22for people
38:23might become
38:24a reality
38:24raises a
38:26provocative
38:26question
38:27time travel
38:29might be
38:30possible
38:30but if
38:31that is
38:31the case
38:32why haven't
38:33we been
38:33overrun
38:34by tourists
38:35from the
38:35future
38:36this argument
38:38I find
38:39very dubious
38:40it might
38:41be that
38:42time
38:42travel
38:42into the
38:43past
38:43is
38:44possible
38:45but they
38:46haven't
38:46gotten
38:47to our
38:47time
38:48yet
38:48they're
38:48very far
38:49in the
38:49future
38:49and it's
38:50the further
38:50back
38:51in time
38:51you go
38:52the more
38:52expensive
38:52it is
38:53then there's
38:54the possibility
38:55that they're
38:56here all
38:56right
38:56but we
38:57don't see
38:58them
38:58they have
38:59perfect
38:59invisibility
39:00cloaks
39:01or something
39:01if they're
39:02so smart
39:03if they have
39:03such
39:03highly developed
39:04technology
39:05then why
39:05not
39:06then there's
39:07the possibility
39:07that they're
39:08here
39:08we do
39:09see them
39:09but we
39:10call them
39:11something else
39:11UFOs
39:12or ghosts
39:14or
39:14I think
39:16that if
39:16people from
39:17the future
39:18were going
39:19to show
39:19themselves
39:20they would
39:21do so
39:21in a more
39:22obvious way
39:23what would
39:24be the point
39:25of revealing
39:25themselves
39:26only to
39:27cranks
39:27and weirdos
39:28who wouldn't
39:29be believed
39:30but
39:34physics does
39:35put a
39:35limit
39:36on how
39:36far
39:37into
39:37the
39:37past
39:37a
39:38time
39:38traveler
39:38could
39:39go
39:39relativity
39:40theory
39:41says
39:41in
39:41general
39:42that
39:42once
39:43you've
39:43made
39:43a
39:44time
39:44machine
39:44you
39:44can
39:45never
39:45use
39:45it
39:45to
39:45go
39:45backward
39:46in
39:46time
39:46before
39:46the
39:47period
39:47when
39:47it
39:48was
39:48made
39:48there's
39:51no way
39:51I
39:51can
39:51use
39:52it
39:52to
39:52travel
39:52back
39:52to
39:52the
39:53age
39:53of
39:53the
39:53dinosaurs
39:54or
39:54even
39:54back
39:55to
39:55the
39:56time
39:57of
39:57my
39:57own
39:57birth
39:58because
39:59I
39:59didn't
39:59make
40:00the
40:00time
40:00machine
40:00until
40:01recently
40:01so
40:03relativity
40:03forbids
40:04one of
40:04the
40:04most
40:04beloved
40:05scenarios
40:05of
40:05science
40:06fiction
40:06from
40:06ever
40:07happening
40:07because
40:08you
40:08cannot
40:08go
40:09back
40:09before
40:09the
40:10point
40:10at
40:10which
40:10the
40:11time
40:11machine
40:11was
40:12made
40:12I
40:14don't
40:14have
40:15to
40:15worry
40:15about
40:15the
40:15possibility
40:16of
40:16my
40:16going
40:16back
40:17and
40:17killing
40:17my
40:17father
40:18before
40:18I
40:18was
40:18conceived
40:19what
40:20I
40:20have
40:20to
40:20worry
40:21about
40:21is
40:21my
40:21grandson
40:22going
40:23back
40:24in
40:24time
40:24and
40:24killing
40:24me
40:25before
40:26he
40:29there's
40:29no
40:30possibility
40:30of
40:30changing
40:31our
40:31past
40:32but
40:32in
40:32the
40:32future
40:33one
40:33can
40:34change
40:34the
40:35future's
40:35past
40:36but
40:38there
40:38is
40:38a
40:38loophole
40:39that
40:39might
40:39still
40:40make
40:40even
40:40the
40:41most
40:41far
40:41fetched
40:42scenarios
40:42possible
40:43according
40:46to
40:46quantum
40:46theory
40:47uncertainty
40:48and
40:48chance
40:48rule
40:49the
40:49subatomic
40:50world
40:50you
40:52can
40:52never
40:53know
40:53the
40:53exact
40:54position
40:54and
40:55speed
40:56of
40:56particles
40:56at
40:57the
40:57same
40:57time
40:58instead
41:00you
41:00can
41:00only
41:01give
41:01probabilities
41:02for
41:02their
41:02behavior
41:03in
41:05a
41:05huge
41:05extrapolation
41:06some
41:07people
41:07think
41:07that
41:08all
41:08these
41:08probabilities
41:09are
41:09occurring
41:09simultaneously
41:10anything
41:12that
41:12might
41:13happen
41:13actually
41:14is
41:14happening
41:15quantum
41:16mechanics
41:16is a
41:17theory
41:17of
41:17many
41:18parallel
41:19universes
41:19some
41:21of
41:21them
41:21are
41:21alike
41:21and
41:22some
41:22of
41:22them
41:22are
41:22very
41:23unlike
41:23there
41:24are
41:24nearby
41:25universes
41:25that
41:26differ
41:26from
41:26this
41:27one
41:27only
41:28in
41:28the
41:28position
41:28of
41:28one
41:29photon
41:29or
41:29one
41:29electron
41:30there
41:31are
41:31other
41:31more
41:31distant
41:31universes
41:32where
41:32we're
41:32not
41:33filming
41:33here
41:33at
41:33all
41:34and
41:34there
41:34are
41:34others
41:35where
41:35I
41:35was
41:35never
41:35even
41:35born
41:36this
41:43is
41:43a
41:43radical
41:43interpretation
41:44of
41:44quantum
41:45mechanics
41:45but
41:49David
41:49Deutsch
41:50finds
41:50the
41:50evidence
41:50he
41:51needs
41:51for
41:51parallel
41:52universes
41:52in
41:53a
41:53well
41:53known
41:53physics
41:54experiment
41:54I
41:59first
42:00saw
42:00this
42:00experiment
42:00demonstrated
42:02when I
42:02was an
42:02undergraduate
42:03in fact
42:04it's a
42:04very old
42:05experiment
42:05it was
42:05first done
42:06in
42:061909
42:07this
42:08is
42:09our
42:09light
42:09source
42:09for
42:10the
42:10experiment
42:10the
42:11light
42:11is
42:11being
42:12steered
42:12by
42:12these
42:12mirrors
42:12onto
42:13the
42:13slits
42:13here
42:14and
42:14there's
42:15two
42:15slits
42:15in
42:15this
42:16slide
42:16and
42:16that
42:16produces
42:17the
42:17young
42:17slits
42:18interference
42:18pattern
42:18which we
42:19see
42:19on
42:19the
42:19camera
42:19the
42:21interference
42:22pattern
42:22is the
42:23set
42:23of
42:23faint
42:23vertical
42:24stripes
42:24at
42:25the
42:25center
42:25of
42:26the
42:26red
42:26spot
42:26these
42:27only
42:27appear
42:28when
42:28two
42:29slits
42:29are
42:29open
42:29one
42:30slit
42:30two
42:31slits
42:32one
42:34slit
42:34two
42:37with
42:38two
42:38slits
42:39open
42:39the
42:39single
42:40beam
42:40of
42:40light
42:40is
42:41split
42:41into
42:41two
42:41beams
42:42which
42:42overlap
42:43like
42:43ripples
42:43on
42:44a
42:44pond
42:44in
42:46some
42:46places
42:46the
42:46ripples
42:47reinforce
42:47each
42:48other
42:48while
42:48in
42:49others
42:49they
42:49cancel
42:49each
42:50other
42:50out
42:50creating
42:51the
42:51pattern
42:51of
42:52stripes
42:52on
42:52the
42:52screen
42:53but
42:53what
42:54happens
42:54if
42:54the
42:55intensity
42:55of
42:55the
42:55beam
42:56is
42:56reduced
42:56by
42:56a
42:57filter
42:57so
42:58that
42:58only
42:58one
42:59photon
42:59at
43:00a
43:00time
43:00can
43:01reach
43:01the
43:01slits
43:02now
43:02you
43:03can't
43:03see
43:03the
43:03beam
43:03at
43:03the
43:03moment
43:04but
43:04if
43:04I
43:04introduce
43:05some
43:05liquid
43:05nitrogen
43:05we
43:06should
43:06be
43:07able
43:07to
43:07pick
43:07that
43:07up
43:07so
43:10now
43:10you
43:10can
43:10see
43:10the
43:10beam
43:11scattered
43:11by
43:12the
43:12nitrogen
43:12but
43:13you
43:13see
43:13nothing
43:13after
43:13the
43:14filter
43:14the
43:14filter
43:15essentially
43:15stops
43:15everything
43:16we
43:16can
43:16see
43:17but
43:17the
43:18camera
43:18can
43:18pick
43:18up
43:18the
43:19few
43:19photons
43:19that
43:19are
43:19arriving
43:20the
43:23photons
43:23arrive
43:24at
43:24the
43:24slits
43:25one
43:25at
43:25a
43:25time
43:26so
43:26those
43:27that
43:27get
43:27through
43:27and
43:28reach
43:28the
43:28screen
43:28on
43:28the
43:29other
43:29side
43:29should
43:30form
43:30just
43:30two
43:30bright
43:31lines
43:31they
43:32shouldn't
43:33interfere
43:33and
43:33produce
43:34the
43:34full
43:34pattern
43:34of
43:35stripes
43:35but
43:36they
43:36do
43:36and
43:39this
43:39is
43:39how
43:40the
43:40stripes
43:40appear
43:40on
43:40a
43:40computer
43:41screen
43:41when
43:43one
43:43does
43:43the
43:43experiment
43:44with
43:44individual
43:45photons
43:45the
43:47pattern
43:47that
43:47builds
43:48up
43:49after
43:49one
43:50has
43:50passed
43:50many
43:50photons
43:51through
43:51the
43:51apparatus
43:51is
43:52exactly
43:52the
43:52same
43:53as
43:53it
43:53was
43:53in
43:53strong
43:54light
43:54and
43:55that's
43:55something
43:55we
43:55just
43:56don't
43:56understand
43:56as
43:58an
43:58undergraduate
43:58we
43:59were
43:59told
43:59oh
44:00this
44:00is
44:01because
44:01the
44:01photon
44:02behaves
44:03partly
44:03as
44:03a
44:04particle
44:04and
44:04partly
44:04as
44:05a
44:05wave
44:05now
44:06that
44:07just
44:07doesn't
44:07make
44:07sense
44:08it's
44:08gibberish
44:08it's
44:09saying
44:10that
44:10the
44:10photon
44:10is
44:10both
44:11in
44:11one
44:11place
44:12and
44:12spread
44:12out
44:12at
44:13the
44:13same
44:13time
44:13Deutsch
44:20with
44:20other
44:20photons
44:21that
44:21we
44:21can't
44:22see
44:22the
44:23photon
44:23that
44:24we
44:24can't
44:24see
44:24is
44:25a
44:25photon
44:25in
44:26a
44:26parallel
44:26universe
44:27which
44:28a
44:28nearby
44:28parallel
44:29universe
44:30which
44:30is
44:30interacting
44:31with
44:31the
44:31photon
44:31in
44:31our
44:32universe
44:32and
44:32causing
44:33it
44:33to
44:33change
44:34its
44:34direction
44:34the
44:38result
44:38of the
44:39single
44:39photon
44:40interference
44:40experiment
44:41is
44:42the
44:42strangest
44:43thing
44:43I
44:43know
44:43it
44:45is
44:45conclusive
44:45evidence
44:46that
44:47reality
44:47does
44:48not
44:48consist
44:48of
44:49just
44:49a
44:49single
44:49universe
44:50because
44:51that
44:51result
44:52could
44:52not
44:53come
44:53about
44:53unless
44:54there
44:54were
44:54another
44:55nearby
44:55universe
44:56interacting
44:57with
44:57ours
44:57if
44:59true
45:00this
45:00idea
45:00has
45:01profound
45:01implications
45:02for
45:02time
45:02travel
45:03when
45:03one
45:04travels
45:04back
45:04in
45:04time
45:05one
45:05does
45:05not
45:06in
45:06general
45:06reach
45:07the
45:08same
45:08universe
45:08that
45:09one
45:09starts
45:09from
45:10one
45:10reaches
45:11the
45:11past
45:11of
45:11a
45:12different
45:12universe
45:13could
45:16this
45:16explain
45:16why
45:16we
45:17haven't
45:17met
45:17any
45:17time
45:18travelers
45:18I
45:20don't
45:20believe
45:21it
45:21personally
45:21mainly
45:22because
45:23this
45:23idea
45:23is not
45:24very
45:24constructive
45:25there
45:25are not
45:26many
45:26consequences
45:27of
45:28this
45:29idea
45:30which
45:30we
45:30can
45:31test
45:31in
45:32real
45:33experiment
45:33I
45:34think
45:34we
45:34can't
45:34say
45:35for
45:35certain
45:35but
45:36if
45:36I
45:36had
45:37to
45:37lay
45:37my
45:37bets
45:37I
45:38like
45:38to
45:38lay
45:38bets
45:39I
45:39would
45:39lay
45:39my
45:40bets
45:40on
45:41the
45:41simpler
45:42non-parallel
45:43universe
45:43resolution
45:44another
45:48person
45:48who
45:48likes
45:49to
45:49lay
45:49bets
45:49is
45:49Stephen
45:50Hawking
45:50he
45:54and
45:54Kip
45:55Thorn
45:55have
45:55been
45:56betting
45:56on
45:56the
45:56nature
45:56of
45:57the
45:57universe
45:57for
45:58years
45:58this
46:00bet
46:00here
46:00is
46:01probably
46:01the
46:01most
46:01famous
46:01one
46:02it
46:02was
46:02a
46:03bet
46:03I
46:03made
46:03with
46:03Stephen
46:04Hawking
46:04in
46:041974
46:05over
46:06whether
46:06or not
46:07Cygnus
46:07X1
46:08is
46:08actually
46:09a
46:09black
46:09hole
46:09and
46:10Hawking
46:11after
46:1116
46:12years
46:12of
46:13this
46:13bet
46:13sitting
46:13here
46:14finally
46:14conceded
46:15Stephen
46:18Hawking
46:18and I
46:19have
46:19occasionally
46:19discussed
46:20making
46:20a bet
46:21over
46:21whether
46:21or not
46:22the
46:22laws
46:23of
46:23physics
46:23permit
46:24you
46:24to
46:24go
46:24backward
46:24in
46:24time
46:25we
46:27haven't
46:27been
46:27able
46:27to
46:27conclude
46:28a
46:28bet
46:28because
46:29almost
46:29all
46:30the
46:30time
46:30we're
46:31on
46:31the
46:31same
46:31side
46:31of
46:31the
46:32issue
46:32we
46:32both
46:33think
46:33it
46:33is
46:33probably
46:34impossible
46:35Stephen
46:36Hawking
46:36has
46:37been
46:37skeptical
46:37of
46:38time
46:38travel
46:38for
46:38years
46:39but
46:39even
46:40he
46:40cannot
46:40rule
46:41it
46:41out
46:41for
46:41sure
46:42to
46:43make
46:43time
46:43travel
46:44possible
46:45one
46:45seems
46:46to
46:46need
46:46both
46:46general
46:47relativity
46:48which
46:48describes
46:49the
46:49large
46:50scale
46:50structure
46:51of
46:51the
46:51universe
46:52and
46:52quantum
46:53mechanics
46:53which
46:54governs
46:55very
46:55small
46:56scales
46:56these
46:57two
46:58theories
46:58are
46:58inconsistent
46:59with
46:59each
47:00other
47:00as
47:00they
47:00stand
47:01so
47:01we
47:02have
47:02to
47:02find
47:02a
47:03new
47:03theory
47:03that
47:03combines
47:04them
47:05cosmic
47:08relationships
47:09like
47:09the
47:09motions
47:10of
47:10planets
47:10are
47:11governed
47:11by
47:11Einstein's
47:12theory
47:12of
47:13gravity
47:13general
47:14relativity
47:14but
47:16these
47:17laws
47:17do
47:17not
47:17work
47:18on
47:18the
47:18sub
47:18atomic
47:18level
47:19just
47:19as
47:20the
47:20laws
47:20of
47:20quantum
47:21mechanics
47:21do
47:22not
47:22work
47:22on
47:22the
47:22larger
47:23scale
47:23so
47:26physicists
47:27are
47:27searching
47:27for a
47:28way
47:28to
47:28unite
47:29the
47:29two
47:29in
47:29a
47:29single
47:30theory
47:30called
47:30quantum
47:31gravity
47:31this
47:32is
47:33the
47:33holy
47:33grail
47:34of
47:34modern
47:34physics
47:34a
47:35unified
47:36description
47:36of the
47:37nature
47:37of
47:37space
47:38and
47:38time
47:38on
47:39all
47:39scales
47:40so
47:40I
47:40would
47:40like
47:40to
47:40discuss
47:41the
47:42scientists
47:44are still
47:45in the
47:45process
47:46of
47:46crafting
47:46this
47:47theory
47:47and
47:48whether
47:48quantum
47:48gravity
47:49will
47:49permit
47:49time
47:50travel
47:50through
47:50a
47:51wormhole
47:51is
47:52unclear
47:52yet
47:54they
47:54can't
47:54resist
47:54speculating
47:55having
48:00discovered
48:00that
48:01it is
48:01possible
48:02according
48:02to
48:02general
48:03relativity
48:03to
48:03take
48:03a
48:04wormhole
48:05with
48:05two
48:05mouths
48:06and
48:06turn
48:06it
48:07into
48:07a
48:07time
48:07machine
48:08my
48:08students
48:08and
48:09I
48:09then
48:09searched
48:10for
48:10some
48:10way
48:11some
48:12mechanism
48:12to
48:12prevent
48:13this
48:13from
48:13really
48:13happening
48:14in
48:14the
48:14real
48:14universe
48:15because
48:16we
48:17thought
48:17it
48:17was
48:17ludicrous
48:18you
48:18shouldn't
48:18be
48:18able
48:18to
48:18do
48:19that
48:19and
48:20we
48:20actually
48:20found
48:21a
48:21mechanism
48:22that
48:22in
48:22fact
48:23we
48:23were
48:23able
48:23to
48:23show
48:24always
48:24intervenes
48:25whenever
48:25you
48:26try
48:26to
48:26make
48:26a
48:26time
48:26machine
48:27suppose
48:27that
48:28I
48:28have
48:28two
48:28wormhole
48:29mouths
48:29that
48:31I'm
48:31going
48:31to
48:31turn
48:31into
48:32a
48:32time
48:32machine
48:32of course
48:33I
48:34do
48:34it
48:34by
48:34taking
48:34that
48:35mouth
48:35and sending
48:35it off
48:36at high
48:37speed
48:37out into
48:38space
48:38it turns
48:39around
48:40and it
48:40comes
48:40back
48:41and just
48:41as it's
48:41on its
48:42way
48:42back
48:42it suddenly
48:43becomes
48:43a time
48:43machine
48:44and one
48:44can travel
48:45through it
48:45and get
48:46back
48:46before one
48:46started
48:47right
48:48as it's
48:49coming back
48:49and settling
48:49in
48:50just before
48:51the moment
48:51when it
48:51can become
48:52a time
48:52machine
48:53suddenly
48:54according
48:55to calculations
48:56we've done
48:56it explodes
48:57poof
48:59no more
49:01wormholes
49:01no more
49:01time
49:02machine
49:02question
49:03is why
49:04what made
49:05that happen
49:06what seems
49:07to happen
49:07is that
49:08particles
49:09can keep
49:09coming
49:10around
49:10again
49:11to the
49:11same
49:11event
49:12in
49:12space
49:12time
49:13this
49:13makes
49:14the
49:14energy
49:14density
49:15very
49:15large
49:16which
49:17warps
49:17space
49:17time
49:18so
49:18that
49:18travel
49:19into
49:19the
49:19past
49:20is
49:20no
49:21longer
49:21possible
49:22it
49:24seems
49:24that
49:24energy
49:25might
49:25pile
49:25up
49:26inside
49:26the
49:26wormhole
49:27until
49:27it
49:27self
49:27destructs
49:28you get
49:30in fact
49:30but when
49:31Thorne
49:32applied
49:32what is
49:32known
49:33of
49:33quantum
49:33gravity
49:34the
49:35picture
49:35changed
49:35again
49:36it
49:37appeared
49:37to us
49:37that
49:37just as
49:38that
49:38mechanism
49:38was about
49:39to in
49:39fact
49:40make
49:40your
49:40time
49:41machine
49:41explode
49:42at the
49:42moment
49:42you're
49:42trying
49:43to
49:43activate
49:43it
49:43right
49:44at that
49:44moment
49:45our
49:46calculations
49:47would fail
49:48because
49:48the new
49:48laws
49:49of
49:49physics
49:49the new
49:50laws
49:50of
49:50quantum
49:50gravity
49:51would
49:51take
49:51over
49:52and
49:52the
49:52techniques
49:52we
49:53had
49:53of
49:53computing
49:54what
49:54happens
49:54would
49:54no longer
49:56be
49:56any
49:56good
49:57so
49:58perhaps
49:59quantum
49:59gravity
50:00might
50:00allow
50:00the
50:01time
50:01machine
50:01to
50:01exist
50:02after
50:02all
50:02Thorne
50:06decided
50:06to see
50:06what
50:06Hawking
50:07made
50:07of
50:07this
50:07he
50:09got
50:10rather
50:11disturbed
50:11because
50:12Stephen
50:12is a
50:12defender
50:13of
50:13the
50:13establishment
50:13he
50:14really
50:14believed
50:14that
50:15this
50:15really
50:15would
50:16be
50:16the
50:16mechanism
50:17to
50:17prevent
50:18you
50:18ever
50:18from
50:18making
50:19a
50:19time
50:19machine
50:19and
50:20said
50:20they
50:20have
50:20not
50:20correctly
50:21read
50:21the
50:21tea
50:21leaves
50:22of
50:22what
50:22quantum
50:22gravity
50:23is
50:23going
50:23to
50:23do
50:23ultimately
50:25after
50:26going
50:26back
50:26and
50:26forth
50:27with
50:27Stephen
50:27several
50:28times
50:28we
50:29converged
50:30to
50:30an
50:30agreement
50:30that
50:31he
50:31was
50:31probably
50:31right
50:32we
50:32were
50:32wrong
50:32the
50:33explosion
50:33probably
50:34will
50:34destroy
50:34the
50:35time
50:35machine
50:35but
50:36we
50:36can't
50:36be
50:37absolutely
50:37sure
50:38we
50:38won't
50:38know
50:39absolutely
50:39for
50:39sure
50:40until
50:40we
50:41have
50:41the
50:41full
50:41laws
50:42of
50:42quantum
50:42gravity
50:42in
50:43our
50:43hands
50:43and
50:43we
50:44can
50:44explore
50:44just
50:44what
50:45they
50:45say
50:45about
50:46the
50:46end
50:46point
50:46of
50:46the
50:46explosion
50:47in
50:51the
50:51meantime
50:51Hawking
50:52remains
50:52skeptical
50:53of
50:53time
50:53travel
50:54a
50:55belief
50:55he
50:56puts
50:56forward
50:56in
50:56what
50:57he
50:57calls
50:57the
50:57chronology
50:58protection
50:59conjecture
51:00the
51:01chronology
51:02protection
51:03conjecture
51:04would
51:04make
51:05the
51:05world
51:05safe
51:05for
51:06historians
51:06the
51:10most
51:10likely
51:10mechanism
51:11that
51:11would
51:11enforce
51:12Hawking's
51:12conjecture
51:13is
51:15the
51:15explosion
51:15of
51:16the
51:16wormhole
51:16but
51:21this
51:21is
51:21just
51:22the
51:22conjecture
51:22it's
51:24a
51:24kind
51:24of
51:25informed
51:26guess
51:26which
51:27may
51:27be
51:28mistaken
51:29that's
51:29what
51:30a
51:30conjecture
51:31is
51:31and
51:32the
51:32word
51:33conjecture
51:33in
51:33mathematics
51:34is
51:34often
51:35used
51:35as
51:35a
51:36challenge
51:36to
51:37future
51:37generations
51:37to
51:38try
51:38to
51:39prove
51:39or
51:39disprove
51:39it's
51:40not
51:40a
51:40principle
51:41it's
51:41not
51:41a
51:41theorem
51:42to
51:42conjecture
51:43because
51:43we
51:44do
51:44not
51:44yet
51:44know
51:45for
51:45certain
51:45that
51:46there
51:46is
51:46a
51:47mechanism
51:47in
51:47nature
51:48that
51:49will
51:49always
51:49enforce
51:50it
51:50well
51:51if
51:51you
51:51ask
51:52me
51:52do
51:53time
51:53travels
51:54really
51:55be
51:55possible
51:56in
51:56the
51:56future
51:57I
51:59must
51:59give
52:00the
52:00positive
52:00reply
52:02you see
52:02it
52:03will
52:04be
52:04possible
52:05I
52:06do
52:07not
52:07see
52:07a way
52:08to
52:08make
52:08time
52:08travel
52:09possible
52:10however
52:11if
52:13I
52:14heard
52:14that
52:15somebody
52:15else
52:16some
52:17other
52:17country
52:18was
52:20doing
52:20this
52:20I
52:21think
52:21I
52:21might
52:22plunge
52:22into
52:22the
52:23game
52:23I
52:24myself
52:24believe
52:24that
52:25there
52:25will
52:25one
52:25day
52:26be
52:26time
52:26travel
52:26because
52:27when
52:28we
52:28find
52:29that
52:29something
52:29isn't
52:30forbidden
52:30by
52:31the
52:31overarching
52:32laws
52:33of
52:33physics
52:33we
52:34usually
52:34eventually
52:35find
52:35a
52:35technological
52:36way
52:36of
52:36doing
52:37it
52:37I
52:37believe
52:38we
52:38will
52:38know
52:39in
52:3910
52:40to
52:4015
52:40years
52:41when
52:41we
52:41have
52:41the
52:41full
52:41laws
52:42of
52:42quantum
52:42gravity
52:42in
52:42our
52:43hands
52:43my
52:44best
52:44guess
52:45and
52:45I
52:45would
52:45be
52:45willing
52:46to
52:46lay
52:46a
52:46fairly
52:46heavy
52:47odds
52:47on
52:48this
52:48on
52:48a
52:48bet
52:48with
52:48hawking
52:49but
52:49he
52:49won't
52:49take
52:49the
52:50other
52:50side
52:50my
52:51best
52:51guess
52:51is
52:52that
52:52when
52:53we
52:53have
52:53those
52:53full
52:54laws
52:54in
52:54our
52:54hands
52:55they
52:55will
52:55say
52:55no
52:56you
52:56cannot
52:57make
52:57a
52:57time
52:58machine
52:58and
52:58go
52:58backward
52:59in
52:59time
52:59ever
52:59but
53:00until
53:00we
53:00have
53:01those
53:01full
53:01laws
53:01we
53:01just
53:02have
53:02to
53:02leave
53:03it
53:03as
53:03a
53:04possibility
53:05that
53:06remains
53:07a
53:07possibility
53:08I
53:09wouldn't
53:09take
53:10a bet
53:10against
53:10the
53:11existence
53:11of
53:12time
53:12machines
53:13my
53:14opponent
53:15might
53:15have
53:15seen
53:16the
53:16future
53:16but
53:17know
53:17the
53:17answer
53:18Einstein
53:28showed
53:29that
53:29space
53:30is
53:30curved
53:30and
53:31time
53:31is
53:32relative
53:32on
53:33NOVA's
53:34website
53:34do a
53:35simple
53:35thought
53:35experiment
53:36and learn
53:37to think
53:37like the
53:38century's
53:38greatest
53:38scientist
53:39next
53:41time
53:41on
53:41NOVA
53:42the
53:42case
53:43that
53:43inspired
53:43the
53:44fugitive
53:44did
53:45Dr.
53:45Sam
53:46Shepard
53:46murder
53:46his
53:47wife
53:47he
53:48was
53:48an
53:48easy
53:48target
53:49new
53:50evidence
53:51on
53:51the
53:51killer's
53:52trail
53:52educators
53:59can order
53:59this
54:00or any
54:01other
54:01NOVA
54:01program
54:02for
54:02$19.95
54:03plus
54:03shipping
54:04and
54:04handling
54:04call
54:05WGBH
54:06Boston
54:07Video
54:07at
54:081-800-255-9424
54:11NOVA
54:13is a production
54:41of WGBH
54:42Boston
54:43major
54:49funding
54:49for NOVA
54:50is provided
54:50by
54:51the
54:51Park
54:52Foundation
54:52dedicated
54:53to
54:54education
54:54and
54:55quality
54:55television
54:56cnet
55:04bringing
55:04the
55:05digital
55:05age
55:05into
55:06focus
55:06cnet.com
55:10the
55:10source
55:10for
55:11computers
55:11and
55:12technology
55:13this
55:15program
55:16is
55:16funded
55:16in
55:17part
55:17by
55:17Northwestern
55:18Mutual
55:18Life
55:19which
55:20has
55:20been
55:20protecting
55:20families
55:21and
55:21businesses
55:21for
55:22generations
55:22have
55:23you
55:24heard
55:24from
55:24the
55:24quiet
55:24company
55:25Northwestern
55:26Mutual
55:27Life
55:27and
55:29by
55:29the
55:30Corporation
55:31for
55:31Public
55:31Broadcasting
55:32and
55:32viewers
55:33like
55:33you
55:33this
55:42is
55:42PBS
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